Chapter 15
FLOOD DAMAGE PREVENTION
Articles:
15-2 Storm Drainage Requirements
Article 15-1
Floodplain Regulations
Sections:
15-1-1 Designation of Authority
15-1-2 Adoption of Floodplain Regulations
Section 15-1-1 Designation of Authority
A. The city elects not to assume the responsibility of floodplain management from the flood control district of Maricopa County as provided for in A.R.S. §§ 48-3609 and 48-3610. The city engineer is designated as the NFIP floodplain administrator for the city, is responsible for coordinating with the flood control district of Maricopa County, and will serve as the city’s point of contact on NFIP issues for county, state and federal officials.
B. The floodplain administrator shall also be, at a minimum, responsible for the following:
1. Keep and maintain current flood insurance studies and flood insurance rate map(s) applicable to the city;
2. Keep and maintain copies of the most current “Floodplain Regulations for Maricopa County” at the office of the city clerk;
3. Keep and maintain elevation certificates (or other evidence of compliance) for all structures within special flood hazard areas. (Ord. 20-243 § 1; Ord. 07-129 § 1; Ord. 05-103 § 1)
Section 15-1-2 Adoption of Floodplain Regulations
A. Those public records entitled “Flood Insurance Study for Maricopa County, Arizona, and Incorporated Areas dated November 4, 2015” with accompanying flood insurance rate maps for Litchfield Park dated October 16, 2013, and all subsequent amendments and/or revisions, three copies of which shall be kept on file in the office of the city clerk, are hereby adopted by reference, as the basis for establishing the special flood hazard areas for floodplain management in the city of Litchfield Park. The special flood hazard areas documented in the flood insurance study and flood insurance rate maps are the minimum area of applicability of the floodplain management regulations and may be supplemented by studies for other areas as allowed in the regulations.
B. That public record entitled “Floodplain Regulations for Maricopa County, Arizona, Dated January 17, 2018,” three copies of which shall be kept on file in the office of the city clerk, is hereby adopted as the legal basis for implementing floodplain management in this community. (Ord. 20-243 § 1; Ord. 18-220 § 1; Ord. 07-129 § 1; Ord. 05-103 § 1)
Article 15-2
Storm Drainage Requirements
Sections:
15-2-1 Purpose
15-2-2 Conceptual Drainage Map and Report
15-2-3 Storm Drainage
15-2-4 Underground Storm Water Retention Facilities
Section 15-2-1 Purpose
A. The purpose of these requirements is to ensure that developments in the city are not subject to flooding nor will they contribute to the flooding potential of properties both upstream and downstream, during construction and after full development has occurred.
B. It is not the intent of these stipulations to abrogate sound engineering judgment, but to establish some guidelines and criteria. In general, unless modified herein, the criteria and calculations shall be as specified in the “Hydraulic Design Manual for Maricopa County, Arizona.”
Section 15-2-2 Conceptual Drainage Map and Report
The purpose of the map and report is to define the runoff, both before and after development, and indicate provisions proposed to handle on-site and off-site flows. In general, the plan is to accept off-site flows, handle these flows and the water that falls on the site in such a way that flows leaving the site shall exist in the same manner and with less velocity and quantity than occurred prior to development. In lieu of this channel, detention/retention structures or other methods could be constructed downstream, providing the developer has control over the downstream property and makes provisions to conduct the waters to a proper disposal site, such as a natural stream or a government-controlled drainage structure, and obtains written permission of the governmental agency having jurisdiction.
Section 15-2-3 Storm Drainage
A. Design Frequency. All developments must provide retention of the storm runoff generated by the one-hundred-year, six-hour storm (three inches).
B. Drainage Area. The area to be considered as generating runoffs to be retained shall be the development itself and the contributing adjacent streets.
C. Street Capacity. Streets will be designed to carry runoff from a five-year peak storm between the curbs. Arterial and major collectors (roads with four lanes for traffic or greater) shall be designed to concentrate the five-year storm runoff such that one lane in each direction is free from runoff. The peak flows from the one-hundred-year storm shall be carried within the cross-section between right-of-way lines and must not exceed four inches above top of curb. Inverted crown streets are not permitted.
D. Storm Sewer. In cases where the street flow from the design storm exceeds the street capacity, underground pipes or aesthetically pleasing channels, of sufficient size, shall be installed.
E. Retention. The right-of-way areas shall not be used for retention purposes. The retention areas shall be landscaped and shall have a maximum water depth of 3 feet and a maximum side slope of 4:1. Storm water shall not be retained in the basins longer than thirty-six hours. The basins must be drained by a gravity line.
F. Floor Elevations. Finished floor elevations for houses or other buildings shall be elevated above the runoff expected from a 100-year storm. Minimum floor elevations shall be 14 inches above the top of the low curb and a minimum of 6 inches above the top of the high curb. Basements may be approved if they are flood-proofed to a point above finish floor elevation. A registered professional engineer or architect shall certify the means of flood-proofing.
G. Culverts and Bridges. Culverts or bridges for street and alley crossings of drainageways shall be sized to carry the 100-year storm.
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Retention Calculations. |
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V= |
AC 4 |
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V =Volume to be retained (acre feet or cubic feet) |
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A =Drainage Area (acres or square feet) |
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C = Runoff Factor (see below) |
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Runoff Coefficient (for retention and rational formula use) |
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General |
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Pavement (asphalt, concrete, brick, etc.) |
0.95 |
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Roof |
0.95 |
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Grass Lawns (less than 7% slope) |
0.20 |
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Grass Lawns (more than 7% slope) |
0.35 |
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Desert Lawn or rock Lawn |
0.70 |
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Farm Land |
0.10 |
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Bare Ground (vacant lots) |
0.25 |
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Undeveloped Desert |
0.40 |
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Commercial, Industrial Area |
0.80 |
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Residential Area |
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Ranch Area 18,000 SF or Larger |
0.35 |
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Single Family Areas Less than 18,000 SF |
0.40 |
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Multi-unit Area |
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Townhouses, Mobile Home Park |
0.50 |
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Apartments |
0.60 |
H. Compliance. It is the responsibility of the developer and his engineer to comply with these provisions and to design a project which will comply with high engineering standards. City review is not to be construed as endorsement or assurance that the plans comply with these standards. The responsibility for the proper drainage of the developer’s property and the protection of adjacent property from flooding remains with the developer and his engineer.
Section 15-2-4 Underground Storm Water Retention Facilities
A. Applicability. For the purpose of this section, the following definitions shall apply unless the context clearly indicates or requires a different meaning:
1. An underground storm water retention facility shall be allowed only for a commercial land development project with a property owners association or property management company, or for a site with a single owner.
2. Underground retention facilities shall not be located within public right-of-way.
3. Underground retention facilities shall not be located under building structures or significant landscaping such as trees.
4. Underground retention facilities are prohibited for storm water runoff from areas that include fuel dispensing or fuel storage facilities. For drainage areas that include fuel dispensing or fuel storage facilities, only surface retention basins may be used along with an Envibro-style drywell(s) for disposal of stored storm water.
5. Underground retention facilities must be located a sufficient distance from property lines so as not to negatively impact existing or future development on the adjacent parcel or parcels.
B. Design.
1. Underground retention facilities shall be designed to store one hundred percent of the one-hundred-year, six-hour storm rainfall volume runoff from the drainage area served by the facility, as determined by the applicable NOAA Rainfall Atlas as currently adopted by the Flood Control District of Maricopa County.
2. Design shall be by a registered civil engineer, who shall prepare and seal a design report for submittal to the city along with submittal of the construction plans for the development project.
3. An overland flow route (ultimate outfall) for storms greater than the one-hundred-year, six-hour storm shall be provided.
4. Underground retention facilities shall have an interior height or diameter of at least six feet. There shall be a minimum of three feet of cover from tank to pavement subgrade or in accordance with the manufacturer’s recommendations.
5. If corrugated metal pipe is used for underground retention facilities, it shall be coated as specified by the design engineer to achieve a facility life of at least seventy-five years. Appropriate bedding and backfill materials and density shall be specified on the plans. The engineer shall provide a letter from the manufacturer, as part of plan submittal, which confirms service life. Pipe gauge, coating, and corrugation size shall be called out on the plans.
6. Alternate pipe materials or designs for underground storm water retention facilities shall be submitted to the city engineer for review prior to the submittal of construction plans.
7. Underground retention facilities shall be drained by gravity to drywells of the dual chamber design. At least one drywell shall be provided, with additional drywells required in order to dewater the underground facility within thirty-six hours following each storm event up to and including the one-hundred-year, six-hour storm. For planning purposes, a dewatering rate of 0.20 cfs per drywell shall be used, with the final number of drywells to be determined based on the installed performance of each drywell divided by two, with a maximum allowable rate per drywell of 0.50 cfs.
8. There shall be a minimum of two access points into an underground retention facility, with forty-eight-inch minimum manhole shafts, and thirty-inch manhole frames and covers with a concrete collar detail designed to not directly bear on the manhole shaft.
9. There shall be a note on the plans specifying that all joints in the underground retention tank system(s) will be water-tight, manufactured joints.
C. Operation and Maintenance.
1. Training Required. The owner of an underground storm water retention facility shall:
a. Ensure that all workers entering the underground retention facility are protected in accordance with OSHA requirements for confined space entry; and
b. Employ maintenance staff with expertise in operating, inspecting, and maintaining an underground storm water storage system; or
c. Retain a civil engineer with storm water management experience to provide an operations and maintenance manual for the system that includes:
i. A schedule for inspections and maintenance; and
ii. Provisions for emergency operations due to power failure, pump failure, and clogged outlet structures; and
iii. A log of the inspections and required maintenance services.
2. Inspections and Maintenance Required. Owner shall:
a. Inspect the entire drainage system semiannually, preferably before summer and winter rains, and take the following actions:
i. Remove accumulated trash and debris from inlet and outlet structures as needed to ensure free flow of storm water;
ii. Inspect all other elements of the drainage system (pipes, geotextiles, and stone rip rap) and repair/replace elements as needed for the storage system to operate at peak efficiency and to prevent erosion.
b. Have the underground facility, including all associated drywells, inspected and cleaned once every five years by a company experienced in cleaning drywells, in order to restore the original design capacity of the underground facility and its drywells.
i. The date of the five-year inspection and cleaning shall more or less coincide with the date the underground retention facility was placed into initial operation.
ii. For those facilities that have been in service for more than five years on the effective date of this section, the owner shall schedule the first five-year inspection and cleaning to coincide with the date of initial operation during the next calendar year.
iii. The inspection and cleaning company shall complete and submit to the city engineer within thirty days following the date of the five-year inspection and cleaning a report made on a form prepared by the city engineer, which shall meet the following minimum requirements:
(A) The report shall be signed by a manager of the inspection and cleaning company, as well as by the owner of the underground retention facility.
(B) The report form shall include a certification that the underground retention facility has been restored to its original design storage capacity.
(C) A description of any non-storm water materials or liquids found in the underground retention facility.
(D) Certification that all materials removed from the underground retention facility have been properly disposed of in a landfill licensed by the state of Arizona.
3. Failure to submit a five-year inspection and cleaning report to the city engineer shall constitute a civil infraction, and shall be prima facie evidence of noncompliance with the inspection, cleaning, and maintenance requirements of this section.
4. Noncompliance with the inspection, cleaning, and maintenance requirements of this section is injurious to the public health, safety, and general welfare, is considered a public nuisance under the provisions of this code, and is subject to abatement in accordance with Article 9-5. (Ord. 22-255 § 1)