Subdivision7

Subdivision Regulations

ARTICLE 29-32 DRAINAGE

Section 29-32-1 Purpose

The topography and drainage patterns in Thatcher necessitate special attention for controlling storm water. Regulatory controls and measures are identified in this Article to minimize storm water problems.

Section 29-32-2 Exceptions

See ARTICLE 29-31 for procedures for exceptions.

Section 29-32-3 Enforcement

No building permit shall be issued when the Building Official has determined that a drainage permit for the site is required by this Ordinance and one has not been issued. No building permit shall be issued unless the building's proposed finish floor elevation is in compliance with this Ordinance. The Building Official shall not issue an occupancy permit unless the Town Engineer has determined that the drainage permit has been complied with; or when a drainage permit is not required under this Ordinance, the Building Official has determined that the building's finish floor elevation is in compliance with this Ordinance.
The failure to construct, improve, develop, maintain, or repair drainage improvements necessary to comply with the approved drainage plan and drainage permit shall constitute a violation of this Ordinance and penalties per ARTICLE 29-25 shall be imposed upon conviction.

Section 29-32-4 Conceptual Drainage Plan

A conceptual storm water collection and handling plan shall be submitted with any preliminary plat or site development plan to be approved by the Planning Commission. This plan may (with the Public Works Engineer’s recommendation) be required to be prepared by a civil engineer registered in the State of Arizona. The purpose of the conceptual drainage plan is to include drainage concepts in the planning process as necessary to outline the solutions to foreseen drainage problems and ensure that the proper amount of land within the development has been reserved for onsite retention and detention. If basins are to be dedicated to and maintained by the Town of Thatcher, the Town council shall approve the conceptual drainage plan and tentatively accept the basin.

Section 29-32-5 Drainage Plan and Drainage Permit

When a drainage permit is required, the application for such permit shall be supported by a plan. The drainage plan shall show surface grades, street grades, curbs, gutters, catch basins, storm sewers, and drainage structures to explain the operation of all surface and subsurface drainage systems. Submitted with the plan shall be calculations to support the Engineer's judgment concerning the estimated height of ponding, basin capacity, and the capacity of subsurface pipe delivery systems. Such drainage plan shall be prepared under the direction of a civil engineer registered in the State of Arizona, except as hereinafter provided.

Exception: Where the permittee is the owner of a lot or parcel of one (1) acre or less which is zoned residential, and where the owner proposes to build one single family residence on this parcel, or on public property adjacent to such lot for driveway purposes, plans and calculations prepared by a professional engineer will not be required. The property owner shall submit a sketch showing the proposed work. The Public Works Engineer is authorized to assist the property owner in preparing such a sketch and making any computations which may be required.

Section 29-32-6 General Drainage Requirements

No improvements shall increase the flow rate or quantity (or flow prior to the adoption of this Ordinance) of runoff onto adjoining land. Basins or sumps per Section 29-32-8 may be required to comply with this Subsection.

Increased flow is permissible in downstream channels, provided the Developer shall furnish downstream facilities adequate to handle the total flow without adverse effect on other properties. Easements must be provided to the satisfaction of Town staff and the Town Engineer must approve the drainage concept.

Unless diversion of water is required to conform to a comprehensive drainage plan for a drainage district, runoff shall be received and discharged at the locations which existed prior to development and as nearly as possible in the same manner which existed prior to development. Should diversion be required, sufficient work shall be done upstream and/or downstream to provide all affected properties at least the same level of flood protection that existed prior to the diversion. The developer shall determine any changes in backwater that will be caused by the proposed developments and any effects the backwater might cause.

High water ponding in public streets in excess of twelve inches (12") during a ten-year storm will not be permitted. The water depth for ponding shall be measured at the gutter invert to yield maximum depth.

Street gutter grades flatter than 0.30% will not be accepted for Town maintenance without Public Works Director approval.

Paved streets with curb and gutter are required for all streets to be dedicated to the Town and are required on private streets and commercial alleys when the grade is flatter than 0.50%.

All building's finish floor elevations shall be at least twelve inches (12") above the crown of the street unless the applicant for a drainage permit can establish with sound engineering practice a lower safe finish floor elevation that complies with SubSection 29-32-6-H. Carports and garages must be at least eight inches (8") above the crown of the street. Basement structures shall be provided with flood protection to the elevations determined for finish floor elevations.

Exceptions: The provisions of this Subsection shall not apply to existing buildings or structures, or additions and expansions to a building or structure, provided that the existing building or structure does not have a history of flooding. Buildings or structures with a history of flooding must comply with the aforementioned regulations.

All drainage facilities other than those accepted for maintenance by the Town shall be maintained by the individual property owner or an entity with assessment powers.

Drainage structures and open channels serving onsite drainage needs will typically be designed and evaluated using a peak runoff from a ten-year storm. For drainage areas 1.0 square mile or larger, a 24 hour duration storm will be used. For smaller watersheds, a 6 hour duration will be used. The evaluation of drainage structures shall show that all adjacent improvements shall be at least six inches (6") above any flood hazards due to a ten-year storm. If the proposed site contains a significantly large channel (man-made or natural), a larger storm event may be required as a design criterion. Bridges or culverts crossing major washes will be designed to a 100-year storm event of 24 hour duration while providing a 12" freeboard with the developer to coordinate details with the Town Engineer on a case by case basis.

All constructed open channels shall drain by gravity; bank protection shall be provided when soil instability appears likely or when circumstances require protection. Levees are undesirable but may be permitted if special conditions require them.

Storm water lift stations are not permitted.

Open channels must conform to Town of Thatcher Standards established by the Town Engineer.

All ponding of water inside basins exceeding two feet (2') shall be fenced in unless all side slopes are four horizontal to one vertical (4 horizontal : 1 vertical) or flatter. All basins shall be fenced in when the maximum depth exceeds three and one-half feet (3.5'). (Compare 29-32-8.F)

Onsite stormwater storage is required for all development except for areas adequately served by existing retention facilities, and developments excluded under Section 29-32-6F.2 of this ordinance.

Sections 29-32-6 F and H apply to runoff from onsite flows only. In addition to these requirements, the engineer shall design drainage and building structures in accordance with local, state, and federal laws related to 100 year flood plains established by FEMA. Further, the engineer shall design improvements to be safe from all offsite flows by either designing the first floor of any building to be one (1') foot above an estimated 100-year flood level or alternately by designing berms or walls acting as a levy to protect development from offsite flows in accordance with standards adopted by resolution of the Thatcher Town Council. The first floor levels of manufactured housing shall be designed in accordance with FEMA criteria for manufactured homes. The developer shall be required to build all drainage improvements within the development designed by the engineer to comply with the requirements of this section,

Section 29-32-7 Public and Private Basins

All basins that are to be maintained by the Town shall be improved by the Developer in accordance with this Ordinance along with any landscaping, seeding, fencing, or sprinkler requirements of the Public Works Department. Basin landscaping will be addressed on a case by case basis. The property shall be deeded over to the Town. The basins shall be designated as easement areas for storm water purposes and shall have a recorded restrictive covenant requiring perpetual maintenance.

All basins to be accepted by the Town for maintenance shall be constructed with a minimum of four to one (4:1) side slopes. The Town will not accept sumps for maintenance. Collection of runoff from outside the Development may be required by the Town with adequate compensation provided to the Developer. Two (2) or more Developers may join together to provide a common facility. A Letter of Agreement by all Developers participating in the facility must be presented to the Department of Planning and the plan shall indicate that the area is a joint facility. The joint facility area shall meet all criteria as a single area.

Some areas drain to existing facilities owned by the Town of Thatcher or to privately owned facilities. If these facilities were designed to accept the some or all of the runoff from the proposed development, the developer will be permitted to drain to these facilities to their design limits. The Public Works Engineer shall inform the developer of design requirements for his property related to developments which drain to Town owned or privately owned storm water facilities.

Section 29-32-8 Design Criteria for all Basins and Sumps

Introduction:

The Developer’s engineer shall design all facilities to comply with this drainage ordinance and summarize the design calculations and assumptions into a drainage report for the development. The drainage report shall include maps illustrating the onsite and offsite drainage areas related to the development. The report shall give peak 10-year flows for the onsite drainage structures, ditches, gutters and pipes. The report shall give peak 10, 25, 50, and 100-year flows for major channels and basins related to the development. The calculations shall document depths of flow and ponding for all structures, ditches, gutters, pipes and basins related to the development. The report shall document onsite and offsite flows and explain how the design manages these flows to conform to the drainage ordinance. The engineer shall provide culvert or storm sewer calculations and designs are required. Culverts and storm sewers handling offsite flows shall be designed as directed by the Public Works Engineer. Where offsite waters drain through onsite storm water basins, the engineer shall communicate this in the preliminary drainage report as required herein to the Public Works Engineer concerning the required size of the basin. The Town and the developer will discuss sizing the basin for the offsite area. Where large washes or offsite drainage flow through the site, but not through onsite basins, the Public Works Engineer shall give the design criteria for these washes to the developer’s engineer.

Pavement drainage structures shall be designed and evaluated using a peak runoff from a hydrological event that has an average recurrence interval of ten (10) years. When ponding from runoff occurs in the streets, the limits of the ponding shall be identified, and the escape path used to identify the limits of ponding shall also be identified and a drain provided.
Basins shall be designed to detain flows from offsite drainage plus pre-development onsite flows as well as retain post-development onsite flows. Basins shall be designed to contain below the spillway the required storage volume of S = 72 CA (measured in acre-feet where 72 is a constant, "C" is the runoff coefficient, and "A" is the area in square miles). Retained water shall remain in the bottom of a basin to percolate or evaporate. Detained water shall be drained by gravity at a point above the retained water level to an adjacent ditch or storm sewer. The outlet pipe must be designed so that water from the ditch or storm sewer does not back up into the basin when flow occurs into the ditch or storm sewer. The basin detention water outlet structure shall have an orifice designed to slow release of captured storm water to a peak outflow no greater than the pre-developed rate from a ten-year storm when the basin is filled to the depth of the spillway. The spillway shall have sufficient capacity to manage a ten-year storm of any duration when the basin is full to spillway height.
If used, sumps shall be designed to contain below the spillway three (3) times the volume of runoff from an event that has an average reoccurrence interval of ten (10) years, and shall have twice the sufficient spillway capicity to manage a ten-year storm of any duration with the sump full to the spillway height.
The maximum permitted depth of water in basins and sumps (measured from the spillway elevation) is eight feet (8') if the area is to be fenced in, and three and one-half feet (3.5'), with four to one (4:1) sideslopes if the area is not to be fenced in.
Basins and sumps must also be completely draining within seven (7) days. Stormwater disposal using dry wells and trenches are acceptable only to provide a method to completely drain a facility within the allowed period. The Developer must provide the Town a copy of the required ADEQ dry well permit if one is used.
All ponding of water in excess of two feet (2') shall be fenced in unless all sideslopes are four horizontal to one vertical (4 horizontal : 1 vertical) or flatter. All ponding of water in excess of three and one-half feet (3.5') shall be fenced in. (Compare 11-31-6.L)
The engineer shall design head walls for all culvert ends including proper erosion control structures in accordance with standards adopted by resolution of the Thatcher Town Council. The engineer shall design basin inlet and outlet headwalls with trash racks to the satisfaction of the Public Works Engineer.

Section 29-32-9 Basins Must Be Drained; Right of Town to Drain Basins

It shall be unlawful for any person owning or controlling a retention basin to permit storm water to stand therein longer than seven (7) days. In addition to any penalty provided by law, should the person owning or controlling any privately owned and maintained basin fail, neglect or refuse to drain said retention basin within seven (7) days, it shall be the right of the Town to enter upon the privately owned retention basin property and take such action as may reasonably be necessary to drain said basin. The draining of said basin shall be at the expense of the owner or person controlling such basin. Such expense shall include all costs related to draining said basin including legal fees.

Section 29-32-10 Inspection of Drainage Improvements

The Town Engineer is authorized to inspect all drainage improvements and shall notify the owner of conditions not complying with this Drainage Ordinance or the accepted drainage plan and drainage permit.


	PO Box 670, 3700 W Main St, Thatcher, AZ 85552
	Phone (928) 428-2290 Fax (928) 428-7061
	Subdivision Regulations ARTICLE 29-32 DRAINAGE Section 29-32-1 Purpose The topography and drainage patterns in Thatcher necessitate special attention for controlling storm water. Regulatory controls and measures are identified in this Article to minimize storm water problems. Section 29-32-2 Exceptions See ARTICLE 29-31 for procedures for exceptions. Section 29-32-3 Enforcement No building permit shall be issued when the Building Official has determined that a drainage permit for the site is required by this Ordinance and one has not been issued. No building permit shall be issued unless the building's proposed finish floor elevation is in compliance with this Ordinance. The Building Official shall not issue an occupancy permit unless the Town Engineer has determined that the drainage permit has been complied with; or when a drainage permit is not required under this Ordinance, the Building Official has determined that the building's finish floor elevation is in compliance with this Ordinance. The failure to construct, improve, develop, maintain, or repair drainage improvements necessary to comply with the approved drainage plan and drainage permit shall constitute a violation of this Ordinance and penalties per ARTICLE 29-25 shall be imposed upon conviction. Section 29-32-4 Conceptual Drainage Plan A conceptual storm water collection and handling plan shall be submitted with any preliminary plat or site development plan to be approved by the Planning Commission. This plan may (with the Public Works Engineer’s recommendation) be required to be prepared by a civil engineer registered in the State of Arizona. The purpose of the conceptual drainage plan is to include drainage concepts in the planning process as necessary to outline the solutions to foreseen drainage problems and ensure that the proper amount of land within the development has been reserved for onsite retention and detention. If basins are to be dedicated to and maintained by the Town of Thatcher, the Town council shall approve the conceptual drainage plan and tentatively accept the basin. Section 29-32-5 Drainage Plan and Drainage Permit When a drainage permit is required, the application for such permit shall be supported by a plan. The drainage plan shall show surface grades, street grades, curbs, gutters, catch basins, storm sewers, and drainage structures to explain the operation of all surface and subsurface drainage systems. Submitted with the plan shall be calculations to support the Engineer's judgment concerning the estimated height of ponding, basin capacity, and the capacity of subsurface pipe delivery systems. Such drainage plan shall be prepared under the direction of a civil engineer registered in the State of Arizona, except as hereinafter provided. Exception: Where the permittee is the owner of a lot or parcel of one (1) acre or less which is zoned residential, and where the owner proposes to build one single family residence on this parcel, or on public property adjacent to such lot for driveway purposes, plans and calculations prepared by a professional engineer will not be required. The property owner shall submit a sketch showing the proposed work. The Public Works Engineer is authorized to assist the property owner in preparing such a sketch and making any computations which may be required. Section 29-32-6 General Drainage Requirements No improvements shall increase the flow rate or quantity (or flow prior to the adoption of this Ordinance) of runoff onto adjoining land. Basins or sumps per Section 29-32-8 may be required to comply with this Subsection. Increased flow is permissible in downstream channels, provided the Developer shall furnish downstream facilities adequate to handle the total flow without adverse effect on other properties. Easements must be provided to the satisfaction of Town staff and the Town Engineer must approve the drainage concept. Unless diversion of water is required to conform to a comprehensive drainage plan for a drainage district, runoff shall be received and discharged at the locations which existed prior to development and as nearly as possible in the same manner which existed prior to development. Should diversion be required, sufficient work shall be done upstream and/or downstream to provide all affected properties at least the same level of flood protection that existed prior to the diversion. The developer shall determine any changes in backwater that will be caused by the proposed developments and any effects the backwater might cause. High water ponding in public streets in excess of twelve inches (12") during a ten-year storm will not be permitted. The water depth for ponding shall be measured at the gutter invert to yield maximum depth. Street gutter grades flatter than 0.30% will not be accepted for Town maintenance without Public Works Director approval. Paved streets with curb and gutter are required for all streets to be dedicated to the Town and are required on private streets and commercial alleys when the grade is flatter than 0.50%. All building's finish floor elevations shall be at least twelve inches (12") above the crown of the street unless the applicant for a drainage permit can establish with sound engineering practice a lower safe finish floor elevation that complies with SubSection 29-32-6-H. Carports and garages must be at least eight inches (8") above the crown of the street. Basement structures shall be provided with flood protection to the elevations determined for finish floor elevations. Exceptions: The provisions of this Subsection shall not apply to existing buildings or structures, or additions and expansions to a building or structure, provided that the existing building or structure does not have a history of flooding. Buildings or structures with a history of flooding must comply with the aforementioned regulations. All drainage facilities other than those accepted for maintenance by the Town shall be maintained by the individual property owner or an entity with assessment powers. Drainage structures and open channels serving onsite drainage needs will typically be designed and evaluated using a peak runoff from a ten-year storm. For drainage areas 1.0 square mile or larger, a 24 hour duration storm will be used. For smaller watersheds, a 6 hour duration will be used. The evaluation of drainage structures shall show that all adjacent improvements shall be at least six inches (6") above any flood hazards due to a ten-year storm. If the proposed site contains a significantly large channel (man-made or natural), a larger storm event may be required as a design criterion. Bridges or culverts crossing major washes will be designed to a 100-year storm event of 24 hour duration while providing a 12" freeboard with the developer to coordinate details with the Town Engineer on a case by case basis. All constructed open channels shall drain by gravity; bank protection shall be provided when soil instability appears likely or when circumstances require protection. Levees are undesirable but may be permitted if special conditions require them. Storm water lift stations are not permitted. Open channels must conform to Town of Thatcher Standards established by the Town Engineer. All ponding of water inside basins exceeding two feet (2') shall be fenced in unless all side slopes are four horizontal to one vertical (4 horizontal : 1 vertical) or flatter. All basins shall be fenced in when the maximum depth exceeds three and one-half feet (3.5'). (Compare 29-32-8.F) Onsite stormwater storage is required for all development except for areas adequately served by existing retention facilities, and developments excluded under Section 29-32-6F.2 of this ordinance. Sections 29-32-6 F and H apply to runoff from onsite flows only. In addition to these requirements, the engineer shall design drainage and building structures in accordance with local, state, and federal laws related to 100 year flood plains established by FEMA. Further, the engineer shall design improvements to be safe from all offsite flows by either designing the first floor of any building to be one (1') foot above an estimated 100-year flood level or alternately by designing berms or walls acting as a levy to protect development from offsite flows in accordance with standards adopted by resolution of the Thatcher Town Council. The first floor levels of manufactured housing shall be designed in accordance with FEMA criteria for manufactured homes. The developer shall be required to build all drainage improvements within the development designed by the engineer to comply with the requirements of this section, Section 29-32-7 Public and Private Basins All basins that are to be maintained by the Town shall be improved by the Developer in accordance with this Ordinance along with any landscaping, seeding, fencing, or sprinkler requirements of the Public Works Department. Basin landscaping will be addressed on a case by case basis. The property shall be deeded over to the Town. The basins shall be designated as easement areas for storm water purposes and shall have a recorded restrictive covenant requiring perpetual maintenance. All basins to be accepted by the Town for maintenance shall be constructed with a minimum of four to one (4:1) side slopes. The Town will not accept sumps for maintenance. Collection of runoff from outside the Development may be required by the Town with adequate compensation provided to the Developer. Two (2) or more Developers may join together to provide a common facility. A Letter of Agreement by all Developers participating in the facility must be presented to the Department of Planning and the plan shall indicate that the area is a joint facility. The joint facility area shall meet all criteria as a single area. Some areas drain to existing facilities owned by the Town of Thatcher or to privately owned facilities. If these facilities were designed to accept the some or all of the runoff from the proposed development, the developer will be permitted to drain to these facilities to their design limits. The Public Works Engineer shall inform the developer of design requirements for his property related to developments which drain to Town owned or privately owned storm water facilities. Section 29-32-8 Design Criteria for all Basins and Sumps Introduction: The Developer’s engineer shall design all facilities to comply with this drainage ordinance and summarize the design calculations and assumptions into a drainage report for the development. The drainage report shall include maps illustrating the onsite and offsite drainage areas related to the development. The report shall give peak 10-year flows for the onsite drainage structures, ditches, gutters and pipes. The report shall give peak 10, 25, 50, and 100-year flows for major channels and basins related to the development. The calculations shall document depths of flow and ponding for all structures, ditches, gutters, pipes and basins related to the development. The report shall document onsite and offsite flows and explain how the design manages these flows to conform to the drainage ordinance. The engineer shall provide culvert or storm sewer calculations and designs are required. Culverts and storm sewers handling offsite flows shall be designed as directed by the Public Works Engineer. Where offsite waters drain through onsite storm water basins, the engineer shall communicate this in the preliminary drainage report as required herein to the Public Works Engineer concerning the required size of the basin. The Town and the developer will discuss sizing the basin for the offsite area. Where large washes or offsite drainage flow through the site, but not through onsite basins, the Public Works Engineer shall give the design criteria for these washes to the developer’s engineer. Pavement drainage structures shall be designed and evaluated using a peak runoff from a hydrological event that has an average recurrence interval of ten (10) years. When ponding from runoff occurs in the streets, the limits of the ponding shall be identified, and the escape path used to identify the limits of ponding shall also be identified and a drain provided. Basins shall be designed to detain flows from offsite drainage plus pre-development onsite flows as well as retain post-development onsite flows. Basins shall be designed to contain below the spillway the required storage volume of S = 72 CA (measured in acre-feet where 72 is a constant, "C" is the runoff coefficient, and "A" is the area in square miles). Retained water shall remain in the bottom of a basin to percolate or evaporate. Detained water shall be drained by gravity at a point above the retained water level to an adjacent ditch or storm sewer. The outlet pipe must be designed so that water from the ditch or storm sewer does not back up into the basin when flow occurs into the ditch or storm sewer. The basin detention water outlet structure shall have an orifice designed to slow release of captured storm water to a peak outflow no greater than the pre-developed rate from a ten-year storm when the basin is filled to the depth of the spillway. The spillway shall have sufficient capacity to manage a ten-year storm of any duration when the basin is full to spillway height. If used, sumps shall be designed to contain below the spillway three (3) times the volume of runoff from an event that has an average reoccurrence interval of ten (10) years, and shall have twice the sufficient spillway capicity to manage a ten-year storm of any duration with the sump full to the spillway height. The maximum permitted depth of water in basins and sumps (measured from the spillway elevation) is eight feet (8') if the area is to be fenced in, and three and one-half feet (3.5'), with four to one (4:1) sideslopes if the area is not to be fenced in. Basins and sumps must also be completely draining within seven (7) days. Stormwater disposal using dry wells and trenches are acceptable only to provide a method to completely drain a facility within the allowed period. The Developer must provide the Town a copy of the required ADEQ dry well permit if one is used. All ponding of water in excess of two feet (2') shall be fenced in unless all sideslopes are four horizontal to one vertical (4 horizontal : 1 vertical) or flatter. All ponding of water in excess of three and one-half feet (3.5') shall be fenced in. (Compare 11-31-6.L) The engineer shall design head walls for all culvert ends including proper erosion control structures in accordance with standards adopted by resolution of the Thatcher Town Council. The engineer shall design basin inlet and outlet headwalls with trash racks to the satisfaction of the Public Works Engineer. Section 29-32-9 Basins Must Be Drained; Right of Town to Drain Basins It shall be unlawful for any person owning or controlling a retention basin to permit storm water to stand therein longer than seven (7) days. In addition to any penalty provided by law, should the person owning or controlling any privately owned and maintained basin fail, neglect or refuse to drain said retention basin within seven (7) days, it shall be the right of the Town to enter upon the privately owned retention basin property and take such action as may reasonably be necessary to drain said basin. The draining of said basin shall be at the expense of the owner or person controlling such basin. Such expense shall include all costs related to draining said basin including legal fees. Section 29-32-10 Inspection of Drainage Improvements The Town Engineer is authorized to inspect all drainage improvements and shall notify the owner of conditions not complying with this Drainage Ordinance or the accepted drainage plan and drainage permit.