DDG prepared this modeling study as a sub-consultant to Trigon Associates for the purpose of evaluating existing drainage conditions and proposed drainage improvements in the Abita Nursery Subdivision in St. Tammany Parish, Louisiana. A 1D / 2D existing conditions drainage model was prepared using the software PCSWMM based on topographic data collected within the subdivision and downstream drainage laterals. The existing conditions model extends from the subdivision to the Abita River. The model was calibrated against regional regression equations and compared with a history of drainage complaints in the subdivision and was in general agreement. The existing drainage system interior to the subdivision, consisting of a network of ditches, driveway culverts, and cross drains is in poor condition and does not have adequate capacity to move water from the subdivision to downstream drainage laterals. As a result, a good portion of the subdivision experiences nuisance flooding associated with relatively small storm events.
DDG modeled proposed improvements including replacing culverts, excavating and re-sloping channels, and increasing outfall capacity at the various points of discharge from the subdivision. The proposed improvements provide significant decreases in peak water surface elevation. To address the increases in downstream peak discharge and water surface elevations, new detention ponds were designed to mitigate the improvements in interior drainage.