Manvel commission narrowly approves pump-detention plan for proposed freestanding ER near Sonic

The Manvel Planning, Development and Zoning Commission voted 4-3 on March 10 to recommend that city council allow a pump-detention system for a proposed 1.54-acre freestanding emergency facility on the south side of Highway 6 near Sonic.

City staff told commissioners that the city’s engineering design criteria generally discourages pump detention because of possible pump failure and extra maintenance, but that pump detention can be considered by council on a case-by-case basis when certain conditions are met. “Generally, pump detention has been discouraged due to potential failure of the pump system, the need for more frequent maintenance and responsibility in maintaining the system, and the potential of flooding hazards that can come from reliance on a complex system to handle a large amount of storage volume,” staff said during the presentation.

John English of Reca Engineering, the project engineer for the applicant, said the site requires pumping because the outfall along Highway 6 has only about a 2-foot-deep drainage ditch. “The reason why this project requires, needs, pumped detention is because we only got a 2 foot, drainage ditch out front to outfall into,” English said. He described a design with three outfalls (gravity for mitigation, gravity for a detention pond, and a separate outfall for the lift station), a backup natural-gas generator, and a maintenance covenant.

Commissioners and the applicant discussed redundancy and monitoring. The applicant said the system will use dual pumps and a generator; “We have a dual pump system,” the applicant said, and added the pumps are sized to empty the pond in about 15 hours under expected conditions. The applicant also said the owner will hire an operator to perform monthly checks and submit reports; Brazoria Drainage District No. 4 inspectors and city staff would be invited to witness inspections.

Staff reiterated it is not in favor of pump detention in general but said the application meets the manual’s listed criteria — nonresidential use, no residence within 1,000 feet of the pump facility, a proposed backup generator, and the applicant’s pledge that at least 50% of discharge will be by gravity — and noted final permit approval will require the drainage district’s sign-off.

After discussion, a commissioner moved to recommend approval of the pump-detention modification; the motion passed 4-3. The commission recorded no named roll-call votes; the tally was recorded as 4 in favor and 3 opposed. The action will be forwarded to city council with the commission’s recommendation and the documentation the applicant provided, including a maintenance plan and draft maintenance covenant.

The commission’s packet notes the site is within the AO and X unshaded flood zones and that Brazoria Drainage District No. 4 and TxDOT are jurisdictional reviewers for outfalls along Highway 6. City staff and the applicant said final approval and operation will depend on the drainage district’s concurrence and permit review.