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I' I. I I I I I I I I I I I I I I I II I A product number high hazard zone is bounded by points within the lOO-year floodplain where the product of lOO-year depth times lOO-year velocity exceeds a value determined to be unacceptably hazardous. The degree of unacceptable hazard is a policy determination established by the governing body. FEMA has adopted the one foot rise floodway, as have many municipalities across the country. FEMA enforces the one foot rise criterion through the National Flood Insurance Program (NFIP), which probably explains its wide acceptance. Those communities that wish to panicipate in the NFIP must meet FEMA's minimum floodplain/floodway criteria and regulations. The UD&FCD has adopted the half foot rise floodway and encourages municipalities within its District to do likewise. The UD&FCD has legal authority, derived from State Statutes, to enforce its floodplain/floodway ordinance, but has not actively enforced its requirements when member municipalities within the UD&FCD regulate through their own floodplain/flood way ordinances. Boulder has adopted Ordinance 5035 whicb temporarily suspends the issuance of flood way permits for new habitable structures or for substantial improvements to habitable structures within the half foot rise flood way. This ordinance is intended to be replaced by subsequent regulations which will govern the placement of habitable structures within areas of the floodplain where an unacceptable hazard exists. Discussion All floodplains are considered hazardous, but the hazard in some areas may not exceed acceptable levels when compared to potential land use. Floodway and land use zoning regulations must consider evaluation of the high hazard areas of the floodplain and must establish restrictions for those areas which protect the health, safety and welfare of the public. Minimum standards for communities panicipating in the NFIP require that floodways be defined according to FEMA's one foot rise criterion. This criterion was developed by FEMA for use throughout the United States. The FEMA criterion does not necessarily represent the high hazard area for the flash floods which occur in Boulder due to steeper stream gradients, higher flood velocities, smaller storm volumes, and other flash flooding conditions. FEMA's regulations for floodways are intended to preserve conveyance of flood waters in the interest of protecting property. This is accomplished by regulating encroachments by development within the floodplain. There is an implied life safety hazard associated with floodway areas due to the increased depths and velocities inherent within the regulated conveyance area. However, the life safety hazard is not defined by FEMA for specific flood depth or velocity criteria. The rise criterion which is relative to base flood elevations is not directly tied to hazard based on specific flood velocities or depths. Similar depths and velocities can and will occur outside of the rise floodway delineation. Because of the flash flooding condition when little time for warning is available, support for a floodway criterion which is more restrictive than FEMA's standards can be justified. This would appear to lend support for a half foot rise criterion or a product number criterion. The existing City of Boulder floodway ordinance did make an effort to quantify hazard associated with flooding by the addition of the two foot depth or two foot per second velocity criteria. These additional criteria lend themselves to establishing a high hazard zone based on velocity and depth. Using velocity and depth as a product may best define a high hazard area. Velocity in itself does not necessarily pose a threat in the case of shallow flows, and depth below the point of buoyancy is not necessarily hazardous unless there is some velocity which creates a force in the direction of flows. For every discrete depth of flow however, there is some threshold velocity which will cause an individual to lose their balance and be swept away. Therefore, using the product of velocity and depth to regulate high hazard areas would appear to be a logical choice. .2-