2A-4. Stormwater Management Criteria
This section presents a set of recommended minimum criteria for
stormwater management for development activities in the state of Iowa in those
communities with regulations. The
overall aim is to provide an integrated approach to address both the water
quality and quantity problems associated with stormwater runoff due to urban
development.
The goal of a
set of minimum stormwater management criteria for areas of new development and
significant redevelopment is to reduce the impact of post-construction
stormwater runoff on the watershed. This can be achieved, as discussed in
Section 2A-1, by
- Maximizing the use of
site design and nonstructural methods to reduce the generation of runoff
and pollutants
- Managing and treating
stormwater runoff though the use of structural stormwater controls
- Implementing pollution
prevention practices to limit potential stormwater contaminants
It should be noted that the criteria presented here may be used in all
communities in Iowa.
They may be adopted by local
jurisdictions as stormwater management development requirements as part of the
jurisdiction post-construction runoff control ordinance and/or may be modified
to meet local or watershed-specific stormwater management goals and objectives.
Please consult your local review
authority for more information.
The minimum guidelines for development are designed to assist local
governments that are regulated, to comply with regulatory and programmatic
requirements for various state and federal programs, including the National
Pollutant Discharge Elimination System (NPDES) Municipal Separate Storm Sewer
System (MS4) permit program, and the National Flood Insurance Program under
FEMA.
- Applicability. The stormwater
management guidelines for new development and redevelopment are intended
to apply to any development site in a regulated agency that meets one or
more of the following criteria and are intended to assist in the
development of measurable goals for the post-construction runoff minimum
control measure:
a.
New development and/or redevelopment that involves
land disturbing activity of 1 acre or more. Based on local conditions and the need
for additional measurable water quality improvement, jurisdictions may consider
including controls on development based on the addition of a threshold amount
of impervious area. Examples of
impervious area threshold values in other states are 5,000 ft2 and 10,000 ft2. A jurisdiction may consider an impervious
area threshold to provide an incentive to reduce the increase in developed
impervious area and encourage better site design planning.
b.
Any
commercial or industrial new development or redevelopment, regardless of size,
with a Standard Industrial Classification (SIC) code that falls under the NPDES
Industrial Stormwater Permit program or a hotspot land use as defined below.
2.
Definitions.
a.
New development is defined as
land disturbing activities, structural development (construction, installation,
or expansion of a building or other structure), and/or creation of impervious
surfaces on a previously undeveloped site.
b.
Redevelopment is defined as
structural development (construction, installation, or expansion of a building
or other structure), creation or addition of impervious surfaces, replacement
of impervious surface not part of routine maintenance, and land disturbing
activities associated with structural or impervious development. Redevelopment does not include such
activities as exterior remodeling.
c.
Hotspot is defined as a
land use or activity on a site that produces higher concentrations of trace
metals, hydrocarbons or other priority pollutants than are normally found in
urban stormwater runoff. Examples
of hotspots include gas stations, vehicle service and maintenance areas,
salvage yards, material storage sites, garbage transfer facilities, and
commercial parking lots with high-intensity use.
3.
Special
Requirements. New development
or redevelopment in critical or sensitive areas, or as identified through a
watershed study or plan, may be subject to additional performance and/or
regulatory criteria. Furthermore,
these sites may need to utilize or restrict certain structural controls in
order to protect a special resource or address certain water quality or
drainage problems identified for a drainage area.
The following guidelines are recommended minimum stormwater management
requirements for new development or redevelopment sites falling under the
applicability criteria.
- Use of enhanced design practices for stormwater management. Site designs are developed to preserve the
natural drainage and treatment systems and reduce the generation of
additional stormwater runoff and pollutants to the fullest extent
practicable. All site designs
are encouraged to implement a set of practices collectively known as "stormwater
better site design" and/or "low impact development" (LID) to
the fullest extent possible. Through the use of these practices
and techniques, the impacts of urbanization on the natural hydrology of the
site and water quality can be significantly reduced. The goal is to reduce the amount of
stormwater runoff and pollutants that are generated, provide for natural
on-site control and treatment of runoff, and optimize the location of
stormwater management facilities.
Better site design concepts can be viewed as both water quantity
and water quality management tools and can reduce the size and cost of
required structural stormwater controls.
- Stormwater runoff quality. The post-construction stormwater runoff from the
development site is managed to improve the water quality. A common water quality goal is to
remove at least 80% of the calculated average annual post-development
loading of total suspended solids (TSS) from the site. However, based on local water
quality conditions, jurisdictions might use other parameters, i.e.,
nutrients. This can be
achieved through the use of site design practices and structural stormwater
controls. This requirement may
be quantified and expressed in terms of engineering design criteria
through the specification of a water
quality volume (WQv) that is treated to the 80% TSS removal
performance goal. The water
quality volume is equal to the runoff generated on a site from the design
rainfall event. The water
quality volume is one of the unified stormwater sizing criteria, which are used in
conjunction to size and design stormwater management facilities to address
stormwater impacts. The
unified stormwater sizing criteria and methods to calculate the WQv are discussed
in Part 2B.
It
is presumed that a stormwater management system complies with this guideline
if:
a.
It is sized to capture and treat the prescribed WQv. The design rainfall event that is
recommended for computing the WQv is 1.25 inches and is equal to the 90%
cumulative frequency rainfall depth for the area. In numerical
terms, it is equivalent to the rainfall depth in inches (the 90% cumulative
frequency rainfall depth) multiplied by the volumetric runoff coefficient (Rv)
for the site, and the site drainage area.
A statewide WQv of 1.25 inches is recommended for use or jurisdictions
can use a similar value derived from an analysis of local historical rainfall
data, i.e., adjusted based on location factors in Iowa (climate districts 1-9).
b.
Appropriate structural stormwater controls are
selected, designed, constructed, and maintained according to the specific
criteria in this chapter.
c.
This
design guideline is based on treatment of the WQv from a site to reduce
post-development TSS loadings by 80%, as measured on an average annual basis. This performance goal is based upon EPA
guidance and has been adopted nationwide by many local and statewide
agencies. TSS is used as the
representative stormwater pollutant for measuring treatment effectiveness for
several reasons:
1)
The
use of TSS as an indicator pollutant is well established.
2)
Sediment
and turbidity, as well as other pollutants of concern that adhere to suspended
solids, are a major source of water quality impairment in Iowa surface waters
due to urban development and agricultural production activities.
3)
A
large fraction of many other pollutants of concern are either removed along
with TSS, or at rates proportional to the TSS removal.
4)
The
80% TSS removal level is reasonably attainable using well-designed structural
stormwater controls (for typical ranges of TSS concentration found in
stormwater runoff).
d.
Runoff from hotspot land uses and activities is
adequately treated and addressed through the use of appropriate structural
stormwater controls and pollution prevention practices.
Provide for treatment of the WQv for all developments where
stormwater management is required. A
minimum WQv of 0.2 inches per acre should be met at sites or in drainage areas that
have less than 15% impervious cover. Drainage areas having no impervious cover
and no proposed disturbance during development may be excluded from the WQv calculations. Designers are encouraged to use these
areas as non-structural practices for WQv treatment. Structural stormwater
controls are sized and designed to treat the WQv. Depending on their removal efficiency or
site constraints, more than one structural control may need to be used in
parallel or in series (treatment train) to meet the water quality treatment
requirement. Further, this guideline
assumes that structural stormwater controls will be designed, constructed and
maintained according to the criteria in this chapter. Stormwater discharges
from land uses or activities with higher or special potential pollutant
loadings may require the use of specific structural controls and pollution
prevention practices. A detailed
overview of structural stormwater controls is provided in Part 2D.
- Stream channel protection. Protection of stream
channels is accomplished through three complementary criteria:
a.
Extended detention of the 1-year, 24-hour storm for
a period of 24 hours using structural stormwater controls. It is known that the increase in runoff
due to development can dramatically increase stream channel erosion. This standard is intended to reduce the
frequency, magnitude, and duration of post-development bank full flow
conditions. The volume to be
detained is also known as the channel protection volume (CPv). The channel protection volume is one of
the unified stormwater sizing criteria
which are used in conjunction to size and design stormwater
management facilities to address stormwater impacts. The use of nonstructural site design
practices that reduce the total amount of runoff will also reduce CPv by a proportional
amount. This requirement may be
waived by a local jurisdiction for sites that discharge directly into piped
stormwater drainage systems, larger streams, rivers, wetlands, lakes, or other
situations where the reduction in the smaller flows will not have an impact on
streambank or channel integrity.
b.
Implement velocity control, energy dissipation,
streambank stabilization, and erosion prevention practices and structures as
necessary in the stormwater management system to prevent downstream erosion and
streambank damage.
c.
Establishment of riparian stream buffers on the
development site. Stream buffers
not only provide channel protection but also water quality benefits and
protection of streamside properties from flooding. It is recommended that 100-foot buffers
be established where feasible. For
new development, an appropriate stream buffer is established by requiring a
development setback from the centerline of the stream. In previously developed areas, where
erosion of the bank material is an issue, structural controls may be required
for bank stabilization. Providing a
buffer strip planted with native vegetation in the boundary area between the
developed property and the streambank can provide effective control.
4.
Overbank flood protection. Overbank flood protection
for downstream channels, and/or flooding from surcharging of downstream piped
conveyances, is provided by controlling the post-development 5-year, 24-hour
storm peak discharge rate (denoted Qp5) from exceeding the predevelopment (or natural
conditions) discharge rate using structural stormwater controls. The overbank flood protection peak rate
is one of the unified stormwater
sizing criteria, which are used in conjunction to size and design
stormwater management facilities to address stormwater impacts. The use of nonstructural site design
practices that reduce the total amount of runoff will also reduce Qp5 by a proportional
amount. See also the related
discussion on minor and major design storms later in this section.
Smaller storm events (e.g.,
2-year and 10-year) are often effectively controlled through the combination of
the extended detention for the 1-year, 24-hour event (channel protection
criterion) and the control of the 25-year peak rate for overbank flood
protection. These design guidelines
are intended to be used together.
If the control of the 1-year, 24-hour storm under guideline #3 is
exempted, then for overbank flood protection, peak flow attenuation of the
2-year (Qp2) through the 50-year (Qp-50) return frequency storm events must be
provided. This guideline may be
adjusted by a local jurisdiction for areas where all downstream conveyances and
receiving waters have the natural capacity to handle the full build-out 50-year
storm through a combination of channel capacity and overbank flood storage
without causing flood damage.
Evaluation of the impact of peak rate control under this guideline is
evaluated in conjunction with guideline #6 to ensure the downstream effect on
timing of release rates from single or multiple detention structures does not
increase downstream flooding.
5.
Extreme flood protection.
Extreme flood protection is provided by controlling and/or safely
conveying the 100-year, 24-hour storm event (denoted Qf). This is accomplished either by:
- Controlling Qf through
structural stormwater controls to maintain the existing 100-year floodplain,
or
- Sizing the onsite
conveyance system to safely pass Qf
and allowing it to discharge into a receiving
water whose protected floodplain is sufficiently sized to account for
extreme flow increases without causing damage.
In this case, the extreme flood protection
criterion may be waived by a local jurisdiction in lieu of provision of safe
and effective conveyance to receiving waters that have the capacity to
handle flow increases at the 100-year level.
The extreme flood protection peak rate is one of the pan
style='mso-bidi-font-style:italic'>unified stormwater sizing criteria, which
are used in conjunction to size and design stormwater management facilities to
address stormwater impacts. The use
of nonstructural site design practices that reduce the total amount of runoff
will also reduce Qf by a proportional amount.
6.
Downstream analysis. A downstream hydrologic
analysis is performed to determine if there are any additional impacts in terms
of peak flow increase or downstream flooding while meeting guidelines #1-#5. Due to peak flow timing and
runoff volume effects, some structural controls fail to reduce discharge peaks
to predevelopment levels downstream from the development site. A downstream peak flow analysis may be
needed to the point in the watershed downstream of the site or the stormwater
management system where the area of the site comprises 10% of the total
drainage area. This is to help
ensure that there are minimal downstream impacts from the developed site. The downstream analysis may result in the
need to resize structural stormwater controls, or may allow the waiving of some
unnecessary peak flow controls altogether. The use of a downstream analysis and the
"ten-percent" rule are discussed in Part 2C.
7.
Groundwater recharge. Recharge to groundwater is implemented
to the extent practicable through the use of nonstructural better site design
techniques that allow for recharge of stormwater runoff into the soil. The annual recharge from the
post-development site should approximate the annual recharge from the
pre-development or existing site conditions, based on soil types. Stormwater runoff from a hotspot should
not be infiltrated without effective pretreatment.
The recommended stormwater runoff volume to be recharged to groundwater should
be determined using the existing site (pre-development) soil conditions. The recommended rates of recharge for
various hydrologic soil groups are as follows:
|
NRCS Hydrologic Soil Group
|
Volume to Recharge (x total
impervious area)
(in acre-inches of runoff)
|
|
A
|
0.51
|
|
B
|
0.34
|
|
C
|
0.17
|
|
D
|
0.08
|
Groundwater recharge is included as part of the water quality volume and is
computed as recharge volume (Rev). Additional
information is provided in Part 2B.
More information on site design practices that promote infiltration is found in
Part 2E. Annual groundwater
recharge rates should be maintained to the extent practicable through the use
of nonstructural methods.
8.
Construction
erosion and sediment control. All new development and redevelopment sites
should meet the regulatory requirements for land disturbance activities under
the "Iowa
Erosion Regulations" (i.e. < 5 tons/acre/year). See Iowa Code Section 161A.64,
subsection 2 and/or the applicable NPDES General Permit #2 for construction activities.
This involves the preparation and
implementation of an approved Stormwater Pollution Prevention Plan (SWPPP),
including appropriate best management practices, during the construction phase
of development. Further guidance on
practices for construction site erosion and sediment control can be found in
Chapter 7 - Erosion and Sediment Control, and IDNR's Iowa Construction Site
Erosion Control Manual, which can be found at www.ctre.iastate.edu/erosion.
Better site design practices and techniques that can reduce the total amount of
area that needs to be cleared and graded should be implemented wherever
possible. It is essential that
erosion and sediment control be considered and implemented in stormwater
concept plans and throughout the construction phase to prevent damage to
natural stormwater drainage systems and previously constructed structural
stormwater controls and conveyance facilities.
9.
Stormwater
management system operation and maintenance. Implement a comprehensive operation and maintenance
plan for the stormwater management system. An operation and maintenance plan is one
of the required components of the post-construction minimum control measure for
permitted MS4 jurisdictions. This
is to include all of the stormwater management system components, including
drainage facilities, structural stormwater controls, and conveyance systems. To ensure that stormwater management
systems function as they were designed and constructed, the operation and
maintenance plan provides:
-
A clear
assignment of stormwater inspection and maintenance responsibilities
-
The
routine and non-routine maintenance tasks to be undertaken
-
A
schedule for inspection and maintenance
-
Any
necessary legally binding maintenance agreements
10. Pollution prevention. Consider pollution prevention measures in the design
and operation for new development and redevelopment sites, and prepare a
stormwater pollution prevention plan. Specific land use types and hotspots may
need to implement more rigorous pollution prevention practices.
11. Stormwater management plan.
Develop a stormwater management site plan for all new development and
redevelopment sites. The stormwater
site plan provides a narrative, technical information, and analysis, indicating
how the proposed development meets minimum guidelines #1-10 (or the applicable
local jurisdiction stormwater requirements).
The concept of minor and major design storms is related primarily to the
conveyance capacity design for storm sewer and surface drainage systems. Part 2C provides a discussion of
rainfall/runoff analysis and the selection of the appropriate design storm for
a particular component of the stormwater management system. The concept of the unified sizing criteria
is covered in Part 2B. This
discussion of minor and major design storms is related to the selection of the
overbank flood protection (Qp), which is one of the five components of the unified
sizing criteria. Every urban area
has two separate and distinct drainage systems, whether or not they are
actually planned for and designed.
One is the minor system corresponding to the minor (or ordinary) storm
recurring at regular intervals, generally two to 10 years. The other is the major system
corresponding to the major or extraordinary storm, generally 50- to 100-year or
greater storm event. Since the
effects and routing of stormwater for the major storm may not be the same for
the minor storm, all storm drainage plans submitted for approval should be
submitted showing the routing path and effects of the major storm.
Table 1: Chance of a storm equaling or exceeding a
given frequency during a given time period
|
|
Time
Period in Years
|
|
Frequency
(years)
|
1
|
5
|
10
|
25
|
50
|
100
|
|
2
|
50%
|
97%
|
99.9%
|
99.9%
|
99.9%
|
99.9%
|
|
5
|
20%
|
67%
|
89%
|
99.9%
|
99.9%
|
99.9%
|
|
10
|
10%
|
41%
|
65%
|
94%
|
99.9%
|
99.9%
|
|
25
|
4%
|
18%
|
34%
|
64%
|
87%
|
98%
|
|
50
|
2%
|
10%
|
18%
|
40%
|
60%
|
86%
|
|
100
|
1%
|
5%
|
9.6%
|
22%
|
39%
|
64%
|
1.
Minor storm provisions. The minor storm drainage system should be designed to provide protection
against regularly recurring damage, to reduce street and stormwater conveyance
maintenance costs, to provide an orderly urban drainage system, and to provide
convenience and protection to the urban residents. Storm sewer systems consisting of
underground piping, natural drainage ways and other required appurtenances
should be considered as part of the minor storm drainage system.
2.
Major storm provisions. The major storm drainage system should be designed to not cause major
property damage or loss of life from storm runoff expected from the major
storm. The effects of the major
storm on the minor drainage system should be noted.
Design storms for drainage facilities are described below. A minimum cleaning velocity of 2 ft/s should
be used for the 2-year storm, and 3 ft/s for the design storm. When detention or overland flow
provisions for storms greater than 10 years are not available, regardless of
the street system, the 100-year or greater storm is required for the design to
minimize impact to private properties.
1.
Intakes should have a
minimum capacity to convey the 5-year storm under developed conditions for
local streets and minor collectors during the peak flow rate. The Engineer may require 10-year
frequency for intakes for major collectors, arterials, expressways, and
freeways.
2.
Storm sewers should have
capacity to convey a 5-year storm under developed conditions within the pipe
for local streets and minor collectors.
The Engineer may require 10-year frequency for storm sewers for major
collectors, arterials, expressways, and freeways. Provisions should be made for the
minimum 100-year storm, greater in critical areas, when overland flow is not
allowed or available to prevent damaging private property. Storm and/or surface water conveyance
easements should be provided to the Jurisdiction.
3.
For those storm sewers that will handle footing
drains, the following discharge (Q) values should be used:
a.
For less than 50 houses, Q=5.0 gpm per house.
b.
For greater than 50 houses, Q=250 gpm plus 2.5 gpm
per house for each additional house over 50.
4.
Culverts should have
capacity to convey the following:
a.
10-year storm without the headwater depth exceeding
the diameter of the culvert
b.
50-year storm without the headwater depth exceeding 1
foot over the top of the culvert
c.
100-year storms should be conveyed through the
culvert without the headwater depth exceeding one foot below the low point of the roadway/embankment, unless there
are other, more restrictive elevations.
d.
For culverts that drain areas over two square miles,
the IDNR rules and regulations will apply.
5.
Ditches should have
capacity to convey a 50-year storm within the ditch banks. Provisions should be made for the
100-year storm to flow overland within the flowage easement. Surface water flowage easements should
be provided to the Jurisdiction for all designed drainageways. For ditches that drain areas over two
square miles, the IDNR rules and regulations will apply. Additional design guidance for vegetated
swale BMPs is provided in Part 2I.
6.
Detention basins should have the
capacity to retain a 100-year storm at critical duration or safely pass the
100-yr discharge over an auxiliary spillway. The top of the detention dike should be
a minimum of 1 foot above the 100-year storage elevation. The detention basin design requires the
IDNR approval for 18 acre-feet of storage or greater. Additional design guidance for detention
basins is provided in Parts 2C and 2G.
1.
Street capacity
for minor storms.
a.
Pavement encroachment for minor design storms should
not exceed the limitations set forth in Table 2:
Table 2: Allowable pavement encroachment and depth
of flow for minor storm runoff
|
Street
Classification
|
Maximum
Encroachment1
|
|
Local
|
No curb overtopping. Flow may spread to crown of street.
|
|
Collector/Minor Arterial
|
No curb overtopping. Flow spread must not encroach to
within 8 feet of the centerline of a two-lane street. The flow spread for more than two-lane
streets must leave the equivalent of two 12-foot driving lanes clear of
water; one lane in each direction.
For one-way streets, a single 12-foot lane is allowed.
|
|
Major Arterials
(4 lanes or greater)
|
No curb overtopping. Flow spread must not exceed 10 feet
from the face of the curb of the outside lane. The flow spread for more than two-lane
streets must leave the equivalent of two 12-foot driving lanes clear of
water; one lane in each direction.
For one-way streets, two 12-foot lanes are required. For special conditions, when an intake
is necessary in a raised median, the flow spread should not exceed four feet
from the face of the median curb for an inside lane.
|
|
Where no curbing exists, encroachment
shall not extend past property lines.
|
b.
The storm sewer system will commence upstream from
the point where the maximum allowable encroachment occurs. When the allowable pavement encroachment
has been determined, the theoretical gutter carrying capacity for a particular
encroachment will be computed using the modified Manning's formula for flow in
a small triangular channel as shown in Part 2M. An "n" value of 0.016 will be
used unless special considerations exist.
2. Street capacity for major storms. The allowable depth of flow and
inundated area for the major design storm should not exceed the limitations set
forth in Table 3:
Table 3: Allowable depth of flow and inundated area for 100-year storm runoff
|
Street
Classification
|
Allowable
Depth and Ponded Area
|
|
Local and Collector
|
The ponded area should not exceed the
street right-of-way and the depth of water above the street crown should not
exceed 6-in. There may be
situations where other restrictions are necessary.
|
|
Major and Minor Arterial
|
A 12-ft lane is the minimum travel lane
to be passable in the center of the street.
|
3. Cross street flow. Cross street flow
(called cross pan) can occur by two separate means. One is runoff which has been flowing in
a gutter and then flows across the street to the opposite gutter or inlet. The second case is flow across the crown
of the street when the conduit capacity beneath the street is exceeded. If the inundated area exceeds the street
right of way, flow easements must be obtained. The maximum allowable cross street flow
depth based on the worst condition should not exceed the limitation stipulated
in Table 4.
Table 4: Allowable cross street flow
|
Street Classification
|
Initial Design Storm Runoff
|
100-Year Design Storm Runoff
|
|
Local
|
6-inch depth at
crown or in cross pan
|
9-inch depth at
crown or in cross pan
|
|
Collector
|
Where cross
pans are allowed, depth of flow or in cross pan should not exceed 3"
|
6-inch depth at
crown
|
|
Arterial
|
None
|
3-inch or less
over crown
|