§ 15.01.150. Drainage system plan.

The drainage plan shall include an analysis of potential drainage problems, along with a proposal indicating how the surface water will be disposed of. Detention basins may be required to alleviate the impact on existing drainage facilities. Said plan shall also include the projected quantity of waters anticipated for a ten-year storm (piping), one-hundred-year storm (detention facilities, if required), and one-hundred-year storm (retention facilities). All drainage facilities shall be installed in conformance with approved city drainage master plans.

Storm drainage pipe design standards to be in harmony with the American Fork City Storm Water Technical Manual. Thereby requiring piping of the 100-year return frequency storm event with the provision that storm water conveyance pipes may be sized for the 25-year return frequency storm event provided the 100-year event can be routed overland to a catchment directing the 100-year event to the detention or retention basin.

The development shall include necessary culverts, drain pipes, basins, and drainage channels. In order to insure the safety of the occupants of a subdivision, the planning commission may require the developer to cover or fence culverts and canals.

In areas where the highest water level in the ground is no closer than eight feet to the ground surface and percolation rates are high and the area is designated on the city's drainage master plan, pre-treatment sumps may be used to dispose of surface waters. They must be designed for the ten-year storm and an auxiliary excess drainage system provided. All design data including ten-foot soil log, percolation tests, etc., must be submitted with the drainage plans. The sump design shall comply with applicable city and county requirements. In general, all discharges from development should be limited to a maximum of 0.2 cfs/acre with the utilization of on-site detention except as approved otherwise by the city engineer.

Drainage basins (detention or retention) shall be designed for a one-hundred-year storm and have a one-foot freeboard, 3:1 slope (max), and grass covering with a sprinkler system unless otherwise approved. Where possible, the design shall incorporate pipe networking to minimize open channel flow through all drainage basins.

For single lots or small areas, the above may be waived so that pre-treatment sumps can be installed or drainage directed on to private property with a drainage easement.

Allowable use of streets for initial storm runoff in terms of pavement encroachment are as follows:

Street Classification	Maximum Encroachment
Minor street, collector, arterial	No curb over-topping. Flow may inundate the shoulder only.

 

Inlet grating maximum design capacity for a standard grate is 5.0 cfs.

All drainage piping for surface and subsurface drainage (twelve-inch minimum size) shall have manholes at four-hundred-foot spacing and at angle points. Minimum slopes shall be the same as required by the Utah State Division of Health for sanitary sewers. Piping, testing, etc., shall comply with specific requirements as defined in the section of these specifications covering storm drainage requirements unless otherwise approved by the city engineer.

In addition to the standards stated herein, all design and construction of storm drainage systems shall comply with the latest version of the American Fork Storm Water Technical Manual, American Fork City Storm Water Management Program manual, American Fork City Storm Drain Element of the General Plan, and any other associated or applicable storm water management manuals or ordinance as adopted by American Fork City.

Roof drainage systems:
Commercial and Multi-Family Applications: Roof drainage systems shall be designed such that all run-off from primary collection roof drains shall be routed to the underground storm water management system without creating a collected flow condition either across sidewalks or parking lot pavement systems. In all cases, roof drainage systems shall be designed in accordance with the adopted city building codes.

Roof drain retention systems are acceptable provided that they comply with all grading provisions of the adopted city building code and are sized to contain the one-hundred-year design event with no percolation used for calculation purposes to account for frozen conditions in American Fork City's winter climate. Roof drain retention systems are not allowed in sensitive lands areas of the city as defined by the city sensitive land ordinance.

Single-family residential applications: Roof drainage systems shall be constructed in accordance with the adopted city building codes. These systems shall be installed to prevent collected flow conditions on sidewalks or driveways and shall not discharge onto the adjacent property. Generally, roof drainage shall be either discharged to an approved street drainage system or retained on site and are sized to contain the one-hundred-year design event with no percolation used for calculation purposes to account for frozen conditions in American Fork City's winter climate. Roof drain retention systems are not allowed in sensitive lands areas of the city as defined by the city sensitive land ordinance.

General property drainage: American Fork City code prohibits the discharge of storm waters onto an adjacent site. Where minor off-site or off-property landscape sheet-flow storm drainage discharge occurs, drainage systems shall comply with the laws of the State of Utah regarding off-site discharge of water. Applicant shall prove compliance with applicable laws where such drainage is proposed to occur. Substantial or any type collected drainage is prohibited from leaving a given site outside of an approved storm drainage management and discharge system.

(Ord. No. 2007-07-31, § 1.7, 7-10-2007; Ord. No. 2011-06-17, § 1, 6-28-2011; Ord. No. 2012-05-15, § 1, 5-8-2012)