Urban Stormwater Retrofit Friendship Park – Winchester, VA

K. Choi, K. Davis, and D. Laird Biological Systems Engineering, Virginia Tech

Introduction

Exposed and shallow bedrock limit excavation, and known karst conditions in the region require special design considerations. Therefore, the design includes plastic liner for each pool to prevent problems associated with karst, such as sinkholes, on the site.

Expected Removal Rates

Acknowledgements This project is a partnership with state and federal agencies and local watershed groups. Funding comes from the Chesapeake Bay Targeted Watershed Grants Program administered by the National Fish and Wildlife Foundation in cooperation with the Chesapeake Bay Program and the CSREES Mid-Atlantic Regional Water Quality Project.

Special thanks to Dr. W. C. Hession, Dr. David Sample, Andrea Ludwig, Jim Lawrence, and Natural Resources Advisory Board of Winchester, VA

Similarly designed natural areas have achieved high levels of pollutant removal. Expected sediment removal rates range from 45 - 95%. phosphorous removal rates can range from 15 - 75%, and nitrogen removal rates can exceed 45% after implementation of similar practices.

 

Runoff from urban areas can carry many pollutants. Nitrogen and phosphorous from fertilized lawns can cause excessive algae growth and deplete oxygen available for stream organisms. The stormwater can also carry sediment and bacteria, which has caused Opequon Creek to be listed as an impaired water in Clean Water Act Section 303(d). Retrofitting the existing stormwater structures at Friendship Park can improve water quality and provide aesthetic and recreational benefits for the local community.

ACKNOWLEDGEMENTS

Site Constraints

Retrofit Site

Final wetland and water quality swale design from AutoCAD.

Currently, the 0.68 (ha) dry detention basin (left) and the 104 (m) long rip rap channel (right) at Friendship Park provide minimal treatment of pollutants as they are carried to Ash Hollow Run and Abrams Creek, tributaries to Opequon Creek. The implementation of this retrofit wetland and water quality swale design will increase retention time, allowing for sediment deposition, phosphorous reduction, and denitrification.

Community Outreach

•The team created a pamphlet to educate local residence about the project

•The team presented at 2 meetings of Winchester’s Natural Resources Advisory Board – where decision makers gave valuable feedback

• The team created a design specification sheet for informed stakeholders

 

Design Elements

Cross-section A-A: Water quality swale design profile.

Cross-section B-B: deep pool design profile (left); D: berm design (right).

B

B

A

A

C

A. Water quality swale in place of existing channel to convey water and encourage water infiltration

B. Deep receiving pool to slow the velocity of water entering the dry pond area and allow sediment to settle

C,E. Micro-pools to encourage microbial processes for nutrient removal

D. Vegetated berm to lengthen water flow path and spread stormwater throughout the area for enhanced treatment

F. Lengthened flow path between pools to encourage flow and prevent water stagnation

Hydrologic Analysis

The hydrologic analysis was completed using the modeling software WinTR-55. The peak runoff rates, volume, stage, and hydrograph were calculated for the 2-yr and 10-yr, 24-hr design storms.

2-yr, 24-hr 10-yr, 24-hr

Peak Flow (m3/s) 0.86 (30.4 ft3/s) 1.00 (35.3 ft3/s)

Storage Volume (m3)

2500 (88300 ft3) 8800 (311000 ft3)

Stage (m) 1.46 (4.8 ft) 2.85 (9.4 ft)

Analysis Parameters:

Drainage Area: 34.3 ha (84.6 ac)Frederick Soils (HSG C)Length of Channel: 104 m (340 ft)Site Area: 0.68 ha (1.68 ac)Building Area: 4.4 ha (10.0 ac)Roads Area: 6.0 ha (15.8 ac)Total Impervious Area: 10.4 ha (25.8 ac)Watershed Imperviousness: 30.4% CN = 80Tc = 0.56 hours

* All units in meters

WinTR-55 output hydrograph (left) and table (right).

Flow paths used for time of concentration.

D

E

F