Monday, January 30, 2012

Low Impact Development and Oaks III Project


The Oaks III project was approved by the Prince William County Board of Supervisors after a public hearing on January 10th 2012. This proposal to rezone almost 18 acres of land adjacent to the Oaks II development and near the Town of Occoquan is an example of how Prince William County is attempting to continue to grow under the demands of the Total Maximum Daily Load (TMDL) mandated by the EPA. Prince William County finds itself along with a large portion of Virginia, Maryland, Washington DC and portions of several other states needing to reduce the amount of run off and better manage existing storm water to meet the goals of the Chesapeake Bay pollution diet, the TMDL mandated by the EPA.

Excessively high levels of nitrogen, phosphorus and sediment in the Chesapeake Bay cause algae blooms that consume oxygen and create “dead zones” where fish and shellfish cannot survive, block sunlight that is needed for underwater Bay grasses, and smother aquatic life on the bottom. The result is fish kills and murky water that threaten the fishing and shellfish industries and recreational use of the bay. The high levels of nitrogen, phosphorus and sediment enter the water from a variety of sources, including agricultural operations, urban and suburban runoff, wastewater treatment facilities, septic systems, air pollution, and minor contribution from natural processes. However, the largest share of nutrient and sediment pollution results from man: suburban development, cars and roadways, agricultural activities to feed man and human and animal waste.

The TMDL sets a total Chesapeake Bay watershed limit for the six states and Washington DC of 185.9 million pounds of nitrogen, 12.5 million pounds of phosphorus and 6.45 billion pounds of sediment per year which is a 25% reduction in nitrogen, 24% reduction in phosphorus and 20 % reduction in sediment from the current levels. The pollution limits are then partitioned to the various states and river basins based on the Chesapeake Bay computer modeling tools and monitoring data. The TMDL addresses only pollution from excess nitrogen, phosphorus and sediment and does not address toxic, carcinogenic or endocrine disruptors that may be present in the Watershed.

The Virginia Watershed Implementation Plans (WIP) Phase I and II lay out a series of pollution control measures called best management practices, BMPs that need to be put in place by 2025, with 60% of the BMPs completed by 2017. While it will take years after 2025 for the Bay and its tributaries to fully heal, EPA expects that once the required BMPs are in place there will be gradual and continued improvement in water quality as the BMPs reduce the nutrient and sediment run off and better control storm water so that the Chesapeake Bay ecosystem can heal itself.

About 37% of Prince William County is served by the HL Mooney Waste Water Treatment Plant which after its recent expansion and upgrade is state of the art in waste water treatment with monthly discharge averages are less than 0.1 for phosphorus, TSS 1 mg/l, BOD non- detect and nitrogen is currently 3 mg/l. Nonetheless, Prince William County needs to reduce the amount of nitrogen, phosphorus and sediment released to the Chesapeake Bay each year to meet the demands of the Virginia Watershed Implementation Plan because the cost to upgrade every waste water treatment plant and every municipal storm sewer system in the state was estimated at three times the cost of implementing best management practices throughout the state.

So, Prince William County finds itself needing to reduce existing runoff from the remaining agricultural operations within the county, urban runoff from towns, suburban runoff, septic systems, and air pollution (if possible) and still be a vibrant community. The Oaks III is the first example of the steps that Prince William County is taking to implement low impact development, LID, features with new growth and use the opportunity of new development to implement BMPs on older projects. LID is the latest catch phase in ecologically friendly site development and consists of five elements: preserving open space and minimizing land disturbance; protecting natural drainage ways, soils and sensitive areas; incorporating natural site elements like wetlands, stream corridors, and woodlands as site features; reducing the size of traditional infrastructure; and decentralize and manage storm water at its source.

The almost 18 acre parcel will be divided into four areas, the largest, 13.6 acres, will be a conservation area. Though public access to the area was not outlined in the proposal, this will serve to preserve open space and limit land disturbance on site. The reduced size of the development planned for the site will result in 8.5% of the site to be covered with impervious surfaces (roads, buildings, parking lots and sidewalks). The developer intends to use LID techniques to manage storm water and runoff on-site including methods to slow storm water flow rates. Instead designing the storm water management system so that it rapidly drains the site, low-impact development relies on design tools and control practices to preserve the natural hydrologic functions of the site. The specifics of the design will be addressed during site plan review in consultation with the developer’s engineer. In addition, the developer will use BMPs to restore 400 feet of the stream channel of the existing on-site intermittent stream. Then, the developer will be required to go back and improve the storm water management on the Oaks II development by installing a new stilling basin in the conservation area where the Oaks II storm water outfall is located. This is planned to slow the storm water flow during large storm events to allow water to infiltrate the soil. The Department of Watershed Management will approve the design to make sure that these BMPs improve the existing storm water management and generate “credit” under the TMDL.

Together the development of Oaks III should result in additional commercial space, additional housing and a reduction in storm water peak flow by using on-site infiltration, on-site bio-retention ponds, grass swales, rain water cisterns and French drains-all tools in the LID technique to mimic natural drainage through distributed control of storm water throughout the entire site. The challenging soils and slope at the Oaks III project should test the effectiveness of implementing these strategies.

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