Many activities related to man´s utilization of soil, mineral, and water resources accelerate the rate of soil erosion and the silting of water reservoirs. Since a river basin is a functional unit, environmental changes in one part of a basin affect other parts of the basin. Rivers, lakes, and man-made water reservoirs often respond to water regulations and associated hydrological changes by bank caving. Often a formation and widening of shallow bank terraces takes place followed by a colonization and stabilization by vegetation, after which the bank retreat ceases.

Bank erosion in the regulated river Umeälven at Hemavan in northern Sweden.

According to the well-known diagrams of Hjulström and Sundborg, which show the relationship between particle size and critical erosion velocity for uniform material, a minimum in the critical erosion velocity is observed in the 0.1 - 0.5 mm of particle size interval if the finer grained material is compacted. The critical flow velocity for erosion is, inter alia, also dependent on the grain-size composition and the degree of consolidation of the sediment, and one also has to consider the binding effect of vegetation.

As is obvious from the comprehensive and annotated bibliography of Zeman (1983), the erosion of fine-grained, more or less cohesive sediments, is very complicated and poorly understood. Many aspects of the erosion resistance still require better understanding.

The erosion diagram I have constructed for calculating the erosion resistance of cohesive, water saturated sediments, relates critical erosion velocity to void ratio and grain size. According to the calculations, recently deposited fine-grained, often underconsolidated matter may be eroded and resuspended by rather weak currents. The resistance to erosion increases during the process of compaction, the result of the expulsion of water from the interstices between sediment grains under load. The interparticle electrochemical forces increase in strength with the packing and with increasing specific surface area of the deposits. Therefore, void ratio as a measure of the compaction and grain size as a measure of the specific surface area are the two most crucial physical parameters in studies of the erosion resistance of cohesive, water saturated sediments. Sediment erodibility also varies with many other factors, such as mineral and organic composition, sedimentary structures, type and activity of bioturbation, redox conditions, etc. The relationships shown by the erosion diagram below are therefore somewhat uncertain.

Calculated relationships of critical erosion velocity to void ratio for particles and aggregates with given grain size. Modified after Axelsson 1992 (UNGI Rapport Nr 81).

The erosion diagram is based on calculated average values from published and unpublished field and laboratory data. The relationship of void ratio to water content and bulk density (exemplified by some tabulated values) was used to calculate probable void-ratio values for water saturated sediments. The logarithmic formulas for the velocity distribution over a sediment bed were used to calculate the critical erosion velocity 1 m above the bed.

This diagram (also published 2002 in a modified form in Geo-Mar Lett 21, p 243) has, among others, been used for determining the erodibility of the heavily mercury-polluted bottom deposits in lake Ala Lombolo in northern Sweden. The two large black spots in the diagram above mark median values (X-ray densitometrically calculated) for the hardest and softest layer (disregarding the surface layer) with a thickness of 1 cm in the upper 0.3 m of 10 sediment cores from the lake. The hardest, less polluted layer was in 1997 situated at a median sediment depth of only 6 cm, that is above the softiest, heavily polluted layers and gives therefore a certain protection against erosion of the underlying, more easily erodible sediment layers.

In tropical and sub-tropical water reservoirs hard crusts may be formed during subaerial exposure of the mud flats. The thickness of the dehydrated layers may decrease to about one-half and sometimes to one-third of its former value, which also means a considerable decrease in void ratio. Apart from the reduction in sediment volume, a very important result of the subaerial exposure of the mud flats during periods with low water levels is a substantial decrease in erodibility of the sediment surface. The stability of the sediments also increases due to the presence of epipelic algae and to the accumulation of mucilage at the surface of the exposed mud flats. However, this effect is partly counteracted by the mud cracks, simultaneously formed as volume is reduced during dewatering.

Radiographs of varved clay below an erosional surface capped by a thin sand layer topped by bottom water. Uppermost part of sediment cores 1124 and 1126, sampled at depths of 24 and 35 m respectively between the bight Pukaviksbukten and the archipelago of Hällaryd, a coastal area of the south-western Baltic sea situated in Blekinge. The distance between these two coring sites amounted to 6.7 km. Observe the graded density distribution within the clay varves. From Axelsson 2002 (Geo-Marine Letters 21/4).

Time gaps in the sedimentary sequence due to erosion are characteristic of high-energy environments, as exemplified by the radiographs above. A rather thin, mainly sandy and partly gravelly top layer, varying in thickness between 1 and 4 cm, was found in all of the seven cores sampled on the same day in this exposed coastal area. A flow velocity of about 0.5 m/s at a distance of 1 m above the bed would probably be enough to erode this mainly sandy top layer. It is thus probable that, during heavier storms, this coarser top layer will be transported in suspension and/or as bed load, accompanied by the development of bed forms.

The very large time gap recorded close to the sediment surface in this coastal area indicates that erosion has dominated over deposition for a long time, and that newly deposited fine-grained material therefore has a short residence time in the for waves and currents exposed parts of this area. The void ratio is rather low (around 3) and the resistance to erosion therefore high in the varved clay below the thin, coarse-grained top layer. By contrast, recently deposited fine-grained matter often has void ratios higher than 20 and in some areas even higher than 100, e.g. in the East Gotland Deep of the Baltic Sea.

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