City of Frisco Engineering Standards

Engineering Standards
Downstream Assessments shall be prepared and submitted to the City with the construction plans.
The study shall demonstrate the development will produce no adverse impacts. No adverse
impacts may include, but are not limited to:
a. No new or increased flooding of existing insurable (FEMA) structures (habitable
buildings).
b. No increases in water surface elevations for the 2, 5, 25, and 100 year storm events unless
contained in existing channel (with freeboard), roadway, drainage easement and ROW.
Dry lane and gutter capacity requirements set forth in Section 4.04.A shall also be met.
c. Post-development channel velocities shall not be increased by more than 5% above predevelopment
velocities. Exceptions to these criteria require a certified
geotechnical/geomorphologic study that provides documentation that a higher velocity
will not increase erosion.
d. No increases in downstream discharges caused by the proposed development that, in
combination with existing discharges, exceeds the existing capacity of the downstream
storm drainage system.
e. The Downstream Assessment shall extend to a point downstream where the proposed
development creates no adverse impacts. The downstream point is known as the Zone of
Influence. The Engineer shall determine how far downstream the analysis for the
Downstream Assessment shall extend. For properties less than 20 acres, the Downstream
Assessment may use the 10% rule to determine the Zone of Influence. For all other
properties, the Zone of Influence will be defined by a detailed hydrologic and hydraulic
modeling analysis.
f. The 10% rule states the Zone of Influence can be considered to be the point where the
drainage area controlled by the drainage facility comprises 10% of the total drainage area.
As an example, if a structural control drains 10 acres, the Zone of Influence ends at a
point where the total drainage area is at least 100 acres.
g. If a portion of a larger property is being developed, the Zone of Influence shall be
determined based on the entire property.
4.04 Street Capacity
A. Straight Crown Streets:
1. All straight crown street capacities shall be hydraulically designed using Manning’s equation:
Q = 1.486
n A R2 3 S 1 2
Q = Gutter flow (cfs)
n = Manning’s roughness coefficient, (0.0175 for concrete street)
A = Cross section flow area (ft 2 )
R = Hydraulic radius of the conduit in feet, which is the area of the flow divided by
the wetted perimeter (R=A/P)
P = Wetted perimeter (ft)
S = Slope of the hydraulic gradient (ft/ft)
Section 4 – Drainage Design Requirements May 2012 Page 4-9