Corps using Recovery Act funds to improve Howard Hanson Dam
Two companies are performing work required to improve the safety of Howard Hanson Dam. The U.S. Army Corps of Engineers, which owns the dam, is using about $9.7 million from the American Recovery and Reinvestment Act to fund this work.
The dam, on the Green River in Washington, impounds water for drinking water supply for the city of Tacoma, as well as providing flood control and storage.
The work involves building a seepage barrier and improving drainage tunnel function. Nicholson Construction is building the barrier under a contract worth nearly $8.9 million, and Jensen Drilling Company is doing drainage tunnel work under a contract worth more than $900,000.
"Seepage concerns at Howard Hanson Dam are among the Corps' highest priorities," says Col. Anthony O. Wright, commander of the Seattle District of the Corps. The goal of this work is to reduce downstream flood risk.
After a record high level of water behind the dam in January, Corps personnel observed two depressions on the right abutment, increased water levels in groundwater monitoring wells, and silty water entering the abutment drainage tunnel.
The Corps restricted flood storage behind the dam, excavated the depressions, installed additional monitoring equipment, and conducted tests at the summer conservation pool level. The Corps determined there is no imminent risk of dam failure.
However, the Corps is not confident about using the full flood storage capacity of the dam, Wright says. And, should a major flood event occur with a limited flood storage capacity, levees in the lower valley could be overtopped, the Corps says.
The seepage barrier will consist of a grout curtain to reduce seepage through the dam abutment. The barrier to be installed is a 450-foot-long grout curtain into the right abutment. The grout curtain will vary from 90 to 170 feet deep. The anticipated volume of grout to be placed is more than 20,000 cubic feet.
Improvements to drainage in the tunnel are intended to direct remaining seepage through a safety pathway, Wright says. These improvements involve installing two 6-inch vertical drains and 13 3.7-inch horizontal drains within the right abutment. The work is scheduled to be complete by November 2009.
USSD releases report on seismic evaluation of concrete dams
The U.S. Society on Dams (USSD) announces availability of a report, "Numerical Models for Seismic Evaluation of Concrete Dams."
Numerical modeling is not just a tool for computation of deformations and stresses, the report says. Numerical modeling allows for identification and appraisal of potential failure modes and determination of the availability or lack of a margin of safety for the dam.
For these reasons, the report says, numerical modeling must be formulated to: capture the correct conditions and load path, model failure mechanisms that control stability of the dam, accurately account for interaction effects, and select and supply seismic inputs that are representative of the earthquake ground motions expected at the dam site.
The report critically evaluates the process of seismic safety assessment of concrete dams using numerical models. It highlights fundamental characteristics and assumptions of different solution approaches and describes the fundamental concepts behind the mathematical formulations and corresponding solutions.
Topics covered in the report include:
- — Numerical modeling methods and considerations;
— Earthquake input;
— Reservoir water modeling issues;
— Foundation modeling issues; and
— Evaluation of seismic performance.
The USSD Committee on Earthquakes prepared the report.
— The report costs US$16 for USSD members and US$26 for non-members. To order, visit the website: www.ussdams.org/pubs.html.
Funds available to improve dam safety in Wisconsin
Lawmakers in Wisconsin have budgeted $4 million for repairs designed to ensure the safety of dams in the state.
The state's 2009-2011 Biennial Budget bill included this new funding for Dam Safety Aid programs. Of these funds, about $500,000 has been allocated but the remainder is available under a competitive bidding process. The majority of the new funding will be made available to municipal dam owners to repair, reconstruct, or remove dams.
Grants using this money are expected to be awarded before the end of 2009, so repairs can begin in the spring of 2010.
The Wisconsin Department of Natural Resources (DNR) has jurisdiction over dams in the state that are not used to produce electricity. There are about 3,800 dams in Wisconsin, of which only about 200 have hydroelectric facilities. DNR says many of the dams in the state were built in the late 1800s and early 1900s and require maintenance and/or repairs to ensure their structural adequacy.
— For more information, visit www.dnr.state.wi.us.