Detention Ponds Bacterial Concentrations HOWARD CHANG CE 394

Detention Ponds & Bacterial Concentrations HOWARD CHANG CE 394 K SPRING 2008

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

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%20 examples/pond/title%20 figure. gif

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

Analysis 200 87 342 90 201 12056 12069 12062 12068 12060 Average E. Coli concentrations in org/100 m. L Decay Rate 12062 – 12060 – 12056 0. 144 /hr 0. 017 /hr

Where to Place Detention Pond 342 90 87 <90 201 Assume linear increase in bacteria concentration 12069 – 12068 200 74 12068 – 12062 Allowable conc = 126 org/100 m. L http: //www. epa. gov/waterscience/standards/wqsli brary/tx/tx-wqs-20061215. pdf 12062 12069 12068 126 0 0. 671. 94 mi mi 2 mi 5. 16 mi 3. 22 mi 12060

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 time Side slope Depth Width: Length Average flow 12 hrs brings 126 to 22 3: 1 6 feet 2: 3 36 cfs inflow = outflow

Sizing the Detention Pond Needed storage volume = 36 ac-ft Detention Pond dimensions Base Top Lo 604’ L 640’ 6. 5 ac Wo 403’ W 439’

Readjust Detention Pond Location 342 112 87 <112 201 200 96 12062 12069 12068 126 22 0. 67 mi 1. 17 mi 12060 Next Steps Check locations Make sure it is suitable for placing detention pond Add safety factor

Summary Place two detention ponds Storage volume of 36 ac-ft Detention time of 12 hours Reduces bacteria concentrations to 22 org/100 m. L First pond 0. 67 miles downstream of station 12069 Second pond 1. 17 miles downstream of station 12068

Questions