City of Frisco Engineering Standards

Engineering Standards
F. Storm Drain
1. Alignments of proposed storm drain systems shall use existing easements and rights-of-way.
If located within an easement, the storm sewer shall be centered within the easement. If
located within rights-of-way, the centerline of the storm sewer shall be located under paving
7’ from the back of curb. No part of the storm sewer is to be designed within the lime-treated
subgrade of a proposed pavement.
2. Horizontal and vertical curve design for storm sewers shall take into account joint closure.
3. A minimum full flow velocity of 2.5 fps and a minimum slope of 0.5% shall be maintained in
the pipe unless otherwise approved by the Director of Engineering Services.
4. Only standard sizes shall be used. The minimum allowable pipe size is 18”. Pipe sizes shall
not be decreased in the downstream direction, unless otherwise approved by the Director of
Engineering Services.
5. In situations where only the lower portion of an enclosed storm sewer system is being built,
stub-outs for future connections must be included.
6. The required storm drain capacity to meet existing and future needs, if applicable, shall be
provided.
G. Manhole Placement
The following is a list of guidelines governing the placement of storm sewer manholes to ensure
adequate accessibility of storm drainage system:
1. Storm sewer lines shall have points of access no more than 500’ apart. A manhole shall be
provided where this condition is not met.
2. A manhole shall be required where two or more pipes connect into a main at the same joint.
3. A manhole shall be located where pipe sizes or slopes change unless otherwise approved by
the Director of Engineering Services.
H. Outfall Design
The Engineer shall demonstrate the drainage from the site is conveyed to an adequate outfall. An
adequate outfall is a structure or location that is adequately designed as to not cause adverse
flooding conditions, erosion, or any other adverse impacts. An adequate outfall shall also have
capacity to convey the increased runoff.
4.09 Detention/Retention Facility Design
A. Detention facilities shall be designed based upon the following minimum criteria:
1. Detention shall be provided for the 2, 5, 25, and 100 year design storms based on the results
of a downstream assessment. Sites without a downstream assessment will be required to
provide detention to undeveloped runoff rates.
2. Pilot channels may be required for detention facilities for maintenance purposes if the bottom
slope is less than 2%. The Engineer shall provide a maintenance plan for the detention
Section 4 – Drainage Design Requirements May 2012 Page 4-25