Thursday, June 30, 2011
The Quantico Watershed Study and the Chesapeake Bay TMDL
On June 23, 2011 I attended the Prince William Department of Public Works meeting to hear the results of the recently completed Quantico Watershed Study . Warren High from the County’s engineering consultants, MACTEC, presented the results. The study was used to assess the current stream conditions, examine existing storm water management facilities, and to identify future Capital Improvement Projects. They used a standardized system of scoring streams and stormwater basins, ponds and retention ponds called RSAT. http://www.stormwatercenter.net/monitoring%20and%20assessment/rsat/smrc%20rsat.pdf The meeting was attended mostly by representatives of community groups and elected and appointed officials of Quantico Bay area who have fought long and hard to try to restore the Quantico Bay that suffers from excessive sedimentation and hydrilla, an invasive aquatic plant. http://web.ead.anl.gov/ecorisk/case/docs/FINAL_Quantico_Creek_Risk_Screening.pdf
Prince William County at the edge of the greater Washington DC metropolitan area, is part of the greater Chesapeake Bay watershed and has 10 major watersheds of its own covering 360 square miles that are in turn subdivided into 222 sub water sheds. The Quantico Watershed Study is the forth watershed study to be completed. The watershed studies evaluate CIP Planning and Storm Water Management Facilities, Regulatory Compliance with federal and state regulations including EPA, CWA, VA DCR and DEQ and the Army Corp of Engineers. The studies typically examine a small number of sub-watersheds to characterize the watershed as a whole. The Quantico study examined six sub-watersheds that included Quantico Creek, South Fork, Dewey’s Creek and Swans Creek. The elevations of the sub-watersheds range from sea level to 450 feet elevation and cover an area occupied by Prince William Forest Park at the northwestern most portion of the watershed to Quantico Marine Corp Base and Dumfries as well as several residential neighborhoods.
MACTEC found culverts impeding fish passage, supercharging the water flow resulting in the scrubbing the soil during rain events. There were mid channel bars caused by woody debris, lawn cuttings and trash. Utility corridors and low head dams where corridor encroachment had occurred due to the lack of a riparian buffer which has lead to severe bank erosion of 2-3 feet per year. Swans Creek has 30 foot cuts and the extreme erosion has almost buried the stream. Every time there is a storm event all the soil is mobilized. Depositing of eroded soil and sedimentation are filling in the estuary.
MACTEC recommends several steps to increase the stormwater retention volume and detention time to prevent further sedimentation in Quantico Bay. Legacy stormwater retention ponds were designed to discharge at a “2 year storm rate” unfortunately streams are shaped by 1 ½ year storm rates so that all the existing retention ponds are shaping our creeks and streams. So the most basic recommendation is that the on-site stormwater capture and retention needs to be beefed up to slow the discharge rate and capture the first flush pollutants. The older holding basins do not do that, but are nonetheless in compliance with the design standards that were in effect when they were built. Larger basins could slow the flow, provide wildlife habitat and increase groundwater infiltration and recharge.
Open channel recommendations were the other major area that MACTEC felt needs to be addressed. These include infrastructure repair, debris removal channel restoration or enhancements, riparian buffer restoration and finally preservation and monitoring of the enhancements. Overall, MACTEC identified 30 open channel problems and 16 stormwater basin maintenance problems in the 6 sub-watersheds and estimates that these improvements and repairs will cost $15.7 million. Stabilizing the stream beds to slow the catastrophic erosion rate of 2-3 feet per year to a more natural erosion rate in the area of Prince William Estates and Dewey’s Creek is the only way to ameliorate the rate of sedimentation in the Quantico Bay. The only way to restore the bay would be first to stabilize the up-stream stream beds and then dredge the bay. While a certain amount of erosion is entirely natural and is part of the natural cycle, development, pavement, destruction of the riparian buffers have resulted in extreme erosion.
Most of the repairs recommended by MACTEC are to address the open channel problems and will have to be maintained and monitored. These are the costs to address just one of the ten watershed basins in Prince William County Virginia. estimating total costs for the entire county from that number is hundreds of millions of dollars. In thinking about how to finance and maintain theses improvements, credit trading and sale of credits for the EPA mandated TMDL seems one source (beyond a direct surcharge tax on property) for funding these improvements. If these types of improvements could be quantified within the Chesapeake Bay Model modules for compliance with the TMDL targets there might be a way to fund some of these activities without resorting to command and control regulatory model so popular to the north of us. Unlike MS4 (municipal separate storm sewage system) and waste water treatment plant permits which have measured results that can be traded, these would have to be given “model credit” for open channel recommendations, on-site stormwater capture and detention etc. if they are to be traded, but like farm BMPs are probably low lying fruit. It might be possible to utilize the Scenario Builder within the Chesapeake Bay model to generate simulations to quantify watershed restoration.