Stability of erosion pits in Nidelva, Trondheim

1. Stability of erosion pits in Nidelva, Trondheim Klas M. Sundelin and Geir Vatne Norwegian University of Technology and Science Department of Geography Introduction A number of erosional pits were discovered in Nidelva in 2002 during flood inundation mapping of the area. As large areas of Trondheim is underlain by quick clays and prone to quick clay slides, there is a concern about the stability and dynamics of these pits. The quick clay deposit under the area of Bakklandet was classified with the highest class of risk, class 5.One large pit upstream Gamle bybro was in 2003 filled by 6500 m3 rock to prevent further erosion. In 2003/2004 most of the river bed downstream Nedre Leirfoss was mapped to map the existence of erosion pits. The mapping revealed at least 10 abnormal deep erosian pits. In my master project I will study these pits thoroughly and compare data collections from 2003, 2004, and 2005 to try to understand the stability (or instability) of these pits. Mapping methodology Changes in bed topography were mapped by Novatech AS using differential GPS and advanced eccolocation, which combined provide high resolution (< 10 cm) contour maps. Figures 1-3 show maps with contour lines from the first mapping in 2003/2004 at Sluppen , Mølledammen and Stavne respectively. The colours show changes in bed elevation between oktober 2003 and november 2005, with red colours indicating erosion and blue colors sedimentation. The erosion maps are based on a 30X30 cm resolution grid where the last map is subtracted from the first map. The bed sediment was mapped by NVE (Norvegian water and energy board) using ground penetrating radar (GPR) with 50 and 100 MHz antannes. The bed echo from the GPR was calibrated using divers and digital video ccamera in the pits at Stavne (Figure 2) and Mølledammen (figure 3). Figure 5: ground penetrating radar image Figure 2: Pit upstream of Stavne bru Figure 4:Pit downstream of Kroppan Figure 3: Pit next to mølledammen Preliminary results Geometry Changes The ovarall picture is that the erosion pits have not changed significantly over 2 yeras. The pattern is however irregular and locally the erosion and sedimentation are up to 1 m. There is however evidence for significant bed changes near Sluppen bru where there is a net erosion of 40 -80 cm in 2 years. This is paralleled by a net sedimentation of 20 – 40 cm near Mølledammen. The erosion and sedimentation pattern is however complex and the pattern for the entire bed downstream Sluppen bru will be analyzed in detail using GIS software. Thickness and grain size of alluvial sediments The strong bed reflection and lack of internal reflections from the GPR indicate a thin layer of alluvial sediments underlain by marine clays. The river bed may thus be vulnerable to erosion as a disturbance in the armouring layer may cause exposure of clay and subsequent erosion of the underlying clay. The bed in the Stavne pit show large areas of exposed clay, only covered in patches by well rounded cobbles and pebbles. During flood conditions the coarse material may be mobilized causing erosion in the underlying clay.