In the Houston area, water is a constant challenge. Residents often face droughts and floods, and the Pecan Lakes subdivision is no different. Jones Creek runs adjacent to this high-end housing development and due to the developments' close proximity to the Brazos River, water from the Brazos River inundates Jones Creek and poses a significant flooding threat to the neighborhood.
In 2007, Federal Emergency Management Agency (FEMA) began remapping the area to update the flood insurance rate maps. As a result of the remap, all 325 residents of Pecan Lakes subdivision would be without flood protection.
Searching for a solution to reinforce the creek banks and protect the neighborhood, the Pecan Grove Municipal Utility District partnered with District Engineer Jones|Carter to find a solution to reinforce the creek channel and provide flood protection that would be on par with the rest of the District.
Initially, officials thought that the project could not be successful or cost effective without acquiring approximately 26 homes, due to unstable soil conditions and the limited right of way along Jones Creek. However, innovative thinking from Jones|Carter allowed the District to provide a solution that negated the need to acquire the homes - saving $1 million. The overall solution included a multi-component system consisting of an earthen levee, a structural floodwall, and a pump station.
For the structural floodwall, Jones|Carter devised a unique solution that included a gravity retaining wall system coupled with a complex dual level sub-surface sheet piling wall for soil stabilization. It proved the best solution to minimize short and long term impacts to residents.
"In 2013 when this project started construction, Texas had been in a drought for several years. Vegetation was dying and engineers were concerned that if a large storm event came through, you'd see massive erosion," explained Jamie Johnson, PE of Redi-Rock International. "Because of the poor soil conditions, there was a significant concern that flooding events would saturate the bank and as floodwaters receded, you'd have an unstable slope condition."
Jones|Carter specified Redi-Rock® as the gravity retaining wall system for the project. Redi-Rock is a large block retaining wall system using massive, one ton blocks that stack up similarly to Lego blocks. Gravity walls rely on the mass of each block for structural support and are a huge benefit in applications like this because they can be built close to property lines.
"The way Redi-Rock came into play was that it was able to give us an alternative so that we could build the flood wall behind the homes with enough space to allow annual maintenance and operations. We also had room to transition the walls back down to the bottom of the existing creek. Using gravity walls allowed us to avoid using soil anchors which would have had to extend into private property and beneath existing homes. That's where Redi-Rock became very enticing because we no longer had to worry about subsurface connections beneath people's homes," said Craig Kalkomey, PE of Jones|Carter.
The soil along the creek is a mixture of heavy clay with high plasticity index, mixed with pockets of very poor sand and gravel. The sheet pile component of the bank stabilization was required to cut off these sand and gravel seams. Kalkomey explained that as water rises in the channel, designers wanted to avoid water passing through the walls and underneath homes because that could lead to material washing out from underneath residences. A 20 foot (6.1 meter) sheet pile cutoff wall was designed to increase overall global stability.
"It was really designed as a full system - the wall and sheet piles were designed to function together," Johnson said.
Above the sheet pile walls, "the Redi-Rock walls were set on a gravel underdrain system and backfilled with gravel. Essentially we did the two tiered wall to make the design a little safer. We still have the vertical drops, but instead of having one big vertical drop this allowed us to tier it, giving an amenity look with intermediate grass," Kalkomey said.
The design team utilized Redi-Rock nine inch setback blocks to increase the batter of the walls.
"We felt that the 9" setback would help give a better aesthetic rather than two completely vertical walls. It also provides flexibility and stability during ingress and egress for cleaning and maintenance purposes. It helps us better inspect the system because it actually serves as mini stairs to safely access the bottom," Kalkomey said.
These walls, all of which feature the Limestone face texture, stretch over a quarter mile along the edge of the housing development and encompass a total of 38,525 square feet (3,579.1 square meters) of retaining walls. The lower wall stands 13.5 feet (4.1 meters) tall and the upper wall stands up to 6 feet tall. The 9 inch setback walls give the channel a trapezoidal shape, which creates an efficient hydraulic radius that leads to improved open channel flow rates.
The overall system is designed based on a 100 year storm level, which was tested when a major storm hit the area in May of 2015, shortly after the project was completed. Based upon the effective information on the flood, experts have classified the storm between a 50-year and 75-year event.
"Parts of the wall were underwater for nearly a month," Kalkomey said. "Since the May event, we've had multiple successive events and the system has been soundly tested. Fortunately, everything worked out very well. For the portion ultimately supplemented with the Redi-Rock, we could not take a chance. We had to stabilize it; we had to make sure movement would not occur," Kalkomey said.
As a testament to the success of the project, it was awarded the Silver Award from the American Council of Engineering Companies - Houston in the water resources category. This project was also named Water Application Wall of the Year in the annual Redi-Rock Rocky Awards.
Project: Pecan Lakes/Jones Creek Channelization #180 Owner: Pecan Grove Municipal Utility District Manufacturer: Wilbert Vaults of Houston General Contractor: Lecon, Inc. Engineer:Jones|Carter Location: Richmond, Texas Year Built: 2013-2014