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Insurance Act of 1968, and further with the paaaaie of the Flood Diaaster ~rotection ~ct of 1973. 7he 19&8 Act provided tor the availability ot tlood insurance within communities that were willing to adopt tlood plain management programs to mitigate future tlood 108aea. The Act also required the tlood plain studies within the United Stat.. and the ..tablishment of flood-risk zones within those areas. ar.a. are not conaidered, i.e. flood routin9 process is not included. The water surface profile i. stationary in time. 3. flow i. one-dimenaional, the only velocity component is in the direction ot flow and the w.ter s~rfsce i. horizontal over the entire croa. ..ctlonJ FLOOD INSURANCE StUDY GUIDELINES AND SPECIFICA~IONS The current Flood Insurance Study Guidelines and Specifications are provided for work involvin9 standard professional practice for flood hazard evaluation, whereas specific instructions are alao provided for work unique to flood insurance studies. The studies should provide communities with 6uffi~ient technical information to enahle them to adopt the floodplain management measures and to establish actuarial flood insurance premiume. A flood insurance study generally involves appraising a community" flood problems, estimating floodflow frequency of flooding aouroe. (hydrology), establishin9 tlood elevation profiles (hydraulic.), plotting flood Doundaries (floodplain mapping), computing flood hazard factor. (insurance rate.), and delineating floodways. The hydraulio analysia in the flood insurance atudy determines the flood elevations along the stream channel u.ing . hydraulic mod.$lin-; technique. Currently F'EMA speolly the U.S. Aray Corp. ot En9ineera Hydrol09ic Engineering Center's generalized computer progr.. K&C-2 i. recommended. The use of alternate computer programa must be approved by the p~oj.ct officer of FEMA. 5. flow i. gradually varied, i~lyin9 a hydrostatic pressure diatribution, the -...11- .lope tcose !! 1) is anwned, 50 tha'C the ve:ttical clep ot water i. repre..ntative of the pressure head, and 6. The Manning equation tor uniform flow applies to 9radually varied flow. 4. Accordin9 to the comments of FENA's st~dies from public an local officials 1n Region VIII, the major defioiencies of the current water surface .1evat:Lon a-ter1ll.ination model are the fi.xed-Pad, st.eady state and 0 dimensional assumptions. EXPERIENC~D MAJOR HYDRAULIC PROBLEMS IN REGIONS VIII, IX and X, FEMA The major hydrauliC problema of flood insurance studies in FEMAle west region. are .~ried .. follov., . stream channel bottom deqradation or aqqreqation _ stream ohannel bank erosion and under=ininq _ stre" channel lateral migration or b:taid1n9 LIMITATIONS OF HEC-2 HYDRAULIC MODEL The HEC-2 was developed to determine water .urf.c. elevationa by standard step method for given discharges in river channels of any eras. section for The effecta of natural _ serious eroaion proble=. at bydraulic structureS _ large &IllQuot of sediments transported in the strellIn _ debris and .ediment blockage at roadway crossings simulated by to flow, floodplain encroachment, and hydraulio structures may be the program. The assumptions or limitations are the following I _ sediment problema :Ln the alluyial fan areas _ landslide in upper basins with ~dflow in channels and on alluvial fa. sUbcritical or supercritical steady-stable flow. obstructions 1. a ri9id or fixed boundary, i.e. no sediment soour or deposition in the channel, _ sedimentation problems in ur~an areas _ channel and overbank storage effects on flood st.age predication ,. flow is steady, 1..e. the passage of a flood wave cannot be accurately analyzed, the flood storage in the channel and overbank As indicated above, all hydraulic problems of FIS in FEMAls western regions are associated with erodible channels and sedimentation. Therefo~ it is important to clarify the applicability of current simple fixed-bed 3.30 3.31 /.