Transcript Howard Chang

Detention Ponds & Bacterial
Concentrations
HOWARD CHANG
CE394K
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%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
200
87
342
90
201
12056
12069
12062
12068
12060
Average E. Coli concentrations in org/100mL
Decay Rate
12062 – 12060
12060 – 12056
0.144 /hr
0.017 /hr
Where to Place Detention Pond
342
90
87
<90
201
200
74
Assume linear increase in bacteria
concentration
58.1org / 100mL
12069 – 12068
mile
12068 – 12062
25.3org / 100mL
mile
Allowable conc = 126 org/100mL
http://www.epa.gov/waterscience/standards/wqsli
brary/tx/tx-wqs-20061215.pdf
12062
12069
12068
126
0
126
0
0.671.94
mi mi 2 mi5.16 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
3:1
6 feet
2:3
36 cfs
brings 126 to 22
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
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/100mL
 First pond 0.67 miles downstream of station 12069
 Second pond 1.17 miles downstream of station 12068
Questions