This article sets forth the minimum requirements for development of a detention or retention drainage facility within the city limits. The purpose for stating the minimum requirements is to achieve the following:
(1) 
Protect human life, health, and property.
(2) 
Minimize expenditure of public funds for maintenance of these types of drainage facilities.
(3) 
Help maintain a stable tax base and preserve land values.
(4) 
Preserve the natural beauty and aesthetics of the community.
(Code 1975, § 10¼-71)
Any person, firm, corporation, or business proposing to develop property or improve property within the city is subject to the provisions of this article. This article would also apply to anyone wanting to install a detention basin or retention basin within an existing easement, right-of-way, or FIA/FEMA floodplain.
(Code 1975, § 10¼-72)
The adoption of this article does not change any of the previous provisions of the Subdivision Ordinance No. 1982, the Public Works Design Manual (Resolution No. 95-11) or the Flood Damage Prevention Ordinance No. 2498 (article II of this chapter]. This article shall clarify, allow, and reasonably control the design, construction, and maintenance requirements of these facilities.
(Code 1975, § 10¼-73)
Detention/retention ponds will only be allowed if the proposed development is discharging into a developed downstream system that does not have the capacity to handle the runoff as mandated in the city's drainage criteria. A developed downstream system may include state department of transportation drainage facilities and/or existing city subdivisions that have drainage improvements that were constructed after 1984. Detention/retention ponds will be allowed to discharge to an undeveloped street or property if one of the following criteria is met and the development is approved by city council as part of a special use permit (SUP).
(1)
Future downstream drainage infrastructure is not required or cannot be constructed at time of development due to lack of easements or right-of-way, or
(2)
The pond discharges directly into a studied recognized watercourse or one percent chance floodplain, contributing less than ten percent of the fully developed flow into said watercourse, or
(3)
Future downstream improvements fall outside of the city's jurisdictional boundaries.
In all cases:
(1)
The pond shall be designed such that it can be abandoned if/when a downstream system is constructed.
(2)
The pond discharge shall not create adverse flooding or erosion conditions downstream and is in all cases subject to the approval of the public works department.
(3)
Discharge shall arrive on downstream properties in a similar manner as it occurred under pre-developed conditions.
(Code 1975, § 10¼-74; Ordinance 3228, § 1, adopted 12/10/2012)
The following words, terms and phrases, when used in this article, shall have the meanings ascribed to them in this section, except where the context clearly indicates a different meaning, subject to final interpretation by the public works department:
Concentrated flow
means the flow of stormwater in a swale, ditch or channel, typically occurring after an outlet point or along a recognized water course.
Design flood
means the flood having a one percent chance of being equaled or exceeded in any given year based upon fully developed watershed conditions. This is commonly known as the 100-year frequency flood, or base flood. Fully developed conditions shall be based on the most current future land use assumption plan for the city, or current zoning map, whichever produces the higher runoff coefficients for the drainage area generating the design flood discharge.
Detention basin
means a dry basin or depression constructed for the purpose of temporarily storing stormwater runoff and discharging all of that runoff over time at a rate of flow equal to or less than which would have occurred prior to installation of the basin. This definition includes all structural components proposed for the basin (i.e., inlet structures, outlet structures, walls, fences, piping, headwalls, etc.)
Federal Emergency Management Agency (FEMA)
means the federal agency which has and does assist the Federal Insurance Administration administering the National Flood Insurance Program.
Federal Insurance Administration (FIA)
means the division of the federal government which oversees the Flood Insurance Program with the help of FEMA.
Freeboard
means the vertical distance between the design flood surface and the top of an open channel, dam, levee, detention or retention basin. The freeboard allows for wave run-up, wind tide, hydraulic jump, or other design conditions without overtopping the structure.
Frequency
means the reciprocal of the exceedance probability. For example, a 100-year frequency storm is one which has a one percent chance of being equaled or exceeded in any year, and a five-year storm has a 20 percent chance of being equaled or exceeded in any year.
Hydraulics
means concerned with the determination of the maximum stage or elevation reached by the waters of a flood at a given location. A flood is said to have occurred when the maximum stage or elevation results in an overflow upon lands that are traditionally usable by man and not normally covered by water.
Hydrology
means concerned with the magnitude and frequency of the flood flow. The magnitude of the flood flow is the statement of the quantity of water that results from a given storm, whereas, the frequency is the statement of the average return or occurrence of a flood event.
Inflow hydrograph
means hydrographs used to determine the stormwater flow volume into a detention basin or retention basin. A hydrograph describes the variation in flow rate over a fixed period of time.
Landscaping plan
means a plan that identifies how the detention basin or retention basin will be landscaped. This plan will need to include the types of plants, trees, shrubs, grass, decorative fencing, etc., that will be used and will need to be approved by the city.
Normal pool elevation
means the level at which a retention basin was designed to be prior to a rainfall/runoff event. In a retention basin, this elevation will be the water surface of the conservation pool.
Open channel
means a channel, branch, creek, or stream in which water flows with a free surface.
Pond
means a still body of water located on the surface of the earth.
Rational formula
means a means of relating runoff from an area and the intensity of the storm rainfall as defined in the Public Works Design Manual.
Retention basin
means a pond which has been designed to have both a conservation pool for holding water indefinitely, and a flood storage pool for storing stormwater runoff on a temporary basis, for the purpose of reducing the peak discharge from the basin. This definition includes all structural components proposed for the basin (i.e., inlet structures, outlet structures, walls, retaining walls, fences, piping, headwalls, aeration systems, etc.).
Surface water
means water on the surface of the ground, the source of which is so temporary or limited that it cannot maintain for any considerable time a stream or body of water having well-defined and established existence. Surface water is derived from falling rains and melting snows, and continues to be such until it reaches some well-defined channel in which it concentrates and flows with other waters, whether derived from the surface or springs, and then it becomes the running water of a stream and ceases to be surface water.
Swale
means a shallow waterway. Swales are required above underground storm drains with capacity, along with the storm drain, to carry a 100-year frequency storm. The city's Public Works Design Manual requires that such swales shall be concrete lined.
Time of concentration
means the estimated time in minutes required for runoff to flow from the most remote section of the drainage area to the point at which the flow rate is to be determined.
Watercourse
means a stream of water of such well-defined existence as to make its flow valuable to the owners of the land along its course. A "recognized" watercourse is further defined as a channel, creek, or underground storm drain which has at least a ten-year conveyance of capacity without flooding adjacent property.
Watershed
means the area contributing storm runoff to a stream, pond, or drainage system.
(Code 1975, § 10¼-75; Ordinance 3228, § 1, adopted 12/10/2012)