Figure 1: Time of Concentration (Tc) Worksheet

Project:                                                                                      By:             Date:              

Location:                                                                                   Checked             Date:        

Circle one: Present   Developed                                                                           

Circle one: Tc      Tc through subarea                                                                         

NOTES:   Space for as many as two segments per flow type can be used for each worksheet.

      Include a map, schematic, or description of flow segments.

 

Sheet Flow (Applicable to Tc only)         Segment ID
1.   Surface description (Table 8)
2.   Manning's roughness coeff., "n" (Table 8)
3.   Flow length, L (total L ≤ 300 feet)         ft
4.   Two-year, 24-hour rainfall, P2         in
5.   Land slope, s                  ft/ft
6.   Tt = {0.007(nL)0.8}/{P20.5s0.4}   Compute Tt   hr

 

 

Shallow Concentrated Flow            Segment ID
7.   Surface description (paved or unpaved)
8.   Flow length, L                  ft
9.   Watercourse slope, s               ft/ft
10.   Average velocity, V (See equations, Table 8)   ft/s
11.   Tt =L/(3600V)      Compute Tt         hr

 

Channel Flow                  Segment ID
12.   Cross sectional flow area, a            ft2
13.   Wetted perimeter, pw               ft
14.   Hydraulic radius, r=a/pw   Compute r      ft
15.   Channel slope, s               ft/ft
16.   Manning's roughness coeff., "n"
17.   V = (1.49 r2/3s½)/n      Compute V      ft/s
18.   Flow length, L                  ft
19.   Tt = L/(3600V)      Compute Tt      hr
20.   Watershed or subarea Tc or Tt (add Tt in steps 6, 11 and 19)      hr