City of Frisco Engineering Standards

Engineering Standards
The Engineer shall include a completed Storm Drain Calculations Spreadsheet (see Figure 3) in
the construction plans. A description of the runoff calculations is provided below followed by a
description of the hydraulic design calculations:
• Column 1: Enter the downstream storm drain station number.
• Column 2: Enter the upstream storm drain station number. This is the design point.
Design should start at the farthest upstream point.
• Column 3: Enter the distance (in feet) between the storm drain stations.
• Column 4: Enter the designation of the drainage area(s) at the design point in Column 2
corresponding to the designations shown on the drainage area map.
• Column 5: Enter the area in acres for the drainage area identified in Column 4.
• Column 6: Enter the total drainage area in acres within the system corresponding to
storm drain station shown in Column 2.
• Column 7: Enter the runoff coefficient “C” for the drainage area shown in Column 5.
• Column 8: Multiply Column 5 by Column 7 for each area.
• Column 9: Determine the total “CA” for the drainage system corresponding to the inlet
or manhole shown in Column 2.
• Column 10: Determine inlet time of concentration.
• Column 11: Determine flow time in the storm drain in minutes. The flow time is equal
to the distance in Column 3 divided by 60 times the velocity of flow through the storm
drain in ft/sec.
• Column 12: Total time of concentration in minutes. Column 10 plus Column 11. Note
that time of concentration only changes at a downstream junction with another drainage
area(s). It remains the same from an inlet or junction to the next inlet or junction picking
up additional drainage areas. The junction of two paired inlets with each other is not a
downstream junction.
• Column 13: The intensity of rainfall in inches per hour for the 100-year storm frequency.
• Column 14: The 100-year storm runoff in cfs. Column 9 times Column 13.
• Column 15: The proposed inlet carryover from upstream inlets during a 100-year storm.
This should generally correspond to the carryover flow in Column 10 of the Inlet
Spreadsheet (minor variances may occur due to travel time routing in the Hydraulics
Table).
• Column 16: The proposed inlet carryover during a 100-year storm. This should
generally correspond to the carryover flow “q” in Column 38 of the Inlet Spreadsheet
(minor variances may occur due to travel time routing in the Hydraulics Table).
• Column 17: Design Discharge for the storm drain system (“Qpipe”) in cfs
Section 4 – Drainage Design Requirements May 2012 Page 4-23