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I I I I I I I I I I I I I I I I I I I AYRES ASSOCGATES grid cell is best described as a floodplain surface that can have a channel contained within it. The HEC-RAS model will be developed and calibrated for in-bank conditions only and will not include floodplain. The HEC-RAS model will be run for a range of flows and flow hydrographs to develop, calibrate and validate the channel portion of the FLO-2D model. The HEC-RAS and FLO-2D models are not coupled and once the FLO-2D model has the channel information included, it will serve all remaining hydraulic modeling purposes. The FLO-2D model grid cells will also have other small channels, levees, embankments, and structures included to fully represent the South Boulder Creek flood flow conditions. The grid cell size for the FLO-2D model will be selected based on several criteria. These criteria include channel size, topographic variability, accuracy of the topographic data, and computer simulation time. Too small a grid size would result in unnecessarily long run times and may not actually improve model results. This is because slope information may not accurately be reflected in a very small grid size. For example, a 1-centimeter error in grid cell elevation between 1-meter cells would result in a ground slope error of 0.01. The error would be greater than the slope of much of this modeled area. A 1-centimeter error between 1 O-meter cells would result in a ground slope error of 0.001, which is approaching acceptable levels. Larger grid cells do not require as accurate topographic information to depict slope trends, but detailed topographic information is still important. For too large a grid cell, the model may not fully include all of the actual topographic variability. For his dissertation research, Dr. Zevenbergen developed a grid-based rainfall-runoff model with several similarities to FLO-2D. He also developed a procedure to identify the optimum grid cell size based on range of topographic variability and the accuracy of the elevation data. This procedure could be adapted to FLO-2D grid development for this project. We anticipate that a grid cell size of between 20- and 30-meters will be appropriate for this project. The FLO-2D model would be run for the 10- through 500-year flow hydrographs plus specific scenario floods. Although this project is intended to identify, but not fix, the flooding problems along South Boulder Creek, the FLO-2D model is ideally suited for testing alternatives for flood control and response should this be a future need. Ayres Associates will develop a variety of flow visualization products for use in conveying the results to City of Boulder staff, study partners and for public meetings and presentations. These products will extend beyond those available within the FLO-2D modeling software and will include CADD analysis, flow visualization using FLO-2D output converted for compatibility with finite element analysis, and other software developed by Ayres Associates. The flow visualizations will be incorporated into the GIS resource atlas for used by the City of Boulder and other project sponsors. The results of the 2-dimensional modeling will be used to develop accurate flood delineations, high hazard areas (depth> 4.0 ft or velocity times depth> 4.0 ff/s), and input for the risk assessment. The previous hydraulic modeling for South Boulder Creek will be compared with the modeling developed for this project. The comparisons will be for in-channel flows only and for discharges up to bankfull. This comparison will identify whether results are consistent with the previous work. If there are differences, the source of any discrepancies will be investigated. Surveyed channel cross sections will also be compared with cross sections from the previous modeling to assess channel vertical and lateral stability over the past 20 years. 23