Coastal Hazard Bulletin

Issue No. 16

The purpose of the Coastal Hazard Mailing List is to provide up-to-date coastal flood hazard information pertinent to the Federal Emergency Management Agency and the National Flood Insurance Program. Subscribers will receive new mailings on a monthly basis.

CHAMP Application - Open-Ocean Wave Height-Dominated Case

The next two Coastal Hazard Listserv bulletins will address the CHAMP modeling and coastal engineering data preparation techniques typically associated with determining coastal flood hazards for a non-restricted fetch (open ocean) wave height-dominated case. A general description of data requirements for coastal flood hazard analyses was provided in the July 2002 Listserv, and should be referred to prior to starting during this exercise.

In this month's bulletin we will discuss the methodologies for determining several coastal parameters for this non-restricted fetch (open ocean) wave height dominated case, including:

Geomorphic Setting
Topographic Data Source
Stillwater Elevation (SWEL) Determination
Initial Wave Height and Period Determination
Wave Setup Determination
Erosion Treatment

This information will then be applied to the first three Programs in CHAMP, including:

Program 1 - Transect Description/Parameter Entry
Program 2 - Transect Data Entry and Editing
Program 3 - Erosion Treatment

Please download the zipped file named by clicking here. It includes a champ database which has been developed through Program 3 - Erosion Treatment. Extract the folder named 'openoceancase' and the file named 'openoceancase.mdb' from 'ch_aug02.zip', and save both to your local directory (C:\Program Files\Champ). The openoceancase.mdb database can be opened by clicking on the 'Open' button on the CHAMP main menu button bar. You may wish to develop the remainder of the CHAMP database (Programs 4-6) to prepare for the September 2002 Listserv.

Geomorphic Setting
Franklin County, Florida, is located in the 'Panhandle' of Florida in the northwest part of the State. The coastal areas of the county are characterized predominantly by undeveloped barrier islands fronting a low-lying mainland. The barrier islands are characterized by a nearly continuous dune ridge with crest elevations ranging from approximately 10 to 20 ft. Most of the barrier island, such as the example provided below, is inundated by flood waters during a 1% annual chance (100-year) storm event.

Topographic Data Source
Topography used for wave height modeling was obtained from U.S. Geological Survey (USGS) 7.5-Minute Series Topographic Quadrangles (Scale 1:24,000), and Florida Department of Environmental Protection (DEP) quarterly surveyed beach profile information, which is available at: http://www.dep.state.fl.us/beaches/.

SWEL Determination
The SWEL used in this example was taken from the effective Flood Insurance Study (FIS) report dated June 17, 2002, and is determined from a numerical simulation of storm surge elevations with a surge model. Further discussion of the methodology used to determine the SWEL is provided in the July 2002 Listserv. The SWEL used for the case provided in this bulletin is 8.1 ft.

Initial Wave Height and Period Determination
The initial wave heights used in the effective FIS report for this area were determined using hurricane statistics for the 1% annual chance storm (a summary of which can be found in coastal FIS reports), and the U.S. Army Corps of Engineers (USACE) Shore Protection Manual (SPM) equations 3-59a and b, and 3-60a and b (located on pages 3-83 through 3-84). Hurricane statistics considered in these equations include hurricane radius, forward speed, maximum sustained windspeed, maximum gradient windspeed, and coriolis parameter. This analysis resulted in an initial wave height and period of 27.20 ft and 11.02 sec, respectively. An alternative source for initial wave data is the USACE Wave Information Studies (WIS) data.

Wave Setup Determination
Wave setup, which is the superelevation of the water surface owing to waves alone and not including storm surge, is 1.8 ft for this case . The amount of wave setup was calculated using the USACE SPM. A worksheet for wave setup calculation for the general Franklin County area is provided below:

Given:
   -H_mo = 27.20 ft
   -To = 11.02 s
   -Ave Nearshore Slope = 1/58 (calculated from Florida DEP survey data and USGS Quadrangle maps)

Compute deepwater wavelength:
-L_o = g * T_p²/2
       = 32.2 ft/s² (11.02s)² /6.28 = 629.5 ft

Compute wave steepness:
-H_mo / L_o = 27.20ft/629.5 ft = 0.043

Using figure 3-53 on page 3-109 of the USACE SPM, find S/Hmo, for:
-0.0368 for slope 1/30 at d_s/H_mo = 0.5 -0.0345 for slope 1/100 at d_s/H_mo = 0.5

Interpolate S/H_mo for a slope of 1/58
-S/H_mo = 0.0359 for a slope of 1/58 at d_s/H_mo = 0.5

Compute 2 * (S/H_mo at d_s/H_mo = 0.5) = 2 * 0.0359 = 0.0718. This determines wave setup at the shoreline (d=0) which is estimated to be twice that at d_s/H_mo = 0.5.

Compute S = H_mo * (S/H_mo) = 27.20 ft * 0.0718 = 1.95 ft - Approx setup height

The final wave setup values used for the Franklin County, FIS (effective date of June 17, 2002), were interpolated between this value of 1.95 ft determined above, and values used for adjacent counties of 1.50 ft. The wave setup value used for this specific case study is 1.8 ft.

Erosion Treatment
This example is treated as a dune removal case as the frontal dune reservoir is less than 540 square feet.

The information discussed above has been entered into the CHAMP database provided in the attached files. Please download these files to view the modeling scenario for the above example.

In the September 2002 Listserv, we will discuss the wave height modeling, Primary Frontal Dune delineation, and flood hazard mapping. This discussion will include the application of the Program 4 - Wave Height Analysis, and Program 6 - Summary Tables and Graphics for Mapping. No discussion of Program 5 (Wave Runup Analysis) is needed as runup is not a factor in this open coast wave height dominated case.

Hot Topic #1 - Coastal Flood Insurance Rate Determination

Available resources for the calculation of flood insurance premiums (including V Zones) are provided below:

Flood Insurance Manual

Flood Insurance Manual Rate Tables

Hot Topic #2 - Advanced Circulation Model for Coastal Ocean Hydrodynamics (ADCIRC) Accepted for Hilton Head, South Carolina

ADCIRC has been approved by FEMA for use for Hilton Head Island, South Carolina. ADCIRC, which was developed in cooperation with the USACE, is a two-dimensional finite element hydrodynamic model used in conjunction with the Empirical Simulation Technique (EST) to calculate storm stage-frequency relationships for determining recurrence interval storm surge elevations. ADCIRC will be used at Hilton Head Island to model the 1% annual chance flood elevations. At present, Hilton Head is the only area for which ADCIRC has been approved.

For additional information on models accepted by FEMA on a national level, view FEMA's Accepted Model List at http://www.fema.gov/plan/prevent/fhm/en_coast.shtm, or call the FEMA Map Assistance Center at 1-877-FEMA MAP.

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Last Modified: Thursday, 21-Jun-2007 11:57:20 EDT