FOR ENGINEERS REVIEWING THIS PLAN

     This plan utilizes standard stream flow formulation to project flood profiles thru the 11 mile metropolitan reach of the Pearl River. Benefits are derived by increasing the cross-sectional area thru the riverbottom land, existing levees and bridges by removing sand. Additional benefits are derived by creating a lake which prevents the regrowth of trees. Because standard stream flow formulation will be used, calculated benefits by different parties should work out to be very similar. If there are differences they can be compensated for by the removal of less or more sand. The important consideration in evaluating this plan is judgmental. If the reservoir prevents sediment form entering the lake form upstream and overdredging prevents sediment form entering the lake from tributaries, what will be the effect of flow upon the remaining sand in the lake? The lake must be sized such that it lows the velocity of flow down. The velocity in the lake cannot disturb or carry this sand.
     The flood control lake is designed to have a mean velocity of 0.4 feet/second during yearly floods and a mean velocity of 2.3 feet/second during a 100 year flood. Our judgement is that this velocity is sufficiently low and is based upon observation of channel erosion over many years. Channel banks in the north Jackson reach of the river consist of a top layer of clay (about 9 feet thick) underlain by a soft white sand. Each year, these banks are subjected to channel flow which has a mean velocity of about 3.4 feet/second. On the outside of the channel bends this velocity  increases to over 5.0 feet/second. In five year floods, when water tops these banks, velocities exceed 6.0 feet/second on the outside of channel bends. Trees that have been undermined and left standing with half their roots exposed are observed to stand for several years before sloughing into the river channel, usually during a five year or greater flood. This slow rate of erosion at much higher velocities enables us to adjudge that redistribution of sand in the flood control lake will not create a problem for over 100 years.
     Please contact John McGowan or Mart Lamar at 601-982-3444 for any additional data that may be necessary to evaluate this plan.