OVERVIEW OF THRESHOLD AND ALLUVIAL CHANNEL DESIGN

Abstract

The purpose of this paper is to outline a systematic hydraulic design methodology and design techniques for hydraulic engineers involved in design of stable streams and channels. The techniques that will be presented are divided into those that are for threshold channels and those for alluvial channels. The objective of the methodologies that will be described is to fit the channel design into the natural system within the physical constraints imposed by other project objectives and constraints. A threshold channel has essentially rigid boundaries. Threshold channels include cases where the streambed is composed of very coarse material, erosion resistant bedrock, clay soil, or grass lining. In an alluvial channel, there is an exchange of material between the incoming sediment load and the bed and banks of an alluvial channel under design or normal flow conditions. Essentially, where a threshold channel is fixed under design conditions, an alluvial channel is one that is free to change its shape, pattern and planform in response to short or long term variations in flow and sediment. The design approach for a threshold channel is to select a channel where the stress applied during design conditions is below the allowable stress of the channel boundary. This paper describes threshold channel design procedures based on the permissible velocity and tractive stress approaches. Channel design becomes more complicated in alluvial channels where the bed is mobile and where bed-material sediment inflow is significant. In addition to water-surface elevation, efficient transport of sediment becomes a focus in the hydraulic design of alluvial channels. The recommended design methodology for alluvial channels that will be briefly outlined includes analytical solutions of resistance and sediment transport equations in combination with application of geomorphic principles. When possible, alluvial channels are sized for the channel-forming discharge. However, the response of the design channel to the entire range of natural flow needs to be evaluated.