BMP Monitoring Sites - Urban Drainage and Flood Control District

UDFCD BMP MONITORING SITES 

Overview 

Overview 

At all of the Best Management Practice (BMP) sites monitored by the District, 

inflow and outflow samples are collected - along with rainfall and runoff data – to 

determine the Event Mean Concentration (EMC) of different constituents and to 

assess the functionality of each BMP on stormwater runoff reduction and water 

quality. 

The monitoring sites are set up in April and taken down in October with periodic 

maintenance throughout the sampling season. 

Types of BMPs Monitored by District


Pervious Pavement 

PERVIOUS PAVEMENT 

Pervious pavements encompass a variety of stabilized surfaces that can be used 

for the movement and parking of vehicles and storage of materials and 

equipment. It differs from conventional pavement: it is designed to infiltrate 

stormwater runoff instead of shedding it off the surface. Pervious pavement 

offers the advantage of decreasing the effective 

imperviousness of an urbanizing or redevelopment site, 

thereby reducing runoff and pollutant loads leaving the site. 

Pervious pavement can be designed with or without 

underdrains. Whenever underdrains are used, infiltrated 

water will behave similar to interflow and will surface at a 

much reduced rate over extended periods of time. All types 

of pervious pavements help to return stormwater runoff 

hydrology to more closely resemble pre-developed 

conditions. However, the actual consumptive use of water falling onto the ground 

is considerably less than under pre-developed conditions and for grass lawns in 

urban areas. Consult with a geotechnical engineer as to the suitability of 

each type of pervious pavement for the loads and traffic it will support and carry 

and the geologic conditions the pavement will rest upon. 

Pervious Pavements Monitored by District


Pervious Concrete Pavement 

PERVIOUS CONCRETE 

PAVEMENT 

Description Location Typical Details Installation 

Photo Gallery Monitoring Data Other Examples 

Description 

General 

Pervious concrete pavement is a relatively new type of permanent surfacing. It is 

a monolithically poured pervious concrete pavement that has 15% to 21% of its 

volume as void. These voids within the concrete are 

achieved by eliminating the fine sand aggregate 

from the concrete mix. They provide the flow paths for 

rainwater from the surface of the pavement to the 

base course underlying it. Because the integrity of the 

concrete structure may be harmed by standing water 

during freezing weather, the use of pervious concrete 

pavement is not recommended for use in 

pervious pavement detention installations. It is 

critical that sufficient aggregate base course layer is 

provided under the pervious concrete slab to store the 

runoff and allow it to infiltrate slowly into the ground 

and drained using an underdrain pipe system. Having a sufficiently thick layer of 

aggregate base course is particularly critical during the months of the year when 

freezing of water can occur. 

Site Specific 

The pervious concrete pavement site monitored by the 

District is located in Lakewood. Modular block pavement 

was monitored in this location from 1994-2004. In 2005, 

the modular block was replaced with the pervious concrete 

pavement that is currently in place. Two separate pads of 

pervious concrete pavement were placed using different 

aggregate sizes for the base course. The east pad used



AASHTO #67 and the west pad used AASHTO #8. Below the surface, there 

are two lateral flow barriers in the form of concrete walls. At each barrier, the 

filtered water is collected in perforated pipes and carried to a manhole located 

adjacent to the pervious concrete pavement. Inside the manhole a riser pipe with 

an orifice is sized to drain the entire gravel pore volume of each cell 

in 6 hours or more. A riser pipe with a levelogger is located just upstream of 

each manhole to measure the flow coming from each cell. All flows from the 

pervious concrete pavement are combined and discharge east of the site through 

a V-notched weir where a pressure transducer measures the depth of flow 

and sends the data into the sampler. 

Adjacent to the pervious concrete pavement watershed is a control 

watershed of traditional asphalt pavement used to compare results of treated 

runoff to untreated, direct runoff. Stormwater runoff from the control watershed is 

collected in a sump catch basin at the northeast corner of the site. Water is 

conveyed a short distance to the outfall and flows through an H-flume where 

flow is measured by a pressure transducer. 

The sampling equipment is stored in a shed near the outlet works. A rain gage 

on top of the shed measures rainfall and signals the ISCO samplers inside the 

shed to begin sampling after 0.1 inches of rain falls. The samplers then draw a 

sample of water from both the pervious concrete pavement runoff and the control 

runoff after ??? cfs has passed, and then every 15 minutes until 12 hours after 

the storm has ended.



Test Site Location 

The pervious concrete pavement test site is in the Lakewood City Shops 

maintenance buildings located at 850 Parfet Street. The pervious concrete 

pavement itself is located in the parking lot on the east side of the property, east 

of the building. 

Map this Location 

The watershed area tributary to the pervious concrete pavement is 9050 square 

feet, with the pervious concrete taking up 2000 square feet of that area. The 

watershed consists primarily of impervious areas, including buildings, parking lots 

and paved areas. 

The test site consists of three main parts: the control shed, the pervious 

concrete pavement itself, and the outlet.

Pervious Concrete Watershed 


Pervious Concrete Pavement



Typical Details 

Use utility vault w/ 

lid to collect 

underdrains. 

Monolithically poured porous 

concrete. 

1in 

5"** 

D = 0.67'(8") min.** 

1" (min.) C-33 Sand 

6" (min.) 

Underdrain 

trench 

6" (min.) trench depth 

16 MIL (mon.) plastic impermeable membrane 

on top of subgrade when required and at bottom 

of underdrain trench. 

Woven geotextile fabric meeting: 

ASTM D4751-AOS US Std. Sieve #50 to #70, 

ASTM D4633 min. trapezoidal tear strength 100 x 60 lbs, 

Minimum COE specified open area of 4%. 

Section A-A 

** For personal vehicles & pickup trucks. 

Thicker section may be required for 

heavier vehicles. 

Consult with pavement engineer for 

needed thickness of concrete slab. 

*** Base course: AASHTO #67, #8 or #4 aggregate 

with all fractured faces. 

So = 0% to 2% (max.) 

Lmax = D/(1.5*So) 

A 

Monolithically poured porous concrete 

(mix of AASHTO #8 or #67 gravel 

and portland cement per specifications 

in the BMP Details and Specifications 

Chapter of this Manual). No phosphorous 

based admixtures allowed in the PCP mix. 

5"** 

D = 0.67'(8") min.** 

So = 1% (min.) 

Install 16 MIL (min.) 

impermeable membrane 

under pipe & wrap it on d/s 

side to within 1in(+ 2"/-0") 1 of 

top of gravel to serve as 

horizontal flow barrier. 

A 

When certified tests show percolation rates of less than 

60 minutes per inch of drawdawn under the PP bottom 

and infiltration is allowed, eliminate the bottom sand 

layer and underdrains. 

When Type C soils are present and when infiltration is 

allowed, unless percolations show otherwise, eliminate 

the bottom sand layer, use underdrains and geotextile 

liner instead of an impermeable one under the gravel. 

When the underlying soils are NRCS Type D or 

expansive, when existing or proposed building is within 

10 feet, and/or when land uses pose risk to groundwater 

contamination, use 16 mil minimum thickness 

impermeable liner under and on sides of the pavement 

sand and gravel media. 

Pervious Concrete Pavement Section



38.00 

Flow 

26.00 

54.00 

Pervious 

concrete 

surface 

Cutoff wall 

Flow 

Concrete 5'' 

Gravel 7'' 

Sand 3'' 

Gravel 2.6'' 

Flow (to South 

Utility Vault) 

Underground 

overflow weir 

Flow (to North 

Utility Vault) 

Pervious Concrete Layout– Isometric 

North Utility Vault Section 

South Utility Vault Section



Installation Guidelines 

Design Thickness 

Design the thickness of the pervious concrete slab to 

support the traffic and vehicle types the pavement will 

have to carry. 

Mix and Installation Mix of AASHTO #67 or #8 

Aggregate and Portland 

Cement. Use low cement to 

water ratio and no less than 

6.5 sacks of Portland Cement 

per yard. No fly ash or 

phosphorous containing 

admixtures shall be used. 

Strictly adhere to the pervious concrete mix specifications 

provided in the Manual. 

Base Course 

The base course shall be AASHTO #3 or #4 (CDOT 

Section 703) coarse aggregate. Assume 30% of total 

volume is open pore space. Unless an impermeable 

membrane liner is required, at least 6 inches of the 

subgrade underlying the base course shall be sandy and 

gravely material with no more than 10% clay fraction. 

Sand Filter Layer Whenever the pervious concrete 

pavement is being installed over 

expansive soils and underdrains are 

required, an ASTM C33 gradation sand 

filter layer shall be installed to remove 

most of the fine particulate 

pollutants from the water column 

before the water reaches the 

underdrains. 

Geotextile Fabric 

Place a woven geotextile fabric on top and bottom 

of the base course. Use a geotextile material that meets 

the following requirements: 

◊ ASTM D-4751 – AOS U.S. Std. Sieve #50 to #70 

◊ D-4633 – Trapezoidal tear strength ≥ 100 x 60 lbs 

◊ COE specified minimum open area ≥ 4%



Place by rolling fabric parallel to the contours starting at 

the most downstream part of the pavement. Provide a 

minimum of 18” of overlap between adjacent sheets. 

Bring up geotextile and impermeable 

membrane to the top of perimeter 

walls. Attach membrane and fabric to 

perimeter walls with roofing tar or other 

adhesive or concrete anchors. Provide 

sufficient slack in the geotextile and 

membranes to prevent stretching them 

when sand and/or rock is placed. Seal 

all joints of impermeable membrane to be totally leak 

free. 

Impermeable Liner 

Contained Cells 

When expansive or NRCS Type D soils are present, or 

potential for groundwater contamination exists, install a 

16 mil thick impermeable liner on the bottom 

and sides of the basin under the pavement. If soils are 

not expansive (i.e. NRCS Type A, B or C), use a woven 

geotextile material that meets the requirements specified 

under Geotextile Fabric above. Products that meet these 

requirements are: 

◊ US Fabric US 2070 and US 670 

◊ Mirafi Filterweave 500 and 700 

◊ Carthage Mills Carthage 6%. 

Install lateral flow cut-off barriers using 16 mil or 

thicker PE or PVC membrane 

liner or concrete walls installed 

parallel to the contours (i.e. 

normal to the flow) to prevent flow 

of water downstream and then 

surfacing at the toe of the 

pervious concrete pavement 

installation. Distance (Lmax) 

between these cut-off barriers 

shall not exceed: 

D 

L max = 1. 5∗ So



◊ Lmax = maximum distance between cut-off 

membrane normal to the flow (ft) 

◊ So = slope of the base course (ft/ft) 

◊ D = depth of gravel base course (ft) 

Subdrain System 

When the pervious concrete pavement is located on 

NRCS Type D soils, when the Type B or C soil sub-base 

is to be compacted for structural reasons, or when an 

impermeable membrane liner is needed, install a 

subdrain system using HDPE pipe. Locate each 

perforated pipe just upstream of the lateral-flow cut-off 

barrier. Do not exceed 20-foot spacing. Use a control 

orifice sized to drain the pore volume of empty each cell 

in 6 hours or more. 

Design Area Ratio The design area ratio shall not exceed 2.0 (ratio = 

And Effective contributing impervious area/pervious pavement area). 

Imperviousness The interim recommendations for effective 

imperviousness are given in the Manual.



Photo Gallery 

Installation Photos – April 2005 

Sand used for filter layer Geotextile fabric above sand AASHTO #67 base course 

filter layer 

Spreading the concrete AASHTO #67 mix Rolling the concrete 

#67 mix vs. #8 mix 

Current Condition Photos 

April 2007 (2 years after install) 

Concrete in good condition Upper cell manhole and Lower cell manhole and 

Levelogger riser 

Levelogger riser



Control watershed inlet 

V-notch and H-flume at outlet 

July 2007 (2 years, 3 months after install) 

Concrete in good condition Concrete with #67 aggregate Concrete with #8 aggregate 

Sampling tube misplaced 

in H-flume 

Levelogger



Monitoring Data 

2006 Flow Data Summary 

Storm 

Number 

Rainfall 

(in) 

Start Time 

End Time 

Peak HF 

Depth (ft) 

Peak VN 

Depth (ft) 

WQ 

Samples? 

1 0.31 4/6 555 4/7 941 0.113 0.142 no 

2 0.16 4/27 417 4/28 1032 0.028 0.109 no 

3 0.09 4/29 2250 5/1 712 0.042 0.125 yes 

4 0.11 5/2 1820 5/5 1725 0.025 0.111 yes 

5 0.34 5/8 2223 5/10 850 0.154 0.152 yes 

6 0.05 5/19 1306 5/20 1625 0.057 0.04 no 

7 0.03 5/21 2117 5/22 2211 0.033 0.033 yes 

8 0.16 6/7 1324 6/8 1359 0.184 0.121 no 

9 0.58 6/23 1833 6/24 2040 0.287 0.323 yes 

Extra N/A 7/4 7/4 N/A N/A yes 

10 0.08 7/19 1624 7/20 851 0.136 0.129 no 

11 0.03 7/22 2156 7/23 2204 0.049 0.046 no 

12 0.05 7/24 1956 7/25 902 0.091 0.101 no 

13 0.18 8/2 1455 8/3 1450 0.238 0.142 yes 

14 0.07 8/4 1716 8/6 303 0.067 0.054 no 

15 0.07 8/12 1943 8/13 1953 0.168 0.143 no 

16 0.1 8/18 557 8/19 1857 0.106 0.112 no 

17 0.05 8/23 1723 8/24 1135 0.119 0.113 no 

18 0.15 8/25 639 8/26 1532 0.091 0.121 no 

19 0.19 9/6 1710 9/9 1236 0.13 0.161 no 

20 0.09 9/10 2036 9/11 2131 0.221 0.122 no 

21 0.27 9/20 28 9/21 703 0.144 0.111 yes 

22 0.05 9/21 2037 9/22 2049 0.248 0.138 no



2007 Flow Data Summary 

Storm 

Number 

Rainfall 

(in) 

Start Time 

End Time 

Peak HF 

Depth (ft) 

Peak VN 

Depth (ft) 

WQ 

Samples? 

1 0.16 3/30 1229 3/31 1339 0.038 0.01 no 

2 0.18 4/8 1559 4/9 1749 0.015 0.033 no 

3 0.03 4/10 440 4/10 1205 0.07 0.062 yes 

4 0.44 4/16 2313 4/17 1105 0.132 0.127 yes 

5 1.31 4/23 1602 4/26 1109 0.147 0.184 yes 

6 0.22 5/1 1542 5/2 1642 0.203 0.138 yes 

7 0.01 5/7 721 5/8 1856 0.037 0.019 yes 

8 0.46 5/14 1856 5/15 1050 0.265 0.121 yes 

9 1.03 5/22 1042 5/24 1553 0.235 0.142 yes 

10 0.48 5/29 1252 5/30 1235 0.041 0.127 yes 

11 0.31 6/12 615 6/13 900 0.234 0.132 yes 

12 0.04 7/3 1523 7/4 1550 0.13 0.039 yes 

13 0.15 7/8 1502 7/9 1518 0.212 0.124 yes 

14 1.02 7/27 1753 7/29 201 0.084 0.217 yes 

15 0.43 8/2 302 8/5 2144 0.244 0.159 yes 

16 0.32 8/5 2239 8/7 1012 0.167 0.144 yes 

17 0.16 8/10 1232 8/10 2233 0.271 0.126 yes 

18 0.08 9/24 838 9/25 540 0.184 0.121 yes



Other Pervious Concrete Pavement Examples 

The following are additional sites in the Denver Metro area that have used 

pervious concrete pavement but are not monitored by the District. The District’s 

involvement in the design, construction and maintenance of these pervious 

concrete pavement sites was either minimal or non-existent, but a photo 

inventory of the sites will be kept up-to-date on this website for informational 

purposes as to the longevity and durability of the pavement. 

Safeway 

14 th and Krameria in Denver 


Approximate installation date: January 2005 

Installation January 2005 

April 2005 (3 months after install)



August 2007 Repairs (2 years, 7 months after install) 

April/June 2008 (3 years, 5 months after install)


Pervious Concrete Pavement



Wal-Mart 

I-70 and Tower Road in Aurora 


Approximate installation date: 2005 

April/June 2008


Porous Asphalt Pavement 

POROUS ASPHALT PAVEMENT 



Description 

General 

Porous asphalt pavement may be substituted for conventional 

pavement on parking areas and areas of light traffic provided the appropriate 

grades, subsoil, drainage characteristics and groundwater conditions are all 

suitable. Porous asphalt consists of an open graded hot mix asphalt that contains 

less than 3% of fines passing a #200 U.S. Standard Sieve. The absence of 

fines creates permeability. The infiltration bed below the asphalt is 

comprised of choker course 

aggregate, course aggregate 

and a sand bed. As with any 

asphalt pavement installed 

in Colorado, the freeze/thaw 

cycles and the 

expansive soils must 

be taken into account. It is 

recommended that porous asphalt be placed between April and October 

(when the ambient air temperature is 55 degrees Fahrenheit or greater) with 

sufficient base course aggregate, an underdrain trench and an impermeable liner 

to prevent failure. Proper site evaluation shall take place prior to approval of a 

site for porous asphalt application and shall include tests for soil permeability, 

porosity, depth of seasonal high water table, and depth to bedrock. Slopes 

should be flat or very gentle and shall not exceed 5 percent. 

Site Specific 

The porous asphalt pavement site monitored by the 

District is located at the Denver Wastewater Management 

Division building. The asphalt was placed in April of 2005 by 

companies who donated time and materials under the leadership 

of the Colorado Asphalt Pavement Association. The design 

included 3” of open graded hot mix asphalt and 18.5” of #67



gravel, #3 gravel and C-33 sand. In a trench beneath the sand, a perforated pipe 

collects the filtered water and conveys it to a catch basin. There is an orifice 

plate at the end of the pipe where a pressure transducer measures the flow into 

the catch basin from the porous asphalt pavement. There is 

also a weir plate at the outlet of the catch basin with a 

levelogger that measures the flow leaving the catch basin. 

The difference between the flow passing the orifice plate and 

the flow passing the weir plate is equal to the runoff that 

bypassed the porous asphalt pavement and entered the catch 

basin through the grate. 

The sampling equipment is stored in a metal box in the island 

adjacent to the porous asphalt pavement. A rain gage on a post 

near the storage box measures rainfall and signals the ISCO 

sampler inside the box to begin sampling after 0.1 inches of rain 

falls. The sampler then draws a sample of water from the porous 

asphalt pavement runoff after a designated flow has passed, and 

then every 15 minutes until 12 hours after the storm has ended. 

There is also a control watershed located a few hundred feet northeast of the 

porous asphalt pavement watershed. The control watershed consists primarily of 

traditional asphalt pavement and is used to compare results of treated runoff to 

untreated, direct runoff. Stormwater runoff from the control watershed is collected 

in a grated catch basin located in the northeast corner of the parking lot. The 

water leaves the catch basin by passing through a weir 

plate where a pressure transducer measures the flow. 

The sampling equipment is stored in a metal box 

adjacent to the parking lot in a manner similar to the 

porous asphalt pavement sampling configuration.




The porous asphalt pavement test site is at the Denver Wastewater 

Management building located at 2000 W. 3 rd Avenue in Denver. The 

pavement is located in the turn-around in front of the building’s main entrance, on 

the east side of the island. 


The watershed area tributary to the porous asphalt pavement is 12, 180 square 

feet, with the porous asphalt taking up 1840 square feet of that area. The 

watershed consists primarily of impervious areas, including buildings, parking lots 

and paved areas.




5.0' 

20.0' 

92.0' 

20.0' 

TEMPORARY ORANGE FENCE 

POROUS 

ASPHALT 

25' 

18" RCP 

@ 1.0% 

INLETS: 

3.5' x 2.0' 

DWMD 

BUILDING 

CONVENTIONAL 

ASPHALT 

Porous Asphalt Pavement Plan 

AASHTO #67, all fractured 

faces; (CDOT SECT. 703-2, 

#67 course aggregate) 

Open-Graded HMA 

ASTM C-33 Sand 

Base Course: #3 Aggregate 

(CDOT Section 703-2) 

2% Min. 

18.0" Min. 

2.5" 

2.0" 

7.0" 

1.0" 

6.0" 

Fill under-drain trench 

around pipe with 


AASHTO #67 stone 

ASTM D4751 - AOS U.S. STD. sieve #50 to #70 

ASTM D4633 - Min. trapezoidal tear strength 100 X 60 lbs, 

Minimum COE specified open area of 4% 

6.0" 

6.0" 

1" Thick sand 

cushion layer 

16 mil impermeable liner 

under all paved areas 

wrapped to top of pavement 

Schedule 40 HDPE 

4" Under-drain 

Porous Asphalt Pavement Section




Design Thickness 

Asphalt Binder 

Design the thickness of the porous 

asphalt slab to have a finished 

thickness of 2.5 inches with a 

bituminous binder of 5.5% to 6.5% by 

weight dry aggregate. 

Use neat asphalt binder modified with an elastomeric 

polymer to produce a binder meeting the requirements of 

PG 76022. The elastomeric polymer shall be styrenebutadiene-styrene 

(SBS) or approved equivalent, applied 

at a rate of 3% by total weight of the binder. The 

composite materials shall be thoroughly blended at the 

asphalt refinery or terminal prior to being loaded into the 

transport vehicle. The polymer-modified asphalt binder 

shall be heat and storage stable. 

Aggregate Gradation Aggregate in the asphalt mix shall be a minimum 90% 

crushed material and have the following gradation: 

U.S. Standard Sieve Size Percent Passing 

0.5-inch (12.5 mm) 100 

0.375-inch (9.5 mm) 75-95 

#4 (4.75 mm) 25-35 

#8 (2.36 mm) 10-15 

#16 (1.18 mm) 5-10 

#30 (600 µm) 1-5 

#200 (75 µm) 1-3 

Batch Testing 

To test draindown, air voids and abrasion, prepare 

three batches containing asphalt binder contents of 

5.5, 6.0 and 6.5 percent by dry weight aggregate. The 

asphalt content that provides the best results according 

to the following table shall be selected as the final mix. 

Test the final mix for Moisture Susceptibility and 

Resistance to Stripping. If the final mix does not meet the 

requirements for moisture susceptibility and resistance to 

stripping, add hydrated lime until the requirements are 

met.



Asphalt Test Requirement Comments 

Binder 

Downdrain 

(ASTM D 6390) 

Air Voids of 

Compacted Mix 

Cantabro 

Abrasion Test 

(unaged sample) 

Moisture 

Susceptibility by 

the modified 

Lottman method 

(AASHTO T 283) 

Resistance to 

Stripping by 

Water (ASTM D 

3625) 

0.3% Maximum Test at 15ºC 

higher than 

production 

temperature 

20% Minimum Compact using 

50 gyrations of 

Superpave 

gyratory 

compactor 

20% Maximum None 

80% Minimum 

Tensile Strength 

Ratio (TSR) 

95% Minimum 

Coating Area 

Only required 

for final mix 

Only required 

for final mix 

Choker Course The choker course aggregate shall be AASHTO #67 

Aggregate 

with all fractured faces. The choker 

course aggregate shall have a 

uniform thickness of 2 inches. 

Base Course 

Sand Layer 

The base course aggregate shall be 

AASHTO #3 with a uniform 

thickness of 7 inches. The base 

course aggregate shall rest on top 

of a 1-inch thick sand cushion 

layer in addition to the required sand layer and on top of 

the geotextile fabric. 

The bottom sand layer shall be ASTM C-33 sand and 

will be installed under the base course and above the 

underdrain trench, when one is used.




The geotextile fabric shall be installed below the sand 

layer. The woven fabric shall conform to the following 

requirements: 

o ASTM D 4751 – AOS U.S. STD sieve #50 to #70 

o ASTM D 4633 – Minimum trapezoidal tear strength 

100 x 60 lbs 

o Minimum COE specified open area of 4% 

Possible geotextile fabric may be US 670, Mirafi 

Filterweave 500, or approved equivalent. 


Fabric and Liner 




16 mil impermeable 

liner on the bottom and 

sides of all areas to be 

paved including the 

bottom of the underdrain 

trench. The impermeable 

liner shall be wrapped to the top of the perimeter walls 

and attached to the sides securely. 

If soils are not expansive (NRCS Types A, B or C) use 

woven geotextile fabric (as described above) as a liner. 

Place fabric and liner by rolling parallel to the contours 

starting at the most downstream part of the pavement. 

Provide a minimum of 18” overlap between adjacent 

sheets. Bring up geotextile fabric and impermeable liner 

to the top of the perimeter walls and attach with roofing 

tar or other adhesive or concrete anchors. Provide 

sufficient slack in the geotextile fabric and impermeable 

liner to prevent stretching during the installation of the 

infiltration bed, choker course and wearing course 

aggregate. Seal all joints of the impermeable liner to be 

totally leak free. 


thicker PE or PVC membrane liner or concrete walls 

installed parallel to the contours (i.e. normal to the flow) 

to prevent flow of water downstream and then surfacing 

at the toe of the porous asphalt pavement installation. 

Distance (L max ) between these cut-off barriers shall not 

exceed:



D 

L max = 1. 5∗ So 

◊ Lmax = maximum distance between cut-off 

membrane normal to the flow (ft) 

◊ So = slope of the base course (ft/ft) 

◊ D = depth of gravel base course (ft) 


When the porous asphalt pavement is located on NRCS 

Type D soils, when the Type B or C soil sub-base is to be 

compacted for structural reasons, or when an 

impermeable membrane liner is needed, install a 

subdrain system using perforated HDPE pipe. 

Locate each perforated pipe just upstream of the lateralflow 

cut-off barrier. Do not exceed 20-foot spacing. Use a 

control orifice sized to drain the pore volume of empty 

each cell in 6 hours or more. 


And Effective contributing impervious area/porous pavement area). The 

Imperviousness interim recommendations for the effective 

imperviousness are given in the Manual.



Photo Gallery 

Installation Photos – April 2008 

Site demo Excavated pit Coring into the catch basin 

Placing the #3 aggregate Layer of #67 aggregate Looking south 

Looking north Monitoring equipment box Porous vs. traditional asphalt 

ISCO Samplers Rain gage Rain gage mechanism



Performance Check – July 2008 (3 months after install) 

Denver Public Works supplied Water dispensed from truck Flow over asphalt 

the water 

Outlet weir in catch basin Flow through orifice plate Equipment in catch basin




2008 Flow Data 

2008 Water Quality Data



Other Porous Asphalt Pavement Examples 


porous asphalt pavement but are not monitored by the District. The 

District’s involvement in the design, construction and maintenance of these 

porous asphalt pavement sites was either minimal or non-existent, but a photo 


purposes as to the longevity and durability of the pavement. 

Wal-Mart 

I-70 and Tower Road in Aurora 


Approximate installation date: 2005 

April/June 2008 (3 years after install)


Permeable Interlocking Concrete Pavement 

PERMEABLE INTERLOCKING 

CONCRETE PAVEMENT 



Description 

General 

Permeable interlocking concrete pavement (PICP) consists of concrete block 

units with open surface voids laid on a gravel subgrade. These voids occupy 

at least 20% of the total surface area that are filled 

with sand or sandy loam turf that has at least 50% 

sand by weight in its volume. However, unless the 

pavement will be watered regularly (i.e., using a 

sprinkler system) to keep the vegetation viable, 

concrete sand infill is the recommended 

material. Permeable interlocking concrete 

pavement may be sloped or flat. PICPs 

have been in use in the United States since 

the mid-1970s. Although field data that 

quantify their long-term performance are somewhat limited, the data 

collected locally, and at other parts or the country and the episodic 

reports from Canada, Australia, Asia and Europe indicate that properly 

installed PICP are reliable and have experienced few problems under a 

wide range of climates. An alternate application of PICP provides for a 

surcharge zone above its surface to detain runoff and provide storage 

space for the water quality capture volume. 

Site Specific 

The permeable interlocking concrete pavement site monitored 

by the District is located at the Denver Wastewater 

Management Division building. The PICP was placed 

in May of 2005 by Rocky Mountain Hardscapes. The design 

included a 3 1/8” paver on top of 16 inches of #8 gravel, #67



gravel and C-33 sand. In a trench beneath the sand, a perforated 

pipe collects the filtered water and conveys it to a catch basin. 

There is an orifice plate at the end of the pipe where a pressure 

transducer measures the flow into the catch basin from the PICP. 

There is also a weir plate at the outlet of the catch basin with a 

levelogger that measures the flow leaving the catch basin. The difference 

between the flow passing the orifice plate and the flow passing the weir 

plate is equal to the runoff that bypassed the PICP and entered the 

catch basin through the grate. 

The sampling equipment is stored in a metal box in the island adjacent 

to the PICP. A rain gage on a post near the storage box measures 

rainfall and signals the ISCO sampler inside the box to begin sampling 

after 0.1 inches of rain falls. The sampler then draws a sample of water 

from the PICP runoff after a designated flow has passed, and then 

every 15 minutes until 12 hours after the storm has ended. 

There is also a control watershed located a few hundred feet northeast of the 

PICP watershed. The control watershed consists primarily of traditional asphalt 

pavement and is used to compare results of treated runoff to untreated, direct 

runoff. Stormwater runoff from the control watershed is collected in a grated 

catch basin located in the northeast corner of the parking lot. The water leaves 

the catch basin by passing through a weir plate where a pressure transducer 

measures the flow. The sampling equipment is stored 

in a metal box adjacent to the parking lot in a manner 

similar to the PICP sampling configuration.




The permeable interlocking concrete pavement test site is at the Denver 

Wastewater Management building located at 2000 W. 3 rd Avenue in Denver. 

The pavement is located in the turn-around in front of the building’s main 

entrance, on the west side of the island. 


The watershed area tributary to the PICP is xxxx square feet, with the pavers 

taking up 1840 square feet of that area. The site consists primarily of impervious 

areas, including buildings, parking lots and paved areas.




18" RCP 

@ 1.0% 

25' 

SAMPLING 

EQUIPMENT BOX 

CONDUIT 

INLETS: 

3.5' x 2.0' 

TRADITIONAL 

ASPHALT 

PAVEMENT 

20.0' 

5.0' 

20.0' 

TEMPORARY ORANGE FENCE 

92.0' 

COBBLESTONE 

BLOCK 

PAVEMENT 

Permeable Interlocking Concrete Pavement Plan 

Base Course: AASHTO #67, all fractured faces; 

(CDOT SECT. 703, #67 course aggregate) 

In-fill and leveling course: AASHTO crushed #8 (CDOT 

sect. 703, #8 course aggregate), all fractured faces 

Cobblestone blocks with at 

least 8% surface area open 

3.125" 

2.0" 

7.0" 

1.0" 

6.0" 

Wrap all geotextile 

fabric and impermeable 

liner to top of cobble 

block. Attach to wall or 

side of trench securely. 

1" Thick sand 

cushion layer 

Note: Extend 

geotextile fabric 

and minimum of 

18" beyond trench 


ASTM D4751 - AOS U.S. STD. sieve #50 to #70 

ASTM D4633 - Min. trapezoidal tear strength 100 X 60 lbs, 

Minimum COE specified open area of 4% 

Fill under-drain trench 

around pipe with 

AASHTO #67 stone 

Schedule 40 HDPE 

Perforated 4" Under-drain 

16 mil impermeable liner 

under all pavement wrapped 

to top of pavement 

ASTM C-33 Sand 

Permeable Interlocking Concrete Pavement Section




Select Blocks Select PICP blocks that have 8% 

open area or more. Follow 

manufacturer’s installation 

instructions, except that infill and 

base course materials and 

dimensions specified in this section shall be strictly 

adhered to. 

Infill Materials and The PICP openings shall be filled with AASHTO #8 

Leveling Course fractured aggregate and shall be placed on a one-inch 

thick leveling course of same #8 aggregate. 

Base Course 

The base course shall be AASHTO #67 coarse 

aggregate; all fractured surfaces. For volume 

calculations assume 30% of total volume to be 

open pore space. Unless an underdrain is 

provided, at least 6 inches of the subgrade 

underlying the base course shall be sandy and 

gravelly material with no more than 10% clay 

fraction. 

Sand Filter Layer The sand filter layer shall be ASTM C-33 

sand and will be installed under the base 

course and above the underdrain trench 

when one is used. 


Place a woven geotextile fabric on top and bottom 

of the base course. Use a geotextile material that meets 

the following requirements: 

o ASTM D-4751 – AOS U.S. Std. Sieve #50 to #70 

o D-4633 – Trapezoidal tear strength ≥ 100 x 60 lbs 

o COE specified minimum open area ≥ 4% 




16 mil thick impermeable liner on the bottom and 

sides of the basin under the pavement. If soils are not 

expansive (i.e. NRCS Type A, B or C), use a woven 

geotextile material that meets the requirements specified



under Geotextile Fabric above. Products that meet these 

requirements are: 

o US Fabric US 2070 and US 670 

o Mirafi Filterweave 500 and 700 

o Carthage Mills Carthage 6%. 

Fabric/Liner 

Installation 

Place by rolling fabric parallel to the contours starting at 

the most downstream part of the pavement. Provide a 

minimum of 18” overlap between adjacent sheets. 

Bring up geotextile and impermeable membrane to the 

top of perimeter walls. Attach membrane and 

fabric to perimeter walls with roofing tar or 

other adhesive or concrete anchors. Provide 

sufficient slack in the geotextile and 

membranes to prevent stretching them when 

sand and/or rock is placed. Seal all joints of 

impermeable membrane to be totally leak free. 

Perimeter Wall 

Recommend that a concrete perimeter wall 

be installed to confine the edges of the PICP 

block areas. 



thicker PE or PVC membrane liner or concrete walls 

installed parallel to the contours (i.e. normal to the flow) 

to prevent flow of water downstream and then surfacing 

at the toe of the PICP installation. Distance (Lmax) 

between these cut-off barriers shall not exceed: 

L max 

= 1. 5∗ 

D 

So 

o Lmax = maximum distance between cut-off membrane 

normal to the flow (ft) 

o So = slope of the base course (ft/ft) 

o D = depth of gravel base course (ft) 


When the PICP is located on NRCS Type D soils, when 

the Type B or C soil sub-base is to be 

compacted for structural reasons, or 

when an impermeable membrane 

liner is needed, install a subdrain



system using perforated HDPE pipe. Locate each 

perforated pipe just upstream of the lateral-flow cut-off 

barrier. Do not exceed 20-foot spacing. Use a control 

orifice sized to drain the pore volume of empty each cell 

in 6 hours or more. 


And Effective contributing impervious area/pervious pavement area). 

Imperviousness The interim recommendations for the Effective 

Imperviousness are provided in the Manual.



Photo Gallery 

Installation Photos – May 2005 

Before construction Excavated pit with concrete Sampling tube conduit 

perimeter wall 

#67 aggregate ASTM C-33 sand Placing the sand layer 

Permeable interlocking One unit of paver Placing the pavers 

concrete pavers 

Finished pavement



Performance Check – July 2008 (2 months after install) 

Denver Public Works supplied Water dispensed from truck Flow over asphalt 

the water 

Water never reached gutter Flow through orifice plate Outlet weir in catch basin 

Equipment in catch basin








Other Permeable Interlocking Concrete Pavement 

Examples 


permeable interlocking concrete pavement but are not monitored by the 

District. The District’s involvement in the design, construction and maintenance 

of these PICP sites was either minimal or absent, but a photo inventory of the 

sites will be kept up-to-date on this website for informational purposes as to the 

longevity and durability of the pavement. 

Wenk Associates 

1335 Elati Street in Denver 


Approximate installation date: April 2004 

Installation Photos - April 2004



August 2004 (4 months after install) 

April 2005 (1 year after install) 

June 2008 (4 years, 2 months after install)


Permeable Interlocking Concrete Pavement


Sand Filters 

SAND FILTERS 

A sand filter extended detention basin is a stormwater filter that consists of a 

runoff storage zone underlain by a sand bed with an underdrain system. 

During a storm, accumulated runoff ponds in the 

surcharge zone and gradually infiltrates into the 

underlying sand bed, filling the void spaces of the sand. 

The underdrain gradually dewaters the sand bed and 

discharges the runoff to a nearby channel, swale or storm 

sewer. 

A sand filter is generally suited to onsite configurations 

where there is no base flow and is put in operation 

when the upstream catchment no longer has construction 

or grading/landscaping activities. 

Primary advantages of sand filters include effective water quality enhancement 

through settling and filtering. The primary disadvantage is a potential for 

clogging if a moderate to high level of silts and clays are 

allowed to flow into the facility. For this reason, the sand filter 

should not be put into operation while construction or major 

landscaping activities are taking place in the tributary catchment. 

Also, this BMP should not be located close to building 

foundations or other areas where expansive soils are a concern, 

although an underdrain and impermeable line can ameliorate 

some of this concern. 

Since an underdrain system is incorporated into this BMP, sand filters are 

suitable for about any site; presence of sandy subsoil is not a requirement. This 

BMP has a flat surface area, so it may be more challenging to incorporate it into 

steeply sloping terrain. 

Sand Filters Monitored by District


Open Bed Sand Filter 

OPEN BED SAND FILTER 



Description 

The open bed sand filter that is monitored for flow volume 

and water quality by the District is located in Lakewood and 

was constructed in March/April of 2007. The vault is 

made of reinforced concrete with two cells separated by a 

concrete wall. The sand filter cell is 100’ long by 5’ wide by 

4’-4.5’ deep. The outlet cell is 4’ long by 5’ wide by 5’ deep. 

The sand filter cell contains a 15”-18” layer of sand, a nonwoven 

geotextile fabric, and a 5”-8” layer of aggregate. 

At the bottom of the sand filter cell there is a 4” perforated underdrain pipe 

that collects the filtered water and carries it to the outlet cell. An orifice plate with 

a 5/8” hole controls the outflow under regular flow conditions and a sharp-crested 

weir at the top of the wall separating the cells controls flow under high flow 

conditions. Pressure transducers located in the middle and at the end of the sand 

filter cell measure water depths. 

The sampling equipment for the inlet is stored in a wooden 

box near the upstream end of the sand filter and the equipment for 

the outlet is stored in a shed adjacent to the outlet of the sand 

filter. A rain gage on top of the shed measures rainfall and 

signals the ISCO samplers inside the box and the shed to begin 

sampling after 0.1 inches of rain falls. The samplers then draw a 

sample of water from the inlet and outlet after a designated 

amount of flow has passed, and then every 15 minutes until 12 

hours after the storm has ended. 

The total cost to construct the open bed sand filter was $74,000 in 2007. The 

costs included reinforced concrete, reinforced concrete pipe, removal and 

replacement of asphalt, earthwork, curb and gutter, geotextile, C-33 sand, #67 

aggregate, grating, orifice and weir plates, and perforated pipe.



Updates 

At the end of the first storm season, the sand filter was not 

draining properly. In the spring of 2008 the water in the 

sand filter was pumped out and the top layer of sand was 

scraped off. By July of 2008 the sand filter was again not draining. The pumping 

and scraping process was repeated. 

In the spring of 2008 the sharp-crested weir was replaced 

with a Cipoletti weir and lowered to the correct height. 

In 2008, a Cipoletti weir was added to the catch basin at the opening of the pipe 

that conveys flows captured in the catch basin to the 

sand filter. The intent of the weir is to allow a means to 

measure the amount of flow into the sand filter. A 

pressure transducer was also placed in the catch basin 

to measure the depth of flow over the weir.




The open bed sand filter test site is on the City of Lakewood maintenance 

buildings property located at 1050 Quail Street. The site consists primarily of 

impervious areas used for maintenance and storage of vehicles and equipment. 


The watershed area tributary to the sand filter is approximately 6,000 square feet 

(0.15 acres). The site contains a car wash which is regularly used, so many 

contaminants (oil, soap, etc.) drain into the sand filter.


Open Bed Sand Filter




Catch Basin 

Sand Filter Inlet 

Orifice Plate 

Sand Filter Cell 

Outfall to Stream 

100' 

Outlet Cell 

106' 

Open Bed Sand Filter Plan 

2-1/2" Dia 

x5" deep sleeve 

for fence post 

49" 13,5" 

2-1/2" Dia 

x5" deep sleeve 

for fence post 

49" 13,5" 

8" 

8" 

67" 

6" 

8" 

8" 

12" 

43" 

60" 

76" 

12" 8" 18" 

29" 

WATER LINE 

9.0'-10.5' 

FROM BACK 

OF CURB 

3.7' APPROX. 

DEPTH 

73" 

43" 

60" 

12" 8" 18" 

35" 

WATER LINE 

9.0'-10.5' 

FROM BACK 

OF CURB 

3.7' APPROX. 

DEPTH 

76" 

Upstream End 

Downstream End 

Open Bed Sand Filter Cross Sections




Basin Storage Volume Provide a storage volume equal to 100 percent of the 

WQCV based on a 40-hour drain time, above the 

sand bed of the basin. 

A. Determine the WQCV tributary catchment’s percent 

imperviousness. Account for the effects of DCIA or 

other runoff volume reducing facilities, if any, in the 

tributary catchments. 

B. Find the required storage volume (watershed inches 

of runoff): Determine the Required WQCV (watershed 

inches of runoff) based on a 40-hour drain time. 

C. Calculate the Design Volume in acre-feet as follows: 

⎛WQCV 

⎞ 

DesignVolu me = ⎜ ⎟ * Area 

⎝ 12 ⎠ 

In which: 

Area = Area tributary to the sand filter 

Basin Depth/Design 

Filter’s Surface Area 

Maximum depth for the Design Volume shall be 3 feet. 

Calculate the minimum sand filter area (As) of the basin’s 

bottom using: 

⎛ DesignVolume ⎞ 

As = ⎜ 

⎟ * 43,560 (square feet) 

⎝ 3 ⎠ 

Sand Media 

Provide, as a minimum, an 18-inch 

layer of C-33 sand. Keep the top 

surface flat. If side slopes need to be 

steeper than 3h:1v (4h:1v or flatter is 

preferred), use vertical walls. 

Granular Base and 

Underdrains 

Granular material shall have all fractured 

faces and meet the technical requirements of AASHTO 

#3, #4 or #67 aggregate.



Impermeable 

Membrane and 

Geotextile Liners 


when standard percolation tests show percolation 

drawdown rates exceeding 60 minutes per inch, or 


16 mil thick impermeable liner on the bottom and 

sides of the basin. If vertical walls are permeable or of 

stacked blocks, extend the impermeable liner behind the 

walls. 

If soils are not expansive (i.e., NRCS Type A, B or C), 

use a woven geotextile material that meets the 

following ASTM requirements: 

o D-4751 – AOS U.S. Std. Sieve #50 to #70 

o D-4633 – Trapezoidal tear strength >= 100x60 lbs 

o COE specified open area >= 4%. 

Products meeting requirements: US Fabric US 2070 and 

US 670, Mirafi Filterweave 500 and 700, Carthage Mills 

Carthage 6%. 

Wrap all liners to top of the sand filter basin and attach 

firmly with staples to the soil vertical wall using staples or 

concrete anchors. Provide sufficient slack so that the 

liners are not stretched when rock and sand/peat mix is 

placed. If tears are seen or discovered, repair them as 

recommended by manufacturer with no less than 18 

inches of overlap on all sides of the tear. 

Outlet Works 

When underdrains are needed, the outlet 

works consists of 4” perforated HDPE 

pipe to convey water to the overflow outlet 

structure. Space perforated pipe on 20 foot 

centers or less. At the outlet of the HDPE 

pipe into the box, install an orifice sized to 

empty the WQCV above the sand in no less 

than 24 hours. 

Provided an overflow outlet pipe out of the 

overflow structure to convey flows away from 

the filter basin when the runoff volume exceeds 

the WQCV at rates required by local 

jurisdiction to control the flood detention, 

typically the 10- and the 100-year storm.



The WQCV basin may be oversized to contain the 

EURV, in which case design the orifice outlet to drain the 

EURV in 72 hours and control the 100-year volume as 

described in the Manual. 

Inlet Works 

Provide an energy dissipating outlet for all inlet 

points into the sand filter. Use an impact basin for pipes 

and a baffle chute or grouted sloping boulder drop if a 

channel or swale is used. Install a Type VL or L riprap 

basin at the inlet over a geotextile fabric that is wrapped 

up on the sides to the top of the sand layer. Fill all rock 

voids with filter sand.



Photo Gallery 

Installation Photos (March/April 2007) 

Pre-construction site Concrete walls Finished sand filter 

During a storm Inlet sampling equipment Outlet sampling equipment 

Sharp-crested overflow weir Concrete wall separating cells Flow over weir 

Flow through outlet orifice Sampling tube at outlet Inflow vs. outflow samples



September 2007 (5 months after install) 

Outlet cell 

Debris in overflow weir 

June 2008 (1 year, 2 months after install) 

New Cipoletti overflow weir Installing the weir Catch basin at inlet 

New Cipoletti weir in inlet 

catch basin 

July 2008 (1 year, 3 months after install) 

Lubri-seal in sand filter Outflow to stream Sand filter not draining





2008 Water Quality Data 


Storm 

Number 

Rainfall 

(in) 

Start Time 

End Time 

Peak Outlet 

Depth (ft) 

Samples 

1 N/A 6/11/07 6:18 6/12/07 9:29 1.019 No 

3 N/A 7/2/07 15:22 7/3/07 15:52 N/A Yes 

2 N/A 7/7/07 15:01 7/8/07 14:30 0.693 Yes 

3 N/A 

7/31/07 

20:15 

4 N/A 8/9/07 13:54 

5 N/A 

8/13/07 

19:34 

6 0.08 9/23/07 8:38 

8/6/07 10:46 1.44 Yes 

8/12/07 

12:39 

8/15/07 

11:02 

9/24/07 

11:58 

0.19 No 

0.398 Yes 

0.153 No 




Other Sand Filter Examples 

The following are additional sites in the Denver Metro area that have used sand 

filters but are not monitored by the District. The District’s involvement in 

the design, construction and maintenance of these sand filter sites was either 

minimal or non-existent, but a photo inventory of the sites will be kept up-to-date 

on this website for informational purposes as to the longevity and durability of the 

sand filter.


Extended Detention Basins 

EXTENDED DETENTION BASINS 

An extended detention basin is a sedimentation basin designed to totally 

drain dry sometime after stormwater runoff ends. It is an adaptation of a 

detention basin used for flood control. The primary difference is the 

outlet design: the extended detention basin uses a much smaller 

outlet that extends the emptying time of the more frequently 

occurring runoff events to facilitate pollutant removal. The extended 

detention basin’s 40-hour drain time for the brim-full water 

quality capture volume is recommended to remove a significant 

portion of fine particulate pollutants found in urban stormwater 

runoff. Soluble pollutant removal can be somewhat enhanced by 

providing a small wetland marsh or ponding area in the basin’s bottom to 

promote biological uptake. The basins are considered to be “dry” because they 

are designed not to have a significant permanent pool of water remaining 

between storm runoff events. However, extended detention basins may 

develop wetland vegetation and sometimes shallow pools in the bottom 

portions of the facilities. 

An extended detention basin can be used to enhance 

stormwater runoff quality and reduce peak 

stormwater runoff rates. If these basins are 

constructed early in the development cycle, they can also 

be used to trap sediment from construction activities 

within the tributary drainage area. The accumulated 

sediment, however, will need to be removed after 

upstream land disturbances cease and before the basin 

is placed into final long-term use. An extended detention basin can sometimes be 

retrofitted into existing flood control detention basins. 

Extended detention basins can be used to improve the quality of urban runoff 

and are generally used for regional or follow-up treatment. Extended detention 

basins are most applicable for catchments with a tributary impervious area of 10 

acres or more. 

Extended Detention Basins Monitored by District


Orchard Pond EDB 

ORCHARD POND EDB 



Description 

The Orchard Pond extended detention basin, located at Grant Ranch in 

Denver, was constructed in 1998 by the Bowles Metropolitan District. The District 

has been collecting water quality and flow data from the pond since 2004. 

By studying the design of the basin and the parameters of the watershed, the 

statistical data will be used to develop relationships between the tributary 

catchment characteristics, the extended detention basin design parameters and 

their effects on water quality. With the results from the monitoring, we are able to 

determine how effectively the basin stores different stormwater volumes and to 

study the hydraulic functioning of the outlet structure.



Since the initial construction, the following modifications have been made to 

improve the facility: 

o In 2004, a micropool was 

incorporated into the basin and the 

outlet structure was improved with 

a perforated plate. 

o In 2005, the inlet structure was 

modified with an improved 

forebay and an energy dissipater 

in order to improve sedimentation. 

Orchard Pond is directly upstream of Bowles Lake, a large facility used for 

recreation, fishing and boating. Therefore it is important to protect these waters 

from metals, oil, grease and sediment that is typically found in urban stormwater 

runoff. 

UDFCD has been collecting water quality and flow data at this site since 2004. 

Wright Water Engineers is responsible for running and maintaining the 

sampling equipment and taking the samples to the lab after a storm.




The Orchard Pond extended detention basin is located in the northeast corner of 

the Grant Ranch residential development in Denver. Specifically, the basin lies 

between South Harlan Way and West Prentice Circle, east of South Jay Circle. 


The watershed area tributary to Orchard Pond is approximately 16.9 acres. The 

watershed contains single-family residential homes, paved roads and open 

space, with a total site imperviousness of 50%. 

Legend 

Impervious area 

Pervious area 

Surface runoff 

Orchard detention pond 

Inlet




Orchard Pond EDB – Plan View

Orchard Pond EDB – Flow Dissipater and Forebay 


Orchard Pond EDB




Basin Storage 

Volume 

Provide a storage volume equal to 120% of the WQCV 

based on a 40-hour drain time, above the lowest outlet 

(i.e. perforation) in the basin. The 

additional 20 percent of storage 

volume provides for sediment 

accumulation and the resultant 

loss in storage volume. 

A. Determine the WQCV tributary 

catchment’s 

percent 

imperviousness. Account for 

the effects of DCIA, if any, on 

Effective Imperviousness. 

Using runoff volume reduction 

practices in the tributary catchment, determine the 

reduction in impervious area to use with WQCV 

calculations. 

B. Find the required storage volume (watershed inches 

of runoff): 

Determine the Required WQCV (watershed inches of 

runoff) based on a 40-hour drain time. 

Calculate the Design Volume in acre-feet as follows: 

⎛WQCV 

⎞ 

DesignVolu me = ⎜ ⎟ * Area 

⎝ 12 ⎠ 

In which: 

Area = Area tributary to the sand filter 

1.2 factor = Multiplier of 1.2 to account for the 

additional 20% or required storage for sediment 

accumulation 

Outlet Works 

The outlet works are to be designed to release the 

WQCV (not the Design Volume) over a 40-hour 

period. Refer to the Manual for 

schematics pertaining to structure 

geometry; grates, trash racks and 

screens; outlet type (orifice plate or 

perforated riser pipe); cutoff collar size 

and location; and all other necessary 

components.



For a perforated outlet, calculate the required area per 

row based on WQCV and the depth of perforations at the 

outlet. The lowest perforations should be set at the water 

surface elevation of the outlet micro-pool. The total outlet 

area is calculated by multiplying the area per row by the 

number of rows. 

Minimize the number of columns and maximize the 

perforation hole diameter when designing outlets to 

reduce chances of clogging by accepting the orifice site 

that will empty the WQCV in 36 to 44 hours. 

Trash Rack 

Provide a trash rack of sufficient size to prevent 

clogging of the primary water quality outlet. Size 

the rack so as not to interfere with the hydraulic 

capacity of the outlet. Using the total outlet area 

and the selected perforation diameter (or 

height), determine the minimum open area 

required for the trash rack. 

Use one-half of the perforated plate’s total outlet 

area to calculate the trash rack’s size. This 

accounts for the variable inundation of the outlet 

orifices. 

Basin Shape 

Two-Stage Design 

Shape the pond whenever possible with a gradual 

expansion from the inlet and a gradual contraction toward 

the outlet in order to minimize short circuiting. It is 

best to have a basin length to width ratio between 2:1 to 

3:1. To achieve this, it may be necessary to modify the 

inlet and outlet points through the use of pipes, swales or 

channels to accomplish this. 

Always maximize the distance 

between the inlet and the 

outlet. 

A two-stage design with a pool 

that fills often with frequently 

occurring runoff minimizes 

standing water and sediment 

depostition in the remainder of 

the basin. The two stages are 

as follows:



A. Top Stage: The top stage should be one or more 

feet deep with its bottom sloped at 1 to 2 percent 

toward the trickle flow channel. 

B. Bottom Stage: The dry weather water surface of the 

active surcharge volume of the bottom stage should 

be 0.5 feet or more below the bottom of the top stage, 

but no less than 4-inches below the invert of the 

upstream trickle channel, and store no less than 0.5 

percent of the WQCV. 

Provide a permanent micro-pool below the active 

storage volume of the lower stage in front of the outlet. 

The pool should be ½ the depth of the top stage depth 

described above, or 2.5 feet, whichever results in the 

larger depth. Line bottom of the micro-pool with concrete 

paving at least 6-inches thick or with grouted boulders 

grouted to the top of the boulders. 

Low-Flow Channel 

Basin Side Slopes 

Dam Embankment 

Conveys low flows from the forebay to the 

bottom stage. A concrete-bottomed 

channel is recommended to provide 

maintainability. Otherwise line its sides with 

buried Type VL soil riprap and bottom with 

concrete. Make it at least 4-inches deep if 

concrete lined sides and 8-inches if buried 

riprap sides are used. At a minimum provide capacity 

equal to twice the release capacity at the upstream 

forebay outlet. 

Basin side slopes should be stable and gentle to facilitate 

maintenance and access. Side slopes should be no 

steeper than 4:1 and the use of flatter slopes is 

recommended: the flatter, the better and safer. 

Design the embankment not to fail during a 100-year and 

larger storms. Embankment slopes should be no steeper 

than 3:1, preferably 4:1 or flatter, and planted with turf 

forming grasses. Poorly compacted native soils should 

be excavated and replaced. 

Embankment soils should be 

compacted to at least 95 

percent of their maximum 

density according to ASTM D 

698-70 (Modified Proctor). 

Spillway structures and



overflows should be designed in accordance with local 

drainage criteria and should consider UDFCD dropstructure 

design guidelines or the use of buried soil riprap 

or reinforced turf mats installed per manufacturer 

recommendations. 

Vegetation 

Access 

Inlet 

Forebay Design 

Bottom vegetation provides erosion control and sediment 

entrapment. Pond bottom, berms and side sloping areas 

may be planted with native grasses or irrigated 

turf, depending on the local setting and needs. 

All-weather, stable access to the bottom, forebay and 

outlet works area shall be provided for maintenance 

vehicles. Grades should not exceed 10 percent, and a 

solid driving surface of gravel, rock, concrete, 

gravel-stabilized turf, or Type VL soil riprap should be 

provided. 

Dissipate flow energy at 

pond’s inflow point(s) to 

limit erosion and promote 

particle sedimentation. 

Inlets should be designed 

in accordance with UDFCD 

drop structure criteria, 

impact basin outlet details 

or other types of energy 

dissipating structures. 

Provide an opportunity for larger particles to settle out in 

the inlet in an area that has a solid surface bottom to 

facilitate mechanical sediment removal. A rock berm or 

concrete wall should be constructed between the forebay 

and the main extended detention basin. 

The forebay volume of the permanent 

pool should be about 3 to 5% of the 

WQCV. A pipe throughout the berm to 

convey water the main body of the basin 

should be offset from the inflow 

streamline to prevent short circuiting and 

should be sized to drain the forebay 

volume in 3 to 5 minutes, respectively. 

The floor of the forebay should be 

concrete or grouted boulder lined to define sediment 

removal limits.



Flood Storage 

Multiple Uses 

Combining the water quality facility with a flood control 

facility is recommended. The 5-year, 10-year, 100-year or 

other floods may be detained above the WQCV. 

When desirable and feasible, incorporate the extended 

detention basin within a larger flood control basin. Also, 

whenever possible, try to provide for other urban uses 

such as active or passive recreation and wildlife habitat. 

If multiple uses are being contemplated, use the multiplestage 

detention basin design approach to limit 

inumdation of passive recreational areas to one or two 

occurrences a year. The area within the WQCV is not 

suited for active recreation activities such as ballparks, 

playing fields, and picnic areas. There are best located 

above the WQCV level.



Photo Gallery 

Constructed in 1998 (no construction photos available) 

July 2001 – 20 hours after 1 inch rainfall (3 years after install) 

Full micro-pool Storage in trickle channel Trash rack submerged 

March 2004 – Maintenance Photos (6 years after install) 

Trickle channel cleaned with Mucking out the micro-pool Vactor truck used for 

spray gun 

maintenance 

Sediment in micro-pool



February 2005 – Inlet Modification (6 ½ years after install) 

Forming the energy dissipater Forming new manhole New manhole 

upstream of pond 

New Palmer-Bowlus in pipe 

(looking downstream) 

New Palmer-Bowlus in pipe 

(looking upstream) 

June 2005 (7 years after install) 

Facing west Multi-staged outlet structure Sediment in forebay 

Facing east



July 2006 (8 years after install) 

Trickle channel to outlet Trickle channel to forebay Facing west 

Pond inlet Energy dissipater Forebay 

Mirco-pool Micro-pool and outlet Multi-staged outlet structure 

Trash rack and orifice 

Sampling equipment box








Other Extended Detention Basin Examples 


extended detention basins but are not monitored by the District. The 

District’s involvement in the design, construction and maintenance of these 

extended detention basins was either minimal or non-existent, but a photo 


purposes as to the longevity and durability of the water quality facility. 

Silverado II

Erie Regional Detention Pond 1045 


Orchard Pond EDB

BMP Monitoring Sites - Urban Drainage and Flood Control District

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