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Riding the Wave of Impending Stormwater Regulations: Water Quality

by Aislinn Creel, EIT, LEED AP Friday May 28, 2010

A paradigm shift is occurring in the way in which we control pollutants generated by stormwater runoff. This shift is evident by examining the existing and proposed stormwater regulations and design standards. Stormwater runoff (non-point source) pollution is one of the most difficult to quantify and manage, yet critical when it comes to protecting our receiving waterways from harm.

Current standards for water quality criteria in the Commonwealth of Virginia (applicable to all state agency projects and every land-disturbing activity regulated under §10.1-603.8 of the Code of Virginia) target a reduction of total phosphorus and are based on a comparison of existing and post-developed pollutant loading or pollutant removal efficiencies. Percent impervious cover is used to determine pollutant loading in both the performance-based and the technology-based methods and various recognized BMPs are then selected based on assigned pollutant removal efficiencies. To date, BMPs are sized to treat peak discharge rates and rely heavily on the principals of detention time and sedimentation for treatment.

By contrast, the 2007 Energy Independence and Security Act mandates for all federally-owned project sites (>5,000 ft2) that design strategies to maintain or restore the pre-development hydrology of the property with regard to temperature, rate, volume and duration of flow be employed. This aforementioned maintenance or restoration of pre-development hydrology can be accomplished by either controlling the 95th percentile rainfall event, or by analyzing pre- and post-development hydrographs.  Either way, the federal mandate requires that 100% of the excess runoff determined be either infiltrated, evapotranspired or captured and used.

Similarly, LEED® standards (Leadership in Energy and Environmental Design) for water quality treatment of stormwater runoff require that 90% of the average annual rainfall is treated using acceptable BMPs and that the BMPs used are capable of removing 80% of the average annual post-developed TSS loads. To obtain LEED® credit for stormwater design:  quality control, a stormwater management plan that reduces impervious cover, promotes infiltration or captures and treats the runoff must be implemented.

The focus has shifted from management of discharge rates to management practices focused on volume that better mimic natural systems, i.e., infiltration, evapotranspiration and capture and use. Rather than assuming that retention and sedimentation will primarily treat harmful pollutants liberated by stormwater runoff, regulators are now focusing on mandates to treat entirely on-site the “first flush”; that is, zero discharge of runoff from small (1”-2”) storm events is allowed.  EPA is currently in development of a new stormwater rule which may address performance standards for new and re-development; expanding coverage of regulated stormwater discharges to areas with rapid development; provisions for existing discharges (retrofits); permitting and non-permitting alternatives; and special provisions for Chesapeake Bay and maybe other sensitive waters. Based on the already mandated standards for federal sites and similar philosophies used in LEED criteria, it seems likely that the EPA will use a similar philosophy in development of the new rule.  We’ll be tracking the progress of this new rule and you can, too, at www.epa.gov/npdes/stormwater/rulemaking.


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Posted in Stormwater Management

What’s So Important About Riprap Outfall Protection?

by Charlene Harper, PE, LEED AP Monday Feb 1, 2010

It may not be glamorous, but erosion control design is a critical component to managing water quality both during and after site construction.  The transition between a manmade drainage system and a natural channel is one area of potential erosion that must be given special attention during design.  A structure is needed at this point to dissipate both the velocity and the energy of the concentrated storm flows and to protect the downstream channel from scour.  Although it is not the only measure available, the most typical transition to alleviate potential erosion is a riprap outfall protection (ROP) apron.

The process of designing and specifying ROP is well detailed in every state erosion control manual; however, one requirement that is often overlooked is the need to install the apron at a zero percent slope.  It may seem like a small detail; however, the absence of slope is absolutely essential for the ROP to function properly.  The pictures below show a drainage system failure resulting from the installation of ROP  at a slope greater than zero percent.

Timmons Group was asked to investigate the cause of the issue and identify potential solutions. The original riprap apron was installed at a slope of approximately three percent at an outfall which drained into a stormwater management basin.  Stormwater began to scour the downstream end of the riprap and then undercut the entire section, causing it to drop over three feet in less than five years.  After consideration of several possible alternatives, the sandy soils and failure magnitude led us to make the following recommendation:  install a drop manhole with a new outfall pipe, construct a flat (zero percent slope) riprap apron with a cutoff wall at the downstream end, and re-grade and stabilize the failed slopes.

The problem shown above is not uncommon, but it is easily avoidable.  Whether using riprap outfall protection or alternative methods (e.g., concrete or gabion aprons or reinforced turf mats), designing this delicate transition from manmade elements to natural systems is vital to the protection of water quality in our receiving waters and the overall success of your project.


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Posted in Stormwater Management