PLT12 - Protecting A Reservoir from Flooding Using a Bi-Directional Spillway and High-Performance Turf Reinforcement Mats

Abstract Description: Hazeltine Reservoir is the largest of five reservoirs that make up Denver Water’s North Complex of reservoirs along the South Platte River northeast of Denver, Colorado. The reservoir is located within the South Platte floodway. It is currently empty but will store up to 8,000 acre-feet of South Platte water. Hazeltine shares an embankment separating it from a smaller reservoir, Howe-Haller B, located immediately to the south.

In 2013 and 2015, flooding along the South Platte River overtopped the banks of Howe-Haller B, significantly scouring and undercutting the interior reservoir slopes, which were completely exposed because the reservoir was empty. The embankment separating Howe-Haller B from Hazeltine Reservoir was not impacted during these events. However, hydraulic analyses showed that either the South Platte River could overtop the embankment separating Hazeltine Reservoir from the river, or that the flooded reservoir could overtop the embankment and release water into the South Platte during floods less a 100-year event.

To minimize the possibility of an embankment breach caused by overtopping flows both into and out of the reservoirs during a flood event, a 1,600-foot-long bi-directional spillway was designed and constructed. A shallow, two-foot-deep notch into the embankment was designed to limit excavation and filling. This design successfully mitigated impacts to the floodway such that a Conditional Letter of Map Revision (CLOMR), which would have delayed the Project schedule, was not required. The project also benefitted from the cost-effective use of a high-performance turf reinforcement mat (HPTRM) to armor the spillway, as an alternative to articulating concrete block mats, concrete, or riprap.