Predicting the Effects of Storm Surges and Abnormal River Flow on Flooding and Water Movement in Mobile Bay, Alabama

Abstract

The threat of man-made and natural disturbances to the coastal environment is a continuing and perplexing problem. With the advent of rapid, numerical simulation models describing coastal water behavior, the ability to better understand these regions and to provide data to offset the adverse impacts caused by these disturbances, has greatly improved.

This paper discusses the recent numerical modeling activities of the Mobile Bay system under severe conditions. Results are presented in terms of changes that occur in water elevation and movement, and, in salinity distribution patterns when the bay is subjected to river flooding inflows and storm surges.

At a river flood stage of 7000 m³/s, water behavior in the northern and central portions of the bay is totally governed by the fresh water inflow. A salinity level of 5 ppt is restricted to the lower bay at a point 15 km from the Main Pass. Usual salinity values under normal conditions are in the range of 15 to 20 ppt in this area. A critical river flow rate of 8500 m³/s is also identified. At or above this flow, saline water intrusion in the lower bay becomes stabilized at 10 ppt on a line 6 km north of the Main Pass.

Conversely, large amounts of saline water enter the bay under storm surge conditions investigated in this study. Conditions typical of those caused by Hurricane Camille in 1969 were used in the modeling exercise. Salinity levels as high as 26 ppt were predicted for the northern bay area. This high saline water intrusion is caused by the development of the surge hydrograph at the gulf/bay interface as the storm approaches the coastline.

In both cases the model results were shown to be representative of bay behavior. Comparisons with existing field observations were made to calibrate and verify the models.