EFFE CTS OF TRAFF I C AND DI TCH MAINTE NANCE ON
FOREST ROAD SEDI MENT P R ODUCTIO N

Charles H. Luce, Research Hydrologist, and Thomas A. Black, Hy drologist, 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 sedim e nt yield from ro ad segm ents in the Oregon Coast Range show that either heavy traffic during ra inf a ll o r blad in g the road d i tch will inc r ease e r osion f

r om f

o rest roads. For the fine soils and high quality aggr eg ate surfacin g on the stu dy plots, ditch blading
increased sedim e nt yield m o re th an traffic equivalent to 12 log trucks per day. The combination of ditch blading and heavy traffic did not pr oduce significantly m o re se diment than simply blading the ditch, a finding withim portant im plications for sedim e nt m o deling and erosion control design. Increases in sedim e nt production caused by traffic persisted after traffic ceased.

INTRODUCTION

Road m a intenance and traffic are two of the primary activities affec ting sedim e nt production
from forest roads. Given the large base of exis tin g roads in f o restlands, it is im portant that we
understand how these activities affect sedim e nt yiel d from road system s. Wh ile it is generally agreed that either traffic or ditch m a inte nance increase sedim e nt production, observations showing the com b ined effects relative to individual effects are lacking.

Maintenance keeps ro ad s in a cond ition suitable f o r trave l an d prevents severe erosion from failure of the drainage system . Unfortunately, road grading can break up arm o r layers on the
road surface or the ditch and tem porarily increase road surf ace erosion (Burroughs and King, 1989; Black and Luce, 1999; Luce and Black, 1999). Burroughs and King (1989) noted increased erodibility of the traveledway followi ng 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 sedim e nt production from a com p lete road segm ent, while blad in g of the ditc h, cutslo p e , and trav eled way s ubstantially increased sedim e nt yield from road segm ents. The recovery from di tch blading occurs rapidly during the first three y ears (Lu ce and Black, 2001) in an exponential pattern si m ilar to that found by Megahan (1974) for new roads. Observations of vegetati on and ditch particle size suggest that m u ch of the reduction over
tim e is due to arm o ring rather than revegetation (Black and L u ce, 1999).

The role of traffic in increasing road sedim e nt production is likewise well recognized and has
had attention from several re searchers (Reid and Dunne, 1984; Swift, 1984; Bilby et al., 1989; Burroughs and King, 1989; Coker et al., 1993; F o ltz , 1999; Ziegler et al ., 2001), who report a range from doubled sedim e nt production to 30 tim e s as m u c h. Results are comm onl y reported as ratios between yields from roads with and w itho u t tr af f i c, conceptua lly n o rm alizing f o r precipitation and allowing genera lization of the results beyond th e particular events studied. Many of the studies are f r om rainfall sim u lation; som e experim e nts were on the entire road prism

, and som e isolated sedim e nt from the traveledway. Those studies where concentration

sam p les were taken sho wa rela tive l y brief ef f ect from traffi c passing during a storm

, with significant recovery occurring on a tim e scale of te ns of m i nutes. The postulated processes for

Pre p ri nt o f L u c e , C

. H. an d T. A. B l ac k, 2 0 0 1

, Ef fect s of Tra ffi c an d Di t c h M a i n t e nance o nF o r e st R o a d Sedi m e nt
Pro d u ct i o n. I nPr ocee di n g s o f th e Se vent h F e deral Int e rage ncySe di m e nt at i on C o n f ere n c e , M arc h 25

- 2 9

, 2 0 0 1

, Ren o , N e v a d a

. pp . V6 7–V 74

.2

the increase in sediment yield are through an incr eased 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 th e effects are cumulative. Some models use the ratios from the studies to estimate the effect, one factor is applied for th e 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 Boar d, 1995). Another way to model the effect is through addition, where the traveledway and ditch co ntributions are calculated separately based on their individual
treatments and then added. A third alternative wo uld suggest that there mi ght be tradeoffs, that the total effect may be less than the sum of the pa rts. Increasing the availa bility of sediment in the ditch and on the traveledway may be somewhat redundant. If the sedi ment transport capacity of the ditch is fully sated by ma terial easily detached in the di tch, the additional available fine material on the traveledway may have littl e 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 se diment 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 woul d not only allow ditch erosion, but also allow passage of the traveledway sediment. Th is logic would support the idea that there is a positive non-lin ear interaction (e.g. multiplicative) between road surface treatments and ditch treatments on sediment yield. Alte rnatively, Burroughs and King (1989) noted that substantial reductions in traveledway sedimentproduction 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 unpr otected (or heavily tr aveled) traveledways, supporting the tradeoff hypothesis. Another observation is thattraffic forms ruts, causing sediment produced on the road surface to travel on the road surface independently of ditch sediments, supporting a simple addi tive model. Some would be qui ck to point out, however, that
this also robs the ditch of much of its water as well, and if the road surf ace 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 asso ciated with the question of combined effects be reduced. Unfortunately, the ques tion cannot be answered with phys ically 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 eff ects 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 E ugene, Oregon. The twelve plots were broken3

into four categories, those withno traffic and no ditch grading (N TNG), those with traffic and no ditch grading (TNG), those with no traffic but w ith a graded ditch (NTG) and those with both traffic and a graded ditch (TG). Traffic was applied to one con tiguous set of six road segments for practical reasons, and the gradi ng treatment was assigned randomly.

The road segments all had similar characteristic s 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 blad ed on all plots. All di tches 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 read er is referred to Luce and Black (1999) for more details on plot construction and soil attributes.

Traffic was provided by a short l og truck and, later, a dump truck. Both vehicles had similar wheel arrangements, with two axles in the rear wi th 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 thos e on full sized log trucks. Th e trucks made 10 round trips per weekday over the 6 traffic plots during the peri od November 15 to December 14. During this period traffic occurred on both ra iny and dry days and on saturate d and dry road surfaces. The traffic was roughly equivalent to 12 loaded full-length log trucks pe r 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 11 thand again on June 13 th . Tanks with the greatest amounts of sediment were weighed withsediment and water, emptied and weighed again filled only with water to obtain the subm erged weight of sedime nt (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 disturban ce of the sediment). The sedi ment was transferred to small steel buckets for weighing on a more precise scale, allowing a more precise determination for the small sediment amounts. Precipitation betweenNov 15 and Dec 14 was 351 mm. 151 mm fell
between Dec 14 and Jan 11. 589 mm fell betweenJan 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 concentra tion. 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 additi on, 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 sta tistical significance of specific contrasts. The transformation was used because earlier research has suggested that erosion is log-normally distri buted (Megahan et al., 2001).4

RESULTS AND DISCUSSION

Mean sedim e nt yield during the first sam p le period, Nov. 15 to Jan. 11, was least for the plots with no traffic and no ditch grading, f o llowed by the plots with traffic but no grading, and the plots with grading but no traffic, and the m ean sedim e nt yield was the gre a tes tf o r seg m ents with both traffic and a graded ditch (F igure 1). All contrasts were st atistically significan texcept for
the difference between th e traffic and no traffic plots given a graded ditch (Table 1). T h ese results suggest that blading the di tch has a greater effect than tra ffic on the sedim e nt yield. This particular result m a y differ given a different soil texture in the ditch or poorer aggregate quality. More sign if icant is the re sult th at the traf f i c ef f ect depends on whether the ditch is graded. Given
that the statistic al tests w e re f o r log tr ansf orm e d data and the low power inherent in a design with
three s a m p les per treatment, we can reject th e m u ltip licative cum u lative ef fect hypothesis, but we cannot statistically discern between the sim p l e additive cu mulative effect and the tradeoff hypothesis. Looking at the pattern of the scatte r from the No- T raffic-Graded (NTG) plots, we
can see that the m ean and scatter are strongly in fluenced by one plot wi th low sedim e nt yield (Figure 1). The other two NTG pl ots actually produced m o re sedim e nt than any of the plots with both traffic and grading. It is worth noting th at the sedim e nt production of the one plot is uncharacteristically low for a grad ed plot given earlier observati ons (Black and Luce, 1999; Luce and Black, 1999, 2001) and concurrent observations from sim ilarly treated plots. For exam ple three shorter plots (42 m

, 42 m

, and 60 m ) with sl ightly steeper, 10%, slopes, graded ditches, and no traffic measured during the sam e period produced 2467, 4533, and 2970 kg/km r e spectively.

Ar m e d with this additional inform ation, rem ovi ng som e weight from the low observation, there is greater support for the tradeoff hypothesis thanfor the sim p le additive effect hypothesis.

These results underscore the im portance of collecting observations of sedim e nt yield from the entire portion of road prism that is contributing w a ter and sedim e nt when evaluating th e effects
of treatm e nts, an idea also suggested by Burroughs and King (1989). Observations of individualSediment Production (kg/km)

Not Graded 0 400 800 1200 1600 2000 2400

No T r affic T raffic
Graded No T r affic T raffic Mean

Data

Figure 1: Sedim e nt yield from road segm ents during the traffic period, Novem b er 15, 1999 to January 11, 2000. Bar graphs show m ean and da ta points show specific observations.5

Table 1: Statistical S i gnificance of sp ecific con trasts.

Effe c tof G i v e n N ov

- - Dec J an- J u n
G r a d ing No Tra ffic 0 .02 0

. 0 2

G r a d ing Traffic 0 .03 0

. 0 2

T r a ffic N o G r ading 0.02 0.04

T r a ffic G r adin g 0.49 0.53 p va l u e

parts of the road prism can be m i sleading if th ere is potential f o r interaction of water and
sedim e nt from different parts of the road prism

. It is also im portan tto recognize th e condition of the ditch and the cutslope/ditch contribution to the road segm ent sedim e nt yield when interpreting results from studies. For exam ple, it is useful to know that the results of Reid and Dunne (1984) showing s ubstantial effect from traffic had little contributio n from ditch and cutslope erosion.

Im plications for m odeling are fairly clear; indepe ndent factors applied for ditch m a intenance and traffic are not appropriate. Nor would it appear that separate calcu la tion of traveledw a y and ditch/cutslope contributions is the best opti on. The stronger support fo r the tradeoff hypothesis
im plies that sedim e nt yield increases modeled to result from traffic m u st consider the condition
of the ditch.

There are important im plications for the design of BMPs or forest prac tice regulations. Ditch grading can increase sedim e nt yields on a level com p arable to or greater than wet weather hauling. Ditch grading is an im portant and nece ssary step in the m a intenance of roads when significant sedim e nt inputs (e.g. from a slum p or upslope gully) block the ditch, how ever indiscrim i nate ditch grading to clean ditches m a y not be the bestuse of equipm ent time. The practice of placing rock in d itches and design criteria for ditch rocking w e re proposed by Burroughs and King (1989), and our results support their suggesti on. The question of whether
wet weather haul increases sedim e nt yields on re cently constructed or reconstructed roads is
im portant for BMP design. W e t weather haul restrictions provide little and uncertain benefit on roads with recently blad ed ditch e s. Sedim e ntdelivery control through crossdrain placem ent is probably the preferred design, but at locations where delivery is likely (e.g. stream crossings), thorough control of sedim e nt woul d require protection of both th e 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 surf acing m a y be severely dam a ged by

tr affi c.

For the post traffic period, January 11 to June 13, rankings are sim ilar to those seen in the traffic period, although there w a s less eros ion (Figure 2). The reduced erosion is due in part to

arm o ring during the previous m onths. The contrast s are sim i lar in s t atistical sign ifican ce (Table 1). The difference in graded versus ungraded plot s is exp ected because we know that the effect of grading persists for more than one year in th e increas ed av ailab i lity fro mditch grad ing (Luce and Black, 2001). The fact that the pattern of differe nces is m a intained suggest that traffic
effects m a y persist beyond the ti m e scale of a few events. Reid and Dunne (1984) noted som e persistence beyond the event tim e scale in their “tem porary non-use” segm ents.6Sediment Production (kg/km)

Not Gr aded 0 100 200 300 400 500 600

No T r affic T raffic

Gr aded No T r affic T raffic Mean

Data

Figure 2: Sedim e nt yield from road segm ents during the post-traffic period, January 11 to June 13, 2000. Bar graphs show m ean and da ta points show indivi dual observations.

Exam ination of sedim e ntconcentrations during an event sho w ed a rise in concentration as several passes were m a de, and a rapid drop to lower concentrations after the traffic stopped (Figure 3). Plot runoff showed substantia lly lower concen tra ti ons th an th e peaks m easured by
the rut sam p ling because of dilution f r om cutsl ope runoff areas of the road surface where the truck had not recently passed. These patterns agree with other observatio ns of event s cale
variations in sedim e nt availability (e.g. Reid and Dunne, 1984; Ziegler et al., 2000, 2001). As the vehicle passed, fines were pressed into the tr e a d of the tire while th e lugs pressed larger pieces of gravel in to the m a trix of fines and grav el com p rising the road b e d. The tread ed pattern
of fine m a terial was quickly di ssipated by precipitation and flowi ng 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 ag gregate was rare.

0 10 20 30 40 50 60

9:30 10:00 10:30 11:00 11:3 0 12: 00 12:30 13:00

Ti meSediment Concentration (g/L) W heel Rut

Plot Runoff

Figure 3: Concentration of sedim e nt over tim e during one set of passes by a heavy vehicle.7

Some persistence in availability of fines would be e xpected on traffic plots, as the supply of fines from the road over the traffic period would reduc e the degree of armoring during that period, and the composition of the material in the ditch beds a nd wheel ruts should be finer at the end of the traffic period. It is not necessa ry that the increased availabilityof 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 equiva lent availability could account for the observations. A more likely scenario is an exponential decline in the ava ilability 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 act ivities 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 mainte nance and traffic affect sediment yield from
forest roads.

Grading of the ditch increased sedi ment yields more than heavy tr affic on a road built in a finegrained parent material with hi gh quality basalt aggregate. The combination of both traffic and ditch grading produced on average more sedimentthan either treatment alone, however the difference between grading-only an d grading-with-traffic was not st atistically 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 s upport to the hypothesis th at there is little difference in sediment yields between traffi c and no-traffic plots gi ven 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 effect s 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 increasi ngly common best management practice that is effective in reducing sediment production from exis ting roads. Proscriptio n 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 con centrations in runoff and, consequently, sediment yields varied on a time s cale of 10s of minutes following traffic. Longer term observations in this study re vealed 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 de signed to trap traffic-e nhanced sediment yields
must be maintained after the traffic ends.8

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56-64. Foltz, R. B., 1999, Traffic and no-traffic on an aggregate surfaced road: sediment production -
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