City of Frisco Engineering Standards

Engineering Standards
The Engineer shall compare the calculated time to the time listed in Table 4.1. If the
calculated Tc differs from the value in Table 4.1, the Engineer shall provide information to
justify the Tc calculations.
D. Rainfall Intensity (I)
The rainfall intensity (I), shall be based on the National Weather Service Rainfall Frequency Data
presented in Technical Memorandum NWS Hydro-35, dated June 1977 (2 to 100 year) and U.S.
Geologic Survey Frequency Data presented in Water Resources Investigations Report 98-4044,
dated 1998 (500 year). The intensity for a particular duration can be obtained using the
coefficients from Table 4.3. If the calculated inlet time differs from the value in Table 4.2, the
Engineer shall provide information to justify the inlet time calculations.The equation used to
determine the intensity values for various storm events and durations is provided below.
I =
b
(Tc + d) e
Refer to the following table for b, d, and e.
Table 4.3 Intensity Coefficient Table
2-year 5-year 25 year 100- year
B 81.319 82.686 106.665 112.783
D 15.788 15.497 18.069 17.572
E 0.864 0.820 0.806 0.771
E. Unit Hydrograph Method (Drainage Areas > 200 acres)
1. The use of a unit hydrograph method shall be based upon standard and accepted engineering
principles normally used in the profession subject to the approval Engineering Services.
Acceptable methods include the Soil Conservation Services (SCS) Technical Release
Number 55 (TR-55) for drainage areas 200 acres to 2,000 acres and SCS’s Technical Release
Number 20 (TR-20), or the United States Army Corps of Engineers HEC-HMS models for
drainage areas 200 acres or more.
2. The post development unit hydrograph method shall be based upon fully developed
watershed conditions assuming no effects from upstream or on-site detention facilities, unless
the requirements set forth in Section 4.03.B are met, or as directed by Engineering Services.
The Engineer should discuss the approach method with Engineering Services prior to design.
3. Circumstances that may require the use of a unit hydrograph method include open channels,
reclaiming floodplains, creating lakes, regional detention/retention facilities or building other
types of drainage related facilities on major drainage courses. The city requires fully
developed watershed conditions be used for all modeling. FEMA’s flows shall not be used as
the flows are generally based upon existing watershed conditions.
Section 4 – Drainage Design Requirements May 2012 Page 4-5