Storm and Sanitary Analysis - Time of Concentration

[jeffY7U5X]

I am using the EPA SWMM Hydrology in SSA.  The reviewer has pointed out (rightly so) that the ToC does not compute. If a sub-basin is 100% pervious or impervious it is straightforward.  But anything else, it is some combination of the two.  I have not been able to find any documentation detailing how the total ToC is tallied.  I have tried combinations of add/sub, averaging, weighting...to no avail.  Any help would be greatly appreciated. Example below:

• EPA SWMM Time of Concentration Computations Report 
• Tc = (0.94 * (L^0.6) * (n^0.6)) / ((i^0.4) * (S^0.3))
• Where: Tc = Time of Concentration (min)
• L = Flow Length (ft)
• n = Manning's Roughness
• i = Rainfall Intensity (in/hr)
• S = Slope (ft/ft)
  • Subbasin SB_DP-1
  • Flow length (ft): 112.89
  • Pervious Manning's Roughness: 0.40000
  • Impervious Manning's Roughness: 0.01500
  • Pervious Rainfall Intensity (in/hr): 0.19167
  • Impervious Rainfall Intensity (in/hr): 0.19167
  • Slope (%): 1.00000
  • Computed TOC (minutes): 65.13

[Hidden_Brain]

what is your:

1. total area (ac)

2. Eq. width (ft)

3. % impervious

4. routing set to: direct to outlet, IP2perv or perv2IP?

[jeffY7U5X]

1. Total Area = 0.38ac
2. Eq. Width = 145ft
3. % Imp = 14%
4. Direct to Outlet

[Hidden_Brain · Accepted solution]

I calculated 64.84 mins as the watershed TOC with your parameters.

Ac = 0.38 = 16553 SF

Eq. Width = 145 ft (this is the main input parameter in SWMM for TOC)

Length = 16553/145 = 114 ft (calculated)

%IP = 14

IP length = 0.14*114 = 16 ft

Perv Length = 114-16 = 98 ft

Using kinematic wave eqn:

IP

L = 16 ft

uniform rain = 0.19167 in/hr

S = 1% = 0.01 ft/ft

N = 0.015

TOC = 3 mins

PERV

L = 98 ft

uniform rain = 0.19167 in/hr

S = 1% = 0.01 ft/ft

N = 0.4

TOC = 64.84 mins

Routing Option - IP direct to outlet

Watershed TOC = larger of IP and Perv = 64.84 mins