Officials with the Harris County Flood Control District revealed several methods to deal with flooding in the Cypress Creek and Addicks Reservoir watersheds at a Nov. 7 public meeting based on studies that have taken place over the past year.
While floods in these watersheds do not occur often under current conditions, HCFCD is looking for ways to prepare for future development in west Harris County, which is expected to make flooding more of an issue. Census data for this particular segment of west Harris County projects population to increase from 300,000 to 540,000 over the next 50 years.
"Right now, Cypress Creek overflow is occurring in areas that are predominately undeveloped or partially developed," said Dena Green, study manager in HCFCD's engineering and construction division. "However, we've seen a lot of information that indicates west Harris County will undergo a rapid increase in population. If that occurs, we think there are going to be some pretty dramatic land use changes."
Cypress Creek overflow—when water flows southward out of Cypress Creek Watershed into the Addicks Reservoir Watershed—happens approximately every eight to 10 years, Green said. During especially heavy rains, overflow will continue into the tributary system and ultimately drain into the Addicks reservoir.
"Although the Addicks Reservoir has a large storage capacity, we need to be cognizant of its limit and rate of discharge," Green said. "If too much [water] is released, you have flooding downstream on Buffalo Bayou. If you don't release enough, you're going to impact the property upstream."
The ongoing study involves measuring the ability of three different land types to absorb rainwater and analyze how future development could impact water infiltration into the soil. Two monitors measuring runoff and absorption were placed in areas identified as "highly developed," "open space" and "prairie." The overall study area encompasses 400 square miles, from east Waller County down to and including the Addicks Reservoir.
"There are some theories that the native prairie grass helps increase the infiltration capacity of the soil," Green said. "They help absorb water as rainfall and runoff goes across the land. That helps reduce the overall volume of water draining into the tributaries."
HCFCD has been monitoring the six sites for about a year and expects to have a preliminary report in December. Green said HCFCD will continue to monitor the sites for another five years as development continues.
In the meantime, HCFCD officials have proposed several projects that would help mitigate overflow and protect populations from increased flooding. Alan Potok, director of HCFCD's engineering and construction division, said the objective was to come up with something that was both financially feasible and could be implemented in a timely fashion.
A steering committee with members representing the City of Houston, Waller County, Harris County precincts 3 and 4 and the U.S. Army Corps of Engineers, among other entities, are helping guide the process. The most appealing concept so far involves three steps.
First, a berm—or raised barrier—would be created between Cypress Creek and Addicks watersheds to collect water and provide relief for the overflow area. Second, overflow conveyance mechanisms would be developed to help convey water downstream to existing channels such as Bear Creek. Third, a 4,000-to-8,000-acre upstream storage facility would be created in Waller County to store between 11,000 and 26,000 acre-feet of water. The study also recommends setting aside 3,500 acres for conservation purposes.
"If we combine the holding basin upstream in Waller County and collect and convey the rest of [the overflow] down Bear Creek, we reduce the overflow actually occurring and control the flow rate into the reservoir," Potok said.
The proposed projects would cost an estimated $325 million for land acquisition, construction and contacted professional services. It would likely be completed over an extended period of time to avoid having to pay the entire cost at once. Potok suggested charging impact fees to developers looking to build in the floodplain as one way to raise revenue to help fund projects.
The next step for HCFCD is to continue to develop these concepts into a draft that can be presented to the Texas Water Development Board and Harris County Commissioners Court for approval. The draft is expected to be ready by early spring 2014. A third public meeting will take place after the draft is composed, but before it is finalized and submitted to TWDB.