Discussion of paper by E. Roy Tinney, ‘The process of channel degradation’

Utilizing analytical procedures, Dr. Tinney has developed an idealized degradation relationship that incorporates the equation for continuity of sediment movement in an alluvial channel, the DuBoys sediment transport equation, the tractive force equation, and the Manning discharge equation. The relationship is successfully tested by application to the flume study results of Newton [1951]. He is to be commended for his comprehensive research. A different combination of existing theory and equations was used by Mostafa [1955] to derive a somewhat different degradation prediction relationship. Others have used the various bed-load equations and incipient motion relationships to predict degradation amounts, with time, under idealized conditions. In contrast, investigators such as Lane [1934], Hathaway [1948], Shulits [1934], and Borland and Miller [1960] have studied the data on actual happenings, and in most cases the degradation amounts and patterns that have occurred, although seeming to follow in general the known laws of stream dyanmics, could not have been accurately predicted using the idealized approach only. The reasons are not difficult to comprehend, and Dr. Tinney has identified many of the limiting factors in his paper. Very few streams are homogeneous. Even if they were, our limited ability to correctly define the physical, hydraulic, and hydrologie characteristics of a stream greatly hinders realistic determinations. Superimpose on this the present inadequacies of bed-load and shear stress relationships, as they apply to avariety of stream conditions, and the limits on estimates of degradation are evident.