General sediment input

The number of sizes chosen to represent the sediment depends on the size grading of the river bed material. If only a small range of sediment sizes is present, then a single size in the model would be sufficient. For widely graded sediments, several sizes will be needed to adequately represent the sediment fractions. As a general rule, a factor of about 1.5 to 2 can be taken between the sediment sizes.  If this rule results in the limit on the number of sizes being exceeded, then fewer sizes can be used to represent the coarser sediments.

The default distribution, entered at the head of the sediment input file, should represent a typical bed material grading. At least 5 or 10 bed material samples should be used to determine bed material composition, because large variations can occur, even in straight regular channels.  Techniques are available for determining bed material grading in rivers where stones or boulders are too large to allow easy sample collection (BS 3680, Parts 10C & 10E, 1993).

The porosity of the channel bed can be set to between 0.4 and 0.47 for sand or gravel beds. Considerably larger values, even up to 0.9, can be expected for finer cohesive sediments. The value will be related to the degree of consolidation that is expected.  Good consolidation, such as is achieved when the channel dries out for extended periods, would imply porosities of 0.4 to 0.5, while poor consolidation with porosities around 0.9 would apply when a few metres of silt or clay deposition occurred in a few weeks without the channel drying out.

Sediment densities should be set to 2650kg/m3 for quartz sand or when no site specific information is available.  It is recommended that the same value is used for all sediment sizes.