City of Balch Springs Hickory Creek Flood Protection Planning Study Final Report Page: 19
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Table 8: Manning's Roughness Coefficients by Type
Channel Overbank
n-Values n-Values
Concrete channels 0.013 --
Concrete pilot with maintained grass banks 0.025 --
Maintained grass 0.035 --
Weeds and brush, irregular channel (overflow) 0.055 --
Medium tree canopy 0.065 --
Heavy tree canopy 0.075 --
Grass -- 0.045
Gravel, construction -- 0.055
Scattered trees, flow obstructions -- 0.060
Light to medium trees with some open space -- 0.070
Medium tree cover -- 0.080
Medium residential with large streets in flow direction -- 0.090
Heavy tree cover with some open space -- 0.090
Industrial, commercial with some open space (no fencing) -- 0.090
Heavy tree cover, residential with some open space -- 0.100
Dense residential (small, fenced lots) -- 0.120
3.5. Results of Hydrologic and Hydraulic Analyses
3.5.1. Peak Discharges
Peak flood discharges calculated for this study include the 2-, 5-, 10-, 25-, 50-, 100-,
250-, and 500-year flood frequencies for both existing land use conditions and projected
ultimate land use conditions. Ultimate land use conditions were analyzed with channel
flood routing data based on existing channels and bridges. Peak discharges were
computed at the downstream end of each sub-basin. Tables 9 and 10 display peak
discharges in cubic feet per second (cfs) for existing and ultimate conditions,
respectively.
3.5.2. Water Surface Elevations
Peak WSELs were calculated for the 2-, 5-, 10-, 25-, 50-, 100-, 250-, and 500-year flood
frequencies for both existing land use conditions and projected ultimate land use
conditions. Flood profiles and HEC-RAS output tables (Existing and Ultimate
conditions) are included in Appendix E.
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Halff Associates. City of Balch Springs Hickory Creek Flood Protection Planning Study Final Report, report, July 29, 2011; Richardson, Texas. (https://texashistory.unt.edu/ark:/67531/metapth838691/m1/30/: accessed July 18, 2024), University of North Texas Libraries, The Portal to Texas History, https://texashistory.unt.edu; crediting UNT Libraries Government Documents Department.