Non-equilibrium bedload transport by steady and non-steady flows (1980)
Type of ContentTheses / Dissertations
Thesis DisciplineCivil Engineering
Degree NameDoctor of Philosophy
PublisherUniversity of Canterbury
AuthorsBell, Robert G.show all
The temporal and spatial response of an alluvial reach to imposed steady and non-steady flows is examined under non equilibrium conditions, specifically the case where the bedload input at the upstream boundary is zero.
The experimental research to date in the areas of initial motion, steady flow and non-steady flow non-equilibrium bedload transport and general mathematical models for non-equilibrium alluvial conditions are reviewed. Some of the assumptions of the models are questioned, particularly the use of an equilibrium transport formula under transient conditions.
A qualitative approach to understanding the physics of the bed response is achieved by the development of conceptual models. Comparison procedures are also developed to resolve differences in bed-load transport rates, due to firstly non-equilibrium effects and secondly non-steady flow effects, from appropriate experimental data sets.
An experimental programme was designed to test the assumptions of the mathematical models by examining the bed response for different reach lengths and for both steady and non-steady flows.
The steady flow non-equilibrium transport rates were compared with comparable local equilibrium transport capacity rates at selected time intervals and spatial positions. In the comparison process, the same mean flow velocity is common to both states, enabling the transport rates to be compared. The difference between local equilibrium capacity rates and non-equilibrium rates exhibited a spatial and temporal variation, being a maximum at x =O+ and decreasing to a small value as the end of the local scour hole is approached.
The non-steady flow types tested were triangular translation waves and step changes in discharge. Non-steady flow non-equilibrium transport rates are compared with comparable steady flow non equilibrium transport rates at selected time intervals. In this comparison process the mean stage and bed levels are equated for both states. Differences in transport rates between these two states were observed, due to a non-steady flow bed response effect. For step changes in discharge an initial transient phase is present, characterised by a temporal lag and a transport rate difference.
For translation waves, the bed response effect is more significant for the steeper waves, given the same wave peak discharge. The bed response effect is a maximum at the downstream end of the local scour hole and diminishes as the end of the general scour hole is approached. The bed response effect is intimately connected with the delayed response of the dune bedforms downstream of the local scour hole.
The performance of the mathematical model for the non equilibrium state will be poor when the reach length is less than the general scour hole length due to the bed response effect and the use of an equilibrium transport formula.
Other aspects of reach response which receive attention include initial motion, bedforms, flow resistance and stage discharge relations.