Engineer: debris basin limits and swale bypass could reduce flood risk quickly

Lane Jensen of France & Civil reviewed the town's hydrology and said the debris basin and associated splitting structures are the critical control points for short-term flood abatement. He told the council that Utah Dam Safety had previously documented seepage and that the basin's auxiliary spillway would overtop at flows modestly above the 100-year event in a burned watershed.

Jensen recommended options that could be implemented quickly: (1) create a swale to bypass undersized culverts at the road to get water into the north flood channel earlier, (2) install log booms and debris screens near the primary spillway and auxiliary outlets, and (3) partially fill the debris basin to allow sediment to settle before the outlet works. "If you leave it open, then that debris will go right to the outlet works and block it immediately," Jensen said, describing how sediment could negate other efforts unless the debris basin and split structures are managed.

He described capacity numbers from his models: in unburned watershed scenarios design flows were just over 10,000 cfs and his team simulated routing 7,000–8,000 cfs through the north channel in some runs; with targeted ditch and splitting-structure changes, he said the town could manage significant flows through existing channels with modifications. Jensen said many of the required maps and calibrated models already exist from ongoing PL 566 work, which would speed design and help the town avoid building models from scratch.

Councilors asked about cultural clearance, culvert procurement and timing; Jensen noted cultural clearance for many ditch areas had already been completed under PL 566 appendices, giving the town flexibility to act quickly on short-term measures. He said targeted modeling and elevation setting for a swale could take less than a week, while a more durable box culvert would be a longer-term procurement.