Reprinted from theSOUTHERN JOURNAL OF APPLIED FORESTRY

Vol. 8, No, 4, November 1984

Soil Losses from Roadbeds and

Cut and Fill Slopes in the

Southern Appalachian Mountains

Lloyd W. Swift, Jr.

ABSTRACT. Soil losses were measured on the cut, fill, androadbed surfaces of a forest road at Coweeta Hydrologic Lab-oratory. Before grass was planted or gravel spread, roadbedsurfaces had the least loss per unit area and loss was primarilywaterborne fine particles. A large part of the soil loss from Jillslopes was due to slippage oj wet soils in early spring. Surfaceerosion of fills was negligible because storm water from theroadbed was not spilled across loose soil. The cut slopes erodedmost, principally because soils were loosened by diurnal cyclesof freezing and thawing in -winter. This study shows that in-clined surfaces of cut and fill slopes are potential sources oflarge soil loss but these losses can be mitigated by early estab-lishment of grass cover and. by design features to control stormwater. Soil loss from roadbeds was greatly reduced by gravelsurfacing.'

1 The Southern Region of the Forest Service and nationalforests in North Carolina contributed to this study by con-structing the two test sites and providing surveys and soil testdata. Storm and erosion data collected during this study wereshared with the Engineering Research Center, Colorado StateUniversity. Their contract to adapt and demonstrate an erosionprediction model for southeastern conditions has not beencompleted.

Jxoads are often the major source of soil erosionfrom forested lands (Patric 1976). The roadway,or area of exposed soil of a newly constructedroad, consists of three distinct surfaces with dif-ferent characteristics that affect their erodibility.These surfaces are the cut slope, the roadbed, andthe fill slope. Measurements were made on anaccess road to a Forest Service timber sale toincrease the information available on comparativeerosion rates from the cut, bed, and fill of forestroadways. Generally, soil loss is greatest duringand immediately after construction. In this study,the time between construction and seeding grassand spreading gravel was longer than is recom-mended so that soil losses from bare slopes androadbeds could be quantified during several sea-sons and for different traffic intensities. Althoughmeasurements created some artificial conditions,study results are informative and useful to theland manager and engineer because they point

SOUTHERN JOURNAL OF APPLIED FORESTRY

out major causes, locations, and seasons of soil lossand demonstrate the value of grass and gravel forreducing erosion.

METHODS

The road is at 3,560-ft. elevation, traversing onthe contour a 35% southfacing slope at the CoweetaHydrologic Laboratory in the southern Appala-chian Mountains of western North Carolina. Meanannual rainfall at the site is 73.8 in. distributedthroughout all seasons. The climate is moderatewith mean monthly air temperatures ranging from72°F in summer months to 25°F in January. Someprecipitation falls as snow and usually melts withina few days. The soil series is Chandler, a micaceous,deep sandy loam which has a relatively low hardrock content. The engineering classification forthe nonplastic soil material is A-2-4 under theAmerican Association of State Highway and Trans-portation Officials system and SM for the Ameri-can Society for Testing and Materials system. ThisTypic Dystrochept soil is dominant on the drierupper slopes at Coweeta and common on similarsites throughout the southern Appalachians. Anoak-hickory forest with dense understory is thetypical cover. Undisturbed soils, protected by veg-etation and a litter layer, absorb all precipitation,and surface runoff exists only on disturbed sitesor in permanent and intermittent stream channels.

Two sections of road were reconstructed in 1976to have outsloped, 22-ft.-wide roadbeds drainedby broad-based dips without inside ditches. Thisforest road design was developed at Coweeta (Hew-lett and Douglass 1968) and is implemented inmany types of terrain (Cook and Hewlett 1979).Each test site was defined as a broad-based dipplus the 190-ft. or less length of roadbed drained

by that dip. Size and angle of cut and fill slopeswere similar between the two sites. Cut slopesranged from %: 1 to 1:1 and fills from 1:1 to1 'A: 1. The toe of each fill terminated above abrush barrier made of trees cut from the right-of-way and piled by the bulldozer before earthmovingbegan. Fills were built up of sidecast material andcompacted only by repeated passes of the bull-

209

dozer. Roadbed grades were constant above eachdip at 7% on Site 1 and 5% on Site 2. Separatemeasurements of soil loss were made for each ofthe three surfaces at a test site. Raised bermsseparated the roadbeds from the fills. Thus, allstorm waters flowed off the outsloped and com-pacted roadbeds only at the dips. Strips of plasticfilm placed in the road margins near the dipsinhibited erosion of the berms (Figure Id). Bermspermitted the measurement of erosion from fillsindependent of erosion caused by excess waterflowing off the roadbed. Drop inlets collectedstormflow from the dips, and metal troughs col-lected flows from the cut and fill slopes (Figure1). Installations (Figure le) similar to those de-scribed by Douglass and Swift (1977) measuredstormflows from each of the surfaces and collectedsediment samples for each storm.

Total soil loss was the sum of two measurements.Dry weight of those heavier particles deposited inthe collection troughs below the slopes and in theapproach section ahead of each H-flume was de-termined from volume measurements and bulkdensity subsamples. Stormflows, carrying thelighter particles, were sampled by Coshoctonwheels. Subsamples were filtered and sedimentconcentration determined by weight. The sedi-ment concentration in ppm was multiplied bygauged stormflow to determine the total suspendedsediment. The sum of deposited and suspendedsediments was expressed as dry weight per unitarea of the road or slope surface yielding the soil.Projected horizontal areas (map areas) were usedin calculations for the four slope surfaces.

Precipitation was gauged by one recording andtwo nonrecording gauges, and the latter were readeach time soil was collected. Precipitation totalswere separated into 24-hour amounts.

Forest Service practice is to plant grass as soonas possible after earthmoving ends but this exper-iment was designed to compare soil losses frombare surfaces in several seasons so grass seedingwas postponed for 9.5 months following roadconstruction. In July 1977 after the timber salewas closed, all cut and fill slopes were fertilized,limed, and seeded to the standard roadside mix-ture of annual rye and Ky-31 fescue grass. Bothroadbeds were graded and surfaced with 6 in. ofcrusher run gravel (Figure Id). Measurementscontinued for an additional 13.3 months. Cumu-lative soil loss over the 22.8 months was partitioned

into five time periods: the 2.5 months immediatelyafter road construction (September through No-vember 1976), the 4 winter months (December1976 through April 2, 1977), the 3 spring monthsof 1977 when most of the logging traffic passedover the sites, the 5 summer and fall months aftergrass planting and graveling (July through Novem-ber 1977), and the final 8.3 months to August

1978. Through the entire period of this study,weekly traffic on this road was at least 15 trips by-lightweight cars, vans, or pickup trucks.

RESULTS

Cumulative soil losses for Sites 1 and 2 over eachof the five periods are shown in Figure 2. Dailyprecipitation is given at the top of the figure.About 54% of the roadway was roadbed with thecut and fill slopes splitting equally the remaining46% of disturbed surface. In contrast to Figure 2,the losses in Table 1 are weighted by the areapercentage of each surface to the area of fullroadway and converted to monthly rates. The sumfor the three surfaces is the monthly mean loss forthe entire roadway. Also listed in Table 1 areweighted mean losses for all the bare soil periods,the 13.3 months after graveling and seeding grass,and the entire study.

Postconstruction

Lower soil losses occurred in this initial periodthan during the winter or logging periods (Table1), with the roadbed at Site 1 showing the greatestloss (Figure 2). During this period, roadbeds pro-duced 46 to 66% of the total erosion from eachsite but in most of the later time periods, bedswere the source of less than half the total soil loss.Loose soil material on the surface of new construc-tion was especially vulnerable to movement bystorms. More than half the soil loss on Site 1occurred during the first storm, a 1.9-in. raincomprising only 14% of the rainfall total for thepostconstruction period.

First Winter

The largest soil losses for all surfaces occurredduring the first winter. Precipitation for Januaryand February was below average but 13.74 in. inMarch 1977 makes this the fourth wettest Marchin the 50-year record at Coweeta. In 4 months theroadbeds yielded 42% of their 23-month cumula-tive soil loss, and the fill and cut slopes yielded 52and 82%, respectively. The cut slopes had thegreatest losses, primarily due to frost heaving anddry ravel. Beginning in December, diurnal freez-ing and thawing cycles loosened large amounts of

210 SOUTHERN JOURNAL OF APPLIED FORESTRY

Figure 1. Instrumentation for soil loss measurements on roadbeds and cut and fill slopes at Coweeta HydrologicLaboratory, (a) Bare cut slope, trough and drop inlet, (b) Bare soil roadbed in March 1977, facing downgi-adetoward the dip with cut slope trough on the left and berm on the right, (c) Bare fill slope, trough and headwallof flume. Sampling installations for roadbed and cut slope in the background, (d) View upslope from dip inSeptember 1977 showing grassed fill and graveled roadbed with storm water running into drain inlet, (e) TypicalI-ft. H-flume and 2-ft. Coshocton wheel used to measure flow and extract 0.5% sample.

SOUTHERN JOURNAL OF APPLIED FORESTRY 211

70

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1976 1977 1978

Figure 2. Cumulative soil loss and precipitation at two wad sites on Coweeta Hydrologic Laboratory. Losses forcut slopes, roadbeds, and fill slopes are represented as weight per unit area for each surface.

Table 1. Mean monthly soil losses from three surfaces of a forest roadway at Coweeta Hydrologic Lab-oratory. Loss from each surface is weighted by proportion of that surface to total roadway area.

Time period and site

Postconstruction9/15/76-11/30/76

12

Winter12/1/76-4/2/77

12

Logging4/3/77-6/30/77

12

New grass and gravel7/1/77-11/30/77

12

Established grass12/1/77-8/8/78

12

First 9.5 months9/15/76-6/30/77

12

Last 13.3 months7/1/77-8/8/78

12

Total 22.8 months9/15/76-8/8/78

12

Precipitation Roadbed

Inches/month

5.341.18

.24

7.151.771.13

3.751.481.10

6.25.62.19

4.88.08.01

5.661.52

.89

5.40.28.08

5.51.80.42

Soil loss

Cut slope

from surface

Fill slope Total roadway

Tons/3crp/mnnth

0.27.25

4.3111.19

.33

.37

.641.06

.04

.13

1.994.89

.26

.48

.982.32

0.34.03

3.041.22

.68

.80

1.15.91

.01

.02

1.58.78

.44

.35

.91

.53

1.79.52

9.1213.54

2.492.27

2.412.16

.13

.16

5.096.56

.98

.91

2.693.27

212 SOUTHERN JOURNAL OF APPLIED FORESTRY

soil material from the cuts and much of this debristumbled into the collection troughs during non-storm periods (Figure 3a). Diseker and McGinnis(1967) blame frost action for high losses fromPiedmont road cuts and found slope orientationand soil moisture to be useful predictors of loss.The yield from the cut slope on Site 2 (11.19 tons/acre/month) was more than double that from Site1 (4.31 tons/acre/month). The apparent reason forless slope stability at Site 2 was its more southerlyorientation which caused soil to dry and losecohesiveness earlier and more often. This cut slopelost 9 times the amount yielded by the fill slope,or about 1 in. depth over the entire cut slopesurface, an amount similar to that reported byCarr and Ballard (1980). If this were a road withan inside ditchline, the debris would have filledthe ditch. Thus, the volumes measured here rep-resent the load of material which would have beenremoved by maintenance or carried through aditch and culvert by storm waters. Without aclitchline, debris at the toe of a cut slope normallystabilizes and soil losses offsite would be much lessthan these measurements suggest. In describing

the ditchless road design developed at Coweeta,Yoho (1980) notes that "roadside ditches are man-made gullies and should be used only whereneeded."

While major soil losses from unvegetated cutslopes began with the onset of winter, the largelosses from fills did not occur until early spring.Winter losses from both fill slopes were largerthan roadbed losses, with the greatest loss at Site1. Winter precipitation and snowmelt liquified aportion of this fill which slumped onto the edgeof the collection trough in February (Figure 3c)and later was undercut by stormflows. Typically,new and uncompacted road fills in the southernAppalachian Mountains become wet and unstablein early spring and some slumps occur. Althoughlosses are shown to continue (Figure 2), the slumpof fill soil was a one-time event in this study.Without the collection trough, this soil materialwould have flowed onto the forest floor, lodgedagainst a brush barrier at the toe of the fill slope,and caused minimum downslope disturbance.Haupt et al. (1963) reported heavy erosion of fillson an outslopecl road and cutting was observed insome fills outside the test sites at Coweeta. Fillerosion was not the major factor in the test sitesbecause storm water that normally would havebeen diverted over the soft fill by an outslopedroadbed and dip was, instead, confined on theroadbed and transported to the sampler for mea-surement.

Some rutting of roadbeds occurred during thespring thaw and both roadbeds washed in thelarger March storms (Figure Ib). Less than 20%of the total loss from the roadway originated withthe roadbed. In this and all other periods, Site 1

SOUTHERN JOURNAL OF APPLIED FORESTRY

roadbed lost more soil than Site 2 roadbed. Duringthe first winter, the 7% grade lost 57% morematerial than the 5% grade. Throughout the study,more than half of the roadbed collection was fineparticles transported by water (suspended sedi-ment) whereas less than 20% of the loss from filland cut slopes was in this class. Material from thecut and fill slopes often fell or slid rather thanbeing moved by water and consisted of all sizes ofparticles (Figure 3).

Logging Traffic

Losses from all surfaces were considerably lessduring the April to June period. Small storms anddrier soils typical for this time of year counteractedthe tendency for logging traffic to increase erosionfrom the roadbed. Logging traffic earlier in thespring did increase soil loss from nearby roads ina related study (Swift 1984). Road use in thisperiod averaged 60 axle-counts per week. The fillswere the most erosive surface on a per-unit areabasis (Figure 2) with most soil lost in a 1.6-in.storm in June. However, on the weighted areabasis, half the loss from the roadway came fromthe roadbeds (Table 1). Differences between siteswere minor except that scattered clumps of vol-unteer grass developed on Site 1 fill and restrainedsome downslope soil movement.

New Crass and Gravel

Precipitation was negligible the first 45 days ofthe June-November 1977 period. Although slopes

had been seeded 30 days earlier, grass had barelygerminated before heavy rains (6.46 in. in 5 days)fell in mid-August. Slopes at both sites showedaccelerated losses during this storm. The slumpedfill on Site 1 and the cut slope on Site 2 again lostmore soil than did the other surfaces, with the fillaveraging 5 tons/acre/month. As expected (Swift1984), roadbed losses were reduced by gravel andwere notably lower for Site 2 where the grade was5%. Soil loss from the 2 graveled roadbeds wasobserved to originate in the lightly graveled orbare margins (Figure Id), and the difference be-tween the low levels of loss from the two sites wasassociated with seemingly minor differences inarea of exposed soil. At Site 2, gravel reducedaverage soil loss per month to about 20% of therate for the first 9.5 months. By late September,the grass cover on the slopes of Site 1 was well-established and soil losses were greatly reduced;on Site 2, grass cover was less complete andincrements of soil loss continued into early Novem-ber.

213

Figure 3. Examples of three types of soil erosion measured on roadways, (a) Cut-slope trough filled by materialloosened by soil frost, (b) Cutting of roadbed and deposition of waterborne soil particles near drain inlet, (c)Trough at Site 1 covered by new slump of fill on 14 February 1977.

Established Grass

Winter and early spring peaks of soil loss didnot occur during the final period (December 1977through August 1978) and total soil losses over8.3 months were smaller than any previous period.The soil loss from each of the slopes and roadbedswas only 1 to 3% of the 23-month totals. Theestablished grass protected the fill slopes and evenwet season losses were negligible. The cut slopesdid not undergo the loosening effect of soil freez-ing found during the first winter. Outside thestudy sites where grass was planted at constructiontime, only small amounts of debris had formed atthe toes of cut slopes. The well-established grasscover seemed to prevent much of the frost actionwhich loosened bare slopes. At Site 2, the thinnergrass cover on the cut slope was sufficient to control

soil loss during winter but inadequate to withstandfour storms in spring and summer. Thus, the totalsoil loss for the final period was nearly 4 times thetotal for the other cut slope at Site 1. The roadbedwith a 7% grade continued to have the greaterloss rate. Grass growing in the margins of theroadbeds virtually eliminated the edge source oferoded material. Gravel with grass in the marginsreduced average soil loss from both sites to lessthan 10% of the rate experienced in the 9.5 monthsbefore sites were graveled and grassed and to lessthan 5% of the rate in the first winter period.Even so, loss from the entire roadway was about20 times the normal rate estimated for undisturbedforest by Patric (1976) or roughly half the accept-able rate for agricultural land.

214 SOUTHERN JOURNAL OF APPLIED FORESTRY

IMPLICATIONS FOR MANAGEMENT

Two results are clearly demonstrated by thisstudy. First, a key factor in erosion control forroads is surface protection. Second, the timing andcauses of soil loss differ between cut slope, fillslope, and roadbed.

Surface Protection

• Without protection, heavy rains remove largeamounts of soil from all portions of the road-way. If gravel is spread and grass cover estab-lished, soil losses are minor during largestorms.

• Grass cover reduces winter loss of cut slopesoils by reducing the frost heaving that pro-motes erosion.

• Grass cover restrains downslope movement ofsoil slumps in moistened fill slopes.

• Good gravel surface averts the soil loss andpoor trafficability typical when bare soilroadbeds soften and rut in early spring.

• Grass on the entire roadway, except for theadequately graveled portion, should beplanted and the cover maintained by reseedingor adding fertilizer and lime.

Time and Causesof Soil Loss

• Roadbeds generally encompass the largestportion of disturbed roadway area but ac-counted here for only 10 to 30% of the totalsoil loss. Soil loss from roadbeds was greatestduring winter and the peak periods of loggingtraffic. The importance of keeping grades lowwas shown by consistently larger losses fromthe 2% steeper roadbed of Site 1.

• Cut and fill slopes produce the greatest soillosses. The highest losses come from cut slopesdue to winter frost heaving. On many light-duty roads, losses can be reduced and soilheld on site by using an outsloped roadbeddesign without an inside ditchline. Cut slopestability is enhanced if roadbed maintenancedoes not disturb debris at the toe of a slope.

• Temporary outer berms on outslopedroadbeds can reduce soil loss by keeping stormwaters from flowing over credible fills whilegrass cover is developing. Fills can be mechan-ically protected from erosion at broad-baseddips. Fill slumps occur when soil reaches highmoisture content, generally in late spring orduring extended storm periods. Soil loss fromfills can be reduced by using additional com-paction, designs that avoid long fills, and brushbarriers to restrain soil movement. Where fills

can be kept away from waterways, the chancesare reduced for soil erosion or slippage todirectly enter streams.

« This road had a greater potential for soil lossthan the original design described by Hewlettand Douglass (1968). Less soil was exposedwith those narrower roads where cuts werevertical and shallow. Because less soil wasmoved, fills were smaller. Where the expecteduse does not require a wide roadbed and large-radius curves, a smaller road with lower ero-sion potential should be specified.

Literature Cited

CARR, W. VV. and T. M. BAU.ARD. 1980. Hydroseeding forestroadsides in British Columbia for erosion control. J. Soil andWater Conserv. 35:33-35.

COOK, W. L., JR. and J. D. HEWLETT. 1979. The broad-baseddip on Piedmont woods roads. South. J. Appi. For. 3:77—81.

DISEKEK, E. G. and J. T. McGiNNis. 1967. Evaluation of climatic,

slope, and site factors on erosion from unprotected road-banks. Trans. Am. Soc. Agric. Eng. 10:9-11,14.

DOUGLASS, J. E. and L. W. SWIFT, JR. 1977. Forest Servicestudies of soil and nutrient losses caused by roads, logging,mechanical site preparation, and prescribed burning in thesoutheast. In Watershed Research in Eastern North America,A Workshop to Compare Results, Vol. II, p. 489-503. DavidL. Correll, ed. Smilhson. Inst., Edgewater, MD.

HAUPT, H. F., H. C. RICKARD, and H. L. FINN. 1963. Effect ofsevere rainstorm on insloped and outsloped roads. USDAFor. Serv. Res. Note INT-1, 4 p.

HEWLETT, J. D. and J. E. DOUGLASS. 1968. Blending forest uses.USDA For. Serv.'Res. Pap. SE-37, 15 p.

PATRIC, J. H. 1976. Soil erosion in the eastern forest. J. For.74:671-677.

SWIFT, L. W., JR. 1984. Gravel and grass surfacing reduces soilloss from mountain roads. For. Sci. 30:657—670.

YOHO, N. S. 1980. Forest management and sediment productionin the south—A review. South. J. Appl. For. 4:27-36.

Lloyd W. Swift, Jr. is research forester, USDA ForestService, Southeastern Forest Experiment Station, Cow-eeta Hydrologic Laboratory, Otto, North Carolina28763.

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