Morphodynamic response of a dryland river to an extreme flood Morphodynamics of bedrock-influenced dryland rivers during extreme floods: Insights from the Kruger National Park, South Africa David Milan1,†, George Heritage2, Stephen Tooth3, and Neil Entwistle4 1School of Environmental Sciences, University of Hull, Cottingham Road, Hull, HU6 7RX, UK 2AECOM, Exchange Court, 1 Dale Street, Liverpool, L2 2ET, UK 3 Department of Geography and Earth Sciences, Aberystwyth University, Llandinam Building, Penglais Campus, Aberystwyth, SY23 3DB, UK 4School of Environment and Life Sciences, Peel Building, University of Salford, Salford, M5 4WT, UK ABSTRACT some subreaches, remnant islands and vege- the world’s population (United Nations, 2016). tation that survived the 2000 floods were re- Drylands are characterized by net annual mois- High-magnitude flood events are among moved during the smaller 2012 floods owing ture deficits resulting from low annual precipita- the world’s most widespread and signifi- to their wider exposure to flow. These find- tion and high potential evaporation, and typically cant natural hazards and play a key role in ings were synthesized to refine and extend a by strong climatic variability. Although precipi- shaping river channel–floodplain morphol- conceptual model of bedrock-influenced dry- tation regimes vary widely, many drylands are ogy and riparian ecology. Development of land river response that incorporates flood subject to extended dry periods and occasional conceptual and quantitative models for the sequencing, channel type, and sediment sup- intense rainfall events. Consequently, dryland response of bedrock-influenced dryland ply influences. In particular, with some cli- rivers are commonly defined by long periods rivers to such floods is of growing scientific mate change projections indicating the po- with very low or no flow, interspersed with in- and practical importance, but in many in- tential for future increases in the frequency frequent, short-lived, larger flows. On any given stances, modeling efforts are hampered by a of cyclone-generated extreme floods in east- river, this results in a high ratio of large to small paucity of relevant field data. Here, we com- ern southern Africa, the Sabie and other flows (McDermott and Pilgrim, 1983), resulting bined extensive aerial and field data with Kruger National Park rivers may experience in highly skewed flood frequency distributions, hydraulic modeling to document erosion, additional sediment stripping and vegetation and regional and relative flood frequency curves deposition, and vegetation changes that have removal. Over time, such rivers may transi- that are usually steep, as the slopes are typically occurred during two successive, cyclone- tion to a more bedrock-dominated state, established by a few, very large floods (Tooth, driven, extreme floods along a 50-km-long with significant implications for ecological 2000). Some of these very large floods may be reach of the bedrock-influenced Sabie River structure and function and associated eco- of sufficient magnitude or impact to be termed in the Kruger National Park, eastern South system services. These findings contribute to “extreme” (e.g., Gupta, 2000) or “catastrophic” Africa. Aerial light detection and ranging an improved analysis of the Kruger National (e.g., Thompson and Croke, 2013). ( LiDAR) data and photography obtained Park rivers in particular, but also to growing For many dryland rivers, highly variable flow after extreme floods in 2000 and 2012 (dis- appreciation of the global diversity of dry- regimes are key to their morphological devel- charges >4000 m3 s–1) were used to generate land rivers and the relative and synergistic opment (e.g., Tooth and Nanson, 2011; Tooth, digital elevation models (DEMs) and provide impacts of extreme floods. 2013) as well as the maintenance of important the boundary conditions for hydraulic mod- riparian habitats (e.g., van Coller et al., 2000; eling (flow shear stresses for three discharges INTRODUCTION Kingsford, 2006; Parsons et al., 2006; Strom- up to 5000 m3 s–1). For the Sabie River study berg et al., 2007; Sandercock et al., 2007; Jaeger reach as a whole, DEM differencing revealed In an era characterized by rapid environmen- et al., 2017). In some physiographic settings, that the 2012 floods resulted in net erosion tal change and variability, increasing research variable flow regimes and diverse riparian of ~1,219,000 m3 (~53 mm m–2). At the sub- attention is being directed to the role of extreme vegetation assemblages combine with limited reach scale, however, more complex spatial hydroclimatic events such as storm rainfall, sediment supplies and heterogeneous bedrock patterns of erosion, deposition, and vegeta- flooding, and drought in the shaping of Earth’s morphologies to produce dryland river morphol- tion change occurred, as largely controlled surface. High-magnitude floods are among the ogies and dynamics that differ markedly from by differences in channel type (e.g., degree of world’s most significant natural hazards, and fully alluvial rivers, particularly those in humid bedrock and alluvial exposure) and changing they play a key role in the shaping of riparian temperate regions (van Niekerk et al., 1995; hydraulic conditions (shear stresses widely environments across a wide range of physio- Heritage et al., 1999, 2001; Wohl and Achyu- >1000 N m–2 across the river around peak graphic and hydroclimatic zones (Woodward than, 2002; Tooth and McCarthy, 2004; Jansen, flow). The impact of flood sequencing and et al., 2010). Drylands (hyperarid, arid, semi- 2006). In recent decades, greater research relative flood magnitude is also evident; in arid, and dry subhumid regions) are one of the focus has been directed toward such “bedrock- most extensive hydroclimatic zones, covering influenced,” “bedrock-controlled,” or “mixed †d.milan@ hull .ac.uk. 41% of Earth’s surface and sustaining 36% of bedrock-alluvial” dryland rivers (Heritage et al., GSA Bulletin; November/December 2018; v. 130; no. 11/12; p. 1825–1841; https://doi .org /10 .1130 /B31839 .1 ; 10 figures; Data Repository item 2018152 ; published online 14 May 2018. Geological Society of America Bulletin, v. 130, no. 11/12 1825 © 2018 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/130/11-12/1825/4535543/1825.pdf by guest on 24 September 2021 Milan et al. 1999, 2001; Tooth et al., 2002, 2013; Tooth and et al., 2010). In addition, sophisticated remote rapidly onto the lower-relief Lowveld (~400 m McCarthy, 2004; Keen-Zebert et al., 2013), but survey technologies such as light detection and asl) and Lebombo zones (~200 m asl) in the morphological, sedimentological, hydraulic, ranging (LiDAR) and increased computer-pro- east (Fig. 1B). The middle reach lies within and ecological data remain limited. This paucity cessing capabilities have opened up the possi- the boundaries of the Kruger National Park of data hampers efforts to develop conceptual bilities of capturing high-resolution topographic (Fig. 1C). Annual average rainfall is highest and quantitative models of the morphological, data and embedding these data in more sophis- in the uplands (~2000 mm yr–1) and declines sedimentary, and vegetative response of these ticated morphodynamic models to characterize rapidly toward the South Africa–Mozambique types of dryland rivers to past, present, and fu- river responses to recent and potential future border (450 mm yr–1), where it is exceeded by ture climatic changes, including the importance floods (Milan and Heritage, 2012; Croke et al., average annual potential evapotranspiration of shifts in flood frequency-magnitude relation- 2013; Thompson and Croke, 2013; Baggs Sar- (1700 mm yr–1). Rainfall occurs mainly in the ships, flood timing, and flood sequencing. These good et al., 2015). In recent years, computational austral summer (November through March) data and knowledge gaps are becoming increas- modeling has led to significant insights into the and normally results from convective thunder- ingly significant as drylands are now widely flow and sediment dynamics and consequent storms, although occasional, high-intensity rain- considered to be some of the regions most vul- morphological responses of fully alluvial rivers fall events can result from cyclones that form nerable to future hydroclimatic changes (Obasi, (Nicholas, 2010; Nicholas et al., 2013). To over the Indian Ocean and track inland. For 2005; IPCC, 2007; Wang et al., 2012), and this date, there have not been similar advances in example, the maximum daily rainfall recorded limited model development restricts attempts the under standing of bedrock-influenced river for Skukuza (Fig. 1C) between 1912 and 2001 to develop environmentally sound, sustainable dynamics, where sediment supply limitations, was 103.5 mm (Kruger et al., 2002), while 2 to 5 management practices for such rivers. resistance to scour, and complex roughness and day rainfall totals can exceed 200 mm (Heritage Some previous work, however, provides a flow partitioning can have significant influences et al., 2001). The flow regime of the Sabie River basis for improved model development. For in- on erosion, deposition, and resultant channel, reflects this hydrological regime, with summer stance, morphological responses in some bed- bar, island, and floodplain development. floods being separated by long periods of win- rock-influenced