Background Reading On Stormwater

The city of Portland OR has struggled with stormwater runoff problems for decades and has established expertise on how to best deal with runoff issues. Meeting ASTM standards relating to stormwater has nothing to do with mitigating run off. The idea behind a Low Impact Design (LID) water garden type of storm water storage is to completely prevent the fist 1/4″ of rainfall from leaving the property at all.

There are two methods typically used to generate rainfall patterns when designing stormwater systems. The first, and more traditional method is to generate a statistically based synthetic single-event hydrograph based on regional rainfall histories. The second method is to use a long-term historical rainfall database to generate runoff based on actual (rather than synthesized) data. The first method has the advantages of being widely accepted and relatively straightforward to compute. However some studies have shown that these regionally based hydrographs can differ widely from local conditions. Additionally, a single-event rainfall does not consider the conditions where two rainfalls occur within a short time period. The second method has the advantages of representing true rainfall conditions, which include the occurrences of consecutive rainfalls. However, a long period of detailed (at least hourly) rainfall is required to characterize the runoff at this scale.

Stormwater Management Study by Department of Environmental Programs, Metropolitan Washington Council of Governments, Washington, D.C., March 2003

In short, DG may wish to plug in the numbers for 2, 10, and 100 year rainfall events to generate minimum size detention/retention for any given sized area. Developers would be required to meet or exceed those minimums.

Existing pipe systems typically are large systems placed to supplement above ground detention. A precast modular storage system such as the StormTrap SingleTrap would probably work if it were at some elevation other than where it is (very low). Plus, it would be custom designed. Read $$$$$$.

In short, 36″ pipes are a cheap solution that has never been employed.

Off-line runoff collection, filtration and dispersement practices, such as detention basins and bio-swales, became established as favored practices (otherwise know as “best management practices, or BMPs) to improve runoff water quality and reduce volume loads upon centralized storm sewer systems.

Taking Storm Runoff Out of Sewers, Reconsidering the quality of runoff generated by current engineering practices with the help of stormwater micro-management, By William W. Bohnhoff, ASLA January 1, 2004

It appears at this point in the village history, simply paying for past mistakes is not enough. DG should at least investigate implementing runoff limits from redevelopment so that the expensive repair/upgrade to the stormwater infrastructure is not simply overwhelmed by additional input from impervious ground cover or inadiquately detention/retention from new development.

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