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We also understand that the storm water pipe system is woefully inadequate and incoherent, and that little has been done to mitigate this problem. An Arlington Department of Environmental Services map from 2014 (included in this letter, below) provides a couple examples. Two pipes totaling 11 feet wide connect to a pipe less than 3 feet wide under the Westover parking lot. A similar mismatch appears in Parkhurst Park. Residents should be able to assume that Arlington County is taking care of critical infrastructure. Clearly this is not the case in our watershed.

 

There is an error in the labeling of the pipes on the map in the storm sewer capacity study map.  The map that is referenced is here: https://projects.arlingtonva.us/wp-content/uploads/sites/31/2014/02/DES-Storm-Sewer-Capacity-Westover-Branch-Watershed-Map.pdf .

This map incorrectly labels 7 foot by 10 foot box culverts as being less than 36” in diameter.  The 7-ft diameter pipe and 4-ft diameter pipe join at a manhole and enter a 10’ wide by 7’ tall box culvert. The map has incorrectly labeled the box culvert as a pipe less than 36” in diameter. Parkhurst Park is similar – the 5-ft diameter pipe and 65” x 40” arch pipe connect to a 7-ft diameter pipe. The map incorrectly labels the pipe as less than 36” in diameter.

 

The image below shows the pipes as described in the County’s Geographic Information System, and shows it correctly labeled as a box culvert.   We apologize for this error in the study document.

 

 

Responses to Torreyson Run Questions

The Lee Highway commercial strip is at our headwaters. What can we do to encourage landlords to increase permeability? The Rivendell School on Lee Highway is now rebuilding and expanding. Is there a robust storm water management plan place?

 

The StormwaterWise Landscapes program, which provides incentives for adding stormwater management practices, is open to businesses and HOAs, as well as residents.

 

The Rivendell school will include several stormwater management facilities.  The site plans include 5 stormwater planters to manage runoff from the building, as well as an area of artificial grass that will be constructed as permeable pavement (with gravel storage layers below the artificial grass).

 

Construction firms are covering entire lots with single-family spec houses. One recent example is at the intake of a storm water pipe on Madison and 21st. There must be some creative way, even with the Dillon Rule, to deter a trend that is harming so many in the community.

 

In 2014, Virginia updated the Stormwater Management requirements.   As part of this update, local governments could choose whether to require stormwater management for projects less than 1 acre in size.  Arlington County chose to regulate these smaller projects, which includes single-family home redevelopment projects, because SFHs are the largest source of recent increases in impervious cover in Arlington County.  Since 2014, Arlington County has required stormwater management facilities for new single-family homes and large SFH additions.  The designer/owner can choose the type of facility, such as stormwater planters, permeable driveway, green roof, rain garden, etc., as long as it meets the requirements for the site.   The homeowners must also submit information annually to certify the function of the SWM facility.

 

County staff are currently reviewing the Stormwater management requirements for redevelopment projects to potentially require more stormwater detention and increase protection for neighboring properties.

 

 

The “Mosquito Park” is county park land bound by Lexington, 20th Rd, 21st St., and 22nd St. Would installing a giant storm water detention vault underground, connected to the existing storm water system be a part of the solution?

 

This area is in the center of the map below, circled in red.   There is currently a sanitary sewer and storm sewer line running through this area of County land, as indicated by the green and brown lines on the map.  It may be possible to add a detention vault in this location, although the project would need a feasibility analysis.

 

 

 

Parkhurst Park’s playground is a hardscape covered with rubber, and it is a convergence site in the watershed. The rain garden system was designed to absorb runoff from the playground only. Following construction, adjacent houses immediately witnessed new flooding that has become worse. Can the county build additional rain gardens in the vicinity of the park?

 

County staff are creating an inter-departmental working group to examine how we can work across departments to increase the County’s flood resiliency.  One topic that this group will examine is can we better use our public spaces and park areas for stormwater storage.  The County currently reviews Neighborhood Conservation projects and Transportation projects for opportunities to add stormwater management, such as green street projects.

 

APS will build Reed School over the next two years. Is there more that can be done to minimize impermeable surfaces? Can detention vaults be installed?

 

The current design selected for the Reed school has the lowest loss of open / permeable space of the designs considered (building up instead of out). The Stormwater management planned for this site includes a swale, 4 stormwater planters, 3 rain gardens, and artificial turf with permeable pavement underneath.

 

Current stormwater management requirements mandate that project designers treat runoff from the new impervious areas on the site, but not stormwater from off-site areas.  Although construction on the school has started, the County is working with the APS project managers and Bowman Consulting to review whether the capacity of the planned stormwater management facilities can be increased.  This feasibility work is currently ongoing, and staff can provide an additional update on the results of the analysis to the community in the near future.  The County is also exploring the possibility of adding stormwater vaults under the parking lot following the school construction.

 

A potential future opportunity for review was identified to explore additional underground storage pipe in the right of way of McKinley Street adjacent to the school.  Parkhurst park and other locations upstream will be reviewed for feasibility to provide detention that could assist to mitigate the downstream flooding.

 

https://www.apsva.us/wp-content/uploads/2018/11/181022-REED-E2C2-Final.pdf (page 18)

 

What is current impervious cover in the Torreyson Run watershed and how much as it increased in the past 10 years?

 

Impervious cover in this watershed has increased from 42.3% to 44% from 2007 to 2017.