Wywdc01 color-etna div dam-irr proj.doc Anderson Consulting Engineers, Inc.

Etna Diversion Dam Project

Etna, Wyoming

On June 6, 1990 the staff of

Anderson Consulting Engineers, Inc.

(ACE) provided services in support of a

contract with the Wyoming Water

Development Commission (WWDC)

related to the Level II Etna Diversion

Dam Project. These services were

provided while previously associated

with Lidstone & Anderson, Inc. During

the initial phase (Phase I) of the project

the existing diversion dam was

evaluated, and the necessary

rehabilitation measures or alternative

improvements were identified. The

Phase II study included the preparation

of conceptual design drawings and cost

estimates associated with the

rehabilitation measures or alternative

improvements.

The objectives of the project were to: (1) evaluate the short-term stability of the structure during

the high flow events especially with respect to the potential for local scour; (2) evaluate the long-term

stability of the structure by mitigating the potential for meander migration and bank erosion; (3) evaluate

the capability of the structure to be hydraulically efficient during the low flow events, thus ensuring the

diversion of the full irrigation requirement to the Etna Irrigation District; (4) minimize the impact

associated with flooding upstream of the structure, and; (5) minimize the maintenance requirements and

costs associated with the structure. A detailed geomorphic, hydrologic and hydraulic analysis of the

existing structure and all proposed

improvement alternatives was conducted.

The geomorphic assessment included

identifying areas of active bank erosion

and evaluating the potential for meander

migration during the design life of the

structure (50 years). The hydrologic

analysis involved the development of flow

duration curves and flood frequency curves

as well as an analysis of low flow events.

The hydraulic analysis included an

evaluation of the hydraulic efficiency

during low flow events, flooding impacts

associated with the design flood, and the

potential for local scour near the structure.

The project also involved an evaluation of

water rights impacted by the proposed

improvements. To mitigate the adverse impacts associated with fish migration following the placement

of the diversion dam, a fishway and fish screen were also evaluated and conceptually designed. Three

diversion dam alternatives were designed including structures composed of rock riprap, reinforced

concrete and roller compacted concrete. Cost estimates were prepared and a financial plan presented to

determine the ability of the District to pay for the proposed improvements.

Diversion Structure Prior to Investigation.

New Diversion Structure and Fish Ladder.