Threshold Channel Design

Purpose

Threshold channel design techniques are used for rigid
boundary systems. In a threshold channel, movement
of the channel boundary is minimal or nonexistent for
stresses at or below the design flow condition. Therefore,
the design approach for a threshold channel is
to select a channel configuration where the stress
applied during design conditions is below the allowable
stress for the channel boundary. Many sources
and techniques for designing stable threshold channels
are available to the designer. This chapter provides an
overview and description of some of the most common
threshold channel design techniques. Examples
have been provided to illustrate the methods.

Introduction

A stable threshold channel has essentially rigid boundaries.
The streambed is composed of very coarse
material or erosion-resistant bedrock, clay soil, or
grass lining. Streams where the boundary materials are
remnants of processes no longer active in the stream
system may be threshold streams. Examples are
streambeds formed by high runoff during the recession
of glaciers or dam breaks, streams armored due
to degradation, and constructed channels where channel
movement is unacceptable for the design flow.
A threshold channel is a channel in which movement
of the channel boundary material is negligible during
the design flow. The term threshold is used because
the applied forces from the flow are below the threshold
for movement of the boundary material. Therefore,
the channel is assumed to be stable if the design stress
is below the critical or recommended stress for the
channel boundary. Design issues include assessing
the limiting force and estimating the applied force. A
requirement for a channel to be considered a threshold
channel is that the sediment transport capacity must
greatly exceed the inflowing sediment load so that
there is no significant exchange of material between
the sediment carried by the stream and the bed. Noncohesive
material forming the channel boundary must
be larger than what the normal range of flows can
transport. For boundaries of cohesive materials, minor
amounts of detached material can be transported
through the system.

Threshold channels, therefore, transport no significant
bed-material load. Fine sediment may pass through
threshold streams as throughput. In general, this
throughput sediment should not be considered part
of the stream boundary for stability design purposes,
even if there are intermittent small sediment deposits
on the streambed at low flow.
An additional requirement for threshold channel
design is to maintain a minimum velocity that is sufficient
to transport the sediment load through the project
reach. This sediment may consist of clays, silts,
and fine sands. This is necessary to prevent aggradation
in the threshold channel.

654.0800 Purpose 8–1 654.0801 Introduction 8–1 654.0802 Design discharges 8–2 654.0803 Allowable velocity method 8–3 (a) Calculate average velocity……………………………………………………………………8–3 (b) Determine allowable velocity……………………………………………………………….8–6 (c) Soil Conservation Service allowable velocity approach………………………..8–8 654.0804 Allowable shear stress approach 8–10 (a) Calculate applied shear stress…………………………………………………………….8–10 (b) Calculate allowable shear stress…………………………………………………………8–14 (c) Procedure for application of allowable shear stress method……………….8–23 (d) Limitations and cautions…………………………………………………………………….8–24 654.0805 Tractive power method 8–26 654.0806 Grass-lined channels 8–27 (a) Allowable velocity………………………………………………………………………………8–27 (b) Allowable shear stress………………………………………………………………………..8–28 (c) Species selection, establishment, and maintenance of grass-lined………8–30 channels (d) Determination of channel design parameters……………………………………..8–30 (e) General design procedure…………………………………………………………………..8–32 654.0807 Allowable velocity and shear stress for channel lining materials 8–37 654.0808 Basic steps for threshold channel design in stream restoration 8–38 projects 654.0809 Conclusion 8–43