The project to detain and filter storm water falling in the 50-acre watershed that drains most of downtown Crozet is expected to be complete by July. The project on the south end of downtown, begun in December, is converting a triangular section of bottomland along Powell’s Creek into a six-cell wetland that will hold storm water and allow sediments to sift out into terraced pools before it reaches the creek. The shallow pools, just six to twelve inches deep, will be planted, as will the low berms that divide them. Vegetation will filter the runoff and by the time water does make its way into the creek it will be substantially cleaned.
The project is necessary to enable development in downtown, according to county water resources manager Greg Harper. He said the need came to light when county staffers started looking at drainage for the Crozet Avenue Streetscape project and the new library. Crozet citizens had also been saying that, given the legacy of the parcels and buildings in downtown and their divergence from county storm water regulations, a common solution was required before development was likely to proceed. Without the new wetlands property, owners could have had to bury large storm water holding tanks to catch runoff from their buildings and on many parcels no locations for tanks exist. Thus new businesses headed for Rt. 250 and Old Trail.
Harper said the staff will soon propose a service district for the watershed to county supervisors. Land inside a boundary that follows parcel lines would be eligible to buy into the regional system. Otherwise property owners would have to meet storm water requirements onsite with an internal plan. Fees for the service district would be based on the amount of impervious surface on the parcel, Harper said. The idea would be that the fee to join would be less than the cost of not joining. He posed the question, what would it cost to comply be if the wetlands didn’t exist? He said fees might also be made mandatory.
The $1.35 million project was paid for with $854,000 in county funds, a $74,000 proffer connected to Grayrock, $107,000 from the Rivanna River Basin Commission and $315,000 from the state’s Department of Environmental Quality.
Why should fees be charged if the public has already paid for the project? Harper said so that developers who come into the district don’t, in effect, get subsidies while developers outside the district still have the expense. The county doesn’t want to seem to favor particular developers, he said. The fee saves the developer the cost of hiring an engineer, he said. Harper said the supervisors will have to sort through the issues of charges and incentives.
The system is designed to accommodate the build-out of rooftops and roads foreseen in the Crozet Master Plan, Harper said. The system will handle a once-in-10-years type of rainfall, one that drops about 5.6 inches of rain in 24 hours and results in a flow of about 160 cubic feet per second.
Storm water will emerge from two large pipes just south of Jarmans Gap Road and follow a 20-foot-wide channel, its flow broken up periodically by rock weirs, like rapids, and then enter a one-third acre pond. From there it begins its slow cascade through 1.5 acres of terraced wetlands. The pond also has an emergency spillway. Harper said that given our typical storm events, the spillway may come into play a few times a year, but normally the pond will capture the runoff of a storm. Two footbridges and a plank bridge capable of handling a vehicle will allow property owners access to parts of their property cut off by the channel.
About 13,500 cubic yards of soil were removed to make the cells, or between 800 and 900 truckloads. The top cells will be further excavated before the project is finished. The county will leave a small access road into the site and Harper expects that in 20 years sediment will need to be cleaned out of the cells. Harper said the plantings “will the typical vegetation you see along a creek, a mix of natives depending on where you are in the wetlands and channel.”
Meanwhile the wetland will be open to the public to walk around in. Birdwatchers especially should enjoy it, Harper said. Greenway plans call for a trail along Powell’s Creek someday, that the wetlands would be a link in.
Engineering for the project was done by Kimley-Horn and Associates, Inc. and the contractor was Synchronized Construction Services, Inc. out of Orange.