Technical Support for the Baywide Runoff Reduction Method

The Runoff Reduction Revolution

Runoff reduction has come from out of nowhere to become a major stormwater trend in the Bay states. Bay stormwater managers are rapidly shifting their focus to reduce the volume of runoff rather than merely treating its quality. Recent technical breakthroughs by the Center for Watershed Protection have produced a unified and integrated approach that should revolutionize site design for new developments. Kudos to Dave Hirschman, Kelly Collins, Scott Crafton and others for their pioneering work on this. This short article lays out the basics of the new runoff reduction approach, which is under consideration, in some form, in virtually every Bay state. Technical Bulletin No. 4: Technical Support for the Baywide Runoff Reduction Method provides a more detailed summary of the new method, and the extensive amount of scientific and engineering data that supports it. The Center for Watershed Protection has provided a memo on the topic that is more specific to the Commonwealth of Virginia.

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OK, so what is runoff reduction?

The runoff reduction approach seeks to maintain the same predevelopment runoff volume delivered to a stream after a site is developed. In its simplest terms, this means achieving the same predevelopment runoff coefficient for every storm, up to a designated storm event. Runoff reduction is defined as the total runoff volume that is reduced through canopy interception, soil infiltration, evaporation, rainfall harvesting, engineered infiltration, extended filtration or evapo-transpiration. Extended filtration includes bioretention or dry swales with under drains that delay the delivery of stormwater from small sites to the stream system by six hours or more.

Why is it better than our past approach?

The many benefits of the new runoff reduction approach are that it:

  • Provides an objective way to measure the aggregate performance of environmental site design, runoff reduction and conventional stormwater treatment practices together.
  • Mimics predevelopment hydrology with respect to runoff volume, duration and velocity which should provide an added level of stream channel protection.
  • Enhances the degree and reliability of nutrient load removal, which is a key objective to protect the Chesapeake Bay.
  • Eliminates the use of stormwater credits and makes use of runoff reduction practices an integral element of on-site compliance.
  • Collapses recharge, water quality and, in some cases, channel protection sizing requirements into a single volume, depending on the unique characteristics of the receiving stream and the intensity of its subwatershed development.
  • Explicitly acknowledges the hydrologic difference between forest and turf, and disturbed and undisturbed soils, which creates strong incentives to conserve forests, reduce mass grading, restore soils and reforest sites.

What stormwater practices really reduce runoff?

The CWP and CSN recently analyzed more than 100 research studies to define runoff reduction rates for several common practices. A key finding is that runoff reduction rates are much less variable than pollutant removal rates, and are closely linked to the underlying hydrologic soil groups. The list below shows the breakdown for many popular practices.

Percent Annual Runoff Reduction

Infiltration 50 – 90
Bioretention 40 – 80
Pervious Pavers 45 – 75
Green Roofs 45 – 60
Dry Swale 40 – 60
Rain Tanks/Cisterns 50
Rooftop Disconnects 25 – 50
Grass Channel 15 – 30
Dry ED Pond 0 – 15
Wet Pond 0
Constructed Wetland 0
Sand Filter 0

The Appendix B of the technical support document provides more details.

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How is the runoff method actually applied on an individual development site?

The beauty of the runoff reduction approach is that it enables designers to spatially analyze each site to quickly evaluate a wide range of environmental site design, runoff reduction and conventional stormwater practices on a common “roof to stream” sequence. The CWP has created a simple spreadsheet tool so that designers and plan reviewers can evaluate compliance without resorting to silly BMP math games. The compliance spreadsheet has been reworked at least a dozen times, and is the final stages of testing on a bunch of tough real world sites this Spring. In a nutshell, the design process follows four simple steps:

Step 1: Apply Early ESD Practices: During site layout, designers look at a site map of environmental and soil features to find the easy opportunities to minimize creation of needless impervious cover or mass grading, and maximize protection of permeable soils, forest cover and other natural features.

Step 2: Compute Post Development Land Cover . Designers then input the resulting land cover variables into the spreadsheet (forest, impervious and disturbed soils) to determine their total runoff reduction and phosphorus treatment requirements at the site.

Step 3: Apply Runoff Reduction Practices . The designer can then experiment with combinations of different runoff reduction practices on the site. In each case, they estimate the spatial area to be treated by each runoff reduction practice, and chip away at the required runoff reduction volume for the site.

Step 4: Determine if Further Phosphorus Treatment is Needed : In the last step, designers checks to see whether the phosphorus load reduction has been achieved at the site. Removal by previously entered runoff reduction practices is automatically calculated, and the designer can then add conventional practices such as sand filters or wetlands to boost nutrient removal to protect the Bay.

Runoff reduction design is an iterative process. If compliance cannot be achieved on the first try, designers can return to prior steps, to see what alternative combinations of ESD, runoff reduction or conventional stormwater practices could be used.

How much runoff reduction do you really need?

The answer is that it depends on your community water resource objectives and the intensity of past watershed development. Based on what we know now, up to four increasingly stringent runoff reduction “benchmarks” can be applied:

Cleaner outfalls: Maximize use of runoff reduction for the so-called “first flush” storm, which is equivalent to about a half inch of rain in most parts of the Bay watershed (0.5 inches). While this degree of runoff reduction cannot prevent some increase in runoff pollution, it can sharply reduce overflows in combined sewers, keep trash and floatables out of the water and keep pollutant concentrations in runoff from exerting toxic effects. This benchmark only applies to the most highly urban watersheds that lack space for most runoff reduction practices and have little or no remaining surface streams.

Stream water quality: Maximize use of runoff reduction for the 90% storm, also known as the water quality storm, which ranges from about 0.9 to 1.2 inches of rainfall across the Bay watershed. This amount of runoff reduction sharply decreases runoff pollutant, helps maintain fair to good stream quality, but may not fully protect stream channels from enlarging, according to recent models. This is an appropriate benchmark in many urban watersheds that have already undergone extensive development (e.g. in the 40 to 65% impervious cover (IC) range).

Stream channel stability: Full runoff reduction for all storms up to the one-year design storm which ranges from about 2.2 to 2.6 inches of rain across the watershed. This amount of runoff reduction, when combined with other watershed protection tools, is expected to be able to maintain natural stream geomorphology, and reduce pollutant loads below background levels. This benchmark is most appropriate to maintain stream quality in suburban watersheds (e.g., 15 to 40% IC).

Stream integrity: Full runoff reduction for all storms up to the two year design storm, which ranges from 3.0 to 3.4 inches across the Bay watershed. This amount of runoff reduction, when combined with streamside forests, should help maintain healthy aquatic diversity, cool water temperatures, and natural stream hydrology. This really tough benchmark is needed to protect exceptionally sensitive streams (e.g., trout) in lightly developed watersheds (e.g., less than 15% IC).

As this is being written, regulators in DC, VA, MD, DE and WV are all grappling with how stringent their runoff reduction benchmarks should be and where they should be applied. Each state is also likely to develop slightly different compliance methods. The core issue is the more runoff reduction that you ask for, the harder you make it to comply at individual sites, particularly at higher levels of development intensity. The solution is to apply different runoff reduction benchmarks based on a sliding scale of watershed impervious cover.

Even so, compliance may not be physically feasible at some very small or dense sites regardless of runoff reduction benchmark selected. What does one do in these cases? The first option is to revert to conventional stormwater practices with more limited runoff reduction capability. If this doesn’t work, the developer can pay a mitigation fee to the local government to support an equivalent amount of runoff reduction or restoration elsewhere in the watershed.

How is runoff reduction integrated with quantity control?

The astute reader has probably noticed that the discussion about runoff reduction has been silent on the topic of managing the storms that produce floods, such as the 2, 10 and 100 year return interval design storms. Clearly, both runoff reduction and flood control need to be integrated together, despite the fact that different models are used for each analysis. Work is underway to create this integration, so that runoff reduction volumes can be deducted from the storage volumes needed for either channel protection or flood control purposes. Look for some new ideas in this space in a few months.

How do you make sure that the system of runoff reduction practices will work effectively over time?

While the runoff reduction method promises better stream outcomes than past stormwater design methods, it creates new challenges for designers, plan reviewers, inspectors and maintainers – particularly when it comes to maintaining the future performance of a distributed roof to stream system. It also needs to be emphasized that the spreadsheet only verifies overall site compliance — individual practices in the system still require careful design and plan review. Your help in making the construction and maintenance elements of runoff reduction design specifications more rigorous remains a critical need.

Your comments are appreciated. Please let me know your thoughts in a comment below.