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I- I. . . . . . . . . . . I . I I ,I !I I I ANALYSIS OF A HIGH HAZARD FLOOD ZONE INTRODUCTION The finn 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 panicular, Boulder desires to realistically define high hazard in the vicinity of the floodway as a means to minimize threats to life and property. DIL has been retained to study and evaluate various methods of defining unacceptably high hazard within the floodway. 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 Disnict (UD&FCD) for guidance. This preliminary report summarizes our findings and greatly narrows the remaining issues requiring resolution. DEFINING THE FLOODW A Y As pointed out in the City of Boulder's Flood Utility staff study session report to City Council of March 9, 1987, several definitions of the term "floodway" exist. By way of general definition, it is the area near the channel centerline through which most flood flows pass, characterized by increased depths and high velocities. The definition of the floodway creates a boundary within which it is felt that the risks imposed by flooding are more important than the factors which motivate building there. These factors for building within the floodway could include political pressure, land values, pre-existing development, and more. The current City of Boulder Floodplain Regulation, Chapter 9-9, B.R.C. 1981, defines flcodway as: Those portions of the floodplain required for the passage or conveyance of the one hundred year flood in which waters will be two feet or greater in depth or flows at two feet per second (f.p.s.) of velocity or greater or encroachment of the floodplain to a point where the one hundred year flood profile will be raised by more than one half fool... The defmitions of flood way or hazard zone chosen by City of Boulder Staff and DIL for analysis in this study were the one foot rise floodway, the half foot rise floodway, and various product number high hazard zones. A one foot rise flood way is defined as that area within the 100-year floodplain which,. when encroached from both boundaries of the floodplain for an equal conveyance loss from each boundary, results in a one foot rise in the lOO-year base flood elevations. Similarly, a half foot rise floodway is equal conveyance loss encroachment based, resulting in a half foot rise in the lOO-year base flood elevation. This floodway is wider than the I foot rise floodway. - 1-