Stormwater Management: Quantifying the Benefits

It’s been a wet, wet fall here in St. Paul.  I have rain on the brain.

In a previous post, I discussed storm water management techniques and the use of storm sewer service charges. Conversations about low-impact development (or “LID”) frequently include a claim that treating storm water on-site is cost effective.

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Intuitively, this seems sound:  Less water volume represents less demand for water system  capacity, less of the costly maintenance these systems require, reduced amounts of pollution released into rivers and lakes, et cetera.  But still, a number of planners have asked me, how much savings is there, and for whom?  Is this an issue of private investment for public benefit in the form of cleaner and less voluminous storm water runoff?  Perhaps a public investment for private benefits?  Recent, local evidence gives additional support to the notion that costs for both developers and the public are reduced by trapping storm water on site.

Tom Cesare is the Civil Group Manager with BKBM Engineers, and spoke recently about current policy set by the watershed management districts in the Minneapolis-St. Paul metro area.  The districts’ rules require developments over one acre in size to trap the first inch of rainfall on site as opposed to releasing it into the storm sewer system.  While roughly 60% of storms in this region deliver less than an inch of rain, this requirement is strict in a state and national context – and not simple to address in the development process.  In addition to volume, the rate of flow is also regulated.

Cliff Aichinger, Administrator of the Ramsey-Washington Metro Watershed District, noted last week that reduced runoff does deliver substantive benefits in the form of both quantity and quality of runoff.  For example, on-site infiltration of water through rain gardens, swales, and trenches and use of porous materials for parking surfaces can reduce the phosphorous content of storm water runoff by 90%.  But I have not yet answered the inquiry from planner colleagues:  Can we quantify the benefits?

The answer, put plainly, is yes.  Brett Emmons is a water resources engineer and a principal with Emmons and Olivier Resources, a firm that coordinated a comparison study of LID techniques designed to trap and treat water runoff.  The firm modeled rainfall scenarios ranging up to six inches (a 100-year event in this region).  The analysis compared three approaches:  LID, “conventional” (all quantity and quality control done through detaining water in ponds) and “built” (an improved conventional design where water control is achieved via ponds and a regional infiltration basin).

Om_cost_2The most essential finding of the analysis is that the on-site LID approach to water filtration is less costly to build than the “conventional” or “built” approaches.  In fact, the cost to construct the on-site treatment infrastructure was 10-25% less than the alternatives.  Extended to operations and maintenance costs, the analysis found that LID methods required a 20% higher investment than the built approach, but 16% less than the conventional approach, over thirty years.  The graphs included in this post convey these differentials.

Donjek’s focus is giving clients – urban planners, developers, municipalities and others involved in placemaking – more information about their projects using analytical tools and research.  Quantifying the dollars and cents of storm water management is valuable as you evaluate financial feasibility, but also as you communicate your project to partners and the public.

Harnessing analysis such as the Emmons and Olivier project allows placemakers to go well beyond the sketches and convey to a broad audience that on-site storm water retention and treatment produces public and private benefits.

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