City of Frisco Engineering Standards

Engineering Standards
Version 6/19/12
2. Table 4.1 provides guidelines for runoff coefficients for typical land use within the city;
however, a weighted runoff coefficient may be used for the design if it is more representative
of the site conditions.
3. A lower runoff coefficient may be used if sustainable elements are included in the design.
The Engineer shall notify Engineering Services of the design intent and provide the necessary
data, calculations and design to support the desired runoff coefficient. All sustainable designs
are subject to approval by Engineering Services (refer to Section 4.18 for sustainable design
options).
Table 4.1 Runoff Coefficients and Inlet Time Guidelines
Land Use
Runoff Coefficient Inlet Time
“C”
(Minutes)
Single Family Residential 0.55 15
Two Family, Patio Home, Town Home 0.70 10
Multiple Family 0.80 10
Non-Residential Uses 0.90 10
Park Area 0.35 20
School 0.70 15
Church 0.80 10
Undeveloped 0.30 20
Hospital 0.90 10
Streets 0.90 10
C. Time of Concentration (Tc)
SCS methodology shall be used to determine the time of concentration (Tc). This method
separates the flow through the drainage area into sheet flow, shallow concentrated flow, and open
channel flow. The Tc is the sum of travel times for sheet flow, shallow flow and open channel
flow. The time of concentration flow path and sheet flow path shall be made available to the City
upon request.
1. Sheet Flow: The maximum allowable length for sheet flow is 300’ for undeveloped drainage
areas and 100’ for developed areas. When selecting n for sheet flow, consider cover to a
height of about 0.1’. This is the only part of the plant cover that will obstruct sheet flow. The
T t in minutes for sheet flow is determined using the following equation:
T t = 0.007(nL)0.8
(P 2 ) 0.5 S 0.4
T t = travel time (hr)
n = Manning’s roughness coefficient (Table 4.2)
L = flow length (ft)
P 2 = 2-year, 24-hour rainfall, 3.6in
S = slope of hydraulic grade line (land slope, ft/ft)
Section 4 – Drainage Design Requirements May 2012 Page 4-3