What is a Live Cribwall?
A live cribwall is a wooden log cabin-type structure built into a streambank which is filled with rock, soil and cuttings of willow. The cuttings will sprout and develop a root mass that, in conjunction with the log structure, will armor the stream bank and protect it from erosion. The submerged rocks and logs provide excellent aquatic habitat. Well built cribwalls can last for decades as the live materials take over the structural function of the logs.
The purpose of this Ohio Stream Management Guide is to describe the applicable site conditions and design, installation and maintenance guidelines for live cribwalls. The specifications listed herein are gleaned from literature sources and not based on field experience here in Ohio. As with any construction project in a stream, it is recommended you consult with the applicable local, state and federal authorities listed in Guide 06, Permit Checklist for Stream Modification Projects, prior to construction. The extent of permit requirements will depend on the location and design of your project.
Live cribwalls are often used in conjunction with other practices that further protect streambanks from eroding forces. Compatible practices, described in other Ohio Stream Management Guides, may be selected by finding similar site requirements and applications.
Figure 1. Cross section of a live cribwall.
Where to use Live Cribwalls
Live cribwalls are effective at reducing erosion and streambank instability on: the outside bends of streams with strong currents; where a there is a steep streambank with an unstable toe of the slope; and where immediate protection from erosion is needed while vegetation becomes established. (The toe is where the stream bed meets the bank slope.) They can be used to reduce the steepness of a slope by building a low wall to protect the toe and grading the soil above it back at a gentler grade. Live cribwalls have also been successfully used to prevent the development of a split channel in a stream. If used to protect roads, structures or utilities, a professional engineer should be involved in the design process.
These structures are not effective in locations where the stream is down cutting because the base of the structure will be undermined. The major disadvantage of live cribwalls is that it is labor and equipment intensive. They are therefore used only where the expected benefit warrants the investment.
Site Requirements
Live cribwalls are effective for streambank stabilization on streams up to 75 feet wide. The structures are placed along scoured or excavated areas of the bank. Therefore the streambank must have adequate room between what is to be protected and the edge of the water (existing or as designed) to allow for the placement of log cabins' that are six feet deep. To anchor the structure the channel bed must be excavated, so this practice is not applicable for sites on bedrock. The live cuttings of willow or other rapidly-rooting species require full sunlight for part of the day during the growing season to grow properly. Therefore, the site will need full sun for the cribwall to be effective.
Design
The length of the eroding streambank can be treated with a number of cribwall structures each of which should not exceed 20 feet in length. The series of structures must be designed to start at and end at areas of the bank where there is no active erosion. If it is not anchored to the bank this way, the stream will scour into the soil around the ends of the cribwall, rendering it useless.
The structure's height depends on the elevation of channel-forming flows. The elevation at which water will flow over the low bank into the floodplain is called the bankfull flow and is considered the channel-forming flow. The bankfull flows are the most critical because they have enough energy to carry solid materials and happen frequently enough (on average every 1.5 years) for their impact to add up.
The cribwall needs to be somewhat higher than the elevation of the channel-forming flow in order to protect the outside bank (see Figure 1), but it should not exceed seven feet measured from the excavated base. If the existing bank is significantly higher than the cribwall's design height, the bank area above the live cribwall needs to excavated to a slope of 1 foot rise for every 2 foot of horizontal distance.
Figure 2. Placement of cribwall structures.
Construction and Installation
Step 1
Collect cuttings from willow, red-osier dogwood or other rapidly-rooting species (listed in Guide 07). These cuttings must be taken from live trees during their dormancy (approximately between mid-November and late March). The cuttings can be 0.5 to 2 .5 inches in diameter at the butt end and need to be six to eight feet long. The harvest should be made no more than 48 hours prior to installation. If not using them immediately keep the cuttings moist by wrapping them in wet cloth or blankets and do not allow them to dry out.
Step 2
Construction begins with the excavation of the bank. This excavation should be done during times of low stream flow and just prior to the installation of the live cribwall. Excavation should be about six feet into the bank measured from the water's edge (either existing or as designed) at low flow. Excavation should also extend two to three feet below the existing streambed. The floor of the excavated area should be sloped so that it is one to two feet lower in the back, toward the bank. As shown in Figure 1, this will tilt the weight of the cribwall away from the water's edge, adding to its stability.
Step 3
Use untreated logs or timber members. Place two logs, four to six inches in diameter, parallel to the stream edge four to five feet apart within the excavated area. Place the second two logs perpendicular to and at the ends of the first course of logs. As in a log cabin, allow about 6 inches of overhang in each direction. The four logs should be spiked together with long nails or re-bar. Each successive course of logs parallel to the stream flow should be set back six to nine inches from the log beneath it creating a stair-step effect (see Figure 1). Continue building the log cabin-like structure with alternating courses of logs up to the level of the stream's low flow. Build the remaining number of rectangular structures along the project's length, up to the same level.
Step 4
Place rock fill material in and around the bottoms of the structures up to the existing stream bed level. The weight of the rock will counteract against the log's buoyancy, securing the structures in place. Make sure that the excavated area between the stream bed and the structures is filled with rock to protect the toe of the slope. Fill the remaining area of the crib structures with soil up to the low flow level, i.e., up to the level constructed so far. Compact the soil.
Step 5
Before adding the next course of logs, lay a row of live dormant cuttings on the compacted backfill soil. Lay the cuttings perpendicular to the flow of the stream with the bud ends protruding one to two feet out of the open face of the cribwall. The butt ends must extend into the area behind the structure so their roots will eventually grow beyond the backfilled soil into the undisturbed soil. Repeat this step until the cribwall reaches the designed height.