2011-07-11_PERMIT FILE - M2011001 ‎(12)‎

• Permit No: M2011001
• IBM Index Class Name: PERMIT FILE
• Doc Date: 7/11/2011
• Doc Name: Third Adequacy Review Response
• From: Tetra Tech
• To: DRMS
• Media Type: D
• Archive: No

Type of Land Surface
Conveyance Coefficient, C
Heavy meadow
2.5
Tillage /field
5
Short pasture and lawns
7
Nearly bare ground
10
Grassed waterway
15
Paved areas and shallow paved swales
20
RUNOFF DRAINAGE CRITERIA MANUAL (V. 1)
Equation RO -3 is adequate for distances up to 500 feet. Note that, in some urban watersheds, the
overland flow time may be very small because flows quickly channelize.
2.4.2 Overland Travel Time
For catchments with overland and channelized flow, the time of concentration needs to be considered in
combination with the overland travel time, t,, which is calculated using the hydraulic properties of the
swale, ditch, or channel. For preliminary work, the overland travel time, t,, can be estimated with the help
of Figure RO-1 or the following equation (Guo 1999):
in which:
RO -6
L= length of overland flow (500 ft maximum for non -urban land uses, 300 ft maximum for urban
land uses)
S= average basin slope (ft/ft)
V =CS
V= velocity (ft/sec)
C,, = conveyance coefficient (from Table RO -2)
S„, = watercourse slope (ft/ft)
Table RO- 2— Conveyance Coefficient, C,
(RO-4)
The time of concentration, t is then the sum of the initial flow time, t,, and the travel time, t as per
Equation RO -2.
2.4.3 First Design Point Time. of Concentration in Urban Catchments
Using this procedure, the time of concentration at the first design point (Le., initial flow time, t,) in an
urbanized catchment should not exceed the time of concentration calculated using Equation RO -5.
t= 18 0 +10
(RO -5)
in which:
t, = maximum time of concentration at the first design point in an urban watershed (minutes)
2007 -01
Urban Drainage and Flood Control District