River Channel Stabilization with Submerged Vanes

Abstract

Submerged vanes are an unobtrusive and cost-effective way for river engineers to address many problems associated with river channel stability and river management in general. The vanes are small flow-training structures designed and installed on the riverbed to modify the near-bed flow pattern and redistribute flow and sediment transport within the channel cross section. The structures are laid out so they create and maintain a flow and bed topography that is consistent with that of a stable channel creating optimum conditions for managing the river. A relatively new technology, submerged vanes are a low-impact method for restoring riverbanks, stabilizing or re-meandering river reaches previously modified (straightened) by humans, increasing flood flow capacity, reducing sediment deposits, and for helping maintain or enhance the ecosystem in and around rivers. Following laboratory research and feedback from field installations, guidelines are now available for designs that are effective and sustainable. These guidelines are described in the book by Odgaard River Training and Sediment Management by Submerged Vanes, ASCE Press, 2009. Following a brief summary of the theory with illustrations from the ASCE book (reprinted with permission of ASCE), this chapter presents the latest feedback from field installations and suggestions for future applications.

Glossary

Aggradation

the process by which streambeds, floodplains, and the bottoms of other water bodies are raised in elevation by the deposition of material eroded and transported from other areas. It is the opposite of degradation.

Alluvial

pertains to alluvium deposited by a stream or flowing water (usually sand).

Alluvial stream

a stream whose channel boundary is composed of alluvium, and which generally changes its cross section and bedform due to the interaction of the flow and mobile boundary adjustment.

Bank migration

lateral shifting of the banks of a stream course.

Bedforms

wave-like irregularities found on the bottom (bed) of a stream that are related to flow characteristics. They are given names such as “dunes,” “ripples,” and “anti-dunes.” They are related to the transport of sediment and they interact with the flow because they change the roughness of the streambed. An analog to streambed forms is desert sand dunes.

Bed load

material moving on or near the streambed by rolling, sliding, and sometimes making brief excursions into the flow a few diameters above the bed, i.e., jumping. The term “saltation” is sometimes used in place of “jumping.” Bed load is bed material that moves in continuous contact with the bed; contrast with suspended load.

Bed material

the sediment mixture of which the bed is composed. In alluvial streams, bed-material particles are liable to be moved at any moment or during some future flow condition. Bed material may include grain sizes that travel both as bed load and as suspended load.

Boundary conditions

definitions or statements of conditions or phenomena at spatial or temporal boundaries of a model. Water levels, flows, sediment concentrations, etc. that are specified at the boundaries of the area being modeled. A specified tailwater elevation and incoming upstream discharge are typical boundary conditions.

Braided channel

a stream that is characterized by random interconnected channels divided by islands or bars. Bars that divide the stream into separate channels at low flow are often submerged at high flow.

Channel

a natural or artificial waterway that periodically or continuously contains moving water.

Conveyance

a measure of the flow capacity of a channel section. Flow is directly proportional to conveyance for steady uniform flow.

Cross section

the shape of the channel in which a stream flows on a line perpendicular to the flow or banks.

Cross-sectional area

the wetted area of a cross section perpendicular to the direction of flow.

Degradation

the process by which streambeds, floodplains, and the bottoms of other water bodies are lowered in elevation by erosion of material. It is the opposite of aggradation.

Deposition

the mechanical or chemical processes through which sediments accumulate in a (temporary) resting place.

Depth of flow

the vertical distance from the bed of a stream to the water surface.

Discharge

the volume of a fluid or solid passing a cross section of a stream per unit time.

Dunes

bedforms with triangular profile that advance downstream due to net deposition of particles on the steep downstream slope. Dunes move downstream at velocities that are small relative to the streamflow velocity.

Erosion

the wearing away of the land surface or stream boundaries by detachment and movement of soil and rock fragments through the action of moving water or other geological agents.

Floodplain

normally dryland adjacent to a body of water which is susceptible to periodic inundation by floodwaters.

Fluvial

in this chapter, pertaining to streams.

Fluvial sediment

particles derived from rocks or biological materials that are transported by, suspended in, or deposited by streams.

Grain size

see Particle size.

Local scour

erosion caused by an abrupt change in flow direction or velocity. Examples include erosion around bridge piers, downstream of stilling basins, at the end of dikes or vanes, and near snags.

Meandering

a planform (alluvial) process that generates a series of bends of alternate curvature connected by straight reaches.

Meandering stream

an alluvial stream characterized in planform by a sequence of alternating bends. The bends are usually a result of alluvial processes rather than the nature of the terrain.

Mean velocity

The discharge divided by the wetted area of a cross section.

Particle size

a linear dimension, usually designated as “diameter,” used to characterize the size of a particle.

Planform

the shape and size of channel and overbank features as viewed form directly above.

Point bar

deposits of sediment that occur on the convex side or inside of channel bends. Their shape may vary with changing flow conditions, but they do not move significantly relative to the bends. However, the general magnitude and location of the bars vary with discharge and sediment load.

River training

enhancements made to the banks of a river to constrain the river to a desired course; the enhancements are usually in the form of realignment or regrading of the banks, and installation of bank protection structures such as rock revetments (riprap), dikes, weirs, or submerged vanes.

Scour

concentrated erosive action by water. The enlargement of a flow section or creation of a depression by the removal of bed material through the action of moving water.

Secondary currents (or flow)

the movement of water particles normal to the principal direction of flow.

Sediment

naturally occurring material that is broken down by processes of weathering and erosion and is subsequently transported by the action of wind, water, or ice, and/or by the force of gravity acting on the particle itself. In this chapter, sediment is material (sand) transported by water.

Sedimentation

refers to the gravitational settling of suspended particles that are heavier than water.

Sediment discharge

the mass or volume of sediment (usually mass) passing a stream cross section in a unit of time. The term may be qualified, for example, as suspended sediment discharge, bed-load discharge, or total sediment discharge. See Sediment load.

Sediment load

a general term that refers to material in suspension and/or in transport. It is not necessarily synonymous with either discharge or concentration. It may also refer to a particular type of load; for example, total, suspended, bed, or bed material load.

Sediment particle

solid fragments of mineral material in either a singular or an aggregate state.

Sediment transport (rate)

see Sediment discharge.

Shear stress (boundary shear stress)

frictional force per unit or area exerted on a channel boundary by the flowing water. An important factor in the movement of bed material.

Stable channel

a stream channel that does not change in planform, cross section, or bed profile during a particular period of time (but may over longer periods of time).

Stream bank erosion

the removal of bank material primarily by hydraulic action.

Top width

the width of a stream section at the water surface; it varies with stage in most natural channels.

Validation/Verification

check of the behavior of a model against a set of prototype conditions that was not used for calibration.