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REVISED MODELING APPROACH The primary difference between the approach used in this analysis and that used before is that this analysis blocks out the existing structures and estimates friction factors based on what exists in the actual conveyancE' area. The cross section locations and hydrologic information are unchanged from the currently adopted studies. The base ground card information used in the David J. Love & Associates analysis was obtained from the the City of Boulder and was unchanged in this analysis. Additional ground card points were added to block out structures and to more accurately define conveyance areas. The model's definition of the left and right overbank locations were also changed in order to more accurately reflet actual conditions. The revised friction factors used in this analysis were rE!viewed with Mr. Kevin Stewart of the Urban Drainage and Flood Control District and with Mr. Alan Taylor of the City of Boulder before they were utilized. The revised friction factors in this analysis are consistent with those used in existing analysis adjacent 10 Boulder Community Hospita1. Changes in the outer :floodplain limits are not requested as part of this submittal. FLOODW A Y REVISIONS The revised modeling approach was utilit.ed to morE" accurately define the location of the floodway. For the purpose of this report the 1/2 foot floodway is synonymous with the flood way. Care was taken to utilize as much as possible of the previous flood way modeling approach. Encroachment stations for cross-sections 371, 383, 410, and 420 were unchanged from the Greenhorne & O'Mata report. ']"he same top width was utilized for section 403. In establishing the initial encroachment stations for sections 391 and 401 a Method 4 equal conveyance encroachment analysis was performed. The encroachment station calculated in this analysis resulted in having a very very small portion of the flow (less that 15 feet wide) located in the North side of North Street. Given the fact the entire area between Alpine Street and North StreN was blocked out it made little sense from a regulatory standpoint to show this isobied 15 foot wide strip as f1oodway. Therefore in the final f100dway run which is provided in Appendix 13 encroachment stations were set at the south side of Alpine adjacent 1:0 the existing wall and building edge. In order 1:0 take into account any possible impact our revised modeling approach might have on the base line 100-year flood heights the flood heights as determined from the flood way run were compared to the existing David J. Love &: Associates base flood elevations. All resulting flood heights are less than :I 12 foot greater than the base flood heights. Table 2 provides the comparison. 2