FLUVIAL ABRASION - CONVERTING SIZE-REDUCTION COEFFICIENTS INTO WEIGHT-REDUCTION RATES

Comparison of field and laboratory data on abrasion coefficients is im portant in ascertaining the importance of fluvial abrasion in downstre am fining in gravel-bed rivers. There are two kinds of coefficients, f or size reduction and for weight (mass) reduction, In the case of abra sion and chipping or simply wearing of single sediment particles, we s hould define reduction rates rather than reduction coefficients. Conve rsion from one to the other is simple, A problem arises with discretiz ation of the particle-size distribution, normally by sieve analysis. T wo cases should be distinguished: the weight of the sediment mixture r emains constant, or it is reduced. What we usually measure is a size-r eduction coefficient, but what we should require is a weight-reduction rate for each sediment particle, For that purpose, conversion equatio ns for size reduction coefficients and weight reduction rates were dev eloped, The equations were tested with experimental data on abrasion o f sediment mixtures, and the agreement was satisfactory. The main conc lusion is that we should not determine weight-reduction coefficients f or sediment mixtures by simply multiplying size reduction coefficients determined in the field by a factor of three but by using the convers ion equations presented here and determining weight reduction rates in stead. Until now, field determination of weight reduction coefficients for sediment mixtures has been inaccurate, because grain-size discret ization has not been taken into account. This may be one of the main r easons why fluvial abrasion in some field situations has been given on ly minor importance.