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I I 19 I SECTION IV FLOODPLAINS I GENERAL The cities of Broomfield and Westminster have recognized the importance of floodplain delineation and regulation. Both cities have adopted floodplain ordinances for the good of the private property owner, the general public, and to reduce the cities' liability (Appendix A). Flood discharges and boundaries must be established in accordance with reasonable and accepted engineering methods. Cross sections were located at appropriate intervals on the mapping provided by the Urban Drainage and Flood Control District and were then digitized by Delta Aerial Surveys for input into the computer model, The locations of these cross sections are shown on the plan and profile sheets in this section, Field investigations were undertaken to define the physical char- acteristics of each stream segment from which roughness coefficients could be estimated, and to determine the condition of the existing roadway culverts, I I I I FLOOD FREQUENCY The frequency of occurrence of the regulatory flood for land use regulation purposes is the I-percent flood event (lOO-year flood) with future development. This flood has a I-percent chance of occurring, or being exceeded, in any given year. A detailed survey was conducted at each culvert and this information was used in the computer model to determine the water surface profiles at each struc- ture, In several instances, it was found that the computer model did not accurately simulate the hydraulics of the flow through the culverts. For these cases, hydraulic calculations were made to determine the capacity of each structure and this information was then input into the computer model so that backwater calculations could be continued along the entire 1 ength of the s,tream. Where the capac i ty of a cu I vert was exceeded, ca 1- culations were made to determine the extent of flooding over embankments and roadways, In some places, field inspection showed that debris blockage of certain culverts would likely occur during flooding, usually due to poor entrance conditions, This blockage was accounted for in the studies. I I I The 1 percent event criteria provide for a basic waterway which will function 1 I fl d de a reasonable basis for during larger, as well as sma er, 00 s an i. public safety and protection. I I I FLOOD HAGN ITUDE The peak flow magnitude of the I-percent flood has been determined for the existing and future development and is presented in Section III. The flood compute'r model of each basin to simulate storm events is defined using a I (Appendix B). The flood profi Ie is the most important tool for the study of a parti- cular area subject to flooding, The floodplain map, or plan view, is a pictorial representation of the profile, and thus is of secondary importance when an individual site is under investigation for legal or regulatory pur- poses. For example, in most cases, it is not possible to show spot eleva- tions for all residences, commercial buildings or other structures and, consequently, plan view delineation of the flood profiles involves a combina- tion of hydraulic calculations and engineering judgment. Due to the limita- tions of the plan view mapping, an onsite survey is recommended for any individual structure or parcet which may be subject to detailed evaluation for possible flood hazard. FLOOD PROFILES I f f'l computed uS'lng standard engineering The floodwater sur ace pro 1 es are techniques. The basic procedure in determining the floodplain was the HEC-2 backwater program developed by the U,S, ,~rmy Corps of Engineers. This efficient means of determining flood depths varying hydraulic characteristics and controls, computer program provides an along a natural channel with I I such as constrictions, culverts, and ponds. I