Fractional bed load transport model for nonuniform unimodal and bimodal sediments

•New experimental set-up for collection of fractional bed load transport data.•Verification of existing methods for bed load transport of bimodal sediments.•New relationship proposed for bed load transport of unimodal and bimodal sediments.•Validation of proposed relationship using independent flume and field data-sets.

Experimental programme for collection of flume data on bed load transport of unimodal and bimodal sediments is reported. Existing relationships for fractional bed load transport of nonuniform sediments are assessed for their applicability to bimodal sediments using data collected in the present study and those available from past investigations. Keeping in view the inadequate performance of existing relationships for bimodal sediments, parameters for computation of fractional bed load transport rates of unimodal and bimodal sediments are identified; and a new relationship is proposed based on recent developments on critical shear stress of such sediments. The performance of proposed relationship is verified with independent data on flume and field studies.