For several decades, there is a recognisable trend of a deterioration of the water biocoenosis. An increased sedimentation in the river system from the catchment area leads to an increase in the col- mation. The colmation describes the solidification of bottom substrate. Against the background of anthropogenically increased fine sediment inputs and river training measures, the situation grows even more problematic in the future [5]. This thesis analyses the development of colmation at a selected subarea of the Rothbach stream in the municipality of Markt Indersdorf. For this purpose, we determined the composition of the sedi- ments washed in by erosion. All grain fractions are present in the scope of clay up to the sand are present. Soil samples from the stream bed in a backwater area of a transverse structure indicate an advanced colmation in the case that barely natural bottom substrate is present. Also, as an indication of this, fine fractions of less than 0.063 mm are heavily represented. These can only be deposited when the pore space is narrowed with larger particles. The medium sand share is decisive for the development of the colmation. The processes of formation of a colmation are also investigated in an artificial channel. The aim of the test series is to investigate the influence of the particle size on the development of the colmation and to observe the sedimentation dynamics over time. A natural stream bed, a riffle-pool sequence, is replicated by periodic hills consisting of gravel with the grain size of 4 to 8 mm. The flow condi- tions are adjusted to keep the initially clean bottom substrate at rest. The suspended fine sediments are transported as wash load. The deposition of the clay, silt and fine sand fractions is determined for a specific volume of the bottom substrate by sediment traps installed in the middle of the hills. During the four-hours long experiment, the pore volume in the sediment trap are clogged with clayey mate- rial at 0.10 %, with silty material at 0.37 % and the fine sand at 0.39 % on average. Approximately 17 %, 59 % and 63 %, respectively of the added 500 g of fine sediment are deposited in the whole bottom substrate. The observation of decrease of turbidity of water is consistent with these measure- ments of the amount of deposition. In addition, the vertical concentration profiles of Rouse show a different distribution of the suspended solids across the flow depth depending on the particle size. More particles of these grain sizes can filter into the gravel substrate and accumulate there, Since the largest share of the fine sand and coarse silt fraction is transported as bedload. The measurement of the deposition of silt after one and two hours indicate that most of the deposition occurs immediately after the input of the fine sediment. The direct comparison of the amount of deposit after two and four hours shows an increase of about 1%. Thus, after some time, an equilibrium is established be- tween the concentration of fine sediment in the water and in the bottom substrate.     «

For several decades, there is a recognisable trend of a deterioration of the water biocoenosis. An increased sedimentation in the river system from the catchment area leads to an increase in the col- mation. The colmation describes the solidification of bottom substrate. Against the background of anthropogenically increased fine sediment inputs and river training measures, the situation grows even more problematic in the future [5]. This thesis analyses the development of colmation at a selected...     »