An urbanized area is a land area comprising one or more places --

central place(s) -- and the adjacent densely settled surrounding area --

urban fringe -- that together have a residential population of at least

50,000 and an overall population density of at least 1,000 people per

square mile.

United States Office of Water EPA 833-F-99-004

Environmental Protection (4203) April 1999

Agency Fact Sheet 2.2

Storm Water Phase II

Proposed Rule

Urbanized Areas:

Definition and Description

This fact sheet is based on the Storm Water Phase II Proposed Rule. Therefore, the information provided herein is subject to

change upon publication of the final Phase II rule in November 1999. A revised series of fact sheets will be provided at that

time. A comprehensive list of the current fact sheets is in the text box at left.

As discussed in Fact Sheet 2.1, Who’s Covered? Designation and Waivers of Regulated Small

MS4s, the proposed Phase II regulations would cover all small municipal separate storm sewer

systems (MS4s) located within an “urbanized area” (UA). Based on the 1990 Census, there are 405

UAs in the United States that cover 2 percent of total U.S. land area and contain approximately 63

percent of the Nation’s population. These numbers include Puerto Rico – the only U.S. Territory with

UAs.

UAs constitute the largest and most dense areas of settlement. UA calculations delineate boundaries

around these dense areas of settlement and, in doing so, identify the areas of concentrated

development. UA designations are used for several purposes in both the public and private sectors. 

For example, the Federal Government has used UAs to calculate allocations for transportation

funding, and some planning agencies and development firms use UA boundaries to help ascertain

current, and predict future, growth areas.

What Is an Urbanized Area (UA)?

The Bureau of the Census determines UAs by applying a detailed set of published UA criteria

(see 55 FR 42592, October 22, 1990) to the latest decennial census data. Although the full UA

definition is complex, the Bureau of the Census’ general definition of a UA, based on population and

population density, is provided below.

The basic unit for delineating the UA boundary is the census block. Census blocks are based on

visible physical boundaries, such as the city block, when possible, or on invisible political boundaries,

when not. How an urbanized area is determined varies slightly on the geographic entity, be it a place,

county, Federal Indian reservation, or minor civil division (towns and townships), as explained below:

 Place. A “place,” whether an incorporated place or a Census-designated place, is included in its

entirety even if all of its census blocks are not densely settled. Therefore, this part of the UA

determination is based on place boundaries. In rare cases (128 places), a place is not included in

its entirety, but rather only partly included within the UA, usually due to the existence of large

expanses of vacant or very sparsely populated territory within its incorporated area.

Storm Water Phase II

Proposed Rule 

Fact Sheet Series

Overview

1.0 – Storm Water Phase II

Proposed Rule Overview

Small MS4 Program

2.0 – Small MS4 Storm Water

Program Overview

2.1 – Who’s Covered? Designation

and Waivers of Regulated Small

MS4s

2.2 – Urbanized Areas: Definition

and Description

Minimum Control Measures

2.3 – Public Education and

Outreach Minimum Control

Measure

2.4 – Public Participation/

Involvement Minimum Control

Measure

2.5 – Illicit Discharge Detection and

Elimination Minimum Control

Measure

2.6 – Construction Site Runoff

Control Minimum Control Measure

2.7 – Post-Construction Runoff

Control Minimum Control Measure

2.8 – Pollution Prevention/Good

Housekeeping Minimum Control

Measure

2.9 – Permitting and Reporting:

The Process and Requirements

2.10 – Federal and State-Owned

MS4s: Program Implementation

Construction Program

3.0 – Construction Program

Overview

Industrial “No Exposure”

4.0 – Conditional No Exposure

Exemption for Industrial Activity
Fact Sheet 2.2 – Urbanized Areas: Definition and Description Page 2

1719D

County A

County B

FIR

Town B

Town A

Central Place

Incorporated Place

Federal Indian Reservation (FIR)

Unincorporated “Urbanized

Area” Portion of a Town

(MCD) or County

Urbanized Area

Town or Township as a

functioning Minor Civil Division

(MCD). An MCD is the primary

subdivision of a County.

County

Figure 1

 County/Federal Indian reservation. A county is

included in its entirety only if all of its census blocks,

based on the county’s unincorporated area, meet the UA

criteria. Unlike a place, a county is often “split” into

urbanized and non-urbanized portions, with no regard for

political boundaries. Only small MS4s in the urbanized

portion of a split county would be automatically covered

under the Phase II Proposed Rule. If a county or sewer

district, for example, operates a small MS4 that runs

throughout a split county, it would only be required to

implement the storm water program in the portion of the

county that is within the UA. The same situation applies to

split Federal Indian reservations, as well as minor civil

divisions (see below).

 Minor Civil Division (towns & townships). Twenty

states have functioning subdivisions of counties called

minor civil divisions (MCDs). MCDs are towns or

townships, depending on the State. Like a county, a MCD

is included in its entirety only if all of its census blocks

meet the urbanized area criteria. A MCD is often “split”

into urbanized and non-urbanized portions, with no regard

for political boundaries. Only small MS4s in the urbanized

portion of a split MCD would be automatically covered

under the storm water Phase II proposal. Where a small

MS4 runs throughout a split MCD, the owner/operator of

the small MS4 would only be responsible for implementing

the storm water program in the portion of the MCD that is

within the UA.

How Can Status as a Regulated Small MS4 Be

Determined?

The drawing below (see Figure 1) is a simplified UA

illustration that demonstrates the concept of UAs in relation

to the proposed storm water Phase II regulations. The “urbanized

area” includes within its boundaries incorporated places, a portion

of a Federal Indian reservation, an entire MCD, a portion of

another MCD, and portions of two counties. Any and all owners

or operators of small MS4s located within the boundaries of the

UA would be covered under the Phase II Proposed Rule,

regardless of political boundaries. Owners and operators of small

MS4s located outside of the UA would be subject to potential

designation into the Phase II MS4 program by the NPDES

permitting authority.

Once the concept of a UA is understood, the owner or operator

can proceed to determining their location in relation to a UA.
Fact Sheet 2.2 – Urbanized Areas: Definition and Description Page 3

Owners and operators of small MS4s can determine if they are

located within a UA, and therefore covered by the Phase II storm

water program as proposed, through the following two steps:

— STEP 1 —

Refer to a listing of incorporated places, MCDs, and counties in

UAs. Such a listing, based on the 1990 Census, can be found in

Appendix 6 in the preamble to the proposed rule, although it does

not include MCDs or governmental entities already permitted

under Phase I. A similar listing, based on the 2000 Census, may

be published with the storm water general permit for regulated

small MS4s. If a small MS4 is located in a listed incorporated

place, then it would be covered by the Phase II program. If a

small MS4 is not located in a listed incorporated place, but is

located in a listed MCD or county, then the owner or operator of

the small MS4 would need to follow step (2) below. (Note:

Listings of Phase II incorporated places, MCDs, and counties

located entirely or partially within a UA can be obtained from the

EPA Office of Wastewater Management or downloaded from the

OWM web site.)

— STEP 2 —

Some owners and operators of small MS4s may find their

counties or MCDs on the list but not know, due to the occurrence

of split counties and MCDs, if their systems are within the

urbanized portion of the county or MCD. In such a case, they

should contact one or more of the following institutions for more

detailed information on the location of the UA boundary:

 The State/NPDES Permitting Authority (may be the

State or the U.S. EPA Region)

Storm Water Coordinators: The NPDES permitting

authority may be the State or the U.S. EPA Region. 

The Storm Water Coordinators for each U.S. EPA

Region are listed in the For Additional Information

section in Fact Sheet 2.9. These regional contacts

can assist with UA information and provide the

names of State storm water contacts. Regional and

State contact information can also be obtained from

the EPA Office of Wastewater Management.

State Data Centers: Each State’s Data Center

receives listings of all entities that are located in

UAs, as well as detailed maps and electronic files

ofUA boundaries. The Bureau of the Census web site

includes a list of contact names and phone numbers

for the data centers in each State

(www.census.gov/sdc/www/).

State Planning/Economic/Transportation Agencies:

These agencies typically use UAs to assess current

development and forecast future growth trends and,

therefore, should have detailed UA information

readily available to help determine the exact UA

boundaries in any given area.

 County or Regional Planning Commissions/ Boards

As with State agencies, these entities are likely to

already have detailed UA data and maps to help

determine UA boundaries.

 The Bureau of the Census

Urbanized Areas Staff: 301 457-1099

Web Site: www.census.gov

The site provides information on purchasing UA

maps and electronic files for use with computerized

mapping systems. Obtain free UA cartographic

boundary files (Arc/Info export format) for

Geographical Information System (GIS) use at:

www.census.gov:80/geo/www/

cob/ua.html.

UA Maps: Detailed UA maps are available for

purchase with a $25 minimum order ($5 per map

sheet). Each map sheet measures 36 by 42 inches. 

For prices and a listing of UAs, visit

www.census.gov/mp/www/geo/msgeo12.html. Order

from the Department of Commerce, Bureau of the

Census (MS 1921), P.O. Box 277943, Atlanta, GA

30384-7943 (Phone: 301 457-4100; Toll-free fax:

1-888-249-7295).

 The Local Library (or State Data Center) for Book of

UA Maps

The following book contains small maps of each UA

and detailed listings of all geographic entities located

within them:

1990 Census of Population and Housing. 

Supplementary Reports. Urbanized Areas

of the United States and Puerto Rico.

1990 CPH-S-1-2 (2 sections, sold as a set).

The UA book also can be ordered from the

Government Printing Office by calling 202 512-1800

(approximate cost is $56).
Fact Sheet 2.2 – Urbanized Areas: Definition and Description Page 4

How Will the Year 2000 Census Affect the

Determination of Status as a Regulated Small MS4?

The listing of incorporated places and counties located

within UAs in the United States and Puerto Rico, found

in Appendix 6 of the proposed rule, is based on the 1990

Census. New listings for UAs based on the 2000 Census

are scheduled to be available by July or August of 2001. 

Once the official 2000 Census listings are published by the

Bureau of the Census, owners or operators of small MS4s

located within the revised boundaries of former 1990 UAs,

or in any newly defined 2000 UAs, would be required to

seek NPDES storm water permits.

Additional automatic designations of small MS4s based on

the Year 2000 Census and subsequent census years would

be governed by the Bureau of the Census’ definition of a

UA in effect for that year and the UA boundaries

determined as a result of the definition.

Once a small MS4 is designated into the Phase II storm

water program based on the UA boundaries, it would not be

waived from the program if in a subsequent UA calculation

the small MS4 is no longer within the UA boundaries. An

automatically designated small MS4 will remain regulated

unless, or until, it meets the criteria for a waiver (see Fact

Sheet 2.1 for more information on the regulated small MS4

waiver option).

For Additional Information

Contact

 U.S. EPA Office of Wastewater Management

• Phone : 202 260-5816

• E-mail: SW2@epa.gov

• Internet: www.epa.gov/owm/sw2.htm

Reference Documents

 Storm Water Phase II Proposed Rule Fact Sheet

Series.

• Contact the U.S. EPA Water Resource Center at

202 260-7786 or at waterpubs@epa.gov

• Internet: www.epa.gov/owm/sw2.htm

 Storm Water Phase II Proposed Rule, published on

Jan. 9, 1998 in the Federal Register (63 FR 1536).

• Internet: www.epa.gov/owm/sw2.htm