District Structure Provides Solid Foundation


It’s official. District employees in Tampa now have a new home.

While the subject of maintaining flood control structures caught the nation’s attention after Hurricane Katrina decimated New Orleans, structures have always been at the top of the District’s priorities.

Flood protection is one of the District’s areas of responsibility. The District’s goal is to minimize the potential for damage from floods by protecting and restoring the natural water storage and conveyance functions of flood-prone areas.

The District manages and maintains 83 water conservation and water control structures throughout the District, which includes four canal systems: the Tampa Bypass Canal, the Lake Tarpon Outfall Canal, the Masaryktown Canal and the Lake Tsala Apopka system. The District regularly performs preventative and routine maintenance on these structures as prescribed by the schedules in the District’s five-year plan.

Terry Rigsby, senior tradesworker, conducts and coordinates these inspections.

“We use consultants to inspect about 10 of the major structures, and I inspect the rest,” said Rigsby.

Many of the structures are inspected on a quarterly basis. Other structures, like the Medard and Inglis dams and Inglis Bypass Spillway, are inspected weekly. These three structures are also inspected weekly by other agencies. For instance, the Hillsborough County Department of Parks, Recreation and Conservation manages the Medard Park and Reservoir so county staff also inspect the dam.

During a routine inspection in November, Rigsby noticed some seepage near the P-5 water control structure in Polk County. The P-5 structure is the outfall for Lake Henry, which discharges into Lake Hamilton.

“I almost didn’t notice the problem because recent rains left the ground wet, but I thought something didn’t look right,” said Rigsby.

Because of Rigsby’s findings, the District hired structural engineering consultants to conduct further tests. District staff, in cooperation with the Lakes Region Lake Management District, slowly released water from Lake Hamilton to allow storage for water from Lake Henry so piezometers (test wells that measure pressure) could be installed to collect data about the site’s groundwater characteristics and the surrounding soil. The information collected from the piezometers supported Rigsby’s findings that there is seepage on the upstream side of the structure. It also showed that there is some unobservable erosion under the downstream slopes.

“We looked at several repair options,” said Dale Ravencraft, District structure operations manager. “The first one came in at over $400,000. After subsequent discussions between District and consulting engineers, we decided on a reasonable, cost-effective plan to repair the structure.”

Repairs are now expected to cost approximately $175,000 and will be completed before the start of the 2006 hurricane season. The water in Lake Henry will be kept at a slightly lower level until the repairs can be completed.

The Peace River Basin Board approved the plan in December. The repair plan involves partially removing the soil in the problem area and pumping in a slurried grout to fill the space. A stone drainage blanket will be installed on the downstream embankments.

A few years ago Rigsby discovered a problem at the S-155 structure. This structure reroutes Hillsborough River floodwaters down the Tampa Bypass Canal and out to Tampa Bay.

Rigsby noticed one of the walls on the embankment looked like it was beginning to bow out. Tests showed the wall was beginning to bend, so the District installed a soil−anchor system on each side of the structure. A soil-anchor system involves installing large steel cables from one side into cement anchors placed under the embankment. The cables are tightened and grout is pumped in to hold them in place. Since the system has been installed, the walls have not moved.

“I will measure the deflections (angles) of the walls periodically,” said Rigsby. “But other than the slight changes that occur because of a change in temperature, there hasn’t been a problem.”

“The work Terry does for the District is very important,” said Gary Kuhl, District operations director. “In a sense, his thoroughness helps keep citizens safe, as well as water levels at the optimum levels.”