Tech Notes

Technique to measure water content in the snow cover

A new snow gauge developed by Electricité de France (EDF) uses cosmic radiation to measure the amount of water in the snow cover. These measurements can be used in a variety of ways, including predicting the influence of snow melt on a hydroelectric system.

The cosmic ray snow gauge performs differential measurement of the thermal neutrons produced by the interaction of cosmic rays with the atmosphere and water. The site sensors measure the cosmic radiation attenuated by the water contained in the snow cover. A snow-free reference sensor measures incidental variations in cosmic radiation to account for events such as solar flares.

A standard installation consists of a cosmic ray sensor positioned flush with the ground, an atmospheric pressure sensor, a temperature sensor, an ultrasonic snow depth gauge, an acquisition unit, a transmission assembly, and batteries supplied by solar panels. A pole adapted to the maximum snow height supports the associated sensors, electronic boxes, solar panels, and antenna.

Dedicated software is used to acquire, process, store, and display the measurements from the sites, providing real-time measurement of the snow water equivalent. An initial calibration by snow core sampling
is recommended.

From 1998 to 2005, EDF installed 41 devices in the mountains of France to provide a forecasting network. In general, the cosmic ray snow gauges were installed on sites that were previously monitored with manual use of snow gauges. EDF uses the data from the cosmic ray snow gauge network to estimate the amount of flow to expect at its hydro projects as a result of the snowmelt runoff.

Each complete cosmic ray snow gauge costs about 30,000 euros (US$35,300).

Bulgarian organization seeks abstracts for symposium

The Bulgarian National Committee on Large Dams (BUNCOLD) seeks abstracts by November 20, 2007, of proposed papers for an international symposium, “Problems with Dams’ Operation.” The one-day symposium in Sofia, June 4, 2008, is one of numerous events planned for the International Commission on Large Dams’ 76th Annual Meeting in Bulgaria. BUNCOLD is hosting the ICOLD annual meeting.

Click here to enlarge image

Submit abstracts of 300 words in English via E-mail to: buncold@ or Fax: (359) 2-8668287. Abstracts will be reviewed by the symposium’s organizing committee. If accepted, authors will be invited to submit a paper by March 1, 2008.

Other events scheduled for the annual meeting are technical committee meetings, the annual executive meeting, and six study tours.

— For more information, visit the website:

Predicting internal erosion in glacial moraine core dams

A researcher with Vattenfall Power Consultant AB and the Royal Institute of Technology in Stockholm, Sweden, is developing a new method to more accurately predict the potential for internal erosion to occur in an embankment dam with a glacial moraine core. This method incorporates results from internal stability assessments of 40 existing embankment dams in Sweden, says Hans F. Rönnqvist, a consultant engineer with Vattenfall Power Consultant and PhD student at the Royal Institute of Technology.

The typical embankment dam in Sweden is earthfill or rockfill with an impervious core composed of glacial moraine and a downstream filter of sand and gravel, often widely graded. Glacial moraine normally is a broadly graded mixture of large boulders, cobbles, and gravel in a matrix of finer particles. However, dams comprising these types of materials appear to develop sinkholes more frequently than dams with materials of other origins. This finding correlates well with the many cases of sinkholes that occur in Swedish dams, Rönnqvist says.

Most research assessing internal stability and erosion is performed on materials with completely different properties than materials used in Swedish dams, such as narrowly graded granular materials or cohesive materials like clays. Vattenfall was concerned that there is a “research gap” regarding typical Swedish dam characteristics, and thus a need to develop a way to more accurately assess, and then predict, internal erosion.

The researcher studied 40 existing embankment dams in Sweden, some with detected and documented internal erosion and some without. To determine the best method for assessing potential for internal erosion, he applied four existing methods to records of each dam. In many cases, the researcher developed additional gradation curves for the core and filter material. He classified the dams into three categories:

  • Probable occurrence of internal erosion (nine dams). The dam has visible sinkholes and settlements on the surface of the dam and leakage with dirty water or other observed signs of internal erosion. Internal erosion has been documented using test pits or drill cores.
  • Observations may be signs of internal erosion (seven dams). Sinkholes and settlements have occurred, but no leakage with eroded material has been noted.
  • No observations have been made to indicate internal erosion (23 dams). No sinkholes or settlements have occurred, and no leakage with eroded material has been noted.

Results indicated that none of the four methods tested provide a distinct indication whether a dam is prone to internal erosion. However, by combining aspects of several different testing methods, the researcher developed a new assessment method. Results from application of the new method show that nearly 80 percent of the dams with documented and confirmed internal erosion (seven of nine) now could have been detected. Further research will focus on substantiating these preliminary results by testing additional existing assessment methods on more embankment dams.

— For more information, contact Hans Rönnqvist at (46) 8-7396342, extension 46; E-mail: hans.ronnqvist@

Tests validate design of small fish-friendly turbine

IT Power Ltd. in Hampshire, United Kingdom, has validated the design of a fish-friendly small propeller-type hydroelectric turbine. This turbine can be used in run-of-river applications at sites with potential for 10 kw to 1 mw.

IT Power completed the conceptual design of the turbine using computational fluid dynamics (CFD), the STRIKER fish injury prediction model from Jacobs Babtie Aquatic in Glasgow, United Kingdom, and traditional design tools. The company used a siphon turbine installed at Derwent Hydroelectric Power Limited’s 10-kw Borrowash site as the baseline for the analysis. CFD modeling was performed to assess the turbine performance and generate flow data to be fed into the STRIKER model. The complete turbine, including the guide vanes and draft tube, was modeled using CFX software from Ansys Inc. in the United Kingdom.

The main focus of the new turbine design is to reduce fish injury by blade strike. Based on results from CFD data and the STRIKER model, IT Power concludes that the efficiency of the new design is comparable with that of the existing baseline turbine design.

IT Power research indicates the new turbine design will cost an estimated 30 percent more than the existing turbine. However, because fish can pass through the new turbine with a reduced risk of injury, less stringent screening will be required. This is expected to decrease costs associated with installing and cleaning the screens, making overall capital and operating costs comparable.

The company is now seeking a development grant to build a prototype of the new design.

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