Many streams and rivers require restoration and/or stabilization measures due to urbanization impacts associated with the watershed.
Stream restoration is used to improve the environmental health of the river or stream. This process aims to restore the natural state and functioning of the river system to support of biodiversity, recreation, flood management and landscape development.
Channel stabilization incorporates natural channel design principles to construct stream improvements with natural elements and vegetation to help stabilize the open channel streams and ditches so that they are non-erosive and self-maintaining.
Figure #1: A side by side comparison of the Kinnickinnic River in Northwest Wisconsin, before and after stream restoration efforts
Stream restoration and channel stabilization can be achieved using both structural and non-structural measures. If in-stream structures are designed and constructed properly, they can provide the following benefits:
- Channel Bed/Bank and Floodplain Scour Protection
- Improved Hydraulic Conveyance
- Effective Sediment Transport
- Habitat Creation or Enhancement
- Nutrient Processing
- Biogeochemical Processing
- Utility/Infrastructure Protection
- Aesthetic Enhancements/Blending into the Existing Landscape
This article describes the in-stream structures most commonly used for stream restoration and channel stabilization as well as how to analyze these structures using HEC-RAS.
Stream Restoration and Channel Stabilization Techniques
A wide variety of strategies and methodologies are implemented in stream restoration projects. Stream stabilization techniques, including in-stream structures, typically are designed based upon the bankfull geomorphic condition. This is the condition that represents the average morphological characteristics (dimension, pattern, and profile) of a channel that are most critical in long-term channel maintenance.
Some practitioners focus on hard structures, constructed of concrete and quarried rock, while others prefer natural materials. The major structures that are likely to be useful in stream restoration and stabilization are described briefly below.
A spur dike can be defined as an elongated obstruction having one end on the stream bank and the other end projecting into the stream channel. It may be permeable, allowing water to pass through it at a reduced velocity; or it may be impermeable, completely blocking the flow. Spur dikes may be constructed of permanent materials such as masonry, concrete, or earth and stone; semi-permanent materials such as steel or timber sheet piling, gabions, or timber fencing; or temporary material such as weighted brushwood fascines. Spur dikes may be built at right angles to the bank or current, or angled upstream or downstream. Two to five structures are typically placed in a series along straight or convex bank lines where the flow lines are roughly parallel to the bank. The effect of the spur dike is to reduce the current along the streambank, thereby reducing the erosive potential of the stream and in some cases inducing sedimentation between dikes. These structures are used to control natural meandering at a river bend, to channelize wide rivers, and to convert poorly-defined streams into well-defined channels.
Figure #2: Spur Dikes
A bendway weir is an installed spur, intended to be overtopped by design discharges. They extend linearly from the outside of a bank, either perpendicular to flow or angled slightly upstream, and are comprised of short riprap or other angular material sized to resist transport for a design discharge. They are designed to control and redirect currents through a bend and immediately downstream of the bend. Their purpose is to deflect high velocity near-bed flow away from the outer bank; inhibit helical secondary current motion in the bend; and redistribute momentum near the outer bank. They reduce near-bank velocity by redirecting the current and adding form roughness along the bank. Bendway weirs should consist of a filter fabric layer for preventing soil movement into and through the feature, undermining its footing, and a series of barbs or spurs of angular stone material. Bendway weirs differ from spurs and vanes (barbs) in that they capture the flow field and redirect flows away from the bank.
Figure #3: Bendway Weirs
Vanes or Barbs
Vanes are a subcategory of barbs. They are discontinuous, transverse structures angled into the flow. They are implemented with an upstream orientation of 20 to 30 degrees from the tangent to the bank line, have a crest elevation at or just below the bankfull elevation, and sloped at 2 to 7 degrees dip towards the tip. Dip angle increases with increasing stream slope and bed material size. In-stream tips of vanes are usually low enough to be overtopped by nearly all flows. Vanes can be constructed of either rock and/or logs. They can be used for bank protection, as well as for providing variable depth and velocity that can benefit aquatic organisms. Vanes redirect flow, provide toe protection, reduce local bank erosion, and result in bed scour downstream of the axis of the vane and near their tips.
Figure #4: Cross Vanes
J-Hook vane is a single arm, low profile vane structure that directs flow away from the stream banks. It decreases the velocity, shear stress, and stream power in the near bank region while creating habitat by encouraging pool development through flow variability. The J-Hook vane is generally built on the outside of the meander bends and consists of angular and blocky rocks placed in such a way that the shape resembles the letter ‘J’. The arms occupy one-third of the bankfull channel width, and the ‘hook’ occupies one-third of the bankfull channel width. J-Hook vanes are well suited for lower gradient stream systems. It should be avoided in bedrock channels or highly unstable streambeds and deeply incised and entrenched channels.
Figure #5: J-Hoke Vane
Defining Flow Training Structures in a HEC-RAS 2D Model
Follow these steps to define flow training structures in a HEC-RAS 2D mesh:
- From the Input ribbon menu, select the 2D Flow Areas menu item and then choose the Draw 2D Flow Training Structures command.
- The Draw 2D Flow Training Structures dialog box will be displayed.
The following sections describe the Draw 2D Flow Training Structures command and how to interact with the above dialog box.
Drawing a 2D Flow Training Structure
This command is used to interactively draw a polyline on the Map View representing the flow training structure. In addition, input parameters are defined to specify key dimensions, such as the top width and height of the flow training structure as well as its side slope angle. The Create curvilinear polyline checkbox permits bent flow training structures, such as J-Hook vanes, to be more easily drawn.
To define a flow training structure, follow these steps:
- Click the [Draw] button.
- The Draw 2D Flow Training Structures dialog box will temporarily disappear. A prompt will be displayed on the status line, informing the user what to do next.
- Within the Map View, draw a polyline starting from the river bank to halfway across the river, in the direction of the flow.
- Right-click and choose Done from the displayed context menu or press the Enter key.
- The Draw 2D Flow Training Structures dialog box will redisplay and automatically name the structure.
- Define the flow training structure’s dimensions. Generally, the structure’s top elevation is a little bit lower than the bank elevation. Also, the side slope entry defines the sloping angle of the structure’s sides.
- Click the [Apply] button.
- The software will then construct the 2D flow training structure.
- Repeat the above steps to draw additional flow training structures.
Incorporating the Flow Training Structures into the 2D Mesh
After the flow training structures have been defined, the software will stamp them into the 2D mesh and refine the mesh to account for each flow training structure shape. Follow these steps:
- From the Map View, double-click on the 2D mesh. This will display the 2D Flow Area Data dialog box.
- Click the [Update] button.
- The software will incorporate the 2D flow training structures into the 2D mesh.Figure #6: Flow training structure in 2D view modeFigure #7: Flow training structure in 3D view mode