The project goals were to design and install a subsurface drainage system in the Melius subdivision of Reserve, LA to replace existing roadside ditches, undersized driveway culverts, and single lot frontage subsurface systems with the purpose of reducing the flooding risk for lots and residences within the neighborhood. The 10-year storm was selected as the design storm in to provide adequate relief for street flooding with sufficient overflow capacity to reduce the flooding risk to residences during larger storm events. The final design had eight trunk lines, one on each side of four streets, all with an equivalent diameter of 42 inches. Reinforced concrete pipe was selected as the pipe material to ensure longevity of the system with superior durability.
BH performed complex modeling to accurately establish the tailwater condition. Nine conduits already cross the railroad and the new drainage system discharges into the railroad right-of-way. To determine the tailwater elevation, the railroad right-of-way was modeled as a detention pond. We used the peak tailwater elevation for the design storm. The pipe was designed at the minimum slope for cleaning velocity and because of cover constraints, we specified concrete arch pipes. Even then, the last 200 feet is surcharged at design flow.
During design, BH video recorded the sanitary sewer to quantify the shallow sewer service lines which require conflict structures. Identifying these conflicts before bidding was a significant cost saver. We added a pay item for exploratory test pits to incentivize the contractor to locate the services before laying the storm sewer, allowing field adjustments to the locations of catch basins, which doubled as conflict boxes. Pay items for various sewer service line adjustments empowered the contractor to select the best adjustment based on his test pits.
We also designed a tee inlet to be installed directly over the pipe to collect small drainage areas between driveways. These became popular and many residents requested additional inlets for their yards.