Arroyo Chico Multi-Use Project

Pima County Regional Flood Control District and the City of Tucson in cooperation with the U.S. Army Corps of Engineers (ACOE) have jointly undertaken a multi-phase flood control, environmental restoration and recreation project called the Tucson Drainage Area/Arroyo Chico Multi-Use Project. The project area encompasses approximately 6 miles of the Arroyo Chico (also known as Tucson Arroyo) Wash from Alvernon Way to its confluence with the Santa Cruz River near St. Mary Road. The phases of the project are: Phase 1, Randolph South Detention Basins which were completed in April 1996 by the Pima County Regional Flood Control District and the City of Tucson; Phase 2A, Cherry Field Detention Basin which was completed in November 2008 by the ACOE; and Phase 2B, Park Avenue Detention Basin Complex which is to be constructed by the ACOE in the near future. The total estimated cost of the project is $65 million.

The Arroyo Chico/Tucson Arroyo and its tributaries -- High School Wash, Railroad Wash, Citation Wash, Paseo Grande Wash and Naylor Wash -- drain an area of 11.4 square miles located in central and downtown Tucson. These ephemeral watercourses drain a watershed which is fully developed and contains a mix of residential, commercial and industrial areas. A segment of the lower watershed main channel is conveyed through an underground two-barrel, 10 feet wide by 8 feet high concrete box culvert for approximately 1.7 miles that was originally constructed in the 1920’s. Because of the increased runoff due to urbanization of the contributing watersheds, the capacities of the open channel/culvert sections are generally inadequate to convey the peak flows caused by intense thunderstorm events, resulting in frequent and severe flooding of residential, commercial and industrial areas along the entire length of the arroyo. Flood damages to both private properties and public infrastructure are estimated by the ACOE at $2.7 million (1998 prices) annually, resulting in a benefit-cost ratio of 1.3:1.

The construction of the Phase 2B Park Avenue Detention Basin Complex provides the opportunity for environmental restoration of degraded riparian ecosystem, preservation of acceptable existing native habitats, and recreational improvements for the neighboring communities. The City of Tucson Citizens Advisory Committee (CAC) provided 18 points that were adopted in April 1998 by the Mayor and Council for the development of the Park Avenue Basins. These recommendations have been used as guidelines for the design of the Park Avenue Basins by the ACOE.  Final construction plans and specifications were completed in March 2005.  At this time, federal priorities have shifted funding from local Civil Works projects to projects in Iraq and Afghanistan, creating delays to the development of this complex.

PROJECT FEATURES

Phase 1 (Randolph South Detention Basin) (Completed April 1996)

The Randolph South detention basin complex constitutes Phase 1 of the overall Arroyo Chico Project. It was designed and constructed by Pima County Flood Control District and the City of Tucson, and represents the local sponsors’ contribution to the overall project under a Section 104 Credit Agreement with the ACOE.

The basin complex consists of six interconnected detention basins that were constructed and integrated into the existing Del Urich Municipal Golf Course (formerly the Randolph South Golf Course). The basins intercepts flood flow from a 3.5 square mile drainage area of the upstream reach of Arroyo Chico and its tributaries. The basins are interconnected by weirs and reinforced concrete pipes and provide storage of 530 acre-ft of storm water during a 100 year event. The basin complex reduces the 100 year discharge of 3,400 cubic feet per second (cfs) that would be discharged without the project to 269 cfs. The outflow (269 cfs) from the last basin is conveyed through a concrete box culvert under Randolph Way and discharges into the existing channel of the Arroyo Chico. The Randolph South basins have worked successfully during flood events since its construction was completed in April 1996.

Phase 2A (Cherry Field Detention Basin and Railroad Wash Confluence)

Tetra Tech, Inc. designed Phase 2 under a contract with ACOE. Due to limited funding Phase 2 of the project was divided into smaller projects, the first of these was the Cherry Field Detention Basin and Railroad Wash Confluence. Cherry Field is Tucson Unified School District’s (TUSD) athletic fields for Tucson High Magnet School’s baseball and softball program. Development of this basin required the cooperation of TUSD, Pima County Regional Flood Control District, the City of Tucson and the ACOE.

The project removed 500,000 cubic yards of dirt from the Cherry Field site to create the offline flood detention basin. The new flood control basin can contain 223 acre-feet of storm water during a 100- year event or half the capacity needed under Phase 2. Flows from an event smaller than the 18-year recurrence interval will be conveyed through the culvert on the Cherry Avenue alignment that was constructed at the end of the existing concrete drainage way for the Arroyo Chico and the Railroad Wash confluence. Larger events will begin to spill into the Cherry Field Detention basin using the combined side weir spillway and concrete bleacher on the south side of Cherry Field. During the receding phase of a large flood, flows out of Cherry Field are conveyed back into Arroyo Chico through a low-flow drain at the culvert outlet. The maximum time to drain the basin is 30 hours during a 100- year flow event, or shorter during smaller flow events.

The project provided TUSD with 2 new baseball and 2 new softball fields with sections of removable fence to allow for soccer and football practice during the off-seasons. The new athletic fields have the latest high efficiency, low reflection sport field lights, and the new field irrigation system uses reclaimed water providing environmental benefit and savings to the taxpayers. A new locker room/classroom/refreshment stand building, a maintenance building and parking area were constructed outside of the detention basin. The new facility meets current standards and codes.

Sidewalks were provided to connect Cherry Field with Miles Elementary School along Miles Street, Cherry Avenue, and 13th Street. Pedestrians and cyclists now have access over the Arroyo Chico at Cherry Avenue. Portraits of the Miles and Barrio San Antonio neighborhoods have been installed by the City Tucson on the Winsett/15th Street floodwall.

Phase 2B (Park Avenue Basin Complex) - Future Improvements

The Park Avenue Basin complex and the associated channel/culvert improvements constitute Phase 2B of the project, and include the following elements:

• Three in-line detention basins, called Basins 1, 2, and 3 located along Arroyo Chico between Park Avenue and Cherry Avenue.
• Realignment of High School Wash box culvert (underground) along 3rd Avenue and 8th Street at Tucson High School.
• Enlarged concrete box culverts under Campbell Avenue at Arroyo Chico Wash.
• Improved channel along Arroyo Chico between Campbell Avenue and Parkway Terrace, with culverts at existing dip sections.

The inlet to the basin complex is located at the new culvert at the Arroyo Chico and Railroad Wash confluence, followed by Basin 3, Basin 2 and Basin 1 in the downstream direction. The basins are interconnected with concrete arch culverts. The depth of the basins below the adjacent streets varies between 16 and 18 feet, with the bottom approximately at the same depth as the existing arroyo channel invert. During the receding phase, flows out of the completed Cherry Field Basin will drain to Basin 3. During a 100 year flood, storage within the four basins reduces the flood peak to a level that can be contained within the existing Tucson Arroyo underground concrete box culverts and open channel segments. The maximum time to drain the basin complex completely is 30 hours during the 100 year event, or shorter during smaller flow events.

Construction of Phase 2B-Park Avenue Basin Complex would provide flood control benefits as well as ecosystem restoration/enhancement, recreation and other benefits. The project would provide environmental restoration of degraded desert habitat and riparian ecosystem within the Park Avenue Basins. Environmental impacts would include preservation of 2.85 acres of existing riparian habitat, restoration of 12.95 acres of riparian environment, and 5.55 acres of riparian mitigation. Revegetation of Basins 1, 2 and 3 includes a combination of native riparian and upland species, as well as turf areas for use by residents of the local neighborhoods. A paved bicycle and maintenance path will encircle the project. The existing dip in the roadway crossing at Highland Avenue will be replaced with a culvert.

Channel improvements in the Arroyo Chico reach between Campbell Avenue and Parkway Terrace will also include replacement of existing dip crossings at Norris Avenue, Olsen Avenue and Plumer Avenue with culvert crossings, thereby providing all weather access and improved roadway safety benefits.

PROJECT BENEFITS

This multi-purpose project provides flood damage reduction, environmental restoration and recreation benefits. Approximately 1,048 residential, commercial, and industrial structures will be removed from the 100 year floodplain by the completion of the second phase, compared with 241 removed by the first phase.