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Storm Water Management Ordinances

flashy eroding stream
Environment, Great Lakes, and Energy

Storm Water Management Ordinances

Pushed by increasing civilization and accumulated experience, storm water management continues to evolve. According to Andy Reese, Stormwater Paradigms, the process was:

  1. Run All of It in Ditches (1800s), result: unsanitary eyesores
  2. Run All of It in Pipes (1900s - 1940s), result: unsanitary streams
  3. Run the Storm Water in Storm Water Pipes (1940s - 1970s), result: flooding
  4. Keep the Storm Water from the Storm Water Pipes (1970s - mid-1980s), result: poorly-designed detention that still caused downstream flooding
  5. Well, Just Don't Cause Flooding (mid-1980s - current), result: streams still polluted
  6. Oh, and Don't Pollute Either (current), result: unclear what is natural/healthy
  7. It's the Ecology (current), result: need to manage by watershed boundaries
  8. Water is Water is Watershed (current), result: watershed-scale efforts often overwhelming
  9. Green and Bear It (current), result: structural and institutional practices integrated within watershed to create environmentally-friendly, sustainable, and beautiful living environments

Complexity increases with each paradigm step, but so do the benefits. Keys to success are passionate advocates working at manageable scales and integrated storm water management that mimics acceptable hydrology, enhances natural diversity and beauty, and balances ecological preservation and conservation with economic growth and development.

Adding water quality requirements moves the ordinance to paradigm step 6. Pollutant concentrations in storm water runoff tend to decrease with time during larger storms. Thus treating the "first flush" of the runoff, often defined as 0.5" of runoff, is often a requirement. First flush sizing criteria generally is only effective for a single site. For multiple sites or watershed wide design it is best to design to capture and treat 90% of the runoff producing storms. This "90% rule" effectively treats storm runoff that could be reaching the treatment at different times during the storm event. It was designed to provide the greatest amount of treatment that is economically feasible.

Impervious runoff photo Managing water quality typically involves treating the first flush of storm water runoff 

Adding channel protection criteria to prevent accelerated erosion moves the ordinance up to paradigm step 7. Channel-forming flows are the flows that control the channel shape. The stream channel responds to increases in these flows by widening and deepening, which becomes apparent by increased erosion. In a morphologically stable stream, Channel-forming flows will be bankfull flows and have a recurrence interval of 1 to 2 years on average. Controlling these flows protects the upstream channel reaches from extensive, excessive erosion and downstream reaches from increased sediment deposition. Controlling the duration of the channel-forming flows may be as important as controlling the peak flow. Exposing the streambanks to bankfull flows for longer time periods can also weaken the streambanks.

Stream channel erosion photo Protecting the channel from increased erosion involves controlling the channel-forming flows and volumes

Paradigm step 8 occurs when political entities within a watershed scale integrate and coordinate storm water management.

Paradigm step 9 occurs when local governments incorporate proven low impact Best Management Practices (BMPs), such as rain gardens, green roofs, grass swales, infiltration basins, etc., throughout the watershed to maintain consistent ecologic conditions.

Flooded home photo Limiting flooding requires managing runoff from large storms

Sample Ordinances 

Other Guidance 

  • The Center for Watershed Protection, Inc. is a national non-profit organization dedicated to fostering responsible land and water management through applied research, direct assistance to communities, award-winning training, and access to a network of experienced professionals.