On the erodibility of fine grained sediments in an infilling freshwater system

On the erodibility of fine-grained sediments in an infilling freshwater system

T.J. Andersen a, E. J. Houwing b & M. Pejrup ~
aInstitute of Geography, University of Copenhagen, Oster Voldgade 10, DK-1350 K~benhavn
K, Denmark.
bInstitute for Inland Water Management and Waste Water Treatment (RIZA), Rijkswaterstaat,
van Leeuwenhoekweg 20, 3316 AV Dordrecht, The Netherlands

The erodibility of fine-grained sediments from the Hollandsch Diep freshwater system has been
measured using a portable EROMES system. The erosion experiments were carried out on natural
sediment surfaces sampled with a box-corer. The measured erosion thresholds varied between
0.16 and 0.70 N m 2. The erosion thresholds were positively correlated to the dry density of the
bed material (r 2 = 0.33) whereas no significant dependence was found on either grain size, organic
content or chlorophyll a content. The measured erosion rates varied between 0.04 and 1.68 g m -2
s -1 and a strong positive correlation with bed shear stress was observed. The equivalent settling
diameters of the eroded material were generally 5 to 10 times larger than that of the primary
particles which shows that the material was eroded as aggregates. However, a strong positive
correlation between the two settling diameters (r z = 0.74) also indicates that the coarser modes
of the primary grain size distributions affect size or density of the faster settling aggregates. The
lack of correlation between the erodibility and organic content and chlorophyll a content indicates
that microphytobenthos is not significantly modifying the erodibility of the sediments at the study
sites. The macrofaunal community is also poorly developed with few species and small numbers
of individuals and it is concluded that the erodibility is mainly controlled by accumulation rate,
dry density of the bed material and perhaps maximum current velocity at the sites. This is further
supported by the similarity with earlier measurements carried out on settled beds in laboratories.
Keywords: Erosion threshold, erosion rate, sediment settling, Hollandsch Diep, The Netherlands

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