BBWKWanlessVKStatement8813-1

8/22/2019 BBWKWanlessVKStatement8813-1

1/12

1

1

STATEMENT ON VULNERABILITY OF VIRGINIA KEY

TO EROSION AND INUNDATION

Harold R. Wanless, Ph.D.

Registered Florida Professional Geologist #985

Even if significant sea level rise were not forecast for this century, Virginia Key is a highly

vulnerable barrier island which will be severely eroded, inundated, and the shore set back as the

century progresses. With ten inches of sea level rise having occurred since 1930, with a current

rate of sea level rise of greater than one foot per century, with sea level rise accelerating because

of accelerating ice melt from glaciers and the Greenland and Antarctic Ice Sheets (Blunden and

Arndt, 2013), and with very significant and accelerating sea level rise projected in the coming

decades and through the century and beyond (NOAA, 2012), it is ill-advised and dangerous to

consider putting tax-payers money into maintaining and upgrading the Central Miami-Dade

Water and Sewer treatment facility on Virginia Key.

There are four reasons why Virginia Key is an un-necessarily risky place for a major water and

sewage treatment plant now and will become totally lost as a coastal barrier island within 50

years: 1) severe depletion of sand resources to Virginia Key has left the island severely

vulnerable; 2) most of remaining nearshore sand is isolated from the active shore-transport

system; 3) an accelerating sea level rise that is projected to overwhelm the seaward side of

Virginia Key within 32-53 years; and 4) an island topography configured to force accentuated

storm surges through the M-DWASD Central Treatment facility.

1. Severe Depletion of Sand Resources To Virginia Key

Severe mismanagement of the sand resources seaward of Virginia Key and Fisher Island over

the past century has resulted in nearly complete erosion of the natural sand resources seaward of

Virginia Key and Fisher Island and eliminated any natural source of sand to maintain the beach

and barrier island of Virginia Key or to provide any natural beach replenishment relief during

erosion. The construction of Government Cut in 1905, the progressive extension and hardening

of the jetties, and the deepening of the channel has totally eliminated the southward transport of

sand by long shore drift. Literature estimates for natural longshore drift of sand in the area

suggest that 80,000 to 300,000 cubic yards of sand would naturally move south past a point each

year. My measurements in monitoring the region are that the region south of Government Cuthas been deprived of 80,000-100,000 cubic yards of sand per year because of the block by

Government Cut. This is reflected in the severe to complete loss of sand offshore of Fisher

Island and Virginia Key, except in depressions in the underlying limestone. Over the past

century, this is 8,000,000 to 10,000,000 cubic yards of sand that has moved on south past the

area and into Biscayne Bay through the tidal passes of Norris Cut and Bear Cut. This lost sand

has not been and cannot be replaced by longshore drift from the north because of Government

Cut.


2/12

2

2

This has left the area seaward of Virginia Key totally sediment starved. The shallow sand

platform seaward of Virginia Key used to have a broad, seagrass-stabilized shallow sand

platform well seaward of the beach, shallow sand platform that extended seaward of an

underlying limestone ridge, and active sand renourishment to the shore and the sand platform by

littoral transport from the north.

That ended with the construction of Government Cut and associated jetties. The beach has

progressively eroded, the shallow sand platform deepened, and the sand resources disappeared as

sand was shifted south and into the Norris Cut and Bear Cut inlets and not replaced. This severe

sand starvation reality alone should be reason to be gravely concerned for the future of the M-

DWASD Central facility on Virginia Key.

References:

Blunden, J., and Arndt, D.S., Eds., 2013. State of the Climate in 2012.Bulletin of the American

Meteorological Society, v.94, no.

8, p. S1S238.

NOAA, 2012. Global Sea Level Rise Scenarios for the United States National Climate

Assessment. NOAA Technical Report OAR CPO-1. Climate Program Office, Silver Spring

MD, 27p. (done cooperatively with United States Geological Survey, Department of Defense,

Environmental Protection Agency, Department of Energy, and the US Army Corps of

Engineers).

2. Most of Remaining Nearshore Sand Is Isolated From the Active Shore-transport System.

The remaining sand on the deepening sand platform seaward of Fisher Island and Virginia Key is

below the level of the seaward limestone ridge and is not playing a role in the sediment budget of

the Fisher Island or Virginia Key beach system

Illustrations by the author documenting the origin of these barrier islands and the severe sand

starvation discussed in points #1 and #2 are shown in Figures 1-7. Research for Figures 4-6 were

done as a student research project S. Oleck and H. Wanless in 1996.


3/12

3

3

FIGURE 1 . As sea level rose into Biscayne Bay (top), quartz-rich beach sands (yellow) flowed

south along the seaward side of an emergent limestone ridge (the same ridge that forms the

Florida Keys to the south). This ridge fades for a stretch in the vicinity of south Virginia Key

and northern Key Biscayne Bay where the gap formed the geologic entrance to Biscayne Bay.

This ridge positioned the sandy barrier islands of southeast Florida but was gradually submerged

and covered with rising sea level (Bottom). Virginia Key lies west of the limestone ridge. Sand

starvation and erosion south of Government Cut has re-exposed this southward deepening ridge

(see Figure --). Yellow is quartz-rich beach sand. Orange is skeletal sand of the seaward shelf.

Brown is mangrove peat.


4/12

4

4

FIGURE 2. Sketch maps of the barrier islands of Miami-Dade County showing the natural

nearshore and inlet sand transport (left) and modifications to the shore system that have modified

the shore dynamics and beach stability.


5/12

5

5

FIGURE 3. Map of historical changes in the southern Miami Beach to Virginia Key area. The hurricane of

1835 formed Norris Cut, separating Virginia Key from Miami Beach. Government Cut, formed in 1905

and quickly fortified with jetties to prevent sand infill resulted in the total block of southward sandtransport since for the past 100 plus years. As a result Fisher Island and Virginia Key have seriously

eroded. In addition Norris Cut has widened and deepened as sand has been swept into Biscayne Bay. A

beach renourishment project in 1973 together with the emplacement of granite groins attempted to

offset this erosion as have subsequent sand and engineering projects. Sand from the major sand

renourishment project on Miami Beach in the late 1970s to early 1980s has not reached south because

of the complete block to natural southwards transport by the jetties of Government Cut.


6/12

6

6

FIGURE 4. Maps showing the shoreline and offshore depth contours in 1852 and 1919 based on

surveys by the US Coast and Geodetic Survey. The light blue offshore is all shallow beach

quality sand, the ebb-tidal delta of Bear Cut and Norris Cut. Note the significant shore erosion

by 1919 following the construction of Government Cut and the initial jetties to block sand

infilling and inadvertently blocking any southward transport of sand.


7/12

7

7

FIGURE 5. Maps showing the shoreline and offshore depth contours in 1960 and 1996 based on

a University of Florida survey and a survey by the author respectively. In the 1960 survey the

littoral platform is deepening and the Virginia Key shore has eroded. In the 1996 survey, the

shallow sand platform has significantly deepened through sand being eroded and transported

southward and into the inlets (Bear Cut and Norris Cut) but no sand coming in from the north. In

the 1996 study we hand probed with a metal rod to determine the thickness of loose sand over

limestone bedrock. The results from Transects #2, #3, and #4 which intersect Virginia Key are

shown and discussed in Figure 6.


8/12

8

8

FIGURE 6. Cross-sectional profiles of the shallow platform seaward of Virginia Key showing

the bottom depth profiles from the 1852, 1919, 1960 and 1996 surveys. Location of these

Profiles #2, #3, and #4 are shown in the 1996 map in Figure 5. Based on probing with a metal

rod in the 1996 survey, the depth of the underlying limestone rock and thickness of the loose

sand in 1996 was determined. The remaining sand as of 1996 is shown in yellow and the

underlying limestone is brown. This shallowly submerged limestone ridge is the same ridge that

forms the Florida Keys. In this area, the limestone ridge served to position the sandy barrier

islands of Miami Beach, Virginia Key and Key Biscayne. Historically sand moved southward bylongshore transport along the seaward (eastern) margin of the ridge and capped the ridge. As

sand supply was cut off by the Government Cut jetties, sand continued to move southward and

into inlets but was not replaced by the natural sand transport from the north. As a result there is

little or no sand on the seaward flank of the limestone ridge or capping it. This historical change

in the profiles is the result of that sand starvation. Importantly, remaining sand west (left) of the

ridge is below the level of the ridge and not and active part of the sand transport in the system.


9/12

9

9

FIGURE 7. An underwater LiDAR image of area seaward of south Miami Beach, Government

Cut, Fisher Island, Virginia Key and northern Key Biscayne showing the geomorphic features on

the sea floor. The shallow sand platform seaward of Virginia Key and northern Key Biscayne is

an ebb-tidal delta of Bear Cut and Norris Cut smoothed off by wave energy from the seaward

(east, right). Since construction of Government Cut in 1905, the southward longshore drift of

sand out of the area and the block of new sand into the area by Government Cut has resulted in

deepening of the sand platform seaward of Fisher Island and Virginia Key, significant erosion of

the ocean beach, and removal of sand from the seaward side of the limestone ridge. The sand

remaining west of the ridge is basically hiding behind the ridge and not playing any role in the

sediment budget (see Figure 6).


10/12

10

10

3. An Accelerating Sea Level Rise Projected to Inundate Virginia Key by the Middle of the

Century.

By 2063, there is projected to be a further sea level rise of 55 to 92 cm (1.8-3.1 feet) according to

the most recent NOAA projections (Figure 8 and Table 1). With a further 1.5-2 feet of sea level

rise there will be no natural Virginia Key left. This will happen within the next 32-53 years.Winter storms and hurricanes will have eliminated all of the low beach sand, low barrier island

sand and low mangrove from the seaward side of the M-DWASD facility on Virginia Key. At

these higher sea levels, storms and stronger tides will have moved this eroded sediment of the

sediment starved barrier island into Biscayne Bay through Norris Cut to the north and Bear Cut

to the south. Stronger tides will occur because of increased tidal prism (enlarged area within

Biscayne Bay to be fed by flooding tides) because of inundation of low-lying areas within the

Bay. This will basically leave The M-DWASD Central facility on Virginia Key exposed to the

open ocean and with higher land elevations to the north and south (the dredge-spoil fill area to

the north and the old un-lined dump to the south).

A further 1.5 feet of sea level rise is forecast to occur within 32-44 years; a 2 foot rise by 2050 to

2066 (Figure 8 and Table 1). These are simply imminent and must be considered thoroughly

when planning for the future of the M-DWASD Central facility on Virginia Key. As the system

is sediment starved, loss of the island must be considered a certainty early in future sea level rise.

Importantly, sea level rise will continue to accelerate through this century and beyond.

Accumulated ocean heat and accelerating ice melt makes that very certain. Planning must not

assume that sea level will become stable after some small amount of rise. All U.S. government

and U.N. Intergovernmental Panel on Climate Change projections forecast continued sea level

rise acceleration. Influence of sea level rise on the M-DWASD Central facility has beendiscussed in more detail in a previous statement affidavit by the author and maps provided

showing conditions with different sea level rise elevations.

4. An Island Topography Configured To Accentuate Storm Surges to Central Treatment

Facility.

Storm surges will be exceptionally high across the M-DWASD Central facility on Virginia Key

because of the high elevation areas to the north and south. As shown in Figure 7, the M-

DWASD Central facility is bordered by an old dump with elevation of 25 feet just to the south

and an area of high elevation fill to the north (spoil from a deepening of Government Cutchannel). These high areas will serve to force exceptionally strong and high storm surge waters

into and across the lower elevation areas between them (as has been observed by the author

between condominiums in Hurricane Andrew and other storms). In other words, major storm

flooding events will treat the M-DWASD Central facility on Virginia Key as a sluiceway. With

rising sea levels this will only be exacerbated.


11/12

11

11

FIGURE 8. Graph of the sea level projections published by NOAA in December 2012 withadded decadal time lines an10 centimeter elevations lines above the 1992 zero elevation shown.

The year 2063, 50 years from now, is also shown. Elevations are in centimeters on the left of the

diagram and in feet on the right. Only the Intermediate High and Highest projections are

meaningful. The Low projection is the historical rate (20 cm/century) projected into the future.Sea level is already rising more than 50% faster than that. The Intermediate Low projection

assumes no melt from the Ice Sheets of Greenland and Antarctica during the century. Greenland

and Antarctica are already providing a significant component to sea level rise and areaccelerating. Intermediate High projection assumes limited contribution of ice melt to sea

level rise. The Highest projection assumes the present highest anticipated contribution of icemelt to sea level rise. Table 1 below summarizes the timing projected one-foot to six-foot risebenchmarks for the Intermediate High and Highest projections.

Reference is: NOAA, 2012. Global Sea Level Rise Scenarios for the United States National

Climate Assessment. NOAA Technical Report OAR CPO-1. Climate Program Office, SilverSpring MD, 27p. (done cooperatively with United States Geological Survey, Department of

Defense, Environmental Protection Agency, Department of Energy, and the US Army Corps of

Engineers).


12/12

12

12

TABLE 1.

Global Sea Level Rise Projections from NOAA (2012)

+1 ft +2 ft +3 ft +4 ft +5 ft +6 ft

Intermediate High 2042 2067 2084 2100 2112 2121

High 2030 2050 2063 2074 2084 2093

______________________________________________________________________________

SUMMARY

There is no evidence that there are any plans or financing for meaningfully protecting or

elevating the M-DWASD Central facility on Virginia Key against erosion from extreme sandstarvation, against major hurricane surges, or against the rising sea levels forecast during this

century. In fact there is no evidence provided that such protection is economically feasible.

This area is simply too vulnerable and too at risk, both now and even more so in the near future,

to spend taxpayers and citizens money on maintaining or upgrading the M-DWASD Central

facility on Virginia Key. In addition, such a vulnerable facility will be at increasing risk for

resulting in a source for a major pollution release.

Respectfully submitted,

Harold R. Wanless, Ph.D.

Registered Florida Professional Geologist #985

1231 Genoa St., Coral Gables, FL 33134

August 8, 2013

BBWKWanlessVKStatement8813-1

Documents

Transcript of BBWKWanlessVKStatement8813-1