The design criteria shown or referenced in this division are the minimum requirements to be allowed. The detention/retention basins are to be designed, signed, sealed, and dated by a professional civil engineer registered in the state, and shall meet all of the city's design standards including, but not limited to, those specified in the Public Works Design Manual and the current practices and policies followed by the public works department. The public works director may, in some situations, require the owner/developer's engineer to follow more stringent design guidelines if, in the opinion of the public works director, the situation warrants more than the minimum. Geotechnical and/or soil studies and reports may be requested from the owner/developer's engineer if, in the opinion of the public works director, it is warranted for the proposed design. Any deviation from this criteria must be reviewed and approved by the public works department for use by the developer and his engineer.
(Code 1975, § 10¼-81)
Detention/retention facilities shall be designed based on a 100-year frequency storm runoff for the upstream drainage watershed area.
(1) 
Rational method.
The "rational method" can be used to calculate the runoff rate and volume for drainage areas less than 250 acres. The formula for this method is Q = CIA.
(2) 
Hydrograph method.
The "hydrograph method" will be used to calculate the runoff rate and volume for drainage areas equal to or greater than 250 acres. The HEC-HMS method developed by the Hydrologic Engineering Center (HEC) of the U.S. Army Corps of Engineers shall be used. Use of other hydrograph methods must be approved by the public works department prior to beginning design.
(Code 1975, § 10¼-82; Ordinance 3228, § 1, adopted 12/10/2012)
The flood routing computer program to be used shall be either HEC-2 or HEC-RAS from the U.S. Army Corps of Engineers. Use of other computer program routing methods must be approved by the public works department prior to beginning design.
(Code 1975, § 10¼-83; Ordinance 3228, § 1, adopted 12/10/2012)
(a) 
Inlet.
The inlet structure shall allow for the upstream 100-year runoff to be discharged to the pond in a manner which minimizes erosion.
(b) 
Outlet.
The outlet structure for detention basin/retention basin shall be constructed to minimize erosion and allow for the 2-, 5-, 25-, 50- and 100-year storm runoff to be discharged in a controlled manner. An outlet control structure will be installed on detention/retention basins to achieve the design discharge equal to or less than the runoff rate which existed from the watershed prior to this proposed development. On the outlet structure for retention basins, an emergency outlet valve and pipe shall be installed. The valve and pipe shall be at least eight inches in diameter and meet all city requirements for water system pipe and valves. Installation of the emergency outlet valve and pipe shall allow the retention basin to be drained to within a maximum of one foot of its design bottom. In addition:
(1) 
Outlet structures shall be located at least 50 feet from a property line and shall be designed to minimize potential erosion.
(2) 
Discharge rates shall not increase for the 2-, 5-, 25-, 50- and 100-year storms.
(3) 
Outlet velocity: No significant increase (maximum five percent) is allowed in concentrated flow velocities for the 2-, 5-, 25-, 50- and 100-year storms. Post-development concentrated flow velocities cannot be increased by more than five percent above pre-development velocities. If existing concentrated flow velocities exceed six feet per second, no additional increase in velocities will be allowed.
(4) 
Flood heights (depth of flow) shall not increase across downstream properties.
(c) 
Storage.
The detention/retention storage basin shall be designed to store that volume required to reduce the discharge rate out of the basin to not more than the runoff rate which existed from the watershed prior to this proposed development which includes the basin. The detention/retention storage volume, excluding conservation pool, shall drain out completely within 24 hours of the end of a rainfall event.
(d) 
Freeboard.
A minimum freeboard of one foot shall be required. This will be the difference in elevation between the 100-year storm design water surface elevation, plus increased elevation created by wave action, wind time or hydraulic jump, anywhere on the detention/retention basin or decorative pond, and the lowest point on the surrounding embankment within the drainage easement.
(e) 
Side slopes.
The embankment side slopes for the different basins or ponds will be as noted below. The top of the side slope shall be a maximum of 12 inches above the surrounding ground. The embankment of the basin shall not "act" as a levee. The 100-year event shall not be higher than the surrounding ground.
(1) 
Detention basin.
Slopes shall be five to one or milder.
(2) 
Retention basin.
Slopes shall be four to one below normal pool depth and five to one above.
(f) 
Overbank.
The overbank of the retention basin (and all detention basins) shall be block sodded with a public works department-approved native grass. The grass must be relatively established prior to final approval of the construction by the city.
(g) 
Depth.
(1) 
Retention basin.
The minimum depth of the retention basin shall be four feet from the normal pool elevation to the bottom between the toe of slopes. The maximum depth of a retention basin shall be no greater than ten feet.
(2) 
Detention basin.
The maximum depth of the detention storage in a parking lot can be designed is eight inches. For all other detention storage basins, the maximum depth shall not exceed ten feet.
(h) 
Velocity.
The average velocity of flow through the detention or retention basin shall be no greater than five feet per second.
(i) 
Erosion control.
Erosion control upstream and downstream of the basins shall be considered in the design. Erosion control systems shall be installed where necessary as determined by the public works department.
(j) 
Aeration.
All retention basins will require aeration systems to be installed. The aeration system will be designed to keep the standing water from stagnating. All electrical service and maintenance costs for an aeration system shall be the owner/developer or homeowners association responsibility to pay. The aeration system will need to be approved by the public works department with the approval of the basin. The owner/developer's engineer will provide appropriate design calculations and/or shop drawings showing that the aeration system is "sized" adequately for the pond. The aeration system will need to have a screening/filter device to minimize pump blockage.
(k) 
Water supply.
Provisions shall be made to keep the water surface elevation in a retention basin at the normal pool elevation. This will require the developer to submit plans and specifications for installation of a water well. Another alternative is a separate irrigation service connection to the city's water system to provide the water supply to recharge the basin or pond as needed. The cost of the water used to recharge the basin shall be the owner/developer or homeowners association responsibility to pay. Recharge design shall comply with all state and federal requirements.
(l) 
Type of detention.
The city will consider different types of detention facilities. Underground detention and parking lot detention are two examples of alternative detention facilities that may be allowed.
(1) 
Underground detention.
Will need to include one or more access points for maintenance, and be designed to minimize maintenance.
(2) 
Parking lot detention.
Will need to be designed in the "extra" parking spaces. No detention will be allowed in the required parking spaces. As stated previously, the maximum depth for parking lot detention shall be eight inches.
(Code 1975, § 10¼-84; Ordinance 3228, § 1, adopted 12/10/2012)