Howard%20Chang

1
Detention Ponds Bacterial Concentrations

• Howard Chang
• CE394K
• Spring 2008

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/des
ign20examples/pond/title20figure.gif
4
Approach
E. Coli data from TCEQ stations Travel time from
NHD plus flow data Calculate decay rate Size
detention pond
5
Analysis
200
342
342
87
90
201
12056
12062
12060
12069
12068
Average E. Coli concentrations in
org/100mL Decay Rate 12062 12060 0.144
/hr 12060 12056 0.017 /hr
6
Where to Place Detention Pond
Assume linear increase in bacteria
concentration 12069 12068 12068
12062 Allowable conc 126 org/100mL http//www.
epa.gov/waterscience/standards/wqslibrary/tx/tx-wq
s-20061215.pdf
342
90
87
201
lt90
200
74
12062
12069
12060
12068
126
0
126
0
1.94 mi
5.16 mi
3.22 mi
0.67 mi
2 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 time 12 hrs brings 126 to 22
• Side slope 31
• Depth 6 feet
• Width Length 23
• Average flow 36 cfs inflow outflow

8
Sizing the Detention Pond

• Needed storage volume 36 ac-ft
• Detention Pond dimensions
• Base Top
• Lo 604 L 640
• Wo 403 W 439

6.5 ac
9
Readjust Detention Pond Location

• Next Steps
• Check locations
• Make sure it is suitable for placing detention
  pond
• Add safety factor

342
112
87
201
lt112
200
96
12062
12069
12060
12068
126
22
126
22
0.67 mi
1.17 mi
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