ERDC/CHL TR-12-16 "Lake Michigan: Prediction of Sand Beach and Dune Erosion for Flood Hazard Assessment"

The present recommendations for dune removal or dune retreat on the Great Lakes for Federal Emergency Management Agency (FEMA) flood mapping purposes are based on a simple geometric method as outlined in FEMA (2009). The simple procedure establishes a relationship between dune survival and storm intensity. The method was adopted several decades ago when numerical models were inadequate for predicting beach profile change and dune retreat. Herein, a robust and efficient model for predicting nearshore waves, circulation, water levels, sediment transport, and nearshore morphology is provided as an option in replacing the geometric model. The numerical model is compared with results from the existing FEMA guidelines for storm-induced beach change at three sites with different nearshore characteristics. A typical storm with detailed modeled hydrodynamics has been used to provide a basis for boundary conditions, and use is made of scaling to approximate a variation in intensity. A change in water levels is also included in the analysis through a reasonable fluctuation of the static lake level. In general, the methodology in FEMA guidelines computes much larger eroded volume than the numerical model predictions. Additionally, the dependence of the volumes on recurrence interval is determined to be much stronger utilizing the FEMA method. Generalizations are difficult, however, and the predicted volume from the numerical model was larger for some cases of moderate storm intensity. The numerically predicted results were shown to depend on the details of both the subaqueous profiles and dune configuration, and simple universal predictions may suffer gross error. DISCLAIMER: The contents of this report are not to be used for advertising, publication, or promotional purposes. Citation of trade names does not constitute an official endorsement or approval of the use of such commercial products. All product names and trademarks cited are the property of their respective owners. The findings of this report are not to be construed as an official Department of the Army position unless so designated by other authorized documents. DESTROY THIS REPORT WHEN NO LONGER NEEDED. DO NOT RETURN IT TO THE ORIGINATOR. ERDC/CHL TR-12-16 iii