FLOOD08875

• County: Boulder
• Basin: South Platte
• Title: Analysis of a High Hazard Flood Zone
• Date: 10/1/1987
• Prepared For: Boulder
• Prepared By: David Love & Associates
• Floodplain - Doc Type: Floodplain Report/Masterplan

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The problem is then to choose a product number which best represents a significant hazard to the
general populace. A product number of 4 was selected for one of the floodways for this analysis
since it reflects the product of the two foot depth and two foot per second velocity criterion of the
current City of Boulder ordinance. Flood waters flowing 4 feet deep at 1 foot per second velocity
or 1 foot deep at 4 feet per second velocity are very hazardous. Examples of scientific calculations
quantifying this hazard are given elsewhere in this document

Appendix A provides a discussion of the methods typically used in flood way delineation. Samples
of mapping for reaches of selected floodplains are included with floodway delineations to contrast
the difference between the one foot and the half foot rise floodway boundaries and the product
number hazard l'U'ea for a product of loo-year depth times velocity equal to 4. The sample mapping
shows that there are some hazard zones defined by the 4 product number outside of the half foot
rise floodway. DIL has also delineated product number hazard zone for product numbers of 2, 3,
3.5, 5 and 6.

Flood hazards may be reduced in flood prone l'U'eas by mitigation efforts such as major
drainageway improvements, site grading and flood protection measures. This process may result
in realignment of the flood hazard zone.

INVESTIGATION OF FLOODPLAIN ORDINANCES IN FORCE
IN OTHER CITIES

In order to evaluate the present "state-of-the"art" in floodplain regulation, DIL contacted the
following entities and obtained their respective floodplain ordinances:

Sacramento, California
Arvada, Colorado
Boulder County, Colorado
Columbine Valley, Colorado
Loveland, Colorado
Boise City, Idaho
Las Vegas, Nevada
Tulsa, Oklahoma
Rapid City, South Dakota
Salt Lake City, Utah
Cheyenne,VVyoming
Aurora, Colorado
Golden, Colorado
Urban Drainage & Flood Control District - Denver, Co.

Many of these entities have experienced recent flash flooding with loss of life and high damage. In
addition to the above ordinances, a "model" ordinance was found in 16 Natural Resources at 665
(1976). A copy of the "model" ordinance is anached as Appendix B.

By far, the most common definition of the floodway is the "one foot rise" flood way. In fact, all of
the ordinances enumerated above except the UD&FCD use a one foot rise, or no specific flood way
defmition at all.

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