THE EFFECT OF ALL STRUCTURES ON 1979 FLOOD ELEVATIONS

     Flood elevations north of the levee system were elevated 5-1/2 feet by the cumulative effect of the levees, bridges, road fills, sewer lagoons, railroads, sanitary landfills and restriction to flow created by the reservoir dam. The following pages use the HEC-2 computer program to show the relative difference in flood levels between the original flood plain with all of these structures removed and the flood levels computed from the current accepted HEC-2 model of the flood plain in its present condition.

     Also, included is an explanation of how the currently accepted HEC-2 program misstates the present condition of maintenance within the levee system and its effect relative to the actual flood elevations experienced during the 1979 flood.

     In order to compare the Pearl River’s various flood levels in its present condition to that of its original state, we started by generating flood profiles for various flowrates using the existing "HEC-2 PearlFwy.Dat" data. We then removed all of the cross-sections from river mile 285.1 through 292.549. These cross-sections contained the land fill at river mile 285.1, all of the Pearl River bridges including Lakeland, and the entire Jackson Pearl River levee system with the cutoff channel. These cross-sections were replaced with cross-sections which were generated from the U S Geological Survey quadrangle maps of the area. All of the bridges, levees, landfills, and the cutoff channel were left out of the cross-sections. The river was shown in its original state. Reach distances and river distances were modified to follow the river as it flowed before construction of the levees and cutoff channel. The north Jackson sewer lagoon was also removed from the cross-sections at river miles 295.19 and 295.49.

     Comparing the various flood profiles (following pages) for these two conditions it can be seen that for the lower magnitude floods such as a 10,000 CFS event, the cutoff channel actually lowers the water level about four feet at river mile 289.5. In a two year flood (26,800 CFS) the benefit is less than two feet and the combination of Lakeland Drive and the sewer lagoon have brought levels to a foot above natural conditions. For a five year flood (43,800 CFS) the effect of the levee system and cutoff channel is about equal to the rivers original condition. In a 25 year flood (75,000 CFS) the structures and levees cause an additional two feet at river mile 293. For 100 year (106,000 CFS) and 200 year (128,000) events the additional water is four and five feet respectively. To summarize these profiles it can be said that the cut off channel lowers water levels in lesser flood events but the loss of half of the lower level meander belt where the levees are located plus the additional effect of the bridges and encroachments overcome the channel’s benefit for a net gain of water elevations during a major flood event.