This document provides an overview and agenda for a training module on extended dry detention basins and infiltration practices. The first lecture reviews watershed planning concepts from Module 1 and introduces extended dry detention basins. The second lecture covers the design of extended dry detention basins through a design example and activity. The third lecture discusses infiltration practices including infiltration basins, trenches, and porous pavement. The training aims to explain structural best management practices for treating stormwater runoff.

1. BMP Training Module 4
Extended Dry Detention Basin
and Infiltration Practices
Sponsored by: MARC
Presenters:
Andy Sauer, P.E. (CDM)
Brenda Macke, P.E. (CDM)
February 20, 2009

2. Agenda

Lecture 1: Review of Module 1

Review Module 1 and WQv definition

Overview of Extended Dry Detention Basin (EDDB)

10-Minute Break

Lecture 2: Extended Dry Detention Basin (EDDB)




Design Example
Design Activity
10-Minute Break
Lecture 3: Infiltration BMPs

Infiltration Basins

Infiltration Trenches

Porous Pavement

3. Lecture 1 Overview

Review watershed planning and BMP value rating process
(Module 1)

Overview of extended dry detention basins (EDDB)

4. Best Management Practice
(BMP)



Best – State of the Practice
 No definitive answer
 Past experience, testing, research,
 Unique to site
Management – Responsible Parties

Improve water quality, meet NPDES Phase II
 Jurisdictional specific

Meet specific requirements of a regional
Practice – Action or Implementation

Practice = defined to carry out, apply, or to
do or perform often.

5. Basic BMP Principles

Plan for stormwater management




Mimic natural hydrology



Sustainable and “be green”
Provide a level of service
Improve water quality
Increase initial abstraction
Promote infiltration, retention & ET
“Treat” the stormwater runoff


Natural processes
Treatment trains

6. BMP Evaluation Process
PLAN
MIMIC
TREAT
Extended detention
(40 hours) to
increase treatment
and decrease peak
flows

7. TREAT
Detention and Treatment

Structural BMPs
detain runoff

Extended Detention
Basins
• Wet
• Dry



Extended Detention
Wetlands
Infiltration basins
Typically used as
larger, centralized
facilities

8. TREAT
Example site
l
ne
n
ha
C
in
Ma
Design Documents
e
i dg
Br
– APWA 5600
– BMP Manual
– Watershed
Master Plans
Grass Swale
Streambank
Biostabilization
Culvert
Roa
d
w ay
Commercial
Building
BioFilters
Dry
Detention

9. Structural BMP Consideration









Pollutant removal efficiency
Water quality volume
Site suitability
Tributary area
Dimensions (depth, length-width ratio)
Outlet
Emergency spillway
Maintenance easement
Routine and non-routine maintenance

10. an
Qu
er
er
Aesthetics/Amenity
y
tit
W
at
at
W
Qu
ali
ty
BMP Evaluation
General Rule

11. BMP Manual
DRAFT – In Progress

12. BMP Manual
Level of Service

Reduce Volume
• Infiltration
• Evapotranspiration (ET)

Remove total suspended solids (TSS)
• Settling

Temperature Reduction
• Urban heat island

Remove oils and Floatables
• Screening and netting

13. Value Rating System –
Based on BMP Goals
Condensed Table 5
Value Ratings
BMP
Vegetation
Rain Garden
Infiltration Practices
Bioretention
Pervious or Porous Pavement
Extended Detention Wetland
Media Filtration Practices
Wetland Swale
Bio-Swale
Extended Wet Detention
Native Vegetation Swale
Extended Dry Detention Basin
Turf Grass Swale

Median
Expected
Effluent
EMC TSS
Water
Quality
Value
Volume
Reduction
Temperature
Reduction
Oils/Floatables
Reduction
Overall
Value
N/A
< 10
< 10
< 10
10 - 20
< 10
< 10
10 - 20
10 - 20
10 - 20
10 - 20
20 - 50
10 - 20
5.25
4
4
4
3
4
4
3
3
3
3
2
3
2
2
2
1.5
1.5
2
0
1.5
1.5
2
1
1
0
1
1
1
1
1
0
0
0
0
-1
0
0
0
1
2
2
2
2
1
2
2
2
1
0
1
0
9.25
9.0
9.0
8.5
7.5
7.0
6.0
6.5
6.5
5.0
4.0
4.0
3.0
BMP value table is based on the 4 goals of
BMPs

14. Post Development BMP
Selection
Value Ratings
BMP
Vegetation
Rain Garden
Infiltration Practices
Bioretention
Pervious or Porous Pavement
Extended Detention Wetland
Media Filtration Practices
Wetland Swale
Bio-Swale
Extended Wet Detention
Native Vegetation Swale
Extended Dry Detention Basin
Turf Grass Swale
Median
Expected
Effluent
EMC TSS
N/A
< 10
< 10
< 10
10 - 20
< 10
< 10
10 - 20
10 - 20
10 - 20
10 - 20
20 - 50
10 - 20
Water
Quality
Value
5.25
4
4
4
3
4
4
3
3
3
3
2
3
Volume
Reduction
2
2
2
1.5
1.5
2
0
1.5
1.5
2
1
1
0
Temperature
Reduction
1
1
1
1
1
0
0
0
0
-1
0
0
0
Oils/Floatables
Reduction
1
2
2
2
2
1
2
2
2
1
0
1
0
Overall
Value
9.25
9.0
9.0
8.5
7.5
7.0
6.0
6.5
6.5
5.0
4.0
4.0
3.0

15. BMP Selection Flowchart
Level Of Service
BMP Value Rating
Water Quality
Volume/sizing
Placement,
maintenance

16. Water Quality Volume (WQv)

Water Quality Volume
(WQv): The storage needed
to capture and treat 90% of
the average annual storm
runoff volume

Water Quality Storm: The
storm event that produces ≤
90% volume of all daily
storms in a year

Extended dry detention
basin design and infiltration
system design is based on
the WQv
WQv

17. Kansas City Water Quality
Storm
Young and McEnroe
(http://kcmetro.apwa.net)
Daily Precipitation (in)
2.
7
2.
5
2.
3
1.
9
2.
1
1.
5
1.
7
1.
1
1.
3
0.
5
0.
7
0.
9
45
40
35
30
25
20
15
10
5
0
0.
1
0.
3
Water Quality
Storm = 1.37 in
# of days > or=
2003 Kansas City Precip events

18. Why Use the WQv to size
BMP?

Retain runoff long enough to get
water quality benefits



Infiltrate
Maintain vegetation
Reducing erosive flows from
smaller runoff events

Less applicable

19. Water Quality Volume
Calculation

Two methods

Short-Cut Method
•
•

Sites < 10 acres
Only 1 predominant cover type
Small Storm Hydrology Method
•
Larger or more heterogeneous drainage
areas

20. WQv Short-cut Example

Given

Tributary area (ATributary) = 2.5 acres

%impervious = 80%
WQv = 1.37in * [0.05 + (0.009 * 80%)] = 1.06 in

Multiply by ATributary to get volume
1.06 * 1ft/12in * 2.5 acres = 0.22 ac-ft

If only 50% impervious WQv = 0.14 ac-ft

21. WQv Calculation

Small Storm Hydrology Method
WQv = P*Weighted Rv



Weighted Rv = Σ(Rvi*Aci)/Total area (ac)
Rvi = Volumetric runoff coefficient for
impervious cover type (table)
Aci = Area of impervious cover type i (ac)

22. Rv Table
BMP MANUAL SECTION 6, TABLE 5
VOLUMETRIC COEFFICIENTS FOR URBAN RUNOFF FOR
DIRECTLY CONNECTED IMPERVIOUS AREAS
(CLAYTOR AND SCHUELER 1996)
Flat roofs and
Rainfall large unpaved
(inches)
parking lots
Pitched roofs and
large impervious
areas
(large parking lots)
Small
impervious
areas and
narrow
streets
Silty
soils
HSG-B
Clayey
soils HSGC and D
0.75
0.82
0.97
0.66
0.11
0.20
1.00
0.84
0.97
0.70
0.11
0.21
1.25
0.86
0.98
0.74
0.13
0.22
1.37
0.87
0.98
0.75
0.14
0.23
1.50
0.88
0.99
0.77
0.15
0.24
Note: a reduction factor may be applied to the Rv values for disconnected
surfaces, consult the BMP manual hydrology section

23. WQv Small Storm Example

Given: ATributary = 26 ac
Cover Type
0.87
1.6
Parking lots
0.98
8.8
Narrow streets
0.75
3.3
Silty soil

Area (acres)
Flat roofs
WQv = ∑
Rv
0.14
12.3
( 0.87 ×1.6 + 0.98 × 8.8 + 0.75 × 3.3 + 0.14 ×12.3) ×1.37 = 0.749in
Rvi × Aci
×P =
Total Area
26
Multiply by ATributary to get volume

24. Overview of Extended Dry
Detention Basin

25. Extended Dry Detention
Basin (EDDB)

26. Why the term “Extended”
Detention?
Extended: Designed to release the WQv over a period of 40 hours



Allows time for more particles and associated pollutants to
settle out
Reduces the downstream velocity and erosive conditions
More closely imitates natural release rates and duration

27. Geomorphic Effects of
Uncontrolled Urban Runoff

28. Exceedance Frequency for
Detention
1000
Developed
Uncontrolled
7-yr
2/yr
6/yr
100
Flow
20/yr
10
Undeveloped
1
0.1
§
·
q·
more frequent than 1-yr
B
1-yr
2-yr
Storm Return Interval
Ú
10-yr
100-yr
y·

29. 40-Hour Drawdown Impacts
1000
Developed
Uncontrolled
100
Flow
0.80 psf
Developed
Controlled
10
0.26 psf
•10-year control
•1-year control
•WQv – extended
detention with 40 hr
drawdown
1
Undeveloped
0.1
0.01
0.1
more frequent than 1-yr
1
1-yr
2-yr
Storm Return Interval
10
10-yr
100
100-yr

30. March 2008 Manual
Extended Detention

Water Quality (40-hr)

Pollutant removal through
• Settling
• Biological uptake (more for
wetland)
• Detain and promote
infiltration

Stream Sustainability (40-hr)


Mimic undeveloped
conditions for full range of
hydrology
Can meet flood control
objectives

31. EDDB Major Components

32. EDDB Inlet/Forebay
Forebay

33. EDDB Inlet/Forebay



Traps sediment and trash and slows inflow velocities
Forebay (optional) should be at least 10% of WQv and
separated from the main basin by an acceptable barrier.
Use energy dissipaters at inlets to reduce scour potential

34. EDDB Pilot Channel
Pilot Channel

35. EDDB Pilot Channel

Conveys low flows
to the outlet

Recommend lining
with riprap
Olathe, KS

36. EDDB Main Basin
Main Basin

37. EDDB Main Basin

Designed to hold the WQv
with a depth of 2 to 5 ft

Does not maintain a
permanent pool

Shallow basins with larger
surface area have higher
performance

Basin bottom should be at
least 2 ft above the wet
season water table

For KC Metro, can be used for
limited passive recreation
such as trails

38. EDDB Outlet Structure
Outlet

39. EDDB Outlet Structure

Release the WQv over a
period of 40 hr

Protected by well screens,
trash racks or grates

Located as far from inlet as
possible

Various outlet structure
types

Single Orifice

Perforated Riser or Plate

V-notch Weir
Source: Hubbard Brook LTER

40. EDDB Outfall
Outfall

41. EDDB Outfall and Emergency
Spillway

Used to convey
flood flows safely
without overtopping
the basin

Required unless
main outlet is
designed to pass
1% design storm
Olathe, KS

42. EDDB Maintenance Access
Maintenance
Access

43. EDDB Vegetation


Function of facility
determines
vegetation selection
Vegetation types


Native grasses
(preferred)
Turf

44. EDDB Vegetation
Buffalo Grass
Woodland Sedge
Big Bluestem
USDA-NRCS PLANTS
Database / Hitchcock, A.S.
Robert H. Mohlenbrock @
USDA-NRCS PLANTS Database
Jennifer Anderson @ USDANRCS PLANTS Database

45. EDDB Site Selection

Soil permeability will
impact performance

Clay soils with low
depths to bedrock pose
siting limitations

Basin bottom must be
at least 1-2 ft above wet
season groundwater
table

Backfilling with high
permeable soil should
be considered

46. EDDB Site Selection

Off-line, outside of
stream corridor

Can be located within
larger flood control
facilities

Not on fill sites or steep
slopes (unless
enhanced)

Olathe, KS
Flood Control Volume
Use fences and
landscaping to impede
WQv
access

47. Incorporating Flood Control
Benefits
x-section 100-yr pool
slotted weir for
control of WQv
outlet pipe sized to
control 100-yr outflow
WQv

48. EDDB Advantages

Relatively easy to construct
and inexpensive

Settling of suspended
solids

Flood control via peak
discharge attenuation

Control of channel erosion
by reducing downstream
flow velocities
California Stormwater Quality Association

Recreational benefits
(mainly trails)

49. EDDB Disadvantages

Not as aesthetically
pleasing as other
BMPs

Not effective at
removal of soluble
pollutants

Difficult to identify
sites with sufficient
infiltration capacity

50. Questions?
10 minute break

51. Lecture 2: EDDB Design
Example and Activity


Water quality storage volume
Outlet structure








Orifice
Perforated riser or plate
V-notch weir
Trash rack
Basin shape
Forebay (Optional)
Side Slopes
Vegetation

52. Design Example



Design an EDDB for a 26-acre commercial development.
Size the EDDB to capture the WQv.
Size an outlet structure to release the WQv over 40
hours.

53. Step 1: Calculate Water
Quality Storage Volume WQv
Two methods

Short-Cut Method
•
•

Sites 10 acres
Only 1 predominant cover type
Small Storm Hydrology Method
•
Larger or more heterogeneous drainage
areas
As tributary area is 26 acres, Small Storm
Hydrology Method will be used.

54. Equation: WQv
Small Storm Hydrology Method
WQv = (P)*(Weighted Rv)
Weighted Rv = Σ(Rvi*Aci)/Total area (ac)
•
•
Rvi = Volumetric runoff coefficient for cover type (Table
7)
Aci = Area of cover type i (ac)

55. Rv Table
TABLE 7
VOLUMETRIC COEFFICIENTS FOR URBAN RUNOFF FOR
DIRECTLY CONNECTED IMPERVIOUS AREAS
(CLAYTOR AND SCHUELER 1996)
Rainfall
(inches)
Flat roofs and
large unpaved
parking lots
Pitched roofs and
large impervious
areas
(large parking lots)
Small
impervious
areas and
narrow
streets
Silty
soils
HSG-B
Clayey soils
HSG-C and
D
0.75
0.82
0.97
0.66
0.11
0.20
1.00
0.84
0.97
0.70
0.11
0.21
1.25
0.86
0.98
0.74
0.13
0.22
1.37
0.87
0.98
0.75
0.14
0.23
1.50
0.88
0.99
0.77
0.15
0.24
Note: a reduction factor may be applied to the Rv values for disconnected
surfaces, consult the BMP hydrology section

56. Water Quality Control
Volume
Cover Type
Rv
Area (acres)
Flat roofs
0.87
1.6
Parking lots
0.98
8.8
Narrow streets
0.75
3.3
Silty soil
0.14
12.3
Rvi × Aci
WQv = ∑
×P =
Total Area
∑ ( .87 ×1.6 + .98 × 8.8 + .75 × 3.3 + .14 ×12.3) ×1.37 = 0.749in
26

57. Water Quality Storage
Volume


Convert WQv from inches to ac-ft by converting
inches to feet and multiplying by the tributary area
Add 20 percent to account for silt and sediment
deposition
= (0.749)*(1ft/12in)*26ac
= 1.62*1.20

58. Step 2: Determine Outlet
Structure
Single Orifice
V-notch Weir
Perforated Riser or Plate

59. Outlet Structure


Outlet sized to release
WQv (ac-ft) within 40
hours
Locate outlet as far away
from inlet as possible



Avoid short-circuiting
The facility must bypass
1% storm event
Provide at least 1ft of
freeboard above WQV
stage

60. Option 1: Single Orifice Outlet

61. Single Orifice Outlet
i.
Depth of water quality volume at outlet (ZWQ)

ii.
Dependent on site conditions – designer determined
Average head of WQv over invert of orifice, HWQ (ft)
HWQ = 0.5*ZWQ
iii.
Average water quality outflow rate, QWQ (cfs)
QWQ = (WQV * 43,560) / (40 * 3,600)

62. Single Orifice Outlet
= 0.5*3.0ft
= (1.62*43,560)/(40*3600)

63. Single Orifice Outlet Co
iv. Set orifice coefficient
(Co) depending on orifice
plate thickness
Do must be or = 4
inches to prevent
clogging
Co = 0.66 if plate
thickness is Do
Co = 0.80 if plate
thickness is Do

64. Single Orifice Outlet
v.
Orifice diameter (Do) must be greater than 4
inches, otherwise use weir or riser
Do = 12 * 2 * QWQ / Co * π *
(
2 * g * HWQ
)

65. Single Orifice Outlet Sizing
Do=12*2*(0.49/(0.66*π*(2*32.2*1.5)0.5))0.5

66. Option 2: Perforated Riser or
Plate Outlet
Photo taken by Larry Roesner
Photo taken by Larry Roesner

67. Perforated Riser or Plate
Outlet

Calculate outlet area per row of
perforations (Ao)
Ao (in2) = WQv / (0.013 * ZWQ2 + 0.22 * ZWQ – 0.1)

Assuming a single column calculate the
diameter of a single perforation for each
row
D1 = (4 * Ao / π)1/2

If D1 is greater than 2 inches add more
columns
nc = 4

68. Perforated Riser or Plate
Outlet
= 1.62/(0.013*3.02+0.22*3.0–0.1)
= (4*2.4/π)1/2

69. Perforated Riser or Plate
Outlet

Use number of columns to determine exact
perforation diameter
Dperf = (4 / π * Ao / nc)1/2

Using a 4” center to center vertical spacing and
ZWQ, determine number of rows (nv)
nv = ZWQ / 4
nv = 5

70. Perforated Riser or Plate
Outlet
= 1.62/(0.013*3.02+0.22*3.0–0.1)
= (4*2.4/π)1/2
= (4/π*2.4/1)1/2
= (ZWQ*12in)/4

71. Option 3: V-Notch Weir Outlet
Dr. Robert Pitt
Source: Hubbard Brook LTER

72. V-Notch Weir Outlet Design
i.
Depth of water quality volume at outlet (ZWQ)

ii.
Dependent on site conditions – designer determined
Calculate HWQ over weir notch
HWQ=0.5*ZWQ
iii.
Calculate the average water quality pool outflow
rate QWQ (cfs)
QWQ = (WQv * 43,560)/(40 * 3,600)

73. V-Notch Weir Outlet Example
= 0.5*3.0ft
= (1.62*43,560)/(40*3600)

74. V-Notch Weir Outlet Design

Calculate required v-notch weir angle
θ = 2 * (180 / π) * arctan (QWQ/(Cv * HWQ5/2))
CV = V-notch weir coefficient = 2.5

If θ is 20º set θ to 20º

Calculate top width of v-notch weir
(WV)
θ
Wv = 2 * ZWQ * Tan (θ / 2)
Source: Hubbard Brook LTER

75. V-Notch Weir Outlet Example
= 2*(180/π)*actan(0.49/(2.5*1.5 ))
5/2
= 2*3.0*tan(20º*π/(2*180))

Since θ 20º set θ to 20º
20º
1.1

76. Step 3: Basin Shape
3W
W
California Stormwater Quality Association

77. Step 4: Forebay (Optional)

Volume (VolFB) should be at least 10% of WQv

Sides and bottom paved or hardened

Surface area (AFB):
AFB = VolFB / ZFB

78. Forebay (Optional)
= 0.10*6.23
= 0.62/3.0

79. EDDB Design Activity

80. Activity
Design an extended dry detention basin (EDDB) to capture the
WQv from a 52-acre development. Design a single orifice
outlet to release the WQv over 40-hours.
Cover Type
Area (acres)
Commercial Center
Flat Roofs
5
Large Paved Parking Lots
6
Clayey Soils
1
Streets
2
Medium Density Residential
Pitched Roofs
15
Paved Driveways
7
Clayey Soils
11
Streets
5
Totals
52

81. Activity Solution

82. Questions?
10 minute break

83. Lecture 3: Infiltration BMPs



Infiltration basin
Infiltration trench
Pervious pavement
I’Lan Park, Leawood, KS
www.lowimpactdevelopment.org

84. Infiltration Practices
Advantages

Provides 100% load reduction for captured runoff
volume

Flood control via peak discharge attenuation

Control of channel erosion by reducing
downstream flow velocities

85. Infiltration Practices
Disadvantages

Sediment can clog an infiltration facility

Tributary area should be stabilized

Not suitable in areas with high water table (12 feet from ground surface)

Soils must have a minimum saturated
hydraulic conductivity

Risk of contaminating groundwater

86. Caution

Infiltration capacity of soils in the MARC region
is general low (0.5 in/hr)

High water tables are also a common concern
related to these practices

Be very careful in site selection for infiltration
basins or trenches

87. Infiltration Basin
Vegetated
basin floor
Emergency
Spillway
Pretreatment
Outfall
Backup
Drain
Outlet

88. Infiltration Basin
WQv

89. Infiltration Basin Pretreatment


Used to remove as many
of the suspended solids
as possible
Various types




Grit Chambers
Swales with Check Dams
Filter Strips
Sediment Forebays
Pretreatment

90. Infiltration Basin Floor
•Basin floor should be as level as
possible for even distribution
Vegetated basin
floor

91. Infiltration Basin
Emergency
Spillway
Outfall
Outlet

92. Infiltration Basin
Outlet/Overflow Structures


Must pass flood flows
without damaging the
structure
Options


Weir
Overflow pipe
Portland OR
(www.lowimpactdevelopment.org)

93. Infiltration Basin Backup
Drain

Used to
drain the
basin if
ponding
persists for
more than
72 hours
Backup Drain

94. Infiltration Basin

95. Infiltration Basin Key Design
Criteria





Maximum of 2 acre tributary
area
Off line, outside of stream
corridors
Where soil permeability
and water table is suitable
Minimum of 150 ft from
drinking water wells
Minimum 10 ft downgradient
and 100 ft upgradient from
building foundations

96. Infiltration Basin
Key Design Criteria




Use a length to width
ratio of at least 3:1
Grade basin bottom as
flat as possible
Side slopes not to
exceed 3:1
Install pretreatment
device
(forebay/swale/filter strip)

97. Design: Infiltration Basin
Depth

Calculate ponding depth (d)
d=f*t

Where:
• f = percolation rate of surrounding soil (in/hr)
• t = retention time (hr)


72-hour maximum ponding time (24-hr
recommended)
Depth should be less than 2 feet

98. Design: Infiltration Basin
Area

Calculate bottom area (A)
A = 12 * V / (f * t),

Where
• V = volume to be infiltrated (ft3)
V = WQv(ac-ft) * 43,560

99. Design: Infiltration Basin
Example
Size an infiltration basin to treat a WQV of 0.15 ac-ft
over 72 hours, if the surrounding soil percolation
rate is 0.35 in/hr




d = (0.35 * 72) / 12 = 2.1 ft
Set d = 2.0 ft
t = 12 * d / f = 12 * 2.0 / 0.35 = 68 hours
A = (12 * 0.15 * 43560) / (0.35 * 68) = 3,300 ft2

100. Infiltration Basin Vegetation

Plant native vegetation on side slopes and
bottom of infiltration basin

Can increase infiltration rate

Use plants listed in the BMP Manual Appendix A
“Recommended Plant Materials for BMPs”

Select species that can withstand drought and
long periods of ponding
DO NOT use sod


101. Infiltration Basin
Maintenance


Inspect at least twice a year
Initially inspect more frequently



Look for sustained ponding
Maintain vegetation
Remove sediment

102. Infiltration Basin Maintenance

Regular inspections





Stabilize areas of erosion in tributary area
Remove trash and debris at beginning and end of
wet season
Remove dry sediment from basin



Preferably once per month
Assess length of time water is ponded following a
storm
Use light equipment
Wait until sediment Is cracking and readily
separating from bottom
Weed trimming to maintain plants

103. Infiltration Trench

104. Infiltration Trench Plan View
Pretreatment
Bypass
Structure
Infiltration
Trench
Overflow
Vegetated
Channel

105. Infiltration Trench
Pretreatment
Pretreatment

106. Infiltration Trench
Pretreatment


Pretreatment increases the life of the trench
Remove as much of the suspended solids as
possible




Grit Chambers
Swales with check dams
Filters strips
Sediment Forebays

107. Infiltration Trench Bypass
Overflow


Bypass
Structure
Convey flows over
the WQv around or
over the trench
safely
Prevent erosion
Overflow

108. Infiltration Trench
Infiltration
Trench

109. Infiltration Trench




Designed to Infiltrate the WQv within 72 hours (24
hours recommended)
Filled with clean stone 1.5-2.5 inches in diameter
Lined with filter fabric
Under drain can be incorporated

110. Infiltration Trench Filter
Fabric

Use non-woven filter fabric layer close to the surface
to prevent majority of substrate from getting clogged
with sediment

Line the trench walls and bottom with filter fabric

111. Infiltration Trench
Monitoring Well




Used to monitor the
infiltration rate
Determine if trench
needs cleaning
4 to 6 inch diameter
PVC
Anchored to bottom of
trench
www.lowimpactdevelopment.org

112. Outlet
Commercial Area Drainage

113. Overflow Weir
Riprap covered
outlet

114. Infiltration Trench Example

115. Infiltration Trench Example
Portland OR
(www.lowinpactdevelopment.org)

116. Infiltration Trench Design
Considerations





Tributary area must be less than 5 acres
If runoff comes in as sheet flow orient the trench
perpendicular to the flow
If runoff is channelized orient the channel parallel
to the channel
Don’t use limestone or shale as backfill material
Surrounding soil should be less than 40% clay

117. Design: Infiltration Trench
Volume

Calculate the volume of the trench (VTR)
VTR = WQv / n

where:


WQv = water quality volume (ft3)
n = void space in trench media (0.4 for clean
stone, 1.5-2.5in diameter)

118. Design: Infiltration Trench
Area

Calculate bottom area (A)
A = 12 * WQv / (f * t)

where:



WQv = water quality volume (ft3)
f = percolation rate of surrounding soil (in/hr)
t = retention time (hr)

119. Design: Infiltration Trench
Depth

Calculate trench depth (D)
D = VTR / A

where:



VTR = volume of the trench (ft3)
A = area of the trench (ft2)
The depth should be 3 to 8 ft

120. Infiltration Trench Design
Length

If WQv enters as sheet flow position the
trench perpendicular to the flow

If stormwater enters as channel flow orient
parallel to flow

Maximize the length of the trench for both
flow types

121. Design: Infiltration Trench
Example

Size an infiltration trench to treat a WQV of 0.15
ac-ft over 48 hours, if the surrounding soil
percolation rate is 0.35 in/hr
VTR = (0.15 ac-ft * 43,560) / 0.4 = 16,335 ft3
A = (12 * 0.15 * 43560) / (0.35 * 48) = 4,667 ft2

Assuming a sheet flow width of 250ft
Ltrench = 250ft
W
= 7,780/250 = 19ft

122. Infiltration Trench
Maintenance

Regular inspections




Preferably once per month
Assess length of time water is ponded following a
storm (monitoring well)
Stabilize areas of erosion in tributary area
Remove trash and debris at beginning and end of
wet season

123. Infiltration Trench
Maintenance

If sediment is visible in top layer, remove top
layer of stone, filter fabric and sediment



Wash stone
Reinstall filter fabric and washed stone
If standing water persists for more than a few
days


Remove and clean or replace all stone aggregate
Replace filter fabric

124. Pervious Pavement

125. Pervious Pavement
Pervious surface
www.oregon.gov/ODOT/TD/TP_RES/docs/2006_NWTC/2C_Cahill.pdf

126. Pervious Pavement Types






Permeable
Interlocking
Concrete Pavers
Concrete Grid
Pavers
Pervious Concrete
Pervious Asphalt
Cast-in-Place
Plastic Turf
Reinforcing
Geowebs

127. Pervious Pavement
www.lowimpactdevelopment.org

128. Pervious Pavement

Design to infiltrate the WQv
I’lan Park, Leawood KS
Concrete Promotions Demo

129. Pervious Pavement
Advantages
 Reduce flooding potential
 Can be more aesthetically pleasing
Disadvantages
 May cost more
 Can be a more uneven driving surface

130. Cast-In-Place Concrete
Slabs


Reinforced slab, suitable
for heavy loads
Poured on site
Lenexa, KS
www.lowimpactdevelopment.org

131. Precast Concrete Grids


Permeable Concrete
pavers with void areas
separating pieces
Higher percentage of
permeable surfaces
www.lowimpactdevelopment.org

132. Modular Unit Pavers


Pavers themselves are impermeable
Porous material places in gaps between pavers
Concrete Promotions Demo

133. Geowebs

Traditionally used for soil
stabilization

134. Pervious Pavement
Considerations







System must be able to sustain traffic load
15% Void space with infiltration rates 12in/hr
Subbase – 36 to 42% voids compacted at 95
proctor
Subbase – ¾ inch clean rock with 2% passing
#200 sieve
A minimum subbase thickness of 8 inches
Use non-woven geotexile fabric between subbase
and soil
Use a uniform grade material to maximize voids

135. Pervious Pavement Design
Criteria


Only use certified ready-mix companies
Request certified contractor or




Require test placement (4 yd3) to verify mix and
installation procedures
Do not use pervious pavements in areas where
heavy trucks will turn
2:1 impervious to pervious area is good rule of
thumb
Use an underdrain to dewater subbase for events
greater than the water quality event

136. Design: Pervious Pavement

Design volume (Dv)
DV= WQv / n
where WQv = volume (ft3)
n = void space

137. Design: Pervious Pavement

Calculate the minimum required surface area
(SAmin) to infiltration the WQv into the soil
SAmin = 12 * WQv / (f * t)
where:
• WQv = water quality volume (ft3)
• f = percolation rate of surrounding soil (in/hr)
• t = retention time (hr)

138. Design: Pervious Pavement
Example
Size a permeable pavement parking area to
capture and infiltrate a WQv 0f 1.37 inches over a 0.5
acre tributary area
 Assume 100% impervious tributary area
 Short-cut Method

WQv = (1.37in)*(0.05+0.009(100%)) = 1.3in
Water quality volume to be infiltrated in 12 hrs into
subsurface soils with infiltration rate of 0.35 in/hr

139. Design: Pervious Pavement
Example
Using the previous example:
WQv = 1.3in
WQv (1.3 / 12)*0.5*43,560 = 2,360ft3
DV = 2,360 ft3 / 0.4 = 5,899 ft3
SAmin = 12 * 2,360 ft3 / (0.35 in/hr * 12 hrs)= 6,742 ft2
6,742 ft2 / 43,560 = 0.15 ac ( 2:1 ratio)
Depth for the WQv = 5,899 ft3 / 6,742 ft2 = 0.88 ft

140. Pervious Pavement
Maintenance

Stabilize areas of erosion in tributary area

Don’t salt the 1st year

Street sweeping with vacuum truck
 3 times per year

April, July, and November

Inspect underdrain outlets annually

Snow plowing acceptable but need to educate
operators

141. Pervious Pavement
Resources

Center for Transportation Research and
Education, Iowa State University
• www.ctre.iastate.edu

Concrete Promotions
• www.concretepromotion.com

Univeristy of Missouri – Kansas City
 John Kevern, Ph.D.

BMP Subcommittee Update this Guidance

142. Designer
Review Team
Planning Phase
– Environmental Site
Assessment
– Select Post
Construction BMPs
– Flood Control Study
– Establish Long-term
Maintenance Agreements
Plat
Approval
Planning
Engineering
Parks Recreation
Environmental Specialists
Attorney
Design Phase
– Erosion and
sedimentation
controls
– Post-construction
BMPs
– Flood control
improvements
Building
Permit
Review Team
Planning
Engineering
Code Compliance
Inspectors
Review Team
Planning
Engineering
Parks Recreation
Environmental Specialists
Operations Maintenance
Construction Phase
– Inspect and maintain
BMPs for construction
activities
– Construct Post
Construction BMPs
– Maintain agreements for
post-construction BMPs
Occupancy
Permit

143. Questions?
Comments.
Thank You