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<br />Wednesday, May 20 10:30 - 11 :45 AM Track 5 - Hydraulks - Moderator: Jim Boulton
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<br />METHOD TO ESTIMATE EFFECTS OF FLOW INDUCED VEGETATION CHANGES ON CHANNEL
<br />CONVEYANCES OF STREAMS IN CENTRAL ARIZONA
<br />Jeff V, Phillips and Anne Tillery
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<br />Proper estimation of Manning's roughness coefficients, n, in open-channels is necessary to reliably estimate channel conveyance-an important
<br />element of an open-channel hydraulic stldy, However, proper estimation ofn-values is difficult for many localities in the arid to semi-arid
<br />southwestern US because peak flows can dramatically alter the roughness characteristics oflbe channel by I) flattening or laying over of
<br />vegetation, which acts to increase conveyance, and 2) removal of vegetation in response to degradation of substrate, which also acts to
<br />increase conveyance, Data collected from 26 peak flows at 19 sites were used to develop semi-empirical relations to assist in estimation
<br />ofn-values for sites where flood induced changes in vegetation are considerable, To investigate the potential for the flattening or laying over
<br />of vegetation in response to a flood, a site-specific vegetation index was developed, This index is a function of the type, size, and distribution
<br />of vegetation, as well as the relation between depth of flow and vegetation height. It was detennined that the flexural stiffness or bending
<br />moment of vegetation is the primary control of the potential for the flattening or laying over of vegetation, The degree to which vegetation
<br />is flow affected can be evaluated using Ihe relation between the vegetation index and stream power, which is a measure of energy transfer.
<br />To investigate the potential for vegetation removal, evaluation of substrate degradation is required. The potential for substrate degradation
<br />is related to bolUldaIy shear stresses. It was fOlUld, for alluvial or sand dominated stream channels, that substantial bedload transport during
<br />a flood is the primary factor that determines the removal of vegetation, Channel conveyance calculations made for pre- and post-flow channel
<br />conditions when considerable changes in vegetation were observed indicate that incorrect assessment of vegetation conditions during the
<br />pelll< flow can result in channel conveyance differences as much as 100 percent. This can lead to erroneous calculations of water-surface
<br />elevations and thus erroneous delineation of flood prone areas,
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<br />FLOODPLUN MANAGEMENT OF INTERIOR AREAS USING HEC-IFH
<br />Martin J, Teal
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<br />Management of interior areas is a diffictiltlask for floodplain administrators (an interior area is defined as an area where local precipitation
<br />runoff and/or other flows are blocked from reaching an exterior stream by a bamer, usually a levee or f100dwall). Typical questions to be
<br />answered include "How do I detennine the 1 DO-year floodplain?" and 'What combination of pumps and gravity drains is econotilically
<br />justified by the reduction in flood damages?' This presentation will describe the author's experiences on several interior flood studies in
<br />California, Arkansas and Missouri and will rely heavily on use of the computer program HEC-IFH (Interior Flood Hydrology), This
<br />program, created by the Hydrologic Engineering Center of the U,S, Anny Corps of Engineers, is largely unknown among floodplain
<br />professionals but is a very useful tool for floodplain management. The program can simulate rainfall-runoff processes, streamflow routing,
<br />auxiliary inflow, diversions and seepage. and complex combinations of gravity outlets and pumping facilities. Interior elevation-frequency
<br />relationships can be determined for various alternative plans by using continuous simulation or hypothetical stenn event analysis, In keeping
<br />with one of the cooference themes of using current technology, a sample flood study will be presented with HEC-IFH. Both input parameters
<br />(e.g" precipitation. exterior stage records) and output parameters (runoff, interior elevation-frequency cwves, pump and gravity drain use,
<br />etc,) will be shown in color graphics usinj! HEC- IFH and a computer projector.
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<br />DISCUSSm:'l OF TECHNIQUES FOR ANALYSIS OF ICE-JAM FLOODING
<br />Wilbert 0, Thomas, Jr., Kira L. Crockett, and Alan Johnson
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<br />In northern regions of the United States, the hydrologic and hydraulic considerations of ice-jam flooding during the winter season should
<br />be evaluated in detennining base flood elevations for Flood Insurance Studies. Ice jams often occur at bridges and/or other constrictions in
<br />the channel, causing base flood elevations to increase above those in open-water conditions, The hydrologic considerations of ice-jam
<br />flooding include determining the magnitude and frequency of flooding during the open-water and ice-affected seasons. The hydraulic
<br />considerations include determining ice-jam locations, the type of jam, and the most appropriate modeling scenario. The hydrologic and
<br />hydraulic modeling techniques for ieo'-jam flooding are described in Flood Insurance Study Guidelines and Specifications for Study
<br />Contractors, Federal Emergency Management Agency (FEMA 37), Depending on the availability of ice-jam stage data, the hydrologic
<br />analysis includes the use of direct and indirect approaches for determining base flood elevations, When ice-jam stage-frequency information
<br />must be developed for areas where known stage-frequency data are not available, as in the case of the indirect method discussed above,
<br />hydraulic modeling of ice jams is necessary to establish the base flood elevations. The most important factor in determining the ice-jam
<br />location and the type of ice jam is the review of historical data. Channel characteristics such as slope changes, flow obstructions, and channel
<br />bends may also playa crucial role in determining ice-jam locations and the type of ice jam, The methods available for modeling floating
<br />and grounded ice jams and the applicability of hydraulic models such as the U,S. Anny Corps of Engineers' ICETIIK ice utility program,
<br />the existing ice cover option in HEC-2, and the future ice modeling capabilities ofHEC-RAS, are discussed. Analyses of the Platte River
<br />at North Bend, Nebraska will be used to illustrate that revisions are needed in Appendix 3, Analysis ofIce Jam Flooding, ofFEMA 37-
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