This Plan-EA has been prepared under the Authority of the Watershed Protection and Flood Prevention Act of 1954 (Public Law [PL] 83-566), as amended. This Plan-EA has also been prepared in accordance with the National Environmental Policy Act (NEPA) of 1969, PL 91-190, as amended (42 United States Code [U.S.C.] 4321 et seq.).

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The goal of the Project is to provide flood prevention or flood damage reduction throughout the MMC by completing a continuous drainage system for Magma Wash, located in Pinal County, Arizona. The project would involve channelizing the MMC to reduce flood damages (and associated sediment damages) to residential, commercial, and agricultural areas; canals and irrigation systems; and road and railway transportation corridors through implementation of flood damage reduction measures and sound flood risk management planning.

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The Federal Objective as set forth in the Water Resources Development Act of 2007 specifies that “Federal water resources investments shall reflect national priorities, encourage economic development, and protect the environment by (1) seeking to maximize sustainable economic development; (2) seeking to avoid the unwise use of floodplains and flood-prone areas and minimizing adverse impacts and vulnerabilities in any case in which a floodplain or flood-prone area must be used; and (3) protecting and restoring the functions of natural systems and mitigating any unavoidable damage to natural systems.” The project would address the following PL 83-566 authorized Federal purpose – flood prevention (flood damage reduction) – as listed in Title 390, NWPM, Part 500, Subpart A, Section 500.3.

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The process of formulating alternatives followed the Council of Environmental Quality’s (CEQ’s) regulations for implementing NEPA and the procedures outlined in the USDA NRCS National Watershed Program Manual (NWPM; NRCS 2015) Parts 500 through 506, USDA-NRCS National Watershed Program Handbook (NWPH; NRCS 2014) Parts 600 through 606, and other recent USDA-NRCS watershed planning policies, including NRCS Circular 290-21-1 and USDA Departmental Manual 9500-013.

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Background Information for the Planning Area

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A Watershed Work Plan (WWP) was completed for Magma Wash in the 1960s. The WWP identified four primary components of the Magma system – the Magma Flood Retarding Structure (FRS), the Upper Magma Channel (UMC), the MMC, and the Lower Magma Channel (LMC). Within the last five years, MFCD was able to rehabilitate three of the four components (FRS, UMC, and LMC) within the Magma Wash system at a cost of nearly $27.5 million (the majority of which was funded by the NRCS).

 

The Middle Magma Watershed has never been addressed and this project is seeking to provide a comprehensive flooding solution for the Middle Magma Wash. The lack of a formal flood control solution for the MMC has left the Middle Magma Watershed vulnerable to adverse flooding impacts, with damage potential for rural community residential areas, agricultural lands, a major water delivery system (the CAP Canal), and roadway and railway transportation corridors.

 

Specifically, the MMC through Wild Horse Estates is an existing natural drainageway that does not meet the current safety and performance standards of the Pinal County Drainage Ordinance and Pinal County Drainage Manual (Volumes I and II; Pinal County 2004) due to high flow velocities through residential areas and excessive channel side-slopes. Public safety issues also arise from floodwaters blocking emergency vehicle access to Wild Horse Estates. 

 

This project seeks to implement an alternative that addresses future flood damage from the natural drainageway by improving the channel to appropriately convey the Middle Magma Watershed drainage and provide a continuous solution for the MMC from its upstream extent to its downstream terminus. The chosen solution would only consist of measures "to improve natural drainageways or channels to prevent significant flood damage to existing developed property," as per NWPH 600.42C. ​

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Preferred Alternative

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Approximately 4 miles of the MMC would be channelized to contain and convey the 100-year flood event. The project alignment extends from the downstream end of the Upper Magma Channel to the CAP Canal overchute; across lands managed by ASLD; and through the residential community of Wild Horse Estates and undeveloped private and BLM lands. The MMC terminates at the Anthem at Merrill Ranch community where there is already a fully constructed channel for the MMC from Franklin Road to its downstream terminus.

 

This alternative only consists of measures "to improve natural drainageways or channels to prevent significant flood damage to existing developed property" (NWPH 600.42C). Therefore, PL 83-566 cost-share funding is available for the features proposed as part of this alternative, and project implementation is eligible for 100 percent of construction and engineering costs. Coordination between this project and that of the Mesquite Trails development has taken place to ensure that the channel seamlessly connects to the planned Mesquite Trails channel and the same design standards would be followed through Mesquite Trails.

 

It should be noted that Mesquite Trails will independently move forward with development regardless of whether this project is implemented or not. Implementation and operation/maintenance of this alternative would require land acquisition or easement from various agencies or private landowners. A delineated flow corridor containing all improvements resulting from this alternative would be owned, operated, and maintained by MFCD. This level of oversight would be consistent with the goal of safely conveying floodwaters through the entire Middle Magma Watershed.

 

Watershed and Resource Opportunities and MFCD Objectives

 

The preferred Alternative would:

1. Address flooding in the Middle Magma Watershed through channelization of the MMC or other effective flood damage reduction measures.

2. Provide an improved, maintained, and continued solution to convey floodwaters of the Middle Magma Watershed from the upstream extent of the MMC to its downstream terminus.

3. Develop a solution to address public health and safety issues along the MMC, as it does not meet current safety and performance standards (Pinal County 2004).

4. Acquire easements and/or purchase land to implement, and maintain floodwater and sediment damage reduction measures, as well as channel and vegetation maintenance to confine floodwaters and sediment and increase flow conveyance of the MMC. These actions are also intended to provide concurrent solutions to known problems within the drainage system, including:

   1. Issues upstream of Magma Wash’s CAP Canal overchute on BLM and private lands, where historic flanking of the training dike causes water to breach the CAP Canal and flood adjacent agricultural lands.

   2. Issues downstream of the CAP Canal overchute on ASTL where Magma Wash has uncontrolled flow and excessive vegetative growth.

   3. Issues in the residential community of Wild Horse Estates where the natural drainageway does not meet current safety and performance standards and can become quickly overwhelmed with floodwaters, causing damage to 24 residential properties and structures in the area.

   4. Issues related to an abandoned channel and failed farmer’s dike downstream of Wild Horse Estates on private and BLM lands, which was likely constructed many decades ago to protect agricultural property from flooding.

 

Potential Impacts

 

The Preferred Alternative would be planned, designed, and installed to have long-term net beneficial effects on residential structures, agricultural production, infrastructure protection, and public safety. Implementation of the Preferred Alternative may result in minor, short-term and long-term, adverse effects during construction to vegetation, wildlife, wetlands, floodplains, and soils along the channel. Short-term adverse effects would result from construction activities. Long-term adverse effects, primarily to soils and vegetation, would occur in the channel footprint. The Sponsor would work closely with partners, contractors, and affected landowners to incorporate measures to avoid and minimize short-term, adverse effects. More detailed discussions of each potentially impacted resource and the corresponding issues are provided in the Watershed Plan-EA document.