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Description: The BLE process involves using best available data and incorporating automated techniques with traditional model development procedures to produce regulatory quality flood hazard boundaries for the 1-percent annual chance event as well as estimates of flood hazard boundaries for multiple recurrence intervals. The source digital terrain data used for surface model development in support of hydrologic and hydraulic analysis as well as mapping activities were leveraged from various local, State and Federal partners. Details regarding the different datasets used are provided below in Section 1.1. Flood discharges for this study were calculated using both United States Geological Survey (USGS) regression equations and gage analyses, where stream gages with sufficient records exist. Regression equations obtained from the USGS Scientific Investigations Report (SIR) 2009-5087, Regression Equations for Estimation of Annual Peak-Streamflow Frequency for Undeveloped Watersheds in Texas Using an L-moment-Based, PRESS-Minimized, Residual-Adjusted Approach (2009) were used. PeakFQ version 7.1 was used to perform Flood Frequency Analysis (FFA) for the three gages along Denton Creek. The Hydrologic Engineering Center’s River Analysis System (HEC-RAS) program version 4.1 was used to compute water surface elevations on a stream by stream basis. All hydraulic models were computed using 1-D steady state analysis. The stream mile network that was validated for these watersheds was compiled using FEMA’s Community Needs Management Strategy (CNMS) inventory. CNMS is an inventory of flood hazard studies and flood hazard mapping needs for areas where a study is needed. This data is helpful for community officials in analyzing and depicting flood hazards to enhance the understanding of flood risks. Communities may use this information to make informed decisions on their planning and flood mitigation efforts. Table ES - 1 lists the stream miles associated with this validation analysis.

Copyright Text: The Texas Water Development Board (TWDB) contracted Halff Associates (Halff) to complete a Base Level Engineering (BLE) analysis for the HUC-08 watershed No. 12030102 – Lower West Fork Trinity in North Texas, to support the U.S. Department of Homeland Security’s Federal Emergency Management Agency’s (FEMA) Discovery process and validation of effective Zone A Special Flood Hazard Areas (SFHAs). The BLE process involves using best available data and incorporating automated techniques with standard 2-dimensional (2D) model development procedures to produce flood hazard boundaries for the 1-percent annual chance event as well as estimates of flood hazard boundaries for multiple recurrence intervals. Texas Water Development Board (TWDB) contracted AECOM to complete a Base Level Engineering (BLE) analysis for the Upper West Fork Trinity Hydrologic Unit Code (HUC) 8 in North Central Texas, to support FEMA’s Discovery process and validation of effective Zone A Special Flood Hazard Areas (SFHAs). This BLE study will provide significant data for several Texas counties previously lacking modernized flood models. FEMA Region 6 contracted Compass to complete a Base Level Engineering (BLE) analysis for Area K – North Central (TX), to support FEMA’s Discovery process and validation of effective Zone A Special Flood Hazard Areas (SFHAs). Area K includes the Elm Fork Trinity HUC-8 and Parker County in the Lower West Fork Trinity HUC-8. This report is focused on the Elm Fork Trinity HUC-8 of Area K. This BLE study will provide significant data for several Texas counties previously lacking modernized flood models. North Central Texas Council of Government (NCTCOG) contracted AECOM, through Halff Associates’ prime contract, to complete a Base Level Engineering (BLE) analysis for the Denton HUC-8 watershed in North Central Texas, to support FEMA’s Discovery process and validation of effective Zone A Special Flood Hazard Areas (SFHAs). The BLE process involves using best available data and incorporating automated techniques with traditional model development procedures to produce regulatory quality flood hazard boundaries for the 1-percent annual chance event as well as estimates of flood hazard boundaries for multiple recurrence intervals.

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