Segmental retaining walls fortify Arizona detention basin flood control infrastructure
The American Southwest’s climate is arid and much of the land is wide-open and sparsely populated. Its geography is miles of desert, plateaus, mesas, mountains, broad basins and plains. Torrential rainfall and flooding are not weather conditions that first come to mind, but for longtime residents and state and county governments responsible for land management and public safety, they are a recurring natural hazard that must be reckoned with.
There are two types of floods: regional, which last for weeks or months, and flash floods, which as the name implies, happen suddenly and have durations of merely hours. Both are destructive and can cause damage to homes, property, infrastructure and sometimes injuries and loss of life.
Floods in Arizona can be either type. The monsoon season has sudden, torrential, localized rainfall that descends on a hard, sunbaked landscape scarred by washes and gullies, and the water is not absorbed into it.
From a floodplain-management viewpoint, the biggest challenges arise from flooding on alluvial fans, where channels diverge downstream, and water overflows banks, causing flooding in broad areas. Flood control divisions of local counties share a common mission: to reduce the risks of death, personal injury and property damage through the identification, regulation and remediation of flood hazards.
Using Detention Basin Infrastructure to Mitigate Flooding
One of the most common infrastructure approaches to address flooding problems and protect public and private land is the use of detention basin systems. They are considered storm water best management practices (BMPs), providing general flood protection that can also control flash and extreme floods. The ponds help manage excess urban runoff that results from newly constructed impervious surfaces like roads, parking lots and rooftops. They function by allowing large flows of water to enter, but limit the outflow volume simply by reducing the size of the outflow opening at the lowest point of the structure. Outflow volumes are determined by the ability of underground and downstream culverts and washes to handle released basin water.
Lake Havasu City is one of the largest and fastest growing in Mohave County. Just outside and upstream of the city is the Horizon Six subdivision, a residential development started in 1961, prior to development oversight in rural Arizona. Uncontrolled grading activities had occurred along the wash by area homeowners, which contributed to adverse flooding impacts.
In an effort to combat these seasonal flood issues in Horizon Six, Mohave County officials commissioned a comprehensive existing conditions analysis that included hydrology, hydraulics and sediment transport. A 3.9 square mile watershed area directs water to this area. The conclusions of the analysis led to the construction of a detention basin system and flood control channels that would, among other things, eliminate the threat of flooding for more than 100 properties within the subdivision. Rains had not only been causing flash floods throughout the region but also caused significant erosion and sediment runoff from the hills surrounding Lake Havasu, leading to discoloration in the lake’s traditionally clear blue waters.
Construction of the $4.5 million project was performed in two phases. A series of detention basins and channel improvements were built to minimize flooding through a portion of Lake Havasu City. Channel improvements involved a creek that runs through existing housing developments during flood events. Sedimentation basins and detention basins were added at the entrance to the improved channel. This design reduced the required width of the channel from 50 to 20 feet wide. The channel runs through an existing neighborhood, so reducing the width was required to minimize the disturbance to the properties adjacent to the channel.
Beginning in August 2012, roughly 210,000 cubic yards of earth were excavated from the project site. The size of the detention basins is estimated to accommodate as much as 74 acre-feet of water, or the amount of water produced by a 25-year storm. The basins required the support of several large retaining walls.
Coe & Van Lou Consultants, a civil engineering firm, was tasked with improving the channel. The final design minimized the width of the channel cross section through the use of a detention basin made up of cascading basins. This approach lowered right of way acquisition costs. It also reduced the design flows by 50%, in turn reducing the material costs by 50%. Ultimately, the savings totaled $2 million.
The project would temper the uncontrolled Mockingbird Wash that cuts through the subdivision and effectively eliminates the Horizon Six residents from the current floodplain.
“During the project design development, available options were reviewed, and the local history and previous engineering efforts guided us in selecting the best engineering solution, which also turned out to be the most cost-effective," said Dave West, P.E. and Mojave County lead civil engineer on the project. "We realized that a detention basin would help both reduce the size of the channel we build downstream as well as reduce the impact of that channelization.”
The Role of Segmental Retaining Walls
Nine MSE segmental retaining walls ranging in height from 5 to 25 feet were constructed in the basins and flood control channels, using roughly 64,000 square feet of Keystone Compac II units. Battered walls (roughly 20,000 square feet) were constructed within the detention basins, while the near-vertical walls were built along the channels.
The project was originally specified and designed around a different segmental retaining wall. The original wall specified would have required two different blocks in order to complete the project: one to build the near vertical walls and another for the battered setback walls. The unit, using a fiberglass interlocking pin system, could accomplish both.
The units on this project provided a uniform design adaptable for constructing both phases — the same unit could be used for the walls for the basins and for the channel. Each unit has near-vertical and battered pin hole configurations. Therefore, on both sides of the channel that winds its way through the Horizon Six residential area, the near-vertical wall batter minimized the use of property needed to upgrade the channel. The typical 1-inch setback of a lipped or lug segmental unit would have used more land on each side for the channel phase of the project.
The basins required the 1-inch setback, which was possible using the unit with the fiberglass pins and back pin holes.
“Space was limited on the job site,” said Mike Pruden, KeyWest Retaining Systems, the contractor responsible for installing the walls. “Using two different blocks wasn’t going to be an option. There was too much already taking place on site.”
Changing the product required a redesign of the project, and Keystone engineers were engaged to assist with the process. Once Keystone’s revised design was approved, construction on the project began.
The Keystone Compac wall is a well-known structural segmental retaining wall used on a broad range of applications.
Soils & Backfill
Lake Havasu, geologically, is a desert oasis surrounded by dry, granular cobble material. The local site soils were suitable for reinforced backfill, which also reduced the overall cost. Additional costs are typically associated with importing adequate reinforced backfill material from other sources.
Construction on the tallest wall began in August 2012, a time when temperatures could easily reach 110ºF.
“The biggest thing is getting water into the material so it can be compacted,” Pruden said.
The dry material, however, proved to have some advantages. Soil on the project site was so coarse, allowing for free drainage, that a draining zone was not required behind the units. A crew of six to 11 workers installed an average of 760 square feet each day.
“We actually ran into situations where we were depleting the backfill supply faster than it could be graded,” Pruden added.
The Mohave County detention basin two-phase project was completed in 2013. It demonstrated an effective means of flood control in the area and a cost savings for the channel infrastructure phase that, in the final analysis, paid for the construction of the basin.
“By combining the two projects (basin and channelization), we were able to get the best use out of funding that was available,” West said. “We reduced the impact downstream and reduced the liability of Mohave County.”
West, who is no longer with Mohave County, recently reflected on the project he spent a lot of time on seven years earlier. It has performed well in the intervening years, he said, and needed very little maintenance.
The combination of soil-cement channel bottoms and segmental retaining walls has proven to be an effective design. While the system has not yet been impacted by any extremely large storm events — a good thing for the region — it has functioned admirably in the storm events that have occurred.
Since then, the Mohave County Flood District has managed and completed similar detention basin projects in the ongoing effort to tame floodwater tides, proving that if you do things right upstream, then downstream the results can be exemplary.