Coastal Engineering Report for Lower Honoapiilani Road ...

Coastal Engineering Report for Lower Honoapiilani

Road Erosion at Kaopala Bay

Lahaina, Maui, Hawaii

September 2018

Prepared for:

County of Maui, DPW

Engineering Division

200 S. High Street

Kalana O Maui Bldg. 4th

floor

Wailuku, HI 96793

Prepared by:

Sea Engineering, Inc.

Makai Research Pier

Waimanalo, HI 96795

Job No. 25619



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TABLE OF CONTENTS

1. INTRODUCTION ................................................................................................................. 1

2. PROJECT SITE DESCRIPTION ........................................................................................ 2

2.1 SITE LOCATION ................................................................................................................. 2

2.2 SITE VISIT AND SHORELINE DESCRIPTION ......................................................................... 2

2.2.1 Profile North ............................................................................................................. 9

2.2.2 Profile South ........................................................................................................... 10

2.3 OFFSHORE BENTHIC HABITATS ....................................................................................... 14

2.4 HISTORICAL SHORELINE ANALYSIS AND SHORELINE SETBACK ...................................... 15

3. OCEANOGRAPHIC SETTING AND DESIGN PARAMETERS ................................. 18

3.1 WIND CLIMATE ............................................................................................................... 18

3.1.1 Extreme Winds ........................................................................................................ 19

3.2 CURRENTS AND CIRCULATION ........................................................................................ 21

3.3 WAVE CONDITIONS ......................................................................................................... 21

3.3.1 Deepwater Wave Heights ........................................................................................ 23

3.3.2 Nearshore Wave Transformation ............................................................................ 26

3.3.3 Hurricanes and Tropical Cyclones ......................................................................... 28

3.4 TIDE AND WATER LEVEL RISE ........................................................................................ 28

3.4.1 Tides ........................................................................................................................ 28

3.4.2 Mesoscale Eddies and Other Oceanographic Phenomena ..................................... 29

3.4.3 Wave Setup .............................................................................................................. 30

3.4.4 Storm Surge ............................................................................................................. 30

3.4.5 Sea-Level Rise ......................................................................................................... 31

3.4.6 Estimating the Effect of Sea-Level Rise on the Shoreline ....................................... 35

3.4.7 Combined Stillwater Level ...................................................................................... 36

3.4.8 Wave Height at the Shoreline ................................................................................. 37

3.5 TSUNAMI ......................................................................................................................... 38

3.6 FLOOD INSURANCE RATING ............................................................................................ 40

3.7 COASTAL PROCESSES DISCUSSION .................................................................................. 41

4. SHORE PROTECTION ALTERNATIVES ..................................................................... 43

4.1 VULNERABILITY TO COASTAL HAZARDS ........................................................................ 43

4.2 SHORE PROTECTION ALTERNATIVES ............................................................................... 43

4.2.1 Emergency Shore Protection .................................................................................. 46

4.2.2 Seawall .................................................................................................................... 49

4.2.3 Rock Revetment ....................................................................................................... 52

4.2.4 Beach Nourishment ................................................................................................. 54

4.2.5 Shore Protection Impacts ........................................................................................ 55



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4.2.6 Shore Protection Impact Mitigation ....................................................................... 57

4.3 REGULATORY ENVIRONMENT ......................................................................................... 58

4.3.1 Overview ................................................................................................................. 58

4.3.2 Federal Requirements ............................................................................................. 58

4.3.3 State of Hawaii Requirements ................................................................................. 59

4.3.4 County of Maui Requirements ................................................................................ 60

5. RECOMMENDATIONS .................................................................................................... 62

5.1 RECOMMENDATIONS FOR FOLLOW-ON WORK .................................................................. 63

5.1.1 Additional Design ................................................................................................... 63

5.1.2 Environmental Review ............................................................................................ 63

5.1.3 Permits .................................................................................................................... 63

6. REFERENCES .................................................................................................................... 66

LIST OF FIGURES

FIGURE 1-1. ROADWAY THREATENED BY UNSTABLE SHORELINE ESCARPMENT .............................. 1

FIGURE 2-1. KAOPALA BAY REGIONAL OVERVIEW ......................................................................... 4

FIGURE 2-2. TMK MAP OF PROJECT AREA ....................................................................................... 4

FIGURE 2-3. KAOPALA BAY OVERVIEW AND PROFILE LOCATIONS .................................................. 5

FIGURE 2-4. TRANSITION FROM ROCK SUBSTRATE TO CLAY/SILT ALLUVIUM .................................. 6

FIGURE 2-5. ROCK OUTCROP IN PROJECT REACH (SEE FIGURE 2-3 FOR LOCATION) ......................... 6

FIGURE 2-6. BEACH AT NORTH END OF KAOPALA BAY FROM START OF THE ROCK REVETMENT ...... 7

FIGURE 2-7. 54-INCH DRAINAGE OUTLET ........................................................................................ 7

FIGURE 2-8. EXISTING REVETMENT (VIEW LOOKING NORTH); NOTE STEEPENING NEAR THE CREST .. 8

FIGURE 2-9. END OF ROCK REVETMENT........................................................................................... 8

FIGURE 2-10. START OF REVETMENT ON ROBINSON PROPERTY ....................................................... 9

FIGURE 2-11. PROFILE NORTH THROUGH ROCK REVETMENT ........................................................... 9

FIGURE 2-12. DRAINAGE PIPE AND CRM WALL; NOTE ROCK SUBSTRATE ...................................... 11

FIGURE 2-13. 24-INCH DRAINAGE OUTLET IN 2000. ....................................................................... 11

FIGURE 2-14. RUPTURE OF THE SOIL ADJACENT TO THE ROADWAY .............................................. 12

FIGURE 2-15. SHORELINE EROSION SCARP AND COBBLE BEACH SOUTH OF THE 24-INCH DRAINAGE

OUTLET; NOTE COLLAPSING TREE AND ROOT BALL ................................................................. 12

FIGURE 2-16. START OF ROCK REVETMENT ON SOUTH END OF THE PROJECT REACH ...................... 13

FIGURE 2-17. PROFILE SOUTH THROUGH SILT/CLAY EMBANKMENT ............................................... 13

FIGURE 2-18. OFFSHORE GEOMORPHOLOGY AT THE PROJECT SITE ................................................. 14



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FIGURE 2-19. OFFSHORE BIOTA IS PRIMARILY MACROALGAE ........................................................ 15

FIGURE 2-20. EROSION RATE MAP AT KAOPALA BAY ................................................................... 16

FIGURE 2-21. A PORTION OF THE 1912 T-SHEET SHOWING THE PROJECT AREA ............................. 17

FIGURE 3-1. WIND ROSE FOR KAPALUA AIRPORT (STATION PHJH) .............................................. 19

FIGURE 3-2. COMMON WAVE TYPES AND APPROACH DIRECTIONS IN HAWAII................................ 23

FIGURE 3-3. WAVE EXPOSURE AT KAOPALA BAY .......................................................................... 23

FIGURE 3-4. VIRTUAL BUOY LOCATIONS ........................................................................................ 24

FIGURE 3-5. A) WAVE HEIGHT ROSE AND B) WAVE PERIOD ROSE FOR NOAA HINDCAST STATION

81102 ..................................................................................................................................... 25

FIGURE 3-6. A) WAVE HEIGHT ROSE AND B) WAVE PERIOD ROSE FOR NOAA HINDCAST STATION

81115 ..................................................................................................................................... 25

FIGURE 3-7. 50-YEAR NORTH SWELL TRANSFORMATION AROUND OAHU AND MAUI NUI .............. 27

FIGURE 3-8. NOS TIDE RECORD FOR AUGUST, 2017, SHOWING ANOMALOUS SEA LEVEL RISE ....... 30

FIGURE 3-9. GLOBAL SEA LEVEL TRENDS ................................................................................... 33

FIGURE 3-10. MEAN SEA LEVEL TREND, KAHULUI HARBOR, 1947 TO 2015 (NOAA, 2017) .......... 33

FIGURE 3-11. SCENARIOS FOR PROJECTED RELATIVE SEA-LEVEL RISE, KAHULUI HARBOR

(USACE, 2014) ...................................................................................................................... 34

FIGURE 3-12. COASTAL EROSION AT KAOPALA BAY ..................................................................... 35

FIGURE 3-13. TSUNAMI EVACUATION MAP, KAOPALA BAY ........................................................... 39

FIGURE 3-14. FLOOD INSURANCE RATE MAP FOR KAOPALA BAY (MAP NO. 1500030264F) ........ 40

FIGURE 4-1. MANAGED RETREAT OPTION WITH NEW MAUKA TRANSIT CORRIDOR USING RAILWAY

RIGHT-OF-WAY ....................................................................................................................... 45

FIGURE 4-2. ELCOROCK REVETMENT LAYOUT ............................................................................... 48

FIGURE 4-3. ELCOROCK REVETMENT SECTION .............................................................................. 48

FIGURE 4-4. TYPICAL SEAWALL SECTION ....................................................................................... 51

FIGURE 4-5. SCHEMATIC OF CRM WALL USED ON REVETMENT PROTECTED REACHES .................. 52

FIGURE 4-6. TYPICAL ROCK REVETMENT SECTION ......................................................................... 54

FIGURE 4-7. AN EXAMPLE OF END EFFECTS .................................................................................... 57

FIGURE 4-8. EXAMPLE OF PASSIVE EROSION AT KAHANA BAY ...................................................... 57

FIGURE 4-9. APPROXIMATE JURISDICTIONAL BOUNDARIES AT KAOPALA BAY .............................. 58

FIGURE 5-1. SCHEMATIC REPRESENTATION OF NEW CRM SEAWALL. ........................................... 64

LIST OF TABLES

TABLE 3-1. ANNUAL MAXIMUM 2-MINUTE WIND SPEEDS AT DANIEL K. INOUYE INTERNATIONAL

AIRPORT ................................................................................................................................. 20

TABLE 3-2. EXTREME VALUE DISTRIBUTION PERIODS FOR 2-MINUTE AVERAGED WIND SPEEDS AT

DANIEL K. INOUYE INTERNATIONAL AIRPORT (1969 TO 2012) .............................................. 21



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TABLE 3-3. DEEPWATER WAVE HEIGHTS BY RETURN PERIOD FOR NORTH PACIFIC AND SOUTH

PACIFIC SWELL ....................................................................................................................... 26

TABLE 3-4. 50-YEAR WAVE PARAMETERS ...................................................................................... 27

TABLE 3-5. WATER LEVEL DATA FOR KAHULUI HARBOR (NOAA) ............................................... 29

TABLE 3-6. ANNUAL MINIMUM ATMOSPHERIC PRESSURE AT HONOLULU AIRPORT AND ESTIMATED

PRESSURE RELATED WATER LEVEL RISE ................................................................................. 31

TABLE 3-7. PROJECTED RELATIVE SEA-LEVEL RISE, KAHULUI HARBOR (NOAA, 2017) ............. 34

TABLE 3-8. NON-STORM POTENTIAL STILL WATER LEVEL RISE AT THE SITE FOR A 50-YEAR RETURN

PERIOD NORTH SWELL ............................................................................................................. 37

TABLE 3-9. 50-YEAR DESIGN WATER DEPTH AND WAVE HEIGHT .................................................... 38

TABLE 4-1. ANTICIPATED ENVIRONMENTAL REVIEW AND REGULATORY PERMITTING

REQUIREMENTS ...................................................................................................................... 61

TABLE 5-1. COMPARISON OF ALTERNATIVES ................................................................................. 65



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1. INTRODUCTION

During the winter of 2017-2018, the shoreline erosion scarp at Kaopala Bay in West Maui began

to encroach on Lower Honoapiilani Road. The County of Maui, Department of Public Works

(DPW) retained Sea Engineering, Inc. (SEI) to evaluate the shoreline erosion processes at this

location and provide alternatives for temporary and permanent protection.

The coastline of West Maui has been severely degraded over the last few years by a combination

of oceanographic factors. The winter wave season is usually tempered by the presence of the

island of Molokai, which blocks much of the wave activity from the northwest. However, the

Pailolo channel is open to the north, and waves from the north-northwest through north-northeast

directions can approach the West Maui coastline unimpeded. Two highly energetic winters

(2015-2017), and persistent wave activity from the north over those winters and during 2018

have contributed to the severe degradation of the West Maui shoreline.

Additionally, sea level was elevated from 2016 to early 2018 across all the Hawaiian Islands,

with tide levels regularly measuring approximately 0.3 to 0.9 ft above predicted levels. This

phenomenon was thought to be related to the 2015-2016 El Niño – Southern Oscillation (ENSO)

event. The effects of elevated sea level include higher wave heights at the shoreline that can

increase erosional effects.

Recent erosion has brought the shoreline escarpment dangerously close to the makai edge of

Lower Honoapiilani Road, with a high potential for undermining the road (Figure 1-1). At the

location shown in the figure, the root ball of a collapsing tree is causing the soil to rupture and

fail. The Maui County DPW has requested assistance with evaluating the existing conditions at

the site and the consideration of potential solutions to reduce the road’s vulnerability to coastal

hazards.

Figure 1-1. Roadway threatened by unstable shoreline escarpment




2. PROJECT SITE DESCRIPTION

2.1 Site Location

Kaopala Bay is located on the west shore of the island of Maui, approximately six miles north of

the town of Lahaina. The physical geography of the region is dominated by the ancient West

Maui Volcano, which has collapsed and eroded into the West Maui Mountains. The nearly

circular shape of the volcano has generated a similarly curved shoreline. The area is part of the

Maui Nui complex, which includes the islands of Maui, Lanai, Molokai, and Kahoolawe. The

islands form a ring of protection that limits wave exposure (see Figure 3-3). The channels

between the islands shape the tide-generated currents, and the prominent land masses, especially

Haleakala volcano, greatly affect the local wind conditions. Kaopala Bay borders the Pailolo

channel, which runs between Maui and Molokai.

A regional location map is shown in Figure 2-1. Kaopala Bay is bounded by Haukoe Point to

the north and Kalaeokaia Point to the south. The project site is approximately 600 ft in length,

and consists of a reach that borders the county road and is between private property boundaries –

the Sarkin Estate to the north (on Haukoe Point), and the Robinson Estate to the South (see TMK

map, Figure 2-2). Kaopala gulch runs from the West Maui Mountains and contains an

ephemeral stream that flows into Kaopala Bay during periods of rain via a 54-inch drainage pipe

toward the north end of the bay. An 18-inch drainage outlet is located at the north end of the bay

in the lee of Haukoe Point, and a 24-inch outlet is located in the middle of the bay near the study

site. Figure 2-3 is an overview of Kaopala Bay showing the project reach and geographic and

geomorphic features described in this report.

2.2 Site Visit and Shoreline Description

Coastal engineers from Sea Engineering, Inc. (SEI) conducted a site visit of Kaopala Bay on

January 31, 2018. The investigation was conducted for DPW due to concern about potential

undermining of Lower Honoapiilani Road. The site work included profile measurements and

mapping of the existing erosion scarp, edge of pavement location, and other shoreline features.

The mapping was done using a survey total station referenced to a previous topographic survey

conducted in approximately the year 2000 that was provided by DPW.

The Kaopala Bay shoreline faces northwest and is a combination of sandy, rocky, and engineered

shoreline features. Haukoe Point is a rock headland that bounds Kaopala Bay on the north. A

narrow sand beach is located on the north side, in the lee of Haukoe Point. However, most of the

beach area is composed of basalt cobbles overlying a rock substrate. The rock grades upward

into cohesive red clay and silt alluvium (Figure 2-4), and the shoreline is characterized by a near

vertical escarpment of this material. Lower Honoapiilani Road approaches close to this

shoreline escarpment in this area as it skirts the bay. The closest point of approach of the




roadway to the escarpment at the time of the site visit was approximately 6 ft. The crest of the

scarp and the roughly horizontal platform that contains the roadway are at an elevation of

approximately 15 to 17 ft above MSL.

The rock substrate varies in competency and elevation. A raised and indurated outcrop within

the project area was used as a survey base station (Figure 2-5).

The north part of the bay at Haukoe Point has a narrow beach (Figure 2-6). The approximate

southern end of the beach is at the 54-inch drainage outlet, although this likely varies over time.

The 54-inch outlet is surrounded by a CRM headwall (Figure 2-7). The pipe and headwall are in

reasonably good condition, although the CRM needs some repair at the beach level.

The shoreline escarpment on each side of the 54-inch drainage outlet CRM headwall is protected

with a basalt rock revetment for approximately 450 ft, from Haukoe Point to a position

approximately 30 ft north of the rock outcrop (see Figure 2-3), and approximately 100 ft north of

the 24-inch drainage outlet. The revetment is in a degraded condition caused by down-slope

movement of armor stone at the toe and lower section. As a result, the revetment profile is steep

near the crest, and more gently sloping near the waterline (Figure 2-8). The 30-ft reach between

the revetment and rock outcrop is unprotected, with an eroding scarp that approaches to within 6

ft of the edge of the roadway (see Figure 2-9).

At the south end of the bay, the shoreline transitions back into another basalt rock revetment that

fronts the Robinson estate and continues south to Kalaeokaia Point (Figure 2-10).

The area of most concern is the unprotected reach between the two revetments, and includes the

24-inch drainage outlet, the at-risk area caused by a collapsing tree shown in Figure 1-1, and the

30-ft reach between the rock outcrop and north revetment (see Figure 2-3). Part of this reach

may include private land of the Robinson property as it also fronts at-risk portions of the road.

Two profiles were surveyed in Kaopala Bay: Profile North was surveyed through the rock

revetment at the middle of the bay and Profile South was surveyed at the location where the road

was closest to the erosion scarp. Profile locations are shown in Figure 2-3.




Figure 2-1. Kaopala Bay Regional Overview

Figure 2-2. TMK map of project area

Haukoe Point

Kalaeokaia

Point

Kaopala

Gulch




Figure 2-3. Kaopala Bay overview and profile locations




Figure 2-4. Transition from rock substrate to clay/silt alluvium

Figure 2-5. Rock outcrop in project reach (see Figure 2-3 for location)




Figure 2-6. Beach at north end of Kaopala Bay from start of the rock revetment

Figure 2-7. 54-inch drainage outlet




Figure 2-8. Existing revetment (view looking north); note steepening near the crest

Figure 2-9. End of rock revetment, taken from rocky outcrop; note exposed scarp on the right




Figure 2-10. Start of revetment on Robinson property

2.2.1 Profile North

Profile North was taken through the rock revetment and is shown in Figure 2-11. The distance

from the makai edge of the road pavement to the top of the shoreline scarp at Profile North is 14

feet. The nominal land elevation between the revetment crest and the edge of the pavement is 17

feet above mean sea level (MSL). The revetment steepness is approximately 1 vertical to 0.5

horizontal (note: standard revetment design steepness is a maximum of 1 vertical to 1.5

horizontal).

Figure 2-11. Profile North through rock revetment




2.2.2 Profile South

The shoreline along the southern portion of the project reach includes a steep red clay and silt

shoreline scarp fronted by a cobble beach. The clay embankment continues south from the rocky

outcrop described in the previous section, and eventually transitions to another rock revetment

fronting the Robinson property (see Figure 2-3).

South of the rock outcrop shown in Figure 2-5, the red clay and silt shoreline scarp approaches

within 10 ft of the makai edge of the road. A 24-inch concrete drainage pipe with a CRM

headwall protrude from the clay embankment, approximately 60 feet to the south of the rock

outcrop (Figure 2-12). The CRM is loosely grouted and appears to be constructed with stone

cobbles from the beach. Figure 2-13 is a photograph of the drainage outlet taken in the year

2000. It shows that a significant amount of erosion of the shoreline scarp has occurred since

that time.

South of the drainage pipe is a series of undermined and partially collapsed trees located between

the clay embankment and the makai edge of the pavement. The undermining has been caused by

erosion of the shoreline scarp. The unsupported trees fall seaward, causing movement of the root

ball and disturbance to the surrounding soil. One of the collapsing trees is causing the ground to

rupture within 6 inches of the makai edge of the pavement (Figure 2-14, see also Figure 1-1).

Others are undermined and near collapse (Figure 2-15).

Further south, the shoreline trends away from the road and is protected by another rock

revetment (Figure 2-16). The rock revetment fronts the privately owned Robinson Estate (TMK

4-3-015:036).

Profile South (Figure 2-17) is located between the 24-inch drainage pipe and the partially

collapsed tree. The distance between the makai edge of the road pavement and the top of the

shoreline scarp was measured to be 8 feet. The elevation of the road at the south profile is 15

feet above MSL.




Figure 2-12. Drainage pipe and CRM wall; note rock substrate

Figure 2-13. 24-inch drainage outlet in 2000; significant shoreline erosion has occurred since that

time.




Figure 2-14. Rupture of the soil adjacent to the roadway caused by the collapsing tree

Figure 2-15. Shoreline erosion scarp and cobble beach south of the 24-inch drainage outlet; note

collapsing tree and root ball




Figure 2-16. Start of rock revetment on south end of the project reach

Figure 2-17. Profile South through silt/clay embankment




2.3 Offshore Benthic Habitats

Benthic habitats offshore of the project site were mapped using data published on-line through

the Pacific Islands Ocean Observing System (PacIOOS) Voyager program, a public resource that

contains maps of the offshore benthic habitats in the Hawaiian Islands. The data consist of both

marine geomorphology and marine biology (http://www.pacioos.hawaii.edu/voyager).

Marine geomorphology of Kaopala Bay presented by PacIOOS is shown in Figure 2-18. The

figure shows that the predominant geomorphology offshore of the project site consists of a

“pavement” seafloor, with sand habitats occurring to the south. Pavement is flat, low-relief,

solid carbonate rock derived from fossil reef platforms.

Marine biota offshore of the project site is shown in Figure 2-19. The figure shows that the area

offshore of the project site is categorized as macroalgae with 10-50% coverage. The sandy area

to the south is un-colonized in some areas. Areas with 50-90% macroalgal coverage are also

offshore of the project site.

Figure 2-18. Offshore geomorphology at the project site consists of a carbonate pavement and

sand bottom (PacIOOS)




Figure 2-19. Offshore biota is primarily macroalgae (PacIOOS)

2.4 Historical Shoreline Analysis and Shoreline Setback

As part of a comprehensive shoreline mapping program, the University of Hawaii Coastal

Geology Group (UHCGG) conducted an historical aerial photographic analysis of Maui beaches

(UHCGG, 2003). The UHCGG used aerial photographs dating from 1949 to 1997 to compare

shorelines and calculate shoreline change rates. The photographs have been ortho-rectified and

geo-referenced, and the low water marks on the photographs digitized to provide a record of

long-term changes. In some areas, survey maps from 1912, known as “T-Sheets”, are also used

for reference. The photographs and maps are used to quantify coastal erosion based on changes

in the low water positions. This methodology captures erosion and accretion trends that include

fluctuations in beach width, as well as erosion of the shoreline scarp. The resulting erosion map

for the project area is shown in Figure 2-20.

The UHCGG data are used to calculate an Annual Erosion Hazard Rate (AEHR) based on

averaged movement of the shoreline reference feature (i.e., the low water position) relative to an

arbitrary baseline. The AEHR at Kaopala Bay is 2.4 feet per year, meaning that the digitized

historical shorelines showed a landward movement trend of 2.4 feet per year during the study

period. To the south of the project site, at Kalaeokaia Point, AEHR’s drop to less than 0.5 feet

per year. The high erosion rate towards the middle of the bay is due to the measured position of




the 1912 shoreline, which may be a statistical and chronological outlier. The 1912 shoreline was

located nearly 100 feet seaward of the 1949 shoreline. Figure 2-20 shows the T-sheet drawing of

the project area, and illustrates the potential problems with trying to accurately scale the

shoreline position. Since 1949, the shoreline has experienced little landward movement.

Erosion trends since 1997 are hard to calculate from aerial images due to the presence of dense

vegetation between the road and the shoreline but it is likely that the present erosion rate is less

than 1 foot per year.

The AEHR calculated by the UHCGG is used by the County of Maui to determine the shoreline

setback line. Only minor construction activities are allowed in the shoreline setback area (the

area between the shoreline and the shoreline setback line) without a shoreline setback variance

(SSV).

Figure 2-20. Erosion Rate Map at Kaopala Bay (Univ. of Hawaii Coastal Geology Group)




Figure 2-21. A Portion of the 1912 T-sheet showing the project area and potential for scaling

inaccuracies

A comparison of the top bank position for the area north of, and adjacent to, the 24-inch drainage

line, as surveyed in the year 2000 and during the recent site visit, indicates between 12 ft and 6 ft

of erosion over 18 years (see Figures 2-12 and 2-13). The erosion rates thus calculated, 0.33

ft/year and 0.7 ft/year, are probably a reasonable range for erosion rates on this shoreline.

Observations of erosion in other locations in West Maui show that the erosion is both seasonal

and episodic, with weather and climate playing a major role.

As noted in the introduction (Section 1), high water levels due to tide, wave setup, and other

phenomena, and wave climate –the combination of swell directions, wave height and frequency

of occurrence, are primary factors that cause erosion of the shoreline. The combined phenomena

will have a highly variable frequency of occurrence, so the erosion rate should not be expected to

be steady. In addition, observations show high lateral variability, with erosion hotspots

developing over limited distances along the shoreline.




3. OCEANOGRAPHIC SETTING AND DESIGN PARAMETERS

Design of shore protection structures is based on knowledge of site specific environmental

parameters which affect the type, size, placement, and composition of a shore protection

strategy. Coastal engineering structures that protect life and property are generally designed for

a “worst case” wave condition such as occurs during a hurricane or large storm, or from a similar

extreme event with a low statistical probability of occurrence. A 50-year recurrence interval

wave event is typically used for coastal engineering design criteria.

Although deep water waves caused by severe storm conditions can be very large, wave heights in

shallow water are physically limited by the water depth. The physical phenomena of waves,

wind, and pressure reduction during storm conditions can cause an elevated water level that will

in turn allow larger than normal waves to reach the shoreline and attack shoreline structures.

These waves are termed depth-limited and serve as a basis for calculating the wave conditions

applicable to shoreline structure design.

The following discussion of oceanographic design parameters is based on existing available data

and used for concept-level design development.

3.1 Wind Climate

The prevailing winds throughout the year in Hawaii are the northeast tradewinds. Tradewind

frequency varies from more than 90% during the summer to only 50% in January, with an

overall annual frequency of 70%. Tradewinds are produced by the outflow of air from the North

Pacific High. The center of this high-pressure system is typically located well north and east of

the Hawaiian chain, but it moves to the north and south seasonally. In the summer months, the

center moves to the north, causing the tradewinds to be at their strongest from May through

September. In the winter, the center moves to the south, resulting in decreasing tradewind

frequency from October through April. During these months, the tradewinds continue to blow;

however, their average monthly frequency decreases to 50%. The blocking effect of the West

Maui Mountains decreases the influence of tradewinds in the Kahana area, and causes the winds

that occur to come from a more northerly direction, following land contours. Offshore, wind

speeds in the channels between Maui, Molokai, and Lanai can be significantly faster due to the

funneling effect caused by the land masses surrounding the Pailolo channel. The shoreline is

typically sheltered, with variable winds near the beach and a wind line offshore of Haukoe Point

during most tradewind conditions.

Wind patterns of a more transient nature increase during the winter months. Winds from extra-

tropical storms can be strong from almost any direction, depending on the strength and position

of the storm. The low-pressure systems associated with these storms typically track west to east




across the North Pacific, north of the Hawaiian Islands. Kona winds generally come from a

southerly to southwesterly direction, and are sometimes associated with slow moving low

pressure systems known as “Kona lows” situated near the island chain. These storms are often

accompanied by heavy rains.

Figure 3-1 is a wind rose diagram for wind data recorded in 2014 at the Kapalua Airport, 1.5

miles south of Kaopala Bay. The wind rose indicates the frequency of occurrence based on wind

speed and direction, and shows the dominance of NE tradewind flow.

Figure 3-1. Wind rose for Kapalua Airport (Station PHJH)

3.1.1 Extreme Winds

The wind record at the Kapalua Airport is not sufficiently complete to calculate historical trends.

The Daniel K. Inouye International Airport on Oahu (formerly the Honolulu International

Airport) is a good proxy for estimating extreme wind that may occur at Kaopala Bay. An

extreme value distribution was applied to the 44 annual maximum 2-minute averaged wind speed

records to develop expected extreme wind speeds for various return periods ranging from 1 to

100 years. The extreme value wind speeds are presented in Table 3-2. The maximum 2-minute

averaged wind listed in Table 3-1 (46 mph) is equivalent to the 50-year wind speed from Table

3-2.




Table 3-1is a list of the yearly maximum 2-minute averaged wind speeds at the airport for the 44-

year period from 1969 to 2012, ordered by wind speed. Some of the annual maxima shown are

associated with specific storm events. The highest value of 46 miles per hour in November of 1982

occurred during the passage of Hurricane Iwa. Other events include Hurricane Iniki in 1992 at 38

miles per hour, and several severe Kona storms with wind speeds up to 40 miles per hour.

An extreme value distribution was applied to the 44 annual maximum 2-minute averaged wind

speed records to develop expected extreme wind speeds for various return periods ranging from

1 to 100 years. The extreme value wind speeds are presented in Table 3-2. The maximum 2-

minute averaged wind listed in Table 3-1 (46 mph) is equivalent to the 50-year wind speed from

Table 3-2.

Table 3-1. Annual maximum 2-minute wind speeds at Daniel K. Inouye International Airport

Year Wind Speed

(mph) Year

Wind Speed

(mph) 1982 46 1972 33

1969 40 1973 33

1970 40 1993 33

2004 40 1999 33

2007 39 2010 33

2011 39 1997 32

1976 38 2003 32

1992 38 2005 32

1977 37 1994 31

2008 37 2006 31

1975 36 1981 30

2002 36 1985 30

1980 35 1986 30

1990 35 1989 30

2000 35 2012 30

2001 35 1987 29

2009 35 1988 29

1971 34 1984 28

1974 34 1991 28

1978 34 1995 28

1979 34 1996 28

1998 34 1983 23




Table 3-2. Extreme value distribution periods for 2-minute averaged wind speeds at Daniel K.

Inouye International Airport (1969 to 2012)

Return Period

(years)

Wind Speed

(mph) 1 33.3

2 35.6

5 38.7

7 39.8

10 41.0

15 42.3

20 43.3

25 44.0

30 44.6

40 45.6

50 46.3

75 47.7

100 48.6

3.2 Currents and Circulation

Local currents in the Hawaiian Islands are generally driven by the semi-diurnal tides. Surface

currents can also be driven by the wind, and currents nearshore are predominately affected by the

presence of reefs and breaking waves. Storlazzi and Jaffe (2006) found that vigorous tradewind

conditions that prevail during the summer season caused relatively strong downwind currents.

During periods of calm, termed “relaxation events”, currents were tide-dominated and skewed to

the northeast. Large wave conditions prevalent during winter months induced offshore flows.

Nearshore wave-derived currents include rip currents and longshore currents that can be

dangerous to swimmers.

3.3 Wave Conditions

Surrounded by the Pacific Ocean, the Hawaiian Islands are subject to wave approach from all

directions. The wave climate in Hawaii is typically characterized by five general wave types.

These include northeast tradewind waves, southeast tradewind swell, southern swell, North

Pacific swell, and Kona wind waves (Figure 3-2). Tropical storms and hurricanes also generate

waves that can approach the islands from virtually any direction. Any and all of these wave

conditions may occur at the same time.

Kaopala Bay is at the center of the Maui Nui complex, which consists of the islands of Maui,

Lanai, Molokai, and Kahoolawe. These islands shelter Kaopala Bay and limit the directions of

direct wave exposure (Figure 3-3).




Tradewind waves occur throughout the year and are the most persistent in April through

September when they usually dominate the local wave climate. They result from the strong and

steady tradewinds blowing from the northeast quadrant over long fetches of open ocean.

Tradewind deepwater waves are typically between 3 to 8 feet in height with periods of 5 to 10

seconds, depending upon the strength of the tradewinds and how far the fetch extends east of the

Hawaiian Islands. The direction and approach, like the tradewinds themselves, varies between

north-northeast and east-northeast and is centered on the northeast direction. Although Kaopala

Bay is sheltered from the direct approach of tradewind waves by the island of Maui, a portion of

the tradewind wave energy reaches the area by wrapping around the north end of the island and

contributes to a generally rough offshore sea state during tradewind conditions. The tradewind

waves may also wrap around West Maui from the south.

Waves can also be generated by the southeastern tradewinds that blow south of the equator and

can occur any time during the year. Southeast tradewind swell has small wave heights on the

order of 1 foot and typical periods less than 12 seconds. These waves are not typically used for

design criteria but may be important for sediment transport in some areas due to their frequency

of occurrence. However, southeast tradewind swell is not expected at Kaopala Bay because of

island shadowing.

Southern swell is generated by storms in the southern hemisphere and is most prevalent during

the months of April through September. Traveling distances of up to 5,000 miles, these waves

arrive with relatively low deepwater wave heights of 1 to 4 feet and long periods of 14 to 20

seconds. Depending on the positions and tracks of the southern hemisphere storms, southern

swell approaches from the southeast through southwest directions.

During the winter months in the northern hemisphere, strong storms are frequent in the North

Pacific in the mid-latitudes and near the Aleutian Islands. These storms generate large North

Pacific swells that range in direction from west-northwest to northeast and arrive at the northern

Hawaiian shores with little attenuation of wave energy. Deepwater wave heights often reach 10

feet and in extreme cases can reach over 20 feet. Wave periods vary between 12 and 20 seconds,

depending on the location of the storm. The island of Molokai shelters Kaopala Bay from the

direct impact of northwest swell. However, swells with a more northerly direction can produce

significant wave energy at Kaopala Bay, creating large shorebreak and strong longshore currents.

Kona storm waves are generated by mid-latitude low-pressure system and occur at random

intervals throughout the year, with higher frequency during the winter months. They approach

from the south through west directions. Some winter seasons have several Kona storms; others

have none. Kona storm waves typically have periods ranging from 6 to 10 seconds; wave

heights are dependent upon the storm intensity, but deepwater heights can exceed 15 feet.




Figure 3-2. Common wave types and approach directions in Hawaii

Figure 3-3. Wave exposure at Kaopala Bay

3.3.1 Deepwater Wave Heights

Historical deepwater wave data were obtained using offshore wave hindcasts. Wave hindcasting

is a tool used to calculate past wave events based on weather models and historical data. With




proper inputs, wave hindcast models can calculate historical wave climates anywhere in the

world. Hindcast model outputs are often recorded for a single location, known as a virtual buoy.

WaveWatch III (WWIII) is a numerical wave model used to forecast and hindcast waves.

Hindcast wind data for a 29-year period (1979-2008) are available at a wide range of locations

through the National Oceanographic and Atmospheric Administration (NOAA/NCEP). For this

assessment, hindcast data were obtained from NOAA virtual buoy 81115, located 60 miles to the

southwest of Kaopala Bay, and virtual buoy 81102, located 62 miles to the northeast of Kaopala

Bay (Figure 3-4). Wave height and period roses for NOAA virtual buoy 81102 and 81115 are

shown in Figure 3-5 and Figure 3-6, respectively.

A Weibull extreme value distribution was applied to the hindcast data in order to determine

recurrence interval wave heights for each directional sector. Table 3-3 shows wave heights by

return period for both North Pacific and South Pacific Swell. The 50-year wave height is a

recurrence value that is expected at the site approximately once every 50 years, or a 2% chance

of occurring during any given year. This wave height is frequently used as the design wave

height for shore protection structures. The 50-year North Pacific swell has a significant wave

height of 32.7 feet, a peak period of 18 seconds, and a direction from the NW. The 50-year

South Pacific swell has a deepwater wave height of 7.2 feet, a peak period of 18 seconds, and a

direction from the SSW.

Figure 3-4. Virtual buoy locations




a) b)

Figure 3-5. a) Wave height rose and b) wave period rose for NOAA Hindcast Station 81102

a) b)

Figure 3-6. a) Wave height rose and b) wave period rose for NOAA Hindcast Station 81115




Table 3-3. Deepwater wave heights by return period for North Pacific and South Pacific Swell

Return Period

(years)

North Pacific Swell

Hs (ft)

(from WIS 81115)

Southern Swell

Hs (ft)

(from WIS 81102)

Typical Seasonal High 15.9 3.4

1 21.3 4.6

2 23.3 5.1

5 26.0 5.7

10 28.0 6.2

25 30.7 6.8

50 32.7 7.2

3.3.2 Nearshore Wave Transformation

As deepwater waves propagate toward shore, they begin to encounter and be transformed by the

ocean bottom. In shallow water, the wave speed becomes related to the water depth. As waves

slow with decreasing depth, the process of wave shoaling steepens the wave and increases the

wave height. Wave breaking occurs when the wave profile shape becomes too steep to be

maintained. This typically occurs when the ratio of wave height to water depth is about 0.78, and

is a mechanism for dissipating the wave energy. Wave energy is also dissipated due to bottom

friction. The phenomenon of wave refraction is caused by differential wave speed along a wave

crest as the wave passes over varying bottom contours, and will cause wave crests to converge or

diverge and may locally increase or decrease wave heights. Not strictly a shallow water

phenomenon, wave diffraction is the lateral transmission of wave energy along the wave crest,

and will cause the spreading of waves in a shadow zone, such as occurs behind a breakwater or

other barrier.

Output from a SWAN (Simulating Waves Nearshore) numerical wave model for the 50-year

North Pacific swell event is shown in Figure 3-7 and illustrates the transformation that a wave

undergoes as it approaches West Maui. Wave models indicate that breaking wave heights

offshore of West Maui for both North Pacific and southern swell is approximately 60% of the

deepwater wave heights. This transmission coefficient (Kt) is the combined effects of wave

transformation phenomena, including wave shoaling, wave refraction, and wave diffraction. A

summary showing the breaking wave height parameters offshore of Kaopala Bay for the 50-year

North Pacific and southern swell events is shown in Table 3-4.




Figure 3-7. 50-year north swell transformation around Oahu and Maui Nui

Table 3-4. 50-year wave parameters

50-year Event North Pacific Swell Southern Swell

Deepwater Wave

Height (ft) 32.7 7.2

Direction 315 190

Period (sec) 18 18

Transmission

Coefficient (Kt) 0.6 0.6

Breaking Wave

Height at Kaopala (ft) 19.3 4.4




3.3.3 Hurricanes and Tropical Cyclones

Tropical cyclones originate over warm ocean waters, and they are considered hurricane strength

when they generate sustained wind speeds over 64 knots (74 mph). Hurricanes that affect the

Hawaiian Islands form near the equator in the region between Mexico and the central North

Pacific. They typically track toward the west or northwest. These storms often pass south of the

Hawaiian Islands, though some have a northward curvature passing near the islands.

Only three hurricanes have passed through the Hawaiian Islands in the past 25 years: Hurricanes

Iwa in 1982 and Iniki in 1992, both passing near or over the island of Kauai as well as Hurricane

Iselle in 2014 passing over the island of Hawaii. These storms caused high surf and wave

damage on multiple shores of the islands. High wave conditions due to hurricanes and tropical

storms passing near the islands is not unusual during the Central Pacific hurricane season (June 1

through November 30).

Severe tropical storms and hurricanes have the potential to generate extremely large waves,

which could potentially damage the shoreline area at the project site. Although not frequent or

even likely events, they may be considered for project design conditions, particularly with regard

to shoreline structures.

3.4 Tide and Water Level Rise

The total water depth at a particular location is composed of the depth below the sea level datum,

plus factors that add to the still water level (SWL) such as the astronomical tide, mesoscale

eddies, wave setup, storm surge (pressure setup and wind setup), and potential sea level change

over the life of the project. The sea level datum used for navigation related projects (such as

harbor design) is the mean lower low water (MLLW) level, which represents the average

elevation of the lowest daily tides. Most topographic work by land surveyors use the mean sea

level (MSL) datum, which is an average of hourly tide levels.

3.4.1 Tides

Hawaii tides are semi-diurnal with pronounced diurnal inequalities (i.e., two tidal cycles each

day with the range of high and low water levels being unequal). Tidal predictions and historical

extreme water levels are given by the Center for Operational Oceanographic Products and

Services (COOPS), National Ocean Service (NOS), NOAA. The nearest tide station to Kaopala

Bay is at Kahului Harbor, 15 miles to the southeast of Kaopala Bay on the north side of Maui.

The water level data from this station is shown in Table 3-5 and is based on the 1983-2001 tidal

epoch.




There is no tide gauge at Lahaina Harbor, located 8 miles to the south of Kaopala Bay; however,

NOAA provides time offsets and height multipliers to predict tides at Lahaina Harbor. High and

low tides at Lahaina Harbor occur after high and low tide at Kahului Harbor. The time offset at

Lahaina Harbor relative to Kahului Harbor is +78 minutes for high tides and +61 minutes for low

tides. The height multiplier is 0.89 times the Kahului high tide (MLLW datum) and 0.81 times

the Kahului low tide (MLLW datum).

Table 3-5. Water level data for Kahului Harbor (NOAA)

Datum Elevation

(ft MLLW)

Elevation

(ft MSL)

Mean Higher High Water +2.3 1.1

Mean High Water +1.9 0.8

Mean Sea Level +1.1 0.0

Mean Low Water +0.3 -0.8

Mean Lower Low Water 0.0 -1.1

3.4.2 Mesoscale Eddies and Other Oceanographic Phenomena

Hawaii is subject to periodic extreme tide levels due to large oceanic eddies and other

oceanographic phenomena that have recently been recognized and that sometimes propagate

through the islands. Mesoscale eddies produce tide levels that can be up to 0.5 feet higher than

normal for periods up to several weeks (Firing and Merrifield, 2004).

A longer term sea level anomaly occurred from 2016 to 2018 across all the Hawaiian Islands,

with tide levels regularly measured to be approximately 0.3 to 0.9 ft above predicted levels. This

phenomenon is thought to be related to the 2015-2016 ENSO event and appeared to dissipate in

early 2018. Figure 3-8 shows predicted versus measured tides at the Kahului tide station in

August 2017.




Figure 3-8. NOS tide record for August, 2017, showing anomalous sea level rise of up to 1 ft (note

“Observed minus Predicted” values).

3.4.3 Wave Setup

During high wave events, the water level shoreward of the breaker zone may be elevated above

the tide level as a result of the wave breaking process. This water level rise, termed wave setup,

may be as much as 10 to 12% of the breaker height.

3.4.4 Storm Surge

During tropical storm and hurricane conditions, with high winds and very low pressures, an

additional water level rise, termed storm surge, can occur. Storm surge on continental coasts can

be amplified by the wide and shallow continental shelf. This type of surge is only minimally

present in Hawaii due to the narrow insular shelf that surrounds the islands. There is no storm

surge component to waves generated by distant storms.

All cyclonic storms, and tropical storms and hurricanes in particular, have a central zone of

lowered atmospheric pressure that causes an upward “bulge” of the water surface in the lower




pressure areas. The water level rise due to reduction of atmospheric pressure associated with a

storm is determined by the following equation:

Sp = 1.14 ΔP

The surge (Sp) is measured in feet, and the pressure units are inches of mercury. A 12-year

record of annual minimum pressure recorded at Honolulu Airport is shown in Table 3-6, along

with the estimated pressure related water level rise. The pressure related water level rise

generally varied between 0.2 and 0.5 feet during this time period. The minimum pressure

recorded in Honolulu during the passage of Hurricane Iniki was not unusually low; however, this

is not surprising as Iniki was a relatively small storm in terms of diameter, and the storm center

passed some 80 miles distant.

Table 3-6. Annual minimum atmospheric pressure at Honolulu Airport and Estimated Pressure

related water level rise

Year Annual Minimum

Pressure (in. Hg)

Pressure Setup

(feet)

1992 (Iniki) 29.52 0.5

1996 29.66 0.3

1997 29.73 0.2

1998 29.78 0.2

1999 29.86 0.1

2000 29.79 0.1

2001 29.71 0.2

2002 29.83 0.1

2003 29.76 0.2

2004 29.51 0.5

2005 29.73 0.2

2006 29.75 0.2

3.4.5 Sea-Level Rise

The effects of climate change have caused a world-wide rise in sea level. The present rate of

global mean sea-level rise (SLR) appears to be accelerating compared to the mean of the 20th

Century (Figure 3-9), but the rate of rise is locally variable (Fletcher et al, 2010). Factors

contributing to SLR include decreased global ice volume and thermal expansion of the ocean due

to warming. Recent climate research by the Intergovernmental Panel on Climate Change (IPCC)

predicts continued or accelerated global warming for the 21st Century and possibly beyond,

which would cause a continued or accelerated rise in global mean sea-level (USACE, 2011).




The present estimated rate of global mean sea level change (SLC) is + .13 in/yr (+3.4 mm/yr)

(NASA, 2017). It is estimated that global SLC may reach +3.3 feet (+1 meter) by the end of this

century, and the National Oceanic and Atmospheric Administration estimates possible SLR as

high as +10.8 feet (+3.3m). Sea level is highly variable, and there is some indication that

Hawaii’s location in the middle of the Pacific Ocean may result in a somewhat higher SLC than

high latitude regions due to reduced gravitation around melting polar ice caps (Sweet et al,

2017). SLC will have a significant impact on the shorelines and low-lying coastal areas.

Sea level trends and prediction scenarios are calculated at tide stations where sea level

measurements are available. The nearest NOS tide station to the project area is at Kahului

Harbor. The sea level trend there for the period of 1947 to present is shown in Figure 3-10

(NOAA, 2017). The rate of sea level change (RSLC) is shown in the figure as being

approximately linear, increasing at a rate of 0.08 in/yr ± 0.016 in (+2.04 ± 0.42 mm/yr) based on

monthly data for the period 1947 to 2015. Figure 3-10 also shows inter-annual anomalies of up

to approximately 0.5 feet (15 cm).

Estimates for sea-level rise for Kaopala Bay using five scenarios provided by the USACE (2014)

and NOAA (2017) for Kahului Harbor are shown in Figure 3-11 and Table 3-7. While the

projections are based on the most current scientific models and measurements, discretion is

necessary in selecting the appropriate scenario. Selecting the appropriate sea level change

projection is a function of many parameters, including topography, coastal setting, criticality of

infrastructure, potential for resilience, budget, and function. As an example, it may be best to

design a power plant or hospital based on the High or Extreme rate, since those are considered

critical infrastructure that would be expensive to modify and damage could have a long, far-

reaching impact. On the other hand, a revetment along a coastal road or park might be designed

based on a lesser rate, and the revetment could be adaptive and reconstructed as sea level rises,

or the road could be relocated as part of future mitigation plans.

The NOAA intermediate rate is considered a reasonable prediction for future sea level at

Kaopala Bay. The calculations predict an increase of 2.3 feet by 2070. For comparison, the low

and high estimates using the same procedure are 1.0 feet and 5.5 feet, respectively.




Figure 3-9. Global Sea Level Trends (State of Hawaii Climate Adaptation Portal)

Figure 3-10. Mean sea level trend, Kahului Harbor, 1947 to 2015 (NOAA, 2017)




Figure 3-11. Scenarios for projected Relative Sea-Level Rise, Kahului Harbor (USACE, 2014)

Table 3-7. Projected Relative Sea-Level Rise, Kahului Harbor (NOAA, 2017) (values in feet)

Year NOAA Low NOAA Int-

Low

NOAA Int NOAA Int-

High

NOAA

High

NOAA

Extreme

2000 0.00 0.00 0.00 0.00 0.00 0.00

2010 0.10 0.13 0.20 0.26 0.30 0.30

2020 0.26 0.33 0.43 0.52 0.62 0.69

2030 0.39 0.49 0.69 0.89 1.08 1.21

2040 0.52 0.69 0.98 1.35 1.74 1.94

2050 0.72 0.92 1.41 1.97 2.56 2.95

2060 0.89 1.12 1.84 2.62 3.51 4.13

2070 1.02 1.31 2.33 3.44 4.63 5.51

2080 1.15 1.51 2.85 4.36 5.94 7.09

2090 1.28 1.71 3.41 5.28 7.22 8.79

2100 1.44 1.90 4.07 6.50 9.06 11.02




3.4.6 Estimating the Effect of Sea-Level Rise on the Shoreline

The recently released Hawaii Sea Level Rise Vulnerability and Adaptation Report (Hawaii

Climate Change Mitigation and Adaptation Commission, 2017) uses a combination of historical

shoreline change trends and the RD-A model of shoreline profile change (Davidson-Arnott,

2005) to estimate shoreline recession due to sea level rise along Hawaii’s beaches. The results

have also been published in the PACIOOS Voyager Sea Level Rise Viewer. With 2.0 feet of sea

level rise (nearly equivalent to the NOAA Intermediate value for 2070), the report estimates that

the shoreline at Kaopala Bay would be located mauka of the current alignment of Lower

Honoapiilani Road for approximately 900 feet (Figure 3-12).

Figure 3-12. Coastal erosion at Kaopala Bay (PacIOOS Hawaii Sea Level Rise Viewer)

Despite being presented on the PACIOOS Voyager site, the model results shown in Figure 3-12

should be used with caution. The RD-A model was developed for sand beaches with well




developed dune systems as found on the east coast of the United States. Its application for

Hawaiian beach systems is constrained by assumptions that reduce its usefulness (see Anderson

et al, 2015). In fact, the cobble beach and clay and silt shoreline scarp existing at the project site

is not a coastal morphology that adapts well to existing shoreline change models.

3.4.7 Combined Stillwater Level

The various water level rise phenomena are additive and may occur at a given time. The total

still water level, therefore, is a linear combination of:

Astronomical tide and other oceanographic phenomena (Sa),

Sea level rise due to atmospheric pressure reduction (Sp),

Wind tide caused by wind stress component perpendicular to the coastline (Sx)

and parallel to the coastline (Sy),

Wave set-up on the beach as a result of the breaking waves (Sw),

Sea level rise (SSLR),

or,

S = Sa + Sp + Sx + Sy + Sw + SSLR

In order to predict a maximum potential water level at Kaopala Bay during a reasonable project

life (i.e. 50 years), best estimates of the above components must be obtained. An astronomical

tide of +2.3 feet (Mean Higher High Water at Kahului Harbor) is considered appropriate for this

discussion due to the frequency of occurrence of this tide level and the duration of distant-

generated swells. The 2070 NOAA Intermediate sea-level rise estimate is +2.3 feet. Mesoscale

eddies and other oceanographic phenomena frequently contribute +0.5 feet to the water level,

based on existing data. This elevation of regional sea level change has been witnessed several

times in the previous decade, and is suitable for planning purposes. The component of the still

water level rise due to astronomical tides and other oceanographic phenomena is thus +5.1 feet.

The combination of pressure setup (Sp) and wind setup (Sx and Sy) is defined as storm surge.

During non-storm conditions, there is no wind or pressure setup component of still water level

rise.

In Hawaii, wave setup (Sw) can be the largest contributor to the still water level rise for design

wave events. For this discussion, wave setup was assumed to be 10% of the nearshore wave

height 50-year return period wave height. Based on an initial deepwater wave height of 32.7 feet

for a 50-year North Pacific swell, and a resulting breaking wave height of 19.3 ft, the wave setup

is calculated to be approximately +1.9 feet at Kaopala Bay.




Nearshore waves are typically depth limited, meaning that the amount of wave energy that

reaches the shoreline is directly tied to the water depth offshore of the shoreline. As a result,

higher water levels generally result in increased impacts on the shoreline. As wave energy

increases with the square of the wave height, even a small water level increase can dramatically

change the coastal processes at a particular shoreline.

Table 3-8. Non-storm potential still water level rise at the site for a 50-year return period north

swell

Parameter Water Level Rise

(ft MLLW)

Tide (Kahului MHHW) +1.1

Sea-Level Rise

(2070 NOAA Int High)

+2.3

Other Oceanographic

Phenomena

+0.5

Storm Surge +0.0

Wave Setup +1.9

Total Stillwater Level Rise +5.8

3.4.8 Wave Height at the Shoreline

Because waves break in water depths proportional to their height, waves in shallow water are

necessarily limited in size. Wave heights are generally highest at the offshore breaking point and

gradually diminish in size as the bottom depths decrease. Attenuation of wave height in the on-

shore direction is due to the combination of wave breaking and friction. Large storm waves will

initially break offshore in deep water, then reform and continue shoreward as progressively

smaller waves, with wave breaking occurring several times before reaching the shore. The wave

height at the shoreline is therefore dependant on the water level at the shoreline. To determine

the actual design wave height at the shoreline, the design stillwater level must be considered.

As waves shoal and their forward speed is reduced, they tend to become higher and steeper.

Waves break when the waveform becomes too steep to be maintained. This occurs at ratios of

water depth to wave height (d/Hb) that generally range from 0.5 to 1.4, and depends on wave

steepness and bottom slope. An accepted value, based upon solitary wave theory, is a ratio of

0.78. In effect, wave heights over a reef flat are depth-limited, meaning there is a maximum

wave height that can occur for a given depth of water. The bottom conditions at the project site

are highly variable, with numerous patch reefs, holes, and sand bars. Based on offshore profiles,

an average MSL water depth of 3 ft is used for calculation of nearshore design wave heights, and

a d/Hb ratio of 0.78 is used for breaking wave criteria.




Table 3-9 shows the calculation of the design water depth and the design wave height given by a

d/Hb ratio of 0.78. Using this ratio the nearshore design wave height for the 50-year North

Pacific swell event would be 6.9 feet.

Design wave height is often the most important parameter for the design of a shore protection

structure. The wave height determines wave forces on structure, which determines various

features of the project, including structure height and armor unit size. The analysis used in this

report is appropriate for development of concept design. For detailed final design, a more

rigorous design wave analysis is recommended. This would include, for example, numerical

modeling of wave transformations across the reef.

Table 3-9. 50-year design water depth and wave height

Parameter Design Conditions (ft)

Nominal Water Depth (MSL Datum) 3.0

Total Stillwater Level Rise 5.8

Design Water Depth 8.8

Design Wave Height 6.9

3.5 Tsunami

Tsunami are sea waves that result from large-scale seafloor displacements. They are most

commonly caused by an earthquake of magnitude 7.0 or greater adjacent to or under the ocean.

If the earthquake involves a large segment of land that displaces a large volume of water, the

water will travel outwards in a series of waves, each of which extends from the ocean surface to

the sea floor where the earthquake originated. Tsunami waves may only be a foot or so in height

in mid-ocean, but they can have wave lengths of hundreds of miles and travel at 500 miles per

hour. As they approach shore, they interact with the shoaling seafloor by slowing in speed and

increasing in height. Tsunami waves can both push inland and recede at considerable speed.

The recession often causes as much damage as the original wave front itself.

Most tsunamis in Hawaii originate from the tectonically active areas located around the Pacific

Rim (e.g., Alaska, Chile, Japan). Waves originating with earthquakes in these areas take hours

to reach Hawaii, and the network of sensors that is part of the Pacific tsunami warning system

can give Hawaii several hours advance warning of tsunami from these locations. Less

commonly, tsunamis originate from seismic activity in the Hawaiian Islands, and there is much

less advance warning for these. The 1975 Halape earthquake (magnitude 7.2) produced a wave

that reached Oahu in less than a half hour, for example.




Walker (2013) reports three historical tsunamis with runup data at Kaopala Bay. The 1946

tsunami had a runup elevation of 24 feet, the 1957 tsunami had a runup elevation of 9 feet, and

the 1960 tsunami had a runup elevation of 10 feet. Fletcher et al (2002) classifies the area as a

“High” tsunami hazard area.

Tsunami evacuation maps are available for all islands in Hawaii and are based on computer

modeling of significant tsunami generating earthquakes that might occur around the Pacific Rim.

The maps are widely regarded as a worst case scenario. Figure 3-13 shows the tsunami

evacuation map for Kaopala Bay. The evacuation zone extends to 550 feet inland to

Honoapiilani Highway. The extreme tsunami evacuation zone extends 750 feet inland.

Figure 3-13. Tsunami evacuation map, Kaopala Bay

Planning

Area No. 1




3.6 Flood Insurance Rating

The National Flood Insurance Program, administered by the Federal Emergency Management

Agency (FEMA), produces maps identifying flood hazards and risks. Figure 3-14 shows the

Flood Hazard Assessment Report for the project site.

The map indicates that coastal area fronting Lower Honoapiilani Road at Kaopala Bay is rated

zone VE with a base flood elevation of 17 feet MSL. Zone VE, the Coastal High Hazard Area, is

the flood insurance rate zone that corresponds to coastal floodplains that have a 1-percent annual

chance of additional hazards associated with storm waves. Portions of Lower Honoapiilani Road

on the north and south ends of Kaopala Bay fall in zone AE, which is the 100-year floodplain of

the Kaopala Stream.

Figure 3-14. Flood Insurance Rate Map for Kaopala Bay (Map No. 1500030264F)




3.7 Coastal Processes Discussion

The Kaopala Bay shoreline is eroding and there is no indication that it will stabilize in the future.

Aerial images dating back to 1949 indicate that the southern portion of the bay has not held a

sand beach in recent history. As is common along many Maui shorelines, there is a clear

offshore beach rock slab that represents a beach location during a previous time, but it is nearly

500 feet offshore of the present day shoreline location.

3.7.1 West Maui Erosion event of 2015 – 2018

Water level and wave climate are primary forcing conditions for beach and shoreline changes.

As noted in Section 3.4.8, nearshore wave height will increase with higher tides and other water

level changes, such as wave setup, mesoscale eddies, and pressure effects. Higher wave heights

mean higher wave energy impacts on the beach, and it must adapt accordingly. Adaptation

examples may include changes in grain size toward a coarser distribution, beach profile

adjustment toward higher berm elevations, and higher rates of sand transport. For coastal areas

that experience seasonal beach loss, additional wave energy may result in temporary beach loss

for a higher percentage of time. Higher wave energy input may also result in higher rates of sand

transport in both the cross-shore and longshore directions.

As discussed in Section 3.3, the wave climate in West Maui is highly seasonal, dominated by

waves from the north in the winter, and waves from the south in the summer. Waves approach

the coastline at high angles of incidence, resulting in strong longshore sand transport in

seasonally opposing directions. If the winter and summer wave conditions are more or less

balanced, the movement of sand in one direction is compensated by movement in the opposite

direction six months later (Eversole & Fletcher, 2003). Strong winter or summer wave seasons

can result in complete depletion of beach sand at the end of the beach cell, with the resultant

exposure and erosion of the shoreline scarp. The erosion can be compounded if the seasonal

wave conditions are not balanced, and beach restoration from the opposing wave direction is not

accomplished.

The El Nino winter of 2015-2016 was characterized by numerous large wave events. In

particular, the spring of 2016 had sustained wave events from the north that were in the exposure

window of the Pailolo Channel (see Figure 3-3). That time period coincided with numerous

shoreline erosion problems in West Maui. The winter of 2016-2017, while not an El Nino event,

was also characterized by a wave climate with sustained swell from the north quadrant.

Conversely, the summers of 2016 and 2017 were notable for the paucity of southern swell

events. The combined effect of strong winter north swell conditions with a lack of balance from

weak summer wave conditions, and the temporary sea level rise effects discussed in Section

3.4.2 have seriously degraded many beaches in West Maui. Under the influence of more or less




one-way transport from winter conditions, sand has moved offshore or around cell boundaries

and left many beaches in a depleted state. The loss of sand, coupled with high water conditions,

enables waves to attack and erode the shoreline.

The beach at the project site is primarily composed of dense rock cobbles, and is therefore a high

energy beach that is not likely to be easily transported. However, elevated water levels have

enabled wave action to attack the vulnerable silt and clay shoreline scarp, causing recession and

the damage. Although the 2016-2018 sea level anomaly appears to be dissipating at the present

time, future sea level rise due to climate change, as well as high tides, wave set-up, and pressure

effects during storms, will likely cause continued erosion of the shoreline scarp and threaten the

roadway. As mentioned in Section 2.4, the erosion is not necessarily steady in time or space, but

can occur during discrete seasonal and climate related wave events, as well as at localized

erosion zones (“hot spots”).




4. SHORE PROTECTION ALTERNATIVES

4.1 Vulnerability to Coastal Hazards

Lower Honoapiilani Road at Kaopala Bay has an immediate need for remediation along an

approximate 100-foot length of road that could otherwise result in the loss of the makai lane.

Future erosion events will only increase the vulnerability of the road to erosion related damage.

While the present erosion at Kaopala Bay and much of the West Maui coastline is probably due

to a combination of events (El Nino winter waves, sustained swell events from the north,

anomalous high sea level), future sea level rise will bring additional erosive conditions that will

strain coastal infrastructure.

The makai side of the roadway is fronted by a vertical erosion scarp; vegetation on and above the

scarp is undermined in some locations. Failure of the scarp or collapse of the vegetation could

cause portions of the road to become undermined and potentially fail. Events that could trigger a

failure of the scarp could be a large north swell event or a Kona storm in combination with high

water levels. The roadway at Kaopala Bay has a long term vulnerability along a 900-foot length

of road that could result in the eventual loss of both lanes.

4.2 Shore Protection Alternatives

Loss of Lower Honoapiilani Road at Kaopala Bay would affect traffic flow to the residential

community around Kaopala Bay as well as infrastructure buried in the roadway. This

infrastructure includes storm drain lines, potable water supply, and sewer lines. Shore protection

options include:

“No action” in the form of managed retreat;

Temporary emergency protection (geotextile sand bags);

Seawall

Rock revetment

Beach nourishment

4.2.1 No Action (Managed Retreat)

Taking “no action” to reduce erosion of the shoreline at Kaopala Bay will likely result in

additional recession of the shoreline and undermining and eventual loss of use of Lower

Honoapiilani Road. Unfortunately, the existing information concerning erosion rates along this

reach is questionable. The AEHR calculated by the UHCGG (2.4 ft/yr) is heavily influenced by

reliance on 1912 T-Sheet information (see Section 2.4), and projected erosion positions from sea

level rise modeling are likely to be not applicable because the modeling used a dissimilar coastal

morphology (see Section 3.4.5). The erosion rate based on topographic surveys in 2000 and for

this report (2018) indicate an erosion rate between 0.3 and 0.7 ft per year.




Observations of erosion at nearby Kahana Bay have shown that the erosion of a silt-clay

shoreline scarp can happen very quickly under adverse conditions. Those conditions may

include seasonal deflation of the sand beach and exposure of the shoreline scarp, high or

sustained wave conditions, and high water levels due to tides and other oceanographic

phenomena. The erosion can occur in localized hot spots, as well as along long reaches. The

presence of falling trees along the shoreline scarp can complicate the erosion by rupture of the

soil around the perimeter of the root ball (see Figures 1-1 and 2-14).

While the actual rate of shoreline erosion remains in question, sea level rise projections make

continued erosion a certainty. With undermining of a portion of the road imminent, future

damage is inevitable. Roadway damage will not only directly affect transportation, but may

damage buried infrastructure, including water and sewer lines.

Coping with the No Action alternative can also be termed “managed retreat”, and this is viewed

as a viable long term option for many shoreline areas. For this reach of Lower Honoapiilani

Road, managed retreat would involve the following:

Abandonment of the roadway, and development of alternative transportation corridors;

Abandonment and re-location of infrastructure, including water lines, drainage lines, and

sewer lines, as well as overhead utilities (power and cable);

It is assumed that all infrastructure lines can be re-located, but the costs will be high. At this

location, alternative transportation corridors can likely also be accomplished. Figure 4-1 shows a

potential plan for a by-pass to move the Kaopala Bay portion of Lower Honoapiilani Road

mauka to service the housing area. The plan shown would use the existing 40-ft railway right-

of-way. While the legal, political, and cost ramifications are not clear, the plan will likely be

complicated and expensive. Although located inland, the right-of–way is within the tsunami

evacuation zone, and crosses the AE flood zone and floodway at Kaopala Stream (see Figures 3-

13 and 3-14) However, all properties fronting the project reach can be accessed by the mauka

alignment. Access to individual homes within the parcels would require new driveway

alignments and would likely be the responsibility of the owners. No Action, and abandonment

of Lower Honoapiilani Road, would also place the private shoreline property mauka of the

existing road at risk from erosion and shoreline recession. Abandonment of Lower Honoapiilani

Road would also likely result in the permanent loss of this road and coastal land along the reach.




Figure 4-1. Managed retreat option with new mauka transit corridor using railway right-of-way




4.2.1 Emergency Shore Protection

Given the apparent imminent undermining of Lower Honoapiilani Road, SEI recommended in a

February 7, 2018 memorandum to DPW that they consider implementing emergency shore

protection.

Emergency shore protection is considered non-engineered protection using temporary methods

and available materials. Temporary methods are not compatible with typical engineering design

criteria, but have been developed by trial and error, and acceptance by permitting agencies. At

present, geotextile sand bags are considered the only viable temporary shore protection option in

open coast environments. Large geotextile sand bags weighing 1 to 5 tons are stacked against

the shoreline scarp. These last for approximately two years before repairs and partial

replacement become necessary due to movement of the bags caused by wave forces and failure

of the geotextile due to sun and weather. In past years, sand bag protection was typically done

using 2 cubic yard woven geotextile bags (Bulklift bags). More recently, ElcoRock geotextile

containers have become available. ElcoRock is a coastal construction system utilizing robust

geotextile containers designed to be filled with sand and then placed to form a stable and durable

structure. The containers are available in 0.75m³ (3,000 lbs) and 2.5m³ (11,000 lbs) sizes. The

containers are efficient to install and remove, and the larger containers provide a stable structure

for Hawaii’s exposed coastal areas. The non-woven geotextile fabric is UV and vandal resistant,

has excellent abrasion resistance, and its soft finish is attractive and non-abrasive.

The preferred emergency shore protection at Kaopala Bay consists of 2.5m³ ElcoRock sand-

filled geotextile containers stacked to protect the shoreline scarp in the vicinity of the threat as

shown in Figure 4-2. The ElcoRock would be stacked on a slope to match the slope of the

erosion scarp and have a crest elevation up to the top of the scarp, which has an approximate

elevation of +15’ MSL. Figure 4-3 shows a proposed sandbag section view. The ElcoRock

weigh approximately 5 tons each, and require a medium-sized excavator for placement (e.g.

Komatsu PC400). While smaller 0.75m³ ElcoRock have been used for most recent temporary

emergency structures in West Maui, the larger containers are recommended for this application

due to the north swell exposure at Kaopala Bay and seriousness of the threat to a public roadway.

The proposed shore protection structure is 80 feet in length, and backed by geotextile filter

fabric. The structure design will require approximately 44 ElcoRock containers and will have a

footprint of approximately 800 square feet. The ElcoRock can be removed upon completion of a

long-term coastal management solution. The preferred fill material is carbonate sand. In the

past this requirement has been filled by dune deposits found in Wailuku and mined by Ameron.

However, due to a moratorium imposed on that resource, an alternative sand sources will likely

be necessary. These may include crushed carbonate, crushed basalt, or dredged sand.




Materials and Quantities

Materials include:

(44) ElcoRock 2.5 m3 sand containers

(144) cubic yards of sand-sized fill material

(2000) square feet geotextile filter fabric underlayment

Access and Staging

Equipment and material access would necessarily be from Lower Honoapiilani Road.

Equipment and materials would be staged on the road and would require at least a one lane

closure along 300 feet of Lower Honoapiilani Road.

Installation

ElcoRock containers would be filled on-site, using the ElcoRock J-Bin filling

apparatus.

Any remaining trees fronting the road in the project area would be cut or removed

to allow unobstructed installation of the ElcoRock. Beach cobbles should be

graded to create a stable, flat surface for ElcoRock placement.

Geotextile underlayment would be rolled out by hand. Installation of the

ElcoRock containers would be accomplished using an excavator. Road plates are

recommended to protect the road surface.

Construction Duration

Construction is anticipated to take approximately 1 week.

Permits for Temporary Emergency Shore Protection

Temporary installations typically require administrative authorization from both the County

Department of Planning and State DLNR-OCCL. As the structure is usually placed seaward of

the upper reach of the wash of the waves, nominal jurisdiction is held by the state. However, the

shoreline zone is designated a special management area regulated by the county. The county

Special Management Area Emergency Permit is designated as SM3 and is issued when there is a

threat to a legally habitable structure, or when public infrastructure is at risk of failure which

would substantially affect public health and safety. DLNR-OCCL operates under similar

emergency criteria, and will issue a letter of authorization for the emergency structure.

If the emergency structure is located below the high water mark, it is considered to also be in

“Waters of the United States”, and will need a letter of authorization from the U.S. Army Corps

of Engineers.




Figure 4-2. ElcoRock revetment layout

Figure 4-3. ElcoRock revetment section




4.2.2 Seawall

A seawall is a vertical or sloping concrete, cement-rubble-masonry (CRM), cement-masonry unit

(CMU), or sheet pile wall used to protect the land from wave damage and erosion. A seawall, if

properly designed and constructed, is a proven, long lasting, and relatively low maintenance

shore protection method. Seawalls also have the advantage of having a relatively small footprint

on the shore.

The impervious and vertical face of a seawall results in very little wave energy dissipation.

Hence, wave energy is deflected both upward and downward, and also a large amount of wave

energy is reflected seaward. The downward energy component can cause scour at the base of the

wall - therefore the foundation of a seawall is critical for its stability, particularly on an eroding

shoreline. Ideally, a seawall should be constructed on a solid, non-erodible substrate. Seawalls

are not flexible structures, and their structural stability is dependent on the stability of their

foundations.

If the foundation of the seawall is breached, hydraulic processes can erode fill material behind

the wall. With the loss of enough fill, the ground surface behind the seawall will collapse into a

sink hole. When a sink hole is observed, repairs should be made as soon as possible or the wall

may eventually fail. Repair methods vary; one method is to excavate behind the wall, reinforce

the foundation with concrete, and replacing the fill with appropriately graded material. To avoid

foundation problems, the seawall foundation should be well below the potential scour level.

The presence of a visible rock substrate at a relatively high elevation near the most at-risk

portion of the shoreline embankment gives strong support for the use of a seawall as a shore

protection option (see Figures 2-4, 2-5, and 2-13).




Figure 4-4 illustrates a potential design cross-section for a CRM seawall structure at Kaopala

Bay. The rock substrate simplifies wall requirements for both a foundation and anchoring,

allowing use of a gravity wall such as the CRM structure shown. This type of structure will also

allow some expansion of the road shoulder by pushing the wall seaward and adding fill behind

the wall.

Advantages of a CRM wall at Kaopala Bay include:

Potential for good foundation conditions;

High foundation elevation minimizes size of the wall;

Relatively simple design and construction;

Compatible with drainage outlets;

Aesthetically well matched with surroundings.

The presence of a high energy cobble beach mitigates some of the negative qualities of a seawall.

Wave reflection will tend to rapidly attenuate due to the high friction properties of the cobbles,

and these also have excellent resistance to scour.




A seawall is particularly applicable to the area of concern shown in Figure 2-3, and as shown in

Figure 4-4. However, it can also be used to protect the revetment reaches by placing the wall

against the vertical clay/silt escarpment as long as a solid foundation is still accessible.

Geotechnical borings are recommended at approximately 100-ft intervals along the reach of the

bay in order to have a complete assessment of the foundation conditions. The remnant stones

that form the over-steepened section against the clay/silt scarp should be removed prior to wall

construction, and place at lower elevations on the beach (Figure 4-5).

Figure 4-4. Typical seawall section




Figure 4-5. Schematic of CRM wall used on revetment protected reaches

4.2.3 Rock Revetment

A rock rubblemound revetment is a sloping uncemented structure built using boulder-sized rock.

The most common method of revetment construction is to place an armor layer of stone, sized

according to the design wave height, over an underlayer and filter designed to distribute the

weight of the armor layer and to prevent loss of fine shoreline material through voids in the

revetment. The armor layer is typically two stone diameters in thickness.

Armor Stone Size

Typical revetment structures are designed as rock rubble mounds with a slope of up to 1.5

Horizontal to 1Vertical (1.5H:1V), which is the steepest slope recommended by the Coastal

Engineering Manual (2006). Stone sizing for the design wave height is given by the Hudson

Formula (Coastal Engineering Manual, 2006):

where,

W = weight in pounds of an individual armor stone

wr = unit weight of the stone, 160 lb/ft3

H = wave height, 6.9 feet (see Section 3.4.8)

KD = armor stone stability coefficient, 2

cot)1( 3

3

rD

r

SK

HwW




Sr = specific gravity of the stone relative to seawater, use 2.5

cot θ = cotangent of the groin side slope, use 1.5

The resultant armor stone weight is approximately 5,000 pounds with a corresponding nominal

diameter of 3.2 feet. A range of ± 25% of the median weight is typically utilized, which yields a

stone weight range of 3,750 to 6,500 pounds. These are preliminary values, and may be

conservative. A more rigorous design wave development including numerical modeling is

recommended for final design parameters.

Revetment Underlayer

Underlayer stone is utilized to transition between the large armor stone and small filter stone or

filter layer. Sizing of the underlayer stone is important for providing sufficient porosity for

energy dissipation rather than reflection, to achieve interlocking between the armor and

underlayer, and to insure that the underlayer material cannot be dislodged through voids in the

armor layer. Underlayer stone is sized at approximately one-tenth the armor stone weight, which

in this case is approximately 500 lbs.

The underlayer stone should be placed over a geotextile filter fabric layer. The geotextile

prevents the migration of fine soil particles through voids in the structure, and permits relief of

hydrostatic pressures within the soils. The underlayer stone protects the geotextile from damage

during placement of the armor layer, and together with the geotextile helps distribute the weight

of the armor stone to provide for more uniform settling. The existing slope should be graded and

dressed prior to revetment construction to provide a 1.5H:1V slope. Rocks and other debris

which might puncture or tear the geotextile should be removed from the prepared slope.

Figure 4-6 shows a potential rock revetment design cross section, assuming a rock substrate for

embedment of the revetment toe.

One major advantage of a rock revetment versus a vertical seawall is that the rough porous rock

surface and relatively flat slope of the structure will tend to absorb wave energy and reduce wave

reflection. Properly designed and constructed rock revetments are durable, flexible, and highly

resistant to wave damage. Should toe scour occur, the structure can settle and readjust without

major failure. Damage from large waves is typically not catastrophic, and the revetment can still

function effectively even if damage occurs. From a coastal engineering perspective, a rock

rubblemound revetment is often the most suitable shore protection alternative for coastal

shorelines.




The project area contains reaches with existing rock revetment protection. The stone is mostly

undersized, and down-slope movement has caused over-steepening of the top portions of the

structure. While new revetment construction is probably not practical for the at-risk portions of

the shoreline due to the wide footprint of the structure, repair of the existing revetment reaches is

a feasible option. It is recommended that additional geotechnical information be acquired in

order decide if a wall or revetment is the best approach.

Figure 4-6. Typical rock revetment section

4.2.4 Beach Nourishment

When sand loss is gradual and the beach has a high economic value for recreation and tourism, it

is sometimes good coastal management policy to replenish the littoral cell with sand from

offshore or other sources. Massive beach nourishment projects have taken place on the eastern

seaboard, Gulf coasts (USACE CEM, 2006), and Southern California (SANDAG, 2000). In the

past ten years “Hawaii sized” beach nourishment projects have taken place on Oahu, Kuhio

Beach Park (2006), Waikiki Beach (2012) and Iroquois Point (2013).

Beach nourishment is expensive, and containment features or structures such as T- head groins

are sometimes necessary to keep the sand from disappearing. Some areas have features such as

headlands or reefs that naturally stabilize the sand. On open ocean or otherwise unprotected

coasts T-head groins both decrease the amount of wave energy reaching the beach, and act as

artificial littoral cells to stabilize the sand.




Beach nourishment requires a supply of sand that is similar in character to the native beach sand.

While sand in Hawaii may seem like a plentiful commodity, the reality is that good quality beach

sand is in very short supply. Inland dune deposits have been used for some nourishment efforts,

but the process of transport by wind preferentially selects a naturally finer grain size, and dune

sand therefore tends to be composed of grains that are too fine for many applications. Offshore

sand deposits also tend to have grain sizes that are finer than many beaches, and many reef-top

deposits are thin and of insufficient volume for meaningful use. However, offshore deposits

have been found that are in some cases suitable. Dredging and recovery operations are

expensive, but have been shown to be effective. The Waikiki Beach Maintenance project (2012)

recovered 24,000 cy of sand from and offshore deposit and pumped it to shore to nourish 1,730

feet of beach in the center of Waikiki, at a cost of $2.4M.

Beaches can be stabilized by beach nourishment, and the addition of sand can slow the erosion of

fast lands. However, in dynamic environments sand that is not held in place by natural

headlands or containment structure can rapidly disappear during adverse conditions. Recent

beach monitoring in Waikiki shows that approximately half of the 2012 nourishment project

sand is no longer on the beach.

In Kaopala Bay, there appears to be a stable beach in the lee of Haukoe Point (see Figure 2-6).

However, the beach along the rest of the bay is composed of rock cobbles with little to no sand.

This morphology is a strong indication that, in general, sand is not stable along this reach of

coast. Implementing a beach nourishment project for the purpose of protecting the shoreline

scarp and protecting the roadway is not likely to be successful.

4.2.5 Shore Protection Impacts

Shore protection is a controversial topic because of both real and perceived impacts to the coastal

environment. Following are brief descriptions of typical impacts that may occur due to

construction of shore protection.

Impoundment

Impoundment is the sequestration of sediment behind a shore protection structure. By

preventing erosion of the upland behind the structure, an eroded beach can be starved of new

sand. Although there are some beach morphologies that may behave in this way, beaches on the

West Coast (California) and Hawaii generally are not directly supplied with sand from coastal

upland erosion. Studies have shown that California beaches derive about 10% of their sand from

the erosion of coastal bluffs, with the majority of beach sand being sourced from river and

stream flow and then transported laterally along the coast (Patsch & Griggs, 2006). In contrast,

Hawaii beaches are primarily composed of carbonate sand that is derived from biogenic




production on fringing reefs and transported to shore by wave action (Moberly & Chamberlain,

1964; Inman et al, 1963).

Placement Loss

Placement loss is the term used to describe the loss of beach due to the footprint of a structure

encroaching on the beach area. The amount of placement loss depends on the structure type and

where it is located. A vertical seawall placed landward of the shoreline would result in virtually

no placement loss, for example, while loss due to revetment construction (see Figure 4-6) may be

20 to 30 ft. However, this figure is misleading since much of the revetment is buried, especially

during seasonal periods when sand is present on a beach.

Flanking Erosion (End Effects)

Flanking erosion is when erosion occurs behind a shore protection structure, and can be a

mechanism for structure failure. Flanking can be accelerated by end effects. End effects are

caused by the radiation of waves from the edge of coastal structure. The waves radiate in a more

or less arcing pattern due to an expanding wave front (i.e. like waves in a pond). Because of this

pattern, as well as attenuation due to wave breaking and bottom friction, the waves lose energy

with distance from the source and results in end effects being a near field process, meaning that

the effects are most pronounced close to the source, in this case the end or edge of the structure.

Figure 4-7 is a photograph of end effect processes, showing waves radiating from the edge of a

temporary structure. The soft sand on the adjacent shoreline is easily eroded. The near-field

property of the phenomenon can be seen by inspection of the eroded shoreline in the photograph.

Passive Erosion

Passive erosion is a term used to describe the effect of erosion that occurs next to a protected

shoreline. With time the unprotected shoreline can erode sufficiently to create an offset and

change the geometry of the coast. Geometry is important for coastal processes, and the offset

can create a de-facto headland that provides a barrier to sand transport and prevents deposition in

front of the protected area. Figure 4-8 shows an example of passive erosion at Kahana Bay:

sand is prevented from being transported and deposited by the offset between the protected and

unprotected shorelines.

Wave Reflection and Scour

Wave reflection from vertical structures is perceived to inhibit the accretion of sand and have a

generally negative impact on beaches, although evidence of this effect has not been clearly

documented (Griggs et al, 1994, CEM, 2006). Nevertheless, it is accepted coastal engineering

practice to minimize wave reflection as much as possible. Wave reflection can also cause a soft

substrate to scour and deepen into a trough in front of the reflecting surface, thereby affecting the

nearshore morphology.




Figure 4-7. An example of end effects

Figure 4-8. Example of passive erosion at Kahana Bay; note shoreline offset – shoreline on the

right was unprotected until recently

4.2.6 Shore Protection Impact Mitigation

Much of the impacts from shore protection can be minimized by understanding the existing

environmental conditions and designing accordingly. For example, a revetment is often a

preferred structure as it tends to reduce wave reflection. However, the substantial cobble beach

that exists at Kaopala Bay is a natural and extremely effective wave absorption mechanism that




will minimize reflection and scour in front of a vertical wall such that the ameliorating effects of

a revetment are redundant. Passive erosion and flanking due to end effects is typically countered

by extending the structure to where it is naturally protected such that these effects are minimize

or eliminated.

4.3 Regulatory Environment

4.3.1 Overview

Hawaii shorelines and coastal waters are governed by a complex array of Federal, State, and

County jurisdictions, rules and regulations. Figure 4-9 illustrates the approximate jurisdictional

boundaries that will determine the scope of the environmental review and regulatory permitting

requirements for a shore protection structure at Kaopala Bay. The U.S. Army Corps of

Engineers (USACE) has jurisdiction seaward of the mean higher high water (MHHW) line and

offshore for 200 nautical miles. The Hawaii Department of Land and Natural Resources (DLNR)

has jurisdiction seaward of the certified shoreline1 to 3 nautical miles offshore, which overlaps

with portions of the USACE jurisdiction. The area landward of the certified shoreline is

managed by the County of Maui.

Figure 4-9. Approximate jurisdictional boundaries at Kaopala Bay

4.3.2 Federal Requirements

The USACE is the designated lead agency with regulatory authority over Navigable Waters of

the United States, which includes the oceans and coastal waters seaward of MHHW.

1 “Shoreline” means the upper reaches of the wash of the waves, other than storm or seismic waves, at high tide during the season

of the year in which the highest wash of the waves occurs, usually evidenced by the edge of vegetation growth, or the upper limit of debris left by the wash of the waves (Hawaii Administrative Rules (HAR) §13-222).”




Construction seaward of MHHW typically requires a Department of the Army (DA) permit

pursuant to Section 10 of the Rivers and Harbors Act of 1899 (33 USC 403) and Section 404 of

the Clean Water Act (33 USC 1344). All work or structures in or affecting the course, condition,

location or capacity of navigable waters, including tidal wetlands, require DA authorization

pursuant to Section 10. In addition, activities involving the discharge of dredged or fill material

(i.e., armor stones) into Waters of the United States requires a DA permit pursuant to Section

404. Activities which require federal permits must also meet requirements of the Coastal Zone

Management Act (CZMA). The MHHW elevation for the project site is approximately 1.2 feet

above mean sea level (MSL). The horizontal location of the MHHW line varies depending on

the beach profile. The rock revetment structure would likely extend seaward of MHHW. Thus,

Federal permits would be required. Federal permits may not be required for the seawall

alternative, assuming it can be located landward of the MHHW line on the shoreline.

4.3.3 State of Hawaii Requirements

The Hawaii Department of Land and Natural Resources (DLNR) is the designated lead agency

with regulatory authority over Conservation District and Public Trust lands, which includes

unencumbered land and submerged land. All lands in Hawaii are classified into four land use

districts: urban, rural, agricultural, and conservation. Hawaii Revised Statutes (HRS) § 205-2

(Coastal Zone Management , or CZM) is the overarching policy that designates the types of uses

permitted in the four land use districts. The area landward of the shoreline at Kaopala Bay is

located in the Urban District. The area seaward of the shoreline in Kaopala Bay is considered

submerged State land and is located in the Conservation District, Resource Subzone, which

extends 3-nautical miles seaward from the shoreline.

The certified shoreline establishes the landward limits of the Conservation District. The Board

of Land and Natural Resources (BLNR) has exclusive jurisdiction to govern land uses in the

Conservation District. Shore protection would be considered an identified land use in the

Resource Subzone of the Conservation District pursuant to Hawaii Administrative Rules (HAR)

§13-5-24-P-15 Shoreline Erosion Control (D-1); therefore, the project will require a

Conservation District Use Permit (CDUP) from the DLNR Office of Conservation and Coastal

Lands (DLNR-OCCL).

The Conservation District Use Application (CDUA) will require submittal of an Environmental

Assessment (EA) and a Finding of No Significant Impact (FONSI) pursuant to Hawaii Revised

Statutes (HRS), Chapter 343, and its implementing regulations. Hawaii Administrative Rules

(HAR) Title 11, Chapter 200, addresses the determination of significance and contents of an

environmental assessment.




Water quality standards and regulations are administered by the Hawaii Department of Health,

Clean Water Branch (DOH-CWB). A National Pollutant Discharge Elimination System

(NPDES) Permit is required any time construction activity covers an area one (1) acre in size or

greater, and is intended to prevent pollutants from reaching coastal waters as a result of storm

water runoff. Information required to obtain a permit includes project specific details and

construction drawings, receiving state water information, storm and non-storm water discharge, a

Best Management Practices Plan (BMPP), and Post-Construction Pollutant Control Measures.

Should any part of the project be constructed below the MHHW mark, a DA Section 404 permit

would be required, which may in turn trigger a requirement for a Section 401 Water Quality

Certification (WQC), which is administered by the Hawaii Department of Health. The WQC

requires the applicant to conduct water quality monitoring before, during, and after construction.

4.3.4 County of Maui Requirements

The County of Maui Department of Planning is the designated lead agency with regulatory

authority over the area landward of the certified shoreline within the Special Management Area

(SMA). Hawaii Revised Statutes (HRS) §205-2 establishes the types of land uses that are

permissible within the SMA, which is considered the most sensitive area of the coastal zone.

The certified shoreline establishes the seaward limits of the SMA and is the baseline that

counties use to calculate shoreline setbacks in Hawaii. It is likely that some portion of proposed

shore protection will be mauka of the certified shoreline, and will therefore require an SMA

permit and a Shoreline Setback Variance (SSV). As with the CDUP, the SMA requires an

Environmental Assessment.

Lower Honoapiilani Road is located in the VE Flood Zone with a Base Flood Elevation of 17

feet, therefore a Flood Development Permit and Coastal High Hazard Area Certificate will likely

be required. The activity may involve ground disturbing activities and/or stockpiling of

materials, so a Grubbing and Grading Permit may be required.

The anticipated environmental review and regulatory permitting requirements for a shore

protection structure at Kaopala Bay are summarized in

Table 4-1, below.




Table 4-1. Anticipated Environmental Review and Regulatory Permitting Requirements

FEDERAL

Department of the Army Permit (DA)

Lead Agency: US Army Corps of Engineers (USACE)

Authority: Section 10 of the Rivers and Harbors Act of 1899 (33 USC 403) and

Section 404 of the Clean Water Act (33 USC 1344)

STATE OF HAWAII

Conservation District Use Permit (CDUP)

Lead Agency: Hawaii Department of Land and Natural Resources (DLNR)

Authority: Hawaii Administrative Rules (HAR) 13-5

Environmental Assessment (EA)

Lead Agency: Hawaii Office of Environmental Quality Control (OEQC)

Authority: Hawaii Revised Statutes (HRS) Chapter 343

National Pollution Discharge Elimination System Permit (NPDES)

Lead Agency: Hawaii Department of Health (DOH)

Authority: Hawaii Administrative Rules, Chapter 11-55

Water Quality Certification (WQC)

Lead Agency: Hawaii Department of Health (DOH)

Authority: Section 401 of the Clean Water Act (33 USC 1344)

COUNTY OF MAUI

Special Management Area Minor Permit (SMA)

Lead Agency: County of Maui Department of Planning

Authority: Hawaii Revised Statutes (HRS) Chapter 205-A

Special Flood Hazard Area Development Permit


Authority: Maui County Code (MCC), Chapter 19.62

Coastal High Hazard Area Certification


Authority: Maui County Code (MCC), Section 19.62.060.G.6.a

Grubbing and Grading Permit

Lead Agency: County of Maui Department of Public Works

Authority: Maui County Code (MCC), Chapter 20.08




5. RECOMMENDATIONS

The site investigations by SEI engineers determined that there is an immediate coastal threat to

Lower Honoapiilani Road. An emergency revetment constructed with ElcoRock 2.5m³

geotextile containers is recommended to mitigate the immediate threat. Planning for long term

shoreline management solution should begin immediately, as design and permitting for

permanent structures can be a multi-year process.

The presence of a hard rock substrate in the area of concern and along much or all of the

shoreline reach makes the CRM seawall option attractive (see Figure 4-4). The wall would have

a relatively small footprint, have an uncomplicated design, and would be aesthetically

compatible with the local environment. Geotechnical borings are recommended to map the

elevation of the hard rock substrate. The environmental effects of a seawall at this location are

negligible. A seawall built close to the existing vertical erosion scarp is physically similar in

terms of coastal processes. Wave reflection off a nearly vertical clay scarp is virtually identical

to reflection off a near vertical seawall, and the narrow footprint of a seawall minimizes

placement loss. There is little to no sand visible in the coastal escarpment, so impoundment of

beach quality sand by the wall structure is extremely unlikely. The effects of passive erosion,

and flanking erosion can be minimized or eliminated by proper treatment of the ends of the

proposed structure.

The existing beach is primarily composed of rock cobbles. These will mostly remain in place,

and tend to collect as a berm in front of the new CRM seawall. The sand beach at the north end

of the project reach exists because that area is somewhat protected from direct wave exposure by

Haukoe Point. The new seawall is also not likely to cause substantial changes in this area.

Figure 5-1 is a schematic representation of a new seawall along the project reach. At minimum,

it is recommended that an approximate 170’ reach along the existing area of concern be

protected with a permanent wall as soon as possible. The wall should extend from the protected

area in the lee of the Robinson revetment and butt into the existing rock outcrop (Figure 2-5).

Eventually the entire Kaopala Bay reach should be protected for a combined distance of about

680 ft. The walls would exist in two reaches (390 ft and 290 ft) that abut either side of the

headwall for the 54” drainage pipe.

A rubblemound rock revetment is not recommended for the project reach due to the wide

footprint and consequent placement loss. This is especially compelling because the smaller

seawall option is relatively straightforward due to the rock substrate. However, if the substrate is

found to be too deep in places for an adequate wall foundation, a rubble mound rock revetment

could be substituted in those locations.




Managed retreat consisting of road and utility relocation is feasible in this location due to the

potential availability of land for a new transit corridor, but is certainly not the most cost effective

solution and would cause extensive disruption to the community.

Beach nourishment is not considered a feasible option for this area. Based on existing

conditions, sand is generally not stable along the project reach and any beach nourishment effort

would need to be accompanied with substantial retention structures such as T-groins. Even then,

beach nourishment would not guarantee stability of the shoreline and protection of the road.

Table 5-1 is a comparison of selected protection options. Project costs listed in the table are

based on recent projects on Maui and Kauai. Costing for shore protection is uncertain as each

project has its own difficulties, and is dependant on the availability of stone and access for heavy

equipment.

5.1 Recommendations for follow-on work

Follow-on work for protection of the Kaopala Bay shoreline include additional design,

environmental review and permitting.

5.1.1 Additional Design

Additional design needs include a new topographic survey of the entire reach, geotechnical

borings (e.g., at 100’ intervals or less) to determine the rock substrate conditions and elevation,

additional oceanographic parameter analysis, and structural engineering design input. These

elements can be presented together in a Basis of Design Report. The report should be prepared

by an engineering firm with experience in coastal structure design.

5.1.2 Environmental Review

An environmental assessment will be required for county and state permits. The assessment will

likely require a marine biological and water quality survey, cultural impact assessment,

archaeological monitoring plan, and a preliminary drainage plan. The environmental assessment

can be prepared by an engineering firm with experience in coastal design, or by a planning firm

with experience in Maui County with assistance from the design engineers and other

subcontractors.

5.1.3 Permits

As indicated in Section 4.3, the project will probably require state (CDUP) and county (SMA,

SSV) permits at minimum. Federal permits may or may not be required, depending on the final

design configuration and location. If the rock substrate is sufficient in elevation, the CRM wall




design may be out of federal jurisdiction. Permit application can either be prepared by a

planning firm with assistance from the design engineers, or by the design engineers.

Figure 5-1. Schematic representation of new CRM seawall.




Table 5-1. Comparison of Alternatives

Alternative SEI Rating Pros Cons

1. CRM Seawall Preferred

Shoreline armoring (maximum protection);

Minimize coastal footprint;

Minimal impact on coastal processes;

Rugged, adaptable structure

Low impact on marine environment

Reflective (but equivalent to existing morphology)

Cost: $5,000 – $6,000 per linear ft

2. Revetment Second Preferred

Shoreline armoring (maximum protection);

Minimize reflection;

Minimal impact on coastal processes;

Rugged, adaptable structure

Wide footprint / placement loss

Cost: $6,000 – $9,000 per linear ft

3. Emergency ElcoRock

Containers

Preferred

(Temporary)

Proven robust temporary protection

Quickly implemented

Installed with Emergency permits

Temporary only – will degrade with time (e.g. 2 yrs) and will need repairs

4. Beach Nourishment Not Appropriate

None

Sand is not stable, would require extensive additional retention structures.

Will not protect roadway

Cost (with structures): $4,000 - $9,000 per linear ft

8. Managed Retreat Not Appropriate Feasible due to available land

No additional structures required

Will allow coastal erosion

Will require relocation of transit corridor

Will require relocation of buried utilities

Very high cost (to be determined)




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