A. General Requirements.
1. The city will not approve any development proposal which does not make adequate provisions for stormwater channels and/or basins. The stormwater drainage system shall be separate and independent of any sanitary sewer system. Storm drainage shall be accommodated in retention basins unless otherwise indicated by the city engineer.
2. All provisions for drainage control shall comply with the master drainage study for the city of Casa Grande. Drainage design shall also comply with drainage policy as promulgated by the city engineer.
3. If offsite flows are to be conveyed in a defined channel across development, the channel must be designed to carry a one hundred-year, two-hour storm, with a minimum freeboard of one foot.
4. Where a development is traversed by a watercourse, drainage way, channel, or stream, it is desirable that the drainage be maintained by an open channel, with landscaped banks and adequate width for maximum potential volume of flow. The watercourse channel shall be contained within a drainage easement dedicated to the city if not contained within a right-of-way. The easement shall be reflected on a subdivision plat or map of dedication.
5. The city shall not be responsible for the design, performance, operation or maintenance of the retention basin, channels or other drainage facilities, unless the maintenance is specifically accepted in conjunction with the dedication, and acceptance of, the drainage easement or drainage tract to the city.
6. Where a proposed drainage system will discharge water onto adjacent private land, outside of the historic flow location, a drainage easement for the water must be secured.
B. Retention Requirements.
1. Location and Design.
a. Stormwater retention and all retention basin capacity calculations shall comply with the master drainage study for the city of Casa Grande as well the city’s drainage policy.
b. All stormwater from a one hundred-year storm of two-hour duration, as established by the National Oceanic and Atmospheric Administration (NOAA 14), shall be retained on site. Additionally, stormwater generated by the street improvements located between the street centerline and the property boundary adjacent to the site shall be retained within the site or the adjacent right-of-way. The city engineer may approve other means to handle the developed flows generated from street improvements.
c. Developments may provide either shared retention basins and/or individual onsite retention for each parcel.
d. Residential developments may use onsite retention basins under the following conditions.
i. The basin is designed to contain the developed onsite and adjacent half-street flows generated by a one hundred-year storm with two-hour duration.
ii. Provisions shall be made that ensure the onsite basins are maintained and preserved in perpetuity by the individual property owners.
e. Shared retention basins shall be protected from further development by a recorded drainage easement.
f. Lakes or ponds used for stormwater collection will be required to meet all retention basin requirements specified herein, except for the water depth, drainage time, and side slopes below the normal water level.
g. Stormwater detention and retention facilities shall be designed and constructed according to the procedures and criteria established in this chapter, No detention or retention basin shall retain standing water longer than thirty-six hours, if the basin has not been designed and constructed to be a permanent body of water, with appropriate health, safety and water quality measures for such a body of water.
h. A maximum of fifty percent of the required retention can be held upon asphalt, concrete or other hard surface except in a special situation and with permission of the city engineer. The maximum depth of water on the hard surface shall not exceed eight inches.
i. Shallow pit percolation tests shall be performed by a licensed geotechnical professional in retention areas to determine natural percolation. Test results shall be reflected within the preliminary or final drainage report prior to approval of drainage plans. Dry wells are permitted to drain surface retention areas only when no other means of disposal is available. Infiltration into the dry well cannot be considered to reduce the size of the retention area. The property owner of record shall be responsible for the design, performance, operation or maintenance of dry wells used with onsite retention. A percolation test must be carried out on the dry well before it is considered acceptable. The percolation test results are to be filed with the city engineer.
j. Maximum Depth. Unless approved otherwise by the city engineer, all retention basins shall have a design capacity to preclude a water depth in excess of three feet resulting from a one hundred-year, two-hour storm. The depth of retention shall be measured from nearest adjacent top of curb. Unless otherwise specially approved by the city engineer, all retention basins shall have at least one foot of freeboard above the maximum water elevation. Unauthorized access should be physically restricted (i.e., by fencing or other appropriate means) where basin depth is greater than four feet and any side slopes steeper than four to one.
k. Side slopes shall conform to the following slope-depth ratios:
i. First four feet - 4:1 maximum slope.
ii. Depth beyond four feet - 8:1 maximum slope.
l. Exceptions to minimum slope requirements will be considered when innovative and aesthetically pleasing design features are presented and public safety is not compromised.
m. Retention basins may be wholly or partially located within the right-of-way, subject to the following conditions:
i. It is determined that the retention basin will not interfere with the current or future use of the right-of-way by the city for public purposes.
ii. The basin is incorporated into the landscape design for the development.
iii. The private development is responsible for the maintenance of the retention basin.
iv. The private development agrees, in writing, to hold the city harmless from any liability associated with the basin.
v. The private development agrees, in writing, to relocate the basin out of the right-of-way at their expense, if it is determined by the city that the basin is interfering with the city’s use of the right-of-way for public purposes.
2. Retention Basin Overflow.
a. The site shall be designed so that all runoff greater than the design storm is routed to the site’s ultimate outlet(s) (the point(s) from which runoff exits the site.)
b. The overflow outlet shall generally be a swale or similar conveyance, with appropriate erosion-resistance surface leading to the ultimate outlet, except that drives and roadways may, upon approval of the city engineer, be used as means of conveying overflows to the ultimate outlet.
c. The invert of the overflow channel shall coincide with the elevation of the maximum volume provided. The overflow channel shall be a minimum one-foot deep, and the maximum depth of flow shall be one-half foot. At least one-half-foot freeboard shall be provided above the depth of the overflow channel.
3. Maintenance.
a. All retention basins shall be privately owned and maintained, with the exception of retention basins dedicated to and accepted by the city. It shall be the responsibility of the developer to establish a system acceptable to the city to provide perpetual operation, maintenance and replacement of all retention areas. This maintenance system may be the formation of a homeowners’ association or some other type of maintenance entity acceptable to the city.
b. In no event shall stormwater stand in the retention basins longer than thirty-six hours.
c. In situations where a basin will not drain and the water needs to be pumped out, it may be drained into existing storm drainage lines, irrigation facilities, or streets, when approved by the city engineer.
4. Right of the City to Drain Basin When Owner Fails to Maintain.
a. It is unlawful for any person owning or controlling a retention basin to permit stormwater to stand therein longer than thirty-six hours. In addition to any penalty provided by law, should the persons owning or controlling any privately-owned and maintained basin fail, neglect or refuse to drain the retention basin within thirty-six hours, it is the right of the city, upon the authorization of the director of public works, or his appointed agent, to enter upon the privately-owned retention basin property and take such action as may reasonably be necessary to drain the basin. The draining of the basin shall be at the expense of the owner or person controlling the basin.
b. Assessment of Drainage Costs by the City. Upon completion of the work, the director of public works shall prepare or cause to be prepared, a verified statement of account of the actual cost of draining of the basin, the date the work was completed, and the street address and the legal description of the property on which the work was done, including five percent for inspection and other incidental costs in connection therewith, and shall serve a duplicate copy of the verified statement upon the person owning or controlling the property.
c. Appeal to City—Terms and Conditions. The owner or person controlling the property shall have thirty days, from the date of service upon him of the assessment, to appeal in writing to the city council from the amount of the assessment as contained in the verified statement. If an appeal is not filed with the city council within the thirty-day period, then the amount of the assessment, as determined by the director of public works, shall become final and binding. If an appeal is taken, the city council shall, at a regular meeting, hear and determine the appeal and may affirm the amount of the assessment, modify the amount thereof, or determine that no assessment at all shall be made. The decision of the city council shall be final and binding upon all persons.
d. Service of Notice to Property Owner. Notice shall be mailed to the owner or person controlling the property, by certified or registered mail, to the address to which the tax bills for the property were last mailed.
e. Lien for Drainage of Basin When. If no appeal is taken from the amount of the assessment, or if an appeal is taken and the city council has affirmed or modified the amount of the assessment, the original assessment or the assessment as so modified shall be recorded in the office of the county recorder, and from the date of its recording, shall be a lien on the lot or tract of land until paid. The liens shall be subject and inferior to the lien for general taxes and to all prior recorded mortgages and encumbrances of record. A sale of the property to satisfy a lien obtained under the provisions of this section shall be made upon judgment of foreclosure or order of sale. The city shall have the right to bring an action to enforce the lien in a court of competent jurisdiction at any time after the recording of the assessment, but failure to enforce the lien by such action shall not affect its validity. The recorded assessment shall be prima facie evidence of the truth of all matters recited thereof. A prior assessment for the purposes provided in this section shall not be a bar to a subsequent assessment or assessments for such purposes, and any number of liens on the same lot or tract of land may be enforced in the same action.
C. Bridge and Culvert Crossing for Private Drives.
1. The street crossing should be designed to convey the one hundred-year, two-hour storm under the road.
2. The construction of a channel crossing must not cause the diversion of drainage flows, except when that diversion is part of an approved plan for modification of drainage patterns.
D. Subsurface Drainage Within Street Right-of-way.
1. Subsurface drainage systems constructed within the public right-of-way shall be discouraged. Where storm sewers are required, the construction cost shall be borne by the developer, and shall be designed and constructed in accordance with standards approved by the city engineer. The city will assume maintenance cost for storm sewer systems constructed within public right-of-way upon dedication and acceptance.
2. All storm drain pipe installed in alleys or streets under curb, gutter and pavement shall be constructed of rubber-gasket, reinforced, concrete pipe, capable of withstanding H-22 highway loads or an alternate as approved by the city engineer.
E. Storm Drainage Design for Streets.
1. Streets and gutters may be used to transport storm drainage to retention basins unless otherwise indicated by the city engineer. It is expected that streets will carry water from adjacent property and from local areas, but they are not to be used as major water carriers in lieu of natural washes or man-made channels. Street runoff may be allowed, at the discretion of the city engineer, in those circumstances where stormwater can be quickly conveyed to established drainage facilities.
2. The design for stormwater carried in the streets shall be in accordance with the following.
a. Calculate street/gutter flow and velocity by using the modified Manning’s equation in Section 3 of the Drainage Manual for Maricopa County, Volume II, Hydraulics.
b. Calculate catch basin and scupper sizes according to Section 3 of the Drainage Manual for Maricopa County, Volume II, Hydraulics, and/or FHWA HEC-12 or HEC-22.
3. Flow of the stormwater shall always be confined to the road right-of-way or to drainage easements.
4. Flows from local streets across any parkway, arterial, or collector streets are not allowed for ten-year-frequency-storm flows. However, cross-street flows are acceptable on local streets.
5. Inverted crowns within public streets and roads are not allowed.
6. Street drainage design shall not increase runoff onto adjacent properties. Parkway, arterial, and collector streets shall not direct surface runoff onto local roadways.
7. Acceptable street drainage facilities include but are not limited to:
a. Curb-opening catch basin constructed to Maricopa Association of Governments (MAG) standard details or approved alternatives.
b. Grated inlets within the public rights-of-way require approval of the public works department and shall be constructed in accordance Maricopa Association of Governments (MAG) standard construction detail 537. All grates must be rated H-20 traffic loading and for bicycle traffic.
c. Under-sidewalk scupper constructed to MAG standard details or approved alternatives.
i. Curbs constructed to Maricopa Association of Governments (MAG) standard detail 220-1/220-2 or approved alternatives.
ii. Outflow pipe head walls with handrails constructed to Maricopa Association of Governments (MAG) standard detail 501-3 or approved alternatives.
iii. Culverts constructed to ADOT Highways Division Structures Section “B” standard details (31-002; June 1992).
F. Channels.
1. Where the capacity of the street section has been exceeded, the stormwater shall be diverted to a drainage channel that shall then route the runoff to either a retention basin or to the ultimate outlet for the drainage area or the project site.
2. In general, drainage channels shall be constructed of materials that will prevent erosion of the side slopes and shall be integrated into the design of the projects.
3. Channels shall be designed with adequate width and depth for maximum potential volume of flow.
4. Channels shall be designed so as to avoid erosion of the soils. The applicant shall provide information, prepared by a licensed civil engineer, of the maximum allowable velocity that can occur without the initiation of erosion in the channel(s). Drop structures shall be added as necessary to achieve the slopes required to maintain permissible velocities.
5. All the channels shall be contained within drainage easement of the appropriate width.
6. All channels shall be maintained per the approved plan by the property owner, unless specifically accepted by the city for maintenance.
7. Existing channels or watercourses shall not be diverted or relocated without specific consent of the directly impacted downstream property.
G. Channels under streets.
1. The crossing structure requirements listed below will normally apply; however, deviations from these requirements may be approved by the city engineer, with a finding that they are appropriate given the type of development or the nature of the terrain.
2. Local and minor collector streets shall have a culvert or bridge capable of carrying all of the peak flow of runoff from a two-year, one-hour storm beneath the roadway, and also capable of carrying enough of the peak flow of runoff from a ten-year, one-hour storm beneath the roads so that the portion of the flow over the road is no more than six inches deep.
3. Major collector and major or minor arterial streets shall have a culvert or bridge capable of carrying all of the peak flow of runoff from a ten-year, one-hour storm beneath the roadway, and also capable of carrying enough of the peak flow of runoff from a twenty-five-year, one-hour storm so that the portion of the flow over the road is no more than six inches deep.
4. Watercourse crossings for roads shall be designed so that all lots and structures within a development will be accessible from the boundary of that development by at least one route during the period of peak flow by runoff from a one hundred-year, two-hour storm. The boundary shall include any adjacent street or streets. Accessibility will be considered to exist if it can be demonstrated by the engineer that, at the time of the peak flow, the depth of flow over the road will be no greater than eight inches.
5. Regardless of the size of the culvert or bridge, the street crossing should be designed to convey the one hundred-year-storm runoff flow under and/or over the road to the area downstream of the crossing to which the flow would have gone in the absence of the street crossing. The construction of a channel crossing must not cause the diversion of drainage flows, except when that diversion is part of an approved plan for modification of drainage patterns.
H. Lowest Permissible Finish Floor Elevation. For structures in areas designated as flood hazard Zone X, the elevation of the lowest floor shall be constructed at an elevation a minimum of fourteen inches above the high point of natural ground within the area bound by a ten-foot perimeter offset from the structure, or fourteen inches above the top of curb elevation, as measured at the lowest portion of the adjacent street.
(Ord. 1397.15.22 § 2 (part), 2019)