In 2013-2015 extensive work was made to increase the dam safety in Höljes. Improved monitoring was required to bring it up to today’s standards but due to the condition of the dam foundations and high downstream water level, installation of a traditional leak detection system was not possible.
HydroResearch proposed that high accuracy temperature measurements be made using Silixa’s ULTIMA-DTS to interrogate optical fibre cable installed in standpipes in the downstream part of the dam. Four standpipes were installed on the two upper terraces of the dam and eleven along the dam toe. All standpipes were drilled through the downstream fill, the permeable soil layers in the old river bed and approximately 1m into the bedrock. Measurements were made with a fine spatial sampling (0.12 m) allowing abnormal water flow to be observed with high resolution in the vertica direction. Using such a technique generates large data sets and to enable effective data collection, HydroResearch’s proprietary web based software (XSeepT) was deployed.
It was established early on that the level of water flow through the dam is low. The seasonal variations were in line with those expected but that significant differences of temperature existed throughout the standpipes indicating differences in water flow. Data continues to be collected.