HomeMy WebLinkAboutCITY OF FORT COLLINS UTILITIES CUSTOMER SERVICES BUILDING - PDP - PDP140005 - REPORTS - DRAINAGE REPORTDecember 24, 2014
PRELIMINARY DRAINAGE AND
EROSION CONTROL REPORT FOR
CITY OF FORT COLLINS UTILITIES
ADMINISTRATION BUILDING
Fort Collins, Colorado
Prepared for:
City of Fort Collins
Prepared by:
200 South College Avenue, Suite 10
Fort Collins, Colorado 80524
Phone: 970.221.4158 Fax: 970.221.4159
www.northernengineering.com
Project Number: 940-002
This Drainage Report is consciously provided as a PDF.
Please consider the environment before printing this document in its entirety.
When a hard copy is absolutely necessary, we recommend double-sided printing.
December 24, 2014
City of Fort Collins
Stormwater Utility
700 Wood Street
Fort Collins, Colorado 80521
RE: Preliminary Drainage and Erosion Control Report for
CITY OF FORT COLLINS UTILITIES ADMINISTRATION BUILDING
Dear Staff:
Northern Engineering is pleased to submit this Preliminary Drainage and Erosion Control Report
for your review. This report accompanies the Project Development Plan submittal for the
proposed City of Fort Collins Utilities Administration Building.
This report has been prepared in accordance to Fort Collins Stormwater Criteria Manual (FCSCM),
and serves to document the stormwater impacts associated with the proposed project. We
understand that review by the City is to assure general compliance with standardized criteria
contained in the FCSCM.
If you should have any questions as you review this report, please feel free to contact us.
Sincerely,
NORTHERN ENGINEERING SERVICES, INC.
Aaron Cvar, PE
Project Engineer
Utilities Administration Building
Preliminary Drainage Report
TABLE OF CONTENTS
I. GENERAL LOCATION AND DESCRIPTION ................................................................... 1
A. Location ............................................................................................................................................. 1
B. Description of Property ..................................................................................................................... 2
C. Floodplain.......................................................................................................................................... 3
II. DRAINAGE BASINS AND SUB-BASINS ....................................................................... 5
A. Major Basin Description .................................................................................................................... 5
B. Sub-Basin Description ....................................................................................................................... 5
III. DRAINAGE DESIGN CRITERIA ................................................................................... 5
A. Regulations........................................................................................................................................ 5
B. Four Step Process .............................................................................................................................. 5
C. Development Criteria Reference and Constraints ............................................................................ 6
D. Hydrological Criteria ......................................................................................................................... 6
E. Hydraulic Criteria .............................................................................................................................. 6
F. Modifications of Criteria ................................................................................................................... 7
IV. DRAINAGE FACILITY DESIGN .................................................................................... 7
A. General Concept ............................................................................................................................... 7
B. Specific Details .................................................................................................................................. 7
V. CONCLUSIONS ........................................................................................................ 8
A. Compliance with Standards .............................................................................................................. 8
B. Drainage Concept .............................................................................................................................. 8
EROSION CONTROL REPORT ............................................................................................ 15
APPENDICES:
APPENDIX A – Hydrologic Computations
APPENDIX B - Water Quality Design Computations
APPENDIX C – Erosion Control Report
Utilities Administration Building
Preliminary Drainage Report
LIST OF FIGURES:
Figure 1 – Aerial Photograph ................................................................................................ 2
Figure 2– Proposed Site Plan ................................................................................................ 3
Figure 3 – Existing Floodplains ............................................................................................. 4
MAP POCKET:
Proposed Drainage Exhibit
Utilities Administration Building
Preliminary Drainage Report 1
I. GENERAL LOCATION AND DESCRIPTION
A. Location
1. Vicinity Map
2. The project site is located in Section 11, Township 7 North, Range 69 West of the 6th
Principal Meridian, City of Fort Collins, County of Larimer, State of Colorado .
3. The project site is located just northeast of the intersection of Laporte Avenue and
Howes Street.
4. The project site lies within the Old Town Basin. Due to the site’s proximity to the
Howes Street outfall, there are no detention requirements for the site. However, the
site still must provide water quality treatment. Several water quality treatment
methods are proposed for the site, and are described in further detail below.
5. As this is an infill site, the area surrounding the site is fully developed.
6. No offsite flows enter the site from the south, west, or east. A small area to the north
of the site sheet flows onto the site.
Utilities Administration Building
Preliminary Drainage Report 2
B. Description of Property
1. The development area is roughly 1.25 net acres.
Figure 1 – Aerial Photograph
2. The subject property is currently composed of existing buildings, and landscaped
areas. Existing ground slopes are mild to moderate (i.e., 1 - 6±%) through the
interior of the property. General topography slopes from northwest to southeast.
3. According to the United States Department of Agriculture (USDA) Natural Resources
Conservation Service (NRCS) Soil Survey website:
http://websoilsurvey.nrcs.usda.gov/app/WebSoilSurvey.aspx,
the site consists of Nunn Clay Loam, which falls into Hydrologic Soil Group C.
4. The proposed project site plan is composed of the development of a City Utilities
Administration Building. Associated site work, water, and sewer lines will be
constructed with the development. Onsite water quality treatment is proposed and
will consist of several features which are discussed in Section IV, below.
Utilities Administration Building
Preliminary Drainage Report 3
Figure 2– Proposed Site Plan
5. There are no known irrigation laterals crossing the site.
6. The proposed land use is a City customer service building.
C. Floodplain
1. The project site is encroached by a City designated 100-year floodplain. The City of
Fort Collins Stormwater Utility has identified a 100-year floodplain and floodway in
Laporte Avenue, as well as a shallow flooding area through the project site. The
proposed building footprint has been located outside of the flood fringe and floodway
identified along Laporte Avenue.
Utilities Administration Building
Preliminary Drainage Report 4
Figure 3 –Area Floodplain Mapping
2. Due to the site’s proximity to the floodway identified in Laporte Ave., and because
modifications to Laporte Ave. are proposed, no-rise modeling is required for the
proposed development. No-rise modeling for the proposed modifications to Laporte
Ave. will be provided at Final.
Utilities Administration Building
Preliminary Drainage Report 5
II. DRAINAGE BASINS AND SUB-BASINS
A. Major Basin Description
1. The project site lies within the Old Town Basin. Detention requirements are to detain
the difference between the 100-year developed inflow rate and the historic 2-year
release rate. However, the site is adjacent to the Howes Street Outfall, and all runoff
will be directed into this outfall. Therefore, detention is not required for this particular
site. Water quality treatment is required and will be provided, as discussed below.
B. Sub-Basin Description
1. The subject property historically drains overland from northwest to southeast. Runoff
from the majority of the site has historically been collected in the existing Laporte
Avenue storm system running along the southern boundary of the site.
2. A more detailed description of the project drainage patterns is provided below.
III. DRAINAGE DESIGN CRITERIA
A. Regulations
There are no optional provisions outside of the FCSCM proposed with the proposed
project.
B. Four Step Process
The overall stormwater management strategy employed with the proposed project utilizes
the “Four Step Process” to minimize adverse impacts of urbanization on receiving waters.
The following is a description of how the proposed development has incorporated each
step.
Step 1 – Employ Runoff Reduction Practices
Several techniques have been utilized with the proposed development to facilitate the
reduction of runoff peaks, volumes, and pollutant loads as the site is developed from the
current use by implementing multiple Low Impact Development (LID) strategies including:
Conserving existing amenities in the site including the existing vegetated areas.
Providing vegetated open areas throughout the site to reduce the overall impervious
area and to minimize directly connected impervious areas (MDCIA).
Routing flows, to the extent feasible, through vegetated swales to increase time of
concentration, promote infiltration and provide initial water quality.
Step 2 – Implement BMPs That Provide a Water Quality Capture Volume (WQCV) with
Slow Release
The efforts taken in Step 1 will facilitate the reduction of runoff; however, urban
development of this intensity will still generate stormwater runoff that will require
additional BMPs and water quality. The majority of stormwater runoff from the site will
ultimately be intercepted and treated using extended detention methods prior to exiting the
site.
Step 3 – Stabilize Drainageways
There are no major drainageways within the subject property. While this step may not
seem applicable to proposed development, the project indirectly helps achieve stabilized
drainageways nonetheless. By providing water quality treatment, where none previously
existed, sediment with erosion potential is removed from downstream drainageway
Utilities Administration Building
Preliminary Drainage Report 6
systems. Furthermore, this project will pay one-time stormwater development fees, as
well as ongoing monthly stormwater utility fees, both of which help achieve City-wide
drainageway stability.
Step 4 – Implement Site Specific and Other Source Control BMPs.
The proposed project will improve upon site specific source controls compared to historic
conditions:
The proposed development will provide LID and water quality treatment; thus,
eliminating sources of potential pollution previously left exposed to weathering and
runoff processes.
C. Development Criteria Reference and Constraints
The subject property is surrounded by currently developed properties. Thus, several
constraints have been identified during the course of this analysis that will impact the
proposed drainage system including:
Existing elevations along the property lines will generally be maintained.
As previously mentioned, overall drainage patterns of the existing site will be
maintained.
Elevations of existing downstream facilities that the subject property will release to
will be maintained.
D. Hydrological Criteria
1. The City of Fort Collins Rainfall Intensity-Duration-Frequency Curves, as depicted in
Figure RA-16 of the FCSCM, serve as the source for all hydrologic computations
associated with the proposed development. Tabulated data contained in Table RA-7
has been utilized for Rational Method runoff calculations.
2. The Rational Method has been employed to compute stormwater runoff utilizing
coefficients contained in Tables RO-11 and RO-12 of the FCSCM.
3. Three separate design storms have been utilized to address distinct drainage
scenarios. A fourth design storm has also been computed for comparison purposes.
The first design storm considered is the 80th percentile rain event, which has been
employed to design the project’s water quality features. The second event analyzed is
the “Minor,” or “Initial” Storm, which has a 2-year recurrence interval. The third
event considered is the “Major Storm,” which has a 100-year recurrence interval.
The fourth storm computed, for comparison purposes only, is the 10-year event.
4. No other assumptions or calculation methods have been used with this development
that are not referenced by current City of Fort Collins criteria.
E. Hydraulic Criteria
1. As previously noted, the subject property maintains historic drainage patterns.
2. All drainage facilities proposed with the project are designed in accordance with
criteria outlined in the FCSCM and/or the Urban Drainage and Flood Control District
(UDFCD) Urban Storm Drainage Criteria Manual.
3. As stated above, the subject property is located in a City designated floodplain. The
proposed project does not propose to modify any natural drainageways.
Utilities Administration Building
Preliminary Drainage Report 7
F. Modifications of Criteria
1. The proposed development is not requesting any modifications to criteria at this time.
IV. DRAINAGE FACILITY DESIGN
A. General Concept
1. The main objectives of the project drainage design are to maintain existing drainage
patterns, and to ensure no adverse impacts to any adjacent properties.
2. Onsite water quality treatment will be provided, as well as other LID features, which
are discussed further below. Water quality capture volume for the purpose of water
quality treatment conforming to porous landscape detention (PLD) criteria will be
provided in a holding cell in the northern portion of the site.
3. Drainage patterns anticipated for drainage basins shown in the Drainage Exhibit are
described below. Drainage basins have been defined for preliminary design purposes
an are subject to change at Final design; however, general drainage patterns and
concepts are not expected to be significantly altered.
Basin 1
Basins 1 will generally drain via overland flow and roof drains into the proposed water
quality facilities along the north side of the site. A drainage swale will direct discharge
from the proposed roof drain system into a porous landscape detention (PLD) basin,
which will provide water quality treatment. Please see further discussion of water
quality and LID features in Section IV.B, below.
Basin 2
Basin 2 will generally drain via sheet flow into a series of proposed Rain Gardens as
shown on the Drainage Exhibit. Please see further discussion of water quality and LID
features in Section IV.B, below.
Basin OS1 and OS2
Basins OS-1 and OS2 consists portions of the site which physically cannot be directed
into the proposed water quality facilities. Runoff from these basins will be directed via
sheet flow into adjacent Right of Way, where it will be conveyed via curb and gutter
into existing storm line systems. We do not anticipate any issues arising from the free
release of these basins into adjacent Right of Way, as there is no significant change
from historic drainage conditions.
A full-size copy of the Drainage Exhibit can be found in the Map Pocket at the end of
this report.
B. Specific Details
1. A porous landscape detention (PLD) holding cell is proposed in the north
portion of the site. This pond will provide standard 12-hour porous landscape
detention (PLD) treatment.
2. A series of Rain Gardens will be provided along the Laporte Ave. frontage of the site.
It is our understanding that the Landscape Architect will design these Rain Gardens,
and will also design an underdrain system which will daylight into the existing Howes
Street Outfall reinforced concrete box (RCB) running along the east side of the site.
Utilities Administration Building
Preliminary Drainage Report 8
We have currently designed the majority of grading within Basin 2 to direct developed
runoff into the Rain Gardens.
3. The following table summarizes LID features and overall percentage of the basin being
treated by the proposed LID features.
Table 1 – LID Summary
Basin
Basin
Area
(sq.ft.)
Portion of Basin
Captured
(sq.ft.)
Treatment
Type
1 37815 37815
Porous Landscape
Detention
2 7777 5274 Rain Gardens
OS1 499 0 None
OS2 8503 0 None
Total 54594 43089 Total % Treated: 78.93%
4. Final design details, construction documentation, and Standard Operating
Procedures (SOP) Manual shall be provided to the City of Fort Collins for
review prior to Final Development Plan approval. A final copy of the approved
SOP manual shall be provided to City and must be maintained on-site by the
entity responsible for the facility maintenance. Annual reports must also be
prepared and submitted to the City discussing the results of the maintenance
program (i.e. inspection dates, inspection frequency, volume loss due to
sedimentation, corrective actions taken, etc.).
5. Proper maintenance of the drainage facilities designed with the proposed
development is a critical component of their ongoing performance and
effectiveness.
V. CONCLUSIONS
A. Compliance with Standards
1. The drainage design proposed with the proposed project complies with the City of Fort
Collins’ Stormwater Criteria Manual.
2. The drainage design proposed with this project complies with requirements for the Old
Town Basin.
3. The drainage plan and stormwater management measures proposed with the
proposed development are compliant with all applicable State and Federal regulations
governing stormwater discharge.
B. Drainage Concept
1. The drainage design proposed with this project will effectively limit any potential
damage associated with its stormwater runoff by providing detention and water
quality mitigation features.
2. The drainage concept for the proposed development is consistent with requirements
for the Old Town Basin.
Utilities Administration Building
Preliminary Drainage Report 9
References
1. Fort Collins Stormwater Criteria Manual, City of Fort Collins, Colorado, as adopted by Ordinance No.
174, 2011, and referenced in Section 26-500 (c) of the City of Fort Collins Municipal Code.
2. Larimer County Urban Area Street Standards, Adopted January 2, 2001, Repealed and
Reenacted, Effective October 1, 2002, Repealed and Reenacted, Effective April 1, 2007.
3. Soils Resource Report for Larimer County Area, Colorado, Natural Resources Conservation
Service, United States Department of Agriculture.
4. Old Town Master Drainage Plan, Baseline Hydraulics, Volume II, Anderson Consulting,
July 15, 2003.
5. Urban Storm Drainage Criteria Manual, Volumes 1-3, Urban Drainage and Flood Control
District, Wright-McLaughlin Engineers, Denver, Colorado, Revised April 2008.
APPENDIX A
HYDROLOGIC COMPUTATIONS
CHARACTER OF SURFACE:
Runoff
Coefficient
Percentage
Impervious Project: 940-002
Streets, Parking Lots, Roofs, Alleys, and Drives: Calculations By: ATC
Asphalt ……....……………...……….....…...……………….…………………………………. 0.95 100% Date:
Concrete …….......……………….….……….………………..….……………………………… 0.95 90%
Gravel ……….…………………….….…………………………..………………………………. 0.50 40%
Roofs …….…….………………..……………….…………………………………………….. 0.95 90%
Pavers…………………………...………………..…………………………………………….. 0.40 22%
Lawns and Landscaping
Sandy Soil ……..……………..……………….…………………………………………….. 0.15 0%
Clayey Soil ….….………….…….…………..………………………………………………. 0.25 0% 2-year Cf
= 1.00 100-year Cf = 1.25
Basin ID
Basin Area
(s.f.)
Basin Area
(ac)
Area of
Asphalt
(ac)
Area of
Concrete
(ac)
Area of
Roofs
(ac)
Area of
Gravel
(ac)
Area of
Lawn, Rain
Garden, or
Landscaping
(ac)
2-year
Composite
Runoff
Coefficient
10-year
Composite
Runoff
Coefficient
100-year
Composite
Runoff
Coefficient
Composite
% Imperv.
1 37815 0.87 0.000 0.087 0.346 0.000 0.435 0.60 0.60 0.75 45%
2 7777 0.18 0.000 0.089 0.004 0.000 0.086 0.61 0.61 0.77 47%
OS1 499 0.01 0.000 0.005 0.000 0.000 0.006 0.56 0.56 0.70 40%
OS2 8503 0.20 0.000 0.163 0.033 0.000 0.000 0.95 0.95 1.00 90%
DEVELOPED COMPOSITE % IMPERVIOUSNESS AND RUNOFF COEFFICIENT CALCULATIONS
Runoff Coefficients are taken from the City of Fort Collins Storm Drainage Design Criteria and Construction Standards, Table 3-3. % Impervious taken from UDFCD USDCM, Volume I.
10-year Cf = 1.00
December 1, 2014
Overland Flow, Time of Concentration:
Project: 940-002
Calculations By:
Date:
Gutter/Swale Flow, Time of Concentration:
Tt = L / 60V
Tc = Ti + Tt (Equation RO-2)
Velocity (Gutter Flow), V = 20·S½
Velocity (Swale Flow), V = 15·S½
NOTE: C-value for overland flows over grassy surfaces; C = 0.25
Is Length
>500' ?
C*Cf
(2-yr
Cf=1.00)
C*Cf
(10-yr
Cf=1.00)
C*Cf
(100-yr
Cf=1.25)
Length,
L
(ft)
Slope,
S
(%)
Ti
2-yr
(min)
Ti
10-yr
(min)
Ti
100-yr
(min)
Length,
L
(ft)
Slope,
S
(%)
Velocity,
V
(ft/s)
Tt
(min)
Length,
L
(ft)
Slope,
S
(%)
Velocity,
V
(ft/s)
Tt
(min)
2-yr
Tc
Rational Method Equation: Project: 940-002
Calculations By:
Date:
From Section 3.2.1 of the CFCSDDC
Rainfall Intensity:
1 1 0.87 11 8 6 0.60 0.60 0.75 2.13 4.21 9.63 1.1 2.2 6.3
2 2 0.18 6 5 5 0.61 0.61 0.77 2.76 4.87 9.95 0.3 0.5 1.4
OS1 OS1 0.01 5 5 5 0.56 0.56 0.70 2.85 4.87 9.95 0.0 0.0 0.1
OS2 OS2 0.20 5 5 5 0.95 0.95 1.00 2.85 4.87 9.95 0.5 0.9 1.9
DEVELOPED RUNOFF COMPUTATIONS
C100
Design
Point
Flow,
Q100
(cfs)
Flow,
Q2
(cfs)
10-yr
Tc
(min)
2-yr
Tc
(min)
C2
Flow,
Q10
(cfs)
Intensity,
i100
(in/hr)
Basin(s)
ATC
December 1, 2014
Intensity,
i10
(in/hr)
Rainfall Intensity taken from the City of Fort Collins Storm Drainage Design Criteria (CFCSDDC), Figure 3.1
C10
Area, A
(acres)
Intensity,
i2
(in/hr)
100-yr
Tc
(min)
Q C f C i A
APPENDIX B
WATER WAWAWATER QUALITY DESIGN COMPUTATIONS
WATER QUALITY DESIGN CALCULATIONS
Water Quality Capture Volume (12-Hr. PLD)
Project: 940-002
By: ATC
Date: 12/1/14
REQUIRED STORAGE & OUTLET WORKS:
BASIN AREA (ac) = 0.870 <-- INPUT from impervious calcs
BASIN IMPERVIOUSNESS PERCENT = 45.00 <-- INPUT from impervious calcs
BASIN IMPERVIOUSNESS RATIO = 0.4500 <-- CALCULATED
WQCV (watershed inches) = 0.160 <-- CALCULATED from Figure EDB-2
WQCV (ac-ft) = 0.012 <-- CALCULATED from UDFCD DCM V.3 Section 6.5
WQ Depth (ft) = ** <-- INPUT from stage-storage table
**To be completed at final design
Calculating the WQCV and Volume Reduction Chapter 3
3-6 Urban Drainage and Flood Control District August 2011
Urban Storm Drainage Criteria Manual Volume 3
Once the WQCV in watershed inches is found from Figure 3-2 or using Equation 3-1 and/or 3-2, the
required BMP storage volume in acre-feet can be calculated as follows:
𝑉 = �
WQCV
12
� 𝐴 Equation 3-3
Where:
V = required storage volume (acre-ft)
A = tributary catchment area upstream (acres)
WQCV = Water Quality Capture Volume (watershed inches)
Figure 3-2. Water Quality Capture Volume (WQCV) Based on BMP Drain Time
Bioretention T-3
November 2010 Urban Drainage and Flood Control District B-1
Urban Storm Drainage Criteria Manual Volume 3
Terminology
The term bioretention refers to the
treatment process although it is also
frequently used to describe a BMP
that provides biological uptake and
retention of the pollutants found in
stormwater runoff. This BMP is
frequently referred to as a porous
landscape detention (PLD) area or
rain garden.
Photograph B-1. This recently constructed rain garden provides
bioretention of pollutants, as well as an attractive amenity for a
residential building. Treatment should improve as vegetation matures.
Description
A BMP that utilizes bioretention is an
engineered, depressed landscape area
designed to capture and filter or infiltrate
the water quality capture volume
(WQCV). BMPs that utilize bioretention
are frequently referred to as rain gardens
or porous landscape detention areas
(PLDs). The term PLD is common in the
Denver metropolitan area as this manual
first published the BMP by this name in
1999. In an effort to be consistent with
terms most prevalent in the stormwater
industry, this document generally refers to
the treatment process as bioretention and
to the BMP as a rain garden.
The design of a rain garden may provide
detention for events exceeding that of the WQCV. There are
generally two ways to achieve this. The design can provide the
flood control volume above the WQCV water surface elevation,
with flows bypassing the filter usually by overtopping into an
inlet designed to restrict the peak flow for a larger event (or
events). Alternatively, the design can provide and slowly release
the flood control volume in an area downstream of one or more
rain gardens.
This infiltrating BMP requires consultation with a
geotechnical engineer when proposed near a structure. A
geotechnical engineer can assist with evaluating the suitability of
soils, identifying potential impacts, and establishing minimum
distances between the BMP and structures.
Bioretention
(Rain Garden)
Functions
LID/Volume Red. Yes
WQCV Capture Yes
WQCV+Flood Control Yes
Fact Sheet Includes
EURV Guidance No
Typical Effectiveness for Targeted
Pollutants3
Sediment/Solids Very Good1
Nutrients Moderate
Total Metals Good
Bacteria Moderate
T-3 Bioretention
B-2 Urban Drainage and Flood Control District November 2010
Urban Storm Drainage Criteria Manual Volume 3
Site Selection
Bioretention can be provided in a variety of areas within new
developments, or as a retrofit within an existing site. This
BMP allows the WQCV to be treated within areas
designated for landscape (see design step 7 for appropriate
vegetation). In this way, it is an excellent alternative to
extended detention basins for small sites. A typical rain
garden serves a tributary area of one impervious acre or less,
although they can be designed for larger tributary areas.
Multiple installations can be used within larger sites. Rain
gardens should not be used when a baseflow is anticipated.
They are typically small and installed in locations such as:
Parking lot islands
Street medians
Landscape areas between the road and a detached walk
Planter boxes that collect roof drains
Bioretention requires a stable watershed. Retrofit
applications are typically successful for this reason. When
the watershed includes phased construction, sparsely
vegetated areas, or steep slopes in sandy soils, consider
another BMP or provide pretreatment before runoff from
these areas reaches the rain garden. The surface of the rain
garden should be flat. For this reason, rain gardens can be
more difficult to incorporate into steeply sloping terrain;
however, terraced applications of these facilities have been
successful in other parts of the country.
When bioretention (and other BMPs used for infiltration) are
located adjacent to buildings or pavement areas, protective measures should be implemented to avoid
adverse impacts to these structures. Oversaturated subgrade soil underlying a structure can cause the
structure to settle or result in moisture-related problems. Wetting of expansive soils or bedrock can cause
swelling, resulting in structural movements. A geotechnical engineer should evaluate the potential impact
of the BMP on adjacent structures based on an evaluation of the subgrade soil, groundwater, and bedrock
conditions at the site. Additional minimum requirements include:
In locations where subgrade soils do not allow infiltration, the growing medium should be underlain
by an underdrain system.
Where infiltration can adversely impact adjacent structures, the filter layer should be underlain by an
underdrain system designed to divert water away from the structure.
In locations where potentially expansive soils or bedrock exist, placement of a rain garden adjacent to
structures and pavement should only be considered if the BMP includes an underdrain designed to
divert water away from the structure and is lined with an essentially impermeable geomembrane liner
designed to restrict seepage.
Benefits
Bioretention uses multiple
treatment processes to remove
pollutants, including
sedimentation, filtering,
adsorption, evapotranspiration,
and biological uptake of
constituents.
Volumetric stormwater treatment
is provided within portions of a
site that are already reserved for
landscaping.
There is a potential reduction of
irrigation requirements by taking
advantage of site runoff.
Limitations
Bioretention T-3
November 2010 Urban Drainage and Flood Control District B-3
Urban Storm Drainage Criteria Manual Volume 3
Designing for Maintenance
Recommended maintenance practices for all BMPs are in Chapter
6 of this manual. During design, the following should be
considered to ensure ease of maintenance over the long-term:
Do not put a filter sock on the underdrain. This is not
necessary and can cause the BMP to clog.
The best surface cover for a rain garden is full vegetation. Do
not use rock mulch within the rain garden because sediment
build-up on rock mulch tends to inhibit infiltration and require
frequent cleaning or removal and replacement. Wood mulch
handles sediment build-up better than rock mulch; however,
wood mulch floats and may clog the overflow depending on
the configuration of the outlet, settle unevenly, or be
transported downstream. Some municipalities may not allow wood mulch for this reason.
Consider all potential maintenance requirements such as mowing (if applicable) and replacement of
the growing medium. Consider the method and equipment for each task required. For example, in a
large rain garden where the use of hand tools is not feasible, does the shape and configuration of the
rain garden allow for removal of the growing medium using a backhoe?
Provide pre-treatment when it will reduce the extent and frequency of maintenance necessary to
maintain function over the life of the BMP. For example, if the site is larger than 2 impervious acres,
prone to debris or the use of sand for ice control, consider a small forebay.
Make the rain garden as shallow as possible. Increasing the depth unnecessarily can create erosive
side slopes and complicate maintenance. Shallow rain gardens are also more attractive.
Design and adjust the irrigation system (temporary or permanent) to provide appropriate water for the
establishment and maintenance of selected vegetation.
Design Procedure and Criteria
The following steps outline the design procedure and criteria, with Figure B-1 providing a corresponding
cross-section.
1. Basin Storage Volume: Provide a storage volume based on a 12-hour drain time.
Find the required WQCV (watershed inches of runoff). Using the imperviousness of the tributary
area (or effective imperviousness where LID elements are used upstream), use Figure 3-2 located
in Chapter 3 of this manual to determine the WQCV based on a 12-hour drain time.
Calculate the design volume as follows:
𝑉𝑉 = �
WQCV
12
� 𝐴𝐴 Equation B-1
Where:
V= design volume (ft3)
Is Pretreatment Needed
Designing the inflow gutter to
the rain garden at a minimal
slope of 0.5% can facilitate
sediment and debris deposition
prior to flows entering the BMP.
Be aware, this will reduce
maintenance of the BMP, but
may require more frequent
sweeping of the gutter to ensure
that the sediment does not
impede flow into the rain
garden.
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Benefits of Shredded Paper in Rain Garden Growing Media
Shredded paper, similar to other woody materials, captures nutrients from the compost and
slowly releases them as the paper decomposes. Compost alone will leach more nutrients than
desired.
As the paper decomposes, nutrients stored in the material are available to the vegetation.
Paper temporarily slows the infiltration rate of the media and retains moisture, providing
additional time for a young root system to benefit from moisture in the growing media.
A = area of watershed tributary to the rain garden (ft2)
2. Basin Geometry: A maximum WQCV ponding depth of 12 inches is recommended to maintain
vegetation properly. Provide an inlet or other means of overflow at this elevation. Depending on the
type of vegetation planted, a greater depth may be utilized to detain larger (more infrequent) events.
The bottom surface of the rain garden, also referred to here as the filter area, should be flat. Sediment
will reside on the filter area of the rain garden; therefore, if the filter area is too small, it may clog
prematurely. Increasing the filter area will reduce clogging and decrease the frequency of
maintenance. Equation B-2 provides a minimum filter area allowing for some of the volume to be
stored beyond the area of the filter (i.e., above the sideslopes of the rain garden).
Note that the total surcharge volume provided by the design must also equal or exceed the
design volume. Use vertical walls or slope the sides of the basin to achieve the required volume.
Use the rain garden growing medium described in design step 3 only on the filter area because this
material is more erosive than typical site soils. Sideslopes should be no steeper than 4:1
(horizontal:vertical).
𝐴𝐴 ≥ (2/3)
V
1 foot
Equation B-2
Where:
V= design volume (ft3)
A = minimum filter area (flat surface area) (ft2)
The one-foot dimension in this equation represents the maximum recommended WQCV depth in the
rain garden. The actual design depth may differ; however, it is still appropriate to use a value of one
foot when calculating the minimum filter area.
3. Growing Medium: For partial and no infiltration sections, provide a minimum of 18 inches of
growing medium to enable establishment of the roots of the vegetation (see Figure B-1). Previous
versions of this manual recommended a mix of 85% sand and 15% peat (by volume). Peat is a
material that typically requires import to Colorado and mining peat has detrimental impacts to the
environment (Mazerolle 2002). UDFCD partnered with the University of Colorado to perform a
study to find a sustainable material to replace peat. The study was successful in finding a
replacement that performed well for filtering ability, clogging characteristics, as well as seed
germination. This mixture consists of 85% coarse sand and a 15% compost/shredded paper mixture
(by volume). The study used thin (approximately 1/4 inch) strips of loosely packed shredded paper
mixed with an equal volume of compost. Based on conversations with local suppliers, compost
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containing shredded paper is not an uncommon request, although not typically provided in the
proportions recommended in this BMP Fact Sheet. Compost suppliers have access to shredded paper
through document destruction companies and can provide a mixture of Class 1 compost and shredded
paper. The supplier should provide the rain garden compost mixture premixed with coarse sand. On-
site mixing is not recommended.
Rain Garden Compost Mixture (by volume)
50% Class 1 STA registered compost (approximate bulk density 1000 lbs/CY)
50% loosely packed shredded paper (approximate bulk density 50 to 100 lbs/CY)
When using diamond cut shredded paper or tightly packed paper, use the bulk densities provided to
mix by weight.
The supplier should premix the rain garden compost mixture (above) with coarse sand, in the
following proportions, prior to delivery to the site:
Rain Garden Growing Medium
15% rain garden compost mixture described above (by volume)
85% coarse sand (either Class C Filter Material per Table B-2 or sand meeting ASTM C-33) (by
volume)
Table B-1 provides detailed information on Class 1 compost. Be aware, regular testing is not
required to allow a compost supplier to refer to a product as a specific STA class. However, regular
testing is required and performed through the United States Compost Council (USCC) Seal of Testing
Assurance (STA) Program to be a STA registered compost. To ensure Class 1 characteristics, look
for a Class 1 STA registered compost.
Other Rain Garden Growing Medium Amendments
The growing medium described above is designed for filtration ability, clogging characteristics, and
vegetative health. It is important to preserve the function provided by the rain garden growing
medium when considering additional materials for incorporation into the growing medium or into the
standard section shown in Figure B-1. When desired, amendments may be included to improve water
quality or to benefit vegetative health as long as they do not add nutrients, pollutants, or modify the
infiltration rate. For example, a number of products, including steel wool, capture and retain
dissolved phosphorus (Erickson 2009). When phosphorus is a target pollutant, proprietary materials
with similar characteristics may be considered. Do not include amendments such as top soil, sandy
loam, and additional compost.
Full Infiltration Sections
A full infiltration section retains the WQCV onsite. For this section, it is not necessary to use the
prescribed rain garden growing medium. Amend the soils to provide adequate nutrients to establish
vegetation. Typically, 3 to 5 cubic yards of soil amendment (compost) per 1,000 square feet, tilled 6
inches into the soil, is required for vegetation to thrive. Additionally, inexpensive soil tests can be
conducted to determine required soil amendments. (Some local governments may also require proof
of soil amendment in landscaped areas for water conservation reasons.)
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Table B-1. Class 1 Compost
Characteristic Criteria
Minimum Stability Indicator (Respirometry) Stable to Very Stable
Maturity Indicator Expressed as
Ammonia N / Nitrate N Ratio < 4
Maturity Indicator Expressed as
Carbon to Nitrogen Ratio < 12
Maturity Indicator Expressed as
Percentage of Germination/Vigor 80+ / 80+
pH – Acceptable Range 6.0 – 8.4
Soluble Salts – Acceptable Range
(1:5 by weight) 0 – 5 mmhos/cm
Testing and Test Report Submittal Requirement
Seal of Testing Assurance (STA)/Test
Methods for the Examination of
Composting and Compost (TMECC)
Chemical Contaminants
Equal or better than US EPA Class A
Standard, 40 CFR 503.13, Tables 1 & 3
levels
Pathogens Meet or exceed US EPA Class A standard,
40 CFR 503.32(a) levels
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4. Underdrain System: Underdrains are often necessary
and should be provided if infiltration tests show
percolation drawdown rates slower than 2 times the rate
needed to drain the WQCV over 12 hours, or where
required to divert water away from structures as
determined by a professional engineer. Percolation tests
should be performed or supervised by a licensed
professional engineer and conducted at a minimum depth
equal to the bottom of the bioretention facility.
Additionally, underdrains are required where
impermeable membranes are used. Similar to the
terminology used for permeable pavement sections, there
are three basic sections for bioretention facilities:
No-Infiltration Section: This section includes an
underdrain and an impermeable liner that does not
allow for any infiltration of stormwater into the
subgrade soils. It is appropriate to use a no-
infiltration system when either of the following is
true:
o Land use or activities could contaminate
groundwater when stormwater is allowed to
infiltrate, or
o The BMP is located over potentially expansive
soils or bedrock and is adjacent (within 10 feet) to
structures.
Partial Infiltration Section: This section does not
include an impermeable liner and, therefore; allows
for some infiltration. Stormwater that does not
infiltrate will be collected and removed by an
underdrain system.
Full Infiltration Section: This section is designed to
infiltrate all of the water stored into the subgrade below.
Overflows are managed via perimeter drainage to a
downstream conveyance element. UDFCD recommends
a minimum infiltration rate of 2 times the rate needed to
drain the WQCV over 12 hours.
When using an underdrain system, provide a control orifice
sized to drain the design volume in 12 hours or more (see
Equation B-3). Use a minimum orifice size of 3/8 inch to
avoid clogging. This will provide detention and slow release
of the WQCV, providing water quality benefits and reducing
impacts to downstream channels. Space underdrain pipes a
maximum of 20 feet on center. Provide cleanouts to enable
maintenance of the underdrain. Cleanouts can also be used to
conduct an inspection (by camera) of the underdrain system to
Important Design Considerations
The potential for impacts to adjacent
buildings can be significantly reduced
by locating the bioretention area at
least 10 feet away from the building,
beyond the limits of backfill placed
against the building foundation walls,
and by providing positive surface
drainage away from the building.
The BMP should not restrict surface
water from flowing away from the
buildings. This can occur if the top of
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ensure that the pipe was not crushed or disconnected during construction.
Calculate the diameter of the orifice for a 12-hour drain time using Equation B-3 (Use a minimum orifice
size of 3/8 inch to avoid clogging.):
𝐷𝐷12 hour drain time = �
𝑉𝑉
1414 𝑦𝑦0.41 Equation B-3
Where:
D = orifice diameter (in)
y = distance from the lowest elevation of the storage volume
(i.e., surface of the filter) to the center of the orifice (ft)
V = volume (WQCV or the portion of the WQCV in the rain garden)
to drain in 12 hours (ft3)
In previous versions of this manual, UDFCD recommended that the underdrain be placed in an
aggregate layer and that a geotextile (separator fabric) be placed between this aggregate and the
growing medium. This version of the manual replaces that section with materials that, when used
together, eliminate the need for a separator fabric.
The underdrain system should be placed within an 6-inch-thick section of CDOT Class C filter
material meeting the gradation in Table B-2. Use slotted pipe that meets the slot dimensions provided
in Table B-3.
Table B-2. Gradation Specifications for CDOT Class C Filter Material
(Source: CDOT Table 703-7)
Sieve Size Mass Percent Passing
Square Mesh Sieves
19.0 mm (3/4”) 100
4.75 mm (No. 4) 60 – 100
300 µm (No. 50) 10 – 30
150 µm (No. 100) 0 – 10
75 µm (No. 200) 0 - 3
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Table B-3. Dimensions for Slotted Pipe
Pipe Diameter Slot
Length1
Maximum Slot
Width
Slot
Centers1
Open Area1
(per foot)
4” 1-1/16” 0.032” 0.413” 1.90 in2
6” 1-3/8” 0.032” 0.516” 1.98 in2
1 Some variation in these values is acceptable and is expected from various pipe
manufacturers. Be aware that both increased slot length and decreased slot centers
will be beneficial to hydraulics but detrimental to the structure of the pipe.
5. Impermeable Geomembrane Liner and Geotextile Separator Fabric: For no-infiltration sections,
install a 30 mil (minimum) PVC geomembrane liner, per Table B-5, on the bottom and sides of the
basin, extending up at least to the top of the underdrain layer. Provide at least 9 inches (12 inches if
possible) of cover over the membrane where it is attached to the wall to protect the membrane from
UV deterioration. The geomembrane should be field-seamed using a dual track welder, which allows
for non-destructive testing of almost all field seams. A small amount of single track and/or adhesive
seaming should be allowed in limited areas to seam around pipe perforations, to patch seams removed
for destructive seam testing, and for limited repairs. The liner should be installed with slack to
prevent tearing due to backfill, compaction, and settling. Place CDOT Class B geotextile separator
fabric above the geomembrane to protect it from being punctured during the placement of the filter
material above the liner. If the subgrade contains angular rocks or other material that could puncture
the geomembrane, smooth-roll the surface to create a suitable surface. If smooth-rolling the surface
does not provide a suitable surface, also place the separator fabric between the geomembrane and the
underlying subgrade. This should only be done when necessary because fabric placed under the
geomembrane can increase seepage losses through pinholes or other geomembrane defects. Connect
the geomembrane to perimeter concrete walls around the basin perimeter, creating a watertight seal
between the geomembrane and the walls using a continuous batten bar and anchor connection
(see Figure B-3). Where the need for the impermeable membrane is not as critical, the membrane can
be attached with a nitrile-based vinyl adhesive. Use watertight PVC boots for underdrain pipe
penetrations through the liner (see Figure B-2).
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Table B-4. Physical Requirements for Separator Fabric1
Table B-5. Physical Requirements for Geomembrane
Property
Thickness
0.76 mm
(30 mil)
Test Method
Thickness, % Tolerance ±5 ASTM D 1593
Tensile Strength, kN/m (lbs/in) width 12.25 (70) ASTM D 882, Method B
Modulus at 100% Elongation, kN/m (lbs/in) 5.25 (30) ASTM D 882, Method B
Ultimate Elongation, % 350 ASTM D 882, Method A
Tear Resistance, N (lbs) 38 (8.5) ASTM D 1004
Low Temperature Impact, °C (°F) -29 (-20) ASTM D 1790
Volatile loss, % max. 0.7 ASTM D 1203, Method A
Pinholes, No. Per 8 m2 (No. per 10 sq. yds.) max. 1 N/A
Bonded Seam Strength, % of tensile strength 80 N/A
Property
Class B
Elongation Test Method
< 50%2
Elongation
> 50%2
Grab Strength, N (lbs) 800 (180) 510 (115) ASTM D 4632
Puncture Resistance, N (lbs) 310 (70) 180 (40) ASTM D 4833
Trapezoidal Tear Strength, N (lbs) 310 (70) 180 (40) ASTM D 4533
Apparent Opening Size, mm
(US Sieve Size)
AOS < 0.3mm (US Sieve Size No. 50) ASTM D 4751
Permittivity, sec-1 0.02 default value,
must also be greater than that of soil
ASTM D 4491
Permeability, cm/sec k fabric > k soil for all classes ASTM D 4491
Ultraviolet Degradation at 500
hours
50% strength retained for all classes ASTM D 4355
1 Strength values are in the weaker principle direction
2 As measured in accordance with ASTM D 4632
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Designing for Flood Protection
Provide the WQCV in rain gardens that direct excess flow into to a landscaped area providing the
flood control volume. Design the flood control outlet to meter the major event (100-year event) and
slowly release the difference in volume between the EURV and the WQCV. (This assumes that the
runoff treated by the rain gardens is routed directly into the outlet or infiltrates.) Providing
treatment in this manner will reduce inundation in the landscaped area to a few times per year,
resulting in an area better suited for multipurpose uses.
6. Inlet/Outlet Control: In order to
provide the proper drain time, the
bioretention area can be designed
without an underdrain (provided it
meets the requirements in step 4) or
the outlet can be controlled by an
orifice plate. Equation B-3 is a
simplified equation for sizing an
orifice plate for a 12-hour drain time.
7. How flow enters and exits the BMP
is a function of the overall drainage
concept for the site. Inlets at each
rain garden may or may not be
needed. Curb cuts can be designed
to both allow stormwater into the rain
garden as well as to provide release of
stormwater in excess of the WQCV.
Roadside rain gardens located on a
steep site might pool and overflow
into downstream cells with a single curb cut, level spreader, or outlet structure located at the most
downstream cell. When selecting the type and location of the outlet structure, ensure that the runoff
will not short-circuit the rain garden. This is a frequent problem when using a curb inlet located
outside the rain garden for overflow.
For rain gardens with concentrated points of inflow, provide for energy dissipation. When rock is
used, provide separator fabric between the rock and growing medium to minimize subsidence.
8. Vegetation: UDFCD recommends that the filter area be vegetated with drought tolerant species that
thrive in sandy soils. Table B-6 provides a suggested seed mix for sites that will not need to be
irrigated after the grass has been established.
All seed must be well mixed and broadcast, followed by hand raking to cover seed and then mulched.
Hydromulching can be effective for large areas. Do not place seed when standing water or snow is
present or if the ground is frozen. Weed control is critical in the first two to three years, especially
when starting with seed.
Do not use conventional sod. Conventional sod is grown in clay soil that will seal the filter area,
greatly reducing overall function of the BMP. Several successful local installations have started with
seed.
Photograph B-2. The curb cut shown allows flows to enter this
rain garden while excess flows bypass the facility. Note: trees are
not recommended inside a rain garden
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Urban Storm Drainage Criteria Manual Volume 3
When using an impermeable liner, select plants with diffuse (or fibrous) root systems, not taproots.
Taproots can damage the liner and/or underdrain pipe. Avoid trees and large shrubs that may
interfere with restorative maintenance. Trees and shrubs can be planted outside of the area of
growing medium. Use a cutoff wall to ensure that roots do not grow into the underdrain or place trees
and shrubs a conservative distance from the underdrain.
9. Irrigation: Provide spray irrigation at or above the WQCV elevation or place temporary irrigation
on top of the rain garden surface. Do not place sprinkler heads on the flat surface. Remove
temporary irrigation when vegetation is established. If left in place this will become buried over time
and will be damaged during maintenance operations.
Irrigation schedules should be adjusted during the growing season to provide the minimum water
necessary to maintain plant health and to maintain the available pore space for infiltration.
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Table B-6. Native Seed Mix for Rain Gardens 2
1 Wildflower seed (optional) for a more diverse and natural look.
2 PLS = Pure Live Seed.
Common Name Scientific Name Variety PLS2
lbs per
Acre
Ounces
per
Acre
Sand bluestem Andropogon hallii Garden 3.5
Sideoats grama Bouteloua curtipendula Butte 3
Prairie sandreed Calamovilfa longifolia Goshen 3
Indian ricegrass Oryzopsis hymenoides Paloma 3
Switchgrass Panicum virgatum Blackwell 4
Western wheatgrass Pascopyrum smithii Ariba 3
Little bluestem Schizachyrium scoparium Patura 3
Alkali sacaton Sporobolus airoides 3
Sand dropseed Sporobolus cryptandrus 3
Pasture sage1 Artemisia frigida 2
Blue aster1 Aster laevis 4
Blanket flower1 Gaillardia aristata 8
Prairie coneflower1 Ratibida columnifera 4
Purple prairieclover1 Dalea (Petalostemum) purpurea 4
Sub-Totals: 27.5 22
Total lbs per acre: 28.9
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Urban Storm Drainage Criteria Manual Volume 3
Reflective Design
A reflective design borrows the
characteristics, shapes, colors,
materials, sizes and textures of
the built surroundings. The result
is a design that fits seamlessly
and unobtrusively in its
environment.
Aesthetic Design
In addition to providing effective stormwater quality treatment, rain gardens can be attractively
incorporated into a site within one or several landscape areas. Aesthetically designed rain gardens will
typically either reflect the character of their surroundings or become distinct features within their
surroundings. Guidelines for each approach are provided below.
Reflecting the Surrounding
Determine design characteristics of the surrounding. This becomes the context for the drainage
improvement. Use these characteristics in the structure.
Create a shape or shapes that "fix" the forms surrounding the improvement. Make the improvement
part of the existing surrounding.
The use of material is essential in making any new
improvement an integral part of the whole. Select materials
that are as similar as possible to the surrounding
architectural/engineering materials. Select materials from the
same source if possible. Apply materials in the same
quantity, manner, and method as original material.
Size is an important feature in seamlessly blending the
addition into its context. If possible, the overall size of the
improvement should look very similar to the overall sizes of
other similar objects in the improvement area.
The use of the word texture in terms of the structure applies predominantly to the selection of plant
material. The materials used should as closely as possible, blend with the size and texture of other
plant material used in the surrounding. The plants may or may not be the same, but should create a
similar feel, either individually or as a mass.
Creating a Distinct Feature
Designing the rain garden as a distinct feature is limited only by budget, functionality, and client
preference. There is far more latitude in designing a rain garden that serves as a distinct feature. If this is
the intent, the main consideration beyond functionality is that the improvement create an attractive
addition to its surroundings. The use of form, materials, color, and so forth focuses on the improvement
itself and does not necessarily reflect the surroundings, depending on the choice of the client or designer.
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November 2010 Urban Drainage and Flood Control District B-15
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Figure B-1 – Typical Rain Garden Plan and Sections
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Figure B-2. Geomembrane Liner/Underdrain Penetration Detail
Figure B-3. Geomembrane Liner/Concrete Connection Detail
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Photograph B-3. Inadequate construction staking may have
contributed to flows bypassing this rain garden.
Photograph B-4. Runoff passed the upradient rain garden, shown in
Photo B-3, and flooded this downstream rain garden.
Construction Considerations
Proper construction of rain gardens involves careful attention to material specifications, final grades, and
construction details. For a successful project, implement the following practices:
Protect area from excessive sediment
loading during construction. This is the
most common cause of clogging of rain
gardens. The portion of the site draining
to the rain garden must be stabilized
before allowing flow into the rain
garden. This includes completion of
paving operations.
Avoid over compaction of the area to
preserve infiltration rates (for partial and
full infiltration sections).
Provide construction observation to
ensure compliance with design
specifications. Improper installation,
particularly related to facility dimensions
and elevations and underdrain elevations,
is a common problem with rain gardens.
When using an impermeable liner, ensure
enough slack in the liner to allow for
backfill, compaction, and settling without
tearing the liner.
Provide necessary quality assurance and
quality control (QA/QC) when
constructing an impermeable
geomembrane liner system, including but
not limited to fabrication testing,
destructive and non-destructive testing of
field seams, observation of geomembrane
material for tears or other defects, and air
lace testing for leaks in all field seams and
penetrations. QA/QC should be overseen
by a professional engineer. Consider
requiring field reports or other
documentation from the engineer.
Provide adequate construction staking to
ensure that the site properly drains into the
facility, particularly with respect to surface drainage away from adjacent buildings. Photo B-3 and
Photo B-4 illustrate a construction error for an otherwise correctly designed series of rain gardens.
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November 2010 Urban Drainage and Flood Control District B-21
Urban Storm Drainage Criteria Manual Volume 3
Photograph B-5. Rain garden is staked out at the low
point of the parking area prior to excavation.
Construction Example
Photograph B-6. Curb and gutter is installed. Flush
curbs with wheel stops or a slotted curb could have been
used in lieu of the solid raised curb with concentrated
inflow.
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B-22 Urban Drainage and Flood Control District November 2010
Urban Storm Drainage Criteria Manual Volume 3
Photograph B-7. The aggregate layer is covered
with a geotextile and growing media. This photo
shows installation of the geotextile to separate the
growing media from the aggregate layer below.
Cleanouts for the underdrain system are also
shown. Note: The current design section does not
require this geotextile.
Photograph B-8. Shrubs and trees are placed outside
of the ponding area and away from geotextiles.
Photograph B-9. This photo was taken during the first
growing season of this rain garden. Better weed control in the
first two to three years will help the desired vegetation to
become established.
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Design Example
The UD-BMP workbook, designed as a tool for both designer and reviewing agency is available at
www.udfcd.org. This section provides a completed design form from this workbook as an example.
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B-24 Urban Drainage and Flood Control District November 2010
Urban Storm Drainage Criteria Manual Volume 3
Sheet 1 of 2
Designer:
Company:
Date:
Project:
Location:
1. Basin Storage Volume
A) Effective Imperviousness of Tributary Area, Ia Ia = 95.0 %
(100% if all paved and roofed areas upstream of rain garden)
B) Tributary Area's Imperviousness Ratio (i = Ia/100) i = 0.950
C) Water Quality Capture Volume (WQCV) for a 12-hour Drain Time WQCV = 0.36 watershed inches
(WQCV= 0.8 * (0.91* i3 - 1.19 * i2 + 0.78 * i)
D) Contributing Watershed Area (including rain garden area) Area = 32,000 sq ft
E) Water Quality Capture Volume (WQCV) Design Volume VWQCV = 954 cu ft
Vol = (WQCV / 12) * Area
F) For Watersheds Outside of the Denver Region, Depth of d6 = in
Average Runoff Producing Storm
G) For Watersheds Outside of the Denver Region, VWQCV OTHER = cu ft
Water Quality Capture Volume (WQCV) Design Volume
H) User Input of Water Quality Capture Volume (WQCV) Design Volume VWQCV USER = cu ft
(Only if a different WQCV Design Volume is desired)
2. Basin Geometry
A) WQCV Depth (12-inch maximum) DWQCV = 12 in
B) Rain Garden Side Slopes (Z = 4 min., horiz. dist per unit vertical) Z = 0.00 ft / ft
(Use "0" if rain garden has vertical walls)
C) Mimimum Flat Surface Area AMin = 636 sq ft
D) Actual Flat Surface Area AActual = 955 sq ft
E) Area at Design Depth (Top Surface Area) ATop = 955 sq ft
F) Rain Garden Total Volume VT= 955 cu ft
(VT= ((ATop + AActual) / 2) * Depth)
3. Growing Media
4. Underdrain System
A) Are underdrains provided?
B) Underdrain system orifice diameter for 12 hour drain time
i) Distance From Lowest Elevation of the Storage y = 2.7 ft
Volume to the Center of the Orifice
ii) Volume to Drain in 12 Hours Vol12 = 954 cu ft
iii) Orifice Diameter, 3/8" Minimum DO = 0.67 in
Design Procedure Form: Rain Garden (RG)
J. Tann
BMP, Inc.
November 24, 2010
Shops at 56th
NW corner of 56th Ave. and 27th St.
Choose One
Choose One
18" Rain Garden Growing Media
Other (Explain):
YES
NO
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References
Erickson, Andy. 2009. Field Applications of Enhanced Sand Filtration. University of Minnesota
Stormwater Management Practice Assessment Project Update. http://wrc.umn.edu.
Guo, James C.Y., PhD, Anu Ramaswami, PhD, and Shauna M. Kocman, PhD Candidate. 2010.
Sustainable Design of Urban Porous Landscape Detention Basin. University of Colorado
Denver
Mazzerolle, Marc J. 2002. Detrimental Effects of Peat Mining on Amphibian Abundance and Species
Richness in Bogs. Elsevier Science Limited.
Sheet 2 of 2
Designer:
Company:
Date:
Project:
Location:
5. Impermeable Geomembrane Liner and Geotextile Separator Fabric
A) Is an impermeable liner provided due to proximity
of structures or groundwater contamination?
6. Inlet / Outlet Control
A) Inlet Control
7. Vegetation
8. Irrigation
NO SPRINKLER HEADS ON THE FLAT SURFACE
A) Will the rain garden be irrigated?
Notes:
November 24, 2010
Shops at 56th
NW corner of 56th Ave. and 27th St.
Design Procedure Form: Rain Garden (RG)
J. Tann
BMP, Inc.
Choose One
Choose One
Choose One
Sheet Flow- No Energy Dissipation Required
Concentrated Flow- Energy Dissipation Provided
Plantings
Seed (Plan for frequent weed control)
Sand Grown or Other High Infiltration Sod
Choose One
YES
NO
YES
NO
APPENDIX C
EROSION CONTROL REPORT
Utilities Administration Building
Preliminary Erosion Control Report
EROSION CONTROL REPORT
A comprehensive Erosion and Sediment Control Plan (along with associated details) will be included
with the final construction drawings. It should be noted, however, that any such Erosion and
Sediment Control Plan serves only as a general guide to the Contractor. Staging and/or phasing of
the BMPs depicted, and additional or different BMPs from those included may be necessary during
construction, or as required by the authorities having jurisdiction.
It shall be the responsibility of the Contractor to ensure erosion control measures are properly
maintained and followed. The Erosion and Sediment Control Plan is intended to be a living
document, constantly adapting to site conditions and needs. The Contractor shall update the
location of BMPs as they are installed, removed or modified in conjunction with construction
activities. It is imperative to appropriately reflect the current site conditions at all times.
The Erosion and Sediment Control Plan shall address both temporary measures to be implemented
during construction, as well as permanent erosion control protection. Best Management Practices
from the Volume 3, Chapter 7 – Construction BMPs will be utilized. Measures may include, but are
not limited to, silt fencing along the disturbed perimeter, gutter protection in the adjacent roadways
and inlet protection at existing and proposed storm inlets. Vehicle tracking control pads, spill
containment and clean-up procedures, designated concrete washout areas, dumpsters, and job site
restrooms shall also be provided by the Contractor.
Grading and Erosion Control Notes can be found on the Utility Plans. The Final Plans will contain a
full-size Erosion Control sheet as well as a separate sheet dedicated to Erosion Control Details. In
addition to this report and the referenced plan sheets, the Contractor shall be aware of, and adhere
to, the applicable requirements outlined in the Development Agreement for the development. Also,
the Site Contractor for this project will be required to secure a Stormwater Construction General
Permit from the Colorado Department of Public Health and Environment (CDPHE), Water Quality
Control Division – Stormwater Program, prior to any earth disturbance activities. Prior to securing
said permit, the Site Contractor shall develop a comprehensive StormWater Management Plan
(SWMP) pursuant to CDPHE requirements and guidelines. The SWMP will further describe and
document the ongoing activities, inspections, and maintenance of construction BMPs.
MAP POCKET
DRAINAGE EXHIBITS
ST
ST
ST ST
FO
FO
FO FO FO FO FO FO
FO
FO FO
FO
FO FO FO
FO
ST ST
ST
ST ST ST
ST ST ST
ST ST ST
ST ST ST ST ST ST
ST
ST
SS SS
SS
SS
SS
SS
SS
SS
SS
SS
ST
ST ST
SS
SS
SS
SS
SS
SS
SS
SS SS SS
ST
ST
ST ST
ST
ST ST ST
ST ST ST
ST ST ST
ST ST ST ST ST ST
FO FO
FO FO FO FO FO FO FO
FO
FO FO FO
FO FO
2" W
2" W
2" W 2" W
2" W 2" W
2" W 2" W
W
RD
RD RD
EXISTING
RIGHT-OF-WAY
EXISTING RIGHT-OF-WAY
1
2
OS1
OS2
PROPOSED FLOWLINE
OF DRAINAGE SWALE
AT MINIMUM 2% GRADE
PROPOSED CONCRETE FOREBAY
FUTURE PEDESTRIAN
WALKWAY
PROPOSED 6" STORM SEWER
CONNECT PROPOSED STORM SEWER
INTO EXISTING BOX CULVERT
PROPOSED WATER QUALITY
OUTLET STRUCTURE
PROPOSED 12" STORM SEWER
PROPOSED 6" STORM SEWER
PROPOSED EMERGENCY
ACCESS ALIGNMENT
1
2
OS2
OS1
EXISTING
RIGHT-OF-WAY
EXISTING 100-YR CITY
HIGH RISK FLOOD FRINGE
EXISTING 100-YR
CITY FLOODWAY
EXISTING
RIGHT-OF-WAY
EXISTING STORM
SEWER MANHOLE (TYP.)
PROPOSED RAIN GARDEN
(RE: LANDSCAPE ARCHITECT)
PROPOSED RAIN GARDEN
(RE: LANDSCAPE ARCHITECT)
PROPOSED SIDEWALK
PROPOSED UTILITIES
ADMINISTRATION BUILDING
FF = 4985.50
RELOCATED
BUTTERFLY
BLDG
FF = 4984.0
4984.1 100-YR FLOOD ELEVATION
(FT.COLLINS NGVD29)
PROPOSED RAIN GARDEN
(RE: LANDSCAPE ARCHITECT)
LAPORTE AVENUE
(100' ROW)
HOWES STREET
(100' ROW)
EXISTING
215 N. MASON BUILDING
EXISTING
214 N. HOWES BUILDING
PROPOSED SIDEWALK
PROPOSED SIDEWALK
APPROXIMATE LOCATION
OF 16' WIDE x5' TALL
CONCRETE BOX CULVERT
PROPOSED CONCRETE TERRACE
(RE: LANDSCAPE ARCHITECT)
PROPOSED SIDEWALK
PROPOSED SIDEWALK
PROPOSED SIDEWALK
PROPOSED
WATER QUALITY
CONTROL POND
Monday, April 14, 2014 10:17 AM
D:\Projects\940-002\Dwg\Drng\940-002_DRNG.dwg
Wegert Provencio
C600
DRAINAGE PLAN
CITY OF FORT COLLINS
222 Laporte Avenue
Fort Collins, CO 80524
UTILITIES
ADMINISTRATION
BUILDING
PROJECT DEVELOPMENT PLAN
1050 17th STREET,
SUITE A200
DENVER, CO 80265
303 295 1717 t
303 292 0845 f
DRAWN BY: CHECKED BY:
A
B
C
D
E
F
G
H
I
J
A
B
C
D
E
F
G
H
I
J
1 2 3 4 5 6 7 8 9 10 11 12 13 14
14 13 12 11 10 9 8 7 6 5 4 3 2 1
15
15
K K
NOTES:
A
B2
1.45 ac
NOT FOR CONSTRUCTION
LEGEND:
( IN FEET )
10 0 10 20 30
1 INCH = 10 FEET
UD
ST
CALL 2 BUSINESS DAYS IN ADVANCE BEFORE YOU
DIG, GRADE, OR EXCAVATE FOR THE MARKING OF
UNDERGROUND MEMBER UTILITIES.
CALL UTILITY NOTIFICATION CENTER OF
COLORADO
R
PROJECT DATUM: NGVD29 Unadjusted (Old City of Fort Collins Datum)
BENCHMARK #1:
City of Fort Collins Benchmark 1-13
Southwest corner of College Avenue and Maple Street, on a concrete traffic signal base.
Elevation= 4976.58
NOTE: If NAVD 88 Datum is required for any purpose, the following equation should be
used: NAVD88 = NGVD29 Unadjusted + 3.17'
PROJECT BENCHMARKS:
the perimeter wall for the BMP
impedes flow away from the building.
Always adhere to the slope
recommendations provided in the
geotechnical report. In the absence
of a geotechnical report, the following
general recommendations should be
followed for the first 10 feet from a
building foundation.
1) Where feasible, provide a slope of
10% for a distance of 10 feet away
from a building foundation.
2) In locations where non-expansive
soil or bedrock conditions exist,
the slope for the surface within 10
feet of the building should be at
least 5% away from the building
for unpaved (landscaped) surfaces.
3) In locations where potentially
expansive soil or bedrock
conditions exist, the design slope
should be at least 10% away from
the building for unpaved
(landscaped) surfaces.
4) For paved surfaces, a slope of at
least 2% away from the building is
adequate. Where accessibility
requirements or other design
constraints do not apply, use an
increased minimum design slope
for paved areas (2.5% where non-
expansive soil or bedrock
conditions exist).
Additional design and
construction steps are required for
placement of any ponding or
infiltration area near or upgradient
from a building foundation and/or
when expansive (low to high
swell) soils exist. This is
discussed in the design procedure
section.
In developing or otherwise erosive
watersheds, high sediment loads
can clog the facility.
Other Considerations
Life-cycle Costs4 Moderate
1
Not recommended for watersheds with
high sediment yields (unless pretreatment is
provided).
3
Based primarily on data from the
International Stormwater BMP Database
(www.bmpdatabase.org).
4
Based primarily on BMP-REALCOST
available at www.udfcd.org. Analysis
based on a single installation (not based on
the maximum recommended watershed
tributary to each BMP).
(min)
10-yr
Tc
(min)
100-yr
Tc
(min)
11No0.25 0.60 0.75 55 2.00% 9.4 5.5 3.9 180 0.50% 1.41 2.1 0 0.00% N/A N/A 11 8 6
22No0.55 0.61 0.77 35 2.00% 4.8 4.3 2.9 75 0.75% 1.73 0.7 0 0.00% N/A N/A 6 5 5
OS1 OS1 No 0.25 0.56 0.70 10 2.00% 4.0 2.5 1.9 0 0.00% N/A N/A 0 0.00% N/A N/A 5 5 5
OS2 OS2 No 0.95 0.95 1.00 25 1.00% 1.4 1.4 0.9 130 1.70% 2.61 0.8 0 0.00% N/A N/A 5 5 5
DEVELOPED TIME OF CONCENTRATION COMPUTATIONS
Gutter Flow Swale Flow
Design
Point
Basin
Overland Flow
ATC
December 1, 2014
Time of Concentration
(Equation RO-4)
3
1
1 . 87 1 . 1 *
S
Ti C Cf L