1. HOWARD CHANG CE394K SPRING 2008 Detention Ponds & Bacterial Concentrations

2. Introduction Study Area: Round Rock, TX Brushy Creek is impaired for bacteria

3. Objectives Bring Brushy Creek back into compliance Put in detention ponds where needed Hold water long enough for bacteria to die off http://www.stormwatercenter.net/Manual_Builder/design%20examples/pond/title%20figure.gif

4. Approach E. Coli data from TCEQ stations Travel time from NHD plus flow data Calculate decay rate Size detention pond

5. Analysis 87 200 342 201 90 Average E. Coli concentrations in org/100mL Decay Rate 12062 – 120600.144 /hr 12060 – 120560.017 /hr 12069 12068 12062 12060 12056 342

6. Where to Place Detention Pond 87 200 342 201 Assume linear increase in bacteria concentration 12069 – 12068 12068 – 12062 Allowable conc = 126 org/100mL http://www.epa.gov/waterscience/standards/wqsli brary/tx/tx-wqs-20061215.pdf 12069 12068 12062 12060 1.94 mi5.16 mi3.22 mi 126 0 74 90 <90 0 0.67 mi2 mi

7. Sizing the Detention Pond A decay rate of.144 takes 34 hours to reach 0 Not feasible: too long & needs large storage volume Parameters Detention time12 hrsbrings 126 to 22 Side slope3:1 Depth6 feet Width: Length2:3 Average flow36 cfsinflow = outflow

8. Sizing the Detention Pond Needed storage volume = 36 ac-ft Detention Pond dimensions BaseTop Lo604’L640’ Wo403’W439’ 6.5 ac

9. Readjust Detention Pond Location Next Steps Check locations Make sure it is suitable for placing detention pond Add safety factor 87 200 342 201 12069 12068 12062 12060 0.67 mi1.17 mi 126 22 96 22 112 <112

10. Summary Place two detention ponds  Storage volume of 36 ac-ft  Detention time of 12 hours  Reduces bacteria concentrations to 22 org/100mL First pond 0.67 miles downstream of station 12069 Second pond 1.17 miles downstream of station 12068

11. Questions