Evaluation of the importance of 100-year flood discharge error bars on the Pájaro River

A Capstone Project

Presented to the Faculty of Science and Environmental Policy

in the

College of Science, Media Arts, and Technology

at

California State University, Monterey Bay

in Partial Fulfillment of the Requirements for the Degree of

Bachelor of Science

by

Bianca Christine Moreno

April 15, 2008

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Abstract Floods for a long time, have threatened riverside livelihood. Today’s levee systems have been built and maintained in accordance to the 100-year flood discharge. The Pájaro River has seen some devastating flooding in the past and has been an area of interest due to the proximity to the City of Watsonville and the small town of Pájaro. A flood control plan proposed to a .9 mile reach of the Pájaro River will only allow a 4 ft levee raise due to the proximity to residential and farmland area. The 100-year discharge as described in the plan is 41,000 cubic feet per second (ft³s). The current channel capacity of the lower reaches of the Pájaro River is approximately 22,000 ft³s. The current levee system of the Pájaro River is out of compliance. The main issue this capstone addresses is whether the error bars (confidence limits) of the 100-year flood are important in considering when delineating floodplains and new levee management plans for the Pájaro River. This finding will determine if the current levee system and the 4 ft levee raise plan will contain the discharges of the 100-year flood interval and protect the livelihood of the river side residents. The 100-year flood was computed using standard Log Pearson III methods, ranged from 16,000 ft³s to 67,922 ft³s through the study reach. Water-surface elevations were determined by inputting stage and discharge data into HEC-RAS, resulting in the 100-year flood discharge being contained by the current levee system yet showed a 4 foot water surface elevation above the surrounding low lying floodplain of Watsonville and town of Pájaro. The high discharge of the 100-year flood resulted in levee breach along the entire length of the study reach. Flood boundaries were then plotted on a 3ft contour tin raster to determine the extent of the 100-year flood interval. It is evident that the current levee plan will not contain the highest discharge of the 100-year flood. However, the 4 ft levee raise will succeed at preventing flood from the highest discharge of the 100-year flood but with high risk, as the elevation of the water within the channel will be well above the surrounding floodplain and increase stress on the levee system. It is suggested that the levee raise should be reconsidered and the best option for Reach 3 of the Pájaro River and its surrounding community members is a levee setback.

Introduction

The wild rivers of California had remained unaltered when the first Europeans

arrived in the 1800’s (Mount 1995), but growing populations have also lead to pervasive

river channel modifications. Following California’s gold rush, early settlers began

colonizing the floodplains along the resource-rich rivers of northern California (Mount

1995). Natural floodplains created and maintained by rivers, are some of the most

productive and diverse ecosystems on Earth (Tockner K and J Stanford 2002).

Floodplains are a valuable resource to urban development and agriculture by providing a

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flat and stable sediment surface (Thorne et al. 1997). Like the early settlers, we continue

to make river floodplains our home. Yet, through the poor management of the Earth’s

rivers, our critical resource is now threatening our existence. Urbanization and misuse of

land and river management alter the hydrologic dynamics of rivers and increase channel

instability and the degradation of rivers (Thorne et al. 1997). Our own disregard for river

dynamics has resulted in more frequent flooding as well as loss of lives and property

damage in some extreme cases. We are only now beginning to realize the importance of

societal and environmental balance along rivers.

Floods have become the number one natural hazard in the United States during

the 20th century and may continue to be number one in the 21st century (Perry 2000).

Periodic flooding of rivers is a natural response to imposed discharge and sediment load.

Extreme modifications to the river channel will result in the instability of the river

(Thorne et al. 1997). Due to our previously poor understanding of fluvial geomorphology

humans have drastically altered rivers for their own convenience. As a result, modified

river channels may no longer contain discharges that it may have in the past. In an effort

to understand flood events and protect the communities from flood disasters FEMA has

created and provided counties, states, and local protection agencies with necessary flood

maps. These flood maps are important to local governments for determining land use

and regulatory purposes (DNR 2004).

Rivers are central to the lives and livelihoods of Californians. We depend upon

our rivers to supply drinking water, irrigate farmland, and generate electricity. We also

value our rivers for their ability to provide us with natural places for recreation. The State

of California has an extensive network of 1,600 miles of levees that protect thousands of

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homes and acres of fertile farmland from floods (Hallissy 2005). The goal of levees are to

increase the margins of the channel, which increases flow capacity to a point that high-

volume flow does not flood the surrounding developed floodplain (Mount 1995). Many

levees are currently being built in contract to the 100-year flood. A 100-year flood is by

definition a flood magnitude that has a probability of 0.01 (1% chance) of being equaled

or exceeded any given year. The 100-year floods maps created by FEMA (Federal

Emergency Management Agency) for flood insurance purposes are based on statistical

historical flood data and are constantly changing as watershed conditions change and new

data are added as new flood events occur.

FEMA Flood Insurance Rate Map (FIRM) and the digital versions (DFIRM) of

the FIRM were designed to be used with digital mapping and analysis software to provide

the user with the ability to determine the flood zone, base flood elevation and the

floodway status for a particular location (FEMA 2008). The National Flood Insurance

Reform Act of 1994 mandates that FEMA assess the need to revise and update all

floodplain areas and flood risk zones no less than once every five years (FEMA, 2002).

However, considering the high expenses associated with a full flood hazard assessment,

many river reaches have not been studied in decades and FIRM maps have not been

properly updated. This can prove to be an inefficient flood mapping technique for

dynamic river systems with high ranges of discharges for any given flood event. It could

also prove to be disastrous for flood protection plans that are based on flood assessment

data provided by FEMA and can result in levees that eventually fail.

The San Joaquin River Delta has seen recent levee failures (Hellissy 2005). The

San Joaquin River Delta now includes more than 1,100 miles of levees to protect the once

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abundant marshlands that have been cleared and drained for major agriculture production,

from flooding. Since 1900, the San Joaquin River Delta levee system has witnessed 163

levee breaches (Department of Water Resources Flood Management 2007). In 2004, the

Department of Natural Resources Colorado Water Conservation Board implemented

guidelines for determining 100-year flood flows for floodplains in Colorado (DNR 2004).

This was done in an effort to accurately outline the flood zones from approximations of

floodplain delineations done by FEMA. In light of the 2005 Hurricane Katrina disaster

there has been a strong outcry for the State of California to thoroughly inspect and, if

necessary, strengthen and repair levees for the protection of its citizens. Understanding

the relationship between levees and natural river sinuosity in channelized rivers is

important for flood risk assessment, monitoring and management. The central coast of

California is currently dealing with failing levees and possible management solutions to

benefit the surrounding urban communities as well as integrating ecological awareness

(Warner 2001). Like most rivers in the Central Coast region, the Pájaro is relatively

small, has a short lag time for flood flows, has peak runoffs, and is subject to winter

flooding (Mount 1995).

The Pájaro River Watershed covers approximately 1,300 square miles, and

overlaps four counties; Santa Cruz, Santa Clara, Monterey, and San Benito (Pájaro River

Watershed Flood Prevention Authority 2002; Curry et al. 2003 ). The Pájaro River,

shown in Figure 1 below, eventually empties into the Monterey Bay Sanctuary, just north

of Elkhorn Slough. Even after rebuilding the levees in the 1940’s by the U.S. Army

Corps of Engineer (USACE), the Pájaro River has flooded repeatedly over the past 50

years (Warner 2001). Recently, substantial flooding occurred in March of 1995 and

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February of 1998, which caused extensive damage to residential areas, agricultural fields,

and the levees along the river (PRWFPA 2002).

California

Figure 1: Regional location map of the Pájaro River Watershed boundary and counties The original levee system along the Pájaro River was completed in 1949 by the

USACE (Warner 2001; Curry et al. 2003). The flood management plan set up by the

USACE has long since changed the shape and function of the river and its floodplain

(PRWFPA 2005). Recurrent flooding and levee damage has led to speculation that flood

maps may be out of date and do not adequately reflect current river conditions. The

USACE has proposed a plan that would set back and raise some of the existing levees as

well as routinely remove gravel and riparian habitat along the channel banks and channel

bottom (Pájaro River Flood Protection Community Planning Process 2001). It is

therefore, imperative that the county use updated flood frequency, recurrence times,

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predicted flood levels, and floodplain zonation in an effort to protect the surrounding

communities from the 100-year flood waters.

The USACE flood protection plan put forth focuses on several projects to increase

flood protection due to the limitations of levee setbacks along the urban areas and

farmland (Pájaro River Flood Protection Community Planning Process 2001). The

specific area of interest in this study, referred to as Reach 3 in the USACE plan, is a 0.9

mile reach of the Pájaro River that flows between the city of Watsonville and the town of

Pájaro (Fig. 2). The flood control plan developed by the USACE for this reach, will only

allow a raise of 4.0 ft of the existing levees as compared to levee setbacks of 100 ft to 225

ft for the rest of the lower Pájaro River. The goal the USACE hopes to achieve is for river

discharge to flow swiftly through the channel while containing the 100-year flood

magnitude (Pájaro River Flood Protection Community Planning Process 2001). To

achieve the accelerated discharge through the channel, the USACE anticipates that the

channel will need to be regularly dredged and reformed (Pájaro River Flood Protection

Community Planning Process 2001). The local residents of Watsonville and Pájaro would

like to be protected with higher levees, rather than the levee setbacks, which would result

in the displacement of many residents from their homes along the river. Due to

economical losses if the levees were set back, the farm owners along reach 3 of the

Pájaro River also prefer the levee raise plan.

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Figure 2 Pájaro River Reaches. Reach 3, Watsonville city limits to creek confluence, outlined. : To conceptualize the extent of the 100-year flood flow range, the Hydrological

Engineer Center (HEC) has developed computer programs for the planning analysis

procedures since 1964 (Hydrologic Engineering Center 1995). HEC RAS (RAS = River

Analysis System) is commonly used for modeling river response to a given storm. The

basic inputs include the river discharge obtained from a local USGS stream gauge and

channel/floodplain geometry obtained from cross-sectional surveys. The program yields a

one dimensional stage (water elevation) model. This output is useful for modeling the

inundation of floodplain and the impacts of river alterations such that of levees.

Flood frequency analyses are used to predict design flood events for sites along a

river. Statistical data calculated from annual peak flow gage date are used to construct

frequency distributions. The Log Pearson Type III Distribution (LPIII) is a standard

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technique used by U.S. Federal agencies such as USGS and FEMA, to calculate flood

recurrences. The LPIII is the recommended flood frequency analysis by the U.S.

Interag

2,000 ft³s which is well below the expected 100-year

flood e

range of the 100-year flood waters at its current system capacity. Due to the dynamic

ency Advisory Committee on Water Data (Klingeman 2005).

The floods of 1995 and 1998 that overtopped the levees protecting Watsonville

and Pájaro also caused damage to the flood protection project itself. The current Pájaro

River Flood Control Project proposes to restore the original set back levee configuration

with minor adjustments. However, data used in the PRFC project dates back to 1995 and

1999 and could not reflect the current channel conditions and whether it is capable of

containing a 100-year flood magnitude. In the 2001 Project Status Report of the Pájaro

River Flood Protection Community Planning Process, the 100-year flood was identified

at roughly 49,000 cubic feet of flow per second (ft³s). The record of flood events on the

Pájaro River since 1940 indicates that the largest flood event occurred in 1998 and was

less than a 25-year flood which is roughly 25,000 ft³s (Pájaro River Flood Protection

Community Planning Process 2001). The current channel capacity of the lower reaches of

the Pájaro River is approximately 2

vent (PRWS 3 & 4a 2005).

Surveys of the river were done soon after the river channel was dredged. If the

river has aggraded in recent years, this would translate into a higher stage for the 100-

year flood discharge. The critical research question addressed in this study asks whether

the confidence limits of the 100-year flood range are important when delineating

floodplains and determining a flood management strategy along reach 3 of the Pájaro

River. This study will also determine if the river channel will contain the possibly large

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nature of the Pájaro River, it is hypothesized that the range for the 100-year flood is large

and is important in planning for maximum levee protection of the 100-year flood.

This finding will help aid the local governments of the Pájaro River Watershed in

determining if the approximate discharge for the 100-year flood used in the Flood

Management Plan can be considered accurate. If so, then the proposed 4 foot levee raise

will succeed in containing the flood waters from inundating the surrounding city and

farm areas. This study will also conceptualize the importance of flood risk to the

residents of Watsonville and Pájaro as well as aid them in the future decisions of levee

designs appropriate for their reach of river.

Methods

The approach used to determine whether the Pájaro River will be able to contain

the high to low estimated discharges of the 100-year flood waters, includes the collection

of cross sectional surveys of the river channel digitally collected in ArcGIS, imported

aerial and DEM files, FEMA flood data, Log Pearson III statistical analysis used to

analyze peak stage data obtained from USGS gaging stations, and HEC-RAS modules to

model and visualize the extent of the upper, average and low discharges of the 100-year

flood event in one dimensional and 3-dimensional space. Model input includes river

channel profiles, profile slope, flow data, geologic controls and existing levees.

Local stream gage data was collected from the gaging station located at

Chittenden, CA upstream from reach 3 of the Pájaro River. USGS gage station 11159000

Pájaro River at Chittenden, CA is located approximately 19,800 meters (12.3 miles)

upstream from the Salispuedes Creek confluence. For this project surface water: peak

stream flow data was retrieved from the USGS Water Resources website at

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http://nwis.waterdata.usgs.gov/ca/nwis/peak?site_no=11159000&agency_cd=USGS&for

mat=rdb. The Log-Pearson Type III analysis, commonly used in computing flood

frequency, was applied in this study to provide the best expectation of the exceedance

probability of any given flow rate. Using the skew from the log transformed data, and to

avoid large skewing of the frequency plot, the USGS gage data was filtered for low and

high outliers. Outliers are considered extreme events that do not follow the trend of the

line. The criterion and formulas used in detecting the outliers were obtained from Bulletin

17B, “Guidelines for Determining Flood Flow Frequency.”

Using a Log-Pearson Type III distribution excel sheet, a statistical datasheet to

determine flood frequency created by Fred Watson of California State University-

Monterey Bay, peak stream flow data was entered and ranked from highest to lowest. The

frequency plot then fit the LPIII distribution to the peak annual flow series. The statistical

datasheet provided the upper and lower confidence limits of the 100-year flood event.

Ten foot digital elevation models (DEM) and three foot contours taken in 2002

were obtained from the City of Watsonville GIS Department. Due to the low resolution

of the imported 10 m DEM, the 3 foot contours taken in 2002 by the City of Watsonville

were used in extracting the necessary cross sections of Reach 3 of the river channel.

Caltrans benchmarks and stream files were downloaded from the Santa Cruz County GIS

website. The data was then imported onto the 3 ft contours. The various data layers were

then clipped to the appropriate size of the study area. A tin was created from the 2002 3

foot contour data in 3D Spatial Analyst and was used for generating cross sectional data.

Using ArcGIS, a transect line was inserted the length of the river reach (0.9 mi)

on top of the 2002 contour layer. Using a random sequence generator, seven transects or

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polylines were then digitally inserted at these random intervals perpendicular to the 0.9

mile transect line. Using 3D spatial analyst in ArcGIS, cross sectional profiles of each

polyline containing distance and elevation data were generated and exported in .xl format

to be input into HEC-RAS. Four additional transects were placed on top of existing

Caltrans cross sections of the river as polylines. The Caltrans cross sections of the river

channel were taken directly upstream and downstream of the Highway 152/Main Street

Bridge and the railroad bridge. These were used for reference purposes. Eleven total

digital surveys were taken of the Pájaro River channel (Fig. 3). The upper most cross

section is approximately 169 meters (.105 mile) downstream of the Salispuedes Creek

confluence. The GPS points of the benchmarks used for the surveys were obtained from

the GIS Resources Department of the Association of Monterey Bay Area Governments

(AMBAG). However, some of the benchmarks were located on residential streets and

were inaccessible. Consequently, they were not used in this study.

Figure 3: Transects in red, taken perpendicular to the river channel on top of contour data in ArcGIS.

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The cross sectional data obtained from the polyline profiles were entered into an

excel spreadsheet and graphs were created for quick evaluation of the river channel

geometry. The cross sectional data was then entered into the HEC-RAS 4.1.1 computer

program along with the peak stream flow data obtained from the USGS Water Resources

website. The data for each cross section comprises X and Y coordinates, downstream

reach lengths, Manning’s n values, main channel bank stations, levee station and

elevation, and contraction and expansion coefficients. The following figure is a 3

dimensional plot of the cross sections on Reach 3 (Fig. 4).

11

10

9 8

7

5

4 3

2

1

Reach 3 Pajaro R iver_bcm02_100yr Plan:

Legend

Ground

Bank Sta

Figure 4: HEC-RAS x-y-z cross sectional plot for Reach 3 of the Pájaro River.

The Pájaro River reach was defined with 11 cross sections numbered from 11 to

1, with cross section 11 being the most upstream cross section. These station numbers are

used only by the program for placement of the cross sections in numerical order.

Distances between the cross sections were obtained from ArcGIS and entered in HEC-

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RAS as left overbank (LOB), main channel and right overbank (ROB). Manning’s n

value for the LOB, main channel and ROB were obtained from the Pájaro River

Community Planning report and are 0.04, 0.05 and 0.04, respectively. Also, the

coefficients of contraction and expansion are 0.10 and 0.30, respectively. The tops of the

left and right levees were defined, as the program will not permit a flow onto the

floodplain until the flow depth over tops the levees.

Before performing a flow analysis, the cross sections were interpolated to produce

smoother channel geometry. Each interpolated cross section was then inspected and

adjusted for geometric accuracy of the actual cross sections they were derived from. This

is because the interpolated cross sections are based on a linear transition between the

input sections. Next, each of the 100–year flood values calculated from the LPIII was run

in HEC-RAS.

Each of the 100-year flood event discharges calculated from the LPIII were

separately run through a steady flow analysis in HEC-RAS. Cross sectional plots and

water surface elevation output from each flow event were analyzed for levee breach. X-

Y-Z plots were generated for each flow events depicting water surface elevation.

The water surface elevations for each flood event was delineated on the 3 ft

contour layer to provide a visual representation of how the 100-year flood event as it

would appear on the floodplain topography that the City of Watsonville and town of

Pájaro sits on. In ArcGIS, points were placed where the water surface elevation for each

flood event was greater than the elevation of the topography. A rough outline was then

digitally drawn on the contour tin layer and specific colors and patterns were assigned to

the lower confidence limit discharge of the 100-year flood, the actual LPIII derived 100-

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year flood discharge and upper confidence limit discharge of the 100-year flood.

Orthoimagery of the Pájaro River and the surrounding counties obtained from the

CSUMB brazil server (\\brazil.csumb.edu\essp$) were imported and the contour tin

layer removed resulting in an image of the 100-year flood range delineations on a

photographic image of Reach 3 of the Pájaro River and the surrounding urban areas.

Results

The USGS Chittenden peak annual stream flow data resulted in a skew of -1.09.

The Bulletin 17b method was then applied for detection of outliers. One outlier was

found and removed. After running the Log-Pearson III model of the peak annual stream

flow data that had been adjusted accordingly for outliers, the resulting skew was -0.97.

This resulted in a frequency plot with curved confidence intervals (Fig. 5).

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100

1000

10000

100000

-3 -2 -1 0 1 2 3

Standard normal deviate of probability of excedance

Mag

nitu

de

5% = 20 yr50% = 2 yr1% = 100 yr

95% = 1.053 yr99% = 1.010 yr0.2% = 500 yr

Figure 5: Frequency plot and confidence intervals plotted on log-normal graph, Pájaro River at Chittenden gauge data.

The upper and lower ranges of the 100 yr flow are 67,922.5 ft³s and 16,003.66

ft³s, respectively (Table 1). The LPIII 100-year flood magnitude has a discharge of

32,969.81 ft³s (Table 1).

discharge (ft³s) upper 95% CI 67,922.5 100yr flood 32,969.81 lower 95% CI 16,003.66

Table 1: Log Pearson III adjusted discharges of the 100-year flood.

The steady flow analysis of the low confidence interval discharge of 16,003 ft³s

produced a water surface elevation of 26.16 ft within the river channel and was below the

surrounding floodplain elevation (Fig. 6).

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Reach 3 Pajaro River_w ithlevees Plan: Plan 12 5/6/2008

Legend

WS PF 1

Ground

Levee

Bank Sta

Ground

Figure 6: Downstream perspective of HEC-RAS x-y-z plot of 16,003.66 ft³s steady flow on reach 3 of the Pájaro River.

The high 100-year flood discharge of 67,922.5 ft³s produced a water surface elevation of

35.76 ft that over topped the levees on each side of the channel (Fig. 7).

Figure 7: Downstream perspective of HEC-RAS x-y-z plot of 67,922.5 ft³s steady flow on reach 3 of the Pájaro River.

Reach 3 Pajaro River_w ithlevees Plan: Plan 12 5/6/2008

Legend

WS PF 1

Ground

Levee

Bank Sta

Ground

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The LPIII 100-year flood discharge of 32,969.81ft³s produced a water surface

elevation of 35.76 ft within the river channel (Fig. 8).

Reach 3 Pajaro River_w ithlevees Plan: Plan 12 5/6/2008

Legend

WS PF 1

Ground

Levee

Bank Sta

Ground

Figure 8: Downstream perspective of HEC-RAS x-y-z plot of 32,969.81ft³s steady flow on reach 3 of the Pájaro River.

The water surface elevation within the river channel lies approximately 4 ft above the

floodplain elevation (Fig.9).

1000 2000 3000 400010

15

20

25

30

35

40

Reach 3 Pajaro River_withlevees Plan: 100yr from ACoE 5/6/2008 Cross Section 723

Station (ft)

Ele

vatio

n (ft

)

Legend

EG Avg flow

WS Avg flow

Crit Avg flow

Ground

Levee

Bank Sta

.04 .04 .04

Figure 9: HEC-RAS cross section 6 of the Pájaro River with LPIII 100-year discharge (32,969.81ft³s).

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The high, actual and low water surface elevations of the 100-year flood are

delineated on orthoimages of the Pájaro River. The high discharge of the 100-year flood

is shown in red, the 100-year flood discharge in orange and the low discharge of the 100-

year flood in blue (Fig. 10).

Figure 10: Delineations of the 100-year flood range: upper CI discharge in red, actual 100-year flood discharge in orange and low CI discharge in blue. Watsonville is located in the upper left corner and the town of Pájaro is located in the lower right corner. Discussion

The HEC-RAS flow analyses of the 100-year flood range provided the necessary

waters surface elevation to determine if the current levee system was able to withhold the

100-year flood range and prevent flooding of the surrounding cities and agricultural

fields. The HEC-RAS X-Y-Z plots provided a simple visual representation of the

possible impacts of the highly dynamic flood flow of the 100-year flood. Results from the

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Log Pearson III analysis shows that the 100-yr flood discharge can be highly

unpredictable and can vary immensely. These results supported my hypothesis that the

100-year flood can have a large range on the Pájaro River. The HEC-RAS results also

showed that the current levee system will not contain the highest discharge of the 100-

year flood.

With a stage of 26.16 ft, it is evident that the low 95% confidence limit of the

100-year flood is within the system capacity of Reach 3 of the Pájaro River. This low

discharge of the 100-year flood is not important when considering levee protection from

the 100-year flood. Further analysis of the individual cross sections of the river channel

with a flow of 32,969.81 ft³s shows that the water surface elevation or stage within the

channel is 4 ft above the surrounding floodplain. This can be of major concern due to

high stress on the levees caused by a high water surface elevation within the river

channel. Any minor scouring, aggradation or degradation with in the channel or change

in the levee structures could result in eventual levee failure. Earthquakes from the nearby

San Andreas Fault could possibly damage the structural integrity of the levee system and

lead to eventual levee failure in the event of the 100-year flood high discharge range. The

addition of the Corralitos Creek gage data, a tributary to the Salispuedes Creek, could

also increase the discharges of the 100-year flood.

The upper 95% confidence limit of the 100-year flood (67,922.5 ft³s) clearly

overtops the levees on each side of the river channel. However, not considered in the

HEC-RAS program is the adjustment for stage on each side of the levee. As the water

tops the levees, the water surface elevation on the floodplain should decreases

significantly as it spreads over the land. This link between the channel water surface

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elevation and the elevation on the floodplain was not correlated in the steady flow

analyses of the high discharge of the 100-year flood. Therefore, the actual stage of the

high confidence limit is in fact lower than what was represented in this study. The

elevation with in the river channel with a high range discharge will put increasingly high

stress on the levee structures. The collapse of a levee with a high water stage is a high

risk the surrounding communities of Watsonville and Pájaro face.

Due to the importance of maximum and effective levee protection needed along

Reach 3 of the Pájaro River, I believe the 4 foot levee raise in plan for the reach will not

be sufficient in providing that protection needed for the 100-year flood range. While the

current levees and future planned levees with a 4 ft rise will protect the urban areas and

farmland against the low and actual 100-year flood discharge it may come at a high price.

A more reasonable option for Reach 3 of the Pájaro River is the levee set back that will

allow the river sufficient floodplain within the river channel as it will increase the width

of the channel. This in turn will decrease the stage of any given flood event and more

importantly, decrease the water surface elevation of the 100-year flood discharges.

The river system will continue to do its job of transporting the sediment provided

to it. It is therefore important to consider all watershed activities going on in the upper

watershed of the Pájaro River. This includes the restoration of Soap Lake, a natural

depression in the landscape that will retain flood waters in the event of high discharge,

resulting lowered discharge flowing to the downstream reaches of the Pájaro River. A

major contributor of sediment is the in stream gravel mining occurring at 2 locations

upstream of the Chittenden gage station. This may result in the aggradation of sediment

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in the downstream reaches. This excess sediment may raise the elevation of the main

channel increasing the stage of any flow event. It is important that

The cross sections derived from the contour data used in this study, provide a

rough sketch of the topographic layout of the Pájaro River channel. By applying peak

discharge data for the 100-year flood event, the resulting image provided a rough

illustration of the 100-year flood extent over the topography. While 3 foot contours

provided by the City of Watsonville GIS department were useful in producing river

channel cross sections that accurately depicted the true landscape of the area in the year

2002, it does not represent current river channel geometry. Due to the dangers of

surveying the river, because of the criminal activity taking place within the channel,

recently taken 2007 Light Detection and Ranging (LiDAR) data will soon be available to

be used in this study. LiDAR is a remote sensing system used to collect topographical

data. The data is collected by aircraft-mounted lasers and can provide 6 in resolution

images. It is important for the accuracy of future flood studies that most current and high

resolution data is used.

The probability of the 100-year flood happening is about 1% every year. This

information and the lack of stage data for large floods give the results a trend of

uncertainty when predicting the water surface elevation of the 100-year flood range. It is

therefore important that the highest discharge of the 100-year flood should be considered

by federal agencies such a FEMA when delineating floodplains. This will result in the

proper implementation of levee systems by local government agencies and the

surrounding areas can trust in the maximum levee protection for the 100-year flood.

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Literature Cited Curry R, Bodensteiner W, Clough L, Gilmore S, Huntington P, Larson J, McMillan A,

Mack S, Mauck CA, Pring S, Roth E, Thistle A, Vincent M. 2003. Pájaro River Watershed Flood Protection Plan. Accessed on April 8, 2007 from http://home.csumb.edu/c/currybob/world/Pajaro/

Department of Natural Resources. 2004. Guidelines For Determining 100-Year Flood

Flows For Approximate Floodplains in Colorado. Version 6 Department of Water Resources Flood Management. 2007. Levee Failures in the

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