Summary - trdoer10271

For example, the 100-year mean maximum erosion thickness for combined

storms for a mound in 73 ft of water that is 8 ft tall with a crest elevation of 65 ft

is 1.7 ft.

Use of Table G10 for evaluating disposal site design parameters such as cap

thickness or site depth should consider both episodic event erosion and net

cumulative erosion. Yearly monitoring of the disposal site should be conducted

to ensure that the cap has maintained its integrity, i.e., cap thickness has not been

reduced by erosion below the minimum safe level. Even with annual

monitoring, the cap should be designed to withstand multiyear erosional events.

Therefore, the disposal site should be designed such that the cap will not be

compromised by either (a) episodic event (tropical) or episodic season (extra-

tropical) erosion of some defined level of intensity such as the 100-year occur-

rence or (b) several years, 5 for example, of normal storm activity.

Summary

In conclusion, vertical erosion frequency and annual cumulative erosion

estimates generated through the techniques described in this appendix can be

used as a basis for designing a capped disposal mound. However, it should be

emphasized that the erosion magnitudes reported can be considered somewhat

conservative for the following reason:

Single event erosion is calculated as the maximum erosion computed at any

location on the cap as a result of the single event. In most cases, this erosion

is limited to the edge of a cap at the intersection of the side slope and the

crest. If localized erosion of the cap were indicated by annual surveys,

maintenance or remedial disposal could easily restore the cap to its design

thickness at the appropriate location. The amount of cap material that would

be required to restore the cap to its original thickness is roughly estimated at

10 to 25 percent of the original cap volume. Computations of average mound

erosion thickness and the area of mound experiencing erosion are recom-

mended to provide additional insight on the potential for cap failure.

The storm-surge frequency analyses described in this study make extensive

use of the EST. The approach requires the generation of a database of storm

responses that, for this analysis, were selected to be vertical erosion. Because

the procedure is a statistical one based on a training set of single-event erosion

magnitudes, the above assumptions leading to conservatism cannot be eliminated

from the analysis. Therefore, the fact that the estimates are conservative must be

considered in the final design.

For specific cap design projects, a comprehensive and rigorous analysis of the

cumulative erosion due to the occurrence of multiple events per year is recom-

mended. This could include either computing a gross erosion reduction factor or

an LTFATE simulation of multiple years of normal storm activity.

G31

Appendix G Procedures for Conducting Frequency-of-Erosion Studies