HR: 16:00h
AN: H54B-01    [Abstracts]
TI: A geometry-based, theoretical estimation of the role of abrasion by suspended sediment in a pothole-dominated knickpoint: Orange River, Republic of South Africa
AU: * Springer, G S
EM: springeg@ohio.edu
AF: Ohio University, 316 Clippinger Laboratories Dept. of Geological Sciences Ohio University, Athens, OH 45701 United States
AU: Tooth, S
EM: set@aber.ac.uk
AF: University of Wales, Aberystwyth, Institute of Geography and Earth Sciences University of Wales, Aberystwyth , Aberystwyth, Cer SY23 3DB United Kingdom
AU: Wohl, E E
EM: ellenw@cnr.colostate.edu
AF: Colorado State University, Department of Geosciences Colorado State University , Fort Collins, CO 80523 United States
AB: A 6-m high knickpoint is retreating through resistant granitic rocks of an anabranch of the Orange River, Republic of South Africa. The knickpoint is densely penetrated by potholes and lacks evidence of quarrying. Pothole depths (d) and radii (r) are related to one another by a simple power law, r=d$^{\epsilon}$, where k and $\epsilon$ are regression coefficients equal to 2.38 and 0.57, respectively. The well-defined relationship between pothole d and r (R$^{2}$ = 0.72; n = 193) precludes pothole removal by lowering of the surrounding bed, which means that potholes are removed wholesale through coalescence and collapse. The relative contributions to pothole growth resulting from abrasion by suspended sediment and milling of pothole floors by grinders can be determined by a geometrical model of pothole growth that exploits the d-r power law relationship. Using the model, at least 53% of all material is removed from pothole walls during pothole growth. Recognizing that suspended sediment is the probable erosive agent in potholes lacking grinders, and on pothole walls above the level of grinders in the few potholes that contain them, we conclude that suspended sediment concentration in floodwaters is a first-order control on erosion of the observed knickpoint. However, our model also shows that abrasion atop the adjacent bed may contribute significantly to erosion of the knickpoint despite the apparent dominance of potholes. By inference, future experimenters, modelers, and field investigators of erosion in knickpoints and atop resistant substrates will need to pay close attention when devising their research plans to the potentially dominant role of abrasion by suspended sediment.
DE: 1899 General or miscellaneous
DE: 1815 Erosion and sedimentation
DE: 1821 Floods
DE: 1824 Geomorphology (1625)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting