Stormwater Management Design – So What Happens After the End-of-Pipe?

Mike Gregory

ABSTRACT

The terminology and state-of-the-practice in stormwater management servicing design in many areas continues to be apparently limited in its functionality to:

  • Source controls that are intended to capture and treat stormwater prior to its discharge into the municipal drain age and collection system;
  • Conveyance controls that are intended to transport and treat stormwater as it is conveyed through the collection system; and
  • End-of-pipe controls that are intended to store and treat stormwater prior to its discharge into receiving waters.

The design function of drainage infrastructure and facilities can be extended to consider downstream impacts on a broader scale (i.e., what the receiving watercourses and waterbodies experience), including:

  • Geomorphology (river form, bank stability, and sediment/flow relationships);
  • Hydrogeology (baseflows, groundwater levels and chemistry);
  • Sediment circulation, nutrient cycling, and temperature regulation;
  • Aquatic resources (fisheries and benthic habitat); and
  • Terrestrial resources (wildlife, vegetation, and soils that are affected by periods of inundation).

Conventional stormwater management design can be improved in two ways. First, by optimizing the hydraulic performance under the full regime of anticipated flows and loading conditions; from the large-volume/low-frequency flows using event-based methods to address flood protection, through to the small-volume/high-frequency flows using continuous simulation to address water balance and maintain minimum baseflows. Water quality and erosion design objectives lie somewhere between these two extremes. Second, additional design objectives and performance indicators (beyond peak flood stages and discharge rates) can be incorporated such as volume, duration, and inundated area as well as frequency curves that characterize flow, velocity, and depth across the full range of hydrologic conditions anticipated.
Recent project examples using SWMM5 will be presented that investigate a variety of receiving water impacts, including:

  • Flooding hazard, by minimizing the exceedance of risk-based flood damage indicators to downstream properties;
  • Erosion hazard, by minimizing the exceedance of erosion thresholds and other geomorphologic indicators in down stream watercourses;
  • Environmental and habitat protection, by minimizing the exceedance of inundation/duration thresholds and other hydroperiod indicators in receiving wetlands or other sensitive waterbodies; and
  • Water balance, by minimizing the exceedance of flow/duration thresholds in watercourses (per California’s “hydro modification” test).

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