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Report in Support of the Recertification of the University of Colorado 's Flood Control Levee Page 2 • The G &O flood plain was adopted by the UDFCD, Boulder County, the Colorado Water Conservation Board, and FEMA. It is also believed that Boulder also adopted the flood plain. At a minimum, Boulder utilized the flood plain in its planning process. • By specific evaluation or by default, the Levee was recognized as an effective flood - control facility with the adoption of the flood plain by all of the responsible parties including FEMA by 1988. • Since its construction, the Levee has provided flood protection to the gravel -mined area that once existed behind it (now open land owned by the University), the Tantra subdivision to the west, and to the Keewayden Meadows area (to the north and west). • It is now owned and maintained by the University of Colorado. • The Levee was raised and strengthened over the years by the gravel mining company that constructed it, and it has been raised and its erosion protection has been reinforced by the University. • The flood plain was studied by Love and Associates in 1996 (B -6) and by Taggart Engineering Associates (TAE) in 1996 (B -7). Both of these studies concluded that the Levee was high enough to not be overtopped by the 100 -year flood (as it existed at that time). • LRE prepared a detailed evaluation of the Levee in support of a request for certification by FEMA in 1999. The evaluation included geotechnical, surveying, and erosion - protection analyses by other engineering companies. LRE divided the Levee into three reaches. • The upstream reach of Levee was certified by FEMA in a letter dated June 15, 2000 after an extensive review of the material submitted by LRE on August 26, 1999. The letter is included here as Appendix A. The 1999 LRE report Appendix B and supporting material Appendices B -1 through B -11 are included with this submittal. • The Levee certification subsequently expired, because additional information that was requested by FEMA was not submitted within 90 days. At the time of the certification, FEMA was aware that Boulder was in the process of restudying the flood hydrology, and that Boulder believed that resulting 100 year flood would "...be significantly higher than the effective discharge that was used to prepare the FIRM ... ". • After several years of study, Boulder submitted the revised 100 -year flood hydrology to FEMA for review and approval in 2007. In the area of the vicinity of the Levee, the "un- routed" peak discharge was reported to be 7,690 CFS. • Boulder also commissioned a detailed evaluation of the 100 -year flood plain that used proprietary one - dimensional (1 -D) and two - dimensional (2 -D) hydraulic models (MIKE Flood). The results of the hydrologic studies were submitted to FEMA in 2007 and 2008, and the review is still underway. After the flood was routed through the flood plain (in the hydraulic study described below), the reported peak flood in the vicinity of the Levee was reduced down to 5,850 CFS, which is somewhat lower than the current "Regulatory" flood (6,200 CFS), and considerably lower than the flood used in the TEA study (9,430 CFS). • This report utilizes the output from the 1 -D and 2 -D hydraulic studies, and the two flood - plain delineations prepared for Boulder to evaluate the freeboard of the Levee. • Based on the hydraulic information from the 1 -D and 2 -D models, LRE identified two reaches of Levee that needed to be increased in height in order for the Levee to continue to meet the required three to three and one -half foot freeboard criteria. Those reaches of © Leonard Rice Engineers, Inc. June 2009 — 207FIP05