Straw Bale Check Dams
Check dam made of straw or hay bales often stacked to provide additional storage capacity. Designed to store sediment and/or prevent downcutting.
Purpose: Straw bale check dams are used to prevent or reduce sediment inputs into perennial streams during the first winter or rainy season following a wildfire. Straw bales function by decreasing water velocity and detaining sediment-laden surface runoff long enough for coarser sediments to deposit behind check dams. The decreased water velocity also reduces downcutting in ephemeral channels.
Relative Effectiveness: Excellent-30% Good-30% Fair-30% Poor-10% (Replies = 10). Straw bale check dams were judged to cover the range from “good” to “poor” effectiveness. They often fill in the first few storms, so their effectiveness can diminish rapidly. However, channel gradients can be easily stabilized, and sediment is stored and released at a slower or diminished rate. They appear to work well in front of culverts, and in semi-arid environments require little maintenance. Structural survival rates of 90 percent have been reported after 1 year with 75 to 100 percent sediment storage, and 95 percent survival after rainfall of 2.4 in/hr (60 mm/hr) for a 10-minute duration. However, a common negative comment was that straw bale check dams tend to blow out in large storms. Failure can occur if the dams are poorly installed or put in locations where they can not contain runoff. Straw bale check dams are considered by many ESR project coordinators to be effective emergency rehabilitation treatments. Straw bale check dams appear to work better than contoured felled logs. Some Forests use straw bales below culverts to disperse flow and trap sediments. They appear to the most successful in channels small enough to require only three bales, but in narrow, steep drainages two-bale wide structures do not function as well.
Others do not recommend use of straw bales because they fill to capacity after small storms. They can be washed out later even when anchored with “U” shaped 1/8 in (3 mm) re-bar are useful only in the upper reaches of watersheds (1st or 2nd order drainages) that are often difficult to access, and can be easily undercut if energy dissipators are not installed. One of the comments on straw bale check dams was there is always a risk of failure in large events. These dams cannot be designed for large storms, and will fail during significant runoff events.
Implementation and Environmental Factors: A large number of comments were made about important implementation and environmental factors that affect the success of straw bale check dams. Regarding implementation, a key factor is having a skilled implementation leader and trained, experienced crews. Straw bale check dams are costly and labor intensive. With such a high investment, the dams must be well-designed, properly placed, and well built.
Generally speaking, straw bales work best in drier regions, on small drainage areas that have low gradients (less than 30 percent), and in channels that are not incised. The bales need to be placed so that they contact the channel bottom, are curved up to and keyed into banks, and are adequately staked or wired to stay in place. Inter-bale spaces need to be filed so that channelized flow does not occur. “U” shaped re-bar seems to work well in stabilizing bales but don’t guarantee that the bales will remain in place. Geotextile fabric works well as an energy dissipator and should be placed starting on the uphill side running over the bales in the center of the channel and downstream in a splash pad. Chicken wire and staking should be used to keep the geotextile in place. Rock, wood, or other straw bales can also be used as energy dissipators but must be large enough or well-anchored to prevent movement during runoff. Straw bale check dams seem to work better and survive longer than silt fences, especially when reinforced with wire on the upstream side.
Other Factors: Because straw bales will break down over time and fail in high flows, maintenance during the first year is very important. Straw bales are not readily available early in the year. After August they are very available. Rice straw bales should be considered because they usually do not contain noxious weeds, and weeds associated with rice crops do not do well on dry hillslopes and ephemeral channels. Straw bale check dams can be destroyed by grazing animals such as cattle and elk. Bears also have a peculiar tendency to indulge in ripping straw bale check dams apart.
Straw Bale Check Dam Implementation
To prevent sediment from entering perennial streams during the first winter following a wildfire.
|To trap and slowly meter sediment release through the system.|
|To decrease water velocity and reduce accelerated stream channel downcutting.|
|General area where straw bale check dams can be effective
|Specific individual dam site locations
CONSTRUCTION AND EQUIPMENT
Placement of the straw bale check dam is the most important part of the construction. Without proper placement the dam is more apt to fail.
Stand in the center of the channel and look upstream. Imagine the water flowing towards you. Attempt to match the existing channel area for placement of the spillway bales. If the bales are placed incorrectly the natural flow of the stream will be altered. The dam must be placed perpendicular to the flow. Do not constrict the size of the area you are using for the spillway bales. If this area is too narrow, the flow of the water going over the spillway bales will be accelerated causing the water to increase in velocity and may cause further erosion downstream. See Diagram 1.
The spillway bale and/or bales should be placed in the channel before the side bales. Use a Mcleod to lightly smooth the ground area upon which you will place the straw bales. This will insure that the bales seat properly, preventing the potential for water to flow under the dam, undercutting the front of the structure.
Use wooden stakes to anchor the spillway bales securely into the ground. Angle the stakes into the straw bales as shown in diagram 2. A 2 1/2 pound single jack sledge hammer is used to pound in the wooden stakes. Make sure that the stake is pounded in at least 2 inches below the straw dam for safety reasons.
The placement of the side bales will be rotated slightly uphill to create a horseshoe shaped structure. This will create a large bowl area for sediment storage as shown in diagram 3. When placing bales, push together tightly to prevent gaps between the bales. If gaps are unavoidable, chink them with small rocks, small woody material and needles.
In some ephemeral and small intermittent streams, it may be necessary to key in the side bales for structural strength.
The crest of the spillway bale must be lower than the bottom edge of the last side bale to insure that the flow of the water (and/or any built up sediment) does not go around the sides of the dam as shown in diagram 3.
To construct the energy dissipater, gather on site natural materials (logs and/or rocks). Place these materials on the downstream side of the spillway bales as shown in all diagrams. To anchor the logs in place, use "U" shaped rebar. To construct the "U" shaped rebar, bend a piece 3/8 inch rebar around a small tree trunk. Pound the "U" shaped rebar into the ground using a 2 1/2 lb. single jack hand hammer. Secure the rebar to the log using 2 inch fencing staples.
As shown in all the diagrams, logs may be placed in front of the side bales. This should be done if there are enough native materials on site. The logs will help to hold the collected sediment in place when the straw dam decomposes. In constructing the energy dissipater and/or the side logs, keep in mind what the structure will look like when the straw decomposes. A large energy dissipater standing alone in a small ephemeral channel is not a desired future condition.
The final step is to place small branches, woody debris and needles on the upstream side of the dam and/or into the channel between structures. These fines will be picked up by the water and, carried by the current, will help plug any gaps in the dam.
ADVANTAGES OF STRAW BALE CHECK DAMS
Straw dams can be built where native materials are not available for other types of erosion control structures.
The straw is sufficiently impermeable to hold water. The bales can be pushed tightly together, reducing the labor cost of chinking large gaps with rock and small woody debris.
Straw dams trap and store significant quantities of sediment when placed in locations that are natural catch basins. They can be built in a horseshoe shape unlike the single log sediment trap.
DISADVANTAGES OF STRAW BALE CHECK DAMS
The cost of straw for the structures more than doubles when access is difficult or limited and the bales need to be air lifted to staging locations along the drainages. The cost also increases when crews have to transport the bales to each specific site.
The structures should not be built in steep channel or as grade control structure.
The straw in the dam is not permanent and decomposes at a faster rate than the log or rock used in the other structures.
SAFETY HAZARDS INVOLVED
When using a single jack hammer to pound in the 3/8 inch rebar used in the energy dissipater and side log construction, small metal flakes from the hammer or the rebar may fly into the workers eyes. All individuals driving in rebar or working in the immediate vicinity must wear safety goggles.
When stakes are not pounded in completely, the protruding ends may cause serious injury to crew members kneeling or sitting on the dam while working. All stakes must be driven in at least 2 inches below the top of the bale as shown in diagram 2.
ESTIMATED COST TO BUILD A 6 BALE STRAW CHECK DAM
|Materials and Labor||Costs|
|6 bales of clean straw at $4.00 per bale||$24.00|
|18 2' long wooden contractor stakes at $.40 each||$ 7.20|
|12 - 4' lengths of 3/81, size rebar at $.70 per length ( used to anchor down dissipater and side logs)||$ 8.40|
|Labor to build the check dam ( 1 laborer for 8 hours )||$96.00|
This estimated figure does not include the cost of the transportation or staging of the straw to the building sites. All figures that were used to achieve this cost estimation were derived from the price list of materials used in the Crystal Peak Burned Area Emergency Rehabilitation 1994. The estimated cost may vary with any changes in any of the above items.