Half-built detention pond eases flooding on Smithville street

A half-built detention pond at the end of Northeast Seventh Street in Smithville held 8 million gallons of water that otherwise would have flooded homes on the flood-prone street during Tropical Storm Harvey, officials said.

“Without the detention pond, flooding would have occurred much sooner and the damage would have been much more severe,” said City Manager Robert Tamble.

Smithville received 23.58 inches of rain from Friday through Monday, as Tropical Storm Harvey pounded South Central Texas. Nine miles east of the city, the storm dropped 29.09 inches of rain, the National Weather Service said Monday.

The pond is only half-built but on track to be completed by October, 2½ years after the grant process to build it began, officials said. It’s meant to alleviate flooding on Northeast Seventh Street, which persistently floods during heavy storm events.

The 3.4-acre grass pond south of Northeast Seventh and Marburger streets is the first phase of a $1.1 million project to provide drainage relief to the 256-acre flood-prone area of the city that culminates on Northeast Seventh Street. It will hold about 13 million gallons once built out, which will be pumped out after each storm event.

The project was 75 percent funded by Federal Emergency Management Agency. The city budgeted the remainder without incurring any debt, Tamble said.

Last week, a day before Harvey, then a Category 4 hurricane, moved north to South Central Texas as a tropical storm, the city held a public meeting to update residents on the status of the pond and explain how at the midpoint of excavation, the pond might still help alleviate flooding.

“You’re still in a lot better situation than before June when this construction started,” said Tim Sanders of Befco Engineering.

About 50 percent excavated and 14 feet deep, the pond filled up within a 24-hour period Saturday.

“It’s not a hole anymore. It’s a lake,” Tamble said Sunday afternoon.

To help curb flooding, city crews put out a temporary pump, nicknamed “Jenny,” with 450 feet of hose to move water from Northeast Seventh Street to the pond, and brought in a Fire Department pumper truck during the heaviest rains to help move water out of the street. “Forrest,” the pumper truck, was staged on the east end of Seventh Street to move water across Faulkner Street.

Still, several homes flooded on Northeast Seventh Street on Saturday.

Construction on the detention pond was met with an unexpected issue earlier in August. When excavating, the contractor struck groundwater.

Core borings down to 30 feet revealed that the groundwater table had risen 8 feet since the last core samples were taken, officials said.

The city considered leaving the design of the pond the same, but adding a clay layer to keep water from rising into the pond, but dewatering the pump station and pond first was cost-prohibitive and would have made the project go over budget, said Smithville Mayor Scott Saunders.

Instead the city has decided to dig the sides of the pond steeper, which will make up the difference in cubic volume from a shallower pond at 13 to 15 feet, over 6 feet higher than originally proposed. The original design also called for gradual slopes. The pump station will be underwater. The grant process for the detention pond started over two years ago after the 2015 Memorial Day weekend flooding. President Barack Obama issued a federal disaster declaration for Bastrop County for that rain event, as well as for another flood in 2015 and two more in 2016.

Phase 2 of the project, which is not funded and is expected to cost about $200,000, would further mitigate flood damages by installing drains on Seventh Street as well as a 60-inch conveyance pipe to the pond.

Tamble said FEMA hasn’t began considering the round of grants the city applied for to fund the additional phase of the project.