Measuring erosion on unsealed roadsthrough the rainforest

The erosion of road surfaces impactson the environment in a number ofways. In the rainforest, road surfacescan be a source of rapid rainfallrun-off leading to the transportationof particles into rivers and streamswhich subsequently affects waterquality and impacts on biota.Resource and road managementauthorities within the Wet TropicsWorld Heritage Area (WTWHA) aregreatly concerned regarding theoperation and management of roadswithin the region.

The Rainforest CRC has thereforeundertaken research to provideresults which will improve decision-making regarding permits, roadmanagement, visitation and use inthe Wet Tropics. The research had twodistinct aims:• To develop an Erosion Potential

Index for the WTWHA, and• To examine the relationship

between the extent of tree canopycover and the actual amount ofroad surface eroded.

Erosion problems onunsealed roadsErosion is a process that occurs in twoparts: firstly, the detachment of soilparticles from the surface, and sec-ondly, the transportation of theseparticles by the erosive agents ofrunning water and wind. Erosion onunsealed roads causes several typesof road surface problems whichinclude sheet, rill and gully erosion,as well as the formation of potholes,ruts and the protrusion of rocks.

The Erosion Potential In-dex (EPI)Three major factors understood toaffect the amount of erosion were in-tegrated using a Geographic Informa-tion System to develop an ErosionPotential Index. These factors wererainfall, slope and soil type. The EPIestimates the potential for erosionacross the Wet Tropics World Herit-age Area and classifies the landscapeinto one of five erosion potentialclasses: very low, low, medium, high,and very high. The percentage of landin each class in the WTWHA is shownon the pie chart overleaf.

The EPI not only provides a usefulindication of which roads will poten-tially suffer more erosion, but can alsobe used to assess the suitability ofareas within the World Heritage Areafor different land uses. For example,managers making decisions regard-ing the use, maintenance or develop-ment of walking tracks, car parks,camping and day-use areas wouldfind the EPI highly valuable.

Canopy cover and the amountof erosionThe EPI was used to identify 30 siteswithin the WTWHA in order toground truth levels of erosion on un-sealed roads. Two segments of roadwere selected at each sampling site,one under tree canopy cover of atleast 60%, and one with no treecanopy cover. This method, known aspaired sampling, strengthens results. Arapid assessment technique was usedto examine the percentage of roadaffected by erosion, the type andseverity of erosion, and percentageand characteristics of canopy cover.

February 2001

pothole erosion

gully erosion

The results indicate that road seg-ments with no tree canopy cover havesignificantly more surface erosionthan road segments with closed treecanopies. At the 30 sites sampled treecanopy cover appears to protect theroad surface from erosion. This pro-tection by tree canopies is likely tooccur due to water being funnelleddown the branches and trunks oftrees and away into the forest ratherthan directly contacting the roadsurface.

These findings conflict with the longheld view that road segments withtree canopy cover have greater sur-face erosion since the amount of sun-light reaching the road is reduced.However, no relationship could beestablished between the amount ofsunlight reaching the road surfaceand the amount of erosion, a factwhich further strengthens these newconclusions.

Further paired sampling would beuseful to continue investigations ofthe relationship between road surfaceerosion, canopy cover and otherfactors.

Implications for Managing the WetTropics

The research clearly indicates thattree canopy cover over unsealedroads in the Wet Tropics reduces theamount of surface erosion and there-fore the environmental impactsassociated with this erosion. EarlierRainforest CRC research has demon-strated that tree canopy cover alsomitigates other impacts of roads,including restriction of movements offauna and changes in microclimate.These new results strengthen the un-derstanding that tree canopy cover isa critical factor in mitigating the im-pacts of roads in the Wet TropicsWorld Heritage Area.

The graph demonstrates that most land in the WTWHA falls into the high or very high

erosion potential categories due to steep slopes, high rainfall and highly erodible soils.

Acknowledgments

This research project is a componentof the multi-disciplinary RainforestCRC research project Impacts of Roadsand Powerlines on the Wet Tropics WorldHeritage Area and was primarilyundertaken by Master of AppliedScience Student Tobi-Ann Bacon. Thesupport of the Wet Tropics Manage-ment Authority GIS division isgratefully acknowledged.

For further information:Associate Professor Steve TurtonTropical Environment Studies andGeographyJames Cook University, CairnsPh: (07) 40 42 1292 Fax: (07) 40 42 1284Steve.Turton@jcu.edu.au