2A-5. Project Drainage Report
The purpose of the Project Drainage Report
is to identify and propose specific solutions to stormwater runoff and water
quality problems resulting from existing and proposed development. The report must include adequate
topographic information (pre- and post-development) to verify all conclusions
regarding offsite drainage. Unless
known, the capacity of downstream drainage structures must be thoroughly
analyzed to determine their ability to convey the developed discharge.
The drainage report and plan will be
reviewed and approved by the Engineer prior to preparation of final
construction drawings. Approval of
these preliminary submittals constitutes only a conceptual approval and should
not be construed as approval of specific design details. The Project Engineer may be required by
law to submit the drainage report and plan to the Iowa Department of Natural
Resources (IDNR) and/or US Army Corps of Engineers (USACE). An application for a permit to construct
will follow the IDNR and NPDES applicable permit requirements and USACE rules
and regulations, and the application will be the responsibility of the Project
Engineer.
1.
Include
a cover sheet with project name and location, name of firm or agency preparing
the report, Professional Engineer's signed and sealed certification, and table
of contents. Number each page of the
report.
2.
Perform
all analyses according to the intent of professionally-recognized methods. Support any modifications to these
methods with well-documented and industry-accepted research.
3.
It is
the designer's responsibility to provide all data requested. If the method of analysis (for example,
a computer program) does not provide the required information, then the
designer must select alternative or supplemental methods to ensure the drainage
report is complete and accurate.
4.
Acceptance
of a drainage report implies the Jurisdiction concurs with the project's
overall stormwater management concept.
This does not constitute full acceptance of the improvement plans,
alignments, and grades, since constructability issues may arise in plan review.
5.
Use
all headings listed in the Contents.
A complete report will include all the information requested in this
format. If a heading listed does
not apply, include the heading and briefly explain why it does not apply. Include additional information and
headings as required to develop the report.
6.
This chapter
does not preclude the utilization of methods other than those referenced, nor
does it relieve the designer of responsibility for analysis of issues not
specifically mentioned.
The following
information contains summaries for hydrology and detention (see Tables 1, 2,
and 3), as well as design considerations for the preparation of project
drainage reports. They are provided
as a minimum guide and are not to be construed as the specific information to
be supplied on every project drainage report, and other information may be
required. Existing and proposed
conditions for each development will require analysis unique to that area.
1.
Site characteristics.
a.
Pre-development conditions.
Describe pre-developed land use, topography, drainage patterns (including
overland conveyance of the 100-year storm event), and natural and man-made
features. Describe ground coverage,
soil type, and physical properties, such as hydrologic soil group and
infiltration. For the
pre-development analysis where the area is rural and undeveloped, a land use
description of "meadow/good condition" is recommended. If a geotechnical study of the site is
available, provide boring logs and locations in the appendix of the report. If a soil survey was used, cite it in
the references.
b.
Post-development
conditions. Describe
post-developed land use and proposed grading, change in percent of impervious
area, and change in drainage patterns.
If an existing drainageway is filled, the runoff otherwise stored by the
drainageway will be mitigated with stormwater detention, in addition to the
post-development runoff.
c.
Contributing off-site drainage.
Describe contributing off-site drainage patterns, land use, and stormwater
conveyance. Identify undeveloped
contributing areas with development potential and list assumptions about future
development runoff contributed to the site.
d.
Floodways,
floodplains, and wetlands. Identify
areas of the site located within the floodway or floodplain boundaries as
delineated on flood insurance rate maps, or as determined by other engineering
analysis. Identify wetland areas on
the site, as delineated by the National Wetlands Inventory, or as determined by
a specific wetland study.
e.
Pre-development runoff analysis.
1)
Watershed area. Describe
overall watershed area and relationship between other watersheds or
sub-areas. Include a
pre-development watershed map in the report appendix.
2)
Time of concentration. Describe method used to calculate the
time of concentration. Describe
runoff paths and travel times through sub-areas. Show and label the runoff paths on the
pre-development watershed map.
3)
Precipitation
model. Describe the precipitation model and
rainfall duration used for the design storm. Typical models may include one or more
of the following:
a)
NRCS Type-II distribution
b)
Huff rainfall distribution (select the
appropriate distribution based on rainfall duration)
c)
Frequency-based hypothetical storm.
d)
Rainfall intensity duration frequency (IDF) curve.
e)
User-defined model based on collected
precipitation data, subject to the Engineer's approval. Total rainfall amounts for given
frequency and duration should be obtained from Bulletin 71, "Rainfall Frequency
Atlas of the Midwest." (See Table 2 in Section 2C-2) This publication supersedes Technical
Paper Number 40, "Rainfall Frequency Atlas of the United States."
4)
Rainfall loss
method. List runoff coefficients or curve
numbers applied to the drainage area.
The Green-Ampt infiltration model may also be used to estimate rainfall
loss by soil infiltration.
5) Runoff model. (See Part 2C). Describe method used to project runoff
and peak discharge. Typical models
are as follows:
a)
Use the Rational Method for drainage areas up to
160 acres, and where flow routing is not required. Often used in storm sewer design.
b)
Use the WINTR-55 Method for drainage areas up to
2000 acres.
¡¤
TR-20
Model
¡¤
Routines
contained in HEC-1 or HEC-HMS computer models
¡¤
Regression
equations and other hydrologic models approved by the Jurisdiction
6)
Summary of pre-development runoff.
Provide table(s) including drainage area, time of concentration,
frequency, duration, peak discharge, routing, and accumulative flows at
critical points where appropriate.
2.
Post-development
runoff analysis.
a.
Watershed area. Describe overall watershed area and
sub-areas. Discuss if the
post-development drainage area differs from the pre-development drainage
area. Include a post-development
watershed map in the report appendix.
Include an analysis of the proposed increase in impervious area. Provide a summary of the total
impervious area and the % impervious are for each sub-watershed/catchment.
b. Time of concentration. The method used will be
the same as used in the pre-development analysis. Describe change in times of concentration
due to development (i.e. change in drainage patterns). Show and label the runoff paths on the
post-development watershed map.
c. Precipitation
model. Storm event,
total rainfall, and total storm duration will be the same as used for the
pre-development model. If IDF
curves are used, describe the change in design rainfall intensity.
d. Rainfall loss
method. Method will be
the same as pre-development analysis.
Describe the change in rainfall loss due to development.
e. Runoff model. The runoff method will be
the same as used in the pre-development analysis, except for variables changed
to account for the developed conditions.
f. Summary of post-development
runoff.
1)
Provide table(s) including drainage area, time
of concentration, frequency, duration, and peak discharge. Summarize in narrative form the change
in hydrologic conditions due to the development. Provide a runoff summary using Tables 1
and 2.
2)
Post-developed discharge should take into
account any upstream offsite detention basins and undeveloped offsite areas
assumed to be developed in the future with stormwater detention.
3)
Provide a summary of the respective volumes and
discharge rates from the unified sizing criteria: WQv, CPv, Qp, and Qf for the project
area.
4)
Calculate the allowable release rate from the
site, based on three conditions:
a)
The peak runoff rate for the 1-year, 24-hour
design storm based on the CPv. See Section
2C-6.
b)
After development, the release rate of runoff
for rainfall events having an expected return frequency of two years and five
years should not exceed the existing, pre-developed peak runoff rate from those
same storms.
c)
For rainfall events having an expected return
frequency of 10 years to 100 years inclusive, the rate of runoff from the
developed site should not exceed the existing, pre-developed peak runoff from a
five-year frequency storm of the same duration. The allowable discharge rate may be
restricted due to downstream capacity.
Include this calculation in the Executive Summary.
3.
Stormwater conveyance
design.
a.
Design information
references. All stormwater
conveyances should be designed according to this chapter, at a minimum. The following references may be used for
supplemental design information:
1)
Federal
Highway Administration (1996) Urban Drainage Design Manual. Hydraulic Engineering Circular No. 22, Washington D.C.
2)
Federal
Highway Administration (1988) Design of Roadside Channels with Flexible
Linings. Hydraulic Engineering
Circular No. 15, Washington
D.C.
3)
Federal
Highway Administration (1985) Hydraulic Design of Highway Culverts. Hydrologic Design Series Number 5, Washington D.C.
4)
US Geological Survey (1968) Measurement of Peak
Discharge at Culverts by Indirect Methods. Book 3, Applications of Hydraulics, Washington D.C.
5)
American Society of Civil Engineers (1986) Design and Construction of Sanitary and
Storm Sewers. Manual of
Practice No. 37, New York, N.Y.
b. Storm sewer.
1)
List design criteria, including storm event and
runoff model. Describe the
hydraulic grade line and whether pressure flow or surcharging is possible. Provide a graphic of the hydraulic grade
line.
2)
List design criteria for intake size and
spacing. Describe the anticipated
gutter flow and spread at intakes.
3)
List any special considerations for sub-drainage
design, such as high water tables.
4)
Provide tables of storm sewer (inlet and pipe)
and intake design data.
5)
Water spread on the street for intake design
year and 100-year elevation in all streets in which the curb is overtopped.
c. Culverts.
1)
Describe culvert capacity, inlet or outlet control
conditions, estimated tailwater and headwater. Determine if 100-year or lesser storm
event will flood roadway over culvert.
2)
Sketch a contour of the 100-year headwater
elevation on a topographic map and/or grading plan. This delineated 100-year flood elevation
is used to determine drainage easement and site grading requirements.
d. Open channel flow
- swales and ditches.
1)
Describe swale and ditch design. State the assumed Manning's roughness
coefficients. State the anticipated
flow velocity, and whether it exceeds the permissible velocity based on soil
types and/or ground coverage. If
the permissible velocity is exceeded, describe channel lining or energy
dissipation.
2)
Discuss design calculations. Depending on the complexity of the
design, these may range from a single steady-state equation (i.e. Manning's) to
a step calculation including several channel cross-sections, culverts and
bridges.
3)
Discuss the overall grading plan in terms of
controlling runoff along lot lines and preventing runoff from adversely flowing
onto adjacent lots.
4)
The limits of swale and ditch easements will be
established based upon the required design frequency. This includes 100-year overflow
easements from stormwater controlled structures.
e. Storm drainage
outlets and downstream analysis.
1)
Discuss
soil types, permissible and calculated velocity at outlets, energy dissipator
design, and drainage impacts on downstream lands. Provide calculations for the energy
dissipator dimensions, size, and thickness of riprap revetment (or other material)
and filter layer.
2)
Include
a plan and cross-sections of the drainage way downstream of the outlet,
indicating the flow line slope and bank side slopes. Identify soil types on the plan.
3)
Perform
downstream analysis. The downstream
analysis will show what impacts, if any, a project will have on the drainage
systems downstream of the project site.
The analysis consists of three elements: review of resources, inspection of the
affected area, and analysis of downstream effects.
a)
During
the review of resources, review any existing data concerning drainage of the
project area. This data will
commonly include area maps, floodplain maps, wetland inventories, stream
surveys, habitat surveys, engineering reports concerning the entire drainage
basin, known drainage problems, and previously completed downstream
analyses.
b)
Physically
inspect the drainage system at the project site and downstream of the site. During the inspection, investigate any
problems or areas of concern that were noted during the review of
resources. Identify any existing or
potential capacity problems in the drainage system; flood-prone areas; areas of
channel destruction, erosion and sediment problems; or areas of significant
destruction of natural habitat.
c)
Analyze
the information gathered during the review of resources and field inspection,
to determine if the project will create any drainage problems downstream or
will make any existing problems worse.
Note there are situations that even when minimum design standards are
met, the project will still have negative downstream impacts. Whenever this situation occurs,
mitigation measures must be included in the project to correct for the impacts.
f. Hydraulic model. If the design warrants
hydraulic modeling, state the method used.
Typical modeling programs include:
1)
HEC-RAS - river analysis systems
2)
HEC-2 - water surface profiles
3)
SWMM - stormwater management model
4)
WSPRO - water surface profiles
5)
HY-8 - hydraulic design of highway culverts
6)
Other commercial or public domain programs
approved by the Jurisdiction
4.
Stormwater management
design.
a.
Design
standards. All stormwater management facilities should
be designed according to these design standards at a minimum. The following references may provide
helpful design information for stormwater detention and water quality issues:
1)
Federal
Highway Administration (1996) Urban Drainage Design Manual. Hydraulic Engineering Circular No. 22, Washington D.C.
2)
American Society of Civil Engineers (1985) Final
report of the Task Committee on Stormwater Detention Outlet Control Structures.
Am. Soc. Civ. Eng., New York, N.Y.
3)
American Society of Civil Engineers (1992) Design and Construction of Urban Stormwater
Management Systems. Manual of
Practice No.77, New York, N.Y.
4)
American Society of Civil Engineers (1998) Urban Runoff Quality Management. Manual
of Practice No. 87, New York,
N.Y.
5)
Stahre, P and Urbonas, B (1990) Stormwater Detention for Drainage, Water
Quality, and CSO Management.
Prentice-Hall, Englewood
Cliffs, N.J.
6)
American Public Works Association (1991) Water Quality Runoff Solutions. Special Report No. 61, Chicago, IL.
b.
Detention basin
location. Describe basin site. Discuss existing topography and
relationship to basin grading.
Determine if construction will be affected by rock deposits. Also determine if a high water table
precludes basin storage. Floodplain
locations should be avoided.
c. Detention
basin performance.
1)
For rainfall events having an expected return
frequency of two, five, and 100 years inclusive, the rate of runoff from the
developed site will not exceed the existing, pre-developed peak runoff from the
5-year frequency storm of the same duration unless limited by downstream
conveyance. Provide a table
summarizing these release rates.
Also provide a stage-storage-discharge table. These tables are shown in Table 3. State the minimum freeboard provided,
and at what recurrence interval the basin overtops.
2)
Discuss the effects on the overall stormwater
system by detention basins in contributing offsite areas. If contributing offsite areas are
presently undeveloped, discuss assumptions about future development and stormwater
detention.
3)
Calculate the basin overflow release rate. This equals the onsite 100-year
post-developed peak discharge plus the contributing offsite 100-year post developed
peak discharge. Include this
calculation with Table 3.
d. Detention
basin outlet.
1)
The single-stage outlet (i.e. one culvert pipe)
is not recommended because of its inability to detain post-developed runoff
from storms less than the 5-year interval.
In many cases, runoff from storm events less than the 5-year recurrence
interval has created erosion and sediment problems downstream of the detention
basin.
2)
A more desirable outlet has two or more
stages. An orifice structure serves
to detain runoff for water quality purposes and release runoff for low-flow
events of a 2-year storm. Greater
storm events are usually discharged by a separate outlet.
3)
Discuss the basin outlet design in terms of
performance during low and high flows, downstream impact (see Section 2C-13 and
Part 2G).
4)
State whether the detention basin volume is
controlled by the required flood control volume or the water quality volume.
e. Spillway and embankment
protection.
1)
Design the spillway for high flows using weir
and/or spillway design methods. The
steady-state open channel flow equation is not intended for use in spillway
design.
2)
Describe methods to protect the basin during
overtopping flow.
f. TR-55 design
limitations. Note the TR-55
method of sizing detention basins may result in storage errors of 25%, and
should not be used in final design.
The detention basin size in final design should be based upon actual
hydrograph routing for the design storms controlled by the basin.
5. Permits. Indicate what permits have
been applied for and received.
Submit IDNR approval letter and report for sites affecting unnumbered
A-zones, as delineated on flood insurance rate maps.
6. References. Provide a list of all
references cited, in bibliographical format.
7. Appendix. Drawings and calculations
in the Appendix should include, but not be limited to:
a.
Drawings.
1)
A
preliminary plat (pre-and post-topography) may be used to show the
proposed development. Minimum scale
of 1 inch = 500 feet or larger to ensure legibility should be used for all
drainage areas. (Drawings no larger
than 24 inches by 36 inches should be inserted in 8-1/2 inch by 11-inch sleeves
in the back of the bound report).
The plat is to show street layout and/or building location on a contour
interval not to exceed 2 feet. The
map must show on- and off-site conditions.
Label flow patterns used to determine times of concentration.
2)
Drainage plans (preliminary plat or topography
map) must extend a minimum of 250 feet from the edge of the proposed
preliminary plat boundary, or a distance specified by Jurisdiction. The limits of swale and ditch easements should
be established based upon the required design frequency. This includes 100-year overflow
easements from stormwater controlled structures.
3)
Soil map or geotechnical information.
4)
Location and elevations of Jurisdictional
benchmarks. All elevations should
be on Jurisdictional datum.
5)
Proposed property lines (if known).
6)
If the preliminary plat does not include proposed
grades, submit a grading and erosion control plan showing existing and proposed
streets, names, and approximate grades.
7)
Existing drainage facilities and structures,
including existing roadside ditches, drainageways, gutter flow directions,
culverts, etc. All pertinent
information such as size, shape, slope location, 100-year flood elevation, and
floodway fringe line (where applicable), should also be included to facilitate
review and approval of drainage plans.
8)
Proposed storm sewers and open drainageways,
right-of-way and easement width requirements, 100-year overland flow easement,
proposed inlets, manholes, culverts, erosion and sediment control, water
quality (pollution) control and energy dissipation devices, and other
appurtenances.
9)
Proposed outfall point for runoff from the study
area.
10) The 100-year
flood elevation and major storm floodway fringe (where applicable) are to be
shown on the plans, report drawings, and plats (preliminary and final). In addition, the report should
demonstrate that the stormwater system has adequate capacity to handle a
100-year storm event, or provisions are made for overland flow.
11) Show the critical
minimum lowest opening elevation of a building for protection from major and
minor storm runoff. This elevation
is to be reviewed with the Jurisdiction to confirm if previous changes were
made to the minimum lowest opening elevation for major storm event.
b.
Calculations.
1)
Determine
runoff coefficients and curve numbers
2)
Total
impervious area (ft2 and % of total drainage area)
3)
Determine
times of concentration
4)
Calculations
for WQv, rev, peak flow rate for the water quality design storm (cfs), and CPv.
5)
Calculations
for intake capacity, sewer design, and culvert design
6)
Peak
discharge calculations - Show results in tabular format and pre- and
post-developed hydrographs
7)
Detention
basin design - Show tabular stage-storage-discharge results and inflow/outflow
hydrographs
8)
Detention
basin outlet design
9)
Open
channel flow calculations
10) Erosion protection design
c.
Computer calculations. Attach computer-generated reports and
output if software was used.
Underline and label results, such as the peak discharge.
d.
Stormwater quantity and quality.
See Part 2A of this chapter for submittal of stormwater quality and
quantity information.
Table 1: Hydrology summary
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Area 1
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Area 2
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Onsite
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Offsite
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Onsite
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Offsite
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Pre
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Post
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Pre
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Post
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Pre
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Post
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Pre
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Post
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Size (acres)
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Predominant land use
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Impervious area (acres or ft2)
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Watershed length
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Time of concentration
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Runoff coefficient (C)
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NRCS CN
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Runoff (Q)
1 yr
2 yr
5 yr
10 yr
25 yr
50 yr
100 yr
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Table 2: Hydrology summary (critical points)
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Design Flows
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Critical Point 1
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Critical Point 2
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Critical Point 3
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Critical Point 4
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1 yr
2 yr
5 yr
10 yr
25 yr
50 yr
100 yr
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Table 3: Detention summary
Detention Basin
A. Inlet
design storm frequency:
B. Outlet
design storm frequency:
Standard Release
Rate
A. Allowable
release rate: _____ cfs
B. Offsite
(developed) rate: _____ cfs
WQv
release rate
_____ cfs (if applicable)
CPv
rate
_____ cfs
Total
release: _____ cfs
Overflow Release
Rate
A. Onsite
pre-developed (100-yr) ______ cfs
B. Offsite
developed (100-yr)* ______
cfs
Total
release: ______
cfs
Structures
A. Inflow
structure: ____________
B. Outflow
structure: ____________
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Stage**
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Storage
(ac-ft)
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Inflow
(cfs)
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Outflow
(cfs)
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Comments
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1
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2
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3
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4
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5
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6
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7
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8
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9
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10
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* Routed through basin **
Max. 1-foot interval