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 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's Road. The two phases of the project are Phase 1 Randolph South Detention Basin which was completed in April 1996 by the Pima County Flood Control District and the City of Tucson, and Phase 2 the Park Avenue Detention Basin Complex, which is to be constructed by the Corps of Engineers beginning in the summer of 2007 and expected completion in four years. The total estimated cost (Phases1 and 2) of the project is $68 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 runoffs 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 infrastructures are estimated by the Corp of Engineers at $5.3 million annually, resulting in a benefit-cost ratio of 1.4.

The construction of the Phase 2 Park Avenue Detention Basin Complex has been divided into two segments: (1) Construction of Cherry Field Detention Basin and improvements to the adjacent Railroad Wash confluence; and (2) all other Phase Two improvements. The Park Avenue Basins 1, 2, and 3 will provide the opportunity for environmental restoration of degraded riparian ecosystem, preservation of acceptable existing native habitats, and recreational improvements for the adjacent neighborhood. 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 with final construction plans and specifications completed in March 2005.

PROJECT FEATURES

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

Randolph South detention basin complex constitutes the Phase 1 of the overall Arroyo Chico Project. It was designed and constructed by the Pima County Flood Control District and the City of Tucson, and represents the local sponsor's contribution to the overall project under Section 104 Credit Agreement with the U.S. Army Corps of Engineers.

The basin complex consists of six interconnected detention basins that were constructed within the existing Del Urich Municipal Golf Course (formerly the Randolph South Golf Course). It intercepts flood flows 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, having a total storage capacity of 530 acre-ft. The basin complex reduces the without-project 100-year discharge of 3400 cfs to 269 cfs. The outflow (269 cfs) from the 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, eliminating flood damages in the downstream areas of Colonia Solana, Broadway Village, Broadmoor, and Parkway Vista.

Phase 2 (Park Avenue Basins Complex)

The Park Avenue Basins complex and the associated channel/culvert improvements constitute Phase 2 of the overall project. The Phase 2 project includes the following elements:

• Three in-line detention basins, called Basins 1, 2, and 3, located along Arroyo Chico between Cherry Avenue and Park Avenue.
• An offline basin, called TUSD Basin, located within TUSD's Cherry Field between Campbell Avenue and Cherry Avenue.
• Realignment of High School Wash box culvert (underground) along 3rd Avenue and 8th Street.
• Modified confluence of Railroad Wash and Arroyo Chico.
• Larger concrete box culverts under Campbell Avenue.
• 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 Arroyo Chico confluence with Railroad Wash, followed by Basin 3, Basin 2, and Basin1 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 or bottom. Flows are conveyed from inlet to the offline TUSD Basin through a sideweir on the north bank at the Arroyo Chico-Railroad Wash confluence. The side weir is designed such that flow enters TUSD Basin only for inflows larger than 18-year events. During the receding phase, flows out of TUSD Basin are conveyed back to the confluence through a low-flow drain to Basin 3. During a 100-year flood (or smaller flows), 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 four basins completely is 30 hours during a 100-year flow event, or shorter during smaller flow events.

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 second phase, compared with 241 removed by the first phase.

Construction of Phase 2, Park Avenue Basins 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.

Other related benefits from the project include improved and modern buildings and athletic facilities that will replace the existing structures in the Cherry Field (TUSD Basin). The new facilities will exceed floor areas and the quality of various equipments and structures located in Cherry Field will be upgraded to meet current standards and codes. The Tucson Unified School District will therefore own a brand new and updated athletic facility at Cherry Field at the completion of project construction. Channel improvement 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.