Guidelines for Streambank Protection
John Poullain, P.E.
online course provides general guidelines and practices for protecting streambanks
from erosion and correcting erosion problems. The nature of streams, erosion
and bank failure processes and methods for reducing damages to the streambed
and bank is discussed. The course covers vegetative stabilization and structural
methods including riprap, gabions, checkdams and bulkheads. Control of surface
water is also necessary to prevent sheet and rill erosion of streambank slopes.
Typical methods used to manage over bank flows including earth berms, diversion
ditches and terraces as well as vegetation are also discussed. The emphasis
of the course is on natural streams and creeks. Navigation channels are not
considered but some of the measures considered here for streambank protection
are appropriate for larger waterways. State or Federal permit approval is required.
This course includes a multiple-choice quiz at the end, which is designed to enhance the understanding of the course materials.
At the conclusion of this course, the student will:
This course is intended for civil engineers and planners.
Benefit to Attendees
The student will
become familiar with methods and techniques used to protect streambanks from
erosion and overbank drainage in order to reduce soil erosion and runoff into
downstream waterways. The student will better understand the application of
interception and diversion methods such as ditches, berms and benches. Other
techniques for erosion protection and vegetation are discussed. The student
will also become familiar with the nature of streams and factors causing streambank
erosion and failure. Also considered are the methods and materials used to establish
vegetation for erosion control. Basic guidance in the selection of the most
appropriate method to use and the advantages and disadvantages are discussed.
Early soil conservation efforts were driven by the wind erosion occurring during the “dirty thirties”. These first measures were aimed at soil conservation but as land near streams increased in value and use for development, stream bank protection became an issue also. Many major waterways are protected by navigation and flood control regulations. Other streams and waterways, which are not protected by these regulations, experience erosion and failure to some degree. State and local governments have soil erosion and sedimentation control regulations that require special precautions for land-disturbing activities.
Streambank problems can be solved in many ways including relocation of endangered assets, conservation and land management practices, streambank removal of stream obstructions, channel relocations and protection measures. For instance if bank erosion threatens a highway in an undeveloped site, rerouting the stream can protect the highway. Excavating a pilot channel can initiate the desired change in alignment, which would be less expensive than a highway relocation or bank protection work. The stream however may become unstable causing future erosion upstream and faster flows downstream. Reaction to a stream cut-off or relocation measure must be considered carefully. All available data for the stream should be reviewed and inspections should be made to be aware of any detrimental changes in flow. In an urbanized area it would be prohibitively expensive to relocate developments or the stream, leaving protection measures to consider. This course mainly covers streambank protection methods.
Impacts of Streambank Erosion Studies have been made to evaluate the economic and environmental losses resulting from severe streambank erosion throughout the nation and the effectiveness of bank protection methods. Erosion of streambanks has the following impacts:
a. Loss of land or poor quality uncompacted “new” land deposited from the eroded soils.
b. RE property boundary litigation if the stream serves as a boundary.
c. Water quality decreases when sediments and chemical compounds such as nitrogen or phosphorous are carried by streamflows. These chemicals cause rapid growth of noxious plants and organisms and declining water quality as they decay.
d. Damages to buildings, road, bridges etc.
e. Wildlife habitat is disturbed or destroyed.
A stream naturally meanders and goes through stages as it matures and adjusts to changing conditions often caused by new construction, changing weather patterns, sand and gravel mining or even uncontrolled cattle grazing or improper stream crossings. As a stream adjusts or stabilizes for the changing events, erosion and streambed scour or sedimentation may occur. Soil particles will be eroded from the stream bank and failure or collapse of the bank may occur. When a stream is in a stable condition it will exhibit relatively unchanging or constant bed elevations and widths, an absence of rough or rushing water and will have smooth meanders without sharp bends. Reaches of rough water indicates headcutting of the bed is progressively working upstream to restabilize the stream. Before undertaking repair or protection of a streambank, the site should be inspected and determined whether the stream is in a stable condition. Besides bank failures the present condition of the stream can be determined by changes occurring in the channel width, streambed elevations or velocities in certain reaches of the stream.
As flows scour the bed and undercut the bank toe, the channel slope will lose support, become unstable and either slide or collapse into the channel. Or if the stream cannot carry the sediment load, sedimentation will occur and the channel width and cross section will be reduced. In order for the stream to convey the next flood, the channel cross section will enlarge from bed scouring and bank erosion. It is important to identify whether the streambed or bank is the cause to avoid wasting time and money on a streambank protection project. If the bed is unstable the factors causing it must be corrected or controlled before other work can be performed successfully for a durable repair. A successful bank protection project will depend on understanding the factors that cause stream instability. Streambank problems are complex and a single identifiable mechanism is rarely the sole cause.
Streambank Erosion Causes Erosion is caused by natural processes and accelerated by the following processes:
Land Development and Land Use Changes. Increased urbanization, more roofs and paving and clearing of woodland and grassland without proper conservation management create erodible land areas, speed up runoff and remove areas available for rainfall infiltration. Vegetation removal at a stream will accelerate more erosion and siltation.
Overbank Drainage and Groundwater Seepage. Overbank stormwater drainage or runoff can severely erode unprotected channel slopes during runoff. Rainfall that does not runoff may seep downward through the bank with a loss in shear strength. This condition may appear as wetness or piping flow down the stream slope. (See Fig 1)
Obstructions Stream obstructions include bridge piers, boat docks, fallen trees, sand bars, trash and debris and improper stream crossings. They constrict the channel, alter the normal flow, create eddys and increase the velocity upstream and through the obstruction causing erosion upstream and also downstream from deflection of normal stream flow. (See Fig 2)
Causes for Streambank Failures Clay soils swell and lose shear strength after absorbing surface and groundwater, resulting in creep or soil movement. Another cause is the erosive effect on the stream size, bottom width and toe. A third cause is from rapid drawdown of stream water. If the soil cannot drain quickly enough as water elevations fall, an imbalance in pressure occurs on the freshly exposed surface. And just as development causes erosive runoff, buildings and roads can overload the bank after the banks are weakened from water infiltration and seepage.
Streambank Management and Protection Methods Streambank management methods include overbank flow diversion with earth berms and ditches, terraces, streambank vegetation, removing obstructions and controlling development along the stream. Structural methods are riprap, cellular mattresses, bulkheads, earth and stone dikes, board fences, gabions, spur dikes.
Diversion Ditches and Berms Overbank flow erosion of streambanks can be reduced by intercepting and diverting water away from the top of bank by diversion ditching, diversion berms or a combination of both. They can be used to protect newly graded slopes until vegetation is established. It may be necessary to temporarily place straw-bale check dams at 50 to 100 foot intervals in the ditches for erosion protection. The diverted runoff is discharged into the stream through a pipe or ditch lined with riprap, portland cement concrete etc. for erosion protection. (See Fig 3)
Grading and Bench Terraces Streambanks that are too steep can be reshaped by grading the slope. There is a balance between over excavating and inadequate excavation, which risks a bank failure. It is more reasonable to terrace the slopes for small reaches. Terracing slows rainfall and runoff by breaking the gradient of long steep slopes in steps. Ditching may not be necessary since the benches are not constructed flat. They typically slope inward toward the bank to divert runoff water along the bench rather than down the slope. Vegetation is then more easily established and maintained.
Vegetation Site specific conditions must be considered to use vegetation. Success of vegetation depends on the stream flow characteristics, bank geometry, site preparation and compatibility with these conditions. Vegetation is one of the most commonly used methods. It is relatively easy to maintain and establish and properly selected plants and grasses are self-maintaining. Erosion control matting may be necessary to hold the seed and soil in place until the vegetation is established. Eroded banks and undercut toes require grading prior to planting. If the toe is being undercut the toe should be protected with riprap or other non-erodible material. Slopes flatter than 1V to 1 ½ H can accommodate vegetation but slopes flatter than 1V on 2H or 3H are preferable especially for ease in maintenance.
Vegetation protects a streambank with the roots and exposed branches, stems. The exposed plants defer flow by deforming the plant in place of removing soil. Over bank flow velocity is reduced and the capacity for infiltration and water withdrawal from the banks are increased. Woody plants provide greater erosion protection but on high banks the roots may not penetrate to the bank toe and a collapse will cause bank failure.
Riprap There are several ways to place riprap. It can be placed conventionally along the bank toe or as a blanket over the bank slope. Riprap can also be placed on top of an eroding bank or buried in the bank as windrow revetment. As the bank is eroded the riprap is undercut; sloughs down the slope into the area of erosion and the bank will stabilize at some angle of repose as the erosion stops. Riprap can be placed where toe scour threatens the bank and the upper bank if not in a bend may be stabilized with vegetation. Riprap mattresses are relatively flexible and can adjust to bank changes from settlement or scour. Minor damage can be easily repaired with additional stone to fill settlement or voids. A rule of thumb for mattress thickness is 1.5 times the thickness of the largest stone being used and to increase the thickness by 50% for stone being placed underwater. Filter fabric or drainage material is placed to protect bank from loss of fine soils and to allow for water seepage from the bank. (See Fig 4)
Check Dams Bed scour can be controlled with check dams or by lining the stream with erosion resistant materials such as riprap. A check dam should be placed across the scouring streambed, bank to bank and downstream from reaches of rough water flow or headcutting. The check dam toe must be protected from undermining and the both ends should be placed into the banks to prevent flanking damage. They are constructed of stone, concrete, metal and sand cement bags. Stream velocity will be reduced and siltation will occur upstream but downstream erosion may occur and should be checked periodically after completion. Lining the streambed with riprap, stone mattresses and filter material is an alternative but would be cost prohibitive to protect long reaches or wide streambeds.
Dikes A streambank is protected by the dike deflecting flow away from from the eroding bank. Dikes can be constructed with stone, rubble or even brush on a smaller scale and lower velocities. The first dike, since one dike will only cause other problems, should be located upstream of the erosion. The other dikes can be constructed to maintain a midstream flow by deflecting the current back towards the middle of the stream. Since the flow characteristics change as the stream flow changes from low stages to flood the positioning of the dikes will require professional design to determine the lengths, heights, position on stream, material type and angle relative the bank. There are also permeable type dikes made from board fence and wire fences that permit deposition of sediment as the stream flows through them. They are most useful on stream carrying heavy sediment loads. However they are vulnerable to damage from debris impact and scour at the post supports and bank tie-ins and may require repairs or adjustments to maintain serviceability.
Bulkheads Bulkheads protect streambanks from erosion and failure, allow river access and can be used without encroaching on the stream channel. They can be constructed from concrete, steel, timbers, aluminum and used tires for economy. Certain measures must be taken to protect the integrity of timber and tire bulkheads, which will require more maintenance than steel or concrete bulkheads. The toes must be riprapped, posts will need deadmen for stability, filter fabric should be placed to keep the bank free draining and to stop soil from leaching through and as with any type of bulkhead the ends should be buried in both banks for protection from stream flanking erosion.
This course is based primarily on Chapter 6 (selected parts) of the Missouri Department of Natural Resources, "Streambank Protection", 25 pages, PDF file and the "Course Introduction".
The following items from EP 1110-1-16 are also included in the
course material, all in PDF files.
You need to open or download above documents to study this course.
This course considers
the techniques and methods used to control surface water and protection of channel
banks and streambeds from erosion. The design, installation, type of materials,
advantage and disadvantages and the effects of the methods and site conditions
are also considered. Vegetative stabilization and structural methods including
riprap, gabions, dikes, checkdams and diversion ditches and dikes are discussed.
Protective actions consist of site management measures to prevent or reduce
the velocity of overbank drainage. The methods used for overbank surface water
are also used to control run-off from construction sites and include interceptor
ditches, earth berms, flumes and terraces and benches. Environmental risks are
mismanagement of surface water and of any contamination or siltation from erosion.
State and federal permits have to be complied with to remove any threat to public
health or to the environment.
For additional technical information related to this subject, please refer to:
"Integrated Streambank Protection Guidelines", Several Washington State and Federal Agencies created these guidelines to assist property owners, planners and designers in protecting and restoring streambanks and wildlife habitat.
Techniques for streambank and waterway protection. Information on riparian, wildlife habitat and waterway development.