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Development of Program 3 - Erosion Treatment


Coastal transects normally undergo FEMA's standard erosion treatment before being used as input to the WHAFIS 3.0 or RUNUP 2.0 models. For cases where the existing transect data already represent the eroded profile, or erosion will not be applicable (i.e., hardened shoreline or rocky coast), the user can bypass this program by proceeding directly to Program 4 - Wave Height Analysis. It is particularly important that the 1-percent-annual-chance stillwater elevation and profile data be firmly established before performing the erosion treatment as the erosion treatment is based on these criteria.

CHAMP assists the user in performing the erosion treatment and adjustment through the Erosion Treatment Program window, which is accessed by choosing the EROSION button on the Main Menu Button Bar.

The Erosion Treatment Program will display the adjusted transect data on a graph of Elevation versus Distance (station), with increasing stations being landward. Horizontal lines representing the 1-percent and 10-percent-annual-chance stillwater elevations (without wave setup) are also shown. The interactive user controls are accessed through the tool bar at the top of the editor window. These controls include the following functions (left to right): • Set Peak • Set Toe • Set Face Slope • Set Approach Slope • Set Seaward Slope • Set Remove Slope • Computed Reservoir Area • Deposit Area • Erosion Profile Status • Eroded Area

In addition, the user must enter or confirm default values for one or more of the following parameters, as applicable: Seaward Slope (1/12), Face Slope (1/1), and Approach Slope (1/40) for "Retreat" profiles; or Removal Slope (1/50) for "Removal" profiles. Note: default values are in parentheses; slopes are vertical/horizontal.

The erosion assessment analysis is performed graphically by selecting points along the profile. The program requires the user to select these points. The following step-by-step instructions for four tasks detailed below outline the procedures for performing a frontal dune erosion treatment within CHAMP.

Establishing Dune Reservoir/Selecting the Peak of the Dune

Click on the Peak field on the top of the window. Click on the profile where the appropriate peak of the dune is located. The coordinates of the point selected will be shown in the data fields next to the Peak field. The program will draw in the dune reservoir and calculate the area of the reservoir and show this value in the Reservoir field. If the peak needs to be moved or reselected, this can be done by clicking on the Peak field again and selecting the new dune peak location on the transect. NOTE: The minimum value of the dune reservoir before the dune is considered removed is the FEMA standard value of 540 square feet for the Atlantic Ocean, Caribbean Sea, and Gulf of Mexico. The program will also show whether the dune is removed or retreated in the Status field.

The reservoir is calculated as the cross sectional area between the 1-percent-annual-chance SWEL (without wave setup), a vertical line drawn through the dune peak, and the transect ground profile for the foreshore dune area (seaward of the dune crest). If this area is less than the critical area of 540 square feet, the profile adjustment will be for dune removal, and noted as "Remove" in the Status field. If the computed area is greater than or equal to the critical area, the Status field will be adjusted to reflect dune retreat, and noted as "Retreat" in the Status field.

If the 1-percent-annual-chance SWEL is above the highest elevation along a profile, the transect will automatically be given a "Remove" status; hence, the method for dune removal should be followed and selecting the peak for dune reservoir calculation can be skipped. A problem occurs when the dune is below the 1-percent-annual-chance SWEL, but the profile rises above the 1-percent-annual-chance SWEL after some back bay type area. In this case, the program will not allow the selection of the dune peak and will not allow an erosion profile to be calculated. The user needs to either remove the points above the 1-percent-annual-chance SWEL past the dune peak or adjust the transect data points as necessary in the Transect Edit Program and skip the Erosion Program.

Method for Dune Removal

If the dune is eroded (Transect 1 of your CHAMP databases), follow the steps below:

Click the Toe field and select the appropriate place along the profile that best represents the location of the dune toe. The toe of the dune can also be approximated by the intersection of the 10-percent SWEL with the seaward side of the ground profile. The dune toe can be changed by clicking on the Toe field again and reselecting the point on the profile.

The program defaults to using the standard slope for an eroded profile as 1 on 50. If an alternate slope is desired, the value can be changed in the Remove field. After the toe is selected, click on the Remove field button. The program will draw the eroded profile and calculate the eroded area as shown in the Eroded field.

Method for Dune Retreat

If the dune is retreating (Transect 2 of your CHAMP databases), follow the steps below:

Define the eroded area of the dune by first clicking the Face field and select the position for the 1 on 1 eroded dune face that is a best initial guess where the eroded area may be equal to 540 square feet. Check the Eroded field to see what the eroded area is. If the eroded area is not equal to or close to 540 square feet, then re-select the position of the eroded dune face and check the Eroded field again. Repeat this until the eroded area is equal to or close to the 540 square feet.

Determine the seaward slope and position that best gives a deposition area that closely matches the eroded area (check the Deposit field). Select the Seaward field and then select the position for the 1-on-12 slope by clicking on the graph. This will place a 1-on-12 slope line on the screen. The seaward slope will be placed at the position of the mouse cursor, and the program will calculate the deposition area.

Check the Deposit field to see if it matches the Eroded field. If they match or are close, the erosion treatment is complete. If they are not close, the selection of the seaward slope can be repeated until the deposition area matches the eroded area.

To clear out the erosion profile in the database, click the Clear Model menu item under the File menu. This can be used when an erosion analysis was performed previously and the user wants to re-do the erosion analysis or if the user wants to run WHAFIS or RUNUP with the non-eroded adjusted transect data.

Exiting the Erosion Program

Exit from the Erosion Graphic Editor window by clicking on the window clos?re 'X' in the upper right corner. As a precaution, the program will ask if the user wishes to save any changes made to the erosion profile. Click Yes if you wish to save changes, or No if you wish to disregard any changes made. The erosion profile can also be saved in the database by selecting the Save Model menu item under the File menu. CHAMP will then create a data table for the eroded profile, which will be used as the WHAFIS 3.0 and RUNUP 2.0 input files in Programs 4 and 5.

Helpful Hint

Once the clear model command is selected, the erosion table in the database is cleared and cannot be undone or brought back. The user needs to perform the erosion analysis again.

Preparation for Future Bulletins

The next two Bulletins (October and November 2001) will address Program 4: Wave Height Analysis, and Program 5: Wave Runup Analysis. These analyses are based on FEMA Accepted Models, including WHAFIS 3.0 and RUNUP 2.0. The user is asked to update their CHAMP databases through the erosion treatment discussed above so that modeling can proceed without delay.

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Last Modified: Tuesday, 19-Jun-2007 11:57:20 EDT