With the onslaught of new water quality devices, ever-changing pollutant removal parameters, and a growing list of acronyms to memorize — such as BMP, LID, and LEED — today's civil and site engineers are challenged to stay on top of their game. Coupled with the economic crunch and the limited resources available to developers, the demands placed on design are more critical than ever, with no room for taboo terms such as “over-budget” and “over-design.”
Optimizing stormwater management with less is well illustrated by a recent development in Blacksburg , Va. , commonly known as First & Main. The high-profile retail development was politicized on a number of fronts, including stormwater management.
Hydrologically, the site presented the types of challenges that could easily make or break the development. First, large amounts of offsite drainage traversed the 27-acre tract, amounting to 200 cubic feet per second (cfs) for the 25-year event. Second, a portion of the pre-developed property was naturally detaining this offsite drainage and was configured as regional stormwater detention. Third, water quality measures upstream of the development were in place, and offsite drainage was already being treated to the satisfaction of the regulating agency. Thus, the objective became storing both the offsite and onsite drainage and treating only the onsite portion of this mixed flow, all in the same “natural” detaining area to mimic pre-development hydrology.
The initial stormwater management design was cost prohibitive, with installation and material costs totaling about $3 million. At that point, the developer did two things:
- commissioned Balzer & Associates to explore alternate methods of calculating storm runoff volumes, peak discharge rates, et cetera; and
- asked Lane Enterprises to provide design-build options for the stormwater management system.
The combination proved effective, lowering related costs to about $1 million.
Balzer & Associates effectively eliminated 40 percent of the stormwater to manage by liberating themselves from the simplified procedures of TR-55, and opting for the tried-and-true Rational Method. It was then left to Lane Enterprises to optimize how the resultant flow quantities would be managed.
A joint venture between Lane Enterprises and Hughes Associates Architects was ultimately successful in providing the design-build option the developer needed. Lane began by upsizing the stormwater management system (SWMS) to 12-foot-diameter corrugated metal pipe. The increase allowed a smaller footprint, less excavation in an area underlain by rock, and correspondingly less material to haul from the site.
The SWMS offered a number of features that all played a part in system optimization. A total of 12 nodes were configured into the HydroCAD modeling software. The critical node was the head-building chamber, a smaller compartment designed to fill rapidly before cresting weirs and filling the SWMS proper. This feature enabled the full orifice head (in this same chamber) to be realized at the beginning of the storm event, eliminating the need for the entire system to fill before achieving the maximum allowable discharge, and correspondingly eliminating unnecessary storage volume.
In the same manner and with similar effects, water that crested the weirs was directed through a Hydro-International Reg-U-Flo Vortex Valve, a device designed to achieve maximum discharge rate at lower-pressure heads while maintaining that same discharge rate at the higher heads via a restrictive vortex created through the outlet of the device.
As long as the main dewatering line was not surcharged, a secondary outlet was available through the operation of a backflow preventer (check valve). This device in full swing revealed another orifice outlet to mobilize additional allowable discharge, which also contributed toward minimizing storage requirements.
Another feature that contributed to minimizing required storage was the ability to use 6 inches of the pipe bedding material for storing the water quality volume ( WQ v ). This was accomplished by adding a full run of 12-foot-diameter perforated pipe into the SWMS, in addition to roughly 360-feet of Lane's 6-inch HDPE under-drain to intercept and outlet the WQ v through a Hydro-International Up-Flo Filter.
Countering much of the design work at this point, system storage was increased to accommodate a more restrictive filtering device. This move enabled removal of a sizeable and costly ($250,000) concrete vault containing nearly 80 filter cartridges, and made way for a more reasonable ($40,000) standard 4-foot-diameter structure with only six filter cartridges. An Up-Flo Filter with capacity to pass 0.33 cfs was all that was needed for the value engineering judgment. Thus, sometimes more is less.
|For more information on stormwater management system design, visit the National Corrugated Steel Pipe Association (NCSPA) website at www.ncspa.org, purchase the NCSPA Corrugated Steel Pipe Design Manual, or contact NCSPA's Chief Engineer Michael McGough, P.E., at 703-812-4701 or email@example.com .|
Jerome S. Silagyi, P.E. , is manager of technical services, Lane Enterprises, Inc., Camp Hill, Pa. He can be contacted at firstname.lastname@example.org.