29Jun99 COA

Report
Sizing Stormwater Control Facilities to
Address Stream-Bank Erosion Control
Anthony M. Dubin, PE
Brown and Caldwell
[email protected]
Major Topics to Address
1. Why Hydrograph Modification
Management is important
2. How Low Impact Development controls
work
3. Describing the technical analysis that
generated the set of pre-sized IMPs
Effects of Urbanization
• Impervious surfaces produce higher runoff rates,
volume and duration of large flows
Effects within the Watershed
•
•
Urbanization alters the watershed
Channels respond with incision and/or armoring
Continuous Hydrologic Modeling Examines
Full Range of Local Conditions
 Sizing to one ‘design storm’ is not enough
Peak Flow Frequency
1.20
Impervious
Pre-Project Site
1.00
0.5Q2
Peak Flow (cfs)
0.80
0.60
0.40
0.20
0.00
0
1
2
3
4
5
6
7
8
9
10
Recurrence Interval (years)
 Identify all HSPF storms in record and rank
Flow Durations
1.00
Impervious
Pre-Project Site
Q10
0.1Q2
0.90
0.80
Flow (cfs)
0.70
0.60
0.50
0.40
0.30
0.20
0.10
0.00
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
% Time Exceeded
 Rank hourly outputs from HSPF model
0.40
Example IMP: In-Ground Planter
18-in sandy loam
Peak Flow Matching Example
1.20
Impervious
Mitigated Post-Project Site
Pre-Project Site
0.5Q2
1.00
IMP Reduces Impervious Runoff
to Less Than Pre-Project Levels
Peak Flow (cfs)
0.80
0.60
0.40
0.20
0.00
0
1
2
3
4
5
6
Recurrence Interval (years)
7
8
9
10
Duration Matching Example
1.00
Impervious
Mitigated Post-Project Site
Pre-Project Site
Q10
0.1Q2
0.90
0.80
Flow (cfs)
0.70
0.60
0.50
IMP Reduces Impervious Runoff
to Less Than Pre-Project Levels
0.40
0.30
0.20
0.10
0.00
0.00
0.05
0.10
0.15
0.20
0.25
% Time Exceeded
0.30
0.35
0.40
BMP Sizing Factor Summary
Under-Drain or Infiltration:
IMP
Sizing Factors
Infiltration Only:
IMP
Sizing Factors
In-Ground Planter Group A: 0.08
Group B: 0.11
Group C: 0.06
Group D: 0.05
Dry Well
Group A:
0.05 to 0.06
Group B:
0.06 to 0.09
Flow-Through
Planter
Group C: 0.06
Group D: 0.05
Infiltration
Trench
Vegetated/
Grassy Swale
Group A: 0.10 to 0.14
Group B: 0.14 to 0.21
Group C: 0.10 to 0.15
Group D: 0.07 to 0.12
Group A:
0.05 to 0.06
Group B:
0.07 to 0.10
Bioretention Basin Group A: 0.13
Group B: 0.15
Group C: 0.08
Group D: 0.06
Infiltration Basin Group A:
0.05 to 0.10
Group B:
0.06 to 0.16
Adjusting IMP Sizing to Account for
Rainfall Variability
0.14
0.12
Sizing Factor
0.10
0.08
0.06
Group A soils
Group B soils
Group C soils
Group D soils
0.04
Group A, y = 0.0020x + 0.08
Group B, y = -0.0005x + 0.11
Group C, y = -0.0022x + 0.06
Group D, y = -0.0022x + 0.05
0.02
0.00
-14
-12
-10
-8
-6
-4
-2
0
2
Mean Annual Rainfall (MAP) Relative to Martinez Gauge (in)
4
6
Sizing Conclusions for Implementation
• IMPs in Group D soil sites are generally
smaller than Group A soil BMPs
• Steep side walls produce smaller sizing
factors
• Sizing factor may be particularly
important for on-site BMPs
• Swales and Bioretention basin footprint
may be less important if BMPs fit into
otherwise undeveloped space
Questions?
Contra Costa Approach to Hydrograph
Modification
• Encourage LID to
control
stormwater flows
• HMP is technically
rigorous and easy
to apply
• Assumes need to
match pre-project
condition
BMP Gallery
BMP Gallery
Instructions for Computing Local Sizing
Factors
1. Describe each DMA on the project site,
including area, soil type, post-project surface
type
2. For DMA's draining to IMPs, select an IMP and
configuration (e.g. swale width, dry well depth)
3. Pick the appropriate sizing factor from the
summary sizing factor table (see handout)
4. Compute the rainfall adjustment using the
regression equations (see handout)
5. Local sizing factor = Sizing Factor x RainAdj

similar documents