Analyses of sediment transport are performed to evaluate the ability of a channel to carry the incoming sediment load. The design goal for mobile bed channel projects is to achieve a state of dynamic equilibrium. This refers to a condition where the channel can transport the incoming sediment load without excessive erosion or deposition. The intent is that the channel retains its planform, shape and profile within an acceptable range of variability without trends. Most frequently for City of Austin applications the analysis is based on including a low flow channel within the active and flood conveyance channels. The Stream Restoration Program promotes the concept of sediment continuity to assist in assessing existing conditions and to design for a state of dynamic equilibrium. The levels of sediment continuity analysis and surrogates thereof may include:
Equilibrium (Steady State) Methods
- Incipient Motion (Threshold)
- Sediment Continuity
- Sediment Continuity
- Sediment Routing
Sediment Continuity Concept
Steady-state and/or dynamic sediment transport models are used for analysis and design of stream stabilization projects. Commonly used models for sediment transport:
- HECRAS Stable Channel Design and Sediment Transport Modules
- SAMwin Hydraulic Design Package for Channels
The initial step in a sediment transport analysis is evaluation of the mobility of the channel bed material. This is accomplished through comparison of the hydraulic shear stress (computed from hydraulic model) and the critical shear stress of the bed material. There are many paradigms for sediment and channel armor mobility. The Shield’s equation is most commonly used for this purpose:
c = critical shear stress to initiate motion of bed material (lb/ft2)
SP = Sheilds Parameter (~0.05 for Austin gravel/cobble streams)
Sg = specific gravity of sediment (~2.4 - 2.65)
Ds = representative diameter of bed material from gradation (ft)
The ratio of hydraulic shear stress (o ) to critical shear stress (tc ) is know as the shear stress ratio. When the "shear stress ratio" (o / c ) exceeds unity or the "excess shear stress" (o -c) is greater than 0 the bed material becomes mobilized and moves downstream. Many sediment transport equations utilize the shear stress ratio concept to determine sediment transport rates.
In mobile bed systems the erodibility of the channel is dependent on the sediment supply from upstream sources and the ability of the design channel to transport the incoming load. Generally there are three cases related to the equilibrium condition of the stream.
Dynamic Equilibrium - the channel can transport the incoming sediment load without excessive erosion or deposition.
Transport Limited – The channel cannot sufficiently pass the incoming sediment load and aggradation results.
Supply Limited – The channel transport capacity exceeds the incoming sediment load and erosion/degradation occurs.
A stream channel is formed by the continuum of flows that the channel receives over time. The channel forming discharge is often selected as a surrogate to this range of flows.
Channel forming Discharge, also known as:
- Dominant Discharge
- Formative Discharge
- Effective Discharge
- Where the Channel is Going
The channel forming discharge is defined as a flow that transports the most sediment over time and determines the principal dimensions and characteristics of a natural channel. The effective discharge has been associated with bankfull discharge in the eastern U.S. However bankfull discharge is less applicable in incising systems and in arid/semi-arid environments. Therefore a collaborative approach including analytical methods, flood frequency and field investigation is used to identify channel forming discharge in the Austin area.
used to identify channel forming discharge in the Austin area.
Analyses of sediment continuity in the channel forming discharge range can be used to develop a family of stable channel dimensions that can provide for a condition of sediment continuity or dynamic equilibrium.
Utilizing sediment continuity requires definition of the upstream sediment supply, which can be expressed with a sediment transport-rating curve for the supply reach. Significant judgment and a thorough knowledge of the system are essential to estimate an appropriate supply loading for a rehabilitation design. A simplified approach in lieu of sediment continuity is the threshold approach setting the design hydraulic bed shear stress (o )to the critical shear stress (c ) at the channel forming discharge. This however may under estimate the sediment supply.
There are multiple combinations of slope, depth and width that could satisfy sediment continuity for a particular reach. More often than not there are space constraints that limit the range of solutions. In other cases one of the channel geometry variables (width, depth, slope) may be selected based on environmental and habitat criteria. Target velocities and/or depth suitable for fishes and bethnic communities may be used to define a template for the channel geometry. Following a stable slope is selected based on the sediment transport analysis.
Sediment transport analysis in combination with observations, experience, hydraulic geometry and planform relations can assist in predicting future channel response and provide design parameters for channel stabilization.