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I I I I I I I I I I I I I I I I I I I HYDRAULIC ANALYSIS Hydraul i c Model The floodplain of the Rio Grande through this area is wide, shallow and analytically complex. The floodplain reported in the FrS is not questioned. The FIS floodplain was determined by separately analyzing the main river channel flow profile, the left overbank flow profile and the ri ght overbank flow profil e. Estimates of probabl e flow splits into the main channel and overbank areas were determined by the Corps to represent the actual flow condition. As reported in the previ ous secti on, the F I S fl oodway was defi ned to be basi ca lly the area between the levees which is also the main river channel area. As FEMA questioned the impact of the fill in the floodway, only the main river channel and floodway portion of the FrS floodplain computer model needed to be evaluated. The new floodway model incorporated six cross sections through the 6100-foot study reach (Station 263+00 to 324+00), compared to the two sections in the FlS model. Riverwood Estates is located, generally, between Station 268+0D and Station 290+00. The floodway model was not calibrated to duplicate the FrS base flood and floodway profiles because of the complexities mentioned above and the feet that the additional cross sections greatly refine the precision of the calculated profiles. The new model used cross sections prepared from the 1976 base maps, as in the FrS. The FIS floodway limits were set in each cross section to limit the lateral distribution of the total 100-year flood flow. For initial comparison purposes, roughness, expansion and contraction coeffi ci ents were the same for both the 1976 and 1986 condi ti ons. Also, the starting water surface level in Section 28.0 of 7544.8 from the FIS was used for both conditions. -8-