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<br />- <br /> <br /> <br />APPENDIX C - ANTECEDENT FLOOD STUDY SOW <br /> <br />CHATFIELD IDF ANTECEDENT FLOOD STUDY <br />SCOPE OF WORK <br /> <br />INTRODUCTION <br /> <br />The purpose of this study is to develop criteria for the antecedent flood to be used <br />in the Inflow Design Flood (IDF) routing for Chatfield Reservoir near Denver, Colorado. <br />As documented in ERI110-8-2 (FR), Corps of Engineers regulations for routing the <br />Inflow Design Flood require consideration of an antecedent flood of 50 percent of the <br />PMP be assumed to occur 5 days prior to the occurrence of the IDF. This study will <br />evaluate the 50 percent criteria to see if it is appropriate or if some other value would be <br />more appropriate. Statistical analysis of streamflow and meteorological data will be used <br />to determine the criteria to be used for the antecedent flood. <br /> <br />This study will be conducted by Hydrologic Engineering Branch ofthe Omaha <br />District, US Army Corps of Engineers and will be funded by the Colorado Water <br />Conservation Board. Results of this study will be incorporated into the Tri-Lakes <br />Reallocation Study, which is currently ongoing and cost shared between the Corps and <br />the State of Colorado. Required tasks for this study are described in the following <br />paragraphs. <br /> <br />REQUIRED STUDY TASKS <br /> <br />1. Develop Natural Flows. This task will involve developing natural or unregulated <br />inflows to Chatfield Reservoir for use in computing the natural flow frequency <br />relationship for Chatfield inflows. Natural or unregulated flows are those flows <br />that would have occurred without any man made effects from reservoirs or <br />diversions. Natural flows are required for the volume probability analysis since <br />historical streamflows reflect significant regulation effects upstream from <br />Chatfield Reservoir. Regulated flows do not meet the Independent and Stationary <br />tests required for statistical analysis. An HEC-HMS model will be configured <br />with records of historical streamflow, diversions, and inflow, outflow and storage <br />from upstream reservoirs. This model will be used to determine and route <br />holdouts to develop the unregulated flows. Once the period of record natural <br />flows are developed, they will be stored in an HEC-DSS database. <br /> <br />2. Natural Flow Volume Probability Analysis. Once the natural flow database is <br />developed, a volume probability analysis will be performed for durations of 1,3, <br /> <br />1 <br />