by Steve Kallesser, Division chair

Sedimentation has been a major concern for the forestry movement since its inception, following the massive industrial forest harvesting of the 1880’s and on. As a result, prior to the federal Clean Water Act, hundreds of studies, field notes, and published anecdotes existed regarding how to avoid sedimentation during forestry operations. Following the need to publish forestry and wetlands Best Management Practices (BMP), many additional studies followed — published and unpublished.  While it is far beyond this article to review each, it can be safely said that as long as an adequate Streamside Management Zone (SMZ) is maintained, the only way for forestry-operation-sourced sediment to enter a stream is if something has gone wrong on a forest road, or a major skid trail – usually at a stream crossing.


Careful planning of forest roads and skid trails are critical to preventing sediment from entering streams and lakes. (photo by Steve Kallesser)

The National Council on Air and Stream Improvement (NCASI) published a report “Assessing the Effectiveness of Contemporary Forestry Best Management Practices: Focus on Roads” to provide education on this critical issue. That report highlighted several areas of concern applicable to New Jersey: (1) road location, (2) road surfacing, (3) mulching, seeding, and stabilizing disturbed sites, (4) road drainage structures, (5) disconnecting roads from streams, and (6) filter windrows.

All the items addressed in road location, and road drainage structures are already addressed in the 1995 BMP manual. Mulching, seeding, and stabilizing disturbed sites is also addressed in the existing manual, (Section IV Access Roads #26 and Section V Skid Trails #12, 13, 14) but it would be nice if the example in Table 2 used native grasses and forbs for stabilization.

In regards to road surfacing, the 1995 BMP manual (Section IV Access Roads #3, 19, 20) contains generalized references, but little specifics. While this is likely to provide flexibility to the professional forester, it would be beneficial to note that the addition of 6 inches of crushed stone on top of a bare soil road reduced sediment yield/production by between 78 and 94%, depending on the study.

As far as road drainage structures, the NCASI report focused on forest road cross drains. These are used when a ditch exists on the uphill side of an in-sloped road. The 1995 BMP manual focuses on out-sloping based features such as water turnouts, broad-based dips, rolling dips, and water bars (but only when retiring temporary access roads). Minimum spacing guidelines are provided.

As far as disconnecting road drainage structures from streams, the 1995 BMP manual addresses this issue at Section II, Filter Strips. However, given the importance of capturing sediment from forest roads, perhaps this section can be given a different name in order to highlight the lesson. This concept is also mentioned in Section IV Access Road #16, however emphasis should be placed on discharging water into any undisturbed buffer, not necessarily the SMZ.

Lastly, filter windrows were not discussed in the 1995 BMP manual vis-à-vis forest roads. These are essentially brush barriers placed at the toe of cut-and-fill roads to capture any erosion coming down the slope. This could be a very simple addition.

The 1995 BMP manual allows bridges, culverts, and fords as acceptable stream crossing methods. Corduroys appear to be classified as fords, but are only allowed if removed immediately following use. Bridges are the recommended crossing method. Fords are restricted to places where the stream bottom is firm enough to support traffic.

A simulated rainfall experiment in Virginia to measure BMP effectiveness resulted in a trace sedimentation at a bridge (where the wood deck was not in direct contact with the soil), but higher rates of sedimentation at a ford, and more at a culvert. That study suggested revisiting the state’s BMP’s for culvert crossings.

A review of forestry best management practices concluded with (among others) the two following recommendations: (1)Sediment control structures applied to stream crossing approaches can significantly reduce runoff and sediment delivery; and (2) Improved stream crossings such as portable skidder bridges and temporary culverts can decrease total suspended solids concentrations and turbidity compared to unimproved stream crossing structures. Such sediment control structures can include filter strips and placement of fine woody debris on skid trails within the SMZ and stream crossings. These are all contained within the 1995 BMP manual.

Another study in Virginia looked at three methods of reducing sedimentation at temporary skidder stream crossings: (1) slash piled at depths of 9 inches to 3 feet, (2) mulch and grass seed, and (3) mulch, grass seed, and silt fence. Both the slash and mulch and grass seed treatments reduced total suspended solids (TSS). The method that included the silt fence increased TSS.

Aside from culvert sizing, the 1995 BMP manual directs foresters to “follow recognized and approved installation and construction methods,” and then directs them to NRCS or SAF’s Forestry Handbook. If temporary skidder bridges are to be encouraged, a sample schematic of proper installation may be included in an update of the manual.