Development of a Simple Method to Predict Discharges in Compound Meandering Channels. Discussion.

Abstract

The research carried out in the Flood Channel Facility at Wallingford on the behaviour of meandering channels is an important addition to the previous work on straight channels (see, for example, Ackers16), and is of great practical significance. This first report on the stagedischarge function in such channels is welcome, although there are several features of the present Paper that are perhaps very much in the nature of an interim statement of progress towards a more reliable method of assessing the conveyance of rivers in flood.40. The general method proposed was based essentially on progressively adding more to the notional wetted perimeter of the suggested divided flow zones until the overall conveyance came within a reasonable tolerance of the observed behaviour, and then further reducing the conveyance by the somewhat arbitrary process of applying the thalweg slope to the main flood plain flow region. However, the confirmation of the method has been relatively restricted: no cases where the flood plain roughness differed from the main channel value; no validation against field information; no geometries including sloping or irregular flood plains. In my view, it would therefore be premature to recommend this method for general use. Simplicity may be seen as an advantage, but only if it can be shown that this simplicity has not led to very restricted application, or to lack of accuracy in cases other than those tested. Simplicity is not synonymous with robustness.41. The concept of adding notional solid surfaces at defined interfaces has some obvious limitations. It is not claimed to be-nor is it in fact-a close simulacrum of the complex processes at work which increase resistance over that for a straight channel of comparable section. There is no equivalence in terms of the generation and decay of cells of powerful secondary currents in the deep channel, so well described in the companion paper by Willetts and Hardwick;17 no equivalence for the momentum exchanges that take place as the flow switches from flood plain to main channel and vice versa; no allowance for an expected dependency of this momentum exchange on the relative roughness of main channel and flood plain, and hence their relative velocities of flow. In the absence of an appropriate hydro-dynamic model as the basis of the methodology, it seems very doubtful that it will be found accurate enough over the full range of conditions of practical interest. 42. One of the difficulties with the simple interposing of notional solid boundaries is the transition from bankfull flow to a depth where the interaction with the flood plains really takes hold. At very shallow flood plain depths, the performance of the main channel has probably deteriorated much less than the sudden addition of the channel width to the perimeter would indicate. This is the reason for the downturn of the percentage error graphs of Fig. 5 as depths on the flood plain reduce below about 10% of bankfull depth.43. It is noted that the roughnesses of the other experimental facilities...