FLOODPLAIN PRESERVATION MANAGEMENT

Portland Metro

Growth Management Services

(503) 797 - 1839

Flood Management

A.	The purpose of these standards is to reduce the risk of flooding, prevent or reduce risk to human life and property, and maintain the functions and values of floodplains, such as allowing for the storage and conveyance of stream flows through existing and natural flood conveyance systems.

B.	This ordinance establishes a Flood Management Area Overlay Zone, which is delineated on the Water Quality and Flood Management Area Map attached and incorporated by reference as a part of this ordinance.

C.	The Flood Management Areas mapped include:

1.	Land contained within the 100-year floodplain, flood area and floodway as shown on the Federal Emergency Management Agency Flood Insurance maps and the area of inundation for the February 1996 flood; and

2.	Lands that have physical or documented evidence of flooding within recorded history. Jurisdictions shall use the most recent and technically accurate information available to determine the historical flood area, such as the aerial photographs of the 1996 flooding and digitized flood elevation maps.

3.	The standards that apply to the Flood Management Areas apply in addition to local, state or federal restrictions governing floodplains or flood hazard areas.

D.	Uses Permitted Outright:

1.	Excavation and fill required to plant any new trees or vegetation.

2.	Restoration or enhancement of floodplains, riparian areas, wetland, upland and streams that meet federal and state standards.

E.	Conditional Uses:

All uses allowed in the base zone or existing flood hazard overlay zone are allowed in the Flood Management Overlay Zone subject to compliance with the Development Standards of subsection G.

F.	Prohibited Uses:

1. Any use prohibited in the base zone or existing flood hazard overlay zone.

2.	Uncontained areas of hazardous materials as defined by the Department of Environmental Quality.

G.	Development Standards

All development, excavation and fill in the floodplain shall conform to the following balanced

cut and fill standards:

1.	No net fill in any floodplain is allowed. All fill placed in a floodplain shall be balanced with at least an equal amount of soil material removal.

2.	Excavation areas shall not exceed fill areas by more than 50 percent of the square footage.

3.	Any excavation below bankfull stage shall not count toward compensating for fill.

(Note: These areas would be full of water in the winter and not available to hold stormwater.)

4.	Excavation to balance a fill shall be located on the same parcel as the fill unless it is not reasonable or practicable to do so. In such cases, the excavation shall be located in the same drainage basin and as close as possible tot he fill site, so long as the proposed excavation and fill will not increase flood impacts for surrounding properties as determined through hydrologic and hydraulic analysis.

5.	For excavated areas identified by the city or county to remain dry in the summer, such as parks or mowed areas, the lowest elevation of the excavated area shall be at least 6 inches above the winter "low water" elevation, and sloped at a minimum of two percent towards the Protected Water Feature. One percent slopes will be allowed in smaller areas.

6.	For excavated areas identified by the city or county to remain wet in the summer, such as a constructed wetland, the grade shall be designed not to drain into the Protected Water Feature.

7.	Minimum finished floor elevations must be at least one foot above the design flood height or highest flood of record, whichever is higher, for new habitable structures in the Flood Area.

8.	Short-term parking in the floodplain may be located at an elevation of no more than one foot below the ten-year floodplain so long as the parking facilities do not occur in a Water Quality Resource Area. Long-term parking in the floodplain may be located at an elevation of no more than one foot below the 100-year floodplain so long as the parking facilities do not occur in a Water Quality Resource Area.

9.	Temporary fills permitted during construction shall be removed.

10.	New culverts, stream crossings and transportation projects shall be designed as balanced

cut and fill projects or designed not to significantly raise the design flood elevation. Such projects shall be designed to minimize the area of fill in Flood Management Areas and to minimize erosive velocities. Stream crossings shall be as close to perpendicular to the stream as practicable. Bridges shall be used instead of culverts wherever practicable.

11.	Excavation and fill required for the construction of detention facilities or structures, and other facilities, such as levees, specifically shall be designed to reduce or mitigate flood impacts and improve water quality. Levees shall not be used to create vacant buildable lands.