Just beyond the grassy fields of Reach 11 Recreation Area in north Phoenix, unnoticeable to most people, are one of the city’s most important insurance policies: the Reach 11 Dikes.
A dike is wall or embankment that is used to hold back water. The Reach 11 Dikes are a system of four dikes constructed in 1977 by the U.S. Bureau of Reclamation (BOR); the primary function to intercept and detain floodwater that originates within the watershed north of the dikes. These dikes also provide cross drainage protection for the CAP canal and flood protection for the residential communities south of the canal.
Originally, the dikes were designed to safely contain the probable maximum flood elevation which would provide protection from an approximate 10,000 year flood. But times and technology have improved and in 2006, by utilizing up-to-date precipitation data and hydrologic procedures, BOR completed a rainfall-runoff study that estimated the minimum overtopping flood recurrence interval for each of the dikes. A recurrence interval is the average time between events with the same intensity. Those recurrence intervals range from 1 to 10 million years for a 96-hour storm.
That doesn’t really mean that it will take 1 million years for a flood like that to occur. Today, they call it a probability. So, instead of saying a 100 year flood, it’s called a 1% flood, meaning there is a 1% chance we could have a flood with that intensity or elevation every year. So if a recurrence interval is 1 million years, it means there is a .0001% chance of having an overtopping flood occurring in any given year.
Underneath the dikes are outlet structures that are designed to release flood water into the canal. All four dikes have their own outlet structure and at each outlet structure there are two pipes that are more than 200 feet in length and 72” in diameter. Each pipe has a gate that controls the flow of water and each outlet structure has a trash rack to catch debris so it won’t hinder the flow of water into the pipes. The gates are currently set so the maximum flow of water into the canal won’t exceed its 3,000 cubic feet per second capacity.
Having a storm that is significant enough to impound large amount of water behind the dikes is a statistic long-shot; however, BOR and CAP inspects the dikes annually and every four years a functional or tabletop exercise of the facility’s emergency action plan (EAP) is conducted.
The dikes have never had water impounded on their upstream slopes but they remain a key piece of infrastructure in the CAP system.
By the Numbers:
- Dike 1 begins at Cave Creek Road and Deer Valley Road, it travels under the 101 and the 51 and ends at Tatum Blvd. It’s 4.5 miles long and is 29 feet high.
- Dike 2 begins on the other side of Tatum Blvd. It passes 56th Street and ends at Scottsdale Road. It’s 3.7 miles long and is 34 feet high.
- Dike 3 begins on the other side of Scottsdale Road. It goes past the Tournament Players Club (TPC) Stadium Course, Hayden Road, and the TPC Champions Course and ends at Pima Road. It’s 2.8 miles long and is 29 feet high.
- Dike 4 begins at the 101 and West World. It passes the Thompson Peak Parkway and ends about ½ mile north of the Taliesin Dr. Bridge. The dike is 3.8 miles long and is 34 feet high.
