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Drainage and Flood Control - A Fact Sheet

This fact sheet is an overview of the Floodplain and Drainage Ordinance and is not intended to be all inclusive or interpreted as regulation. Copies of the ordinance are available from the city's One Stop Shop. Contact the drainage planning staff at 480-312-7696 regarding specific questions

How does the Floodplain and Drainage Ordinance protect my property?
The ordinance establishes requirements for the use and development of land that will minimize potential losses and hazardous conditions that might result from flooding. Additionally, it prohibits development in floodplains and development that would create hazards to life or property by increasing the likelihood of flooding on either the property under development or any other property.

What are some of the main requirements of the ordinance?
Runoff from all storms should enter and exit from a piece of land after its development in substantially the same manner as under predevelopment conditions.

  • The 100-year floodplain of watercourses within the environmentally sensitive lands (ESL) area with a capacity of 50 cubic feet per second (c.f.s.) or greater shall be dedicated to the city by a drainage easement and maintained in their natural state. Outside of the ESL area, drainage and flood control easements are required for all watercourses with a capacity of 25 c.f.s. or greater. These watercourses are not required to be maintained in their natural state.
  • Generally, new development within the city must provide stormwater storage.
  • Access must be provided for all lots within a development (from the property line of that development) during the peak flow of the 100-year event. Access is considered to exist if the depth of flow over the road does not exceed one foot.
  • Watercourses may not be altered unless a professional engineer certifies that the alterations will not increase flood levels or flooding hazards. The obstruction or reduction in capacity of a watercourse by any use or by filling, dumping, constructing, or any other means is prohibited.

How are watercourses classified?
Watercourses are classified by the amount or capacity of water they convey during a 100-year flood. This capacity is measured in cubic feet per second (c.f.s.). In addition, watercourses are classified by their potential flood hazard. Flood hazards are mapped in accordance with Federal Emergency Management Agency (FEMA) criteria. FEMA maps are available at the city's One Stop Shop. The ESL Ordinance also classifies watercourses as major (more than 750 c.f.s.) and minor (between 250 and 750 c.f.s.) for planning purposes.

Are large developments such as the master planned communities subject to the same requirements as other developments?
Yes, by law any development, regardless of its size, within the city is required to adhere to the floodplain and drainage ordinance.

The natural drainage pattern seems to work, why then are detention basins needed?
Storage basins are needed to offset the increase in runoff created by new development, so that flooding potential is not increased on downstream properties.

Can washes be manipulated?
The ordinance states "a watercourse may not be altered" unless a professional engineer certifies that the alterations do not increase the flood levels, and will not increase flood hazards within, upstream or downstream of the altered portion of the watercourse. Watercourses with a capacity of 50 c.f.s. or greater within the ESL area cannot be manipulated since they are required to be maintained in their natural state.

Can I build In a wash?
This is generally discouraged, and is actually prohibited by ordinance in certain situations. The ordinance also prohibits the obstruction of a watercourse or the reduction in the capacity of a watercourse. It is prohibited if the wash is to be preserved in its natural state, such as in the ESL areas.

Why can't the washes be left in their natural state?
They can be if property owners are willing to do so, and in some instances it is required by city ordinance. Within the ESL area, washes with a capacity of 50 c.f.s. or greater are required to be maintained in their natural state. In other parts of the city and on smaller washes within the ESL area there are no requirements that a wash be kept natural; although it may not be altered as described above. Washes and their floodplains when left in their natural state have many beneficial functions in addition to drainage and flood control values. Aesthetics, wildlife habitat, recreation, water quality and ground water recharge are a few examples.

What is a floodplain?

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PLAN VIEW OF FLOODPLAIN AND RIPARIAN HABITAT

Regulatory Floodplain (100-Year Floodplain): the channel and adjacent land areas inundated by the discharge from the flood having a 1% probability of being equaled or exceeded in any given year.

the channel and adjacent land areas inundated by the discharge from the flood having a 1% probability of being equaled or exceeded in any given year.

Floodway : the channel and portions of the adjacent floodplain that must be kept free of encroachments so that the 100-Year flood discharge (base flood) can be conveyed without raising the flood level by more than one foot.

Flood Fringe: the area inundated by the 100-Year flood that is outside the floodway.

the area inundated by the 100-Year flood that is outside the floodway.

COSDP 8/97

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