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Santa Cruz River, Silverlake to Grant Channel Capacity Restoration

Study status:  In Progress

Overview

For more information contact:
Colby Fryar, PE, Civil Engineer
(520) 724-4600

After the completion of the Intergovernmental Agreement (IGA) with the City of Tucson in 2014 which transferred maintenance responsibility of the Santa Cruz River to the Regional Flood Control District (District), the District evaluated the aggradation of the river from Silverlake to Grant and its impact on the channel’s flood carrying capacity. The results of the evaluation revealed that the aggradation, which has occurred over 30 years, has the potential to flood an additional 146 structures and additional property with estimated assessed value of $69 million. In order to identify the extent to which sediment removal activities must be performed, the District evaluated numerous scenarios which provide insight regarding the consequences of leaving otherwise desirable vegetation in the channel. Santa Cruz River channel with sediment build-up. Keeping the vegetation, means the sediment that is underneath the vegetation cannot be removed. The scenarios included full channel clean-out, leaving vegetation in place where flood damage potential is least severe, leaving the most desirable vegetation in place irrespective of flood damage potential, and maintenance near bridges to minimize bridge failure. These evaluations are complete and the District will engage neighbors, Loop users, and other stakeholders to identify the optimal solution regarding performing the necessary flood control maintenance while being environmental stewards. This project is being broken up into three phases: Phase 1 – Speedway Blvd to Grant Road, Phase 2- Silverlake Rd to Mission Lane, and Phase 3 – Mission Lane to Speedway Blvd.

Scope of Work

Phase 1 – Speedway Blvd to Grant Rd (Completed)

The District is fee owner or has other property rights for maintenance for the portion of the river from Speedway to Grant and desires to commence maintenance activities this may after outreach to adjacent neighborhoods, the Loop community and other stakeholders. The Districts final recommendation for maintenance of this section is based on hydraulic evaluation and the results of a vegetation survey that identified the most desirable plant species to preserve was utilized. The goal is to preserve as many of the native trees as possible while restoring the necessary capacity.. It is anticipated that, upon completion of this sediment removal project, the 51 structures that are impacted by the floodplain in today’s conditions (22 commercial, 20 residential, and 9 government structures) will be reduced to 6 or fewer structures. In addition, it is possible that once the lowest floor of these structures, which are owned by the State of Arizona, are measured that one or more will be elevated such that flood damage to the interior is not anticipated The grading plan for channel can be found below.

It is also important to note that the receiving area for the sediment from the river, an old meander north of the Grant Road on the west bank that has been cut off from the river, is planned to become a restoration project after this phase of the sediment removal project is complete. This area, currently a sparsely vegetated hole, will be turned into a neighborhood scale water harvesting project that will serve as an amenity to the adjacent Silver Creek II subdivision as well as a node on the Loop. The placement of the sediment from the river will occur in a way that results in a multi-acre water harvesting basin with terraces that provide for a lush mesquite bosque in the lower areas with hardier desert species in the upper terraces. The diversion of flow from an adjacent small watershed will reduce the need for long term irrigation, while containing the full flow volume of the 100 year flood. See attached concept plan for the restoration, tentatively called Meander Park.

Phase 2 – 29th Street to Mission Lane

When measured in terms of public safety and flood damage potential, the flood risk reduction associated with Phase 2 is comparatively small. However, there are benefits for completing this project in a timely manner, including the stated goal of Tucson Water to begin recharging reclaimed water in this reach by Memorial Day 2019. The District’s preference is to perform the necessary maintenance prior to the discharge of reclaimed water.

With respect to trying to balance the desire to preserve vegetation with the need for public safety, the District is approaching the project the same way as Phase 1. To date, both the hydraulic analysis and vegetation survey have been performed. These have been used to develop a recommended grading plan. There is a net aggradation of approximately 131,000 cubic yards of material in the Phase 2 reach, from 29th Street to Mission Lane. Based on the recommended plan, the amount of material that will remain in the preservation areas after completion is approximately 54,000 cubic yards, with approximately 77,000 cubic yards being removed. The preservation area covers an area of approximately 5.5 acres compared to total river bottom of 24 acres.

It is anticipated that, upon completion of this phase of the project, the 9 structures that are impacted by the floodplain in today’s conditions (8 residential and 1 government structure) will be reduced to zero structures impacted by flooding. This will allow for the discharge of reclaimed water, and associated growth of riparian vegetation, without risk to adjacent structures. The existing conditions and post-project floodplain maps are attached. It is anticipated that the sediment from this reach will be deposited at the A mountain landfill, adjacent locations as the City desires, or in Meander Bend park.