Tactic CR-9: Trenching
Trenching is used to intercept the flow of a spilled substance, to divert a spilled substance around a sensitive area, or to capture and recover water used during flooding and flushing (Figs. 49 and 50). Examining the sidewall of a trench can help determine if spilled substances are moving below the ground surface (Fig. 51). Dig trenches by hand or using a trencher attached to a skid loader, tractor, or other type of heavy equipment.
Dig a trench or series of trenches at right angles to the flow, angled slightly downhill to avoid excessive pooling. Place the excavated material on the downhill side of the trenches. Line the sides and bottoms of trenches with plastic sheeting. A trench can be flooded with water to inhibit contaminant penetration and to promote flow toward a recovery device.
Digging trenches in tundra should be considered a last resort, if no other tactic is available to divert or capture water or contaminants. Do not excavate trenches in an area where the excavation will cause more damage than benefit. Excavating trenches in permafrost terrain will disrupt the thermal regime and cause thermal erosion (thermokarst). It may be necessary to backfill trenches (Tactic TR-12) to reestablish a stable thermal regime, and revegetation may be needed to meet rehabilitation goals for the site.
Considerations and Limitations
- Vehicle use on tundra must comply with applicable tundra travel policies (Tactic P-5).
- The Bobcat trimmer should be used for trenching only if no other options exist. It can cut a maximum depth of about 4 inches per cut; a trench deeper than 8 inches will be as wide as the Bobcat.
- It may be necessary to survey spot elevations before trenching, to ensure that fluids flow into the trenches.
- A permit may be needed from the landowner before trenching.
- Trenching in tundra should be considered a last resort. Trenching may lead to further disturbance if a natural stream, river, or swale intercepts the path of the trench.
- This tactic has been adapted from Tactics R-7 and C-12 in the Alaska Clean Seas Technical Manual ( ).
Equipment, Materials, and Personnel
- Shovels (1 worker per tool) – to hand dig trench.
- Skid loader, or tractor with trenching attachment (1 operator) – to dig trench.
- Visqueen or similar heavy plastic sheeting – to line trench.