A. All development except for the following specific exclusions shall be subject to the requirements of this section:
1. Single-family residential homes being constructed on lots final platted prior to December 31, 2007, or on unplatted properties.
2. Other residential housing being constructed on lots containing two (2) acres or less final platted or on unplatted properties where total ownership prior to December 31, 2007, consists of two (2) acres or less. It is the intent of this section that larger parcels may not be subdivided into smaller parcels to qualify for exclusion.
B. In drainage areas with known downstream flooding of structures, or if it is determined that development of subject property will cause or contribute to flooding or sedimentation of existing structures downstream, the developer shall install detention facilities maintaining a discharge rate not to exceed the historical runoff rate prior to development.
1. For common drainage locations that serve an area with ten (10) or more disturbed acres at one time, a temporary (or permanent) sediment basin providing three thousand six hundred (3,600) cubic feet of storage per acre drained or equivalent control measures, shall be provided where attainable until final stabilization of the site. The three thousand six hundred (3,600) cubic feet of storage area per acre drained does not apply to flows from off site areas and flows from on site areas that are either undisturbed or have undergone final stabilization where such flows are diverted around both the disturbed area and the sediment basin. Where providing three thousand six hundred (3,600) cubic feet of storage per acre drained is not attainable, smaller sediment basins and/or sediment traps should be used.
2. After the construction activities have been completed and site has undergone final stabilization, upon the approval of the manager structures may be removed.
C. In drainage areas where the city has no record of downstream flooding of structures and development of the subject property using a runoff coefficient of seventy percent (70%) would not cause downstream flooding of existing structures, detention will not be required. Detention storage will be required for the increased runoff resulting from development having imperviousness in excess of seventy percent (70%) for all developments.
D. Detention shall not be required in drainage areas where the city has no record of downstream flooding of structures and drainage calculations provided by the developer or the city indicate projected flooding of existing downstream structures would not occur assuming the drainage basin were totally developed utilizing maximum projected land use as indicated in the long range comprehensive plan.
E. When it has been determined on site detention is required, engineering plans and drainage calculations shall be provided to the community development department for review and approval when filing a final plat or applying for a building permit on unplatted property.
F. The required volume for stormwater detention shall be calculated on the basis of the runoff from all storms from the ten percent (10%) chance storm up to the one percent (1%) chance storm. This volume of storage shall be provided for the fully developed watershed that is tributary to the area designated for detention purposes. The stormwater release rate shall be considered when calculating the stormwater storage capacity and the control structure designed to maintain a discharge rate not to exceed the rate of existing conditions regardless of the depth of stormwater in the storage area.
G. Public regional detention facilities may be designed with one percent (1%) chance storm outflow rates equal to or lower than predevelopment values with possible increased flow rates for the twenty percent (20%) chance, ten percent (10%) chance, and two percent (2%) chance storms from predevelopment flow rates under those conditions where downstream areas are not adversely impacted.
H. If the development is situated in such a manner that the stormwater is discharged into a stormwater system which the city manager determines will not be adversely affected, the developer may make a monetary payment or some other form of valuable consideration to the city in accordance with section 13.42.435 of this chapter, in lieu of constructing an on site detention facility.
I. All applications for payment of a fee in lieu of on site detention requirements shall be submitted to the community development department with the following information for review:
1. Plot plan drawn to scale showing location, property dimensions, and proposed construction or development including all impervious surfaces.
2. A summary showing total impervious surfaces in square feet.
After receipt of the above, the community development department may accept payment of a fee in lieu of on site detention facilities or may deny the application and require on site detention or other improvement to the stormwater control system.
J. Fees to be accepted shall be tendered prior to the issuance of a building permit. Should the impervious surfaces on a given property at the time of application for an occupancy or use permit be less than that stated on the building permit, then the permittee may request that the permit fees be recalculated, the excess fees refunded to the payee and the building permit amended. Should the impervious surfaces on a given property be greater than stated on the building permit, additional fees shall be due pursuant to a recalculation of the fees and must be tendered before any occupancy or use permit may be issued.
K. "Impervious surface" shall mean any hard surfaced areas which prevent or retard the entry of water into the soil in the manner and to the extent that such water entered the soil under natural conditions, or where water is caused to run off the surface in greater quantities or at an increased rate of flow than was present under natural conditions. Impervious surfaces shall include, but are not limited to, rooftops except roof overhangs; sidewalks; paving; driveways; parking lots; walkways; patio areas; storage areas; and asphalt, concrete, gravel, oiled macadam or other surfaces which similarly affect the natural infiltration or runoff patterns of real property in its natural state.
L. Impervious surface installed or constructed as a part or portion of a public street or a private or public sidewalk in the public street right of way shall not be calculated as a part of the fee collected in lieu of on site detention.
M. Fees accepted shall be deposited in a separate account; fees shall be utilized for the costs and expenses incurred or to be incurred in evaluating, preventing, reducing, eliminating, or attempting to eliminate, prevent, or reduce the known or projected flooding problems in the watershed of the subject property; and shall be utilized to maintain such facilities and stormwater control systems.
N. Should adequate fees have been accepted and all existing and projected flooding problems in a given watershed been resolved, on site detention requirements, and fees in lieu thereof, shall be utilized to maintain stormwater control systems in that watershed.
O. The fees in lieu of on site detention shall be in the amounts established hereafter.
P. As used in this section, the fees tendered hereunder shall be deemed accepted when the funds required are actually received by the city treasurer.
Q. Whenever a property upon which an impervious surface is installed or constructed lies within two (2) different watersheds, the fees shall be proportionately applied based upon the amount of impervious surface within each watershed. Whenever a property upon which an impervious surface is installed or constructed lies within and without a watershed requiring on site detention facilities, or fees in lieu thereof, the fees shall be accepted and applied based upon the amount of impervious surface within the watershed so requiring.
R. It is unlawful for any person to erect, construct, install, enlarge, move, make, put together, or convert any building, structure, improvement, facility, or impervious surface within the city, or cause the same to be done, without first paying any fee in lieu of detention required by this section.
S. The number of square feet of impervious surface installed or constructed shall be established as follows:
a = b - c | ||
Where: | ||
a | = | The number of square feet of impervious surface for which the applicant must pay a fee. |
b | = | The total square feet of impervious surface at the property as set forth in the new building permit or any recalculation thereof as provided in subsection J of this section. |
c | = | The total square feet of impervious surface on the existing building permit, if any, for the property on the effective date of this section or such subsequent building permit appropriately issued, whichever reflects the greater total square feet of impervious surface already authorized on said property. |
After receipt of the above, the community development department shall make a report listing known downstream flooding locations and projected increase in runoff from the proposed construction site.
T. Changes may be made to streams or channels within the regulatory flood fringe area, subject to approval of the city manager, provided that:
1. The volume of floodwater storage is not reduced;
2. Neither downstream or upstream water surface elevations are adversely increased; and
3. All required permits are obtained prior to starting work.
U. Stormwater detention or other storage facilities shall not be located in street rights of way or easements unless it is placed in an enclosed storm sewer system.
V. Outlet control structures shall be designed as simply as possible and shall require little or no attention for proper operation. Each stormwater storage area shall be provided with a method of emergency overflow in the event that a storm in excess of the regulatory flood occurs. This emergency overflow facility shall be designed to function without attention and shall become a part of the "natural" or surface channel system. Hydraulic calculations shall be submitted to substantiate all design features. Detention storage facilities herein provided for shall not be dedicated to the public for public maintenance unless located on a primary or secondary drainage channel. All private on site facilities shall be properly maintained by the owner such that they do not become a nuisance. Nuisance conditions shall include, but not be limited to, improper storage resulting in uncontrolled runoff and overflow, stagnant water with concomitant algae growth, insect breeding and odors, discarded debris, safety hazards created by the facility's operation and silting of the facility decreasing the design storage capacity by twenty percent (20%) or more. All detention facilities located on private property shall be accessible at all times for inspection.
W. Where development of a property presents the threat of increased flooding or damage by runoff to downstream structures, the facilities for stormwater runoff control shall be constructed prior to any earthmoving or drainage construction on the project site.
X. The construction of the storm sewer water control system shall be accomplished as part of the cost of land development. If the amount of storage capacity can be increased to provide benefit to the municipality, the city may, but is not required to, participate in the construction cost. In any event, the city's participation is subject to the availability of funds and a written agreement with the city.
Y. When it has been determined by the department that alternative methods of protecting downstream properties can be accomplished without causing substantial detriment to the public good, safety or welfare or without being contrary to the spirit, purpose and intent of this chapter, the city manager may accept a fee in lieu of requiring on site detention facilities. (Ord. 2017, 2014: Ord. 1969, 2011)