Title: Design of Infiltration and Detention Basins
1Design of Infiltration and Detention Basins
2Infiltration and Detention Basins
- Description
- Appropriate Applications and Siting Criteria
- Water Quality Volume
- Factors Affecting Preliminary Design
- Workshop Exercises on BMP Design
3Description
Infiltration Basins
- An infiltration basin is a device designed to
remove pollutants from surface discharges by
capturing the Water Quality Volume (WQV) and
infiltrating it directly to the soil rather than
discharging it to receiving waters.
4Typical Infiltration Basin Layout
Infiltration Basins
PPDG Figure B-1, Page B-3
5Infiltration Basins
Infiltration Basins
6Siting Criteria
PPDG Table B-2, Page B-4
- gt 3 m to seasonally high water table
- Soil infiltration rate ? 1.3 centimeters per hour
- Infiltration Rate gt 6.4 cm/hr, contact RWQCB
- Clay content lt 30
- Silt and Clay combined lt 40
- Site should not be located in area containing
fractured rock within 3 meters of basin invert. - Infiltrated water is unlikely to affect the
stability of downgradient structures, slopes, or
embankments - Minimum WQV for basin must be gt 123 m3
7Pre-Screening for the Infiltration Basins
Infiltration Basins
PPDG Figure B-2, Page B-6
8Water Quality Volume
Infiltration Basins
- The Water Quality Volume (WQV) is the amount of
runoff that enters the BMP and must be treated
during the water quality storm event. Use Basin
Sizer or consult with District Hydraulics to
determine water quality storm depth. See PPDG
Page 2-16. - The WQV is the water quality storm depth x the
area tributary to the BMP x the weighted average
runoff coefficient for that area
9Minimum Footprint Area for Infiltration (PPDG
Page B-8)
- Aest 100SFWQV/(Kestt)
- Aest Estimated invert area (m2)
- SF Safety factor of 2.0
- WQV Water Quality Volume (m3)
- Kest Estimated Infiltration Rate (cm/h)
- T drawdown time, 48 hours
10Preliminary Design Factors
Infiltration Basins
- When designing a BMP, it is critical to remember
that the BMP must not negatively impact drainage
of the roadway. - Consult with District Hydraulics to ensure that
the design will not compromise roadway drainage. - It is Caltrans policy to minimize runoff from
pervious areas to Treatment BMPs.
11Preliminary Design Factors
Infiltration Basins
- Provide Maintenance access road around basin and
provide ramp to basin invert - Provide scour protection at inlet
- Use a factor of safety of two (2) to size the
invert area of the basin - Provide an Emergency/maintenance gravity drain,
if practical - Use 13 side slope ratios or flatter
- Provide minimum 0.3 meters of Freeboard
- Size to capture the Water Quality Volume, minimum
volume should be 123 m3
12Infiltration Basin Design Approach
- Determine if location meets siting requirements
listed in Appendix B. - Calculate tributary area.
- Determine Water Quality Volume.
- Calculate minimum footprint area using Equation
1, Page B-8. - Calculate actual footprint area accounting for
natural terrain, desired basin depth, side
slopes.
13Description
Detention Basins
- A detention basin temporarily detains runoff
under quiescent conditions, allowing sediment and
particulates to settle out before the runoff is
discharged. - A detention basin includes a water outlet
structure to provide controlled discharge from
the basin to a surface water.
14Typical Detention Basin
Detention Basins
PPDG Figure B-4, Page B-15
15Detention Basins
Water Quality Outlet
Emergency Outlet
Basin Inlet
16Appropriate Applications and Siting Criteria
Detention Basins
- If conditions are not suitable for an
infiltration basin, then a detention basin should
be considered. - Determine that water stored in the basin does not
cause an objectionable backwater condition in the
storm drain system. - Determine that seasonally high groundwater will
be no higher than the invert elevation of the
basin.
PPDG Table B-4, Page B-20
17Water Quality Volume
Detention Basins
- - Detention Basins are sized to capture the Water
Quality Volume. - - Detention Basin Volumes are determined in
exactly the same manner as Infiltration Basins. - Minimum Detention Basin volume is 123 cubic
meters.
18Preliminary Design Factors
Detention Basins
- The BMP must not negatively impact drainage of
the roadway. - Consult with Caltrans Hydraulics to ensure that
the design will not compromise roadway drainage. - It is Caltrans policy to minimize runoff from
pervious areas to Treatment BMPs.
19Preliminary Design Factors
Detention Basins
- Maximum water level should not cause groundwater
to occur under the roadway within 0.2 m (0.7 ft)
of the roadway subgrade - Provide a Maintenance access road around basin
and provide a ramp to basin invert
20Preliminary Design Factors
Detention Basins
- Provide an Upstream diversion if possible.
- Provide a downstream overflow structure / flood
control outlet (mandatory) - Discharge through a water quality outlet with
debris screen (or equivalent) - Flows should enter at low velocity. Use scour
protection on inflow, outfall and spillway if
necessary. - Use 13 side slope ratios or flatter
- Provide minimum 0.3 meters of Freeboard
21Basin Dimensions
Detention Basins
F Freeboard (0.3m minimum)
L
L (2Zy)
22- The minimum Detention Basin Length to Width (LW)
ratio is 21 (at WQV surface, not at invert).
23Workshop Exercise 1 (15 minutes)
- Required
- For an Infiltration basin, calculate WQV, minimum
footprint, overall dimensions. - Given
- Tributary Area 2.0 hectares (Cave 0.95)
- Water Quality Storm Depth 2.5 cm
- Infiltration Rate 5.0 cm per hour
- Assume
- Vertical Side Slopes
- Square Basin Shape
- Seasonal High Groundwater at 5.0 meters below
Original Grade (O.G.)
24Workshop Exercise 1 Calculations
Infiltration Basins
- Dimensions of Basin
- Step 1 Determine WQV
- WQV
- 2.0 hectares (10,000 m2 / hectare) 2.5 cm
(1 m/100 cm) 0.95 - WQV 475 cubic meters
25Minimum Footprint
Infiltration Basins
- Aest 100SFWQV/(Kestt)
- Aest 1002475 / (5.048)
- Aest 396 square meters
- For sake of calculations, use
- Area 400 square meters
26Basin Dimensions
Infiltration Basins
- WQV 475 cubic meters
- Minimum Area 400 square meters
- Depth WQV / Area 1.12 m
- Groundwater is at 5.0 m, minimum separation is
3.0 meters, therefore maximum depth must be less
than - 5.0 - 3.0 2.0 m
- Design Basin as 20 m x 20 m x 1.2 m deep, volume
480 cubic meters. Remember to add 0.3 meters
of freeboard. Total basin depth becomes 1.2 m
0.3 m 1.5 meters.
27Workshop Exercise 2 (15 minutes)
- Required
- For a Detention basin, calculate WQV, available
depth, overall dimensions. - Given
- Tributary Area 3.0 hectares (Cave 0.85)
- Water Quality Storm Depth 3.0 cm
- Assume
- Vertical Side Slopes
- Rectangular Basin Shape, Length 2 Width
- Groundwater at 4.5 meters below O.G.
28Workshop Exercise Calculations
Detention Basins
- Dimensions of Basin
- Step 1 Determine WQV
- WQV
- 3.0 hectares (10,000 m2 / hectare) 3.0 cm
(1 m/100 cm) 0.85 - WQV 765 cubic meters
29Calculate Available Depth
Detention Basins
- WQV 765 cubic meters
- Groundwater is at 4.5 m, minimum separation is
3.0 meters, therefore maximum depth must be less
than - 4.5 - 3.0 1.5 m
- Minimum freeboard is 0.3 meters.
- Use Detention Basin Depth 1.2 meters
30Basin Dimensions
Detention Basins
- WQV 765 cubic meters
- Initial Design Depth 1.2 m
- Area WQV / Depth 765/1.2 638 m2
- Minimum Flow Path Length / Width ratio is 21
(L/W). - Therefore, Area L W (2W W) 2W2
- 638 2W2, Therefore W2 319, and W 17.9
- Let W 18.0 m, L 36.0 m, Depth 1.20 m
- Then Volume 778 cubic meters
31Basin Dimensions
Detention Basins
F Freeboard (0.3m minimum)
L
L (2Zy)
32Emergency Overflow / Bypass
Overflow / Bypass
- All basins must be equipped with either an
upstream Bypass (off-line) or an Emergency
Overflow (on-line). Bypass structures are
preferred, since they prevent large flows from
scouring out the basins. - Most basins use weirs as the overflow/bypass
structure.
33Emergency Overflow / Bypass
Overflow / Bypass
- Broad Crested Weir Calculations
- Flow Over Weir Q (C)(L)(H1.5) rearrange
terms L (Q)/(C)(H1.5) - Q Design Storm (Q100 or Q25)
- C Weir coefficient
- L Length of weir (perpendicular to flow) Note
Minimum L is 1.0 meter - H is the difference between the basin elevation
at full depth minus the weir elevation (set at
water surface at WQV) - Note See HEC No. 22, Table 8-1 for C values
34Workshop Exercise 3 (15 minutes)
- Required
- For an Infiltration basin, calculate Emergency
Overflow weir length using a broad crested weir. - Given
- Q100 0.4 cubic meters per second
- H 0.3 meters
- (Note for this exercise, H Minimum freeboard)
- C 1.50
35Exercise 3 Solution
Overflow Weir
- Weir Calculations
- Weir Length L (Q)/(C)(H1.5)
- Solve for L (Q100)/ (C)(H1.5)
(0.4)/(1.50)(0.31.5) 1.63 meters (Use L
1.75 meters) - Since 1.75 m gt Minimum weir length of 1.0 meters,
use L 1.75 meters
36Detention Basin Water Quality Outlet Design
Detention Basins
- Refer to PPDG, Appendix B, Page B-18
37Detention Basin Water Quality Outlet Design
Detention Basins
PPDG Page B-17
38Water Quality Outlet DesignPage B-18 PPDG
Detention Basins
For metric units, single row of orifices a
(2106)(A)(H-Ho)0.5 / 3600CT(2g)0.5 Where
a area of drainage orifices (square mm) A
Surface area of the basin at mid elevation (sq.
meters) C orifice coefficient commonly 0.4 to
0.6 T Basin Drawdown Time (normally 40 to 48
hours) g Acceleration due to gravity (9.81
m/sec2) H Elevation of water surface at WQV
(m) Ho Elevation of basin invert (m)
39Water Quality Outlet Design
Detention Basins
For metric units, multiple rows of orifices at
(2106)(A)(hmax) / 3600CT(2g (hmax -
hcentroid))0.5 Where at total area of
drainage orifices (square mm) A Surface area
of the basin at mid elevation (sq. meters) C
orifice coefficient commonly 0.4 to 0.6 T
Basin Drawdown Time (normally 40 to 48 hours) g
Acceleration due to gravity (9.81 m/sec2) hmax
Height from lowest orifice to WQV elevation
(m) hcentroid Height from lowest orifice to
centroid of orifice configuration (m)
40Exercise 4 (15 minutes)
Detention Basins
Calculate orifice configuration (single row)
Find Total orifice area (a), Number of holes,
and orifice Diameter Given A 450 sq.
meters at mid elevation C 0.6 T 48 hours g
9.81 m/sec2 H 101.25 m Ho 100.00 m
41Exercise 4 (Outlet Design)Page B-18 PPDG
Detention Basins
For metric units, single row of orifices a
(2106)(A)(H-Ho)0.5 / 3600CT(2g)0.5 Where
a (2106)(450)(101.25-100.00)0.5
__________________________________________________
___________________ 36000.648(29.81)0.5
a 2,191 square mm Use three orifices, 120
degrees apart. Then, each orifice has an area of
a/3, so each orifice area 730 mm2 Therefore,
orifice diameter (4730)(3.14)0.5 30 mm
42Questions