EFFECTS OF TRAFFIC AND DITCH MAINTENANCE ON 

FOREST ROAD SEDIMENT PRODUCTION 

Charles H. Luce, Research Hydrologist, and Thomas A. Black, Hydrologist, USDA Forest 

Service, Rocky Mountain Research Station, Boise, Idaho 

316 E. Myrtle St., Boise, ID, 83702, cluce@fs.fed.us, 208-373-4382. 

Abstract: Observations of sediment yield from road segments in the Oregon Coast Range show 

that either heavy traffic during rainfall or blading the road ditch will increase erosion from forest 

roads. For the fine soils and high quality aggregate surfacing on the study plots, ditch blading 

increased sediment yield more than traffic equivalent to 12 log trucks per day. The combination 

of ditch blading and heavy traffic did not produce significantly more sediment than simply 

blading the ditch, a finding with important implications for sediment modeling and erosion 

control design. Increases in sediment production caused by traffic persisted after traffic ceased. 

INTRODUCTION 

Road maintenance and traffic are two of the primary activities affecting sediment production 

from forest roads. Given the large base of existing roads in forestlands, it is important that we 

understand how these activities affect sediment yield from road systems. While it is generally 

agreed that either traffic or ditch maintenance increase sediment production, observations 

showing the combined effects relative to individual effects are lacking. 

Maintenance keeps roads in a condition suitable for travel and prevents severe erosion from

failure of the drainage system. Unfortunately, road grading can break up armor layers on the 

road surface or the ditch and temporarily increase road surface erosion (Burroughs and King, 

1989; Black and Luce, 1999; Luce and Black, 1999). Burroughs and King (1989) noted 

increased erodibility of the traveledway following road grading operations. However, Luce and 

Black (1999) noted that blading of only the traveledway on an aggregate surfaced road with well 

vegetated ditches yielded no increase in sediment production from a complete road segment, 

while blading of the ditch, cutslope, and traveledway substantially increased sediment yield from

road segments. The recovery from ditch blading occurs rapidly during the first three years (Luce 

and Black, 2001) in an exponential pattern similar to that found by Megahan (1974) for new 

roads. Observations of vegetation and ditch particle size suggest that much of the reduction over 

time is due to armoring rather than revegetation (Black and Luce, 1999). 

The role of traffic in increasing road sediment production is likewise well recognized and has 

had attention from several researchers (Reid and Dunne, 1984; Swift, 1984; Bilby et al., 1989; 

Burroughs and King, 1989; Coker et al., 1993; Foltz, 1999; Ziegler et al., 2001), who report a 

range from doubled sediment production to 30 times as much. Results are commonly reported as 

ratios between yields from roads with and without traffic, conceptually normalizing for 

precipitation and allowing generalization of the results beyond the particular events studied. 

Many of the studies are from rainfall simulation; some experiments were on the entire road 

prism, and some isolated sediment from the traveledway. Those studies where concentration 

samples were taken show a relatively brief effect from traffic passing during a storm, with 

significant recovery occurring on a time scale of tens of minutes. The postulated processes for 

Preprint of Luce, C.H. and T.A. Black, 2001, Effects of Traffic and Ditch Maintenance on Forest Road Sediment

Production. In Proceedings of the Seventh Federal Interagency Sedimentation Conference, March 25-29, 2001,

Reno, Nevada. pp. V67–V74. 
2

the increase in sediment yield are through an increased availability of fines caused by crushing 

the road surfacing and by pressing larger particles down through a matrix of finer sediment. 

An important question left unanswered by these studies is the combined effect of ditch 

maintenance and traffic. One hypothesis is that the effects are cumulative. Some models use the 

ratios from the studies to estimate the effect, one factor is applied for the time since construction 

or disturbance of the ditch and another is applied for the traffic level (e.g. Cline et al., 1984; and 

Washington Forest Practices Board, 1995). Another way to model the effect is through addition, 

where the traveledway and ditch contributions are calculated separately based on their individual 

treatments and then added. A third alternative would suggest that there might be tradeoffs, that 

the total effect may be less than the sum of the parts. Increasing the availability of sediment in 

the ditch and on the traveledway may be somewhat redundant. If the sediment transport capacity 

of the ditch is fully sated by material easily detached in the ditch, the additional available fine 

material on the traveledway may have little effect on the segment sediment yield. 

Consideration of the flowpath is important in estimating the effects of treatments to roads. 

Burroughs and King (1989) showed an effect on traveledway sediment yield from blading, but 

Luce and Black (1999) found nearly no effect on sediment yield from an entire road segment 

given the same treatment. One explanation may be that sediment from the road surface was 

trapped in the well vegetated ditch, implying that grading the ditch would not only allow ditch 

erosion, but also allow passage of the traveledway sediment. This logic would support the idea 

that there is a positive non-linear interaction (e.g. multiplicative) between road surface treatments 

and ditch treatments on sediment yield. Alternatively, Burroughs and King (1989) noted that 

substantial reductions in traveledway sediment production by placing rock aggregate (~80%) did 

not reduce total plot sediment production as much (~30%) because of increased sediment 

detachment in the unprotected ditch. This observation suggests that high sediment yields can 

come from either unprotected ditches or unprotected (or heavily traveled) traveledways, 

supporting the tradeoff hypothesis. Another observation is that traffic forms ruts, causing 

sediment produced on the road surface to travel on the road surface independently of ditch 

sediments, supporting a simple additive model. Some would be quick to point out, however, that 

this also robs the ditch of much of its water as well, and if the road surface is constructed with 

material that is less erodible than the ditch, traffic could conceivably reduce sediment yields on 

roads with freshly cleaned ditches. 

Because the earlier studies on the individual effects of traffic and maintenance are used as the 

basis of sediment yield models, forest practice regulations, and best management practice design, 

it is important that some of the uncertainty associated with the question of combined effects be 

reduced. Unfortunately, the question cannot be answered with physically based models, because 

any of the three hypotheses can be generated using different choices of parameter values and 

flowpath. Observations showing the individual and combined effects of traffic and maintenance 

are needed to understand the interaction. 

METHODS 

The effects of traffic and ditch maintenance were examined on twelve road segments in the 

Oregon Coast range about 20 km northwest of Eugene, Oregon. The twelve plots were broken 
3

into four categories, those with no traffic and no ditch grading (NTNG), those with traffic and no 

ditch grading (TNG), those with no traffic but with a graded ditch (NTG) and those with both 

traffic and a graded ditch (TG). Traffic was applied to one contiguous set of six road segments 

for practical reasons, and the grading treatment was assigned randomly. 

The road segments all had similar characteristics otherwise. All had lengths of 80 m and were 

isolated by ditch dams and rubber-flap/wood-box water bars and runoff was diverted into 

sediment traps. Road gradient was between 9 and 10 percent, and cutslope heights were 

approximately 2 m on all road segments. The roads were constructed on a silty clay loam soil 

over weathered sandstone. The roads were surfaced with high quality basalt aggregate and had 

inboard ditches. The traveledway was freshly bladed on all plots. All ditches and cutslopes were 

seeded with grass during the previous spring, and the ditches were bladed on October 14, 1999 

for the segments with that treatment. The reader is referred to Luce and Black (1999) for more 

details on plot construction and soil attributes. 

Traffic was provided by a short log truck and, later, a dump truck. Both vehicles had similar 

wheel arrangements, with two axles in the rear with dual tires and a front steering axle. The rear 

sets of duals carried 15,840 kg (33,850 lbs.) and the front axle carried 5,610 kg (12,340 lbs.). 

These weights are similar to those on full sized log trucks. The trucks made 10 round trips per 

weekday over the 6 traffic plots during the period November 15 to December 14. During this 

period traffic occurred on both rainy and dry days and on saturated and dry road surfaces. The 

traffic was roughly equivalent to 12 loaded full-length log trucks per day. On each day of traffic, 

5 round trips (10 passes) were made over a 1 hr period with a 30-minute break followed by 

another 5 round trips. 

Sediment was collected from the tanks on January 11th and again on June 13th. Tanks with the 

greatest amounts of sediment were weighed with sediment and water, emptied and weighed 

again filled only with water to obtain the submerged weight of sediment (see Luce and Black, 

1999 for details). For tanks with little sediment, we decanted the clean water off of the tanks 

using a siphon (avoiding disturbance of the sediment). The sediment was transferred to small 

steel buckets for weighing on a more precise scale, allowing a more precise determination for the 

small sediment amounts. Precipitation between Nov 15 and Dec 14 was 351 mm. 151 mm fell 

between Dec 14 and Jan 11. 589 mm fell between Jan 11 and Jun 13. Average precipitation 

depths for these periods in Eugene, Oregon are 218, 170, and 551 mm respectively. 

During one day of traffic, water samples were collected from the wheel ruts immediately 

following a vehicle pass to capture the peak sediment concentration. An additional sample was 

taken 20 minutes into the break between sets of passes, and three samples were taken at 20 

minute intervals following cessation of traffic to see how concentration in the wheel rut at the 

base of the plot changed with time. In addition, concentration samples were taken from the plot 

outflow (ditch plus tread) at the same time. 

We used t-tests on the log transformed sediment yields to test the statistical significance of 

specific contrasts. The transformation was used because earlier research has suggested that 

erosion is log-normally distributed (Megahan et al., 2001). 
4

RESULTS AND DISCUSSION 

Mean sediment yield during the first sample period, Nov. 15 to Jan. 11, was least for the plots 

with no traffic and no ditch grading, followed by the plots with traffic but no grading, and the 

plots with grading but no traffic, and the mean sediment yield was the greatest for segments with 

both traffic and a graded ditch (Figure 1). All contrasts were statistically significant except for 

the difference between the traffic and no traffic plots given a graded ditch (Table 1). These 

results suggest that blading the ditch has a greater effect than traffic on the sediment yield. This 

particular result may differ given a different soil texture in the ditch or poorer aggregate quality. 

More significant is the result that the traffic effect depends on whether the ditch is graded. Given 

that the statistical tests were for log transformed data and the low power inherent in a design with 

three samples per treatment, we can reject the multiplicative cumulative effect hypothesis, but 

we cannot statistically discern between the simple additive cumulative effect and the tradeoff 

hypothesis. Looking at the pattern of the scatter from the No-Traffic-Graded (NTG) plots, we 

can see that the mean and scatter are strongly influenced by one plot with low sediment yield 

(Figure 1). The other two NTG plots actually produced more sediment than any of the plots with 

both traffic and grading. It is worth noting that the sediment production of the one plot is 

uncharacteristically low for a graded plot given earlier observations (Black and Luce, 1999; Luce 

and Black, 1999, 2001) and concurrent observations from similarly treated plots. For example 

three shorter plots (42 m, 42 m, and 60 m) with slightly steeper, 10%, slopes, graded ditches, and 

no traffic measured during the same period produced 2467, 4533, and 2970 kg/km respectively. 

Armed with this additional information, removing some weight from the low observation, there 

is greater support for the tradeoff hypothesis than for the simple additive effect hypothesis. 

These results underscore the importance of collecting observations of sediment yield from the 

entire portion of road prism that is contributing water and sediment when evaluating the effects 

of treatments, an idea also suggested by Burroughs and King (1989). Observations of individual Sediment Production (kg/km)

 Not Graded

0

400

800

1200

1600

2000

2400

No Traffic Traffic

 Graded

No Traffic Traffic

Mean

Data

Figure 1: Sediment yield from road segments during the traffic period, November 15, 1999 to 

January 11, 2000. Bar graphs show mean and data points show specific observations. 
5

Table 1: Statistical Significance of specific contrasts. 

Effect of Given Nov--Dec Jan-Jun

Grading No Traffic 0.02 0.02

Grading Traffic 0.03 0.02

Traffic No Grading 0.02 0.04

Traffic Grading 0.49 0.53

p value

parts of the road prism can be misleading if there is potential for interaction of water and 

sediment from different parts of the road prism. It is also important to recognize the condition of 

the ditch and the cutslope/ditch contribution to the road segment sediment yield when 

interpreting results from studies. For example, it is useful to know that the results of Reid and 

Dunne (1984) showing substantial effect from traffic had little contribution from ditch and 

cutslope erosion. 

Implications for modeling are fairly clear; independent factors applied for ditch maintenance and 

traffic are not appropriate. Nor would it appear that separate calculation of traveledway and 

ditch/cutslope contributions is the best option. The stronger support for the tradeoff hypothesis 

implies that sediment yield increases modeled to result from traffic must consider the condition 

of the ditch. 

There are important implications for the design of BMPs or forest practice regulations. Ditch 

grading can increase sediment yields on a level comparable to or greater than wet weather 

hauling. Ditch grading is an important and necessary step in the maintenance of roads when 

significant sediment inputs (e.g. from a slump or upslope gully) block the ditch, however 

indiscriminate ditch grading to clean ditches may not be the best use of equipment time. The 

practice of placing rock in ditches and design criteria for ditch rocking were proposed by 

Burroughs and King (1989), and our results support their suggestion. The question of whether 

wet weather haul increases sediment yields on recently constructed or reconstructed roads is 

important for BMP design. Wet weather haul restrictions provide little and uncertain benefit on 

roads with recently bladed ditches. Sediment delivery control through crossdrain placement is 

probably the preferred design, but at locations where delivery is likely (e.g. stream crossings), 

thorough control of sediment would require protection of both the ditch and the traveledway. 

Note that the roads in this study were built well enough that the subgrade showed through the 

aggregate surfacing in only a few places, and the deepest ruts were about 90 mm. The results of 

this study do not apply to roads where the integrity of the surfacing may be severely damaged by 

traffic. 

For the post traffic period, January 11 to June 13, rankings are similar to those seen in the traffic 

period, although there was less erosion (Figure 2). The reduced erosion is due in part to 

armoring during the previous months. The contrasts are similar in statistical significance (Table 

1). The difference in graded versus ungraded plots is expected because we know that the effect 

of grading persists for more than one year in the increased availability from ditch grading (Luce 

and Black, 2001). The fact that the pattern of differences is maintained suggest that traffic 

effects may persist beyond the time scale of a few events. Reid and Dunne (1984) noted some

persistence beyond the event time scale in their “temporary non-use” segments. 
6

Sediment Production (kg/km)

 Not Graded

0

100

200

300

400

500

600

No Traffic Traffic

 Graded

No Traffic Traffic

Mean

Data

Figure 2: Sediment yield from road segments during the post-traffic period, January 11 to 

June 13, 2000. Bar graphs show mean and data points show individual observations. 

Examination of sediment concentrations during an event showed a rise in concentration as 

several passes were made, and a rapid drop to lower concentrations after the traffic stopped 

(Figure 3). Plot runoff showed substantially lower concentrations than the peaks measured by 

the rut sampling because of dilution from cutslope runoff areas of the road surface where the 

truck had not recently passed. These patterns agree with other observations of event scale 

variations in sediment availability (e.g. Reid and Dunne, 1984; Ziegler et al., 2000, 2001). As 

the vehicle passed, fines were pressed into the tread of the tire while the lugs pressed larger 

pieces of gravel into the matrix of fines and gravel comprising the road bed. The treaded pattern 

of fine material was quickly dissipated by precipitation and flowing water. During the course of 

the traffic, wheel ruts developed varying in depth from 10 mm to 90 mm, and exposure of the 

subgrade through the aggregate was rare. 

0

10

20

30

40

50

60

9:30 10:00 10:30 11:00 11:30 12:00 12:30 13:00

Time

Sediment Concentration (g/L)

Wheel Rut

Plot Runoff

Figure 3: Concentration of sediment over time during one set of passes by a heavy vehicle. 
7

Some persistence in availability of fines would be expected on traffic plots, as the supply of fines 

from the road over the traffic period would reduce the degree of armoring during that period, and 

the composition of the material in the ditch beds and wheel ruts should be finer at the end of the 

traffic period. It is not necessary that the increased availability of fine sediments persist into 

June for the observed differences in Figure 2. A brief period of increased erosion followed by a 

long period of essentially equivalent availability could account for the observations. A more 

likely scenario is an exponential decline in the availability of fine sediment as the finer sediments 

are selectively removed from the plot (e.g. Megahan, 1974). 

CONCLUSIONS 

Traffic and maintenance operations are activities normally experienced by forest roads at one 

time or another during their useful life. Both activities increase the susceptibility of a road to 

erosion. In order to better manage the spatial and temporal distribution of sediment inputs to 

streams, it is important to understand how maintenance and traffic affect sediment yield from 

forest roads. 

Grading of the ditch increased sediment yields more than heavy traffic on a road built in a finegrained parent material with high quality basalt aggregate. The combination of both traffic and 

ditch grading produced on average more sediment than either treatment alone, however the 

difference between grading-only and grading-with-traffic was not statistically significant with 3 

samples in each treatment. A closer examination of the individual data points and results from 

similar plots in this year and earlier years provides support to the hypothesis that there is little 

difference in sediment yields between traffic and no-traffic plots given a graded ditch. 

These results suggests that the multiplicative interaction model commonly used to estimate 

effects of multiple treatments on roads overestimates the effect of traffic on new roads or 

recently graded roads. A model of traffic effects that is conditional on ditch condition (e.g. time 

since construction or ditch grading) seems more appropriate. Although the ditch grading effect 

is much larger, its effect is seldom accounted for in road sediment yield modeling whereas traffic 

effects generally are, if only as a traffic regime. For roads with regularly scheduled maintenance, 

it may be desirable to model the effect of a maintenance regime. 

Proscription of wet weather haul is an increasingly common best management practice that is 

effective in reducing sediment production from existing roads. Proscription of wet weather haul 

on roads with high quality aggregate and recently disturbed ditches may have little benefit. 

Reducing the amount of road with unnecessary ditch grading is unequivocally effective in 

reducing sediment production. 

Observations in this study and in previous work show that sediment concentrations in runoff and, 

consequently, sediment yields varied on a time scale of 10s of minutes following traffic. Longer 

term observations in this study revealed that traffic effects may persist for longer periods, as 

armoring of the flow paths is prevented by the abundant fine sediment supply. This indicates 

that a traffic regime model may be appropriate as opposed to needing knowledge of each vehicle 

pass. It further indicates that any mitigations designed to trap traffic-enhanced sediment yields 

must be maintained after the traffic ends. 
8

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