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Saturday, 16 Dec 2017

Sädva Dam

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A Rockfill Dam in Northern Sweden

Seepage Evaluation from Resistivity Monitoring


The Sädva dam is located in the upper part of the Skellefteälven River just south of the Arctic Circle. It has a reservoir storage volume of 625 million m3. The dam and power plant was put into operation in 1985. The total length of the dam is 620m, which is divided in a 210m long main dam across the old river channel and a 410m long dyke along the old river channel. The maximum height of the main dam is 32m. The dam is a rock fill embankment dam with a slightly inclined central core made of fine-grained glacial till.


Installations on Sädva Dam

The monitoring installation comprises full instrumentation for resistivity measurements. Permanent electrodes are installed inside the top of the dam core along the full length of the dams.

Seepage Evaluation From Resistivity Monitoring Data

Four areas, situated on the same depth (d=20 m) but on different distances along the dyke, at 350m, 375m, 450m and 510m, were selected for qualitative evaluation. The dam geometry is nearly identical in all areas, so a similar resistivity variation can be expected for similar seepage flow regimes. Anomalous resistivity areas with large temporal variations may indicate leakage. Here, the area around 450m stands out with lower resistivity. Although not alarming, the considerable high variation may be related to higher seepage flow rate. A quantitative evaluation method was also tested for the area around 450m. This approximate method uses the known relation between resistivity and temperature and is similar to the temperature evaluation approach. Basically, the temperature variation inside the dam is determined from resistivity data and then fitted to the calculated temperature variation generated by an assumed seepage rate (the solid curve). A total seepage flow in the order of one litre per second for the whole area (300m2) around 450m, was obtained using this approximate method.