Thursday, May 19, 2016

How big will the I-49 Connector retention pond be? Where will we put it?

Retention/detention pond behind the new Costco store in Lafayette, Louisiana.
photo credit: M. Waldon, no rights reserved

The Lafayette I-49 Connector project envisions a new 5.5 mile highway project going through Lafayette. Where will its storm water drain? The 2002 Environmental Impact Statement simply says it will drain directly into the Vermilion at its southern end, and into local drainage which flows to the Vermilion in its central and northern sections. Even in 2002, it must have been apparent that this could impact flooding. Today, we generally require retention/detention ponds to hold back flows from new construction so that no added flooding results from new projects. These ponds also reduce water pollution by settling out some pollutants How could this not be a major topic of discussion today?

Let's do a "back-of-the-envelope" calculation to get an idea of how large the pond or ponds must be. The project is 5.5 miles (29,040 feet) long, and average width is, I'm guessing, about 600 feet. Multiplying gives 17.424 million square feet of total project area. There are 43,560 square feet in an acre, and division gives 400 acres for the entire project area.

A big 3-day rain event in Lafayette can drop a lot of rain, and we need to plan for the really big storms to avoid flooding. At times we have gotten over 20 inches of rain in one day, so I will assume for planning that we get 30 inches in a 3-day event. Of this rain, some runs off, and some is retained or evaporates. For a typical residential, industrial, or open area, 20% to 60% of the rain might run off. Here, I will assume that 30% currently runs off to the Vermilion. For developed areas with significant impervious surface and sloped, compacted, and drained soil, 80% to 95% might run off. Here, I assume 85% runs off. So, 55% more of the rain is expected to run off after the project is completed. This is 16.5 inches (1.375 feet) of new runoff.

The total volume of added runoff is therefore 400 acres times 1.375 feet, or 550 acre-feet of water. Therefore, the pond needs to provide a storage volume of 550 acre-feet. If our retention pond has an average depth of 3 feet above the dry weather water level, then the pond must be 183.33 acres. Making room for shoreline and fence line (maybe even a jogging track), I assume the pond and related features will take up about 200 acres or 8.712 million square feet.  If square, this requires a property 2952 feet (0.56 miles) on each side.

Where in developed Lafayette can  we place such a feature? The pond must be downhill from the I-49 Connector to avoid the costs and uncertainties of pumping. Therefore, feasible placement of this pond is limited generally to the area between the roadway and the Vermilion channel. Further, any destruction of wetlands or residential areas should be avoided. Agricultural land might be an ideal choice, but we are unlikely to find such within the developed urban area within a feasible distance.

Thus, the question remains - How will we deal with increased flooding from the I-49 Connector? Will this be addressed by DOTD in their public meetings?