Proceedings

Summit 2000,

Washington Private Forests Forum,

Summer 2000

University of Washington,

Seattle, WA

Sediment and Road Density Reduction

Peter Schiess and Finn Krogstad

Forest Engineering

University of Washington

Abstract

The operational and environmental impacts of a conventional and a long-span yarding approach to forest management and its impact on sediment deliveries to stream networks were simulated and compared in T12N/ R14E in the Ahtanum valley West of Yakima, WA. The conventional approach produced higher revenues at lower costs as expected, but delivered no more sediment to the stream than the long-span, no-new-roads approach. The explanation for this counter-intuitive result can be found in the density of the road network and its proximity to the stream. The road network produced a tenfold sediment increase over background levels, which might suggest a program of elimination and/or surfacing of existing roads. Analysis of this case, however, suggests that the construction of a ridge-based road network will be both environmentally and economically superior. This approach of integrating cumulative environmental impacts into the landscape scale harvest and transportation planning appears promising for identifying management options for reducing salmonid habitat degradation.

Keywords: harvest planning, road density, sediment delivery, stream proximity

Road Density

Sediment eroded from forest roads and delivered to the stream network may impact the habitat of endangered salmon species. This sediment can be estimated for alternate harvest practices using existing models of sediment production and delivery, but our planning objectives and day-to-day decisions must be guided by a simpler measure of road impacts. One convenient measure of the environmental impact of forest roads is road density, defined as the total road length in a landscape divided by the landscape area. A strategy to reduce environmental impacts would presumably include reducing road density, but is this approach valid?

As part of a larger study on the economic an environmental costs of road density reduction in a 36 square mile planning area in Eastern Washington, a 25 year harvest and transportation plan was constructed using a conventional approach and a road minimizing alternative. Contrary to expectations, the road reducing options did not reduce simulated sediment production in these studies.

Road Length vs. Road Use

The utility of road density as a measure of sediment delivery depends on whether sediment is produced by roads or by road use. If we view roads as the source of the sediments, then reducing road density will reduce road sediment. If we view road use as the source of sediments, then reducing road density will not reduce necessary management traffic, or the sediment it produces. The critical question is thus whether road activities (construction and haul) or the roads themselves are the source of sediments. The study findings (that road density reduction did not reduce the delivered sediments) supports the road use explanation.

Figure 1: The road network covers most of the planning area (left: existing roads are solid, proposed roads are dashed). The figure to the left shows traffic volume and pattern over a 25-year period. Most of these roads remain unused over a 25-year period with most traffic concentrated on the major haul roads along the valley bottoms (right: thicker lines represent more haul traffic).

Disturbance is significant, since forest soils in the Pacific Northwest tend to be protected by a layer of vegetation and litter, and thus produce little surface flow or sediment delivery to streams. Soil disturbances such as fires or landslides can expose soils to erosion, but vegetation rapidly covers and eliminates this erosion. Road construction can expose soils to erosion, but these roads will stabilize and re-vegetate if left undisturbed. Vehicle traffic is thus necessary to maintain active erosion and delivery to streams. Much of this traffic is the result of transportation of logs from their stumps to the mill. If the same logs are to be harvested and transported to the same mills, then haul traffic and traffic related sediment generation will not change by eliminating the first few hundred feet of spur road.

Road Elimination

A traditional program of road density reduction identifies and eliminates all road segments that will not be needed in many years. Having no traffic, these roads would have minimal impact on total basin sediments. A traditional program of road density reduction will thus eliminate roads producing minimal sediment, and retain roads producing most of the road network sediment.

A road network has a branching structure, much like a tree or a stream network. Like these other networks, most of the length of a road network is in the smallest branches (or spur roads or first order streams), while most of the volume (traffic or sap flow or streamflow) is carried in a main stem (or primary roads). Eliminating unused spur roads can dramatically reduce the total road network length, and can eliminate the first few hundred feet along the spur road, but not the remaining miles along secondary and primary haul roads.

Road Surfacing

Improving road surfacing can eliminate much of the sediment. Applying gravel or paving roads can be costly however, and even after paving, significant amounts of sediment can still be produced from cut and fill slopes, and road-side ditches. Model results for the study area in Eastern Washington suggested that even a combination of road reduction and gravel surfacing every road produced sediment well in excess of background levels.

Stream Proximity

Road-stream separation is more effective at reducing sediment delivery. While sediment is produced on all forest roads, its delivery to streams is a function of the distance to a stream (Figure 2). The further that sediment has to flow across the forest floor, the more it can be filtered, and the less likely it is to deliver to the stream network. Roadside ditches and culverts that deliver to stream crossings short circuit this filtering, so road alignment should avoid streams wherever possible.

The ridge network is the topographic opposite of the stream network, never crossing and always maximizing its distance from the stream network. A network of primary and secondary roads following ridge networks (and crossing the stream network only rarely) would deliver minimal sediment to the stream network, even if most of the road is native surfaced. Shifting from a riparian-based road network to a ridge-based road network will entail building more roads however, and in some cases such a sediment minimizing alignment might even increase road density.

Figure 2: Sediment delivery decreases with distance from the stream network (darker shade is higher delivery). Primary haul roads (black) that stay in low delivery (white) ridge oriented alignments (see examples in Northwest and Southeast sections) can route much haul traffic with little stream impact.

Conclusion: Road Density, a poor indicator of sediment delivery to streams

Theoretic and simulation analysis suggest that road density is a poor measure of road related sediment delivery to the stream network. A program of road density reduction will tend to eliminate the road sections with the least sediment impact (for example, the elimination of a temporary spur road), and can even inhibit road realignment options that would actually reduce sediment delivery.

It should be noted that sediment is not the only ecosystem impact of roads. Other impacts include peak flows, habitat fragmentation, and mass wasting. Road density might be a valid indicator of road impacts on these processes, or it may be no better than it is for estimating sediment delivery to streams. Similar landscape-scale studies should be conducted to evaluate the validity of using road density as a measure for each of these impacts.

AcknowledgemenTS

The reported project is supported through funds provided by the Washington State Department of Natural Resources. The views expressed here are those of the authors and do not imply agency endorsement.

THE AUTORS

Peter Schiess is Professor of Forest Engineering in the College of Forest Resources, University of Washington, Seattle, WA 98195. Finn Krogstad is a Research Associate and and doctoral candidate with the University of Washington.