Biosystems Engineering (2002) 81 (4), 465d479 doi:10.1006/bioe.2001.0034, available online at http://www.idealibrary.com on SW*Soil and Water Linking Clay Enrichment and Sediment Delivery Processes C. Di Stefano; V. Ferro Dipartimento di Ingegneria e Tecnologie Agro-Forestali, Sezione Idraulica, Universita % di Palermo, Viale delle Scienze, 90128 Palermo, Italy; e-mail of corresponding author:vferro@mbox.unipa.it (Received 28 November 2000; accepted in revised form 14 November 2001) Recent research has directed attention to the size distribution of eroded material because of its in#uence on deposition mechanics and in carrying capacity of pollutant materials. At "rst, in this paper the relationship betweenaggregatebreakdownmechanismanderosionprocessesisbrie#yreviewed.Thenthelinkbetweenthe clayenrichmentratioandthesedimentdeliveryratioatmorphologicalunitscaleisinvestigated.For129soil sampleswelldistributedovertheSicilianSparaciabasin,thevaluesfortheexperimentalclayenrichmentratio calculatedbythemeasuredultimategrain-sizedistributionsarecomparedwiththetheoreticalclayenrichment ratio values obtained by a procedure based on e!ective grain-size distribution. The clay content measurements, carried out in sediment samples drawn out from the sedimentation tank locatedatthebasinoutletduringtheperiodFebruary1996}February1997,areusedtocalculatethecoe$cient appearing in the relationship applied to estimate the sediment delivery ratio of each morphological unit. A relationship for estimating this coe$cient using sediment clay content, measured by a conventional ultimate particle-size distribution, is also proposed. Finally,aMonteCarlotechniqueandcaesium-137measurementscarriedoutintheexperimentalbasinare usedforverifyingtheaccuracyofthesedimentdeliverydistributedmodelwhichisstronglydependentonthe estimate of the coe$cient appearing in the sediment delivery ratio relationship.  2002 Silsoe Research Institute. Published by Elsevier Science Ltd. All rights reserved 1. Introduction Past research on soil erosion has principally focussed on the total amount of material eroded from hillslopes andonthequantityofsedimentwhichistransferred,in a given time interval, from the eroding sources through the channel network to the basin outlet (Walling, 1983, 1988; Slattery & Burt, 1997). Recent work has directed attentiontothepropertiesoftheerodedmaterial,includ- ing grain size, particle shape, density, organic matter content, mineralogy and aggregate stability, because of theirin#uenceonthesedimentdeliveryprocessesandthe importance of sediment-associated transport in non- point pollution problems (Novotny & Chesters, 1989). The sediment size distribution and density of the sediment are the primary factors which determine whether soil particles are transported or deposited for a given #ow condition (Rhoton et al., 1982; Harmon etal.,1989).Thecapacityofthetransportedparticlesto carry various contaminants, that contribute to water pollution (Karickho! & Brown, 1978), is closely related to their speci"c surface area which is in turn largely dependentontheclayandespeciallythemontmorillonite content of the transported particles (Young & Onstad, 1976; Gabriels & Moldenhauer, 1978). Althoughthein#uenceofsoilaggregationandbreak- downmechanismsonsoilparticletransportabilityisnow well documented (Durnford & King, 1991), particle-size dataforsoilsandsedimentsarefrequentlyonlyavailable from standard laboratory measurements where the ma- terialisfullydispersedintoitsprimaryparticlespriorto measurements(ultimategrain-sizedistribution).Ifmostof the sediment moving through a drainage basin is trans- ported as aggregates rather than as discrete separate grains, the size distribution of the transported sediment whichhasbeentermedthe e+ectivegrain-sizedistribution 1537-5110/02/$35.00 465  2002 Silsoe Research Institute. Published by Elsevier Science Ltd. All rights reserved