City of Balch Springs Hickory Creek Flood Protection Planning Study Final Report Page: 14
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Table 6: Land Use and Curve Numbers (cont.)
Land Curve Number
Description Use Impervious (%) Condition
Code A B C D
Brushy Poor BP <50% ground cover 48 67 77 83
Light Commercial LC 50% impervious 69 80 86 89
Commercial C 85% impervious 89 92 94 95
Developing Urban DU Pervious only, no vegetation 77 86 91 94
Industrial IN 72% 81 88 91 93
Institutional IS 72% 81 88 91 93
Multi-Family MF 72% 81 88 91 93
Maintained open area OG good condition, grass cover >75% 39 61 74 80
Unmaintained open area OF fair condition, grass cover 50-75% 49 69 79 84
Single Family 1 Acre SF-1 20% 51 68 79 84
Single Family 1/2 Acre SF-0.5 25% 54 70 80 85
Single Family 1/4 Acre SF-0.25 38% 61 75 83 87
Single Family 2 Acre SF-2 12% 46 65 77 82
Transportation Highway TR completely paved 98 98 98 98
Transportation Road RD paved; open ditches including ROW 83 89 92 93
Water W 100 % impervious 100 100 100 100
Wooded WF Condition fair 36 60 73 79
Wooded WP Condition poor 45 66 77 83
Brushy Good BG > 75% ground cover 30 48 65 73
3.3.4. Unit Hydrograph Method
The SCS Dimensionless Unit Hydrograph method was used and SCS lag times were
computed for each sub-basin to generate runoff hydrographs. Existing time of
concentration (ta) was computed based on TR-55 methodology. Travel times for
channel flow were based on velocities from the hydraulic model.
The time of concentration is the summation of these phases, where:
tc = ts eet + tshalow concentrated + tpipe + tcannel
Lag times were computed using the following equation:
T = 0.6 * t .
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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/m1/25/: accessed July 18, 2024), University of North Texas Libraries, The Portal to Texas History, https://texashistory.unt.edu; crediting UNT Libraries Government Documents Department.