City of Frisco Engineering Standards

Engineering Standards
Section 4 – Drainage Design Requirements
4.01 General
A. This section contains the minimum storm drainage design criteria to be followed in the design of
storm drainage facilities and demonstrates the design procedures to be used on drainage projects
in the City of Frisco.
B. The design factors, formulas, graphs and procedures described are intended to serve as guidelines.
Responsibility for the actual design remains with the Engineer. Deviation from the requirements
of these standards shall be approved by the Director of Engineering Services.
C. The Engineer shall prepare the design plans in accordance with the standard design checklist.
D. It is the responsibility of the Engineer to provide all necessary calculations and designs described
herein. The Engineer shall provide the City the data, calculations, and designs necessary to
demonstrate the design does not adversely impact the surrounding or downstream property and
meet local, state, and federal rules, regulations, and requirements.
E. The Engineer shall use the city’s base models for development along floodplain areas. If a model
is not available, the Engineer shall be required to provide a floodplain model in accordance with
city standards and guidelines.
4.02 Determining Design Discharge
The Rational Method may be used to determine the runoff generated from a property when a
contributing drainage area is less than 200 acres. A unit hydrograph method shall be used to
determine the runoff generated from a property with a contributing drainage area greater than 200
acres. The Director of Engineering Services may require developments with contributing drainage
areas less than 200 acres to use a unit hydrograph method if the hydrologic results of the
contributing drainage area or components within the drainage area more accurately reflect a unit
hydrograph.
A. Rational Formula (Drainage Areas < 200 acres)
The Rational Formula for computing peak runoff rates is as follows:
Q = C ∗ I ∗ A
Q = runoff rate (cfs)
C = runoff coefficient (dimensionless)
I = rainfall intensity (in/hr)
A = drainage area (ac)
B. Runoff Coefficient (C)
1. Runoff coefficients shall be based on the future land use plan, which is included in the City’s
Comprehensive Plan. Runoff coefficients reflecting other conditions may be used based on
the guidelines set forth in Section 4.02.F.
Section 4 – Drainage Design Requirements May 2012 Page 4-2