Small structure in zero or first order channels made of logs designed to stabilize the channel gradient and store a small amount of sediment. which carries surface water flow only after storm events or snow melt.
Purpose: Log dams, like 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. They are constructed of more durable material than straw bale dams. Log dams function by decreasing water velocity and detaining sediment-laden surface runoff long enough for coarser sediments to deposit behind check dams. Decreased water velocity also reduces downcutting in ephemeral channels.
Relative Effectiveness: Excellent-40% Good-60% Fair-0% Poor-0% (Replies = 5). Log dams were rated “excellent” and “good” in their effectiveness as a ESR treatment by the limited number of interviewees who commented on log dams. Well-built log check dams can be 70 to 80 percent effective in trapping sediment and last 15 to 30 years. The amount of sediment trapped is highly variable depending on the size of the dam. In one location, individual log dams were reported to trap up to 40 cubic yd (40 cubic m ) of sediment without failure. They can be very effective in adding to channel stability and keeping sediment onsite. On the negative side, failures due to undercutting, bypassing, and complete blowout have aggravated erosion problems by producing deep scouring at dam sites and release of large amounts of sediment in pulses. Despite these potential problems and situations where 25 percent failed in the first storm, no one rated log dams as “fair” or “poor” in effectiveness.
Implementation and Environmental Factors: Like straw bale check dams, a key factor in log dam construction is having a skilled implementation leader and trained, experienced crews. Log check dams are costly and labor intensive, requiring six to eight times the labor for installation than straw bale check dams. Design features such as appropriate size of watershed, dam orientation, log sizes, lateral keying (1.5 to 3 ft (0.4 to 1 m) into banks), spillways, contact with the stream bed, plugging of gaps, and energy dissipaters are important implementation considerations. Some ESR coordinators recommend that log dams never be put in fully functional channels, but others recommend that log dams can be used to replace coarse woody debris burned out of small perennial channels. Often rocks are used in conjunction with log dams. Other Factors: In some locations, there might not be adequate woody material after a fire to build log dams.
Log Grade Stabilizer (Low Tech "V" Log Dam) Implementation
To prevent sediment from entering perrenial streams during the first winter following a wildfire.
To establish grade control, decrease water velocity, and reduce accelerated stream channel downcutting.
To maintain correct stream channel width/depth ratio.
To trap and slowly meter sediment release through the system.
|General area where straw bale check dams can be effective
|Specific individual dam site locations
CONSTRUCTION AND EQUIPMENT
The proper placement of a dam is the most important part of the construction and is a combination of:
|Locating a specific site where natural material has burned out or gradient control is needed.|
|Locating standing or down wood that is 1) preferably 12 to 20 inches in diameter, 2) cedar, hardwood, Douglas Fir, or other durable wood, 3) reasonably near to the dam site, and 4) light enough to be moved safely, if necessary.|
Stand in the middle of the channel looking upstream. Imagine the water flowing towards you. [Most likely, the channel will be dry or flow only a trickle of water.] If the logs are placed incorrectly, the natural flow of the stream will be altered. It is best to place the wall of the structure perpendicular to the flows, or in a shallow V formation, with the bottom of the V pointed downstream. This is sometimes difficult to accomplish due to safety, the size of the wood pieces, etc, but conform as closely as possible within reason.
Small material - including channel fines and rocks - should be used to seal the gaps between the logs and the channel banks and/or ground. Good seals at the walls of the channel are crucial to success.
If preferred, key in the logs by digging a trench approximately perpendicular to the channel. NOTE: This is usually not necessary and structures have been successful without being keyed in. However, it may be essential where erosion rates are high, where the wood conforms poorly to the cross section or in areas that have burned very hot leaving a deep ash.
To anchor the logs into the ground a straight length of 3/8 inch rebar is pounded into the ground in front of the log or a bent "U" shaped piece is placed over the log and driven into the ground. Up to four pieces of rebar are needed for each log depending on the length of the log and the amount of water that is expected to flow in the channel. To prevent injury, pound the rebar down till approximately 6 inches is left. Bend the 6 inch piece over the log and hold into place using 2", fencing staples. Driving rebar into rocky ground can often be nearly impossible, and in these cases, other treatments should be considered.
As shown in diagram 1 and diagram 2, logs and/or rocks are placed in front of the "V" of the dam to construct the energy dissipater. Do not build the dissipater higher than the logs forming the "V" dam. There should be at least a 4 inch difference in height. Add small branches, woody debris and needles into the channel upstream from each structure. These small woody fines will be picked up by the water and, carried by the current, will help plug any gaps in the dam.
ADVANTAGES OF LOG GRADE STABILIZERS
Construction can be both efficient and economical. A crew of three to four persons with one chain saw can average well over one dam every two hours. This is a higher rate of production than for most other kinds of dams requiring intensive manual labor. It is approximately twice the rate for rock-dams, and three times the rate for straw bale dams.
Equipment and purchased material needs can usually be minimal. A good sawyer with chain saw is all that a crew needs. Unlike the straw bale dam, there is not an additional transportation cost for the materials. All materials can be hand carried to the specific sites.
The design is basically simple. A person with minimal training can be the crew leader or technical advisor of a crew. NOTE: See #3 under disadvantages.
A trench for the single log sediment trap is generally not necessary. Structures have been successful when the logs were not trenched. Time and energy are saved.
A finished dam can be aesthetically pleasing. It can approximate a natural feature, especially after the storage area is filled or after surviving the annual peak flows. Or it can at least blend into the natural setting of a riparian zone better than other types of erosion control structures.
DISADVANTAGES OF SINGLE LOG SEDIMENT TRAPS
The trees that have to be felled and bucked, should all be used in the construction of the dams. Unused pieces of logs and/or branches should be stacked for wildlife use. Beware of sawyers and crews that emphasize the thrill of felling and bucking the largest trees around at the expense of the objective of constructing structures to control erosion.
While the idea of low tech log dams appears to be a basic one, it is more abstract than the idea of rock dams and straw bale dams. The concept of a proper spillway is more difficult to understand. It can be difficult to fit logs, especially hardwoods, to the cross section of the channel. Proper sealing to keep water flowing only through the spillway can be difficult to achieve, although trenching may greatly improve the fit.
With the high rate of production typical of these dams, crews sometimes become so intent on production that they sacrifice quality. ESR is an emergency rehabilitation project and it is difficult to combine quantity and quality.
Crews that have worked on long term watershed restoration projects sometimes have a difficult time with the concept of not building large, permanent, more technical structures. None of the structures described in this handout are ever built higher than 18 inches.
SAFETY HAZARDS INVOLVED
In using power equipment, several special precautions must be followed:
|All non-sawyers should keep their distance from the saw operator and swamper. This includes not being downhill of a sawyer in the riparian zone.|
|Be careful around logs and trees: backs can be strained, arms and legs can be pinned by the wood, or worse can happen. Avoid uncontrolled rolling.|
|A crew should be safe and stay within the limits of its equipment and skills.|
|Use safety glasses when pounding in rebar.|
|Pay special attention when rolling or throwing rock down the hill and into the channel where the structure is being constructed. Also, take special care when placing rocks, many fingers and or hands have been smashed.|
ESTIMATED COST TO BUILD A SINGLE LOG SEDIMENT TRAP
|Materials and Labor||Cost|
|22 - 4' lengths of 3/8" rebar at $.62 per length||$13.64|
|3 pounds of 211 fencing staples at $80.00 per 40 pounds||$ 6.00|
|Labor to build the dam (1 sawyer and 1 laborer/swamper for 2 hours each)||$70.00|
The estimated cost may vary if areas are not easily accessible by roads and require a long walk-in and out time. Crews that are more experienced will require a shorter amount of time to construct each structure. The cost may also vary with any changes in any of the above items.