Report_Coastal_Engineering.pdf

VETERANS DRIVE IMPROVEMENTS – PHASE I

ST. THOMAS, US VIRGIN ISLANDS

COASTAL ENGINEERING REPORT

SUBMITTED TO:

USVI DEPARTMENT OF PUBLIC WORKS

SUBMITTED BY:

PARSONS BRINCKERHOFF

JULY, 2015

PARSONS BRINCKERHOFF II FINAL USVI DEPARTMENT OF PUBLIC WORKS

TABLE OF CONTENTS

1. Introduction

1.1 General

1.2 Project Description & Scope of Work

1.3 Oceanographic Data Sources

1.3.1 Bathymetry

1.3.2 Tides

1.3.3 Sea Level Rise

1.3.4 Hurricane Data

1.3.5 Waves

1.3.6 Tsunamis

2. Modeling of Water Levels, Waves and Current

2.1 Water Levels

2.1.1 Hydrodynamic Model Description

2.1.2 Tide Modeling

2.1.3 Storm Surge Modeling

2.1.4 Future Sea Level Rise

2.2 Waves

2.2.1 Offshore Waves

2.2.2 Nearshore Wave Transformation Modeling

2.2.3 Wave Conditions along the Project Site

2.3 Currents

2.3.1 Tidal Currents

2.3.2 Hurricane Currents

3. Recurrence Interval Analysis

4. Scour

5. Wave Loads

6. Summary

7. References

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TABLES

Table 1: Tide Gage Locations in the Vicinity of St. Thomas Harbor, USVI

Table 2: Charlotte Amalie Tide Gage Elevations on Station Datum

Table 3: Historical Storms Modeled with ADCIRC

Table 4: Historical Storm Events Used in EST Analysis

Table 5: Offshore Wave Parameters for Historical Storm Events

Table 6: Offshore Wave Parameters for Historical Storm Events

Table 7: Observed and Modeled Water Levels for Historical Storm Events

Table 8: EST Station Coordinates and Depths

Table 9: Average Recurrence Interval Values for Water Levels, Wave Heights and Storm

Currents without Sea Level Rise

Table 10: Average Recurrence Interval Values of Water Level and Wave Height with a Low Sea

Level Rise (1.2 feet) Scenario

Table 11: Average Recurrence Interval Values for Water Level and Wave Height with a Medium

Sea Level Rise (2.4 feet) Scenario

Table 12: Average Recurrence Interval Values for Water Level and Wave Height with a High

Sea Level Rise (3.6 feet) Scenario

Table 13: Scour Results for Retaining Wall

Table 14: Wave Loads on Retaining Wall – Base Cases

Table 15: Wave Loads on Retaining Wall by Station

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FIGURES

Figure 1: Veterans Drive Project Site Map

Figure 2: Existing Bathymetry, St. Thomas and Surrounding Areas

Figure 3: NOAA Navigation Chart (No. 25649) for St. Thomas Harbor

Figure 4: ADCIRC Computational Domain and Bathymetry

Figure 5: ADCIRC Computational Domain and Bathymetry

Figure 6: Charlotte Amalie Tide Gage Water Level Comparison

Figure 7: Lime Tree Tide Gage Water Level Comparison

Figure 8: Regression Analysis for Observed and Modeled Water Level at Charlotte Amalie

Figure 9: Storm Track for Hurricane Hugo in the Caribbean

Figure 10: Storm Track for Hurricane Hugo near St. Thomas

Figure 11: Hurricane Hugo Wind Field

Figure 12: Water Level Comparison for Hurricane Hugo (9/10/1989) at Charlotte Amalie Tide

Gage

Figure 13: Water Level Comparison for Hurricane Marilyn (9/12/1995) at Charlotte Amalie Tide

Gage

Figure 14: Water Level Comparison for Hurricane Marilyn (9/12/1995) at Lime Tree Bay Tide

Gage

Figure 15: Water Level Comparison for Hurricane Bertha (7/5/1996) at Charlotte Amalie Tide

Gage

Figure 16: Water Level Comparison for Hurricane Bertha (7/5/1996) at Lime Tree Bay Tide

Gage

Figure 17: Values of Hmo/Hmo max Plotted Relative to Center of Hurricane (USACE, 2002) ... 22

Figure 18: Wave Model Grids

Figure 19: Fine Grid Wave Model Results, Hs (m),for Hurricane Hugo

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Figure 20: EST Station locations inside St. Thomas Harbor

Figure 21: Currents for Hurricane Hugo Inside St. Thomas Harbor

Figure 22: Average Recurrence Interval for Water Levels at Station 15

Figure 23: Average Recurrence Interval for Wave Heights at Station 15

Figure 24: Average Recurrence Interval for Wave Period at Station 15

Figure 25: Average Recurrence Interval for Storm Currents at Station 15

Figure 26: Wave Pressure Distribution with Wave Crest at the Retaining Wall

Figure 27: Wave Pressure Distribution with Wave Trough at the Retaining Wall

APPENDIX A - Wave Load Methodology and Example Calculation

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1. INTRODUCTION

1.1 General

Parsons Brinckerhoff (PB) has been retained by the Government of the Unites States Virgin

Islands (USVI) Department of Public Works (DPW) under the professional services contract amendment no. 6 (PC-PWD-97-94) to undertake engineering design and environmental studies for the widening of Veterans Drive along the waterfront of Charlotte Amalie on the island of St.

Thomas in the USVI. The proposed widening requires design and construction of a retaining wall along the shoreline. This coastal engineering report documents the identification and analysis of oceanographic conditions, including tides, storm surge, waves and currents, scour and wave loads pertaining to the proposed retaining wall along the shoreline.

1.2 Project Description & Scope of Work

At Fort Pladsen, the existing Veterans Drive is a two-lane, undivided facility east of King’s Wharf to W.G. Lewis Lane and a four-lane, undivided facility west of King’s Wharf. Existing sidewalks, although not continuous, are located on both sides of Veterans Drive, and a seawall is located on the south side of the roadway bordering St. Thomas Harbor. Figure 1 is a site map of the proposed improvements along the waterfront.

The Veterans Drive Improvement Project will include reconstruction to add new lanes for a continuous four-lane roadway, consisting of two lanes in each direction. A 20 ft wide promenade between the roadway and the harbor will be constructed from the existing promenade at Tolbod

Gade to the end of the project. Beginning near Tolbod Gade, the new roadway alignment turns to pass on the south side of the Coast Guard Station and the Legislature Building on new fill.

The alignment rejoins existing Veterans Drive just west of Hospital Gade and then continues with widening into the Harbor to a point just west of the W.G. Lewis Lane/Long Bay Road intersection. The widening will require removal of existing shore protection (rip-rap), installation of a new retaining wall, filling, and construction of a widened roadway.

The coastal engineering analysis performed by PB and described in this report was used to identify hydraulic conditions for use in the retaining wall design. Tsunami related hazard does exist at the project site. However, analysis of tsunami history and development of associated recurrence interval statistics is beyond the scope of this study. A brief discussion of tsunami history and risk at Charlotte Amalie is provided based on available literature.

The specific scope of work conducted by PB consisted of the following:

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Identification of existing data sources.

Analysis of hurricane induced storm surge and current at the project site based on historical hurricanes obtained from the National Oceanic and Atmospheric Administration

(NOAA) database and the hydrodynamic model ADCIRC.

Analysis of wave properties along the shoreline including height, period, direction and setup using the numerical model SWAN for wave propagation between deep water offshore of the Island and the project site within the bay.

Calculation of probability of occurrence relationships for storm surge and wave setup combined, wave height, wave period and current using the empirical simulation technique model EST.

Analysis of scour depths and wave loads along the proposed retaining wall for Phase I of the project beginning immediately east of the Legislature building and continuing through the eastern end of the project.

1.3 Oceanographic Data Sources

The various sources of oceanographic data obtained and utilized in the coastal engineering analysis are presented below.

1.3.1 Bathymetry

Bathymetry for the project site was compiled from the following sources:

Hydrographic Survey Data was collected by Brian Moseley & Associates for PB in

December 2006 and February 2008. This data includes bathymetric and topographic data along Veterans Drive in the vicinity of the project location.

Bathymetry within St. Thomas Harbor and the immediate vicinity outside the harbor was obtained from Navigation chart Number 25649 published by NOAA.

Bathymetry data offshore of St. Thomas Island and within the Caribbean Sea was obtained from the National Geophysical Data Center (NGDC).

The remainder of the regional bathymetry used in the model was obtained from an

Atlantic Ocean grid developed by U.S. Army Corps of Engineers (USACE).

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Figure 1: Veterans Drive Project Site Map

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Bathymetry data from the various sources was compiled after conversion to consistent horizontal and vertical datums, specifically US State Plane NAD 83, Puerto Rico/Virgin

Islands (International Feet) and Mean Sea Level (MSL). The resulting regional bathymetry is presented in Figure 2. The NOAA Navigation chart for St. Thomas Harbor

(No. 25649) is provided in Figure 3 with soundings in feet, mean lower low water

(MLLW).

1.3.2 Tides

The National Ocean Service (NOS) maintains four tide gages in the vicinity of St.

Thomas, which are represented in Figure 2 by blue filled circles. The periods of record and tidal ranges for each of the gages are shown in Table 1. The tidal range is defined as the average difference between Mean High Water (MHW) and Mean Low Water

(MLW). Table 2 provides a description of various water level datums for the Charlotte

Amalie Tide Gage referenced to station datum.

Table 1: Tide Gage Locations in the Vicinity of St. Thomas Harbor, USVI

Station Station Number

Latitude Longitude Period of record Tidal Range

(feet)

Charlotte Amalie, VI 9751639 18° 20.1' N 64° 55.2' W 1/1/1979 - Present 0.70 Lime Tree Bay, VI 9751401 17° 41.8' N 64° 45.2' W 2/3/1982 - Present 0.69 Lameshur Bay, VI 9751381 18° 19.2' N 64° 43.5' W 8/3/2006 - Present 0.77

Vieques Island, PR 9752695 18° 5.6' N 65° 28.3' W 8/15/2005 - Present 0.68

Table 2: Charlotte Amalie Tide Gage Elevations on Station Datum

Datum Value (Feet)

Description

MHHW 6.04 MEAN HIGHER HIGH WATER

MHW 6.00 MEAN HIGH WATER

MSL 5.63 MEAN SEA LEVEL

MLW 5.30 MEAN LOW WATER

MLLW 5.25 MEAN LOWER LOW WATER

1.3.3 Sea Level Rise

Sea level rise has been calculated and reported by NOAA (2011) for many tide gages in the United States and its territories including Puerto Rico and the U.S. Virgin Islands.

These estimates were calculated in 2006 and include data from the available tide history of the station. The information includes the expected value of sea level rise, the 95% confidence interval and period of record and results are provided below for the four stations closest to the project site.

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Figure 2: Existing Bathymetry, St. Thomas and Surrounding Areas

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Figure 3: NOAA Navigation Chart (No. 25649) for St. Thomas Harbor

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1. Charlotte Amalie, USVI, 1.20 ± 0.96 mm/yr, 32 years

2. Lime Tree Bay, USVI, 1.74 ± 1.20 mm/yr, 30 years

3. San Jaun, Puerto Rico (PR), 1.65 ± 0.52 mm/yr, 55 years

4. Magueyes Island, PR, 1.35 ± 0.37 mm/yr, 62 years

This data is used in a sea level rise calculations discussed below in Section 2.

1.3.4 Hurricane Data

The North Atlantic hurricane database, HURDAT (Jarvinen et al., 1988) was used in this study to identify historical hurricanes impacting the project site. The database consists of tropical cyclone (i.e. tropical storms and hurricanes) positions and intensities every six hours for storms from 1851 to the present.

1.3.5 Waves

The Wave Information Studies (WIS) performed by the USACE Waterways Experiment Station

(Hubertz, 1992) provide a database of hindcast, nearshore wave conditions covering U.S.

coasts. The hindcast wave data is available for the Atlantic and Gulf coasts from 1980 through

1999. Available data includes hourly wave parameters such as significant wave height, peak period, mean period, mean wave direction, and wind speed and direction. The closest station for which wave data is available from the WIS database is approximately 65 miles southeast of

Charlotte Amalie. The WIS parameter information served as an independent reference for comparison against the methodology used in this study to estimate wind waves generated during hurricanes.

1.3.6 Tsunamis

The project site is exposed to tsunami hazard. The history of tsunamis affecting Charlotte

Amalie is documented in several sources including Lander and Lockridge (1989), Lander et al.

(2002), O’Loughlin and Lander (2004). The largest tsunami to impact the project site occurred on November 18, 1867 with a 2.4 meter runup measured at the wharf (Lander and Lockridge, 1989) and maximum runup ranging from 4.5 m to 6.0 m at other locations within Charlotte

Amalie (Lander et al., 2002). In April of 1690, a large earthquake occurred and was followed at

Charlotte Amalie by a large withdrawl of water approximately 16 to 18 m toward the sea, such that fish were left stranded on the shore. However, there is no mention, in the limited historical record consisting of a letter from 1690, of a subsequent devastating tsunami crest as arrived during the 1867 event. Lander and Lockridge (1989) report two other tsunamis in 1868 and

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1918 with a wave runup of 0.6 m and 0.5 m in Charlotte Amalie, respectively. Several additional tsunamis are reported for Charlotte Amalie but no runup information is available.

Implications of a tsunami, similar to the one that occurred in 1867, in modern day Charlotte

Amalie are presented by Watlington (2004). Issues such as the vast increase in population along the waterfront as well as a thriving tourist industry and industrial related infrastructure lead to much higher risk with respect to loss of life and economic impact relative to conditions in

1867. A Territorial Hazard Mitigation Plan, including tsunami and hurricane related risk, was recently published by the Virgin Islands Territorial Emergency Management Agency (VITEMA, 2011). This plan briefly includes some of the tsunami history outlined above and recommendations for risk mitigation.

Tsunami runup probability for the Caribbean region was analyzed by Parsons and Geist (2009) along shorelines discretized into 20 km squares. Results are limited to the probability of a 0.5 m runup over the next 30 years. For Charlotte Amalie there is an 18% chance in the next 30 years that tsunami related runup will exceed 0.5 m.

2. MODELING OF WATER LEVELS, WAVES AND CURRENT

The following section provides details on the methodology used to model water surface elevations, waves and current along the project site in St. Thomas Harbor.

2.1 Water Levels

Storm surge is defined as water that is pushed toward the shore by the force of storm winds in addition to the mounding of water due to reduced atmospheric pressure. This advancing surge combines with the normal tides to create the storm tide, which elevates the mean water level above normal tide levels. The rise in water level can cause severe flooding in coastal areas, particularly when the storm surge coincides with the normal high tides. The width and slope of the continental shelf can affect the level of storm surge at a particular location. The relatively narrow shelf offshore of St. Thomas that drops rapidly into deep ocean trenches limits the development of storm surge in the vicinity of Charlotte Amalie.

2.1.1 Hydrodynamic Model Description

Water-surface elevations and currents for both tides and storm events were calculated for this project using the large-domain long wave hydrodynamic model ADCIRC (Advanced Circulation

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model; Luettich et al., 1992). ADCIRC is a Finite Element Model (FEM) that includes tidal and atmospheric forcing in the computations.

A finite element computational grid for the study area was developed using bathymetry sources described in Section 1.3.1. The computational domain for the grid is shown in Figure 4. SMS software (Jones and Richards, 1992) was used to develop the grid and input tidal constituents for the boundary conditions. Figure 5 shows the detailed grid in the vicinity of St. Thomas

Harbor. The entire grid consists of a total of 29,548 nodes and 56,184 elements. The minimum grid size is 66 feet along the shoreline inside St. Thomas Harbor.

Figure 4: ADCIRC Computational Domain and Bathymetry

The ADCIRC model results were verified with measured water surface elevations at the

Charlotte Amalie and Lime Tree Bay Tide gages including normal tides and storm events.

Time-series comparisons are presented in the Sections 2.1.2 and 2.1.3.

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2.1.2 Tide Modeling

Tides were simulated within ADCIRC by specifying a surface elevation time series at the open ocean boundary in the Caribbean Sea and the Atlantic Ocean around the perimeter of the model grid shown in Figure 4. Tidal water surface elevations from the ADCIRC model were calculated for the four tide gage locations shown in Figure 2 for verification purposes.

Tidal verification of the model was performed to ensure that the grid resolution, bathymetry and boundary conditions were acceptable to properly simulate conditions in the defined domain.

Verification of ADCIRC tidal simulations was accomplished using the NOS published tidal records for the four gage locations shown in Figure 2 and described in Table 1.

Figure 5: ADCIRC Computational Domain and Bathymetry

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A period with relatively calm conditions was selected to compare the real-time data and

ADCIRC simulated time-series. Figures 6 and 7 show the comparisons for tidal elevation time series for the month of September 2006 from two of the tide gages, Charlotte Amalie and Lime

Tree Bay. Prior to plotting the results and calculating the errors the mean value was removed from each data set to remove the offset in the mean water level due to seasonal variations. The root mean square errors between the ADCIRC model results and the NOS predicted tides are

3.5 cm and 4.6 cm at Charlotte Amalie and Lime Tree Bay, respectively. At Charlotte Amalie the root mean square error normalized by the spring tide range is 10%. A linear regression-analysis between the NOS predicted tides and the ADCIRC model results is shown in Figure 8 resulting in a correlation coefficient of 0.83.

The comparison with NOS predicted water level is better as it does not include local effects such as wind and seasonal fluctuations in water levels. The comparisons are judged to be acceptable based on the agreement to within 10%.

Figure 6: Charlotte Amalie Tide Gage Water Level Comparison

Charlotte Amalie Tide Gage Station

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Date

T id es m

NOS Predicted

NOS Observed

ADCIRC

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Figure 7: Lime Tree Tide Gage Water Level Comparison

Figure 8: Regression Analysis for Observed and Modeled Water Level at Charlotte Amalie

Lime Tree Bay

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Date

T id es m

NOS Predicted

NOS Observed

ADCIRC

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2.1.3 Storm Surge Modeling

The Planetary Boundary Layer (PBL) model (Cardone, 1992) was coupled with the ADCIRC model to simulate storm wind effects at St. Thomas Harbor. Storm specific data were obtained from the HURDAT database for use in the PBL model.

The PBL model simulates hurricane-generated wind and atmospheric pressure fields by solving vertically averaged equations of horizontal motion. The PBL model results were used to specify the wind fields for use in the ADCIRC model. The storm tracks were specified as one hour snapshots of storm location which included the radius of maximum wind (approximated using

Jelesnianski and Taylor, 1973). The PBL model solution was superimposed onto the ADCIRC grid and used to specify the wind field at each node in the grid at one-hour intervals.

Table 3 shows historical storm events identified to be passing within a 100-mile radius from the midpoint of St. Thomas Harbor. These storm events were simulated using the approach explained above with the PBL and ADCIRC models.

Figures 9 and 10 show an example of a storm track for Hurricane Hugo (9/10/1989). In Figure 9, the asterisk (*) relates the storm position at 6-hour intervals. Figure 10 provides another view of

Hurricane Hugo and its proximity to St. Thomas Island with wind speed in knots at each 6-hour interval. Storm surge experienced at the shoreline due to passing of the hurricane is greater in the front right of the hurricane eye (Harris, 1963).

Among the 58 storm events (both tropical storms and hurricanes), 21 events were identified that produced observable storm surge inside St. Thomas Harbor. The storm tracks associated with these 58 events were also analyzed to establish whether the storm winds would result in waves inside St. Thomas Harbor. Table 4 below shows the selected 21 storm events that resulted in storm surge greater than 0.3 feet. ADCIRC results for these storms were used in the EST model to calculate the extreme water levels for various recurrence intervals.

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Table 3: Historical Storms Modeled with ADCIRC

HURDAT

Storm

Number (2006)

Date Storm Name

HURDAT

Storm

Number (2006)

Date Storm Name

8 09/05/1852 Un-Named 582 9/25/1932 Un-Named

101 09/24/1866 Un-Named 589 7/14/1933 Un-Named 111 10/27/1867 Un-Named 650 8/8/1938 Un-Named 140 08/17/1871 Un-Named 716 9/12/1945 Un-Named 169 09/12/1876 Un-Named 744 8/23/1949 Un-Named 192 11/25/1878 Un-Named 825 8/9/1956 BETSY 194 08/13/1879 Un-Named 864 8/29/1960 DONNA 216 08/21/1881 Un-Named 915 8/21/1966 FAITH 284 09/01/1889 Un-Named 918 9/21/1966 INEZ 302 10/12/1891 Un-Named 969 8/20/1971 DORIA 315 08/13/1893 Un-Named 988 8/25/1973 CHRISTINE 330 10/11/1894 Un-Named 1043 7/15/1979 CLAUDETTE 339 08/30/1896 Un-Named 1046 8/29/1979 FREDERIC 354 09/05/1898 Un-Named 1067 9/7/1981 GERT 361 10/27/1898 Un-Named 1094 11/5/1984 KLAUS 364 08/03/1899 Un-Named 1122 8/21/1988 CHRIS 366 09/03/1899 Un-Named 1135 7/31/1989 DEAN 371 8/27/1900 Un-Named 1139 9/10/1989 HUGO 384 9/9/1901 Un-Named 1195 8/22/1995 IRIS 418 8/25/1906 Un-Named 1198 8/27/1995 LUIS 454 9/5/1910 Un-Named 1199 9/12/1995 MARILYN 483 7/10/1916 Un-Named 1207 7/5/1996 BERTHA 486 8/21/1916 Un-Named 1233 9/15/1998 GEORGES 493 10/6/1916 Un-Named 1250 10/17/1999 JOSE 529 8/16/1924 Un-Named 1252 11/13/1999 LENNY 559 9/6/1928 Un-Named 1256 8/19/2000 DEBBY 566 8/31/1930 Un-Named 1271 8/22/2001 DEAN 570 8/16/1931 Un-Named 1320 9/13/2004 JEANNE 572 9/8/1931 Un-Named 1357 8/1/2006 CHRIS

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Figure 9: Storm Track for Hurricane Hugo in the Caribbean

Figure 10: Storm Track for Hurricane Hugo near St. Thomas

A spatial distribution snapshot of the calculated wind field for Hurricane Hugo at the time of the peak surge at the project site is shown in Figure 11.

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Table 4: Historical Storm Events Used in EST Analysis

Storm Number Hurricane Date

101 Un-named 09/24/1866 111 Un-named 10/27/1867 302 Un-named 10/12/1891 330 Un-named 10/11/1894 384 Un-named 9/9/1901 418 Un-named 8/25/1906 483 Un-named 7/10/1916 486 Un-named 8/21/1916 493 Un-named 10/6/1916 559 Un-named 9/6/1928 572 Un-named 9/8/1931 582 Un-named 9/25/1932 744 Un-named 8/23/1949

915 FAITH 8/21/1966

969 DORIA 8/20/1971

1094 KLAUS 11/5/1984

1139 HUGO 9/10/1989

1199 MARILYN 9/12/1995

1207 BERTHA 7/5/1996

1233 GEORGES 9/15/1998

1250 JOSE 10/17/1999

Figures 12 through 16 compare the modeled storm surge with observed and predicted water surface levels at NOAA-operated Charlotte Amalie and Lime Tree Bay Tide Gages, relative to

MSL. Comparisons are provided for three storms events consisting of Hurricanes Hugo, Marilyn and Bertha. The water level data for Hugo for Lime Tree Bay is not presented because the data obtained from tide gage records is incomplete during this hurricane event.

As evident from the figures, the modeled storm surge results in a reasonable agreement with measured results. Although the model predicts the phase of the water level peak well, it under-predicts the peak water level value. The difference in peak water level is due in part to wave setup which is not included in the ADCIRC model. Wave setup is an increase in water level near the shore due to the influence of the wave shoaling process. Water level observations at a site in a tropical storm event are affected by a combination of local tides, storm surge and wave setup. The wave setup calculation is discussed below in section 2.2.4.

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Figure 11: Hurricane Hugo Wind Field

Figure 12: Water Level Comparison for Hurricane Hugo (9/10/1989) at Charlotte Amalie

Tide Gage

Hurricane Hugo Charlotte Amalie

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Date & Time

W a te r

S u rf a c e m

NOS Predicted Water Surface (Tides) NOS Observed Water Surface ADCIRC Storm Surge & Tides

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Figure 13: Water Level Comparison for Hurricane Marilyn (9/12/1995) at Charlotte Amalie

Figure 14: Water Level Comparison for Hurricane Marilyn (9/12/1995) at Lime Tree Bay

Hurricane Marilyn Charlotte Amalie

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Date & Time

W at er S u rf ac e

(m

NOS Predicted Water Surface (Tides) NOS Observed water Surface ADCIRC Storm Surge & Tides

Hurricane Marilyn Lime Tree Bay

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Date & Time

W at er S u rf ac e

(m

NOS Predicted Water Level (Tides) NOS Observed Water Level ADCIRC Storm Surge & Tides

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Figure 15: Water Level Comparison for Hurricane Bertha (7/5/1996) at Charlotte Amalie

Figure 16: Water Level Comparison for Hurricane Bertha (7/5/1996) at Lime Tree Bay Tide

Gage

Hurricane Bertha Charlotte Amalie

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7/7/96 12:00 7/8/96 0:00 7/8/96 12:00 7/9/96 0:00 7/9/96 12:00 7/10/96 0:00 7/10/96 12:00

Date & Time

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NOS Predicted Water Level (Tides) NOS Observed water Level ADCIRC Storm Surge & Tides

Hurricane Bertha Lime Tree Bay

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7/7/96 12:00 7/8/96 0:00 7/8/96 12:00 7/9/96 0:00 7/9/96 12:00 7/10/96 0:00 7/10/96 12:00

Date & Time

W a te r

S u rf a c e m

NOS Predicted Water Level (Tides) NOS Observed Water Level ADCIRC Storm Surge & Tides

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2.1.4 Future Sea Level Rise

Future sea level rise was calculated for the project using methods outlined in an Engineering

Circular published by the USACE (2009). The projection of future sea level rise includes use of historic sea level rise trends from local gages and estimates of global sea level rise trends based on global climate change estimates associated with various emission scenarios. The

USACE recommends use of local records of at least 40 years in order to limit the uncertainty in estimating historic sea level trends. Four stations were reported in Section 1.3.3 including two stations in Puerto Rico with records of 55 and 62 years. In the engineering circular the global sea level trend is given as 1.7 ± 0.5 mm/yr for the expected value and 95% confidence interval.

For both the long term stations as well as Charlotte Amalie the expected trend is less than or equal to the global trend of 1.7 mm/yr. In addition, the upper limit of the 95% confidence interval for all three of these same stations is less than or nearly equal to the global value of 2.2 mm/yr.

Thus, the global value is used for this study based on the trends in local gages.

Sea level rise estimates were calculated for an assumed project life of 75 years assuming construction is completed in 2013. The low, medium and high sea level rise estimates calculated based on USACE (2009) methodology are 1.2, 2.4 and 3.6 feet over an assumed 75 year project life. The low value lies within the range of predictions presented in the IPCC (2007) fourth assessment report. However, the range of values presented in the fourth assessment report did not include possible melting of the Antarctic ice sheet. The USACE (2009) circular recommends consideration of the medium and high estimates during planning and design to account for the possibility of more rapid sea level rise based on uncertainties such as loss of a portion of the Antarctic ice sheet.

The effect of the low, medium and high sea level rise scenarios was tested within the ADCIRC model runs for Hurricanes Hugo, Georges and Jose. The resulting increase in maximum water levels from the ADCIRC model runs was within 2% of simply adding the sea level rise to the

ADIRC model run based on current sea levels. Thus, for this location design water levels are approximated by adding the future sea level rise to the hurricane surge results calculated using

ADCIRC and existing water levels. This approximation is conservative since the ADCIRC runs with sea level rise were either equal to or up to 2% less than taking the ADCIRC run for current sea levels and adding the sea level rise to it.

2.2 Waves

Wave conditions inside St. Thomas Harbor were predicted using the tropical storm generated waves at an offshore position and numerically modeling the transformation of those waves as they propagated to the project site in the harbor. The following sections describe the

PARSONS BRINCKERHOFF 21 FINAL USVI DEPARTMENT OF PUBLIC WORKS

methodology used to obtain offshore waves associated with tropical storms and the modeling of nearshore wave transformations.

2.2.1 Offshore Waves

The offshore wave conditions were obtained using the methodology provided in the Shore

Protection Manual (SPM, USACE, 1984) and the wave height nomograph in CEM (USACE, 2002), shown in Figure 17.

The offshore wave height and wave period were calculated using the SPM methodology when the hurricane was in the vicinity of the project site. The ADCIRC model results for the 21 historical events indicated the incoming wave direction based on the hurricane track and wind direction relative to St. Thomas Harbor. Input waves are assumed to have the same direction as the wind. For these historical events, an adjustment factor was applied to the wave heights using Figure 17 to account for the relative distance of the storm eye to the project site and the direction traveled by the storm. In Figure 17, ‘r’ represents distance of the site from the storm eye and ‘Rmax’ represents radius of maximum wind for the hurricane.

The offshore wave height and wave period were calculated using the SPM methodology when the hurricane was in the vicinity of the project site. The ADCIRC model results for the 21 historical events indicated the incoming wave direction based on the hurricane track and wind direction relative to St. Thomas Harbor. Input waves are assumed to have the same direction as the wind. For these historical events, an adjustment factor was applied to the wave heights using Figure 17 to account for the relative distance of the storm eye to the project site and the direction traveled by the storm. In Figure 17, ‘r’ represents distance of the site from the storm eye and ‘Rmax’ represents radius of maximum wind for the hurricane.

The calculated wave heights, wave periods, wave directions and wind speed associated with the hurricane were used as input at the boundary of the nearshore wave transformation model

SWAN. A description of Simulating WAves Nearshore (SWAN, Ris, 1997) is provided in Section

2.2.3.

Table 5 shows the input parameters used for the SWAN wave model run of each hurricane. In

Table 5, column 9 represents the distance of the storm eye (x,y) in nautical miles. This represents ‘r’ as in Figure 17 with ‘x’ being the distance along the storm movement direction and

‘y’ being the distance perpendicular to the storm movement direction. The wave heights and periods were also calculated for WIS Station 61012 (64.00° W, 18.00° N) using the above methodology for Storms Klaus, Hugo, Marilyn, Bertha, Georges and Jose. The calculated results are compared against the WIS hindcast data in Table 6. Differences between the

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calculated and WIS hindcast results for wave heights and wave periods are less than 13% and

20%, respectively.

Figure 17: Values of Hmo/Hmo max Plotted Relative to Center of Hurricane (USACE, 2002)

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Table 5: Offshore Wave Parameters for Historical Storm Events

Storm Number

Run No. Hurricane Date Pressure

(mb)

Max.

Wind Speed (knots)

SPM Results CEM Results

Hs (ft) Tp

(sec)

Distance from Site (x,y nm)*

Wave propagation direction**

Reduction factor in

Hs

1-Hour Wind Speed at Site

(knots)

Adjusted Hs (feet)

101 1 Un-named 09/24/1866 956 100 41.1 13.7 (36,77) 45 0.68 46.0 28.0

111 2 Un-named 10/27/1867 952 100 45.9 14.5 (24,5) 353 0.70 73.0 32.1

302 3 Un-named 10/12/1891 993 85 24.4 10.5 (46,10) 57 0.92 54.4 22.4

330 4 Un-named 10/11/1894 963 85 34.5 12.5 (57,32) 67 0.91 45.3 31.4

384 5 Un-named 9/9/1901 993 51 28.7 11.5 (21,0) 4 0.50 16.6 14.4

418 6 Un-named 8/25/1906 975 110 30.4 11.7 (46,49) 45 0.80 63.1 24.3

483 7 Un-named 7/10/1916 993 60 26.4 11.0 (27,4) 0 0.85 34.9 22.4

486 8 Un-named 8/21/1916 988 90 29.6 11.6 (10,6) 295 0.58 57.6 17.1

493 9 Un-named 10/6/1916 970 90 30.3 11.7 (51,0) 57 0.95 50.9 28.8

559 10 Un-named 9/6/1928 932 140 53.0 15.5 (60,46) 308 0.88 83.8 46.6

572 11 Un-named 9/8/1931 988 85 27.6 11.2 (34,3) 340 0.92 44.6 25.4

582 12 Un-named 9/25/1932 954 105 44.2 14.2 (45,8) 338 0.92 60.3 40.7

744 13 Un-named 8/23/1949 990 60 30.1 11.8 (45,41) 50 0.80 29.0 24.1

915 14 FAITH 8/21/1966 988 90 31.6 12.1 (54,68 ) 52 0.80 35.4 25.2

969 15 DORIA 8/20/1971 993 30 29.9 11.8 (59,9) 0 1.00 12.6 29.9

1094 16 KLAUS 11/5/1984 993 75 25.9 10.9 (17,27) 0 0.70 46.7 18.2

1139 17 HUGO 9/10/1989 943 130 45.1 14.3 (9,28) 315 0.86 82.0 38.8

1199 18 MARILYN 9/12/1995 955 100 30.0 11.6 (38,0) 12 0.95 61.5 28.5

1207 19 BERTHA 7/5/1996 979 75 33.5 12.4 (40,0) 44 0.92 42.6 30.9

1233 20 GEORGES 9/15/1998 970 90 32.4 12.2 (35,28) 321 0.90 42.6 29.2

1250 21 JOSE 10/17/1999 993 80 25.9 10.9 (39,9) 43 0.91 33.4 23.6

*x,y are measured from the eye of the storm to the project site and aligned with storm movement ** direction of wave propagation in compass degrees (i.e. direction is zero degrees and 90 degrees for waves travelling north and east, respectively)

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Table 6: Offshore Wave Parameters for Historical Storm Events

Storm Number Hurricane Date

Calculated WIS Station 61012

Hs (m) T (sec) Hs (m) T (sec)

1094 KLAUS 11/5/1984 4.7 10.9 4.8 9.1

1139 HUGO 9/10/1989 11.0 14.3 11.9 14.3

1199 MARILYN 9/12/1995 9.1 11.6 10.4 12.5

1207 BERTHA 7/5/1996 7.7 12.4 7.9 12.5

1233 GEORGES 9/15/1998 7.9 12.2 7.9 11.1

1250 JOSE 10/17/1999 6.3 10.9 6.1 11.1

2.2.2 Nearshore Wave Transformation Modeling

The SWAN wave transformation model was used to calculate wave conditions in St. Thomas

Harbor based on the input conditions presented in section 2.2.1.

The SWAN wave transformation model is a finite-difference, phase-averaged, spectral wave model developed at Delft University of Technology (Ris, 1997). This model accounts for many of the physical processes that modify the wave field within coastal and inland waters including refraction, shoaling, friction, wave breaking, energy inputs by wind, and energy transfers between waves. SWAN also includes an approximation to account for wave diffraction. The

SWAN model assumes wave properties vary slowly over a wavelength.

The SWAN model describes the evolution of wave spectra by starting at the offshore model boundary and propagating over geographic space with variations in water depth. Energy input in

SWAN results from wind and forcing specified at the boundaries of the computational domain.

Three mechanisms for dissipation in SWAN include white capping, bottom friction, and depth-induced breaking. Energy is redistributed over the wave spectrum by nonlinear wave-wave interactions.

SWAN utilizes a regularly spaced rectangular grid or a curvilinear grid for computations. For this study, a nested finite difference grid was used consisting of a 20 m fine grid with within a 50 m coarse grid to improve computational efficiency. The model inputs include bathymetry, water levels, friction factors, wind velocities and offshore waves.

Input wave conditions for the coarse grid SWAN model (Figure 18) were obtained from the methodology described in Section 2.2.1 and correspond to data contained in Table 5. The

JONSWAP spectrum was assumed for all the SWAN model simulations. The coarse grid model results were used to specify the boundary conditions for the fine grid model. An example simulation result for Hurricane Hugo is shown in Figure 19.

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The wave conditions corresponding to historical hurricanes shown in Table 6 were simulated using SWAN to obtain output results for both of the models throughout the domain as well as user defined output points along the extent of the project site.

Figure 18: Wave Model Grids

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2.2.3 Wave Conditions along the Project Site

Wave conditions inside the harbor along the project shoreline in Figure 19 exhibit the protection provided by the relatively narrow entrance of the harbor. The wave height near the center of the bay is about 8.2 feet (2.5 m) and decreases to the west and east along the north shore of the harbor. Wave heights range from about 6.5 feet (2.0 m) to 4.9 feet (1.5 m) along the project site.

Depending on the location, these wave heights may be significantly reduced due to depth limited conditions.

Figure 19: Fine Grid Wave Model Results, Hs (m),for Hurricane Hugo

Wave setup values obtained using the SWAN model were added to the modeled storm surge values in order to estimate water levels for the 21 historical storm events. Table 7 provides a tabulation of storm surge including tide, associated wave setup, and the resulting water levels for the storm events. The values are provided at the Charlotte Amalie Tide Gage location as well as a location near the center of the harbor shoreline south of the Coast Guard Station and

Kings Wharf (shown in Figure 3). The modeled water levels compare reasonably well with the observed water levels for the storm events although the observed water levels for Hurricane

Marilyn were 0.8 feet higher than the modeled water levels.

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Table 7: Observed and Modeled Water Levels for Historical Storm Events

Storm Number

Hurricane Date

South of Coast Guard Station Charlotte Amalie Tide Gage

Modeled Surge (feet)

Wave Setup (feet)

Modeled Water

Level (feet)

Modeled Surge (feet)

Wave Setup (feet)

Modeled Water Level

(feet)

Observed Water Level

(feet) 101 Un-named 09/24/1866 0.59 0.81 1.40 0.57 0.81 1.39 NA 111 Un-named 10/27/1867 1.94 0.90 2.85 1.95 0.96 2.91 NA 302 Un-named 10/12/1891 1.24 0.39 1.63 1.32 0.41 1.73 NA 330 Un-named 10/11/1894 0.49 0.16 0.66 0.62 0.16 0.79 NA 384 Un-named 9/9/1901 0.72 0.21 0.93 0.72 0.24 0.96 NA 418 Un-named 8/25/1906 1.05 0.72 1.77 1.21 0.75 1.96 NA 483 Un-named 7/10/1916 0.74 0.49 1.23 0.72 0.53 1.25 NA 486 Un-named 8/21/1916 0.89 0.15 1.04 0.85 0.18 1.03 NA 493 Un-named 10/6/1916 1.21 0.70 1.92 1.25 0.73 1.98 NA 559 Un-named 9/6/1928 0.79 0.04 0.82 0.72 0.02 0.74 NA 572 Un-named 9/8/1931 1.18 0.51 1.69 1.15 0.56 1.71 NA 582 Un-named 9/25/1932 1.74 0.55 2.29 1.67 0.53 2.21 NA 744 Un-named 8/23/1949 0.67 0.72 1.39 0.72 0.74 1.47 NA

915 FAITH 8/21/1966 0.76 0.56 1.33 0.79 0.58 1.37 NA

969 DORIA 8/20/1971 0.39 1.02 1.42 0.39 1.09 1.48 NA

1094 KLAUS 11/5/1984 0.84 0.41 1.25 0.84 0.45 1.29 NA

1139 HUGO 9/10/1989 2.03 0.27 2.30 1.87 0.23 2.10 2.36

1199 MARILYN 9/12/1995 1.97 0.87 2.84 1.90 0.92 2.83 3.61

1207 BERTHA 7/5/1996 1.02 0.71 1.72 1.02 0.71 1.73 1.73

1233 GEORGES 9/15/1998 0.76 0.45 1.22 0.65 0.51 1.16 1.24

1250 JOSE 10/17/1999 0.56 0.55 1.11 0.55 0.58 1.13 0.79

NA=Not Available

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2.3 Currents

2.3.1 Tidal Currents

Currents associated with tides were analyzed using the ADCIRC modeling runs performed in section 2.1.3. Currents were analyzed at 17 stations described in detail in section 3. The coordinates for these stations are listed in Table 8 and the stations are shown in Figure 20. The modeled tidal currents inside St. Thomas Harbor along the project site are very small, less than a hundredth of a knot, and range up to about a twentieth of a knot at Haulover Cut. Currents in the harbor are likely larger than these due to effects such as ocean circulation and winds which were not included in the ADCIRC analysis for tides.

Table 8: EST Station Coordinates and Depths

EST Station Easting

(feet) Northing

(feet)

Elevation of mudline

(feet, MSL) 1 1,174,184 840,499 9.5 2 1,174,284 841,430 17.6 3 1,175,123 841,869 16.9 4 1,175,888 842,011 13.8 5 1,176,811 842,307 14.1 6 1,177,166 842,262 10.4 7 1,177,255 842,037 11.2 8 1,177,359 841,960 11.7 9 1,177,546 841,914 7.8 10 1,177,729 841,980 11.2 11 1,177,871 842,111 8.1 12 1,177,999 842,254 5.0 13 1,178,546 842,153 6.3 14 1,178,799 841,630 9.3 15 1,179,079 841,385 14.0 16 1,179,469 841,380 12.4 17 1,180,026 841,239 13.7

2.3.2 Hurricane Currents

Currents were analyzed using ADCIRC for the 21 historical storm events. Water currents are shown using vectors in Figure 21 for Hurricane Hugo. The current shown in Figure 21 is a snapshot in time when the maximum velocities occur near the project site. The results indicate the largest velocities are oriented to the west in the vicinity of the Legislature building whereas the largest velocities along the project site occur in the eastern half at Stations 14 through 16.

For hurricane Hugo the maximum currents from the ADCIRC modeling results, within the project

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site, ranged from about 0.4 knots at Station 13 to 0.7 knots at Station 15. The analysis of currents for extreme events follows in Section 3.

Figure 20: EST Station locations inside St. Thomas Harbor

The effect of the low, medium and high sea level rise scenarios on the hurricane induced current was tested with the ADCIRC model runs for Hurricanes Hugo, Georges and Jose. The results show a decrease in maximum current as the sea level rise increases. Since the sea level rise has little effect on the hurricane surge the decrease in current with increasing sea level rise is expected since the depths are larger and thus the hurricane induced discharge, which is relatively constant with sea level rise, yield smaller currents due to the increased water depth.

The decrease in velocity ranges up to a maximum of 26% for Jose and is less for the larger events including Georges and Hugo. Not only do the currents decrease slightly to moderately with increasing sea level rise, they are also relatively small along the project site, as shown in

Figure 21, due to its location in the wide part of the protected harbor. Therefore, the maximum currents for the various sea level rise scenarios are conservatively approximated as the maximum current with zero sea level rise.

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Figure 21: Currents for Hurricane Hugo Inside St. Thomas Harbor

3. RECURRENCE INTERVAL ANALYSIS

The Empirical Simulation Technique (EST) was used to perform recurrence interval analyses for tropical storms affecting St. Thomas. EST is a procedure for simulating storm events and their corresponding environmental impacts or any cyclic or frequency-related phenomena (Borgman et al., 1992 and Scheffner et al., 1999).

In order to estimate the frequency of events for this project, all of the tropical storms passing through a 200 nautical-mile radius of St. Thomas Harbor were selected from the HURDAT database. This approach was chosen to account for uncertainties associated with tropical storm development and their tracks. The analysis included 213 tropical storm events representing a time period of 156 years (1851-2006).

Seventeen stations shown in Figure 20 were selected for average recurrence interval analysis using EST. The coordinates (US State Plane NAD-83, Puerto Rico/Virgin islands datum; feet)

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for these 17 stations are provided in Table 8. The following information was used for each storm in EST:

1. Minimum distance from the site to the hurricane eye.

2. Pressure deficit

3. Maximum wind speed

4. Forward speed of storm system

5. Radius of maximum wind

6. Response (water level, wave height, wave period or current)

For each set of responses 100 simulations were conducted for 500 years. The EST model calculates statistics such as the mean and standard deviation of the response based on the number of simulations. A frequency of 1.8 storms per year was used for calculations of water level and current. This frequency is consistent with that used in the studies conducted for the currently effective FEMA (2007) Flood Insurance Study for the U.S. Virgin Islands. Note this is a slightly higher frequency than what would have been based on 213 storms over 156 years but was used for slightly more conservative results (i.e. higher water levels) that are consistent with the FEMA Flood Insurance Study.

For wave heights and wave periods the frequency used in EST is consistent with that based on the response threshold associated with the 21 storms used. Thus, 21 storms over 156 years results in a frequency of 0.135 storms per year. This approach is consistent with recent studies conducted in the South Pacific for typhoon induced wave heights, Thompson and Scheffner

(2002, 2004). This method was used in this study since use of the FEMA frequency of 1.8 events per year yields large values of wave height such that the 50 year event far exceeds the maximum wave height over the entire 156 year record. A frequency of 0.135 storms per year for the wave height results in a 50 year wave height that is consistent with the lowest of the three highest events that occurred over the previous 156 years.

Average recurrence interval values were calculated with EST for water levels, wave heights, wave period and currents for each of the 17 stations Water levels in this section are referenced to MSL. EST stations 12 through 16 are located along Phase I of the project. An example of the

EST output is illustrated in Figures 22 through 26 which are plots showing the water level, wave height, wave period and current for Station 15. The figures include the mean as well as ±1 standard deviation.

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Final modeling results and associated EST analyses were calculated along the project site from

Stations 12 through 16. Table 9 shows the 25, 50 and 100 year EST results for Stations 12 through 16 based on current sea levels. Water levels associated with a 50-year storm event (2% probability of occurrence) are predicted to range between 4.6 feet to 5.0 feet along the project site. Similarly, wave heights, wave periods and storm currents associated with a 50-year storm event are predicted to range from 4.5 feet to 6.9 feet, 12.5 seconds to 12.7 seconds, and 0.7 knots to 1 knot, respectively. As expected, the wave heights and currents are largest near the headland located at Station 15.

The location reported in the FEMA (2007) Flood Insurance Study is closest to EST station 4 shown in Figure 20. The reported water levels in the FEMA FIS were about 8% and 12% higher than the 50 year and 100 year EST results calculated for this study, respectively. These differences are likely due in part to the different choice of storms in the FEMA study as well as the larger depths at the EST station used in this study relative to the nearshore locations used to report flood elevations for the FEMA study. The choice of storms in the FEMA study were motivated by inclusion of the three islands, St. Thomas, St. Croix and St. John whereas

Charlotte Amalie is the focus of this study.

The effect of future sea level rise on water level and wave height is shown in Tables 10 through

12 for the low, medium and high scenarios, respectively. The effect of sea level rise on storm surge and wave setup is minimal. Thus, for all sea level rise scenarios the results are within 0.1 feet of simply adding the sea level rise to the storm surge results calculated without any sea level rise. In contrast, wave heights increase moderately with sea level rise. For the high sea level rise scenario the 50 year significant wave height increases by 1.4 feet (29%) at Station 13 and 1.1 feet (16%) at Station 15.

The peak wave period increases slightly with increasing sea level rise by about 1 to 1.5 seconds between no sea level rise and the high scenario. However, this change is about the same as the uncertainty associated with the spectral frequency and wave period resolution of 10% between adjacent values used in the SWAN model. The wave period should be considered approximate and an allowance of ± 2 seconds be considered with respect to the sensitivity of calculations such as rock stability or wave loads.

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Table 9: Average Recurrence Interval Values for Water Levels, Wave Heights and Storm Currents without Sea Level Rise

Water Level (feet, MSL) Wave Height Hs…