Research finds effects on performance of seepage barriers
Researchers have identified four erosion mechanisms with potential to affect performance of seepage barriers in dams. The mechanisms were discovered through an analysis of nearly 40 dams where seepage barriers have been in place for more than ten years.
The mechanisms are:
- – Erosion due to leaks through seepage barriers;
– Erosion along bedrock joints;
– Erosion of solution void infill; and
– Erosion in internally unstable foundation soils.
Researchers at Virginia Polytechnic Institute and State University studied several types of seepage barriers, constructed with soil-bentonite mixtures, concrete, plastic concrete, deep soil mixed materials, and sheetpiles.
Overall, the seepage barriers appear to be effective in controlling the problems they were designed to mitigate. However, the four mechanisms listed above were found to have affected or have the potential to affect the long-term performance of many of the barriers. These mechanisms can be attributed to buildup of water pressure behind the barrier and the associated increase in hydraulic gradient beneath, around, and through the barrier. The increased hydraulic gradients and velocities can lead to internal erosion and piping in the dam and foundation.
By conducting finite element analysis, researchers were able to determine that the hydraulic conductivity of the seepage barrier materials often is much greater than the design values. Through soil-structure interaction analysis, researchers determined that deformations due to the high differential water pressures across the barrier are likely to result in cracking of rigid seepage barriers.
One of the researchers, Dr. John D. Rice, has moved to Utah State University. He is developing laboratory testing methods to assess the erosion potential of soil infill from bedrock joints and seepage barrier material along cracks and defects in the barrier.
– For more information, contact John Rice, P.E., PhD, at E-mail: jdrice@ engineering.usu.edu.
Hydro keeps CO2 production low in the Pacific Northwest
Carbon dioxide (CO2) production from power plants in the Pacific Northwest is significantly lower than that of other regions because of hydropower, according to results of a study by the Northwest Power and Conservation Council (NPCC). The study results showed that, in 2005, under normal water conditions, power plants in the Pacific Northwest would have produced about 520 pounds of CO2 for each megawatt-hour (MWh) of electricity generated, compared with 900 pounds for the entire Western interconnected power system.
The study and the resulting report, Carbon Dioxide Footprint of the Northwest Power System, were completed to compare CO2 production in 1990 with that of 2005 and to forecast future levels. This information can then be used to determine how various resource development scenarios would affect CO2 production from the Pacific Northwest power system.
In 1990, CO2 production was about 44 million tons, compared with 67 million tons in 2005. Forecasting rates to 2024, NPCC studied several alternative scenarios and their effects on CO2 production in the region. Two of the scenarios studied would result in a decrease in CO2 production:
- – A high renewables scenario to add energy from wind and biomass, reducing CO2 by 4.2 percent; and
– A scenario of no summer spill at the lower Snake River and lower Columbia River hydro projects, reducing CO2 production by 1.1 percent.
The other three scenarios would result in an increase in CO2 production:
- – A low conservation scenario that does not meet the conservation targets set forth in the NPCC’s Fifth Power Plan, increasing CO2 by 4.4 percent (the Fifth Power Plan, completed in December 2004, is a regional plan that directs the power supply activities of the Bonneville Power Administration);
– Removing the four federal hydro projects on the lower Snake River and replacing this generation with natural gas facilities, increasing CO2 production by 3.6 percent; and
– Spilling water from the lower Snake River and lower Columbia River projects in the summer, as ordered by the court in 2006, increasing CO2 production by 0.5 percent.
– To print the report, visit the Internet: www.nwcouncil.org/library/2007/ 2007-15.htm.
CEATI publishes guide to resistivity investigation
CEA Technologies Inc. (CEATI) announces availability of a guide covering use of the resistivity geophysical method for dam monitoring and seepage investigations. The report is titled A Guide to Resistivity Investigation and Monitoring of Embankment Dams.
The resistivity method, which has been used on many embankment dams, involves injecting electrical current into the ground and measuring the voltage potential across a pair of electrodes. The data is analyzed to estimate the resistivity of the ground. Variations in the resistivity values may indicate areas of anomalous seepage flow or soil properties.
The method can be used in two ways. First, one-time resistivity investigations can be used to detect spatially anomalous zones along the dam, or to investigate suspected structural weaknesses. Second, long-term resistivity monitoring uses seepage-induced seasonal variation inside the embankment to detect anomalies in space and in time.
The purpose of this guide is to help users optimize the use of the resistivity method. The guide covers:
- – Background on the method;
– Design considerations for carrying out resistivity surveys on dams;
– Equipment, data acquisition, and data processing;
– Modeling and interpretation;
– Example applications; and
– Lessons learned.
This technology review was developed by CEATI’s Dam Safety Interest Group (DSIG). This group is comprised of more than 30 dam owners who jointly sponsor research and development projects designed to help assess and improve the safety of dams.
– For more information or to purchase this guide, contact (1) 514-866-5371; E-mail: firstname.lastname@example.org.
Research provides design options for stepped RCC spillway
To determine the proper design of a stepped roller-compacted-concrete (RCC) spillway for Renwick Dam in North Dakota, researchers developed and tested a scale model. Stepped spillways present unique design challenges compared with a conventional smooth spillway chute, says Sherry L. Hunt, with the Hydraulic Engineering Research Unit of the U.S. Department of Agriculture’s (USDA) Agricultural Research Service. However, most research has been related to stepped spillways with rather steep slopes (greater than 25 degrees). The spillway proposed for Renwick Dam is rather flat (4H:1V).
The Small Watershed Program, administered through the USDA’s Natural Resources Conservation Service, is providing technical and financial assistance for Renwick Dam, which is used for flood control.
Due to increased urbanization in the area around Renwick Dam, the spillway lacked adequate capacity. The option for this dam was to construct a stepped RCC spillway.
To help design the structure, USDA researchers built a two-dimensional, 1:8-scale physical model of the spillway. Results from the testing indicated:
– Energy dissipation and air entrainment had little significance on the design of the spillway training walls under design flow conditions. This means researchers did not have to account for any flow bulking in the design of the training walls for this particular design.
– A 10-foot difference in spillway drop (from 40 feet to 30 feet) and a 10-foot difference in tailwater resulted in a 42 percent difference in the length of the stilling basin (30 feet to 46 feet) and a 46 percent difference in the diameter of the riprap (10 inches to 16 inches) for the downstream channel. This provides the Natural Resources Conservation Service with several designs to choose from.