Overview
Pima County Regional Flood Control District and the City of Tucson in cooperation with the U.S. Army Corps of Engineers (USACE) 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 4.8 miles of the Tucson Arroyo/Arroyo Chico 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 December 2008 by the USACE; and
- Phase 2B, Park Avenue Detention Basin Complex, which was completed by the USACE in December 2012.
- Increment 3 (Upstream Channel Improvements), in 2022, reconsidered the need to construct additional channel capacity and ecosystem mitigation, and project design intent met without increment 3 construction.
- Increment 4 (High School Wash Storm Drain) was completed April 2015.
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 that 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 1920s. Segments of the lower watershed storm drain have recently been reconstructed as part of the City of Tucson's 8th Street Drainage Improvements and the 4th Avenue Streetcar projects. The City of Tucson is currently under construction with enlarged and reconstructed Tucson Arroyo storm drain system as part of the Downtown Links Project.
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 were estimated by the USACE at $2.7 million (1998 prices) annually, resulting in a benefit-cost ratio of 1.3:1.
Construction of Phase 1 Randolph Park Detention Basin Complex provided opportunity for a multi-use facility with the primary use being a golf course and the secondary purpose as a flood and drainage facility. The detention complex detains floodwater originating upstream of Arroyo Chico and Naylor Wash protecting downstream residents in Colonia Solana, Broadway Village and Broadmoor subdivisions, while creating the Del Urich golf course that includes interesting and challenging topography from one that was originally characterized as a pool table.
Phase 2A Cherry Field Detention Basin and Railroad Wash Confluence provide another opportunity for a multi-use facility, with the athletic fields being the primary use and flood detention being the secondary use. Due to limited federal funding Phase 2A was bid and constructed separately from Phase 2B by the USACE.
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 USACE. Construction plans and specifications were completed in March 2005, but federal funding for Phase 2A was not available until July 2007, and June 2010 for Phase 2B.
Project Benefits and Features
Project Benefits
This multi-purpose project provides flood damage reduction, environmental restoration and recreation benefits. It has been estimated that up to 1,315 residential, multi-family, commercial and industrial structures will be removed from the 100-year floodplain. The most current map revision application to FEMA does not reflect the future drainage infrastructure under construction by the City of Tucson with the Downtown Links Roadway Project. To date 1,047 residences or businesses have been removed with a total assessed property value of $226,932,898. The footprint of the floodplain has been reduced by 72%. Homes removed from the FEMA 100-year floodplain would save individual households between $2,000 to $3,000 annually in flood insurance costs.
There are 71 publicly owned parcels that are not reflected in the assessment above, including 3 schools, and the maintenance and bus yard for Tucson Unified School District. Reduction in flood repairs and inundation costs to the public infrastructure of $2,070,000 annually by providing flood control protection to:
- Union Pacific Railroad, 1.26 track miles
- Interstate 10 corridor and 4.6 miles of local major streets
- The Tucson Modern Streetcar storage facility and route
- State Route 210, the Barraza-Aviation Parkway through downtown Tucson
Project Features
Phase 1 - Randolph South Flood Detention Basins
Phase 1 consists of a series of six (6) interconnected detention basins with a capacity of 530 acre-feet of flood storage during a 100-year event constructed on an existing public golf course in the eastern part of the watershed. Individual basins were located within the fairways and out-of-play areas of the golf course. Tees and greens, which are expensive and easily damaged by floodwaters, were located in the high ground area above the 100-year flood stage. Concrete cart paths with small bridges and carefully designed fairway through the basins make golf play possible during the frequent low flow events. 400,000 cubic yards of material was excavated and removed from the project site to create the basins. Flood flow cascades under gravity through a series of pipes and weirs. A non-jurisdictional embankment at the downstream end collects flows off of the main watercourse and serves as the final control point for remainder of the basins. The final outflow is metered to the existing downstream watercourse via a single 5 feet by 3 feet concrete box culvert. The flood detention complex reduces the peak discharge from 3,100 cubic feet per second (cfs) to 240 cfs.
Construction of the Randolph South Detention Basin began May 1995 and was completed in April 1996. Phase 1 was designed and constructed by Pima County Flood Control District and the City of Tucson. This phase of the project provides a substantial portion of the Flood Control District’s $12 million cost share of the overall project, under Section 104 Credit Agreement with the ACOE.
Phase 2A - Cherry Field Detention Basin and Railroad Wash Confluence
Cherry Field is a Tucson Unified School District (TUSD) athletic field for Tucson High Magnet School’s baseball and softball program. Development of this basin required cooperation from TUSD, Pima County Regional Flood Control District, the City of Tucson and USACE.
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 main inline control culvert at the Arroyo Chico/Railroad Wash confluence along the Cherry Avenue alignment. 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.
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.
The Pima County Regional Flood Control District in cooperation with the adjoining neighborhood provided sidewalks to connect Cherry Field with Miles Elementary School along Miles Street, Cherry Avenue, and 13th Street. The City of Tucson provided public art for the facility in the form of portraits of the Miles and Barrio San Antonio neighbors on the Winsett/15th Street floodwall near the Cherry Avenue intersection.
Construction began July 2007 and was completed in December 2008. The USACE cost for construction of Phase 2A is $21.8 million. Upon completion of the facility by the U.S. Army Corps of Engineers, Tucson Unified School District (TUSD) retained control of the site as an athletic facility. Pima County Regional Flood Control District has a perpetual drainage easement over the property.
Phase 2B - Park Avenue Basin Complex
The Park Avenue Basins consist of three in-line detention basins called Basins 1, 2, and 3 and are located along Arroyo Chico between Park Avenue and Cherry Avenue. Increment 3 (Upstream Channel Improvements) and Increment 4 (High School Wash Box Culvert Storm Drain Improvements) were originally part of the contracted Phase 2B improvements and will be completed separately from the Park Avenue Basin Complex when additional federal funds are authorized.
The inlet to the Park Avenue Basin complex begins with Culvert 4 of the Phase 2A improvements at the Arroyo Chico and Railroad Wash confluence, followed by Basin 3, Basin 2 and Basin 1 in the downstream direction. The basins are defined by and hydraulically controlled by the size and dimension of the outlet of the concrete arch culverts. The depth of the basins below the adjacent streets varies between 12 and 20 feet, with the basin bottom approximately at the same depth as the pre-construction arroyo channel invert. During the receding phase, flows out from Cherry Field Basin drains into Basin 3. During a 100 year flood, storage within the basins reduces the flood peak to a level that can almost be contained within the existing Tucson Arroyo underground concrete box culverts and open channel segments. The completion of the City of Tucson Downtown Links storm drain should fully contain the 100-year event. The maximum time to drain the basin complex completely is 36 hours during the 100 year event, and shorter during smaller flow events.
Construction of Phase 2B-Park Avenue Basin Complex provides flood control benefits as well as ecosystem restoration/enhancement, recreation and other benefits. The project provides 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 two turf areas for recreational use by residents of the local neighborhoods. A paved bicycle and maintenance path encircles the project. The deep dip section at Highland Avenue has been replaced with an all-weather culvert and widened street. Two ephemeral toad-breeding ponds have also been developed at the request of the neighborhood to promote the return of frogs and lizards to the area.
Construction of Phase 2B (Park Avenue Basins Complex) improvements by the U.S. Army Corps of Engineers and their contractor (Hunter Contracting) began September 2010 and were completed December 2012. The USACE cost for construction of the Park Avenue Basin is $11.1 million.
Pima County Regional Flood Control District is the agency responsible for operating and maintaining the Park Avenue Basin Complex. District has provided additional improvements to the site at the request of the adjacent Miles and Barrio San Antonio neighborhoods, which included:
- Public art from 3 different artists. Mary Lucking provided 8 seating tuffets in various location throughout the Park Avenue Basins. John Davis provided a large metal sculpture on the west side of the Highland turf area, near Santa Rita Avenue. And the artist team of Josh Sarantitis and Alex Garza provided a metal , glass and tile sculpture for the turf area at 12th Street and Santa Rita Avenue.
- Additional sidewalks along the east side of Fremont Avenue next to Fremont building, south side of 13th Street between end of Basin 2 pathway and Highland, east side of Highland Avenue to alley, south side of Manlove Street between apartments and Cherry Avenue, and north side of 15th Street from end of pathway at Culvert 4 to Father Kino statue at Kino Blvd.
- Paving across the top of culvert 2 embankment, on the Santa Rita Avenue alignment between 13th Street and Manlove Street.
- Additional handicap access ramp at Santa Rita and Manlove.
- Pedestrian crossing at Highland Avenue.
- Additional traffic and park rules signs.
- A BMX bicycle park area was developed by the District and is being maintained by local volunteers. District provided a water source to the area for dust control.
- Additional trees and shrubs along Fremont Avenue.
- A drinking fountain near the pathway and the Fremont maintenance building and fencing around the Fremont building.
Additional improvements to the Park Avenue Basin Complex since construction completion include landscape and enhancements around the public path as well as placement of rubble-pile clusters to benefit lizard species recovery.
Increment 3 - Upstream Channel Improvements
Increment 3 was recommended by the USACE Feasability Study completed in 1998. The project has been redesigned based on recently completed geotechnical investigations (April 2017) by the USACE. The original plans would have removed a significant area of vegetation and soil, which would have been replaced by a five-foot deep concrete channel. The redesigned project increased the estimate project cost from $2.2 Million to $15.8 Million due to highly engineered soldier pile wall construction needed to address limited right of way and potential impacts to adjacent foundations. The District proposed an alternative construction concept to try to reduce costs and provide a more environmentally sensitive design. The proposed design would require acquisition of adjacent properties and widening the channel. Construction costs would be less but the neighborhood impacts would be greater. The District undertook a more detailed study of the existing conditions with all previous phases in place to determine the benefit of the proposed Increment 3 improvements. The original Feasibility Study states that these channel improvements were needed to avoid stormwater breaking out of the existing channel and bypassing the downstream Phase 2 basins. The District study utilized a combination of one dimensional and two dimensional floodplain mapping along with updated ground survey information to demonstrate that under existing conditions the amount of flow bypassing the basins during the 100-year storm event is minimal. Based on this updated modeling USACE agreed to close out the Arroyo Chico project without construction of Increment 3.
The Flood Control District has completed remapping of the FEMA 100 year floodplain. The previously completed Randolph South Detention Basins, Cherry Field (TUSD) Detention Basins and Park Avenue Detention Basins have significantly reduced the floodplain footprint. Minor flooding outbreaks continue along the upstream Arroyo Chico channel, affecting property immediately adjacent to the wash and along intersecting streets such as Plumer, Olsen and Norris.
Increment 4 - High School Wash Box Culvert Storm Drain
Increment 4 is part of the contracted Phase 2B improvements. Additional Federal funds were authorized June 24, 2013 and construction began June 2, 2014 and was completed on April 9, 2015. The City of Tucson requested the Flood Control District pursue betterments in the form of enlarging a section of the main storm drain to handle additional flood flows, and addition of catch basins to remove street runoff and thereby reduce the limits of the downstream floodplain. On May 13, 2014 the Pima County Board of Directors for the Flood Control District authorized an additional $1,500,000 to pay for construction of the $1,921,165 storm drain betterment.
The High School Wash Box Culvert Storm Drain consists of 776 linear feet of new concrete box culvert (12 ft. x 8 ft. and 10 ft. x 8 ft.) that connect with the existing 1930’s vintage 10 ft. x 7.5 ft. concrete box culvert located under Tucson High School with the recently completed City of Tucson/ Regional Transportation Authority (RTA) 8th Street Drainage Improvements that ended at 4th Avenue. The new storm drain efficiently moves storm flow from the inlet at Euclid east of Tucson High into the new 8th Street storm drain system and ultimately into the Santa Cruz. The project significantly reduces flooding around Tucson High School and the 4th Avenue business district.
Cost for construction of this increment was $4.2 million.
FEMA Mapping
Current mapping activities
A Letter of Map Revision (LOMR) has been approved by FEMA to re-map the floodplains for Tucson Arroyo, Arroyo Chico and the High School Wash, from Fremont Avenue west to the Santa Cruz River. This map revision, LOMR Case # 15-09-2298P, was effective June 13, 2016. For more information on LOMRs, including flood insurance refunds and technical appeals, please read the LOMR Frequently Asked Questions document.
Past mapping activities
The District has previously completed a LOMR for Phase 1- Randolph South Detention Basins (FEMA Case number 00-09-274P, Effective March 29, 2011). This LOMR removed 241 residential properties from FEMA’s Special Flood Hazard Areas (SFHAs) along the Arroyo Chico between Randolph Way and Tucson Boulevard. This LOMR also included a portion of the Citation Wash.
The Phase 2- Park Avenue and Cherry Field Basins LOMR revised the Arroyo Chico and Railroad Wash floodplain and became effective August 29, 2014 (Case # 13-09-3317P) The limits of that map revision started upstream (east) of Tucson Boulevard and extended to Fremont Avenue.
Future drainage improvements
Analysis indicates a shallower floodplain exists from 4th Avenue west to just upstream of Main Avenue. Additional drainage improvements are proposed by the City of Tucson associated with the Downtown Links roadway improvements which will further reduce that flood hazard.
The process of re-mapping the floodplains based future improvements cannot begin until the construction of those projects are finished and “As-Built” construction plans are certified.
Habitat Restoration and Recovery
In the years since construction of the Arroyo Chico Park Avenue Detention Basins project, monitoring of habitat and wildlife continues to show recovery of plants and animals, and increasing species diversity within the project. Wildlife species most benefitting from project habitat restoration include birds, lizards and toads. More recent Projects Updates (see tab) include improvements for migratory and resident bats that provide beneficial mosquito control.
Lizards in particular have recovered well with tiger whiptail, tree lizard, desert spiny lizard, side-blotched, zebra tailed lizard, and regal horned lizards recorded. Habitat improvements have resulted in increased number of ant species, which provide food for the lizard population.
Toads have benefitted from the construction of ephemeral ponds that provide suitable breeding habitat for Couch’s spadefoot, Great Plains, narrow-mouth toads. Populations of macroinvertebrates contribute mosquito control in the ephemeral ponds. However, non-native mosquito fish often wash downstream into Arroyo Chico from Reid Park during flooding impacting the native beneficial macroinvertebrates necessitating monitoring and control of mosquitos within the project.
Native plants have successfully established providing cover and forage for wildlife. The number of native plant species have increased over 200%. Plants continue to mature and a combination of wetter and drier habitat areas within the project support a variety of native trees, shrubs and grasses. Monitoring data to date documents 63 resident and migratory bird species, an increase in diversity and abundance over pre-project levels. These bird species include eight priority Arizona Partners in Flight species (Brewer’s Sparrow, Costa’s Hummingbird, Gilded Flicker, Grasshopper Sparrow, Lucy’s Warbler, Purple Martin, Rufous-winged Sparrow, and Savannah Sparrow).
Monitoring data provides a measure of the ecological success of the habitat restoration and guides adaptive management of native biodiversity. Improvements to project habitat is ongoing with monitoring and control of non-native species and conversion of landscaped rock areas being converting to more natural conditions to benefit regal horned and other lizard species forage and cover needs.
See more photos of important animal species at Arroyo Chico.