Dispersion in Tumbling Flow

Steep mountain streams are often in the tumbling flow regime, that is alternating sub- and super-critical flow with energy dissipation caused mainly by sudden changes in slope and cross section. Though longitudinal dispersion characteristics of tumbling flows are important in time-of-travel, stream gauging, and contaminant spread considerations, very little pertinent information is available. The results of a laboratory study intended to provide preliminary data on tumbling flow dispersion are discussed. To simulate tumbling flow, a 1 m wide flume was divided into 11 pools by placing identical triangular weirs at 3 m intervals. Slugs of fluorescent dye were injected at the first weir and the resulting concentration-time variations measured downstream at every second weir. Test results are analyzed according to existing theoretical models and corresponding parameters evaluated. It is concluded that conventional river dispersion models are possible means of describing tumbling flow dispersion but it is unclear at this time whether storage dispersion models are relistic.