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.. I \1 . . . "I I II I I I I I I ,I I I I EXECUTIVE SUMMARY The firm of David I. Love & Associates, Inc. (DIL) is under contract with the City of Boulder to assist in establishing a new floodplain/flood way ordinance. In particular, Boulder desires to realistically define high hazard in the vicinity of the floodway in such a way that threats to life and property are minimized. DIL has been retained to study and evaluate various methods of defining unacceptably high hazard within the floodplain. In order to accomplish this task we have looked at the various definitions of zones within the floodplain, existing floodplain ordinances from numerous cities, literature related to floodplain regulation, and performed scientific and engineering calculations to quantify hazards. In addition, we have interviewed the flood control regulatory experts from the Federal Emergency Management Agency (FEMA) and the Urban Drainage & Flood Control District (UD&FCD) for guidance. Several definitions of the term "floodway" exist. By way of general definition, it is the area near the channel centerline which conveys the major portion of flood flows. This "higher conveyance" area is characterized by greater depths of flow and high velocities, and is regulated to maintain the flood-carrying capacity of major drainage ways. A number of cities surveyed regulate major drainageways by the use of a "rise" floodway. A rise floodway is defined in terms of conveyance. This is generally arrived at by modeling the loo-year floodplain as it exists to find the Base Flood Elevation (BFE). The conveyance of the floodplain is then reduced incrementally on either side of the main channel by modeling an encroachment from the boundaries of the floodplain until a predetermined. or "target" rise in the loo-year base flood elevation is reached (Appendix A). FEMA has adopted the one foot rise floodway, as have many municipalities across the country. The UD&FCD has adopted the half foot rise flood way and encourages municipalities within its District 10 do likewise. A floodplain regulated by a rise defined floodway may not adequately address the hazard to the life safety of individuals caught in rapidly rising flood waters. Boulder has adopted Ordinance 5035 which 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. - i - Oevld J. Love & A..ocillt.., Inc. 2995 c.ntergrfHHI Court Soulh. Suit. C Boulder. Colorado 8030 1 -- .- ::- ""t.~O-___.....~_ SUnpHU.d HuMan GeOlUll'J -..- ...... Id..llud -Bod}' IIoAoMth- ~ --