Grain and form resistance in gravel-bed rivers

Grain and form resistances for bankfull and near bankfull flows have been determined for 176 gravel reaches using data compiled from several published sources. Partitioning grain and form resistance is based on the original definition of k(s) adopted by Nikuradse (1933). Grain resistance is calculated with the Keulegan equation assuming a grain roughness height equal to D-50. This defines a lower bound to the observed flow resistance. Form resistance, which includes grain protrusion, pebble clusters, dunes, bars and pool-riffle sequences, is shown to be significant at bankfull flow. Computed form resistance comprises up to 90% of the total. No predictive relation for form resistance has yet been developed. The results have implications for river restoration efforts that include the reintroduction of pool-riffle sequences into degraded or channelised rivers, and indicate that constructed pool-riffle sequences would have a significant effect on flood levels.