Big Elk Lakes (Dam Site WP-7) Spillway Design

Background

Dam site WP-7 was identified in the Papillion Creek Watershed Partnership Plan to provide regional detention in the West Branch Papillion Creek Watershed in conjunction with WP-6. This sub-watershed is one of the most rapidly developing watersheds in the metropolitan area, and these sites were selected when the Plan was developed to maximize flood control, given what open ground remained in the area. These two sites were at the top of the list for the Papio-Missouri River Natural Resources District’s (P-MRNRD) re-prioritization study due to the pressure of impending development adjacent to the dam sites.

The Project

FYRA Engineering, now part of Houston Engineering, Inc. (HEI), conducted the preliminary design, permitting, and construction phases for this project. This project description will focus on the spillway hydraulics of the dam embankment.

Due to the narrow valley and location of a major arterial road, in order to fit an earth cut auxiliary spillway around either abutment the dam would have had to be moved almost half a mile upstream, losing a significant portion of the already smaller watershed.
Instead, a one-of-a-kind auxiliary spillway was designed over the center of the dam embankment in lieu of an earth cut spillway. The reinforced concrete structure consists of a fixed ogee crest, two steep chute sections on both sides of a bridge over an arterial road, and a hydraulic jump-type basin at the spillway terminus. The chute walls are independently supported concrete retaining walls to keep the road’s bridge structure separate from the spillway around it.

The structure was designed to pass a full probable maximum flood (PMF) event for all flows in excess of the 500-year storm. The structure had to accommodate varying velocities, hydraulic profiles, and uplift pressures associated with them as well as attached storm sewer systems.

A system of water stops, pressure relief ports, foundation drains, and frost blankets helps to keep the foundation pressures separated from the uplift pressures of the flow. Concrete slab, wall, and joint design were all carefully thought through for the structural design of this project. A reinforced concrete pressure pipe with a US Bureau of Reclamation (USBR) type impact basin handle the normal flows.