SPECIFICATION OF SEDIMENT MAINTENANCE FLOWS FOR A LARGE GRAVEL-BED RIVER

Reservoir releases may be specified to flush interstitial fine sedimen t from gravel beds in the river downstream. Choice of an effective flo w depends on trade-offs among discharge, flow duration, and pool dredg ing as they determine rates of bed mobilization, sand removal, and gra vel loss. A basis for evaluating these trade-offs is developed with an approximate method appropriate to the sparse data typically available . Sand and gravel transport are represented with rating curves. Approx imate methods are introduced for estimating effective gravel entrainme nt, subsurface sand supply, and pool sediment trapping. These are comb ined in a sand routing algorithm to evaluate flushing alternatives for the Trinity River, California. A sediment maintenance flow of moderat e size, just sufficient to entrain the bed surface over the duration o f the release, limits gravel loss and maximizes sand trapping by pools . Larger discharges produce more fines removal but at the cost of grea ter gravel loss and reduced selective transport of fines. Dredged pool s increase sand removal efficiency by providing multiple exits from th e channel and minimize gravel loss if dredged sediment is screened and gravel returned to the river.