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PNW-123

July 1970

STABILIZATION OF NEWLY CONSTRUCTED ROAD BACKSLOPES

BY MULCH AND GRASS-LEGUME TREATMENTS

•by

C. T. Dyrness, Principal Soil Scientist

ABSTRACT

Amounts of soil Zoss froman unprotected newly constructedbacks lope were two to -four timesgreater than Zoss from a compar-able slope 5 years after construc-tion. Of six roadside treatments,studied, the two showing consis-tently Zarge amounts of soil Zossduring the first critical rainyperiod were the only ones withouta straw mulch covering.

In an earlier study,lj

five different mulch and grass-legume applications to a 5-year-old road backslope were equally

lj C. T. Dyrness. Grass-

legume mixtures for roadsidesoil stabilization. PacificNorthwest Forest & Range Exp.Sta. USDA Forest Serv. Res.Note PNW-71, 19 pp., illus.1967.

effective in eliminating erosion.Only the untreated control plotsshowed appreciable amounts of soilmovement during the first year ofmeasurement. At that time, it washypothesized that erosion ratesfrom freshly constructed backslopesare generally substantially higher.Consequently, such slopes may offerconsiderably more resistance tostabilization, especially duringthe critical first year after con-struction.

The present study was con-ducted to test this hypothesis,not only to provide informationon the stability of new slopesbut also to aid in formulatingmeasures to minimize soil loss.Experimental design and treatmentsare essentially the same as thosedescribed in the 1967 report, theprincipal difference being thatthese plots were established in

the fall following road construc-tion.

METHODS

In late August 1967, twoblocks of six backslope plotseach were established along aroad in the Blue River Districtof the Willamette National For-est. The road, constructed dur-ing the early summer of 1967, isat an elevation of 3,100 feet.The soil (Slipout series) is im-perfectly drained and exhibits aclay loam surface grading into asilty clay subsoil. Parentmaterial is largely greenishtuffs and breccias.

Five grass-legume mixturesand a control were replicatedtwice. Plots were 6 feet wideand as long as the backslope--generally about 20 to 25 feet.Species composition and amountsapplied of the seed mixtures areshown in table 1. With the ex-ception of the control plots, ,all plots received a blanketapplication of ammonium phos-phate fertilizer (16-20-0) at therate of 400 pounds per acre bothat the time of seeding (August1967) and the following April(1968). Treatments were asfollows: (1) control--slopeleft untouched, (2) mulch onl y --wheat straw mulch applied at therate of 2 tons per acre, (3)Blue River District mixture--seed at the rate of 25 poundsper acre applied without mulch,(4) Oregon Highway mixture--seedat the rate of 40 pounds per acreand straw mulch at the rate of 2tons per acre, (5) experimentalmixture No. 1--43 pounds per acreof seed and 2 tons per acre of

1,;4.

straw mulch, and (6) experimentalmixture No. 2--43 pounds per acreof seed and 2 tons per acre ofstraw mulch.

Soil movement on the plotswas evaluated by a sloRe

1 profile

measurement technique. ZZ— At thetime of each measurement, percent-age of vegetative cover on eachplot was also estimated. Initialslope profile measurements weremade soon after plot establish-ment in September 1967, and sub-sequent measurements were madein Apr11, June, and Septemberof 1968.

RESULTS AND DISCUSSION

Although these plots wereseeded early in the fall, plantgrowth was rather severely limit-ed prior to and during the cold,wet, winter months. Amountsof grass and legume cover re-mained relatively low untilApril. However, only 2 monthslater, all seeded plots werevirtually fully protected byplant cover (table 1). There-fore, the growth of seeded spe-cies during the fall and wintermonths apparently provides in-sufficient cover for soil stabi-lization, at least at moderateto high elevations.

Soil losses or gains duringthe first rainy season followingroad construction are shown intable 2 in the April column.With the exception of one plotof experimental mix No. , 1, wherea small slump occurred, the onlyplots to show consistently large

2! See footnote 1.

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Table 2.--Average cumulative soil Zoss or gain on 12 backslope plots during the firstyear after construction, compared with soil movement in 1 year on a compar-able backslope 5 years after construction

Treatmentandblock

1968

1-year results from ,,5-year-old backslopelf

April June Sept.

Control:

Inches

1 -0.48 -0.55 -0.83 -0.232 -.45 -.59 -.84 -.44

Mulch only:1 -.06 -.08 t.05 +.132 -.07 -.07 -.14

Blue River District mixture:1 -.72 -.72 -.77 +.052 -.42 -.55 -.31 +.14

Oregon Highway mixture:1 -.12 -.13 -.20 -.132 -.1 1 -.19 -.23 -.04

Experimental mixture No. 1:1 2/..54 -.66 -.65 + . 112 +.10 +.14 +.17 +.10

Experimental mixture No. 2:1 +.01 -.07 -.07 +.142 +.02 +.08 +.11 -.01

1/ Data taken from C. T. Dyrness. Grass-legume mixtures for roadside soil stabili-

zation. Pacific Northwest Forest & Range Exp. Sta. USDA Forest Serv. Res. Note PNW-71,19 pp., illus. 1967.

/ Most of this movement was caused by a small slump near the base of the plot.

amounts of soil loss were thecontrol and Blue River Districtmixture plots. It is significantthat these two treatments are theonly ones which lacked a strawmulch covering. In order to fullyappreciate the magnitude of soilloss, one needs only to realizethat 1 inch of soil is roughlyequivalent to 100 tons per acre.Thus, a 0.45-inch loss representsa soil loss of approximately 45

tons on an acre basis. Althoughmuch of this eroded materialcomes to rest in the roadsideditch, road drainage water duringrainy periods generally carriessome of the sediment into streamchannels. The soil gain figuresshown in the table probably rep-resent deposition of soil whichraveled down from the top of thebackslope on the measured slopesection.

4

By June and September, anearly complete covering ofgrasses and legumes on all plots,including plots treated with theBlue River District mixture(table 1), had substantiallyreduced soil movement. As aresult, the control plots werethe only ones to show continuedhigh rates of soil loss duringthis period (table 2). It isinteresting to note that for thecontrol plots dry season loss byraveling was almost as great asrain-caused soil loss.

The hypothesized greater in-stability of freshly constructedbackslopes is apparently borneout by the data in table 2. One-year soil losses from unprotectednew slopes are roughly two tofour times greater than losses ina 1-year period from a 5-year-oldbackslope. However, since soiland slope characteristics of thetwo sites differ, sweeping gener-al conclusions should be avoided.

CONCLUSIONS

Considerable evidence hasindicated the desirability oftreating newly constructed road-side slopes in the early fall be-fore heavy rains. Probably mostimportant is that the loose, un-protected soil is most vulnerableto erosion during this periodand, therefore, needs protection.For example, Fredriksen-V reported

3j R. L. Fredricksen

[Fredriksen]. Sedimentationafter logging road constructionin a small western Oregon water-shed. U.S. Dep. Agr.. Misc. Pub.970, 56-59, illus. 1965.

that stream sediment contentincreased 250 times during thefirst rainstorms after roadconstruction in a small drainagein the western Cascades. Theopen, porous soils on freshlyconstructed slopes offer a veryfavorable seedbed. On the otherhand, slopes which have beenexposed to the beating action ofrainfall tend to have smooth,sealed surfaces and are lessfavorable for germination andseedling establishment.

Results of this study suggestthat mulching backslopes may beessential for reducing soil lossto a minimum during the first fewcritical months following con-struction. The only mulched plotwhich showed a large amount ofsoil movement did so as a resultof a rather deep-seated slump--indicating, of course, that these •treatments are effective only incurtailing surface erosion.These results also indicate that,contrary to appearances, aluxuriant growth of grass andlegumes during the first growingseason following fall treatmentis not conclusive evidence thatsoil loss was negligible duringthe preceding winter months.

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