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Eastlake-SY130601 (3)DENTON CREEK –KING’S LAKE RESIDENTIAL DEVELOPMENT CLOMR SUBMITTAL JUNE 2013 5.0 Fully Developed Condition 12 K:\DAL_Hydro\069308201-Cummings Tract\Docs\CLOMR Report.docx Copyright © 2013 Kimley-Horn and Associates, Inc. The results indicate that there are no increases in 100-year water surface elevations within the subject reach. Output from the fully developed hydraulic models is included in Appendix E. 5.4 VALLEY STORAGE KHA evaluated the difference in valley storage between pre-developed and proposed conditions for the combined Westhaven and King’s Lake developments using HEC- RAS. The pre-developed condition is defined as existing conditions per the Westhaven CLOMR (Case Number 12-06-3818R). The respective RAS model from the Westhaven CLOMR was used to determine pre-developed valley storage volume. Table 5 summarizes the pre-developed and proposed cumulative volume comparison using HEC-RAS. Table 5: Pre-Developed and Proposed 100-Year Valley Storage Analysis River Station Cumulative Volume (ac-ft) Pre- Developed Proposed Pre- Developed Proposed 394+36 394+36 812.8 765.2 349+50 349+50 216.0 216.0 Volume within project limits:596.8 549.2 Volume Reduction = 8.0% The City Floodplain Ordinance states projects may decrease the 100-year floodplain storage by 15% if the drainage area associated with the floodplain is less than 100 square miles. According to the FEMA Flood Insurance Study, the drainage area of Denton Creek downstream of Grapevine Lake is 10.3 square miles at the project area. Based on the HEC-RAS output for both conditions, the anticipated valley storage loss from pre-developed to proposed condition is 8%. As mentioned in the response to comment letter KHA prepared for the City for the Westhaven CLOMR submittal, KHA acknowledges that the City of Coppell Floodplain Ordinance indicates that valley storage must be computed for the Standard Project Flood (SPF). KHA reviewed readily available information for Denton Creek, including the “City-wide Storm Water Management Study,” but was unable to find a flow associated with the SPF for Denton Creek. KHA also followed up with Russell Erskine regarding this comment for the Westhaven development. During the discussion, he indicated that the Floodplain Ordinance requirement was probably intended for development along the Elm Fork of the Trinity River. With the lack of information DENTON CREEK –KING’S LAKE RESIDENTIAL DEVELOPMENT CLOMR SUBMITTAL JUNE 2013 5.0 Fully Developed Condition 13 K:\DAL_Hydro\069308201-Cummings Tract\Docs\CLOMR Report.docx Copyright © 2013 Kimley-Horn and Associates, Inc. regarding the SPF flow, he recommended not performing the SPF valley storage analysis. Therefore, KHA is requesting that the SPF valley storage requirement be dismissed for the King’s Lake development as well.