Experimental Study on Fine Sediment Infiltration ( <scp>FSI</scp> ) Processes in Different Gravel Mixtures

ABSTRACT Fine sediment infiltration (FSI) can lead to riverbed clogging, thereby degrading important habitats for aquatic biota in the hyporheic interstitial zone of gravel‐bed rivers (e.g., by reducing oxygen availability). This stress on rivers may increase from anthropogenic influences and can be counteracted by artificial gravel augmentation to improve bed morphology like sediment transport dynamics. For optimal functionality of artificial gravel deposits, knowledge about the infiltration capacity of fine sediments is necessary. For this purpose, 1:1 scale experiments were conducted in an outdoor experimental channel with a very high suspended sediment concentration (up to 48 gL −1 ). Three different sediment (gravel) mixtures, all without fines and suitable for salmonid spawning, were studied as gravel filters in the horizontal flow direction during three different exposure times to identify the mass of accumulated fine sediment. Although a very high suspended sediment concentration ( SSC ) was given, the rate of FSI occurred in highly different magnitudes. Narrowly graded gravel mixtures without fine fractions and thus larger voids show the highest, broadly graded gravel mixtures the lowest rates of fine sediment infiltration. The results show that grain‐size distribution properties are crucial for FSI and can be used for adapted spawning habitat restoration concepts by for example, artificial gravel augmentation.