HOWARD CHANG

CE394KSPRING 2008

Detention Ponds & Bacterial Concentrations

Introduction

Study Area: Round Rock, TXBrushy 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%20examples/pond/title%20figure.gif

Approach

E. Coli data from TCEQ stations

Travel time from NHD plus flow data

Calculate decay rate

Size detention pond

Analysis

87200 34

2 201

90

Average E. Coli concentrations in org/100mL

Decay Rate12062 – 12060 0.144 /hr12060 – 12056 0.017 /hr

12069

12068

12062

12060

12056

342

Where to Place Detention Pond

87

200

342 20

1

Assume linear increase in bacteria concentration

12069 – 12068

12068 – 12062

Allowable conc = 126 org/100mLhttp://www.epa.gov/waterscience/standards/wqslibrary/tx/tx-wqs-20061215.pdf

12069 1206

8

12062 1206

0

mile

mLorg 100/1.58

mile

mLorg 100/3.25

1.94 mi

5.16 mi

3.22 mi

126 12

6

0

74

90<90

0

0.67 mi

2 mi

Sizing the Detention Pond

A decay rate of .144 takes 34 hours to reach 0Not feasible: too long & needs large storage

volume

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

Sizing the Detention Pond

Needed storage volume = 36 ac-ftDetention Pond dimensionsBase Top

Lo 604’ L 640’Wo 403’ W 439’

6.5 ac

Readjust Detention Pond Location

Next Steps Check locations Make sure it is suitable for

placing detention pond Add safety factor

87

200

342 20

1

12069 1206

8

12062 1206

0

0.67 mi

1.17 mi

126 12

6

22

96

22

112 <11

2

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

Questions