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4. Crossings — Bridge and culvert crossings were not included in the models, apparently to <br />minimize cost. Bridges and culverts, particularly those undersized, can have a <br />significant impact on flood inundation extents especially along the upstream side of a <br />road. In many instances, it appears that the floodplain delineations do not reflect the <br />potential backwater impacts caused by the crossings. WSB recommends that STARR <br />review and refine the floodplain delineations at all crossing locations to better represent <br />potential inundation limb. <br />5. Definition of Bank Points and Manning's n Values — Manning's n values are <br />incorrectly applied to many cross sections. Bank points are used to define breaks in <br />Manning's n values. Unfortunately, bank points are repeatedly defined incorrectly in the <br />approximate study area models (e.g. too low on the banks, on the bed of the channel, or <br />In some cases completely missing the low flow channel). When bank stations are <br />defined incorrectly, large unreasonable Manning's n values are inadvertently applied to <br />the main channel. This can lead to overly conservative water surface elevations and <br />inundation limits. An example of incorrectly defined Manning's n values is shown in <br />Figure S. If STARR is delineating bank points in GIS, an underlying surface terrain layer <br />should be used to visually determine where the tops of bank are located. If relying <br />instead on aerial photography, the banks should be determined based upon inspection <br />of the cut moss sections. Cross sections In all models should be reviewed to check <br />bank station placement and therefore, Manning's n value assignments. In some cases, <br />field verifications may be necessary. <br />B. Definition of Ineffective Areas -- Ineffective flow limits are often incorrectly defined <br />which allows conveyance through areas that do not convey flow (e.g. backwater areas, <br />areas disconnected from the main channel by natural high ground, etc.) All cross <br />sections, particularly those near road crossings, should be reviewed to determine if <br />ineffective flow limits are needed. <br />T. Downstream Boundary Conditions — Water surface elevations at the downstream and <br />of each model are often incorrect. In each model, either a specified fixed water surface <br />elevation has been applied or the computer is allowed to estimate the water surface <br />elevation using normal depth calculations at the most downstream cross section_ For <br />those models with a foxed water surface elevatlon, the ;same value is frequently applied <br />to all profiles simulated (2 -year through 500 -year events). An example of this is in the <br />Little Creek model and is shown on Figure 3. For models that use normal depth, the <br />slope of the energy grade (often estimated as the slope of the channel profile or valley) <br />must be entered. In many cases, the slope applied is incorrect, which likely has led to <br />either over or underestimation of water surface elevations and inundation limits near the <br />downstream and of the model. An example of this is in the Crystal Creek Cedar Gultch <br />model and is shown on Figure 2. <br />8. Cross Section Spacing — Gross sections should be spaced close enough so that river <br />and floodplain features are adequately defined. It is conventional for cross sections to <br />be fairly evenly spaced with fighter cross sections In areas that need additional definition <br />(for example crossing locations). An example of inadequate cross section spacing <br />3 <br />