appendix 2 - risk analysis

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Design Document

Appendix 2

Risk Analysis


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Town of Palm Beach

Phipps Ocean Park – Beach Restoration & Stabilization

Appendix 2 - Risk Analysis

1.0 Introduction

The vulnerability and risk associated with specific storm events are herein evaluated byapplication of the U.S. Army Corps of Engineer’s (USACE) dune erosion model, Storm-inductedBEAch Change (SBEACH). SBEACH is a two-dimensional numerical model developed tosimulate cross-shore storm effects, including sediment transport primarily due to breaking wavesand changing water levels (Larson and Kraus, 1989). The model was developed based onextensive analysis of beach profile changes produced in wave tanks and in the field. SBEACH isintended to predict short-term, storm-induced profile adjustment. A fundamental assumption ofthe SBEACH model is that profile change is produced solely by cross-shore processes, resultingin a redistribution of sediment across the profile with no net gain or loss of material. Longshore processes are considered to be negligibly small in calculating profile change, and therefore,alongshore wave and current sediment transport is not accounted for by SBEACH.

2.0 Risk Analysis - Methodology

As part of the risk analysis, it is necessary to identify representative profiles, establish native beach sand grain size characteristics, and define the 20, 50, and 100-year storm hydrograph andwave characteristics. These elements of the analysis methodology are described below.

2.1 Native Beach Sand Characteristics

The following is excerpted from Chapter 3 – page 101 of the 2004 SEIS (by Coastal Tech): Native Beach: The U.S. Army Corps of Engineers investigated native beach sandcharacteristics along Palm Beach County beaches in 1979 and published the results in the1987 Palm Beach Island GDM/EIS (Appendix B). The USACE results provide an historicalrepresentation of the sand characteristics of the beaches in the region of the Project Area. Inthe 1987 Palm Beach Island GDM …, the USACE characterized beach sediment in southPalm Beach County as a medium-grain-moderately-sorted sand with a mean grain size of

0.34 mm and a Sorting Coefficient of 0.97 phi. These 1979 conditions best represent thenative beach prior to the extensive erosion that subsequently occurred and resulted inreduction of the fines content of the native beach. As the shoreline has eroded, beachsediments were sorted, whereas the fines were removed, and the mean grain size of the native beach has increased.


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Town of Palm Beach Phipps Ocean Park – Beach Restoration & Stabilization

Risk Analysis

data. The ERM values reflect the sorting of the native beach sediments due to wave actionand erosion.

On behalf of the Applicant, in August 1999, Coastal Tech collected surface grab samplesfrom the native beach in the Project Area at DNR Monuments R-116, R-120, and R-125.Samples were collected at three locations along each transect: (a) at the toe of the dune, (b)at the top of the foreshore slope, and (c) in approximately 5 feet of water. As presented inAppendix K, Supplementary Geotechnical Analysis, the grain size analysis of these sampleswas performed by Fraser Engineering and Testing; the existing beach sediments were foundto have a composite mean grain size of 0.43 mm. Existing beach sediments were found to

consist of moderately sorted coarse sand, with a Sorting Coefficient of 0.99 phi and a siltcontent of less than 1%. As the 1999 data is the most recent data, this data is used to bestcharacterize existing beach sediments.

In the spring of 2011, the Town placed beach fill to repair the dune and berm from damagesincurred in association with Hurricane Fay. The sand for the 2011 beach fill was obtained fromthe “Ortona” sand mine, which has a median grain size of 1.15 phi or 0.45 mm – as identified in post-construction sediment analyzes.

Based upon the above, the Effective Grain Size used in the SBEACH model analysis is 0.45 mm,which is reasonably assumed to represent the pre-2006 beach fill conditions.

2.2 Storm Surge Characteristics and Data

The storm tide data used in the risk analysis came from the Florida State University Beaches and

Shores Research Center website and was specifically obtained from report entitled “CombinedTotal Storm Tide Frequency Restudy for Palm Beach County, Florida” dated January 2007 byS.Y. Wang, Manausa, Dean and Walton (Wang et al, 2007).

Vulnerability is determined - per FDEP rules – via modeling of erosion impacts associated with20, 50, and 100-year return interval storms. The 100-year storm hydrograph profile is publisheddata that can be found on the website. The 20 and 50-year storm “data” used for SBEACH inputwas developed based on the adaptation of the 100 year storm surge hydrograph profile, as

described by the following:o obtaining the 100-year storm surge hydrograph including the astronomic tide from the

Beaches and Shores Resource Center,o separating and scaling the 100-year storm surge (proportional to the 20 and 50-year Peak

Storm Surge elevation) to produce the 20 and 50-year storm surge,o adding back-in the astronomic tide to produce the 20 and 50-year Storm Surge


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Risk Analysis

Table 2-1: Storm Surge Elevation per Return Interval Storm Event

Return Int. Conversion Wave

Surge

ElevationStorm Event Factor Height (m) (ft, NAVD)

100-year 1.00 4.14 9.650-Year 0.87 3.18 8.220-Year 0.68 2.17 6.1

Profile 2 was chosen as the best geographic representative in closest proximity to the Projectarea. Inherent in this method of proportioning the 100 year storm hydrograph for the 20 and 50-year storms is the assumption that the shapes of the three storm hydrographs would be similar. Note that the 100 Year hydrograph from Beaches & Shores Resource Center includesastronomical tide data which was not proportionally reduced in the conversion.

2.3 Wave Data

The U.S. Army Corps of Engineers (USACE) WIS Hindcast data was used in the model

application. WIS hindcast data is a compilation of a 20 year predicted wave record representativeof 1980 to 1999 for a total of approximately 175,000 waves. A statistical analysis of the wavedata from WIS Station 459 was conducted in order to determine the general local wave climatecharacteristics. The representative wave conditions were determined through a ‘binning’ utilityinternal to the Nearshore Evolution Modeling System (NEMOS) developed for the ERDCCoastal Hydraulics Laboratory. The utility is used to process the extensive wave time serieswithin typical limitations of maximum allowed model input records. The waves were binned in22.5 degree intervals for the entire 360 degree data set. However, the dominant wave direction

observed relative to the Town of Palm Beach is from the offshore Northeast direction. Thewaves were ‘binned’ in quarter-meter wave height increments in order to capture the variabilityin the waves and directions.

The twenty year wave record from WIS Station 459 was further analyzed to identify the annual percent occurrence of various wave heights arriving from the corresponding cardinal directions.The percent occurrence and frequency distribution was calculated to identify the representativewave height and period per storm return event for 20, 50, and 100 year storms. Table 2-2

summarizes the calculated wave height and period indicative of each storm return interval, usedin the SBEACH model.

Table 2-2: Calculated Wave Height and Period per Return Interval Storm

Return

Int. Wave Wave Wave


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Risk Analysis

It is important to note that because the WIS data is only a 20 year wave record, the percent

probability of occurrence of each storm event with a return interval greater than 20 years wastaken into consideration when forecasting wave heights for each return interval event. Forexample, the likelihood of occurrence of a 100-year return interval storm event during the 20year period reflected in the WIS data set is 20% or less. Therefore, the exceedance frequencyused to determine the wave height for a 100-year return interval storm is 0.2% (1%*20%).

2.4 Representative Profile Data

The profiles used in this analysis are: R-117, R-120, R-124, and R-127, representative of theProject area. The profile data used in this analysis is the 2006 ‘Pre-Fill Condition’, to representthe “existing” beach conditions expected to represent the most severe or most ‘vulnerable’condition. The presence of an existing seawall was noted along the upland properties at R-monuments 120 and 127. For these profiles, two SBEACH runs were conducted – with andwithout the seawall in place - in order to:

(a) determine the profile evolution behavior for each storm return interval for bothconditions, and

(b)

identify the seaward most limits of erosion without the presence of a seawall.The condition in which the seawall is not in place is considered the most conservative case in theevent that the seawall fails under potential storm conditions. A comparison of both scenarioswas conducted by superimposing both condition on one profile plot. As may be expected, asignificant change in shoreline evolution was observed where the seawall fails. The profile plotsare included in this Appexdix.

2.5 Model Input

The SBEACH model was applied to the representative profiles in order to predict bluff recessionfrom the 20, 50, and 100-year return interval storm. Table 2-3 summarizes all of the input parameters necessary for a model run – including the recommended ranges for each parameterand the default values for each. For this study the default values were used for all parametersexcept the Effective Grain Size, which as noted above was separately researched and input foreach county and segment.

3.0 Model Results

SBEACH produces a post-storm profile from which the predicted limits of dune or bluffrecession for the 20, 50, and 100-year storms can be extracted. These limits are assessed toidentify the landward most profile translation, representing erosion limits, which may threaten


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Risk Analysis

Table 2-3: SBEACH Input Parameters

Reference: “SBEACH Numerical Model for Storm Induced Beach Change, Report 1” by USACE (1989).

Table 2-4: SBEACH Model Results

Dune Bluff Recession (feet)

R-Mon 20-Yr 50-Yr 100-Yr

117 0 0 11

120 54 89 116124 51 74 77

127 10 45 69


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R-123R-124

R-127R-125R-12R-126

2545-2565 S.

Ocean Palm Beach

White House

2500 S.

Ocean

Condo

2580 S. Ocean

Stratford

2600 S. Ocean

The 2600

2660 S. Ocean

Beach Point

2770 S.

Ocean

2774 S. Ocean

Ambassador

South Coop.

2780 S. Ocean

Ambassador

II Coop.

2730 S. Ocean

Ambassador

Hotel

Scale in feet

0 400

M a t c h l i n

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Risk Analysis Maps

VERO SARASOTA MELBOURNE AU. . .Certificate of Authorization Number: 0000

DRAWNENG DATE

MPW AQN 03/14/2012

1. Aerials provided by Town of Palm Beach; aerial date is May 8, 2011.

2. The limits of erosion indicated on these maps:

a. assume existing seawalls will fail during storm events,

b. are base upon application of SBEACH at referenced monuments R-117, R-120, R-124 &

R-127 - with interpolation between the monuments.

appendix 2 - risk analysis

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