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.
The Stream Restoration Program utilizes hydrologic and hydraulic models for estimating runoff quantities, rates and the hydraulic forces impacting a reach of stream. These analyses provide parameters for use in stable channel design. For projects within an existing floodplain, generally the existing FEMA hydrology and hydraulic models are used. For smaller projects models are developed for each specific project. Common models used are:
Hydrology: HEC-1 TR20 FFA
Hydraulics: HEC-2 HECRAS (HECGEORAS) WSPRO Manning’s Equation
Results from the H&H analyses are used to estimate channel boundary shear stresses and sediment transport capacities, which allow for prediction of future short- and long-term channel erosion and provides data for design of channel stabilization measures.
The first step in consideration of a stream stabilization project includes a site investigation or field reconnaissance where an assessment of stream conditions and the problem severity are made. Watershed Protection Department Staff: deduce the physical processes dominating the system, evaluate morphological state of the stream (channel evolution), identify constraints, consider potential solution types and extents, assess appropriate level of engineering analysis, and prioritize with respect to Citywide problem areas.
Fluvial geomorphology is the science dealing with the physical processes and characteristics of rivers and streams:
Some geomorphic factors considered during stream assessment are:
- Lateral and Vertical Channel Variability
- Bed and Bank Material Type, Composition and Stratigraphy
- Bed Forms
- Channel Cross Section Shape
- Channel Gradient
- River Valley Conditions
- Floodplain Conditions
- Riparian Vegetation
- Upstream Watershed Conditions (impervious cover, soils, vegetation)
Geomorphic analysis in the engineering and implementation context is used to quantify channel morphological parameters as they relate to design of a stable system. Geomorphic analysis provides:
- Quantitative channel and stability assessment tools
- Foundation for natural channel design
- Prediction of short- and long term channel change
- Optimize design for stability and natural channel processes
- Estimate maintenance requirements
All of these components interact with each other to form the ultimate channel configuration. In urban channels these elements often become “out-of-phase” with each other as the channel adjusts to imposed watershed conditions.
General Channel Stability There are levels of analyzing channel stability and developing solution types. Generally the approach is based on the extents of the affected processes and constraints typically limit the selected solution type. Channel stability can be looked at on a large watershed scale or down to site specific problems.
Approaches to Channel Stability
- Upland Stormwater Management Controls
- (ponds, disconnected impervious cover, impervious cover limits)
- Channel lengths with common hydraulic/morphologic characteristics
- Site Specific
- Stabilization of isolated stretch of channel (usually for property protection)
General Channel Adjustment For watershed, stream reach based and site-specific situations, the Stream Restoration Program utilizes the concept that a stream reach equilibrium is dominated by the hydrology, hydraulics and sediment load. A relationship proposed by Lane can be used to qualitatively identify these physical processes dominating the system.
Qw = water discharge S = channel slope Qs = sediment discharge Ds = sediment size
Our experience shows that the most common response to urbanization in degrading reaches can be represented as:
The (+) signs indicate an increase in water discharge (Qw) and coarsening of the channel bed material (Ds); the S- indicates the river slope would decrease through meandering (planform adjustment) and/or downcutting (geometric adjustment) and the relative sediment supply would decrease in an incising reach. This qualitative analysis provides a basis for more quantitative analyses.
Channel Planform Channel planform is evaluated to assess the condition of stream meanders and the tendency of the channel to migrate laterally. Channel planform characteristics are most readily assessed using historical aerial photography and mapping. The most commonly used geomorphic planform variables are:
- Meander Amplitude and Belt Width
- Meander Wave Length
- Meander Arc Length
- Meander Radius of Curvature
- Meander Arc Angle
- Riffle-Pool Spacing
- Channel Width
Channel Sinuosity and Meander Belt
Sinuosity is the ratio of the length of the centerline of the channel (CL) to the length of a line defining the general trend of the valley or stream reach (VL) and describes the amount of meandering in a stream.
Sinuosity = CL / VL
Channel Planform Characteristics
Some commonly used relationships for planform in natural stable systems are as follows:
- 10 to 14W
- Riffle spacing 5 to 7W or ½
- rc 2 to 3W
In general pools are located in bends, riffles are located near crossings. It should be noted that the relations for wavelength and radius of curvature have been most often been identified in stable natural systems and should be used with discretion in the urban environment. This is because impervious watershed conditions accelerate the erosion process and can cause a shift from the natural condition. However these relationships are used as a starting point for many channel reconstruction projects. They can be used to determine whether a system is “out-of-phase” and provide design targets for stabilization projects. From historical observation and common planform relationships the Stream Restoration staff are able to ascertain the probability of bank retreat in a particular area.
Channel Geometry and Profile Channel geometry refers to the cross sectional and longitudinal parameters that affect the amount of channel conveyance and hydraulic forces on the channel boundary. Some common channel geometry parameters are:
- Channel Width
- Flow Area
- Hydraulic Radius
- Hydraulic Depth
- Depth of Flow (maximum depth)
- Width/Depth Ratio
- Bank Height
- Channel Profile
The channel geometric parameters vary throughout a stream reach depending on location these can be averaged to estimate the "reach-average" conditions for certain types of evaluation and analysis.
Relationships that relate channel geometry to hydrology are termed “regime equations” and are based on observations of a large group of streams. These relationships usually take the form of:
- W = aQb
- d = cQf
- V = kQm
- S = fQz
For width (W), depth (d), velocity (V) and slope (S)
As with planform channel geometric relations are only relevant in stable systems and should be used with discretion in the urban environment. In areas with rapid land-use change such as developing watersheds relationships such as these may be useless for design. However they may be used for comparative purposes. In older or undeveloped watersheds they may prove more functional. In general more detailed analyses are required to determine appropriate stable channel geometry in areas where watershed land use has been altered.
Drainage area is also used as a surrogate to discharge in channel geometry relationships. It has been observed that the trend is and upward shift in the relationship for channel width to drainage area as a result of urbanization.
The channel adjustment process resulting from urbanization can also be expressed with incised channel evolution model proposed by Schumm (1984).
The critical bank height at which mass failure begins is described as hc. when the bank height (h) exceeds hc (Stages II - III) geotechnical failures can be expected.
Observations in Austin indicate that the progression from Stage I to II occurs quite rapidly (10- 30 years) and the widening and restabilization process (Stage IV – V) occurs over a much longer time frame. Most of our urban streams that have been impacted are currently in stages II & III. This identification allows us to utilize other empirical and analytical methods appropriately. The channel evolution model serves to tell us:
- Where the Channel has Been
- Where the Channel is in its Evolution
- Where the Channel is Going
It is important to identify the channel stage of evolution in order to develop appropriate mitigation strategies and reduce future adverse impacts.
Bed Material Characteristics The size, shape, composition and distribution of material in the channel bed are important to the channel stability. These characteristic are used to determine the mobility of the channel bed and subsequently the erosion potential. In general larger sediment sizes (cobbles/boulders) act to stabilize the channel bed, where smaller particles (sand/silt) are more readily erodible. The distribution of particle sizes in bed material mixtures affects the ability of water to mobilize these sediments. The characteristics of the bed material are analyzed through visual observation and gradations developed from sieve analyses or pebble counts.
Bed Material Gradation Curve
A well-sorted sediment mixture consists of grains that are of uniform size and a poorly-sorted sediment contains particles of many sizes. A poorly-sorted sediment may be indicative of a high energy/flashy system. A poorly-sorted stream may also include large particles that armor the channel bed.
The shape of the bed material affects its stability. Angular particles will provide more stability than rounded particles because of the interlocking and friction characteristics.
The chemical composition of the bed material particles affect how it breaks-down, changes size and shape as the material moves downstream. Weaker materials such as shale and limestone degrade faster than quartz-based sediments.
Bank Composition The type of material and stratigraphy in a channel bank affects its erosion potential. Bank stratigraphy is identified and measured in the field. Geotechnical analyses are performed to analyze the strength characteristics of the bank materials. Many channels in Austin are comprised of composite channel banks with bedrock, clay, alluvium and soils.
Composite Channel Bank in Shoal Creek
Composite Channel Bank in Onion Creek
Riparian Vegetation Vegetation acts to provide channel stability as the root systems strengthen the bank material and resist erosive forces. Deep rooted plants and trees give internal strength to the soil mass comprising the channel bank. Shallow rotted plants such as grasses provide more erosion resistance to surficial forces from flowing water. In addition riparian vegetation is an essential component of the aquatic ecosystem.
Roots in the Channel Banks
Geotechnical analyses are used to determine existing bank stability, anticipate bank failures and to provide design parameters for embankment construction. Bank failure can occur in various modes depending on the bank soil properties and the morphology of the stream. Some bank failure modes include shallow, planar, rotational and cantilever type failures. The most common type of bank failure in our urbanized stream results from removal of soil from the channel toe (undermining) and subsequent slope failure.
Bank Failure Modes
In design of bank stabilization projects the primary components of a slope stability analysis include evaluation of:
- External Stability
- Internal Stability
- Local Stability
- Global Stability
External stability refers to the acting and resisting forces adjacent to stream that influence stability of the constructed slope. External stability analysis evaluates forces related to bearing capacity, base sliding and overturning moments.
Internal stability refers to forces within the channel bank that affect the stability of reinforcements (internal sliding, tensile overstress, and pullout).
Local stability is related to the surficial facing of a channel bank. This also relates to the connection strength between the facing and internal reinforcements in a constructed slope.
Global Stability relates to deep seated rotational failures that are generally outside the limits of a constructed slope.
The amount and type of data obtained for a stream stabilization project depends of the extent of the problem, the geomorphic physical process affecting the system, variability within the problem area and the type of solution envisioned for the project.
The Stream Restoration Program’s objective is to create a stable stream system that decreases property loss from erosion and increases the beneficial uses of our waterways. In this context a stable channel is one that maintains its plan form, profile and channel geometry without excessive erosion or deposition.
Rock riprap is a layer of loose rock used to protect soil from the erosive or scouring flows of water. Rock riprap is sometimes used for bank protection or bed stabilization in stream restoration projects where other erosion control techniques are not appropriate.
In order for the rock riprap to properly function, installing rock of good quality and the proper size gradation is important. Design criteria for rock riprap are listed in the Environmental Criteria Manual (ECM) Section 1.4.6. Rock riprap materials and construction methods are described in City of Austin Standard Specification 591S.
The Watershed Protection Department recommends a field gradation test method for use during construction projects to verify that the specified rock will be installed. The gradation test method is described in the following documents, and an example analysis worksheet is provided. However, engineers may use an alternate preferred method conforming to ECM Section 1.4.6.
- Topographic and Bathymetric Survey
- Bed Material Pebble Counts and/or Sieve Analyses
- Geotechnical Borings and Soil Strength Testing \
- Historical Aerial Photography
- Historical Comparative Channel Cross Sections and Profiles
- Existing Hydraulic and Hydrology Models and Floodplain Information
- Watershed Mapping (GIS)
Components of the engineering analysis for stream stabilization projects include hydrologic and hydraulic modeling, geomorphic analysis, sediment transport analysis, channel adjustment modeling, and geotechnical analysis. The level of engineering analysis for a stream stabilization project is dependent on the size of the problem and solution type to be implemented.
The stream assessment is used to determine the key factors causing the stream instability. This identification may be used to assess whether a long-term solution may be provided on a site-specific, reach based or watershed-scale approach. Constraints such as budget, land availability and temporal factors also significantly affect the type of solution envisioned.
The Stream Restoration Program utilizes both traditional and innovative design techniques to provide channel stability while enhancing natural channel variability to the extent possible. To the extent practical we utilize a natural channel design approach while meeting the ultimate channel stability goals. The attempt in natural channel design is to:
Design with nature - rather than against it and allow the river to participate in its own recovery.
Imitate natural systems - in particular their morphological variability, rather than a rigid homogenous design.
Scientific basis - is a balance between empirical-statistical and analytical (process-based) methods.
Natural channel design includes manipulation of the channel planform, geometry and profile to minimize the amount of hard armor required to provide channel stability. The primary components of stable channel design include consideration of:
- Reach-Average Channel Geometry (Width, Depth, Slope)
- Local Channel Geometry (Composite/Uniform Shape, Pools Riffles)
- Channel Planform
- Bed and Bank Stabilization
- Grade Control
- Local Scour
Space constraints and infrastructure in the urban environment may limit the amount of channel geometry and planform manipulation that can be provided to achieve stability. Beyond these adjustments, channel stabilization and armoring techniques are employed. The primary components of channel stabilization considerations include:
- Bank Stabilization
- Grade Control
- Channel Bed Armoring
- Toe Protection
- Flow Training Structures
Bank Stabilization The Stream Restoration Program encourages use of natural materials for bank stabilization. The combined use of structural elements, i.e., boulders, reinforcing grid, geocells, fabrics, soils and vegetation, create a stable streambank that is resistant to internal and external forces. These stabilization techniques provide flexibility in structure, aesthetic appeal, habitat benefits and potential cost savings over traditional methods.
Conceptual Streambank Stabilization Design
Streambank Stabilization Design Drawing
Grade control is used to inhibit long-term channel degradation which occurs through general incision, head-cutting and nick point migration. Grade controls act as hard-points (artificial geology) in the system.
Conceptual Grade Control Design
Grade controls are designed with stable materials that should not move during extreme flood events. Grade controls can be designed with rocks, boulders, concrete or other materials. A natural channel design approach is “constructed riffles” using rock placed in a similar configuration as natural riffles. The Stream Restoration Program frequently uses limestone boulders for construction of these structures.
Channel Bed Armoring Channel bed armoring refers to placing stable materials continuously throughout a design reach. Traditionally rubble riprap, gabion mattress or concrete have been used by others. The Stream Restoration Program attempts minimize the extents of channel bed armoring when conditions allow. Alternately a series of grade control structures is encouraged instead of continuous channel armoring to allow as much of the native channel bed material to exist.
Toe Protection A critical element to any channel stabilization project is providing protection of the channel toe. Experience shows that this is the initial point of failure and subsequently bank collapse occurs. Toe protection may be provided with a variety of materials including rock riprap, boulders, biologs, etc. The Stream Restoration Program encourages the use of native materials, but toe protection is included in virtually every channel stabilization project.
Flow Training Structures Flow training structures act to alter the flow pattern and divert flow away from a channel bank or structure to be protected. This can be a more cost-effective alternate to continuous bank stabilization in areas where more space for channel adjustment may be allowed. Some common types of flow training devices are spurs and bendway weirs, which are constructed transverse to the flow path. The function is to act as flow deflectors and between which sediment deposition may occur.
Conceptual Spur/Bendway Weir Field
Bendway Weir Design Drawing