Spillway Capacity Innovations at Small Dams

Many older small dams do not have sufficient capacity to comply with current dam safety standards. Engineers often select standard methods of increasing spillway capacity, such as adding tainter gates or lengthening a fixed crest spillway to bring these dams up to standards. However, many times they overlook economical and beneficial alternative methods to increase spillway capacity. This paper will describe some of the successful alternatives that have been used to increase spillway capacity at a number of dams in Minnesota and Wisconsin.
Depending on their site conditions, the spillway capacity of these dams has been increased by using one or more of the following types of gates or spillway modifications.
Stop Log or Flashboard Replacement System
· Steel Crest Gates - Steel crest gates, similar to a bascule type gate, were installed on two dams replacing needle logs and stop logs. The power-operated gates can be easily adjusted to pass changing flows over the gate's crest.
· Steel Split-Leaf Gates - Steel split leaf gates were designed for a new spillway, and retrofit at two other sites. The adjustable upper leaf passes flow over the gates during low flows mimicking stop logs, but the gate can be completely lifted to allow free passage of flood flows.
· Downward Opening Roller Gate - This type of gate was installed in a new spillway and was retrofitted in an existing spillway. The downward opening feature provides much easier control of reservoir level than discharging through a low-level outlet and provides a means of passing floating debris over the spillway.

Increase of Spillway Length
· Labyrinth Spillway - The fixed crest labyrinth spillway was designed and constructed to replace a stop log spillway. The spillway provides an economical means to increase spillway capacity and eliminate gate operation.

Increase of Spillway Capacity in an Embankment
· Drop Inlet Spillway - Spillways at two dams were replaced with fixed-crest drop-inlet spillways to increase spillway capacity and minimize the need to operate gates during flood events. The cost-effective drop inlet spillways replaced undersized overflow spillways.

Dam Raise
· Stair-Step Spillway - A fixed crest spillway was rehabilitated by replacing the crest with a more efficient shape and anchoring it to the existing structure with post-tensioned anchors. A stepped downstream slope was constructed to dissipate energy and protect the downstream channel bottom from scour.

The paper will elaborate on the above project examples and will describe the operational and economic benefits for using each method of increasing spillway capacity. 9 pp.