In the July/August Edition of Stormwater magazine, two articles appear discussing the water quality aspects of detention ponds.

The first article, More Than a Detention Pond by Warren C. High, describes how retrofitting of a typical detention pond into a bioretention pond results in increased effectiveness in removing contamination from sediments for improved water quality, adding needed wildlife habitat and recreation, as well as solving water quantity problems.

The second piece, Sediment Contamination in Stormwater Detention Ponds by John Weinstein, Kevin D. Crawford and Denise M. Sanger, discusses sediment contamination in a typical detention pond and the problems that the accumulated pollutants, concentrated over time, have on ecological health, including humans.

We heartily agree that the overall benefits of a properly designed bioretention pond dwarf the modest functioning of the typical detention basin (pond functions are increased more if the removal of accumulated solids can be integrated into the design of the bioretention pond through a forebay to facilitate contaminant sampling and sediment removal).  The process by which the retrofitting of a typical detention pond into a bioretention pond varies with site conditions and municipal requirements, but the outcome is usually quite positive.  Furthermore, retrofitting performance can be expected to improve over time in all aspects of form and function, as vegetation grows and matures.

For the Retrofit article, go here>>

For the Sediment Contamination article, go here>>

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StormwaterPA’s Editors had their ears open at the Villanova Urban Stormwater Partnership’s (VUSP) Green Infrastructure Open House last week.  Here are a few highlights from Dr. Robert Traver’s report on recent preliminary research findings:

• Plant roots seem to maintain openings in the soil during freeze/thaw events, allowing continued infiltration; use plants with deeper roots, as well as plants with high ET rates.
• Rain garden problems are often caused by poor construction.
• Use of geotextiles between the sand medium and native soil on the bottom of basins/rain gardens may impede infiltration.
• Use a 50/50 mix of native soil and sand for an infiltration medium.
• Rain gardens and basins function well with a sand layer depth of 2 ft, where most plant roots are found, for faster infiltration and less long term compaction.
• Infiltrate relatively clean water through a treatment train design; for small storms (less than 1 in), use a pre-treatment method such as a grass/stone strip or rain garden; for larger storms, use rock bed infiltration or detention storage.
• Perhaps size and depth of storage “bowl” for infiltration can be decreased in size, compensated by an enlarged root zone and sand medium.
• Temperature of water effects infiltration rate (slower in the winter).
• Perhaps both bowl and root zone storage calculations - not just bowl calculations -  can be used.
• To make BMP’s last longer, avoid eroding surfaces from inflow.
• Design for pre-treatment.
• Clean out pre-treatment areas.

Learn More about VUSP here>>

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Volume-Based Hydrology

We’re still digesting an important Andy Reese article in September’s Stormwater Journal and will continue to  comment in upcoming weeks…

As another way to lead into the issue, some thinking needs to be directed toward the concept of design storms which have been a driving force in most stormwater management discussions - really at the heart of peak rate management and flood control objectives.  Current PADEP guidance and regulations typically require pre- to post-development control of peak rates for design storms from smaller to larger storms, usually up to the 100-year storm; in recent years control for smaller storms such as the 1-year and 2-year storm has been added to achieve better stream and streambank protection.

These requirements continue, even as volume control (the PADEP BMP Manual recommends for up to the 2-year storm, pre- to post-development) is also achieved.

Obviously, peak rate control of large storm events such as the 100-year storm typically requires substantial storage and can be challenging  for design engineers to satisfy at development sites. Large numbers of detention basins have been constructed in Pennsylvania municipalities and beyond, often in environmentally sensitive areas and often at considerable cost, to achieve this performance standard.

Although peak rate control for these large storms has come to be accepted as almost a cardinal principle of stormwater management, are there situations where design for these large design storms doesn’t make sense?

Where we aren’t getting much bang for the buck?

• In any situation with tidal dominance, does it make sense to control peak rates for large design storms?
• And a step further, where sites of modest size discharge to large riverine systems, does the rationale for peak rate control become less important?
• If the site discharges directly or nearly directly to a lake where flood control is of no consequence and where no potential flooding impacts exist, does it make sense to spend the stormwater management budget on high performance water quality BMPs, rather than unnecessary flood control basins?

In sum, flood control achieved through management of large design storms is critical in Pennsylvania municipalities, but there may be important exceptions.

Make sure your municipal ordinance allows for this flexibility in the right hydrologic situations.

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From the Chesapeake Stormwater Network… As a follow up to our blog last week on infiltration in and around limestone, read the blog on The Chesapeake Stormwater Network website that discusses Stormwater Solutions for Karst Terrain. Some of the important considerations include:

• Increased risk of new sinkhole formation by stormwater runoff
• Failure of stormwater practices and infrastructure due to sinkholes
• Greatly increased post-development runoff rates when land is paved
• Underground karst features are hard to detect and vary greatly over just a few feet
• Strong and often mysterious runoff/groundwater interaction make it hard to understand flow paths and drainage patterns
• Watersheds have lower stream density, losing streams and karst swales
• Polluted runoff greatly increases risk of groundwater contamination
• Need for special groundwater injection permits
• Changes in recharge or runoff quality can harm endangered species

Click here for the entire article including links to a 30-page technical bulletin and a powerpoint primer on karst terrain and stormwater.

This Week’s Food for Thought

Forests/Woodlands - the Tree as the only Best Management Practice!

“Penn’s Woods” ought to take its forests seriously.  We’ve become increasingly aware that woodlands provide valuable eco-services - critical watershed functions in terms of stormwater quality and quantity, as well as a host of other valuable functions.  Undisturbed forested cover, even in areas of “heavy” clayey soils, is capable of infiltrating remarkably large volumes of precipitation, after interception by forest canopy.   Evapotranspiration is maximized as well.  With such reduction in runoff volumes, water quality benefits as well.  Even in the largest storm events, runoff  from woodlands is surprisingly well filtered.  Not only does saving trees at a development site not increase stormwater runoff volumes, but undisturbed wooded areas can receive runoff and function as perhaps the only truly “best management practice.”  Trees are money in the bank from a stormwater perspective.

The positive role of trees in stormwater needs to be elevated.  Highly regarded research institutions like Pennsylvania’s Stroud Water Research Center are demonstrating that extent of forest remaining intact is perhaps the single most important variable relating to water quality and hydrology (i.e., overall watershed health) and overall watershed health.  And the value of trees is not limited to “mature specimen hardwoods” as has sometimes been thought to be the case.  Woodland areas with relatively immature trees can rank high on the eco-services scale.

In sum, the stormwater principle in terms of woodlands looks something like this:  fit the development into the trees - and if the trees are gone, try to replant and re-forest as much as possible.

Some comments and questions for your reaction:

-Does cutting down trees to make room for detention basins make any sense?

Perhaps the ultimate site planning insult (and absurdity) is clearing trees in order to build detention basins.  Many folks don’t realize that stormwater ordinances need to relate closely to municipal woodlands protection ordinances which should require rigorous tree replacement when trees are cleared from development sites.

-As critical as trees are to riparian buffers, upland forests are critical to watershed water resources, quality and quantity, as well.

In PADCNR’s Growing Greener program, woodlands are assigned a “secondary” resource status, in contrast to the primary status of floodplains and wetlands - should woodlands be taken more seriously in terms of stormwater management?

-PADEPs BMP Manual recommends a crediting methodology for trees and other environmental features.

Is anyone using these credits?  If you’ve used credits, what changes would you make? Are the credits meaningful?

Looking Ahead

Volume-Based Hydrology (VBH) is the very latest innovative thinking for stormwater management and is vastly different from the peak flow approach.  Andrew J. Reese explains this sea change in thinking - read the full article in the September issue of stormh2o.com. And look for more on this From the Editors next week…

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