UNCORRECTED PROOF The in¯uence of rainfall on sediment transport by overland ¯ow over areas of net deposition L. Beuselinck a , G. Govers a,b, * , P.B. Hairsine c , G.C. Sander d , M. Breynaert a a Laboratory for Experimental Geomorphology, Katholieke Universiteit Leuven, Redingenstraat 16, 3000 Leuven, Belgium b Fund for Scienti®c Research, Flanders, Belgium c CSIRO Land and Water and the Co-operative Research Centre for Catchment Hydrology, P.O. Box 1666, Canberra 2601, Australia d Department of Civil and Building Engineering, Loughborough University, Loughborough, Leicestershire, LE11 3TU, UK Received 29 September 2000; revised 21 August 2001; accepted 12 October 2001 Abstract Overland¯owisoftenonlyafewmillimetresdeep.Consequently,thepotentialforraindropimpacttoaffect¯owhydraulics and sediment transport is high. Furthermore, the relative importance of rainfall impact is highest for shallow low-energy ¯ows on low slopes. In such ¯ows net sediment deposition may occur. Therefore, laboratory experiments were conducted to study sediment deposition in the presence of rainfall over a range of hydraulic conditions. In order to investigate the impact of raindrops on sediment deposition by overland ¯ow, these experimental data were compared to the experimental data collected intheabsenceofraindropimpact.Comparisonoftheexperimentaldatashowsthatraindropimpactretardsthe¯owvelocityand has a clear positive effect on sediment delivery. Under rainfall signi®cantly more coarse sediment is transported over areas of net sediment deposition. Subsequently, the experimental results are used to evaluate a multi-class net deposition theory, describing sediment transport and sediment sorting over areas of net deposition in the presence of both raindrop impact and ¯ow-driven processes. The multi-class theory is calibrated using part of the experimental data. Evaluation of the model predictions using the other part of the data shows that the optimised model is able to accurately predict sediment delivery and sediment sorting over areas of net deposition. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Overland ¯ow; Sediment deposition; Sediment sorting; Raindrop impact 1. Introduction Threedifferenttypesofsedimenttransportbywater erosioncanbedifferentiatedMossetal.,1979).Sedi- ment can be transported by overland ¯ow, by rain- splash and by the combination of overland ¯ow and rainfall impact. Beuselinck et al. 1999a) presented experimental results on sediment transport by over- land¯owoveranareaofnetdepositionintheabsence of raindrop impact. These experimental data were used to evaluate the simple settling theory, in which sediment deposition is described as only settling basedonthesettlingvelocitydistributionofthein¯ow sediment Beuselinck et al., 1999b), and the Hairsine et al. 2001) sediment deposition algorithm Beuse- linck et al., 2001a,b). Sediment transport by raindrop impact in the absenceofanoverland¯owlayerhasalsobeenexten- sively studied e.g. Moeyersons and De Ploey, 1976; Savat, 1981; Poesen and Savat, 1981; Moss and Green, 1983). If a raindrop falls on a horizontal soil 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 Journal of Hydrology 000 2001) 000±000 HYDROL4397 0022-1694/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII:S0022-169401)00548-0 www.elsevier.com/locate/jhydrol * Corresponding author. Tel.: 132-16-32-6437; fax: 132-16-32- 6400. E-mail address: gerard.govers@geo.kuleuven.ac.be G. Govers). ARTICLE IN PRESS Journal of Hydrology ± Model 3 ± Ref style 2 ± AUTOPAGINATION 2 A lden 26-10-2001 13:11 article GL