National Flood Insurance Program 
Community Rating System 

CRS Credit for 
Stormwater Management 

2006 


Note on this edition: This document was revised to reflect the following changes in the 
2006 CRS Coordinators Manual: 

 A new requirement for periodic evaluations of stormwater management master 
plans. 
A community interested in more information on obtaining flood insurance premium credits 
through the Community Rating System (CRS) should have the CRS Application. This and 
other publications on the CRS are available at no cost from 

Flood Publications 
NFIP/CRS 

P.O. Box 501016 
Indianapolis, IN 46250-1016 
(317) 848-2898 
Fax: (317) 848-3578 
NFIPCRS@iso.com 

They can also be viewed and downloaded from FEMAs CRS website, 

http://training.fema.gov/EMIWeb/CRS/index.htm 


 Contents 

Section Page 

Introduction .................................................................................................1 
Objective................................................................................................1 
The CRS ................................................................................................2 
Regulations Recognized in Activity 450...............................................2 
Regulations Recognized Elsewhere ......................................................3 
Legal Aspects ........................................................................................4 
Verification Visit...................................................................................5 

451 Credit Points.........................................................................................6 
Stormwater Management Regulations (SMR) ......................................6 
Size of Development (SZ).....................................................................9 
Design Storms (DS).............................................................................11 
Public Maintenance (PUB)..................................................................13 
Alternative Approaches.......................................................................15 
Stormwater Management Master Plan (SMP).....................................16 
Example Programs...............................................................................22 

452 Impact Adjustment ............................................................................23 
The Impact Adjustment Map...............................................................24 
Options for Impact Adjustment...........................................................26 
Impact Adjustment for Extraterritorial Jurisdiction............................29 
Common Problems..............................................................................30 

453 Credit Calculation..............................................................................31 

454 Credit Documentation .......................................................................31 
Regulatory Language for SMR ...........................................................31 
Stormwater Master Plan......................................................................33 
Stormwater Impact Adjustment Map ..................................................34 
Other Communities Regulations........................................................34 
Facility Maintenance Procedures ........................................................34 
Permit Records ....................................................................................35 

455 For More Information........................................................................35 

Hilton Head Island, South Carolina ..........................................................37 
451 Credit Points................................................................................37 
452 Impact Adjustment ......................................................................39 
453 Credit Calculation........................................................................40 
454 Credit Documentation .................................................................40 

Cheyenne and Laramie County, Wyoming ...............................................42 
451 Credit Points................................................................................42 
452 Impact Adjustment ......................................................................46 
453 Credit Calculation........................................................................49 
454 Credit Documentation .................................................................49 

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Credit for Stormwater Management - ii - Edition: 2006 


Introduction 

Objective 

One of the biggest problems of floodplain 
management in urbanizing areas is the increase in 
peak flow caused by development within a 
watershed. As forests, fields, and farms are 
covered by impermeable surfaces like streets, 
rooftops, and parking lots, more rain runs off at a 
faster rate. When an area is urbanized, the rate of 
runoff can increase five-fold or more. 

This problem is compounded by changes in the 
surface drainage system. Stormwater runoff 
travels faster on streets and in storm drains than it 
did under pre-development conditions. As a 
result, flooding is more frequent and more severe. 

Efforts to reduce the impact of increased runoff 
that results from new development in a watershed 
are known as stormwater management. 
Stormwater management also encompasses many 
aspects of water quality, and includes efforts to 
reduce erosion and the entry of sediment and 
pollutants into receiving streams. 

Finally, it is recognized that many buildings 
outside the floodplain are flooded because of 
local drainage problems. Elevating new buildings 
outside the floodplain or providing positive 
drainage away from new development reduces 
much of this damage. 

Among the objectives of the Community Rating 
System (CRS) are flood damage reduction and 
water quality protection. These objectives are 
addressed by the elements in this CRS activity. 

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The CRS 

The Community Rating System (CRS) is a part of the National Flood Insurance Program (NFIP). 
When communities go beyond the minimum standards for floodplain management, the CRS can 
provide discounts up to 45% off flood insurance premiums. 

Communities apply for a CRS classification and are given credit points that reflect the impact of 
their activities on reducing flood losses, insurance rating, and promoting the awareness of flood 
insurance. The Insurance Services Offices ISO/CRS Specialist reviews the communitys 
program and verifies the CRS credit. This includes a review of the written procedures and 
records of an activity and, in some cases, a visit to sites in the field. 

A community applies using the CRS Application. CRS credit criteria, scoring, and 
documentation requirements are explained in the CRS Coordinators Manual. Copies of both are 
available free from the office listed on the inside front cover of this publication. 

Regulations Recognized in Activity 450 

The CRS credit for watershed management regulations, watershed planning,and water quality 
management is provided under Activity 450 (Stormwater Management) in the Coordinators 
Manual. This publication explains the CRS credit criteria and gives examples. 

This activity recognizes five approaches to managing surface water outside of the floodplain. 
They are detailed in Sections 451.a through 451.e of the Coordinators Manual. 

a. Stormwater management regulations require that, in new developments, the increase 
in peak flows that normally results from development be prevented. The acronym used in 
the credit calculation formulae is SMR. The credit criteria are explained in Section 
451.a of the Coordinators Manual. 
b. Even with regulation of new development, the combined flows from a number of 
developments can still increase downstream peak flows. Stormwater master planning 
(SMP) can modify the regulatory standards and/or add structural measures to further 
reduce flood damage throughout the watershed. A master plan may also include elements 
to improve water quality and preserve the natural and beneficial functions of watersheds. 
The credit criteria for SMP are explained in Section 451.b of the Coordinators Manual. 
c. Much of the nations flood damage occurs in B, C, and X Zones. A large portion of this 
damage can be prevented by requiring all new development to be elevated. Requiring 
freeboard for new buildings in C, D and X Zones (FRX) is discussed in Section 451.c 
of the Coordinators Manual. 
d. Sediment control is especially important in watersheds where land is being disturbed by 
construction or farming. Drainage systems cannot operate as designed if they are choked 
with sediment washed in from construction sites. Erosion and sediment control 
regulations (ESC) are discussed in Section 451.d of the Coordinators Manual. 
e. Stormwater runoff picks up dirt, road oil, salt, farm chemicals, and other substances. 
Regulations that require developers to install or implement measures that permanently 
Credit for Stormwater Management - 2 - Edition: 2006 


improve the quality of stormwater are credited under the CRS. Water quality 

regulations (WQ) are discussed in Section 451.e of the Coordinators Manual. 

This publication reviews the first two elements of this activity, SMR and SMP, and explains the 
requirements for recognition by the CRS. 

Stormwater Management (Activity 450) can be made more effective through a comprehensive 
program of channel maintenance and debris removal (Activity 540 under the CRS). These two 
activities work together to reduce flood damage to development outside the floodplain. Activity 
540 (Drainage System Maintenance) explains the credit points for channel maintenance and 
debris removal. It is strongly recommended that a community delineate these procedures in the 
same document, especially because the two maintenance programs are often conducted by the 
same personnel. 

A comprehensive community drainage program should include elements from both Activities 
540 (Drainage System Maintenance) and 450 (Stormwater Management). 

Regulations Recognized Elsewhere 

The regulations credited in Activity 450 are related to reducing flood damage in and originating 
in watersheds, and protecting water quality. Communities may have other regulations related to 
flooding or water resources protection. Many of these are credited under other CRS activities, 
such as those listed below. 

 Requirements that developers or sellers publicize or disclose the flood hazard on their 
properties are credited under Activity 340 (Flood Hazard Disclosure). 
 Requiring permit applicants to develop base flood elevations or study the impact of 
their projects on flood heights or velocities in floodplains where such data are not 
provided by the NFIP is credited under Activity 410 (Additional Flood Data). 
 More restrictive floodway mapping, zero rise floodway, and full urbanization 
hydrology requirements are also covered under Activity 410. 
 Prohibiting new buildings in the floodway, V Zone, or other part of the floodplain is 
credited under Activity 420 (Open Space Preservation). A community can receive 
credit for a prohibitory regulation under either Activity 420 or Activity 430, but not 
under both. Activity 420 provides more credit points than does Activity 430 because 
new buildings are better protected from flooding if they are kept out of the floodplain 
in the first place. Therefore, most communities opt to credit prohibitory regulations 
under Activity 420. 
 Activity 430 provides CRS credit for numerous regulatory standards that reduce flood 
damage, otherwise protect buildings, and generally improve floodplain management. 
 Regulations on dumping or placing debris in stream channels are credited under 
Activity 540 (Drainage System Maintenance). 
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Legal Aspects 

For the purposes of this activity, creditable regulations must be legally enforceable requirements 
placed on watershed development. They do not have to be enforced by the community but they 
do have to be legally enforceable by a government agency. For example, state regulations or 
requirements from a county or regional drainage or flood control district may be credited if those 
entities have jurisdiction in the community. 

In most states, regulations are in the form of state statutes, codes, or regulations or local 
ordinances or bylaws. Plans, such as land use plans and comprehensive plans, are usually 
recommendations, not regulations. A community that submits a plan for credit under this activity 
must also submit its attorneys opinion that the plan has the force of law and is enforced by a 
regulatory office, such as a building or zoning department. 

Most stormwater management regulations appear in a subdivision ordinance or separate 
stormwater management ordinance. They either cover construction projects throughout the 
community or in subdivisions and other developments that require special plan approval. 

In some cases, an ordinance, especially a subdivision ordinance, will refer to state or local 
policies, specifications, a design manual, or other separate document. Many local officials have 
said, developers dont argue, they follow this manual because we tell them to. Unless the 
separate policy document is specifically adopted by reference in the ordinance, the community 
will have to include a statement from its legal counsel that its policies and design standards have 
the force of law. 

Similarly, some regulations state that something may be required or that a permit applicant 
should do something. The CRS only credits clear and explicit regulations that require specific 
actions or standards from a floodplain developer. Generally the word shall indicates such a 
requirement. 

For example, the following language would NOT be credited. 

If, in the opinion of the building official, the soils are not suitable for construction, 
appropriate fill and compaction may be required. 

The following language WOULD be credited. 

The applicant shall provide a soils engineering report based on the results of one soil 
boring for each acre where the following soil types are present . . . 

In general, statements in the purpose or objective section of an ordinance are not acceptable. The 
CRS credits the specific requirement, not a statement about a reason for adopting the ordinance. 
For example, many communities have language that says one of the objectives of the ordinance 
is To prevent fraud and victimization of unwary land and home buyers. Nowhere else in the 
ordinance is there a reference to fraud or a specific disclosure requirement. Therefore, credit 
under Activity 340 (Flood Hazard Disclosure) has not been provided for that language. 

In some cases, state laws provide the authority for a state agency or a community to do 
something. Usually a state agency will implement regulations or a community will enact an 

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ordinance pursuant to the law. It is the subsequent regulations or local ordinance that must be 
submitted for CRS credit, not the authorizing or enabling legislation. 

Sometimes a requirement is meaningless without the definition section of the ordinance or 
regulation. Instead of requiring buildings to be elevated 1 foot above the base flood elevation, 
some communities require them to be elevated above a flood protection elevation. In these 
cases, the community also needs to submit the ordinance section that defines the flood 
protection elevation. 

As with all regulatory issues, the opinion of the communitys attorney or corporation counsel is 
most important. If language is not accepted by the ISO/CRS Specialist because it does not appear 
to be clear, explicit, or consistently enforceable, then the community may submit a letter on its 
attorneys letterhead stating that the debated item has the force of law. 

COMMUNITIES SHOULD BE CAREFUL WHEN USING MODEL ORDINANCES AND THE EXAMPLES IN THIS 
PUBLICATION. A community should not amend its ordinances solely to earn CRS credit points, 
nor should it adopt these examples verbatim. Ordinance language should be carefully written to 
support the communitys goals and the purposes of its regulatory program, to sufficiently 
respond to the watershed situation facing the community, and to conform with state law. ALL 
SUCH LANGUAGE SHOULD BE REVIEWED BY THE COMMUNITYS LEGAL COUNSEL BEFORE ADOPTION. 

Verification Visit 

During the verification visit, the ISO/CRS Specialist will check records such as building permit 
files and subdivision plats to verify enforcement of the communitys ordinance. For Activity 
450, a 5-5-5 sampling method, described below, is used. If the verified credit for an element is 
less than 50%, no credit is given for that element. 

Under the 5-5-5 sampling method, the ISO/CRS Specialist selects a sample of five (e.g., five 
building permit files and/or subdivision plats). If all of the first five developments comply with 
the stormwater regulations, full credit is given for SMR. If one or more items in the sample does 
not qualify for credit, the ISO/CRS Specialist will look at five more examples. 

If the review finds six or more of the sample of 10 to be deficient (less than 50% compliance) 
then five more permits will be reviewed. The final score is based on the sample of 15, provided 
at least eight of them (53% of 15) qualify for the credit. 

Example: A community has stormwater management regulations that require 
detention of the 100-year storm for all new developments that increase the 
impervious area by more than 5,000 square feet. There is no procedure to 
inspect stormwater facilities. The community is applying for the following credit: 

SZ = 20 

DS = 60 

SMR = 80 

The ISO/CRS Specialist asks to see building/development permits for a 
convenience store, a neighborhood shopping center, and three residential 

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subdivisions. Although the convenience store created more than 5,000 square 
feet of new impervious area, it was granted a variance to the detention 
requirement because the lot was crowded with the proposed development. 

Because the detention requirement was not included in all of the first five 
developments, the ISO/CRS Specialist takes an additional sample of five 
developments. All of these are compliant. 

Altogether, nine of the ten developments met the requirements of the stormwater 
regulation. The communitys credit for SMR is reduced to 90% of the credit 
earned by the regulation. The verified score for SMR is 

80 x 0.9 = 72 

This credit is adjusted by the impact adjustment as explained in Section 452. 

451 Credit Points 

This section discusses two of the five elements (stormwater management regulation and 
stormwater master plans) and the corresponding CRS credit points under Activity 450. As in the 
Coordinators Manual, the credit criteria of the CRS Schedule are shown in shaded boxes with 
double line borders. Examples of regulatory language are shown as block quotations in this 
italicized typeface. Scoring examples are shown in this typeface. 

This section also covers how the elements are scored, provides example ordinance language, and 
identifies some of the more common problems and misunderstandings found in community CRS 
applications. 

Stormwater Management Regulations (SMR) 

Why do we need stormwater management regulations? 

The NFIP is based on regulation of floodplain development to reduce future flood damage. 
However, 23% of all NFIP policies are written on buildings outside the floodplains, and these 
buildings have had almost 25% of the flood insurance claims. Some of these buildings are 
adjacent to mapped floodplains, but many are far removed from identified flood hazards. 

Floods are generated from watersheds. Whether precipitation falls as water, ice, or snow, it either 
evaporates, infiltrates into the ground, or runs off of the watershed toward a river, lake, playa, or 
ocean. The proportion that evaporates and infiltrates is dependent on a number of things. For 
rainfall, the primary factor is the intensity of the rainfall. The harder it rains, the less water 
evaporates and infiltrates, and the more runs off over the surface. For snow, the primary factor is 
how fast it melts. This discussion will focus on rainfall and runoff, but it is equally applicable to 
snowmelt. It will also talk about runoff into a river, but most of the discussion applies equally to 
lakes, playas, and oceans. 

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A watershed, also called a drainage 
basin or catchment area, is the 
geographic area where the water for a 
river or lake originates (see figure). All 
lands in a watershed drain downhill 
toward a stream, lake, bay, or other 
body of water. The boundary of a 
watershed is also called a divide. 
Stormwater runoff on one side of the 
divide drains to one body of water and 
runoff on the other side drains 
elsewhere. 

As a watershed receives rainfall, water 
starts to flow on the surface. Water that 
falls next to the river enters the river 
immediately. Water from the top of the watershed does not reach the river until it has made its 
way down the slopes of the watershed. As the rain continues, more and more of the watershed is 
contributing flow to the river, and the river level rises. If the rainfall intensity is steady, and the 
storm lasts long enough for water from the farthest point in the watershed to flow to the river, the 
river flow will quit rising. 

When the rainfall ends, the river will gradually recede as the areas closest to the river quit 
contributing flow. The river will get back to its normal level when the upper parts of the 
watershed quit flowing. 

This process is the same for a quarter-acre city lot with a house and for the Mississippi River 
where it enters the Gulf of Mexico. In the case of the city lot, the time from the start of the storm 
to the maximum rate of runoff is just a few minutes. For the Mississippi River, the time is 
several months. 

Other factors affect runoff. On larger rivers, there are dams that regulate the flow. Much of the 
water that infiltrates during a rainstorm reaches the river as groundwater hours, days, or years 
after the rainfall ends. However, in most cases of flooding, there is a direct and immediate 
connection between precipitation and flooding. 

What we think of as a flood is the event during which a larger-than-normal amount of water is 
flowing in the river. It is the peak flow that causes the most flood damage. 

A flood peak usually occurs when the watershed above the point of flooding receives rainfall for 
a long enough period that the entire watershed is contributing to the flow. At that point, the size 
of the peak flow is directly related to the intensity of the rainfall. 

However, the peak flow at a point on a river is very much dependent on the nature of the 
watershed. In a mountainous area, a one-square-mile watershed might be fully contributing after 
a few minutes, while a one-square-mile watershed on the Great Plains might take several hours. 
The time it takes for the farthest point in a watershed to reach the river is called the time of 

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concentration. If the duration of a storm is equal to or longer than the time of concentration, the 
watershed will produce its highest peak flows for the intensity of that storm. 

Storms have different intensities, but the highest 
intensities cover the smallest area. If a storm is five miles 
across, there will be some smaller areas where the rain is Stormwater facilities do not have to 
heavier than in others. The same is true for a tropical be wasted space in a development 

or in the community. The City of Las

storm or hurricane that is 100 miles or more in diameter. Vegas has nearly 300 soccer fields,Everyone has seen the radar images on television that many of them in city-ownedshow a broad area of rainfall with a few local centers of detention basins. In a recently 

intense rainfall. opened complex, there are five 
soccer fields at different levels, so 

Another factor in the peak flow from a watershed is the that they are not all flooded 

frequently. Only one field is below

condition of the surface of the watershed. Imagine a 10-the 10-year flood elevation in thesquare-mile watershed (that is, about 2  miles wide and detention basin. 
four miles long) covered by dense forest. Much of the 
rainfall is absorbed by the organic litter under the trees. 
There is a relatively small amount of runoff. Then the forest is cleared and the watershed is 
planted in wheat. The runoff increases significantly. After a time, the wheat fields are converted 
to row crops, and the watershed is contour plowed. The runoff decreases dramatically because 
the furrows store the runoff and increase the amount of infiltration. Then, the fields are 
subdivided for residential development. About half of the watershed area is covered by roofs and 
streets. These impervious surfaces allow almost no infiltration, so the amount of runoff increases 
dramatically. Not only does the amount of runoff increase, but the water runs off of roofs, into 
the streets, and into drainage ditches that carry it straight to the river. The time of concentration 
has been decreased significantly. 

When the watershed was forested, the time of concentration was perhaps four hours. When it is 
developed, the time of concentration might be one hour. Now the watershed produces five times 
the amount of runoff, and it arrives at the river four times as fast. The result is a much higher 
peak flow. In the forest, if a ten-square-mile cell of intense rainfall hit the watershed for an hour, 
only a portion of the watershed contributed to the peak flow during that hour. Developed, the 
entire watershed contributes to the peak. 

Stormwater management regulations require that each developer in the watershed ensures that 
the increased runoff from his or her development does not increase the peak flow downstream. 

Scoring SMR 

SMR credit is provided if new developments are required to prevent or reduce the 
increase in runoff that results from urbanization of the watershed. To receive SMR 
credit, the watershed must be subject to a regulation that requires the peak runoff 
from new developments to be no greater than the runoff from the site in its pre-
development condition. Credit may be provided for other approaches to managing 
the impact of development on runoff where the community can show that there is 
no increase in flood damage downstream. 
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SMR credit is the sum of the credit for three sub-elements: 
SMR = SZ + DS + PUB. 
If SZ = 0, then SMR = 0. 
The basic requirement is that the peak flow after development must not be increased as a result 
of the development. Picturing the change from farmland to subdivision, how is this done? 
Generally, the developer builds a storage basin at the lower end of the development to store the 
extra water that runs off. Usually, the developer is allowed to release the water from this storage 
basin at the peak flow rate before development. In most cases, these storage basins will empty in 
a few hours and be ready for the next storm. 

There are other ways to accomplish the desired result of preventing downstream flooding. These 
generally require watershed master planning and are discussed beginning on page 16. 

Example Regulatory Language for SMR 

From the Subdivision Rules and Regulations for Marshfield, Massachusetts: 

. . . the Planning Board shall require an on-site storm drainage analysis for all 
proposed developments that mitigates the 10, 25 and 100 year storm event in 
accordance with the standards contained herein. 

The following information shall be required: 

(x) Demonstration of no increase in the rate of discharge from pre-
development to post-development conditions. 
The Drainage Regulations for Maricopa County [Arizona] state: 

The entire drainage retention and conveyance system shall be designed to 
eliminate or minimize storm water runoff and convey the runoff through the 
development with minimum detrimental effects to the development or to any 
other property. No system shall be approved if the effect may cause an increase 
in the peak discharge, volume or velocity or change the point of entry of drainage 
onto another property during the runoff event. 

Size of Development (SZ) 

SZ credit has a range of from 25 points (all projects are regulated) to 5 points (only projects 5 
acres or larger are regulated). There is no credit if the regulations only affect developments 
larger than 5 acres or with more than 20,000 square feet of impervious surface. The CRS does 
not credit regulations that apply only to these large developments because the cumulative effect 
of a number of small, unregulated developments could have a significant impact on runoff in the 
watershed. 

This sub-element recognizes that many communities do not want to impose the complexity of 
stormwater management on the owner of a small parcel. 

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 1. Size of development (SZ) (Maximum credit: 25 points) 
SZ is based upon the minimum size of areas regulated. Use either: 
(a) 25, if all development is regulated, regardless of size; 
(b) 20, if all development is regulated except single-family residences or 
increases in impervious area of 5,000 square feet or less; 
(c) 15, if all development is regulated except for parcels of 1/2 acre or less or 
increases in impervious area of 10,000 square feet or less; 
(d) 5, if all development is regulated except for parcels of 5 acres or less or 
increases in impervious area of 20,000 square feet or less; or 
(e) 0, if the regulations only apply to development of parcels larger than 5 
acres or increases in impervious area of more than 20,000 square 
feet. If the regulations only cover such large development projects, 
there is no credit for SZ or SMR. 
During verification, SZ is adjusted, if necessary, to reflect different thresholds for different types 
of development. For example, if a community regulates commercial developments larger than 2 
acres and residential developments larger than 5 acres, SZ is pro-rated according to the 
percentage of current land use in each category. 

Example Regulatory Language for SZ 

Maricopa County, Arizona, regulates all commercial, industrial, and multi-family residential 
developments and all subdivisions. This regulatory language receives 20 points for SZ because 
single-family residences are exempt. 

The Village Code of Downers Grove, Illinois, states that all development shall meet the 
stormwater requirements if 

The parcels being developed total three acres or greater for single or two family 
residential land uses, or . . . the parcels being developed total one acre or 
greater for multiple family or nonresidential land uses, . . . 

This regulatory language provides 5 points for SZ, because it exempts up to 3 acres of 
development. 

The Riverdale, New Jersey, code requires a site plan for any building in any business, 
commercial recreation or industrial zone, and for residential subdivisions. Subdivisions and site 
plans require that the post-development rate of stormwater runoff not exceed the pre-
development rate. Because single-family residences are not required to regulate stormwater 
runoff, the credit for SZ is 20. 

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The New Hanover County [North Carolina] Storm Water Management Ordinance regulates any 
development activity that will result in the accumulation of ten thousand (10,000) square feet of 
impervious area on any site. Such development is required to provide stormwater management. 
This language is worth 15 points for SZ. 

Documentation Needed for Verification 

The only acceptable documentation for SZ is the ordinance language requiring development to 
keep post-development peak flows at or below the pre-development peaks. The section of the 
regulation that provides exemptions to the requirement must be included. 

Common Problems 

 The most common problem communities have with this element is not having the 
right community staff member available during the verification visit. Frequently, the 
CRS Coordinator is not familiar with the specifics of the stormwater management 
regulation. 
 Communities fail to understand that regulation of, for example, all subdivisions, is 
management of all development. To be eligible for full credit (25 points) for SZ, a 
community must regulate all development, including single-family residences in 
existing subdivisions, to ensure that pre-development runoff is not increased. 
Design Storms (DS) 

DS credit is provided based on the design sizing of the runoff control facilities. For DS credit, 
the communitys regulations generally must require pre- and post-development hydrology 
calculations and post-development runoff must be limited to pre-development levels. The 
standard used may be peak flow, volume, or a combination of the two. 

DS is the total of the following points based on the design storms used in the 
regulations (i.e., the storms used to measure the impact of new developments). For 
DS credit, the communitys regulations must require pre- and post-development 
hydrology calculations and post-development runoff must be limited to pre-
development levels. The standard used may be peak flow, volume, or a 
combination of the two. 
(a) 60, if detention/retention is designed for the 100-year storm; 
(b) 20, if detention/retention is designed for a storm larger than the 10-year 
but smaller than the 100-year storm; and 
(c) 10, if detention/retention is designed for a 10-year storm. 
Although the 100-year flood is the basis for floodplain management, communities are also 
encouraged to look at the effects of stormwater management on smaller, more frequent storms. A 
design that maintains or reduces the peak flow from only the 100-year storm may increase peaks 
from smaller storms, increasing flood damage. 

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Full credit (90 points) can be obtained in the following situations: 

 If developers account for the runoff from three storms: the 100-year storm, another 
storm larger than the 10-year, and the 10-year storm; 
 If the design storms include all storms up to and including the 100-year; 
 If 100-year retention is required. For CRS purposes, retention (as opposed to 
detention) means that a basin has no outlet and stored runoff must be infiltrated into 
the soil, pumped out for irrigation, or otherwise disposed of on site; or 
 If 100-year detention is required with a release rate based on the pre-development 10year 
runoff. 
The following examples should help explain the formula for DS: 

 Design storms used DS

 2, 5, and 10 10 
25 20 
10 and 25 30 
100 60 
25 and 100 80 
10, 25, and 100 90 

If the regulations are based on inches of rainfall, the value must be converted to a storm 
recurrence interval. One source that can help do this is the Rainfall Frequency Atlas of the 
United States (the National Weather Services Technical Paper 40; accessible at 
http://www.ncdc.noaa.gov/oa/documentlibrary/rainfall.html). 

Stormwater regulations that focus on water quality are generally not adequate for SMR credit 
because they have little or no impact on flood flows. For example, some communities require 
detention of the first 1 inch of rainfall to reduce non-point sources of water pollution. In some 
areas this is less than a 1-year storm, so there is no credit provided under SMR, but it could 
qualify as a water quality measure (WQ) for credit under Section 451.e. 

Example Regulatory Language 

The Maricopa County, Arizona, regulation states that 

The retention system shall be designed to receive and retain the volume 
generated from the 2-hour 100-year run-off event falling over the entire 
development including all rights-of-way, excluding off-site flows. 

Because the stormwater is RETAINED on the development, rather than being DETAINED, this 
language earns 90 points for DS. If the 100-year runoff is retained, all smaller storms are also 
retained. Water that is retained must generally be disposed of on the property, so there is no 
runoff. 

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The Delaware Sediment and Stormwater Regulations require that 

Projects in New Castle County that are located north of the Chesapeake and 
Delaware Canal shall not exceed the post-development peak discharge for the 2, 
10 and 100 year frequency storm events at the pre-development peak discharge 
rates for the 2, 10 and 100 year frequency storm events. 

This language is worth 70 points for DS. Runoff from the 10- and 100-year storms is regulated. 

Hoffman Estates, Illinois, requires that the 100-year storm runoff be detained. 

The District will accept the release rate of not greater than that calculated from a 
storm of three (3) year frequency . . . 

By detaining the 100-year storm and releasing it at a rate lower than the 10-year pre-
development rate, this regulation earns 90 points for DS. 

Documentation Needed for Verification 

The only acceptable documentation for DS is the ordinance language that specifies the design 
storms used to size detention basins. In some cases, the regulation adopts the standards specified 
in a design manual. In this case, appropriate pages from both the regulation and the design 
manual are needed for verification. 

Common Problems 

 The most common problem communities have with this element is determining which 
design storms are used for detention/retention. This problem almost always occurs 
when someone other than the community staff person responsible for stormwater 
management tries to assemble the documentation. 
 Another, less frequent problem arises when a parameter other than the recurrence 
interval for the design storm is used to determine either the required storage volume 
or the allowable discharge rate. For example, the regulation may say, Detention must 
be provided for the first 1.0 inch of rainfall, or the discharge cannot exceed 0.15 
cubic foot per second per acre. In these cases, the community must provide 
documentation that relates the parameter in its regulation to the recurrence interval of 
a storm in its area. 
 Some communities use different design storms for different aspects of development. 
A communitys standards might say that underground storm drains must be designed 
for the 10-year storm, runoff from the 25-year storm must be contained within 
drainage easements, and buildings must be protected from runoff from the 100-year 
storm. CRS credit for DS is based entirely on the storm used to design 
detention/retention facilities. 
Public Maintenance (PUB) 

Frequently, stormwater management regulations leave it up to the developer, the owner, or a 
homeowners association to assume responsibility for maintenance of the required facilities. 
Because experience has shown that private maintenance of stormwater management facilities is 

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not as reliable as public maintenance in the long run, CRS credit is provided to encourage these 
facilities to be maintained by a public agency. This credit is also provided if the community 
inspects all private stormwater facilities at least annually and has the authority to require the 
owners to perform appropriate maintenance. 

PUB = 110, for public maintenance of all stormwater facilities. 
PUB is determined by the nature of public maintenance of stormwater facilities. 
Credit is provided if the community assumes maintenance responsibility for all 
private facilities or if the community inspects all private stormwater facilities at least 
annually and has regulatory authority to require the owners to perform appropriate 
maintenance. 
A community can receive PUB credit through any one or combination of three ways: 

1. The community inspects all new stormwater management facilities at least annually 
and orders maintenance when needed. If the owner fails to perform the maintenance, 
the community does the job and bills the owner; 
2. The owners of all new stormwater management facilities perform the maintenance 
and their engineers certify at least annually to the community that it has been done; or 
3. All new stormwater management facilities (including basins built by private 
developers) are required to be deeded to the community, and the community inspects 
the facilities at least annually and provides maintenance as needed. 
Whichever approach is used, it must be supported by an ordinance or other regulatory authority. 
For example, holding the owner responsible for maintenance must be based on clear legal 
authority, such as the subdivision ordinance, that was known to the developer at the time of 
construction of the stormwater facility. Credit is not provided for a policy or a statement with 
which the community has been able to get compliance in the past. 

If inspection is performed by the community, the community must document its inspection 
program with all documentation required for channel debris removal (CDR) in Section 544. 

Example Regulatory Language 

The drainage regulations for Maricopa County, Arizona, state that 

It is a violation of this Regulation . . . for failure to maintain any natural drainage 
system or any drainage easement, tract or channel or common area created 
pursuant to this Regulation . . . 

Violation of this Regulation is a public nuisance, per se, and subject to all 
remedies as provided by law . . . 

If the property owner fails to remove fill material, rubbish, trash, weeds, filth, 
debris or any obstruction that . . . the Drainage Administrator determines 
obstructs, retards or diverts a natural or improved drainage system, . . . the 

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Drainage Administrator may remove or cause the removal of the fill material, 
rubbish, trash, weeds, filth or debris upon his determination that failure to remove 
or abate the condition constitutes a threat to the public health, safety or welfare. 

If it is verified that the county inspects all detention facilities at least annually, and all necessary 
maintenance is performed, this regulatory language earns 110 points for PUB. 

The Land Development Code for Austin, Texas, states that 

At least once each year, the Transportation and Public Services Department 
shall inspect each water quality basin required to be maintained by the owner . . . 

The ordinance also specifies that the developer must provide a maintenance plan. The city has 
regulatory authority to repair facilities if needed. After verification of the inspection procedures 
and a field visit to a sample of stormwater facilities, PUB was verified at 110 points. 

Documentation Needed for Verification 

Documentation for PUB must include the regulatory language and the same inspection 
procedures and documentation that are required for channel debris removal (CDR) in Activity 
540 as it applies to SMR stormwater facilities. 

 If the community performs inspections of private facilities, the regulatory language 
must show that the community has the right to enter the property for inspections, and 
that the community has the authority to perform maintenance if the owner fails to do 
so. 
 If the private owner of the drainage facilities is required to have an inspection 
performed, the regulatory language must include this provision. 
 If all stormwater facilities are deeded to the community, this must be reflected in the 
regulatory language, and these facilities must be included in the communitys 
drainage system for CDR credit in Activity 540. 
Common Problems 

The most common problem with PUB credit is that communities do not understand that 
inspection of new stormwater facilities is their responsibility. A regulation that requires 
stormwater facilities to be maintained by the owner is not adequate for PUB credit. 

Alternative Approaches 

As noted above, there are other ways to accomplish the desired result of preventing downstream 
flooding. These alternative approaches require a stormwater master plan: 

 If a community is next to the ocean, a large river, or a lake or playa, it may choose to 
get the water out of town as quickly as possible. In this case, the community may 
choose to develop a large drainage system based on anticipated development. 
Credit will be verified for this approach only where the community drains to a large 
receiving body of water, and where the entire watershed affecting the community is 
almost entirely developed. 

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 With advance planning, a community may design a combination of storage facilities 
and drains to accommodate all future development. These facilities may be built as 
the development occurs. They are frequently funded by the developers. This approach 
can be eligible for both SMR credit and credit for stormwater master planning (SMP). 
Verification of this approach will depend upon the planned discharge at the 
downstream end of the system. If it is apparent that the intention of the plan is to limit 
downstream peak flows, it can receive SMR credit. If not, it may be eligible for credit 
for structural flood control projects under Activity 530. 

 As an alternative to using a uniform standard for all areas, some communities 
regulate development according to a master plan that analyzes the combined effects 
of existing and expected development on stormwater and flood flows in the 
watershed. Such watershed-specific regulations may allow different amounts of 
runoff for different areas in order to control the timing of increased flows into the 
receiving streams. A watershed master plan may also be used to preserve wetlands, 
riparian areas, or other important habitats. This approach can also be eligible for both 
SMR credit and credit for stormwater master planning (SMP). 
Verification of SMR credit for this approach generally requires credit for channel 
debris removal (CDR) in Activity 540. This approach may lead to rapid drainage of 
the lower part of the watershed so that it will not be contributing to the runoff from 
the upper parts of the watershed. In this case, maintenance of the channels all the way 
down to the receiving stream is critical to the success of the master plan. 

Stormwater Management Master Plan (SMP) 

Why do we need stormwater management plans? 

Even if each developer controls the runoff from his or her development so that peak flows are 
not increased, downstream peaks still will increase. In regulating the peak flow, when the 
volume of runoff increases, the duration of high flows from the development increases. As these 
high flows move down the watershed and combine with the releases from other developments, 
the combined peaks are higher than the pre-development flow. 

This can be avoided through stormwater management master planning. A stormwater master 
plan estimates the runoff from different parts of the watershed based on fully developed 
conditions. The regulations are adjusted. Perhaps developments in the lowest part of the 
watershed are allowed to pass their runoff into the system immediately, so that it is gone when 
the runoff from farther up the watershed arrives. Perhaps the development from the middle of the 
watershed is only allowed to release at a rate equal to 50% of the pre-development peaks, and the 
upper-most developments must wait a specified time before they can release runoff. Many other 
options can be developed through stormwater master planning. 

A stormwater master plan is the result of a hydrologic and hydraulic study of the watershed, 
usually under both existing conditions and future development conditions, with different 
management scenarios. It usually includes recommendations for a set of management controls 
and/or construction projects to solve existing flooding problems and to prevent new problems. 

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Credit is provided if the community develops and implements surface water runoff regulations 
through a stormwater master plan that ensures that flood damage within and downstream from 
the watershed is not increased by future development. In general, the plan must 

 Have been adopted in the communitys regulatory program, 
 Require that the peak flows of runoff from future development will not increase 
beyond the present peak flows, and 
 Manage all storms up to and including the 25-year storm (no credit is provided for 
SMP for management of storms smaller than the 25-year storm). 
A stormwater master plan can work with other planning tools used by the community to help 
accomplish other community objectives. Knowing the present and future conditions of runoff 
from streams too small to be included on the communitys Flood Insurance Rate Map (FIRM) 
allows for protection of many buildings in the watershed. The community can also use this to 
preserve open space corridors, protect wetlands and other habitats, and for other actions that 
generally improve the quality of life throughout the community. Additional CRS credit is 
provided for some of these planning aspects. 

CRS Credit for Stormwater Management Master Plans 

Stormwater master plans must meet four prerequisites to be eligible for CRS credit. If these four 
prerequisites are met, the community receives a basic credit of 80 points. The community can 
receive additional points for other standards in the plan. 

1. Prerequisites: 
(a) The community must have adopted a stormwater management master 
plan for one or more of the watersheds that drain into the community. 
To receive SMP credit the communitys governing board must have adopted the plan(s). Even if 
the plan does not have the authority of a regulation, adoption indicates that the community 
accepts the standards and other provisions included in the plan and intends to implement them. 

(b) The community has adopted regulatory standards for new construction in 
the watershed based on the plan. 
In general, there must be a connection between the stormwater master plan and the communitys 
stormwater management regulation. There are two ways CRS communities have met this 
requirement: 

1. A community recognizes that ongoing development is increasing downstream 
flooding. It adopts a stormwater management regulation requiring that all new 
development detain or retain increased runoff so that the peak flow from the 
development is not increased. After some time, the community sees that downstream 
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flooding is still increasing as development proceeds. It develops a watershed master 
plan. This plan recommends more stringent detention/retention requirements or 
different requirements for different parts of the watershed. The plan usually also 
recommends certain structural measures to solve existing problems. 

2. A community is experiencing increased runoff from ongoing development. It 
develops a watershed master plan to figure out what to do. This plan recommends 
detention/retention requirements throughout the community, or different requirements 
for different parts of the watershed. The plan usually also recommends certain 
structural measures to solve existing problems. In this case, SMP credit is provided 
only if the community has adopted the recommended stormwater management 
regulation. 
Regardless of how the community got started on master planning, the plan usually must include 
studies of current- and future-conditions hydrology and hydraulic studies of current- and future-
conditions runoff. Current conditions are generally studied to show the existing problems and to 
justify regulation of new development and expenditures on structural elements of the plan. 

(c) The plans regulatory standards manage future peak flows so that they 
do not increase over present values. 
Many documents called stormwater master plans have been developed to guide the construction 
of storm drain systems. These documents are usually comprehensive reviews of a watersheds or 
basins hydrology. However, they are not eligible for SMP credit if they are only used to design 
drainage facilities and do not include regulations or set regulatory standards to prevent new 
developments from aggravating stormwater problems. 

(d) The plans regulatory standards require management of runoff from all 
storms up to and including the 25-year event. 
The purpose of CRS credit for stormwater master planning and stormwater management 
regulation are to reduce flood damage resulting from development in watersheds. Regulation and 
planning for smaller storms is effective in improving water quality, but does little or nothing to 
reduce flood damage. 

(e) In order to maintain SMP credit for any plan that is more than five years 
old, the community must evaluate the plan to ensure that it remains 
applicable to current conditions. The evaluation must address whether 
the dates used for the plan are still appropriate and whether the plan 
effectively manages stormwater runoff. If a stormwater management 
master plan is obsolete, the community must update the plan or the SMP 
credit will be revised accordingly. 
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Like other plans, stormwater master plans become obsolete. This can happen for several reasons: 

 Rainfall data added to an old record can change the design storm(s) that is used to 
generate runoff data; 
 Changes such as channel improvements and new detention basins alter downstream 
flows; 
 Stormwater master plans are often based on projected land use. As time goes by, it is 
necessary to compare actual land use with the projections used; or 
 Over time, new methods are developed for the hydrology and hydraulics used in 
stormwater projections. 
There may be other considerations when deciding if a stormwater master plan is obsolete. 

If a stormwater master plan is more than five years old, the community must certify that the plan 
is still applicable and not obsolete. The certification must address the issues included in the 
documentation requirement, Section 454.j, on pages 35 and 36. 

2. SMP = the total of the following points. Credit must be received for item (a). 
(a) 80, if the stormwater management plan meets all of the prerequisites 
listed in Section 451.b.1. 
After a stormwater master plan meets the four prerequisites, additional SMP credit is awarded 
for seven aspects of planning ((b) through (h), below): 

(b) 25, if the plan manages the runoff from all storms up to and including the 
100-year event. 
Additional credit is provided if the communitys regulations manage all storms up to and 
including the 100-year storm. All storms includes specifically listed storms, such as the 2-, 10, 
25-, 50-, and 100-year storms. Almost all communities with CRS credit for watershed master 
planning have used the 100-year storm, so they get this credit. 

(c) 40, if the plan provides management of future peak flows AND VOLUMES 
so that they do not increase over present values. If the community 
can demonstrate that its stormwater management plan prevents 
damaging increases in peak flows at all points within its watershed(s) 
and downstream, it will receive this credit. 
The more common approaches are when a community disposes of its increased volume of 
stormwater to an aquifer through groundwater recharge, or uses the water for irrigation or other 

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purposes. For this credit, the community must retain the increased runoff due to development, 
although it may be allowed to detain and discharge an amount of water equal to the pre-
development runoff. As with any retention system, sufficient storage must be created to handle a 
later storm that occurs before the retained water is disposed of. 

Communities that discharge directly into an ocean or a Great Lake may receive this credit if they 
have adopted a watershed master plan that models their watershed(s) and prevents increased 
peak flows within those watershed(s). Communities with watersheds that discharge into other 
large lakes or rivers must demonstrate that their discharges will not increase flood elevations in 
the lake or anywhere downstream on the receiving river. 

(d) 25, if the plan manages the runoff from all storms up to and including the 
5-day event. If a community can demonstrate that an event shorter 
than five days is the locally appropriate worst-case runoff event for 
stormwater management, it may receive the credit if it uses that 
event for its regulatory standard. 
There are usually at least three worst-case runoff events for a particular recurrence interval 
storm: one that causes the highest peak discharge from the development; one that causes the 
highest peak discharge from the watershed; and one for the stream into which the watershed 
discharges. Most communities plan for the first two, which may range from a few minutes to a 
few days. Fewer plan for the third, which may range from a few days to several weeks. 

This credit is provided for assuring that the most appropriate modeling techniques are used for 
the location. This is assumed to be a 5-day event unless the community can show that a shorter 
event is more appropriate for local conditions. In some areas this may require continuous-
simulation modeling. If a community, regional, state, or federal agency can demonstrate that, 
say, the 72-hour event provides the worst case runoff for a watershed, the 72-hour event would 
be credited for communities in that area. Generally, the worst case runoff will occur when the 
time of concentration for the watershed is about equal to the duration of the storm event used for 
modeling. 

In many locations, a state or federal agency or a regional stormwater management or flood 
control agency has determined the storm duration that causes worst-case flooding, including 
flooding on larger rivers. If that agency states that a certain storm duration is appropriate for the 
large rivers within its jurisdiction, the community may receive credit for using that storm 
duration. Using continuous simulation modeling is also credited. 

(e) 15, if the plan identifies existing wetlands or other natural open space 
areas to be preserved from development to provide natural 
attenuation, retention, or detention of runoff. 
Preservation of these areas reduces runoff and flood damage and provides other floodplain 
management benefits as well. 

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(f) 10, if the plan prohibits development, alteration, or modification of 
existing natural channels. 
Where the watershed master plan includes undeveloped areas, preservation of the natural 
channels may reduce maintenance costs and provide many amenities to the community. 

(g) 10, if the plan requires that channel improvement projects use natural or 
soft approaches rather than gabions, rip rap, concrete, or other 
hard techniques. 
Even where communities are preserving existing natural drainageways, they may not have the 
needed capacity for a large storm. In these cases, the community may choose to use vegetation 
for erosion control or along the improved channel. Excavated channels may be landscaped as 
natural streams, golf courses, or parks. 

(h) 20, if the plan was prepared in coordination with or as a part of the 
communitys floodplain management plan credited under Activity 
510. 
In many cases, a communitys floodplain management plan points out the need for watershed 
management, and therefore recommends the development and implementation of watershed 
master plans for some or all of its watersheds. In other cases, a watershed master plan can only 
solve part of the communitys flood problems, and its results are input to a floodplain 
management plan. 

This credit is dependent on the communitys receiving credit for a floodplain management plan 
under Activity 510 (Floodplain Management Planning). A floodplain management plan 
developed for Activity 510 usually does not qualify for SMP credit, but a stormwater master plan 
may qualify for credit under Activity 510. A community may receive these 20 points if 

 The floodplain management plan is mentioned prominently in the stormwater master 
plan, and if references in the stormwater master plan demonstrate that it is intended to 
help implement the floodplain management plan; and/or 
 Hydrologic output from the stormwater master plan is used as input for the floodplain 
management plan. 
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Example Programs 

The City of Cheyenne and Laramie County, Wyoming, have developed a common stormwater 
management policy. It is included in the development regulation adopted by both communities: 

It is recognized that surface water drainage problems are primarily defined by the 
natural topography of the land, without regard for legal boundaries, existing 
improvements or proposed improvements to the land. The City and County 
therefore will provide for the necessary control, coordination and implementation 
measures required to achieve the general objectives as well as the specific 
improvements which may be required in order to satisfy the regulations as 
established herein. The City and County shall prepare a Drainage Plan or Plans 
for the entire area, which shall serve as a guide for the development of drainage 
improvements within the City and County . . . The City/County Drainage Plan 
shall be prepared based upon the anticipated land uses as established by the 
official land use plans of the City and County, the measured and recorded 
historic precipitation data, and the expressed objective of minimizing increases in 
surface water run-off and reduction in water quality caused by land development. 
The Drainage Plan may be prepared in several parts and such sections that are 
complete shall be utilized upon formal adoption. 

The City/County Drainage Plan discussed in this regulation was completed in eight sections (one 
for each of the eight watersheds that affect the greater Cheyenne area) and adopted. 

Thurston County, Washington, adopted the Green Cove Creek Comprehensive Drainage Basin 
Plan. The goals of comprehensive drainage planning in Thurston County are 

1. Preserve and/or enhance water quality, stream morphology, wetlands, 
groundwater, fisheries/wildlife habitat, and aesthetic amenities. 
2. Promote sustainable development within each basin (i.e. minimum impact on 
water resources and habitat). 
3. Promote public interest and involvement in water resource management. 
4. Establish short-term and long-term solutions to existing and future stormwater 
quality and quantity problems. 
5. Promote a regional approach for financing, ownership and operations/ 
maintenance of regional facilities and programs. 
Although solutions to . . . stormwater . . . quantity problems is only part of one of these goals, 
the Green Cove Creek plan names flooding as the first problem, and solutions to flood problems 
as the highest priority. The plan recommends measures to deal with increased runoff due to 
urbanization, undersized culverts, sedimentation, inadequate design of existing facilities, and 
areas with high water tables. 

Documentation Needed for Verification 

The documentation for SMP must include documentation that the plan was adopted and those 
pages from the plan that support the credit requested. There should be language in a hydrology 

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or study methods section of the plan to define the design storm(s) used to develop the plan, as 
well as a comparison of current- and future-conditions runoff. This language, along with 
language from the introduction or executive summary relating the plan to stormwater 
management regulations, should meet the prerequisites for CRS credit in Section 451.b.1. 

The hydrology or study methods section of the plan should qualify for credit under Sections 
451.b.2(b), (c), and (d) if the plan is based on the 100-year storm, manages peak flows and 
volumes, and/or is based on a 5-day or worst case storm event. A recommended plan or 
similar section should provide the documentation for credit under Sections 451.b.2(e), (f), (g), 
and (h). 

Common Problems 

A common problem with SMP credit is that the communitys stormwater master plan or 
watershed master plan is a drainage capital improvement plan with no stormwater 
management features in it. Such plans do not receive CRS credit. 

452 Impact Adjustment 

The CRS measures the impact of an activity on the community. It does not matter if the 
stormwater management program is administered by the community or by a regional district. 
What counts is whether the buildings in the community are being protected from increased 
runoff that results from development in the watershed. 

Most communities are in several watersheds. These may include a large watershed that drains to 
a large stream and a number of small watersheds that drain into creeks or ditches that enter the 
community from other locations. 

A community that can regulate all development in all of the watersheds that drain into it should 
receive full credit for this activity. This is not the case for most communities since corporate 
boundaries rarely follow watershed boundaries. Most communities do not have jurisdiction over 
new development outside their corporate limits or outside an extraterritorial limit. Because these 
communities cannot regulate all of the watersheds that drain into them, their CRS credit points 
are adjusted to reflect the limits of their stormwater management programs. 

To do this, Activity 450 has impact adjustments for SMR and SMP that factor in the area of the 
watersheds affected. Impact adjustments for Activity 450 are different than for other CRS 
activities because the effectiveness of stormwater management depends on how much of the 
watershed is affected by the communitys program. The impact of other activities is based on 
how much of the floodplain is affected. While floodplains are mapped on readily available 
FIRMs, most communities will have to delineate their own watersheds. 

The impact adjustments for SMR and SMP are ratios that are multiplied by the elements credit 
points. The ratio for SMR is rSMR and the ratio for SMP is rSMP. There are three ways that an 
applicant can obtain the values for rSMP and rSMR: Options 1, 2, and 3. 

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The Impact Adjustment Map 

An Impact Adjustment Map must be developed for Options 1 and 3. 

The stormwater Impact Adjustment Map shows the community and all watersheds that drain into 
it. Watersheds larger than 50 square miles (as measured where the stream enters the area under 
the communitys jurisdiction) do not need to be shown. Although the purpose of this map is 
similar to the Impact Adjustment Map discussed in Section 403 of the Coordinators Manual, it 
may be quite different in appearance. The community will likely be only a small part of the total 
watershed. 

Areas may be measured in acres or square miles. The area subject to the stormwater management 
regulations is shown as aSMR and the total area of the watershed is aW. aSMR is typically the 
area of the community plus any area subject to its extraterritorial jurisdiction. The area covered 
by the stormwater management plan is aSMP. 

An Impact Adjustment Map is shown on the next page. It shows the approximate corporate 
limits for the City of Alamogordo, New Mexico, (aSMR) and the boundaries of the watersheds 
that affect the city (aW). Based on this map and information provided by the city, the corporate 
limits of Alamogordo (aSMR) include about 19.5 square miles, while the drainage area 
(including the incorporated areas) is about 75 square miles. 

It is typical for an Impact Adjustment Map to be a large map. Although this activity is reviewed 
by a technical reviewer for all CRS communities, in most cases the ISO/CRS Specialist 
examines the Impact Adjustment Map during the community visit and verifies the impact 
adjustments rSMR and rSMP. 

Many communities use Option 2 rather than calculate aSMR, aSMP, and aW for Option 3. 
However, the CRS encourages communities to calculate their points to accurately reflect their 
programs. Here are some ways to make it easier. 

1. Watershed area data are often found in the communitys Flood Insurance Study and 
other stormwater reports, such as 205 studies. 
2. If a community can demonstrate that upstream communities have similar stormwater 
management regulations for the upper portions of their watersheds, it can increase the 
size of aSMR and aSMP. A community in a regional district that regulates all 
watersheds may be able to use Option 1. 
3. The following areas may be excluded from the calculations for aSMR, aSMP, and 
aW: 
a. Watersheds larger than 50 square miles (as measured where the stream enters the 
area under the communitys jurisdiction) may be excluded. The area of the 
watershed within the community remains in the impact adjustment calculation. 
If such large watersheds are outside the communitys jurisdiction, or are not 
regulated, the community will receive more credit by excluding them. If they are 
regulated, the community will receive more credit by including them. 

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Impact Adjustment Map for Alamogordo, New Mexico 

b. If watersheds upstream of the community are effectively reduced by flood control 
structures that control the base flood, the size of the areas affected is reduced 
accordingly. Only dams designed to control the base flood can be used for this 
type of adjustment to aW. 
c. If portions of the watersheds are unlikely to be developed due to their ownership, 
those portions of the watershed may be excluded. Areas that might be excluded 
are nature preserves, state parks, or privately owned land that is dedicated to open 
space use. 
Alternatively, counties may use anticipated population growth to determine rSMP using 
Option 3. In many counties, a large portion of the projected population growth may be 
concentrated in a few relatively small areas of the county. Because stormwater master plans are 

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usually done where a community expects growth, an impact adjustment based solely on area 
does not reflect the value of master planning in these cases. 

If a county uses anticipated population growth to determine rSMP, it is the responsibility of the 
county to document that anticipated population growth in the watershed with SMP credit and the 
anticipated population growth for the entire county. This must be done at the census tract level. 
The community must also provide documentation that shows that its projected population 
estimates are reasonable. 

Example: Big County covers 1,000 square miles. Drainage into the county, 
excluding watercourses with watersheds larger than 50 mi2, is 100 mi2. aW for 
Big County = 1,100 mi2. 

Big County has one large city, Central City, and two small communities that are 
in other parts of the county. Central City and adjacent areas have 80% of the 
countys population. Big County and Central City have jointly completed 
stormwater master plans for all watersheds that drain through Central City. 
These watersheds total 330 mi2. 

Based on area, rSMP = 330 mi2 = 0.30 
1,100 mi2

Big County uses its Comprehensive Land Use Plan 20002025 data from the 
2000 census and its GIS system to estimate population growth in the watersheds 
with master plans. The GIS operator overlays the census tracts and the 
watershed boundaries. If a census tract is at least 50% within a watershed, it is 
counted as being in that watershed. The population within the watersheds with 
master plans is expected to increase by 20,000 between 2000 and 2025. The 
population of the entire county is expected to increase by 25,000. Big County 
uses rSMR = 0.80 based on anticipated population growth. 

Big County provides the methodology in the comprehensive plan as 
documentation to support the projected populations. 

Options for Impact Adjustment 

Option 1 

Option 1 is used by communities with stormwater management regulations that cover all of their 
watersheds. 

1. Stormwater Management Regulations (SMR): If the community, separately or along 
with upstream communities, regulates development within all of the watersheds that 
affect it, rSMR = 1.0. 
2. Stormwater Management Master Plan (SMP): If the stormwater management master 
plan regulates all development within all of the watersheds that affect the community, 
rSMP = 1.0. 
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An Impact Adjustment Map usually is 

The CRS has a procedure for communities in a required for Option 1. The Impact 
metropolitan area, region, or state that are subject to 

Adjustment Map for this activity is 

the same regulations or regulatory requirements. A 
uniform minimum credit can be developed for these discussed above. 
communities based on the minimum requirements. All 
communities in the affected area receive the same There are two cases in which Option 1 iscredit and usually do not have to submit as much 

most often used. The first is where the 

documentation. Documentation would be needed if 
the communitys program exceeds the minimum community actually has jurisdiction over 
requirements and it wanted more credit points. all of its watersheds. An island 

community is one example of this. If an 

For example, most communities in Delaware and 
South Carolina, along with many communities in island were covered by a single 
Colorado, Florida, Illinois, and Washington, receive up community, and it regulated all 
to 90 points for stormwater management regulations development on the island, it would have 
(SMR) and additional credit for ESC and WQ rSMR = 1.0. If a stormwater master plan regulations. Generally, these communities receive the 
credit automatically. They do not have to apply unless had been done for the entire island, it 
they implement their own programs with more would also have rSMP = 1.0. This is 
restrictive standards. Many of these communities use because the entire island, i.e., the entire 
Option 1 because all of their watersheds are covered watershed, is under the communitys 
by the regional or statewide regulatory programs. 

jurisdiction. 

It is possible that many other state and regional 
agencies have regulatory programs that would qualify This can also happen where thetheir communities for uniform minimum CRS credit. community has its corporate boundaries These agencies and/or the communities affected 
should contact their ISO/CRS Specialist for more formed entirely by watershed divides 
information on uniform minimum credits. (ridges) or bodies of water. Such a 

community can regulate all 
developments that will affect the water 
that drains into it. 

The second case is where a regional agency either enforces stormwater management regulations 
or sets minimum standards for local regulations. These are county-wide or multi-county 
organizations, like urban drainage districts or water management districts. Development in 
watersheds subject to these agencies must meet the same stormwater management standards, 
regardless of the community within which they lie. 

Communities are encouraged to manage stormwater in cooperation with adjacent communities. 
If a community only has regulatory jurisdiction over a portion of its watersheds, then it does not 
receive the full benefits of stormwater management. However, if upstream communities also 
manage future development, either independently or through a state or regional agency, all 
communities can benefit. Therefore, if a community can demonstrate that all upstream 
communities in its watershed have similar stormwater management programs, it can use 
Option 1. 

Option 2 

If a community only manages a part of a watershed, it cannot prevent future increases in runoff 
from all of the watersheds development. The impact adjustment reduces the score to account for 
the fact that the community cannot do 100% of the job. Under Option 2, a default value of 25% 
is used for the impact adjustment ratio. 

Credit for Stormwater Management - 27 - Edition: 2006 


1. Stormwater Management Regulation (SMR): If the community does not regulate 
development within all of the watersheds that affect it, it may use the default value 
rSMR = 0.25. 
2. Stormwater Management Master Plan (SMP): If the stormwater management master 
plan does not regulate all development within all of the watersheds that affect the 
community, it may use the default value rSMP = 0.25. 
The default value means that communities will always get at least 25% of the credit for their 
stormwater management regulations no matter how little of the watershed they can actually 
regulate. This default value also helps communities that find it difficult to use Option 3. 

Option 3 

An Impact Adjustment Map is required for Option 3. The Impact Adjustment Map for this 
activity is discussed above. 

Under Option 3, the impact adjustment adjusts the credit for SMR and SMP in accordance with 
the portion of the watershed regulated. 

1. Stormwater Management Regulation (SMR): A community must develop a stormwater 
Impact Adjustment Map to determine the impact of its stormwater regulations (rSMR): 
rSMR = aSMR 
aW

aSMR = the area subject to stormwater management regulations, and aW = the area of all 
watersheds affecting the community. 

It is necessary to show the entire drainage area that affects the community (aW) and the 
community boundary on the Impact Adjustment Map. The area regulated by the 
community (aSMR) is usually the same as the area of the community. 

2. Stormwater Management Master Plan (SMP): A community must develop a stormwater 
Impact Adjustment Map to determine the impact of its stormwater master plan (rSMP): 
rSMP = aSMP 
aW

aSMP = the area regulated in accordance with the stormwater plan, and aW = the area of 
all watersheds affecting the community. 

The Option 2 impact adjustment favors communities in larger watersheds that have jurisdiction 
over less than 25% of the watersheds that drain into them. The Option 3 adjustment favors 
smaller watersheds that are mostly within the communitys jurisdictional area. 

Option 2 is also used to estimate the communitys credit in an initial application for the CRS. 
A community may prepare a stormwater Impact Adjustment Map later with the assistance of 
the ISO/CRS Specialist. If the map has been completed during or immediately after the 

Credit for Stormwater Management - 28 - Edition: 2006 


ISO/CRS Specialists verification visit, the additional credit points provided by using Option 3 
will be added then. 

Impact Adjustment for Extraterritorial Jurisdiction 

Many incorporated communities have full review and permitting authority outside their 
corporate limits. This authority allows them to impose their land use standards on new 
developments adjacent to the community. This is usually called extraterritorial jurisdiction, 
or ETJ. 

The impact adjustment for stormwater management regulation and stormwater master plans 
depends on how the extraterritorial jurisdiction is treated by the Federal Emergency Management 
Agency on the communitys FIRM. 

1. ETJ not shown on the FIRM. If a community regulates new development in its 
extraterritorial jurisdiction according to its stormwater management regulation, and the 
extraterritorial jurisdiction is NOT shown as part of the community on its FIRM, then the 
portion of the extraterritorial jurisdiction upstream from the community is included in 
aSMR. This area is regulated, and the community benefits from its regulation of runoff 
from future development. 
a. The area regulated (aSMR) includes the area of the community plus the portion of the 
extraterritorial jurisdiction that is in the watershed affecting the community. 
b. The area of the watershed affecting the community (aW) is the area of the community 
plus that portion of the watershed that drains into the community. 
In this example, because the 
extraterritorial jurisdiction is not shown 
on the communitys FIRM, only the 
incorporated area and the portion of the 
extraterritorial jurisdiction that drain into 
the incorporated area are counted as 
aSMR. Regulation of the rest of the 
extraterritorial jurisdiction benefits 
property downstream from the 
community. The county or a downstream 
community could receive credit for 
stormwater management in this area. 

Credit for Stormwater Management - 29 - Edition: 2006 


2. ETJ shown on the FIRM. If the 
extraterritorial jurisdiction is 
included on the communitys 
FIRM, the area of the watersheds 
affecting the community (aW) is 
based on the extraterritorial 
jurisdiction. This increases aW and 
usually decreases rSMR and rSMP. 
In this example, the area of the 
extraterritorial jurisdiction is 
considered by the NFIP to be part 
of the community, and NFIP 
policies on buildings in the 
extraterritorial jurisdiction are 
eligible for the CRS premium 
discount provided to the 
community. Therefore, the 
boundary of the extraterritorial 
jurisdiction is used just like corporate limits for most communities when determining aW 
and aSMR. 

In this case, the county in which the community is located cannot receive credit for 

stormwater regulation in the extraterritorial jurisdiction, because buildings in the 

extraterritorial jurisdiction are within the community for CRS purposes. 

Common Problems 

Communities often fail to understand that the impact adjustment for this activity usually includes 
drainage areas outside of the community. The impact adjustment gives the community credit for 
stormwater management based on the portion of the watershed affecting the community that is 
managed. The area with stormwater management regulation, aSMR, is divided by the total 
watershed that produces runoff through the community, aW. aW usually extends beyond the area 
of the communitys jurisdiction. 

The same calculation is done for areas with stormwater master plans. aW is the same for both 
calculations. 

Credit for Stormwater Management - 30 - Edition: 2006 


453 Credit Calculation 

In this section, the credit for stormwater management regulation (SMR) is multiplied by the 
impact adjustment (rSMR). The credit for stormwater master planning (SMP) also is multiplied 
by the impact adjustment (rSMP). The results are the total credit points for each element, cSMR 
and cSMP. 

a. cSMR = SMR x rSMR 
b. cSMP = SMP x rSMP 
For example, if the community uses Option 1 for the impact adjustment for its stormwater 
management regulations, SMR is multiplied by 1.0. If the community chooses Option 2, SMR is 
multiplied by 0.25. Under Option 3, SMR is modified by the value of rSMR as calculated using 
the stormwater Impact Adjustment Map. 

454 Credit Documentation 

For a communitys first application for a CRS classification, worksheet page 32 of the CRS 
Application is submitted along with the documentation described below. 

Subsequent requests for credit are called modifications. Modifications include activity worksheet 
AW-450, along with the documentation described below. These worksheets are also used by the 
ISO/CRS Specialist to calculate the communitys verified credit. Examples of how the credit is 
calculated are provided on pages 3752. 

Section 454 on worksheet page 32 of the CRS Application and on AW-450 is a checklist for the 
documentation listed below. These items are needed to confirm that the communitys program 
meets the CRS credit criteria. If there is more than one item, each should be labeled as 
Attachment 1, etc., for easy reference. 

Several of the documentation requirements are for ordinance or law language. A copy of the 
appropriate pages of the ordinance, statute, and/or plan is sufficient and should be attached to the 
worksheet. The Chief Executive Officers certification that accompanies the application or 
modification is considered to include a certification that the ordinance or statute has been 
enacted into law and is being enforced (see Section 212 of the CRS Application). 

Regulatory Language for SMR 

The community must submit the following: 
a. [Required if the community is applying for credit for SMR under Section 451.a]: 
A copy of the ordinance or law language regulating surface water runoff from 
new developments in the watershed. For SMR credit, the language must require 
that peak runoff from new developments be no greater than the runoff from the 
Credit for Stormwater Management - 31 - Edition: 2006 


site in its pre-development condition. The margin next to where this appears in 
the ordinance must be marked SMR. 
The language submitted must include those factors that are credited: size of 
developments regulated, design storms to be used, and how the maintenance of 
drainage and retention facilities is handled. The appropriate acronym(s) (SZ, 
DS, and PUB) must be marked in the margin of the ordinance sections that 
pertain to each element. 
The community may also be asked to complete an Activity Worksheet that helps 
identify where the credits are due. 
The community must provide a copy of the ordinance or law language that regulates surface 
water runoff from new developments in the watershed. Generally this will be in the form of 
pages excerpted from the local ordinance, statute, or bylaw. 

A local policy, drainage design manual, or other document is not acceptable unless the 
community can document that it has the force of law. When there is doubt, the community 
should provide a letter from its attorney that the documentation submitted has been adopted by 
the governing board and is legally binding on all new development. 

The regulatory language submitted must include those factors for which the community is 
seeking credit. Each appropriate acronym must be marked in the margins. The acronyms are as 
follows: 

SMR identifies the regulatory language that requires that peak runoff from new development 
be no greater than the runoff from the site in its pre-development condition. SMR regulations 
are usually part of a subdivision ordinance, public works design standard regulation, or other 
regulation that sets drainage design standards for new developments. Examples of this 
language as used by the example communities are presented on pages 3752. 

NOTE: This publication contains examples of certifications and ordinance language. 
Communities are advised to have all certifications and proposed ordinances reviewed by their 
attorneys or corporation counsels. 

SZ is put in the margin of the section of the regulations that identifies the size of 
development that is regulated. Usually this language is found at the beginning of the 
regulation in a section that discusses jurisdiction, applicability, and/or exceptions. Examples 
of SZ language for the example communities can be found on pages 3752. 

DS shows the regulatory language that prescribes the design storm(s) used to set the release 
rate from new development. It is usually in the section that discusses the design standards for 
retention or detention basins. It is important to note that DS is not the same as the design size 
for storm sewers. It is the recurrence interval of the storms that are regulated at the outfall 
from the development. Examples of regulations design storms for the example communities 
appear on pages 3752. 

PUB is used to mark the section of the regulations that describes if and when stormwater 
management facilities built by the developer are maintained by a public agency. PUB 

Credit for Stormwater Management - 32 - Edition: 2006 


language must cover retention or detention basins, not just the sewer lines on public rights of 

way. 

Two types of documents can suffice as documentation for this element. The first is a subdivision 
ordinance or other regulatory document that states that stormwater management facilities are 
turned over to a public agency for ownership or maintenance. The other is a regulation that states 
that a public agency may enter the property and perform the maintenance if the owner fails to 
properly maintain the facility. An example of the latter approach is found on pages 3839. The 
community will also need to provide a copy of the procedures used to inspect and maintain the 
facilities (see Section 454.h). These procedures are the same as those required for CRS credit for 
channel debris removal (CDR) in Activity 540. 

Stormwater Master Plan 

b. [Required if the community is applying for SMP credit under Section 451.b] 
Copies of the pages of the stormwater master plan that show the following: 
1. Management of peak flows and volumes so that they do not exceed present 
values. The plan must either include regulations that meet these criteria or 
be based on a rainfall/runoff model that achieves these results; 
2. The recurrence interval of the storm used for the regulations and/or model; 
3. The duration of the storm used for the regulations and/or model; 
4. [Required if the community is applying for credit for Section 451.b.2(d)(f)] 
How the plan utilizes or protects the existing natural stormwater features 
within the watershed; and 
5. [Required if the community is applying for credit for Section 451.b.2(g)] A 
statement by the community official responsible for implementation of the 
stormwater master plan that it was prepared in coordination with or as part of 
the communitys Floodplain Management Plan credited under Activity 510. 
This documentation may be provided from either plan if it is contained there. 
A stormwater management plan is usually a complex and bulky document. There may be an 
introduction or summary describing the area covered by the plan, its objectives, and the 
regulation of surface water runoff. The summary usually is adequate documentation for some or 
all of this credit. If no such summary is available, one must be developed to document this credit. 

NOTE: Sections 454.c through 454.e are not discussed in this publication. Communities that 
apply for CRS credit for erosion and sediment control (ESC), protection of water quality (WQ), 
or freeboard for buildings in the B, C, D, and X Zones (FRX) must meet the documentation 
requirements in Sections 454.c through 454.e in the Coordinators Manual. 

Credit for Stormwater Management - 33 - Edition: 2006 


Stormwater Impact Adjustment Map 

f. [Required if the community calculates the impact adjustment ratio for one or 
more elements by using Option 1 (452.a) or Option 3 (452.c)] An Impact 
Adjustment Map showing watershed boundaries and stormwater management 
jurisdiction. 
If the community chooses either Option 1 or Option 3 for its impact adjustment, the 
documentation must include a map showing the watersheds. The stormwater Impact Adjustment 
Map is explained on pages 2426. The community should also show how aSMR, aSMP, and aW 
were calculated. Example maps and calculations are shown for Cheyenne and Laramie County 
on pages 4549. 

Other Communities Regulations 

g. [Required if the community determines the area of stormwater management 
regulation (aSMR) or the area covered by the stormwater management plan 
(aSMP) to include watershed areas regulated by other communities] 
Documentation that watersheds outside the jurisdiction of the community are 
regulated to similar standards or are subject to the same plan as those within 
the community. 
Under Option 1 and Option 3 of the impact adjustment, a community may receive more credit 
points if other communities in its watersheds have stormwater management regulations that are 
eligible for CRS credit. If the applicant wants the additional credit points, it must document the 
other communities regulations. This can be in the form of copies of the other communities 
regulatory language. 

As an alternative to copies of many regulations, the applicant may submit a statement from the 
regional agency with stormwater management jurisdiction that lists the other communities in 
which the regulations are also in effect. 

Facility Maintenance Procedures 

h. [Required if the community is applying for PUB credit under Section 451.a.3] 
The procedures used to inspect and maintain drainage facilities. 
If the community is applying for PUB credit for stormwater facilities under Section 451.a.3, then 
the submittal must include a copy of the procedures used to inspect and maintain those facilities. 
This applies for both publicly owned and privately owned facilities. The procedures should be a 
part of or coordinated with the communitys procedures for drainage system maintenance (see 

Credit for Stormwater Management - 34 - Edition: 2006 


Section 544.a of Activity 540 (Drainage System Maintenance) in the Coordinators Manual). 
For PUB credit, all such facilities must be inspected at least annually. 

If the community is applying for credit for channel and basin debris removal (CDR) under 
Activity 540, the documentation for that activity should meet the documentation requirements 
for Section 454.h. 

Permit Records 

The community must have the following documentation available to verify 
implementation of this activity: 
i. Development and building permit records that demonstrate enforcement of the 
regulations. If the community applied for credit for public maintenance under 
Section 451.a.3, records that demonstrate implementation of the inspection and 
maintenance requirements. 
To confirm that the regulations have been implemented, the community must be able to show the 
ISO/CRS Specialist the appropriate development and building permit records. The ISO/CRS 
Specialist will select a sample of recent development sites and request the records during the 
verification visit. Permit records are not submitted with an application or modification. The 
applicant should have the records ready for the verification visit. 

For PUB credit, verification includes a review of inspection and maintenance records and a field 
verification of facilities. If the community has credit for channel and basin debris removal (CDR 
in Activity 540), these facilities must be included in the communitys drainage system as defined 
for that activity. If the community does not request credit for CDR, it must still provide the same 
CDR documentation for stormwater facilities. See CRS Credit for Drainage System Maintenance 
for more information. This document is available free (see Appendix E of the Coordinators 
Manual). 

In the unusual case where, as a result of a stormwater master plan, the community has areas in 
which stormwater management does not include detention or retention, PUB will be verified 
based on channel maintenance to the lower edge of the community or to the receiving body of 
water. 

SMP Evaluation 

If it has received credit for a stormwater management master plan (SMP) under 
Section 451.b, the community must provide the following documentation at the time 
of its cycle verification: 
j. An evaluation report that addresses whether the communitys stormwater 
management plans that are more than five years old are still based on 
appropriate data and effectively manage stormwater runoff. In lieu of a formal 
Credit for Stormwater Management - 35 - Edition: 2006 


report, the community may submit a letter signed by a licensed professional 
engineer that addresses the following issues: 
(1) The future conditions at the time the plan was completed: Do these 
conditions still reasonably reflect the actual watershed conditions today? 
(2) The precipitation data used for the plans hydrology: Does the community or 
agency still use the same precipitation that was used in the report? 
(3) Method used for the plan(s): Is the method used to develop the plan(s) 
considered appropriate today by the agency? 
(4) Construction: Has construction of stormwater infrastructure altered actual 
conditions in ways that make the plan(s) obsolete? 
(5) Other factors: Are there other aspects of the plan(s) that make it obsolete or 
otherwise of questionable applicability? 
455 For More Information 

Many metropolitan areas have regional planning commissions or stormwater or sanitary districts 
whose staff have expertise in stormwater management. Some have model ordinances or 
mandatory requirements for local governments. 

Credit for Stormwater Management - 36 - Edition: 2006 


Example 

Hilton Head Island, South Carolina 

Excerpts from the Hilton Head Island stormwater management program are given on the 
following pages to serve as a scoring example. Examples for two other communities are shown 
later. The communities programs are not meant to be models to be copied and adopted. A 
stormwater management program must be tailored to local conditions and developed with 
adequate technical input. 

NOTE: Communities are advised to have all proposed ordinance language reviewed by their 
attorneys or corporation counsels. 

Hilton Head Island has a fairly simple stormwater program. Its regulation requires that all new 
development, except single-family residences on existing lots, ensure that the flows of the 25year 
storm do not exceed the flows from the site under pre-development conditions. Because the 
community is an island, all watersheds are regulated and the community uses Option 1 for its 
impact adjustment. Hilton Head Island also has credit for maintenance of all stormwater 
facilities. 

451 Credit Points 

Stormwater Management Regulations (SMR) 

1. Size of Development (SZ). Hilton Head Islands ordinance is applicable to virtually all 
development except single-family residences, agricultural activities, and temporary work 
done during emergencies. 
Sec. 16-5-501. Applicability; Exceptions 

All development, with the following exceptions, shall include adequate provisions 
for stormwater management in compliance with this Article. 

A. All developments exempted by this Title from development plan review, 
unless other applicable review procedure requires compliance with provisions 
for stormwater management. 
B. The establishment of a single family residential structure, including a 
manufactured home, and its accessory structures that are part of an 
approved subdivision, or additions or modifications to existing single family 
residential structures. Such activities are, however, subject to the flood 
damage control standards as outlined in Title 9 of Title 15 of the Municipal 
Code of the Town of Hilton Head Island. 
C. Existing agricultural activity and new agricultural activity not involving the 
filling or drainage of land. 
Credit for Stormwater Management - 37 - Edition: 2006 


D. Work by agencies or property owners required to meet emergency flooding 
conditions. If practicable, emergency work should be approved verbally by 
the Administrator or other duly appointed official in charge of emergency 
preparedness or emergency relief. Property owners performing emergency 
work will be responsible for any damage or injury to persons or property 
caused by their unauthorized actions. Property owners will restore the site of 
the emergency work within 60 days following the end of the emergency 
period. 
E. Developments that do not disturb more than 5,000 square feet of land in total, 
nor disrupt existing drainage patterns. 
Based on this requirement, SZ = 20. 

2. Design Storm (DS). Hilton Head Islands ordinance requires that the 25-year storm be 
accounted for in all development. 
Sec. 16-5-502. General Standards for Stormwater Management 

A. Each development shall incorporate stormwater management facilities 
sufficient to maintain pre-development peak discharge rates across adjacent 
property lines. In the case of redevelopment, the pre-development condition 
shall be defined as the natural state. This requirement may be waived where 
a suitable means of flow into a downstream tidal discharge point is 
accessible; or where the applicant provides a drainage system with adequate 
capacity to carry site flows to an ultimate downstream tidal discharge point. 
Except as noted above, discharge of runoff into tidal or freshwater wetlands 
shall be limited to pre-development conditions, unless otherwise approved by 
the OCRM. 
Sec. 16-5-505. Design Standards 

A. Twenty-Five Year Storm. The 25-year frequency storm shall be used in the 
design of all drainage systems and runoff detention/retention structures. 
B. On-Site Retention. All applications for development shall provide for on-site 
retention (dry or wet), or percolation of stormwater sufficient to maintain pre-
development 25-year frequency peak discharges after development has 
occurred, or as exempted in Sec. 16-5-501. 
DS = 20. 

3. Public Maintenance (PUB). Hilton Head Island requires owners to maintain stormwater 
facilities. The city has the right to enter these facilities for inspections and to perform 
maintenance if necessary. 
Sec. 16-5-506. Drainage Easements 

A. Assignment. The Administrator may require as a condition for obtaining 
approval of runoff control and drainage plans that the applicant record plats 
or covenants insuring that drainage easements and facilities are assigned to 
a specific entity. Specific entities may include, but are not limited to, the 
developer, a Homeowners or Landowners Association or regime, or a public 
Credit for Stormwater Management - 38 - Edition: 2006 


service district. If the recording of such covenants or plats is not desired by 
the applicant, the Administrator will then require as it deems necessary that 
the applicant solicit dedication to and acceptance by a specific entity of such 
easements and facilities to assure the provision, maintenance and operations 
of said facilities. All required easements or covenants shall be recorded and 
filed in the Beaufort County Courthouse and documented prior to the 
issuance of a Certificate of Compliance. 

B. Maintenance Requirements. General maintenance requirements necessary 
to insure the long-term functions of deeded stormwater runoff controls, 
easements and drainage facilities shall also be described in the documents 
establishing the Homeowners or Landowners Association. The documents 
will also state that the Town or a legal entity having authority over such 
drainage may perform or require the Homeowners or Landowners 
Association to take action under the following conditions: 
1. normal maintenance is not performed which results in an adverse affect 
on drainage flow; or 
2. to alleviate flooding or other emergency drainage problems upstream, or 
downstream of the easement. 
3. The Town will assist the developer in negotiating with the Homeowners 
or Landowners Association and other affected parties on equitable 
distribution of costs incurred in performing or repairing actions taken on 
these easements under such conditions. 
In its documentation for channel debris removal (CDR in Activity 540), Hilton Head Island 
included annual inspection of all stormwater facilities approved under this ordinance. A review 
of permit records and subdivision plats confirmed the implementation of the requirements, and 
field inspection indicated that the maintenance is performed. 

PUB = 110. 

SMR Credit 

The credit for SMR is the sum of the three sub-elements: SMR = SZ + DS + PUB. 

SMR = 20 + 20 + 110 = 150 

Stormwater Management Master Plan (SMP) 

Hilton Head Island has no stormwater master plan, so SMP = 0. 

452 Impact Adjustment 

Because it is an island, the area of Hilton Head Island equals the total area of its watershed. All 
of its watershed is subject to its stormwater regulations. It uses Option 1 for its impact 
adjustment: rSMR = 1.0. 

Credit for Stormwater Management - 39 - Edition: 2006 


453 Credit Calculation 

cSMR = SMR x rSMR 

cSMR = 150 x 1.0 = 150 

454 Credit Documentation 

Section 454 on activity worksheet AW-450 lists the documentation needed. Hilton Head Island 
checks off what is included in the submittal and what it agrees to provide during the verification 
visit. It numbers the attachments to make the review easier. 

The community also marks the margins of the documents with the appropriate acronyms to show 
where the credited elements appear. This is very important to assist the reviewer. If the reviewer 
cannot find the documented support for the credit claimed, the community may not receive the 
credit. 

The community must submit the following: 
a. [Required if the community is applying for credit for SMR under Section 451.a] 
A copy of the ordinance or law language regulating surface water runoff from 
new developments in the watershed. For SMR credit, the language must require 
that peak runoff from new development be no greater than the runoff from the 
site in its pre-development condition. The margin next to where this appears in 
the ordinance must be marked SMR. 
The language submitted must include those factors that are credited: size of 
development regulated, design storms to be used, and how the maintenance of 
drainage and retention facilities is handled. The appropriate acronym(s) (SZ, 
DS, and PUB) must be marked in the margins of the sections of the ordinance 
that pertain to each element. 
Hilton Head Island provides copies of sections 16-5-501, 16-5-502.A, 16-5-505, and 16-5-506 of 
its city code with the acronyms SZ, DS, and PUB marked in the margins. The community 
includes a copy of the cover of the code to identify the source of the copied pages. Each page of 
the code has the latest revision date in the footer, so there is no need for further documentation of 
the adoption date of the code. 

f. [Required if the community calculates the impact adjustment ratio for one or 
more elements by using Option 1 (452.a) or Option 3 (452.c)] An Impact 
Adjustment Map showing watershed boundaries and stormwater management 
jurisdiction. 
Credit for Stormwater Management - 40 - Edition: 2006 


During the verification visit, the ISO/CRS Specialist verifies that Hilton Head Island is entirely 
located on a barrier island. He or she makes a note of this and sends it to the technical reviewer 
with Hilton Head Islands other documentation. 

h. [Required if the community is applying for PUB credit under Section 451.a.3] 
The procedures used to inspect and maintain drainage facilities. 
During the verification visit, the ISO/CRS Specialist reviews the documentation for channel and 
basin debris removal (CDR) under Activity 540. He or she makes a note that these procedures 
have been verified and sends it to the technical reviewer with Hilton Head Islands other 
documentation. 

The community must have the following documentation available to verify 
implementation of this activity: 
i. Development and building permit records that demonstrate enforcement of the 
regulations. [If the community applied for credit for public maintenance under 
Section 451.a.3] Records that demonstrate implementation of the inspection 
and maintenance requirements. 
During the verification visit, the ISO/CRS Specialist asks to see several recent subdivision plats 
and permits for other developments that are required to provide stormwater detention under the 
city code. He or she verifies that the plats and plans attached to the permits show that the 
detention was included in the development. Some of these sites will be included in the field 
verification for channel and basin debris removal (CDR). The ISO/CRS Specialist further 
verifies that annual inspections are done for each privately owned detention facility and for the 
conveyance systems that drain to and from these facilities. 

Credit for Stormwater Management - 41 - Edition: 2006 


Example 

Cheyenne and Laramie County, Wyoming 

Excerpts from the stormwater management programs for the City of Cheyenne and Laramie 
County, Wyoming, are provided on the following pages. The communities programs are not 
meant to be models to be copied and adopted. A stormwater management program must be 
tailored to local conditions and developed with adequate technical input. 

NOTE: Communities are advised to have all proposed ordinance language reviewed by their 
attorneys or corporation counsels. 

Cheyenne and Laramie County have a fairly complex stormwater management program. Their 
regulations require that all new subdivisions ensure that the flows of the 50- and 100-year storms 
do not exceed the flows under pre-development 
conditions. The Cheyenne area experienced a flood 
in 1985 that caused $65 million in damage and took 
12 lives. As a result of that flood, the city and 
county developed and adopted a Drainage Master 
Plan and detailed master plans for each of the eight 
basins in and around Cheyenne. In addition to 
stormwater and floodplain management, these basin 
plans included $85 million in capital improvement 
projects. Planning is complicated to some extent by 
the existence of F. E. Warren Air Force Base within 
the urbanized area. 

451 Credit Points 

Stormwater Management Regulations (SMR) 

The City of Cheyenne and Laramie County adopted the CheyenneLaramie County 
Subdivision/Development Regulations 2000 in June 2000 and July 2000, respectively. Article 
VIII, Section 80.010(b) of these regulations states 

Land subject to flooding, and/or within drainage areas shall be developed in 
accordance with the provisions of the Cheyenne Storm Water Management 
Manual, Design Standards and Regulationsan Amendment to the 
Subdivision/Development Regulations, as well as the Floodplain Management 
Regulations. 

1. Size of Development (SZ). The following definitions are from the CheyenneLaramie 
County Subdivision/Development Regulations 2000. 
Subdivision. (County Only) Means the creation or division of a lot, tract, parcel or 
other unit of land for the immediate purpose of sale, building development or 

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redevelopment, for residential, recreational, industrial, commercial or public uses. 
The word Subdivide or any derivative thereof shall have reference to the term 
subdivision, including mobile home courts, the creation of which constitutes a 
subdivision of land. 

Subdivision/Development. (City Only) The division of a lot, tract, parcel or other 
unit of land into three (3) or more lots, plots, sites or other 
subdivisions/developments of land for the immediate or future purpose of sale, 
building development or redevelopment, for residential, recreational, industrial, 
commercial or public uses. Mobile home parks and cemeteries shall be 
considered subdivisions/developments. The following items are exempt from 
Subdivision/Development Regulations: 

(a) The sale of land where the parcels involved in the sale are thirty-five (35) 
acres or larger. 
(b) The sale of land for agricultural purposes. 
(c) Railroad rights-of-way. 
(d) State and County Highway rights-of-way. 
Because the regulations include virtually all development except agricultural development 
and single family residences on existing lots, DS = 20 for both the City of Cheyenne and 
Laramie County. Agricultural uses usually do not increase stormwater runoff. 

2. Design Storms (DS). Appendix A of the CheyenneLaramie County Subdivision/ 
Development Regulations 2000 has separate sections for design standards in rural and urban 
areas, but the design storms used to size detention/retention structures are the same: 
StorageSubdivisions/Developments shall be planned and drainage facilities 
designed such that the peak rate of stormwater discharge that shall occur, at 
downstream boundary discharge points, after completion of the 
subdivision/development shall not exceed the peak rate of storm water discharge 
that would occur from the undeveloped or existing conditions at the same 
discharge locations, for an event equal to a 50-year frequency storm. . . . 
Freeboard shall be provided for storage impoundments to contain the 100-year 
frequency storm or pass it through spillways and outlets of sufficient capacity that 
the downstream flows do not exceed the predevelopment conditions. 

Because discharges for the 50- and 100-year storms are controlled to the pre-development 
levels, SZ = 80 for both the City of Cheyenne and Laramie County. 

3. Public Maintenance (PUB). From Appendix A of the CheyenneLaramie County 
Subdivision/Development Regulations 2000: 
MaintenanceUpon satisfactory completion of the construction of a drainage 
improvement (ponding area, storm drain, ditch, etc,) in accordance with 
previously approved plans, and subject to the approval of the City and County 
Engineer, and after proper deeding or dedications of the land or easements 
containing the improvement to the City or County, the City or County shall 
assume ownership of the land and the drainage facility located thereon and shall 
become responsible for all repairs that are required, except for such defects or 

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repairs that are covered under warranty by the Contractor or 
Subdivider/Developer who constructed the facility. Such maintenance shall 
include periodic cleaning, weed and grass cutting, repairs to pipe and 
underground structures and all else which is reasonably expected of a publicly 
owned and operated public utility system. The design of a drainage improvement 
shall provide for equipment access for maintenance purposes. 

Based on this regulatory language, PUB = 110. However, the community must provide 
documentation equivalent to that required for channel debris removal credit (CDR in Activity 
540) for all channels and detention facilities approved under this regulation. These facilities 
will be inspected during the CRS verification process, and if it is apparent that some areas 
are not maintained in accordance with the credited procedures, PUB credit will be reduced 
accordingly. 

SMR Credit 

The credit for SMR is the sum of the three sub-elements: SMR = SZ + DS + PUB. 

SMR = 20 + 80 + 110 = 210 

Stormwater Management Master Plan (SMP) 

As a result of record rainfall on August 1, 1985, the Cheyenne area suffered a devastating flood. 
Damage was estimated at $65 million, and 12 lives were lost. The City of Cheyenne and Laramie 
County immediately set out to develop a master drainage plan to reduce the flood threat to lives 
and property. The 1988 Drainage Master Plan that resulted includes drainage master plan reports 
for eight watersheds that drain into and through the greater Cheyenne area. 

To qualify for CRS credit for stormwater master plans (SMP), a plan must meet five criteria 
(Section 451.b.1). 

(a) The community must have adopted a stormwater management master plan 
for one or more of the watersheds that drains into the community. 
The City of Cheyenne and Laramie County have adopted stormwater drainage plans for eight 
watersheds. 

(b) The community has adopted regulatory standards for new construction in the 
watershed based on the plan. 
The CheyenneLaramie County Subdivision/Development Regulations 2000 were adopted as a 
result of the Surface Water Drainage Committees review of the Drainage Master Plan. 

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(c) The plans regulatory standards manage future peak flows so that they do 
not increase over present values. 
Although the recommendations in some of the watershed master plans allow some increase in 
discharge, these plans do not include the detention required of new developments under the 2000 
stormwater regulations. Also, in several watersheds, additional detention was recommended by 
the Surface Water Drainage Committee. It is expected that peak flows on Crow Creek below the 
confluence of the eight watersheds will be reduced as a result of implementation of the plans, as 
modified by the committee. 

(d) The plans regulatory standards require management of runoff from all 
storms up to and including the 25-year event. 
All master drainage plans were based on 2-, 5-, 10-, 25-, 50-, and 100-year storm events. 

(e) In order to maintain SMP credit for any plan that is more than five years old, 
the community must evaluate the plan to ensure that it remains applicable to 
current conditions. 
The City and County understand that their engineers will need to evaluate the plans and sign a 
statement at the next cycle verification visit. 
SMP.b.2(a) = 80 
Additional credit for stormwater master plans is as follows (Section 451.b.2): 

(b) 25, if the plan manages the runoff from all storms up to and including the 
100-year event. 
All master drainage plans were based on 2-, 5-, 10-, 25-, 50-, and 100-year storm events. 
SMP.b.2(b) = 25 

SMP = the sum of the above. 
SMP = 80 + 25 = 105 

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452 Impact Adjustment 

The impact adjustment for Cheyenne and Laramie County is the portion of the area of each 
community eligible for credit (aSMR and aSMP) for each element, divided by the area of 
watershed affecting each community (aW). 

Stormwater Management RegulationCheyenne 

The City of Cheyenne regulates all of its 

incorporated area, and Laramie County 
regulates areas adjacent to the city to the Although the city does not receive credit for 

same standards (SMR = 210). The city does SMR for the area of Warren Air Force Base 

because it does not regulate developmentnot regulate development on Warren Air there, it works actively with the base onForce Base. drainage issues, and the base, the city, and 

the county are co-sponsoring a detention basin 
See the map on the next page. The area of the on the base to reduce flooding in portions of 
City of Cheyenne is 18.9 square miles. The the city. 
area of the developed portion of Warren Air 
Force Base is 9.2 square miles. Since the city 
and the county have the same regulatory standards, the city can include the entire drainage area 
to the west in its calculation of aSMR. However, since a portion of the drainage area for Crow 
Creek is in Albany County, and is not regulated, and because the drainage area of Crow Creek 
where it enters the citys jurisdiction is larger than 50 square miles, the city excludes the Crow 
Creek drainage area from its calculations. The area of Laramie County draining into Cheyenne, 
exclusive of Crow Creek, is 53.6 square miles. The drainage area of Crow Creek is indicated by 
dashed lines on the Impact Adjustment Map on the next page. 

aSMR = the area of the city and county that is regulated. 

aSMR = 18.9 mi2 + 53.6 mi2 = 72.5 mi2 

aW = the area of the watersheds that drain into the city, excluding the watershed for the main 
stem of Crow Creek, because it is greater than 50 square miles. 

aW = 18.9 mi2+ 53.6 mi2+ 9.2 mi2 = 81.7 mi2 

rSMR = aSMR = 72.5 = 0.8874 
aW 81.7 

This is rounded to 0.89 and entered on the worksheet. 

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Impact Adjustment Map for Cheyenne 

Stormwater Management RegulationLaramie County 

Laramie County regulates all unincorporated areas, and the City of Cheyenne regulates all of its 
incorporated area to the same standards (SMR = 210). Development on Warren Air Force Base 
is not regulated. 

See the map on the next page. The total area of Laramie County is 2,660 square miles. All of this 
area is regulated by either the county or the City of Cheyenne, except for Warren Air Force 
Base. For Laramie County, 

aSMR = 2,660 mi2  9.2 mi2 = 2,651 mi2. 

Examination of U.S. Geological Survey quadrangle maps indicates that about 130 square miles 
drain into Laramie County when streams with more than 50 square miles of drainage area where 
they enter the county are ignored. For Laramie County, 

aW = 2,660 + 130 = 2,790 mi2. 

rSMR = 2,651 mi2 = 0.950 
2,790 mi2

This is rounded to 0.95 and entered on the worksheet. 

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Impact Adjustment Map for Cheyenne and Laramie County 

Stormwater Master PlanCheyenne 

All of the watersheds affecting the City of Cheyenne have stormwater master plans. The city 
uses Option 1. rSMP = 1.0. 

The area of Warren Air Force Base is included in aSMP because the watershed master plans 
included the base and determined future runoff based on future growth on the base. 

Stormwater Master PlanLaramie County 

See the map above. Laramie County has completed stormwater master plans for 336 mi2. 
aSMP = 336 mi2. 

The total area of Laramie County is 2,660 square miles. Examination of U.S. Geological Survey 
quadrangle maps indicates that about 130 square miles drain into Laramie County when streams 
with over 50 square miles of drainage area where they enter the county are ignored. For Laramie 
County, 

aW = 2,660 + 130 = 2,790 mi2. 

rSMP = 336 mi2 = 0.120 
2,790 mi2

Laramie County uses Option 2. 

rSMP = 0.25 

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The population of the Cheyenne metropolitan statistical area (MSA) is 73,544, 
compared with a total county population of 78,872. It is likely that Laramie County 
would have a considerably higher impact adjustment (rSMP) if it determined the 
projected population growth of the watersheds that have stormwater master plans. The 
county has a comprehensive plan and GIS capability. It can provide this documentation 
and receive more credit for SMP whenever it is ready to submit a modification for a 
higher CRS classification. 

453 Credit Calculation 
City of Cheyenne 

cSMR = SMR x rSMR. cSMR = 210 x 1.0 = 210 
cSMP = SMP x rSMP. cSMP = 105 x 1.0 = 105 

Laramie County 

cSMR = SMR x rSMR. cSMR = 210 x 0.95 = 199.5 
cSMP = SMP x rSMP. cSMP = 105 x 0.25 = 26.25 

It may seem unfair that the City of Cheyenne receives more credit for stormwater 
management regulation and stormwater master plans than Laramie County does. 
However, this is the purpose of the impact adjustments for these activities. 

Although both communities regulate new development to the same standards, all of the 
watersheds affecting Cheyenne are regulated, while there is no documentation that 
watershed areas outside of Laramie County are regulated. 

Similarly, all watersheds that affect Cheyenne have been included in master plans, and 
as these plans are implemented, flood problems will be reduced. Only a small portion of 
the watersheds in Laramie County are afforded that protection. If Laramie County 
provides the documentation needed to adjust SMP based on where growth is expected, 
the county should receive considerably more credit. 

454 Credit Documentation 

Because the City of Cheyenne and Laramie County both applied for CRS credit for stormwater 
management at the same time, the ISO/CRS Specialist made the verification visit to both 
communities during the same week. When it became apparent how closely tied the two 
stormwater management programs were, the ISO/CRS Specialist arranged a joint meeting with 
the stormwater management experts in the two communities. They agreed to the documentation 
needs for SMR and SMP and provided the documentation for both communities in one package. 

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Stormwater Management Regulation 

1. SMR. The CheyenneLaramie County Subdivision/Development Regulations 2000 include 
Ordinance No. 3408, adopted by the Governing Body of the City of Cheyenne, and 
Resolution No. 000801-04, adopted by the Board of Commissioners of Laramie County. 
These are the instruments by which the city and county, respectively, adopted the subdivision 
regulations. 
The ordinance language that earns CRS credit for stormwater management regulation is 
shown on pages 4244. 

2. rSMR. Documentation for rSMR was not on an Impact Adjustment Map, but was obtained 
from several sources. The Drainage Master Plan documents for each of eight basins had the 
drainage areas of the studies. A map in Implementing the Master Drainage Plan for the 
Greater Cheyenne Area included a map showing the drainage areas and the corporate limits 
of the city. The areas of the city, the county, and Warren Air Force Base were found in the 
citys comprehensive plan on the citys website. Drainage areas entering Laramie County 
from other jurisdictions were estimated from U.S. Geological Survey quadrangle maps. 
If the stormwater technical reviewer had not been able to find all of this information and 
determine the impact adjustments, he or she would have contacted the city and/or the county 
and told them what was needed. If they could not provide it, they would have been verified 
with rSMR = 0.25 and rSMP = 0.25, the Option 2 default values. 

To maximize their credit for these elements, communities are advised to provide all 
documentation specified in the Coordinators Manual. 

Stormwater Master Plan 

1. SMP. The communities provided documentation showing that the eight basin Drainage 
Master Plans were adopted by the governing bodies of both the city and the county. The 
plans were based on hydrologic modeling of the 2-, 5-, 10-, 25-, 50-, and 100-year storms of 
a duration equal to the basin response time (2 to 24 hours). 
The relationship between the stormwater management regulation and the stormwater master 
plans is demonstrated in the documents. 

1. From the CheyenneLaramie County Subdivision/Development Regulations 2000: 
City/County Drainage PlanIt is recognized that surface water drainage 
problems are primarily defined by the natural topography of the land, without 
regard for legal boundaries, existing improvements or proposed improvements to 
the land. The City and County therefore will provide for the necessary control, 
coordination and implementation measures required to achieve the general 
objectives as well as the specific improvements which may be required in order 
to satisfy the regulations as established herein. THE CITY AND COUNTY SHALL 
PREPARE A DRAINAGE PLAN OR PLANS FOR THE ENTIRE AREA, WHICH SHALL SERVE 
AS A GUIDE FOR THE DEVELOPMENT OF DRAINAGE IMPROVEMENTS WITHIN THE CITY 
AND COUNTY. The City/County Drainage Plans shall define and identify the major 
drainage facilities required to store and transport surface drainage run-off 
affecting large land areas. 

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This language in the regulation shows that the regulations will be implemented in 

coordination with structural projects from the stormwater master plan. 

2. From the Drainage Master Plan: Capitol and Holliday Basins: 
Future land use within much of Capitol and Holliday Basins is expected to remain 
the same as existing because these areas are nearly fully developed. Within 
Holliday Basin, the only area for potential change is at the Cheyenne Municipal 
Airport, where future development may result in an expansion of the terminal and 
parking facilities along an extension of Morrie Avenue. FOR HYDROLOGIC 
ANALYSES CONDUCTED HEREIN, IT WAS ASSUMED THAT ANY SUCH EXPANSION INTO 
THIS AREA WOULD BE REQUIRED TO LIMIT ITS RUNOFF TO THE PRESENT HISTORIC 
LEVELS. 

The last sentence in this paragraph shows that the impact of the stormwater management 

regulations is considered in the development of the stormwater master plan. 

Present and future conditions were considered in each basin, as shown in the following 

discussion from the Drainage Master Plan: Clear Creek Basin. 

Clear Creek basin is essentially a rural basin composed of rangeland used for 
grazing sheep and cattle. Land cover in the basin is mainly grasses, forbs, and 
shrubs. About 2,100 acres of land in the basin have been issued permits for 
irrigation. Two reservoirs, with a combined capacity of about 320 acre-feet, store 
irrigation water on Clear Creek upstream of Interstate 25 (I-25) . . . 

Over 85 percent of the Clear Creek Drainage Basin is located west of I-25. 
Except for one industrial plant and some residences, this part of the basin is 
essentially undeveloped. The remaining basin below I-25, containing the Clear 
Creek flood plain study reach, is more urbanized but still has vast open 
areas . . . About 12 percent of the basin below I-25 is impervious area, due to 
roads, residential subdivisions, and industrial and warehouse complexes. 

Future land use within the basin is expected to show the conversion of much of 
the rural land near the City of Cheyenne into an urban use. City and county 
planning documents indicate that much of the area is part of a planned unit 
development (PUD), with areas designated for residential, commercial, and 
industrial uses . . . Based upon the proposed development conditions, future 
basin lag times and impervious areas were estimated to evaluate future basin 
hydrology. All other hydrologic parameters were assumed to remain the same. 
With the planned urbanization of the basin, the amount of impervious area will 
increase from the present 2.6 percent to an expected 16.2 percent. 

Stormwater master plans are not static documents. They should be reviewed and revised from 
time to time as they are implemented. This was done in Cheyenne and Laramie County. 

The recommendations from the watershed master plans were not fully implemented. In 1997, the 
city and the county commissioned a Surface Water Drainage Committee to look into the matter. 
According to Implementing the Drainage Master Plan for the Greater Cheyenne Area, dated 
March 2000, this committee 

 Examined progress and explored ways to implement the Drainage Master Plan; 
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 Developed an organizational means to coordinate city, county, and Warren Air Force 
Base efforts to mitigate the flood hazards; 
 Identified ways to reduce the cost of implementing the master plan by proposing 
revisions of improvement projects using revised design criteria; 
 Assessed the benefits and ramifications of establishing a drainage utility; 
 Developed preliminary recommendations for implementing a comprehensive 
drainage management program; 
 Hosted public meetings and gathered and responded to public comments; and 
 Developed final recommendations for implementing a comprehensive drainage 
management plan. 
A primary finding of the Surface Water Drainage Committee was that funding has not been 
provided to implement the 1988 Drainage Master Plan. The final committee recommendation 
was the development of a surface water drainage utility that would include the city, Warren Air 
Force Base, and portions of the county. This utility could be funded by fees from existing and 
future development. 

The committee stated that it is reasonable to expect that all developments address any drainage 
problems they might create, and it is recommended that design criteria and standards for 
surface water be provided in a single document to be adopted by the City, County and Warren 
Air Force Base. These two stormwater management elements are included, as much as possible, 
in the CheyenneLaramie County Subdivision/Development Regulations 2000. The committee 
noted that Warren Air Force Base will be an important part of the surface water drainage 
program. Warren Air Force Base is currently developing drainage plans and a series of 
environmental restoration projects on the base . . . The degree that Warren Air Force Base 
participates will depend on the Department of the Air Force. 

In reviewing the watershed master drainage plans, the committee recommended several revisions 
to reduce costs. These included the acquisition of some residences instead of far more costly 
structural projects to protect them, and the development of larger, publicly owned detention 
facilities in several locations. Acquisition of floodprone buildings has become a more common 
and more acceptable way to mitigate flood damage since the original plans were completed in 
1988, and the construction of larger publicly owned detention facilities is possible if the 
stormwater utility is established with appropriate fees. 

The preceding discussion shows several things common to stormwater master planning: 

 Implementation of a stormwater master plan that includes structural components can 
be very expensive; 
 Many communities have unavoidable impediments to planning, such as the existence 
of large state, federal, or other development over which they have no jurisdiction; and 
 Periodic review of stormwater master plans is essential to their implementation. 
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