Two Lakes : Dreams Realized

Retention vs. Conveyance

There has evolved, in recent times, a strong school of thought which teaches that
flooding can be controlled by the retention of water in green areas and wetlands within floodplains. Belief in this concept has lead to harsh restrictions by government upon how citizens may use property. However, to our knowledge no attempt has been made to define, through formulation, the exact quantitative amount of benefit that wetlands and green areas provide. If proof of benefits had been required, this is what would have been discovered.

Wetlands and green areas that are situated in vast, flat deltas reduce the level of flooding in bayous and rivers that extend below them by retaining water. When situated in the fast-flowing upland river floodplains such as the Pearl in Jackson, wetlands and green areas do not affect flooding in the same manner.

Water retained within these floodplains, at ground levels, actually has no effect upon the high magnitude floods which cause harm to people. This is because the reaction that results from water's retention or detention in floodplain green areas cannot be timed to coincide with the higher magnitude flood, which occurs later. The reaction happens too early and separates itself from the flood that follows by moving away downstream. For this reason, the reaction to ground level wetlands and green areas, in Jackson's floodplain does not accumulate to any later benefit. Water retained in the initial stage of runoff does not lower the higher stage, damaging floods that occur at a later time. Only water retained at higher stages affects the magnitude of a damaging flood. For this reason, only structures that elevate water to and above the maximum flood heights (such as Barnett Reservoir) can affect the ultimate level of such floods.

However, water retention can be used to reduce high magnitude flood levels when the retained water is situated to have its effect concurrent with the high stage. But the actual, formulated benefit that water retention provides is disproportionately small when compared to its cost in labor and resources. For this reason, retention methods can rarely be justified. On the other hand, when conveyance is improved by construction of ditches and canals benefit is disproportionately large when compared to cost.

For example: the U. S. Army Corps of Engineers had proposed building a dam above the Barnett Reservoir at Shoccoe. This retention project would cause 51,000 acres of rural land to be inundated with water during flood events. The project would also utilize the ability of the reservoir to retain water by filling it to its maximum safe level. This would add 38,000 acres. Yet the loss of this 89,000 acres of rural resource to flood control by retention would only provide Jackson with a benefit of reduced flooding amounting to 2000 acres in another 1979 flood event. Flood levels would be reduced 6.6 feet at Lakeland, leaving numerous homes and businesses in North Jackson still flooded.

By comparison, for less expense, the conveyance of water through the Jackson metropolitan area could be improved by dredging a canal and, to prevent re-growth of trees, making the canal into a lake. This would remove 11 feet of flooding from Northeast Jackson and 4 feet from the Town Creek area. This would end 96% of all flooding in another 1979 flood. In addition, 10,000 acres of land which had previously flooded would become available for development and , through industrious use of the dredged material, a valuable island would be created.

The use of retention to control flooding by preserving green areas and wetlands has become very popular and coincides with contemporary society's appreciation for the natural environment. The use of retention, however, if formulated seldom has any effect on damage causing floods, and retention projects that do have an effect most often extract a net loss from the total human resource. For this reason, flooding has been and always will be, primarily controlled by improving the land's ability to convey water by the construction and maintenance of canals and ditches.