What is SWMM and how does it help?
The topic of Water Quality Management seems to have become a key and critical element in the design of projects, not to also mention the challenges in getting through the review process with agencies. As I understand the goal, a new development needs to “treat” the storm water runoff from the property before it hits the storm water system of a city or county. And a common practice is to create water quality basins within the project, where storm water gathers in these basins and percolates through engineered sand and gravel, thereby cleaning the water of its oils, toxins, and contamination.
A few years ago while in the initial design of a project in San Diego County, my civil engineer suggested the use of a SWMM analysis in order to maximize the amount of usable land. SWMM stands for Storm Water Management Model and is a fairly technical analysis of the soil, slopes, and water quantity to determine the required size of the water quality basin. The County standards, as I understood, would require a basin size equal to approximately 10 – 12 % of the total impervious areas (streets, driveways, patios, rooftops, etc) within the project design. We eventually engaged a hydrologist who specialized in these SWMM calcs and it reduced the basin size to approximately 4 – 5 % of these impervious areas. Our 70-lot development probably would have lost another 4 – 5 lots without these SWMM calcs, thus impacting our overall land value. We submitted this highly technical SWMM document with our project design to the city staff, and the smaller basin size was found to be acceptable. Some consulting dollars were well spent to save these 4 – 5 lots.
If anyone has some experience with SWMM calcs or has questions, feel free to post your comments or questions below.