City of Frisco Engineering Standards

Engineering Standards
4. Coincident peak flows can also be considered using Table 4.4.
Table 4.4: Frequencies for Coincidental Occurrences
Area Ratio
100 year design
Main Stream
Tributary
10000:1 2 100
100 2
1000:1 10 100
100 10
100:1 25 100
100 25
10:1 50 100
100 50
1:1 100 100
100 100
5. Modeling Requirements
HEC-HMS shall be used in developing all hydrologic models. Other hydrologic models may
be used upon approval from Engineering Services. The following criteria should be used:
a. 24-hour storm duration using an SCS Type II distribution.
b. Rainfall values calculated using coefficients provided in Table 4.3.
c. The SCS Curve Number (CN) method shall be used to determine the loss rate. CN
values shall be taken from TR-55.
d. Tc values shall be calculated as shown in Section 4.02.C.
e. Muskingum Cunge method shall be used for routing of the Unit Hydrograph through the
drainage system. Engineering Services may request for other routing methods, such as
the Modified Puls Routing Method, to be used in the design if the method is more
characteristic of the hydrologic and hydraulic conditions of the watershed.
F. Runoff from Off-Site Developments
1. Off-site Flows for Developed Upstream Watershed
The Engineer may take the effects of upstream detention into account if the hydrologic and
hydraulic information for the existing upstream ponds are shown on the construction plans
and the information can be verified by record drawings or a record survey. An emergency
overflow path between the existing detention ponds and the proposed site shall be identified
and clearly indicated on the construction plans. The Engineer shall confirm the hydrologic
and hydraulic effects of upstream facilities in accordance with Section 4.9.
Section 4 – Drainage Design Requirements May 2012 Page 4-6