Stormwater Program

In 2003, the County filed for authorization to discharge stormwater runoff into waters of the U.S. This filing was made with the Arizona Department of Environmental Quality (ADEQ), and places stormwater discharge from select, small municipal separate storm sewer systems (SMS4s) under County jurisdiction. The County also submitted a Stormwater Management Program (SWMP) to ADEQ committing the County to the development of Best Management Practices, a County Stormwater Ordinance, measurable goals, and achievement dates.

Phases I and II of the stormwater program have been implemented and the EPA and ADEQ are trying to reduce water quality impacts before the receiving water body’s physical structure and habitat have been altered. This is being accomplished by designating areas with actual physical boundaries as municipal separate storm sewer systems (MS4s) that are owned and operated by public entities:
  • MS4s have a system of conveyances (curbs, gutters, streets, drains, etc.) to route stormwater runoff into waters of the U.S. (washes, streams, and lakes) or to collection/ treatment systems.
  • MS4s are designated and regulated depending on whether they are large, medium or small.
    • Large and medium MS4s are automatically designated based on the 1990 census.
      • Large MS4s are incorporations or county jurisdictions with a population of 250,000 or greater.
      • Medium MS4s are incorporations and county jurisdictions with a population between 100,000 and 249,999.
    • In the Phase II NPDES stormwater regulations, the determination and designation of small MS4s (SMS4s) is much broader in scope and not merely a function of population. SMS4s include:
      • Inclusion in urbanized areas (UAs)
      • An automatic nationwide designation rule
      • A potential designation based on population and six recommended criteria
      • A potential designation based on a physical interconnection with a regulated MS4
An Urbanized Area (such as the Flagstaff UA) is a land area composed of one or more central places with an adjacent urban fringe that together have a population of at least 50,000 and an overall population density of at least 1,000 people per square mile. The number of SMS4s will continue to change over time based on future censuses and connected areas to UAs that meet the other SMS4 criteria defined in the Phase II regulations.

The Phase II regulations provide for three ways that an area can be designated an SMS4.
  1. Through the automatic nationwide coverage rule for all operators of SMS4s that are partially or totally located within the boundaries of a delineated UA.
  2. Areas outside a UA that serve a jurisdiction of at least 10,000 and have a population density of at least 1,000 people per square mile, depending on a balanced evaluation of the following criteria:
    • Discharge to sensitive waters
    • High population density
    • High growth or growth potential
    • Contiguity to a UA
    • Significant contributor of pollutants to waters of the U.S.
    • Ineffective protection of water quality concerns by other programs
  3. Areas located just outside the boundary of a UA that are contributing measurably to the pollutant loadings of the physically interconnected MS4.
Certain areas within Coconino County are designated and listed as comprising the County SMS4. This was done because they are physically interconnected with the city of Flagstaff UA and/or measurably contributing to pollutant loadings in the common receiving streams. Maps of the affected subdivisions that are subject to the Construction General Permit are as follows: Coconino County as operators of the SMS4s is required to develop and implement a Stormwater Management Program (SWMP). This requirement commits the County to meeting minimum control measures through enforcing and developing best management practices to reduce the physical and chemical pollution from stormwater runoff. To accomplish the enforcement of three of the control measures, the County adopted a Stormwater Ordinance.  

Two of the other control measures involve public education and public involvement. The SWMP also addresses the following: For additional information: