Gabion Revetments Guide No. 15

Streambank erosion is a natural process that occurs in streams. Depending on the soil type and land use, streambank erosion can account for 40 percent or more of total soil loss in some watersheds (Farm Journal, 1992). The major factor accounting for streambank erosion is the velocity of the flow- ing water. Velocity is affected by the stream cross section, stream bed gradient, bank cover, depth of flow and degree of meander. Water flowing at two feet per second can move a cobblestone weighing half a pound, but an increase to ten feet per second can move a stone that weighs one hundred and fifty pounds. There are numerous methods of controlling streambank erosion. When a streambank requires pro- tection from high velocity flows, structural methods should be con- sidered. Two structural methods commonly used are riprap revet- Figure 1. Gabion revetment on streambank ments and gabion revetments. Structural methods are also used the revetment. Projects on larger of Natural Resources recommends when infrastructure, such as utility streams and projects protecting that you consult with the applicable lines, roads or buildings, are endan- structures or utilities should be local, state and federal authorities gered by the eroding stream. When designed by a professional engi- listed in Guide 06, Permit Checklist installing streambank protection, neer. Design advice and limited for Stream Modification Projects, the velocities during everyday flows assistance on smaller projects may prior to construction. The extent of as well as the velocities during large be obtained at your local Soil and permit requirements will depend storm events should be considered Water Conservation District. Prior on the location and design of your in the design process. to purchasing materials, check the project. This Ohio Stream Management stream’s water quality against the Guide describes the basic uses for manufacturer’s recommendations. gabion revetments, their installa- If the water quality would cause pre- Gabion Revetments tion and maintenance. Depending mature deterioration of the baskets, on the skill, equipment and labor other bank protection measures Gabions are stone-filled galva- force available to a landowner, the need to be considered. nized or coated wire baskets placed services of a land improvement As with any project in a stream along a streambank. Gabions are contractor may be needed to install environment, the Ohio Department particularly effective for protecting the submerged part of the stream- bank. They provide the same basic protection as riprap, but can be utilized when the streambank slope cannot be cut back due to physical Lid constraints (e.g., roads, utilities or buildings) or when larger rock is not readily available. Back Sizing

The typical size of a wire basket is three feet wide by three feet high per cell purchased in the desired End Base End length. Wire baskets can be pur- chased prefabricated or requiring assembly. Stone size diameter should be a minimum of four inches to prevent the stone from wash- Front ing through the mesh of the wire basket.

Installation Figure 2. Gabion basket as it arrives from manufacturer for assembly Procedure stone. Tight layers, free of voids, Basic installation consists of the A highly permeable fabric should be should be formed when the stone following steps; detailed installa- used. is in place. A tight layer can be tion specifications, provided by the 2. Make an entrenchment into obtained by varying the size of the manufacturer, should be reviewed the stream bed at least one foot stone and may require some hand and followed during installation. deep and large enough to hold the manipulation. Improper installation can lead to bottom row of baskets. The trench 4. As the baskets are filled with collapse of the gabion revetment. floor should be level along the proj- stone, place wire reinforcements in 1. Place appropriately sized geo- ect length. Pin the empty baskets each direction every twelve inches textile filter fabric on the streambank into the stream bed with several to retain the shape of the basket slope following the manufacturer’s steel rods that are three to four feet (see Figure 3). recommendations. Caution should long. 5. Secure the tops of the bas- be taken not to tear the filter fabric. 3. Fill the wire baskets with kets with a galvanized wire and

WIRE REINFORCEMENTS

3

4

2

1

FRONT FACE Figure 3. Use wire reinforcements to retain rectangular shape and increase stability lace the baskets to each other (see Figure 4). The length of wire needs to be about 1.5 times the length to be laced. Wire longer than 5 feet is difficult to handle. Secure one end at a corner, and lace the wire by alternating single and double loops at 5 inch intervals. Secure the wire again at the end. 6. Stagger the joints of the wire baskets during construction to make the structure stronger and more stable. Each row should be stair- stepped back from the previous Figure 4. Wire the baskets shut and wire them to each other row. 7. Any open area between the The branches must be long enough filter fabric and baskets should be to reach the undisturbed soil. Make The primary factor in maintaining backfilled with gravel to provide sure the buds point out, toward the a gabion revetment is in the durabil- positive drainage. Open areas sunlight. See Guide 07 for informa- ity of the wire used in the basket. should be minimal if gabions are tion on harvesting and using plant A gabion revetment is susceptible properly installed. materials. to wire deterioration. The stream’s 8. Where sunlight is available, water quality should be within the dormant cuttings of willow or other Maintenance manufacturer’s recommendations. rapidly-rooting species may be Broken wire needs to be replaced placed between the layers of bas- Requirements with galvanized or coated wire. kets during construction (Figure 5).

EXISTING VEGETATION, PLANTINGS OR SOIL BIOENGINEERING SYSTEMS

GRAVEL AND COMPACTED FILL MATERIAL

LIVE BRANCH CUTTINGS (1/2 TO 1 INCH IN DIAMETER)

EROSION CONTROL FABRIC

CHANNEL-FORMING FLOW GEOTEXTILE FABRIC

BASEFLOW GABION BASKETS STREAMBED

2 TO 3 FEET

Figure 5. Cross section of vegetated gabion revetment A gabion revetment needs to be References: Each Guide is designed to be easily inspected annually and after high and clearly reproduced and can be flow events. Any displacement or Iowa Department of Water, Air and bound in a notebook. Single copies shifting of the wire baskets needs Waste Management. How To are available free of charge. When to be corrected immediately. Control Streambank Erosion, distributing guides at meetings or 1984. in mailings, please use the printed Advantages editions as a master for reproducing Smith, Darrell. “Raging Waters.” the number of copies you need, or • Designed for high velocity flows Farm Journal, May 1992. you may print high quality originals • Can be installed where tight from PDF files available on-line at: physical constraints exist State of Ohio, Department of Trans- http://www.dnr.state.oh.us/odnr/ • Easily fits contours of stream- portation. Construction and Ma- water/pubs/onlnpubs.html bank terial Specifications, 1995. • Openings provide positive drain- age of streambank U.S. Army Corps of Engineers, • Freeze & thaw sequences shift North Central Division. Help Prepared by the Ohio Department of Yourself, 1978. Natural Resources, Tara Lee, Division of stones with minimal impact to Engineering, principal author. Input from structure staff of several ODNR divisions, and local, • Heavy machinery may not need state and federal agencies are used in the development of the Ohio Stream Manage- to be used ment Guides. Funding for the production • Minimal maintenance costs ~ of the Ohio Stream Management Guides is This Guide is one of a series of provided in part through a grant under Sec- Ohio Stream Management Guides tion 319 of the federal Clean Water Act. Disadvantages Guides are available on-line at: covering a variety of watershed http://www.ohiodnr.gov/soilandwater/pubs/ and stream management issues onlnpubs.html • Wire baskets are susceptible to and methods of addressing stream deterioration related problems. All Guides, in- • Labor intensive cluding an Index of Titles, are • Installation cost may be expen- available from the Ohio Depart-

S O S sive O E I s h L C e i c ment of Natural Resources. For o R r A U u D N o e O s p D S e E R a r W A T R R l t m E r a more information about the project e n t o f N a t u call the ODNR, Division of Soil and An equal opportunity employer--M/F/H. Water Resources at 614/265-6739. Printed on recycled paper