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Definition of Flood Plains Along Arid-Region Rivers
Abstract
Catastrophe theory is appropriate because morphologic change on these river systems is not progressive, but instead proceeds through time in a discontinuous, but perhaps, predictable pattern. In addition, threshold changes in channel pattern can produce transitional channel forms that cause catastrophic sediment yields during subsequent high-frequency flows. -from Editors
... This may in part reflect the use of non-engineering terminology or the perceived 'complexity' of river landscapes where both geography and history matter in their evolving characteristics (Phillips, 2007). Indeed, defining the most relevant geomorphological landform to floods: the 'floodplain' is not a straight forward and unambiguous task (Graf, 1988;Nanson and Croke, 1992;Moody et al., 1999). In reality, the distinction is often complex and the floodplain can be composed of several discrete surfaces of differing elevation that vary in their frequency of inundation (Woodyer, 1968;Pickup and Warner, 1976;Grams and Schmidt, 2002;Heitmuller et al., 2015). ...
... Identification of the genetic floodplain is particularly difficult in settings with high discharge variability (Pickup and Warner, 1976), including arid regions (Graf, 1988). Channels in these hydrological settings experience flood events that, globally, would constitute an extreme event, but are often the norm (Pickup and Warner, 1976). ...
... Channels in these hydrological settings experience flood events that, globally, would constitute an extreme event, but are often the norm (Pickup and Warner, 1976). This hydrological variability often produces a channel form referred to as a macrochannel or compound channel-in-channel which is characterized by a small inner channel and associated benches set within a much larger channel that operates as a conduit for high magnitude floods (Woodyer, 1968;Graf, 1988;Gupta, 1995;Van Niekerk et al., 1995;Gupta et al., 1999;Croke et al., 2013a). The term 'banktop discharge' is a more appropriate expression of the inundation surface in settings where traditional 2.5 year recurrence intervals do not apply (Pickup, 1976;Pickup and Warner, 1976;Williams, 1978;Heritage et al., 1999;Heitmuller et al., 2015). ...
Flood risk management is an essential responsibility of state governments and local councils to ensure the protection of people residing on floodplains. Globally, floodplains are under increasing pressure from growing populations. Typically, the engineering-type solutions that are used to predict local flood magnitude and frequency based on limited gauging data are inadequate, especially in settings which experience high hydrological variability. This study highlights the importance of incorporating geomorphological understanding into flood risk management in southeast Queensland (SEQ), an area badly affected by extreme flood events in 2011 and 2013. The major aim of this study is to outline the hydrological and sedimentological characteristics of various 'inundation surfaces' that are typical of catchments in the sub-tropics. It identifies four major inundation surfaces; within-channel bench [Q~2.33yr average recurrence interval (ARI)]; genetic floodplain (Q=20yr ARI); hydraulic floodplain (20yr <Q≤200yr ARI) and terrace (Q>1000yr ARI). These surfaces are considered typical of inundation areas within, and adjacent to, the large macrochannels common to this region and others of similar hydrological variability. An additional area within genetic floodplains was identified where flood surfaces coalesce and produce an abrupt reduction in channel capacity. This is referred to here as a Spill-out Zone (SOZ). The associated vulnerability and risk of these surfaces is reviewed and recommendations made based on incorporating this geomorphological understanding into flood risk assessments. These recommendations recognize the importance to manage for risks associated with flow inundation and sediment erosion, delivery and deposition. The increasing availability of high resolution topographic data opens up the possibility of more rapid and spatially extensive assessments of key geomorphic processes which can readily be used to predict flood risk.
... Compound channels consist of a single, low-flow, meandering channel inset into a wider, braided flood zone ( Fig. 6) (Graf 1988b). Dramatic channel widening and activation of braided channels accompany extreme flow events; a meandering form develops after a long (i.e., decades) sequence of low to moderate discharges (Kondolf and Curry 1986, Pearthree and Baker 1987, Kresan 1988, Graf 1988b, Friedman and Lee 2002. ...
... Compound channels consist of a single, low-flow, meandering channel inset into a wider, braided flood zone ( Fig. 6) (Graf 1988b). Dramatic channel widening and activation of braided channels accompany extreme flow events; a meandering form develops after a long (i.e., decades) sequence of low to moderate discharges (Kondolf and Curry 1986, Pearthree and Baker 1987, Kresan 1988, Graf 1988b, Friedman and Lee 2002. While similar channel conditions are found in humid climates (Hickin and Sichingabula 1988), compound channel development is enhanced, and their presence more common, in arid climates for at least three primary reasons: the lower density of erosion-resistant vegetation (Graf 1978, Kondolf andCurry 1986), the greater prevalence of non-cohesive sandy soils (Cooke at al. 1993), and a higher ratio between record peak discharges and average annual discharge (Graf 1988b). ...
... Dramatic channel widening and activation of braided channels accompany extreme flow events; a meandering form develops after a long (i.e., decades) sequence of low to moderate discharges (Kondolf and Curry 1986, Pearthree and Baker 1987, Kresan 1988, Graf 1988b, Friedman and Lee 2002. While similar channel conditions are found in humid climates (Hickin and Sichingabula 1988), compound channel development is enhanced, and their presence more common, in arid climates for at least three primary reasons: the lower density of erosion-resistant vegetation (Graf 1978, Kondolf andCurry 1986), the greater prevalence of non-cohesive sandy soils (Cooke at al. 1993), and a higher ratio between record peak discharges and average annual discharge (Graf 1988b). All of these factors promote rapid channel widening during extreme events. ...
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... In general, fluvial processes in the Pilbara, located in the arid subtropics, tend to be driven by infrequent, high intensity and short duration hydrologic events that are related to the occurrence of Tropical Cyclones (Harvey et al. 2014). The morphology of the alluvial sections of the ephemeral-flow, gravel-bed creeks where sediment transport is episodic, tends to be a relic of the last major flow event and results in highly variable channel morphology over both space and time (Graf 1988). Graf (1988) concluded that in arid and semi-arid regions where the flows are ephemeral, infrequent and relatively shortduration hydrological events rarer than the 1 in 100 AEP are the major determinants of overall valley floor morphology, but more frequent events are responsible for defining highly variable channels (macrochannels) within the disturbed landscape. ...
... The morphology of the alluvial sections of the ephemeral-flow, gravel-bed creeks where sediment transport is episodic, tends to be a relic of the last major flow event and results in highly variable channel morphology over both space and time (Graf 1988). Graf (1988) concluded that in arid and semi-arid regions where the flows are ephemeral, infrequent and relatively shortduration hydrological events rarer than the 1 in 100 AEP are the major determinants of overall valley floor morphology, but more frequent events are responsible for defining highly variable channels (macrochannels) within the disturbed landscape. Macro-channel morphology (compound channels) is associated with regions of high hydrological variability (Croke et al. 2013). ...
This book collects papers from the 2020 IMWA Congress that was postponed because of travel restrictions related to COVID19.
... As an integral component of the fluvial landscape, floodplains are genetically related to the modern hydrological regime as periodic deposition and formation occurs during relatively frequent overtopping flows (Williams, 1978). Defining the floodplain in hydrologically variable rivers is less straightforward (Graf, 1988;Croke et al., 2016) as these rivers can often experience extended periods of low-flow conditions punctuated by extreme events. A complex arrangement of alluvial surfaces can form contemporaneously that are not necessarily inregime but the inundation and reworking of each surface may be associated with various discharges (Woodyer, 1968;Pickup and Warner, 1976;Thoms and Olley, 2004). ...
... A complex arrangement of alluvial surfaces can form contemporaneously that are not necessarily inregime but the inundation and reworking of each surface may be associated with various discharges (Woodyer, 1968;Pickup and Warner, 1976;Thoms and Olley, 2004). Accordingly, these rivers have been referred to as having a compound, nested or channel-in-channel morphology (Richards, 1982;Graf, 1988;Erskine and Livingstone, 1999). In cases like these, the channel defined by the outermost banks has come to be referred to as a macrochannel (van Niekerk et al., 1999;Croke et al., 2013a) and in essence is equivalent to an exceptional major channel as defined by Tricart (1960). ...
Along the eastern margin of Australia, hydrological variability reaches a peak in the subtropics of south-east Queensland and many rivers have entrenched characteristics. To address the nature of entrenchment and the relationship with adjacent alluvium, this paper presents the results of detailed chrono-stratigraphic analysis of alluvial units in the partly confined mid-reaches of Lockyer Creek, Australia. Four sites were investigated using topographic, sedimentological and chronological data. Radiocarbon and single grain optically stimulated luminescence dating indicate a large proportion of the valley fill reflects a major phase of aggradation of fine-grained alluvium from ca. 35 ka throughout the Last Glacial Cold Period. Synchronous incision of Pleistocene alluvial fills between 11.5 and 9.3 ka suggests the current entrenched Lockyer Creek formed in response to changes in late Quaternary climate. Holocene floodplains set within the entrenched Pleistocene valley floor have basal ages of ca. 7.5 ka, but whose proximal margins are still actively accreting. This Holocene fill has accreted over the mid- to late Holocene but overlaps with the contemporary hydrological regime. The sedimentary nature of the Holocene fill appears to be related to persistent antecedent controls in the form of bedrock and terrace constriction.
... Most of the streams of northwest Europe and eastern North America that have been intensively investigated are relatively small on a world-wide basis, and though some generalities certainly must apply in many locales, the details may differ. Until the late 1990s, much of the theory for dryland rivers came from experiences in the American Southwest (Graf, 1988), but more recent investigations in Australia by Gerald Nanson, Steven Tooth and others, for example, have shown that the American experience is not applicable in all drylands (Nanson and Knighton, 1996). ...
... How should width be determined in the common circumstance where multiple sets of banks have resulted from episodic incision or simply variable fl ows, which is often the case in arid, semi-arid, arctic or alpine regions. Many legal systems also defi ne the channel as being 'between the banks,' but do not specify which banks to use for the description (Graf, 1988). All geomorphological research includes assumptions which form another source of uncertainty. ...
... In reality, however, stream channels and floodplains are not uniform in character over the length of their courses because of changes in discharge, sediment load, and substrate. This is especially true for arid and semiarid streams, where quasi-equilibrium conditions may never be attained (Graf, 1988a(Graf, , 1988b. Even casual perusal of topographic maps and aerial photos of semiarid regions allows ready identification of stream segments where erosion or deposition locally prevails, the floodplain and stream channel are locally wider or narrower, the channel is straighter or more sinuous, channel pattern changes from a single channel to braided or the reverse, where large tributaries converge with the master stream, where vegetation or springs indicate localized near-surface groundwater, or where streams are locally emergent or submergent. ...
... When present, riverine floodplains are restricted environments in the overall landscape, occurring as ribbons along drainages. Wider floodplains can often be divided into active and geomorphic floodplains (Graf, 1988b; Figure 5), based on regularity of floodwater inundation. The active floodplain is regularly inundated by floodwaters, whereas the geomorphic floodplain defines the area between the active floodplain and the limits of the maximum probable flood, and thus it is always either wider than, or isomorphic with, the active floodplain. ...
Maize agriculture appears to have been introduced into the semiarid southern part of the North American Southwest ca. 2200–2000 B.C., at a time when valley bottoms were aggrading. Although small-scale, high-risk agriculture might have been practiced in many landscape positions, valley-bottom locations having particular geomorphic and/or hydrologic conditions would have provided the places most risk-free for farming. These places, associated with what are here called reach boundaries, have conditions that cause elevated water tables or enhance stream discharge, and have floodplain areas suitable for farming. Reach boundaries are easily identified, are relatively predictable in location and occurrence, and are early settled and/or long occupied. Deposition has been the dominant process in most valley bottoms in the southern Southwest for the last ca. 6000 years. Many streams across the region, however, experienced approximately synchronous periods of erosion when deeply incised channels (arroyos) formed in valley bottoms. Each of these periods of erosion, except for that which began in the late 1800s, was followed by a longer period of arroyo-filling and floodplain aggradation, completing a so-called alluvial cycle. Valley-bottom conditions related to alluvial cycles would have greatly influenced the location, water availability, water management technology, and yield of agricultural ventures. Here we identify more than 280 reach boundaries in parts of the middle and upper Gila River and Mimbres River watersheds in Arizona and New Mexico. Spatial analysis shows a strong 4000-year-long correspondence between reach boundaries and agricultural sites where data are available. Identification and characterization of these locations may prove useful for locating yet unknown, deeply buried early agricultural sites (ca. 2200 B.C.–A.D. 200), in addition to providing explanations for prehistoric settlement patterns through time. © 2011 Wiley Periodicals, Inc.
... Progressive aggradation in braided channels commonly results in the development of secondary channels perched above the main channel (Keppel and Renard, 1962;Graf, 1988a;Reid and Frostick, 2011). During periods of relatively more frequent and lower magnitude flows, braided channels typically develop compound meandering channels inset across the braided valley bottom (Graf, 1988b). Infrequent, high-magnitude floods can completely restructure ephemeral channel geometry and temporary flow characteristics in arid regions (Graf, 1988a), but ephemeral streams tend to maintain similar flow characteristics over longer periods of time (Bull and Kirkby, 2002). ...
... Ephemeral streams may experience aggradation for decades without significant channel-altering flows (Graf, 1981) and undergo substantial erosion during large floods (Graf, 1988a;Kondolf et al., 2001;Friedman and Lee, 2002;Merritt and Wohl, 2003). This makes it difficult to identify a single flow magnitude, or associated channel width or flow stage, that adequately represents channel adjustment over a period of decades (Graf, 1988b;Tooth, 2000). Lack of information on flow magnitude and recurrence intervals for ephemeral streams is exacerbated by the paucity of flow data in arid environments. ...
Despite the global abundance of arid-region ephemeral streams, hydrologic and geomorphic data for these systems are limited compared to their perennial counterparts. High spatial and temporal variability in flow makes hydrologic and geomorphic aspects of dryland ephemeral channels difficult to characterize. Perennial stream classifications have been extended to dryland ephemeral streams but do not adequately describe observed differences in channel geometry and characteristics of ephemeral channels in desert environments. We present a geomorphic classification for ephemeral streams in mountainous regions based on planform, degree of confinement, and composition of confining material. Five stream types were identified in the Sonoran desert of southwestern Arizona: (1) piedmont headwater, (2) bedrock, (3) bedrock with alluvium, (4) incised alluvium, and (5) braided channels. Nonparametric permutational multivariate analysis of variance for 101 surveyed reaches indicated differences (p < 0.001) in channel geometry and hydraulics among the five stream types. Nonmetric multidimensional scaling ordination identified the strongest channel geometry and hydraulic variables capable of distinguishing the five channel types, and a classification tree determined relative importance of these variables in the following order: width-to-depth ratio (W/D), stream gradient (S), stream power (Ω), and shear stress (τ). A classification tree and discriminant analysis used W/D, S, Ω, and τ for 86 study reaches on the U.S. Army Yuma Proving Ground (77% and 77% internal validation hit rate, respectively) to predict stream type of 15 separate study reaches on Barry Goldwater Air Force Range with 67% and 73% external validation hit rates, respectively. Differences in channel geometry among the five stream types reflect likely differences in hydrology, hydraulics, and sediment transport with implications for disturbance regime, channel adjustment to disturbance, and ecological sensitivity.
... Progressive aggradation in braided channels commonly results in the development of secondary channels perched above the main channel (Keppel and Renard, 1962;Graf, 1988a;Reid and Frostick, 2011). During periods of relatively more frequent and lower magnitude flows, braided channels typically develop compound meandering channels inset across the braided valley bottom (Graf, 1988b). Infrequent, high-magnitude floods can completely restructure ephemeral channel geometry and temporary flow characteristics in arid regions (Graf, 1988a), but ephemeral streams tend to maintain similar flow characteristics over longer periods of time (Bull and Kirkby, 2002). ...
... Ephemeral streams may experience aggradation for decades without significant channel-altering flows (Graf, 1981) and undergo substantial erosion during large floods (Graf, 1988a;Kondolf et al., 2001;Friedman and Lee, 2002;Merritt and Wohl, 2003). This makes it difficult to identify a single flow magnitude, or associated channel width or flow stage, that adequately represents channel adjustment over a period of decades (Graf, 1988b;Tooth, 2000). Lack of information on flow magnitude and recurrence intervals for ephemeral streams is exacerbated by the paucity of flow data in arid environments. ...
Because understanding of ephemeral stream characteristics is limited and many stream classifications do not adequately describe them, it is necessary to develop a better understanding of these dryland fluvial systems and develop more precise terminology to discuss their physical attributes. In addition to development of a geomorphic classification system, we examine relationships between basin characteristics and channel geometry that will indicate where these ephemeral stream types might occur. Our conceptual model includes five geomorphic ephemeral stream types; 1) braided washes, 2) incised alluvium, 3) bedrock with alluvium, 4) bedrock, and 5) piedmont headwater channels. Preliminary watershed classification and cluster analysis of the U.S. Sonoran Desert was conducted using NHD 10-digit Hydrologic Unit Boundaries, PRISM precipitation data, state geologic survey lithology, and data derived from 30m DEMs. A total of 85 reaches were surveyed on the U.S. Army Yuma Proving Ground in southwestern Arizona representing the five stream types within three watershed categories. Following delineation of small-scale watershed characteristics using 10m DEMs for each reach location, statistical analysis will be performed to examine correlations and significant relationships among stream type, basin and channel characteristics. We hope to identify physical drivers resulting in the development of distinct geomorphic stream types and predict where the relative abundance of those stream types are likely to exist in arid environments of the southwestern U.S. We posit that locations and relative distributions of the five stream types will correlate significantly to local basin characteristics. Initial findings verify that composition of confining material dictates the level of confinement and largely influences occurrence of the five channel types. Additionally, we expect to see significant differences in width/depth ratios, grain size, stream gradient, basin hillslope gradient, and drainage area among stream types. Field observations and preliminary results support a progressive increase in width/depth ratios, drainage area, and distance from the mountain front with a concurrent decrease in grain size, stream gradient and hillslope gradient from bedrock, bedrock with alluvium, incised alluvium, to braided washes, respectively. Piedmont headwater channels are expected to occur with the smallest of drainage areas and the most variability in hillslope and stream gradient, distance from the mountain front, and grain size. The results of this study contribute to the collective understanding of arid ephemeral streams through the development of a geomorphic classification related directly to physical drivers within dryland watersheds.
... Deltas of such rivers are characterized by channels that cannot be maintained in one location for extended periods. The land form is produced by a shifting locus of deposition similar to that which occurs on alluvial fans in inland areas; the arcuate shape testifies to this tendency of channels to migrate back and forth (Bull, 1968;Graf, 1988;Schumm, 1977). The multiple channels of the deltaic system, called distributary channels, are characteristically active for a period of time, then may become inactive, and then become active again at a later date. ...
... The most recently revised FEMA Guidelines have recognized these characteristics of alluvial fans and note that "Below the apex of the fan . . . the channel will occur at random locations at any place on the fan surface; under natural conditions, it is not more likely to follow a preexisting flowpath than it is to follow a new flowpath" (Federal Emergency Management Agency, 1985a). Graf (1988) has also concluded that "Present geomorphological theory does not provide for the calculation of the probability that any particular distributary channel will receive flow. On natural fan surfaces the flood hazard is therefore largely indeterminate." ...
On February 12, 1992, a portion of the Ventura River, California, flowed through the Ventura Beach RV Resort which had recently been constructed across a major historically active distributary of the Ventura River delta. State and local land-use planners recognized the flood hazards associated with the site, but decision-makers relied on analytical methods developed by the U.S. Army Corps of Engineers and flood-hazard categories developed by the Federal Emergency Management Agency, which did not adequately reflect the mobile nature of the Ventura River channel and distributaries. A better understanding of the historical behavior of the Ventura River could have averted the flood damages experienced in 1992. Low intensity recreational, agricultural, or habitat restoration use of the site would minimize potential flood damages and obviate the need for structural flood protection that would impact the rivers natural resources. Continued operation of the recreational vehicle park could result in additional flood damages in the relatively near future; recognizing the limitations of the flood-modeling methodologies used for the Ventura Beach RV Resort could prevent similar miscalculations of flood potential in comparable situations.
... The massive (structureless) loamy unit V along Wadi Ruth alluvial plain, shows evidence of transport of upstream loess into the dunefield (Fig. 14b) through flows with high suspended sediment concentrations, typical for this area (Alexandrov et al., 2003(Alexandrov et al., , 2007Schwartz and Greenbaum, 2009). Flows over and along floodplains are usually shallow (Schick and Lekach, 1987;Graf, 1988;Reid and Frostick, 2011). However, here the overlying 0.5 m thick unit V (Figs. 8 and 9) suggests aggregation, linked to rapid deposition as the fluvial system flows into the dunefield. ...
Dunefield margins are sensitive to aeolian-fluvial processes that record local environmental conditions and climatic events. A common aeolian-fluvial process along dunefield margins is dune-damming—marking aeolian domination over a fluvial system. This study reports on the character, controls, and chronology of a transition from a pre-dunefield environment to a fluvial-dominated environment, at the intersection of the northwestern Negev desert (Israel) dunefield margins with a medium-sized (100 km2) drainage basin. Relative and absolute Optically Stimulated Luminescence (POSL/OSL) dating along several radiocarbon dates and sedimentological analysis helped decipher the chronostratigraphy of nine sections in the drainage basin where it meets the dunefield, and reconstruct landscape evolution.
The study reveals immediate and lagged fluvial response to aeolian domination during the Heinrich-1 and Younger-Dryas when the region was subject to exceptional wind power. The domination of vegetated linear dune encroachment over the drainage basin led to a temporal and spatial succession of dune-dammed waterbodies. Aeolian-dominated fluvial sedimentation resulted in a sequence of distinct event-based couplet depositions. Decreased wind power following the Younger-Dryas stabilized the properties of the dune-dams, while increasing fluvial activity during the early-Holocene, upon the stabilized dune-dams, led to aggregation of couplets in water bodies. These sediments formed the maximal extent of the dunefield margins alluvial plain. After the dune-dam breaching at the dunefield margins, fluvial penetration into the dunefield resulted in a mid-Holocene dune-damming impoundment within the dunefield. Altogether, the lagged fluvio-sedimentological response to aeolian-domination significantly contributed to the aggregation and extension of alluvial plains, shaping the dunefield margin landscape.
... Flash floods are typically produced in small (o100 km 2 ) catchments as a result of intense convectional precipitation that occurs on steep slopes with bare rock outcrop or crusted surfaces, which can lead to high runoff coefficients and a dominance of Hortonian overland flow. Most research attention has focused on channeled flash floods but unchanneled flash floods ('sheetfloods') have been reported from piedmont settings (McGee, 1897;Jutson, 1919;Davis, 1938;Hogg, 1982;Graf, 1988aGraf, , 1988b. Field measurements and modeling simulations illustrate how flash-flood hydrographs are characterized by steep rising and receding limbs and short time bases that may be only a few hours (e.g., Walters, 1989;Rhoads, 1990;Hassan, 1990a;Dick et al., 1997;Belmonte and Beltrán, 2001;Cigizoglu et al., 2002). ...
The idea that dryland fluvial environments are somehow distinctive or even unique compared to fluvial environments in wetter and/or cooler climatic settings has arisen in much of the literature, despite the fact that drylands are themselves inherently diverse, covering a wide range of local climatic, tectonic, structural, lithological, and phytogeographical settings. Focusing on rivers in warm drylands, this chapter outlines the main characteristics of dryland rivers, including flow and sediment transport conditions, channel forms and dynamics, channel and floodplain sedimentology, and equilibrium and nonequilibrium behavior. Although dryland rivers are commonly ascribed a set of restricted characteristics that have been derived mainly from studies in short, steep, tectonically active catchments, investigations in larger, lower-gradient, tectonically stable catchments have revealed much greater diversity in river character both within and between drylands, and have demonstrated overlap with river characteristics in other climatic settings. With this global perspective, I argue that many previous statements regarding the distinctiveness of dryland river characteristics either can no longer be sustained as generalizations, or, at the very least, the geographical and geomorphological contexts for those generalizations need to be clarified. One of the key priorities for future research will be to identify and explain the spatial extent and frequency of occurrence of river styles across different drylands. In particular, research into the controls of meandering, straight, and anabranching rivers characteristic of some moderate-to low-energy Southern Hemisphere drylands has not only contributed to greater appreciation of the global diversity of dryland rivers but is also leading to a better understanding of rivers in general. Technological and methodological developments (especially in geochronology) are enhancing understanding of dryland river process, form, and change, and this will continue to facilitate interdisciplinary work in diverse scientific and applied contexts.
... In low-gradient dryland rivers, well-defined floodplains may be entirely absent (Tooth, 2000). And, in braided channel systems, floodplains may exist as areas above low terraces (Graf, 1988) or undefined zones adjacent to the channel characterized by a composite mixture of alluvial features (Parsons, 1994). ...
Elevation-based hydrogeomorphic models are widely used to parsimoniously delineate floodplains with limited input data using topographic gradients to distinguish floodplain areas from hillslopes. Hydrogeomorphic models generally use scaling laws to assess flood flow depths as a function of contributing drainage areas. Floodplains are consequently mapped as those areas underlying maximum flood levels. Recent scientific literature has demonstrated that hydrogeomorphic models consistently perform from regional to global scale, validating geomorphic floodplain delineations in diverse morphological and hydroclimatic river settings. Nevertheless, the relationship between model fidelity, basin morphology, and basin hydroclimatic conditions is still unclear. Specifically, further investigations on the applicability of scaling laws in semi-arid and low-gradient basins is needed. In this work we investigated how climatic variability and basin slope can influence the parameterization of the abovementioned scaling laws in support of hydrogeomorphic floodplain modelling. Eleven basins in the west-central United States were selected as case studies. This research demonstrated that sub-basins slope and annual rainfall are the most influential morphometric and climatic parameters on scaling law regressions. Specifically, we found that scaling relationships are inconsistent in defining semi-arid basin floodplains (average annual rainfall lower than 570 mm) with low-gradient valley slopes (lower than 5%).
... These boulders were transported onto the plains to the west as a component of the bedload which was dominated by gneiss-derived fine gravel (Figure 6). At the apex of the terminal floodout (sesu Graf, 1988;Tooth, 1999), unit stream power fell abruptly as the flow widened. A significant number of large boulders (2 to 5.9 m long axis) were transported for several kilometres in the fine-gravel bedload ( Figure 6). ...
The aeolian regime of the 100 km wide, hyperarid Namib Desert has been sporadically punctuated by the deposition of fluvial sediments generated during periods of higher humidity either further inland or well within the desert from Late Oligocene to Late Holocene. Four new Late Cenozoic formations are described from the northern Skeleton Coast and compared with formations further south: the Klein Nadas, Nadas (gravels, sands), Vulture’s Nest (silts) and Uniab Boulder Formations. The Klein Nadas Formation is a trimodal mass-flow fan consisting of thousands of huge, remobilised, end-Carboniferous Dwyka glacial boulders, many >3 m in length, set in an abundant, K-feldspar-rich and sandy matrix of fine gravel. Deluge rains over the smallest catchments deep within the northern Namib were the driving agent for the Klein Nadas Fan, the termination of which, with its contained boulders, rests on the coastal salt pans. These rains also resulted in catastrophic mass flows in several of the other northern Namib rivers. The Uniab Boulder Formation, being one, consists only of huge free-standing boulders. Gravelly fluvial deposition took place during global interglacial and glacial events. The Skeleton Coast Erg and other smaller dune trains blocked the rivers at times. The low-energy, thinly bedded silt deposits of the central and northern Namib are quite distinctive from the sands and gravels of older deposits. Their intermittent deposition is illustrated by bioturbation and pedogenesis of individual layers. Published offshore proxy climatological data (pollens, upwelling, wind, sea surface temperatures) point to expansion of the winter-rainfall regime of the southern Cape into southwestern Angola during strong glacial periods between the Upper Pleistocene and Holocene. In contrast to deposition initiated by short summer thunder storms, we contend that the silt successions are river-end accumulations within which each layer was deposited by runoff from comparatively gentle winter rains that lasted several days.
... mud flood). Flooding has a geomorphic context, and the term flooding may be defined as: the result of flow exceeding channel bankfull capacity through overflow (Graf, 1988). Flooding occurs when water accumulates in a channel as a result of watershed runoff, exceeds bankfull capacity, and overflows on to the surface adjacent to the channel. ...
An assessment of post wildfire runoff and debris flow hazards for pre-fire planning.
... A general increase of vegetation cover over the last century is found also in the wider northern Ethiopia highlands (Descheemaeker et al., 2006;Frankl et al., 2011Frankl et al., , 2013Nyssen et al., 2014). In the Hara catchment, the normal- According to Graf (1988b), during periods of relatively more frequent and lower magnitude flows, braided streams develop compound meandering channels set across the braided valley bottom, which could increase channel width by lateral migration and bank erosion. ...
The magnitudes of flash floods and their effect on channel width changes were investigated for Hara river in the Raya graben (northern Ethiopia). Precipitation was measured using rain gauges evenly distributed over the study catchment. Event peak discharges were measured with crest‐stage gauges at a reference cross‐section. Changes in channel width were measured at 6 monitoring sites along a 1.5 km long reach. Decadal channel width change was analyzed over the period 1965‐2014. The average rainfall depth of the rainy days was 14.2 ± 9.4 mm whereas the highest was 41.9 mm and the lowest, capable to generate some flow in the river, was only 1.8 mm. The maximum calculated discharge peaked at 408 m3 s‐1 whereas the lowest flow was 19.5 m3 s‐1. An average widening of 0.11 ± 0.19 m per flood (for a total of 3.74 m) was measured. The cumulative precipitation and cumulative channel widening suggest that larger and longer lasting rains have substantial influence on channel width change. A marked increase in channel width commonly accompanies two‐three days of flood duration; it is less marked where there is riparian vegetation on the banks. The long term (1965, 1986, 2005, and 2010) measurements indicate that channel width increased rapidly over the last 50 years; nearly 25 m of widening. Generally, channel changes are mainly induced by the duration of floods rather than by their intensity and the data clearly show that ephemeral streams channel dynamics can be triggered also by floods much smaller than bankfull.
... The design of environmental flows (sensu Arthington, 2012) may also benefit from a better understanding of the effects of natural large floods on both abiotic and biotic components of riparian ecosystems, which may greatly differ across rivers. For example, given that the size of large floods relative to base flows is greater in arid regions than in humid regions (Graf, 1988a), native riparian vegetation could be more resistant and resilient to flood disturbance in dryland rivers (Stromberg et al., 1993). For the same reason, the geomorphological effects of large floods tend to increase with aridity (Friedman & Lee, 2002). ...
In December 2010, a 40‐yr flood occurred in the lower Virgin River (SE Nevada, southwestern U.S.), a braided river segment with riparian vegetation largely dominated by invasive shrubs in the genus Tamarix. We assessed geomorphological and vegetation responses to this large magnitude disturbance event by comparing pre‐ and post‐flood remote sensing and field survey data in four river reaches. Analyses of orthophotos and LiDAR‐derived topography showed that both the active channel area and channel width increased between ~80 and 258%, representing an increase from 13% to 30% of the total river corridor area. Erosion predominated in the outer bends of the enlarged channel and deposition in the pre‐flood channel, causing local avulsions of the low‐flow channel. Field‐based topographic data recorded before and after the flood in 385 plots also showed that deposition occurred in parts of the floodplain that were not eroded. Two thirds of woody vegetation cover (mainly dominated by Tamarix, with some native shrub Pluchea sericea) was lost in areas that eroded (~20% of the river corridor). In the remaining ~80% of stable river corridor (aggrading or no change in elevation) Tamarix remained dominant. Following erosion, but also where sediment deposition predominated under the Tamarix canopy, the most common colonizing vegetation in the understory was comprised of annual plants, especially Salsola tragus. Our study supported previous studies describing large floods in braided rivers: we documented the first phase of a cycle of channel widening and increase in vegetation heterogeneity that is commonly followed by narrowing and vegetation homogenization.
... Lichvar & McColley (2008) call it the most common channel type in dry regions. The lower Salt River and portions of the Gila River in Arizona were classic examples of compound channel patterns (Graf, 1988). With respect to navigability, the following apply to compound channels: o The inset meandering low flow channel is of more importance to a navigability determination than the braided flood channel, since the inset, main or low flow channels convey the non-flood, i.e., ordinary, flows. ...
Every State receives title to the beds of its navigable rivers when it joins the Union. The streambeds of navigable rivers become sovereign lands to be held for the public trust and to preserve corridors of trade and travel on the rivers. States own the beds of only the navigable waterways. Therefore, before a State can establish a claim to a specific river, it must first demonstrate that river was, or could have been, navigated by boats or used to float logs as of the time of Statehood. Furthermore, federal law requires that each river’s navigability be determined at a time when it was in an “ordinary and natural” condition, rather the condition of the river at the time of the navigability determination, or at Statehood, if the river’s navigability had already been altered by humans.
While federal courts require consideration of a river’s navigability in its ordinary and natural condition, the courts have provided only minimal direction regarding the scientific definition of the terms “ordinary” and “natural” with respect to navigability. This paper recommends technical definitions for both terms relative to a river’s flow rate, as well as to the physical condition of the river’s boating channel. Ordinary flow is defined as the range of flows between base flow and the flow rate that creates the river’s ordinary high-water mark. The ordinary physical condition of the river is defined as the portion of the floodplain that includes the low flow channel up to the ordinary high-water mark. Natural is defined as the conditions (for both flow rate and channel characteristics) absent the impact of humans. Recommended procedures are provided that can be used to determine if a river is still in its natural condition, or if not, how to obtain information that describes the river’s boating characteristics prior to human impacts on navigability.
In addition, five case histories are briefly summarized to illustrate the types of technical issues that have been raised in recent navigability decisions in Arizona and Alaska regarding the rivers’ ordinary and natural conditions.
... To calculate the peak flow discharged from each catchment, the Soil Conservation Service ( SCS, 1972) method is used. This method was developed originally to be used in small agricultural catchments in USA, and now is adapted to be used in arid areas of Saudi Arabia and USA ( Graf, 1988;Walters, 1990). Also, the SCS method is approved to be used in rural roads protection according to the Egyptian code of practice for urban and rural roads ( ECP, 2008). ...
In the mountainous area of Assiut plateau in southwestern Egypt, a proposed road is designed to pass through a flood path near Durunka village. A reliable and accurate information about natural hazards, could occur in this region especially flash floods, is required. Due to the lack of this information, a runoff model is built adopting the Soil Conservation Service method for an un-gauged watershed of Assiut plateau. The hydrologic characteristics and soil type and cover for the study area are estimated during a site visit. The watershed is delineated and the morphometric parameters are derived from analyzing the SRTM3 Digital Elevation Model (DEM) using Geographic Information System (GIS) to construct a hydrological model. This model provides a good estimate about the magnitude of flash floods including the water velocities and depths using HEC-RAS. Two rainfall events are used in this analysis, the first event is a storm with 50 years return period, and the second event is for highest precipitation value recorded in this area. The model showed that the maximum water depth could occur is 4.01 m and the highest water velocity is 11.75 m/s. The results of this study could help the decision makers in protecting the proposed road and to minimize the flood hazards.
... Each site was located within a confined river setting with confinement provided by either bedrock or highly resistant macrochannel banks. Macrochannels, or compound channel-in-channels, have been described in a range of hydrologically variable settings globally (Woodyer, 1968;Graf, 1988;Gupta et al., 1995Gupta et al., , 1999Van Niekerk et al., 1995) and ...
The application of palaeoflood hydrology in Australia has been limited since its initial introduction > 30 years ago. This study adopts a regional, field-based approach to sampling slackwater deposits in a subtropical setting in southeast Queensland beyond the traditional arid setting. We explore the potential and challenges of using sites outside the traditional physiographical setting of bedrock gorges. Over 30 flood units were identified across different physiographical settings using a range of criteria. Evidence of charcoal-rich layers and palaeosol development assisted in the identification and separation of distinct flood units. The OSL-dated flood units are relatively young with two-thirds of the samples being < 1000 years old. The elevation of all flood units have resulted in estimated minimum discharges greater than the 1% annual exceedance probability. Although these are in the same order of gauged flood magnitudes, > 80% of them classified as ‘extreme event’. This study opens up the renewed possibility of applying palaeoflood hydrology to more populated parts of Australia where the need for improved estimation of flood frequency and magnitude is now urgent in light of several extreme flood events. Preliminary contributions to improve the understanding between high magnitude floods and regional climatic drivers are also discussed. Recognised regional extreme floods generally coincide with La Niña and negative IPO phases, while tropical cyclones appear to be a key weather system in generating such large floods.
... In many dryland rivers, river flow and its associated variability maintains the complexity of the in-channel environment (Graf, 1988;Thoms and Walker, 1993;Thoms and Sheldon, 1997). The development of water resources directly affects the variability of flow and these changes are easy to model (see Maheshwari et al., 1995;Thoms and Sheldon, 2000b). ...
... In many dryland rivers, river flow and its associated variability maintains the complexity of the in-channel environment (Graf, 1988;Thoms and Walker, 1993;Thoms and Sheldon, 1997). The development of water resources directly affects the variability of flow and these changes are easy to model (see Maheshwari et al., 1995;Thoms and Sheldon, 2000b). ...
... But over longer time spans, fl oods are likely to be responsible for fl ushing stored sediment from channel networks and thus maintaining stream char-acteristics ranging from channel longitudinal profi le to channel width/depth ratio and pool volume. Flood magnitude and frequency have been repeatedly identifi ed as key controls on channel form and stability (Graf, 1983(Graf, , 1988a(Graf, , 1988bGrimm et al., 1991;Scott et al., 1996); so management of dryland channel networks may need to include regulation of fl ood magnitude, which is not commonly done at present. ...
Dryland channel networks share many similarities with channel networks in more humid regions, but they are also unique in having: extreme temporal and spatial variability in rainfall, runoff, and both hillslope and channel processes; poor integration between tributary and main channels; dominantly ephemeral or intermittent flow; and lack of equilibrium between process and form. Floods are likely to be particularly important in dryland channels, and riparian vegetation exerts a strong influence on channel processes and form. Land managers working in dryland channel networks particularly need to answer the following questions: What is stable? What is the role of disturbance? How do ecosystems depend on physical form and process? This paper explores methods for determining thresholds and resiliency within a channel network and suggests metrics that can be used to assess the condition of a channel segment or entire drainage network relative to management goals. The management metrics focus on flow regime, sediment supply, bed grain size, bedform configuration, width/depth ratio, bed gradient, channel planform, and extent and type of riparian vegetation. For each of these metrics, geological, historical, and systematic records can be used to define the natural range of variability for a particular channel form in the absence of direct land-use impacts. The range of variability present under land use such as military training activities can then be compared to the natural range to assess whether these activities are negatively affecting the dryland channel network. The Yuma Wash drainage in the Yuma Proving Ground, Arizona, is used as a case study.
... Flash floods are typically produced in small (o100 km 2 ) catchments as a result of intense convectional precipitation that occurs on steep slopes with bare rock outcrop or crusted surfaces, which can lead to high runoff coefficients and a dominance of Hortonian overland flow. Most research attention has focused on channeled flash floods but unchanneled flash floods ('sheetfloods') have been reported from piedmont settings (McGee, 1897;Jutson, 1919;Davis, 1938;Hogg, 1982;Graf, 1988aGraf, , 1988b. Field measurements and modeling simulations illustrate how flash-flood hydrographs are characterized by steep rising and receding limbs and short time bases that may be only a few hours (e.g., Walters, 1989;Rhoads, 1990;Hassan, 1990a;Dick et al., 1997;Belmonte and Beltrán, 2001;Cigizoglu et al., 2002). ...
The idea that dryland fluvial environments are somehow distinctive or even unique compared to fluvial environments in wetter and/or cooler climatic settings has arisen in much of the literature, despite the fact that drylands are themselves inherently diverse, covering a wide range of local climatic, tectonic, structural, lithological, and phytogeographical settings. Focusing on rivers in warm drylands, this chapter outlines the main characteristics of dryland rivers, including flow and sediment transport conditions, channel forms and dynamics, channel and floodplain sedimentology, and equilibrium and nonequilibrium behavior. Although dryland rivers are commonly ascribed a set of restricted characteristics that have been derived mainly from studies in short, steep, tectonically active catchments, investigations in larger, lower-gradient, tectonically stable catchments have revealed much greater diversity in river character both within and between drylands, and have demonstrated overlap with river characteristics in other climatic settings. With this global perspective, I argue that many previous statements regarding the distinctiveness of dryland river characteristics either can no longer be sustained as generalizations, or, at the very least, the geographical and geomorphological contexts for those generalizations need to be clarified. One of the key priorities for future research will be to identify and explain the spatial extent and frequency of occurrence of river styles across different drylands. In particular, research into the controls of meandering, straight, and anabranching rivers characteristic of some moderate- to low-energy Southern Hemisphere drylands has not only contributed to greater appreciation of the global diversity of dryland rivers but is also leading to a better understanding of rivers in general. Technological and methodological developments (especially in geochronology) are enhancing understanding of dryland river process, form, and change, and this will continue to facilitate interdisciplinary work in diverse scientific and applied contexts.
... Su morfología y dimensiones son modeladas por sobrantes energéticos, de manera que quedan ajustadas para poder conducir las crecidas ordinarias entre las orillas. En ríos y ramblas mediterráneos los procesos controlan a las formas en episodios de gran magnitud, mientras que las formas controlan a los procesos con caudales medios y bajos (Graf, 1988). El perfil longitudinal, generalmente resuelto en una sucesión de rápidos y remansos, es también un mecanismo automático que regula el transporte y da unidad y continuidad a todo el sistema. ...
... Su morfología y dimensiones son modeladas por sobrantes energéticos, de manera que quedan ajustadas para poder conducir las crecidas ordinarias entre las orillas. En ríos y ramblas mediterráneos los procesos controlan a las formas en episodios de gran magnitud, mientras que las formas controlan a los procesos con caudales medios y bajos (Graf, 1988). El perfil longitudinal, generalmente resuelto en una sucesión de rápidos y remansos, es también un mecanismo automático que regula el transporte y da unidad y continuidad a todo el sistema. ...
... The research also shows how channel change in dryland river systems is episodic and spatially variableconditions that reflect an inherent lack of equilibrium adjustment in these systems. A practical implication of this research is that the complexity of adjustments between process and form in dryland rivers complicates the definition of floodplains compared to floodplain delineation for stable humid-temperate rivers (Graf, 1988b). ...
This special issue honors the contributions of William L. Graf to geomorphology and river science. A hallmark of Will's work over the course of his career has been a focus on the natural and human structuring of river systems. More broadly, Will has been an innovator and leader who has shaped the way in which geomorphologists conduct research. Through his work, he has made fundamental contributions to basic fluvial theory, to the understanding of human impacts on river systems, and to policy-relevant science. He has demonstrated by example how to pursue policy-relevant science and to participate in science-based policy formulation. His contributions to river science can be classified into several themes: (1) the hydrology and geomorphology of suburban drainage systems, (2) riparian vegetation and river systems, (3) the spatial structure and dynamics of incised channels, (4) the dynamics of dryland river systems, (5) heavy metals in river systems, (6) dams and dam removal, and (7) water and public policy. The papers in this special issue reflect many aspects of these themes and address topics related to (1) the understanding of rivers and other geomorphic systems in the midst of dynamic physical change, (2) human influences on geomorphic processes, (3) the intersection of geomorphology and public policy, and (4) the fusion of geomorphic analysis and GIScience.
... The inset floodplains are discontinuous in nature, and range in length and width 13-226 m and 3-25 m, respectively. Similar compound channels have been reported elsewhere in the Murray-Darling Basin (Thoms and Walker, 1993) and in other dryland river systems (Graf, 1988). Although the geomorphology of these compound channel features is not well known, a number of different inset floodplain forms have been suggested. ...
This paper investigated the textural character of surface sediments across a range of inset-flood plain surfaces on the Barwon Darling River, Australia. Surface sediments ranged in size from clay to coarse sand (-1Φ) - <4.75Φ) but varied in composition between different inset-flood plain surfaces. Multivariate entropy analysis detected five sediment classes based on the grain size distributions of individual samples. River channel sediments were present in two of the entropy classes, whereas the different inset-flood plain surfaces were present in four or more of the identified entropy classes. A number of factors may be influencing the spatial distribution of sediment texture across the inset-flood plain surfaces including: (1) decreasing energy gradients with increasing elevation from the channel; (2) variable sediment supply conditions during flow events; and (3) local sediment inputs. The resulting patterns found in the study area demonstrate there to be a mosaic of sediment texture patches rather than a simple gradient of changing sediment texture with increasing distance from the channel.
... Flash floods are typically produced in small (o100 km 2 ) catchments as a result of intense convectional precipitation that occurs on steep slopes with bare rock outcrop or crusted surfaces, which can lead to high runoff coefficients and a dominance of Hortonian overland flow. Most research attention has focused on channeled flash floods but unchanneled flash floods ('sheetfloods') have been reported from piedmont settings (McGee, 1897;Jutson, 1919;Davis, 1938;Hogg, 1982;Graf, 1988aGraf, , 1988b. Field measurements and modeling simulations illustrate how flash-flood hydrographs are characterized by steep rising and receding limbs and short time bases that may be only a few hours (e.g., Walters, 1989;Rhoads, 1990;Hassan, 1990a;Dick et al., 1997;Belmonte and Beltrán, 2001;Cigizoglu et al., 2002). ...
IntroductionDistinctiveness of dryland riversDiversity of dryland riversReassessing distinctiveness and diversityConclusions
References
... Although the rainfall is episodic in this region, studies indicate that solute flushing from semi-arid uplands and hill slopes in response to summertime rainfall results in elevated streamflow solute concentrations and sediment transport [5,6]. In addition, watersheds in the desert west/southwest often release water almost immediately after a storm [7] due to sparse vegetation, steep topography, complex soils and rapid land development changes. ...
Urbanized areas of the southwestern/western United States are among the fastest growing in the nation and face multiple water resource challenges. Low impact development (LID)/green infrastructure (GI) practices are increasingly popular technologies for managing stormwater; however, LID is often not as common in the southwest/west due to the lack of regulatory and/or economic drivers. There is also a lack of performance evaluation of these practices, particularly at the field scale. This study focused on investigating the hydrologic and pollutant removal performance of field-scale LID/GI systems in arid/semi-arid climates. Nine typical practices were reviewed: rainwater harvest system, detention pond, retention pond, bioretention, media filter, porous pavement, vegetated swale/buffer/strip, green roof, and infiltration trench, as well as integrated LIDs. We evaluate these practices by a cost-effectiveness analysis and also recommend best practices for the arid/semi-arid area. The analysis provides data support and insights for future implementation of LID/GI in the southwest/west.
... Pero la dinámica de estos sistemas hace que sus efectos morfológicos se puedan reproducir en cualquier parte de la cuenca y a diferentes escalas, lo que le confiere a cada tramo del sistema una independencia propia y una capacidad de autoajuste (Poesen et al., 2003). Los procesos geomórficos, así considerados, adquieren una elevada capacidad de retroalimentación y sinergia que hacen difícil su estudio si no es abarcado de forma integral (Graf, 1988;Qu & Duffy, 2007). ...
El presente artículo tiene como objeto mostrar las modificaciones impuestas por el hombre en uno de los sistemas hidromorfológicos semiáridos más activos y peligrosos de la Región de Murcia: la cuenca de la Rambla de las Culebras, Águilas (Murcia). Tras una descripción somera de las condiciones ambientales que controlan el funcionamiento hidromorfológico de esta cuenca, se analizan los rasgos del sistema de ramblas del Renegado y de las Culebras, en particular su régimen hidráulico y los aportes torrenciales asociados a su elevada capacidad de transporte. A continuación, se estudian los cambios morfológicos inducidos por la acción antrópica en el conjunto de la cuenca, entre ellos los ocasionados por la reactivación geomorfológica de las laderas, así como los observados en la red de drenaje, cauces y lechos de inundación.
This paper show the changes imposed by human activities in one of the most dangerous and active semiarid hydromorphological systems on the Region of Murcia: Basin of Rambla de las Culebras (Águilas, Murcia). Systems properties of ramblas del Renegado and las Culebras have been analyzed, mainly hydraulic conditions and torrential inputs associated caused by high transport capacity. Finally, morphological changes induced by anthropic action in the whole basin have been studied, including those caused by hillsides geomorphological reactivation and those observed in the drainage network, stream bed and floodplain.
... En el caso de ramblas mediterráneas, Brakenridge (1988) habla de sistemas fluviales dominados por sucesos extraordinarios, con una "configuración activa", por oposición a la "configuración estable" de zonas húmedas. En estos sistemas los procesos controlan a las formas en episodios de gran magnitud, mientras que las formas controlan los procesos cuando la magnitud de los episodios es baja (Graf, 1988). ...
Mesa de trabajo de la Estrategia Nacional de Restauración de Ríos. Temática: alteraciones geomorfológicas en cauces; la restauración de sistemas fluviales afectados por canalizaciones y dragados. Fundamentos, problemática, soluciones, debate y conclusiones.
... La evidencia histórica ha servido en muchas ocasiones para reconstruir los cambios de modelo de cauce (Hooke y Kain, 1982), llegándose a consultar, con bastante frecuencia, fuentes documentales muy numerosas y de naturaleza diversa; por ejemplo, Graf (1988) utilizó, a lo largo del río Gila, en Arizona, más de quince fuentes históricas para identifi car tramos de estabilidad y de riesgo hidrogeomorfológico. En Devon, Hooke (1977) realizó un estudio detallado del trazado del río, comparando los modelos que aparecían en mapas de planimetría de diezmos con los de algunas ediciones de mapas de gran escala del Servicio Ofi cial de Topografía. ...
Distinguir los impactos humanos en la morfología del cauce a partir del comportamiento natural
de los sistemas fl uviales resulta a veces problemático. A los factores geofísicos que controlan en
mayor o menor grado la forma del cauce hay que añadir la superposición, a menudo compleja,
de las alteraciones impuestas por la acción del hombre. El presente artículo constituye una
revisión del estado actual de la cuestión, sobre todo en lo que afecta a los cambios históricos
recientes de modelos de cauce y a las variaciones de trazado fl uvial en llanuras aluviales,
incluyendo las producidas dentro del ámbito mediterráneo español.
Distinguishing human impacts on channel morphology from the natural behaviour of fl uvial
systems can be problematic. River basins typically encompass wide ranges of geophysical
factors resulting in diverse controls on channel form. They also can incorporate long and complex histories of overlapping human disturbances. This paper shows a current review of the subject, especially in what it concerns the historical and recent adjustments in channel patterns and the planform changes of sinuous rivers in alluvial plains, including those of the Mediterranean Spanish area.
... This results in significant downstream decrease in channel capacity of transporting water and sediment (Tooth, 2000a;Donselaar et al., 2013). It is in these peak discharge periods that the major geomorphological changes and landscape development take place, such as the formation and/ or expansion of crevasse splays, changes in river course, and channel avulsion (Baker, 1977;Graf, 1983Graf, , 1988a, 1988b;Knighton and Nanson, 1997). However, it is difficult to access or directly observe the geomorphological changes, due to the hazardous environment during peak discharge. ...
In this remote sensing-based study, we present the analysis of the geomorphological development at the low-gradient terminus of the modern Río Colorado dryland river system in the endorheic Altiplano Basin (Bolivia). Changes in the river morphology occur after short periods of catastrophic peak discharge which result in the expansion of existing crevasse splays, formation of new crevasse splays and in river path avulsion. Episodic peak discharge events in the study area were pinpointed and quantified by combining daily precipitation records from gauging stations in the vicinity with catchment area analysis from ASTER global DEM (GDEM) remote sensing data. A time series of Landsat imagery for the period 1975–2001 was then used to analyze the river morphology changes after major peak discharge events. Compensational stacking of crevasse splays in combination with river avulsion produced a thin but aerially extensive connected sand sheet at the terminus of the fluvial system.
... phenomenon is very typical of ephemeral desert rivers, which accommodate rapid changes in discharges and consequently undergo radical morphological adjustment (Graf, 1988). Nanson (1986) The environmental signal is strongly modified as it propagates through the fluvial system: passing geomorphic thresholds below the dam and maintaining near-equilibrium conditions as the distance increases, drainage area increases, and discharge wanes (Schumm, 1979). ...
he Nahal Oz Reservoir - in the coastal, semi-arid southwestern Israel was designed to enhance local irrigation of crops using reclaimed sewage water during the dry summer months. On March 2001, part of the western dike of the reservoir was breached and generated a flow release of 3.5*106 m3 of secondary irrigation water that was channeled down the 1st order ephemeral loess stream (Nahal Yare'akh). The consequent 12-hour flood surge, with an estimated peak discharge of 1000 m3s-1, inflicted severe loess erosion, agricultural, property and infrastructural damage downstream ($6 million).
Post-flood mapping documented the geomorphic response to the flood which included channel scour and widening along the initial 2 km downstream of the reservoir where a spillway channel was formed. The increase in the cross-sectional area was about 60% and had an estimated 170,000 m3 of sediment bed, bank and floodplain erosion. Calculated maximum shear stress and stream power along this section are estimated at 300 Nm-2 and 900 wm-2, respectively. The peak discharge at the end of this segment was estimated at 800 m3s-1 indicating only minor attenuation along this segment. Two km downstream of the breach, a wide braided fan indicated deposition of the eroded sediments. At the end of this segment the floodwater diverged into several watercourses and inundated tilled agricultural fields and neighborhoods. Downstream, 9 km from the reservoir, the discharge attenuated to 100 m3s-1, slightly above bankfull. Further downstream and upon reaching the large Shikma stream the flow was already very low. This reduction in discharge is attributed to the anthropogenic infrastructure - roads, neighborhoods and agricultural fields and the large transmission losses typical of sandy ephemeral streams.
The study shows that channels within erodible materials respond to high peak discharges very locally. Erosional thresholds that severely incised the channel are only maintained for 2 km below the breached reservoir and as distance from the source area increases, available energy is substantially reduced due to high transmission losses, gentler valley gradients, very wide shallow flow and the ability of the drainage network to accommodate large discharges. Consequently, the geomorphic impact was limited to the first 2.6 km with only minor erosional or depositional evidence downstream. The current natural flow regime has only minor impact on the newly formed channel geometry below the breached dam. Accordingly, the channel geometry may be preserved for a relatively long time. However, channel recovery in this segment was rapid and within a few years all bare loess surfaces exposed during the flood have been covered by dense vegetation. The analysis also shows that Nahal Oz Reservoir’s dam-break flood transmission losses and flood attenuation rates were extreme in comparison to other case studies. This artificial flood event in a desert environment offers a rare opportunity to quantify channel and bank erosion evolution processes during an extremely high-magnitude flood within highly erodible fine sediment as well as to detect the recovery processes a few years later.
... The vertical loess banks (up to 5 m high) are currently stable except for minor dry mass wasting processes (falls and slides) of the loose material producing conic aprons that are not evacuated by the present channel flows (Fig. 4b). This "inner channel" phenomenon is very typical of ephemeral desert rivers, which accommodate rapid changes in discharges and consequently undergo radical morphological adjustment (Graf, 1988). Nanson (1986) suggested a model in which high energy floods instantaneously destroy floodplains that were slowly built by vertical accretion over decades or centuries. ...
... Due to the temporal and spatial variability of rainfall in dry land areas, most arid rivers are ephemeral, flowing only during storm events and remaining dry for most of the year. As a result, the analysis of streamflow and changes in channel morphology in dry land systems emphasizes flood events (Graf 1988). ...
Several traditional techniques have been used for estimating storm-water runoff from ungauged watersheds. Some of these techniques were applied to watersheds of Rashadia in south-eastern desert of Jordan. When engineers apply rainfall-runoff models for hydrologic design, there are difficulties in defining and quantifying peak discharges that are required to design different types of hydraulic structures. The lack of data presents major difficulties for rainfall-runoff modeling in arid regions. These regions have characteristically high rainfall intensity and consequent flash floods. The specific objectives of this study are: (1) apply synthetic hydrographs for estimating peak discharges from limited hydrological data. (2) Evaluate the reliability of six techniques to accurately estimate storm-water runoff; and, to evaluate the runoff that is required to design hydraulic structures such as bridges, culverts and dams. (3) Estimate the flood resulting from direct runoff after subtracting all the loses such as: the infiltration, interflow and evaporation. (4) Develop a simple regression relationship between peak flow discharges and catchment areas. The results show that there is uncertainty in determining the accuracy of storm-water volume, this is due to several methods were utilizing the estimation the hydrographs base time, but promising results in predicting the peak flow discharge.
... Dryland river deposits are relatively poorly documented in comparison to rivers from other climatic zones. Descriptions of desert environments frequently focus entirely on eolian sediments, and where dryland rivers are described, sandy and gravelly examples often predominate (e.g., Reineck and Singh 1980;Miall 1996;Reading 1996;Graf 1988;Bourke and Pickup 1999;Cohen and Laronne 2005;Hassan 2005). However, arid-zones or dryland rivers display a great variety of form and process (Nanson et al. 2002), and when preserved in the rock record contain mudrock as well as sandstone (e.g., Wells 1983;Cowan 1993;Meadows and Beach 1993;Gierlowski-Kordesch and Rust 1994;Gierlowski-Kordesch 1998;Müller et al. 2004;McKie and Audretsch 2005). ...
Dryland rivers in which fine sediments travel as aggregates are increasingly recognized in modern and ancient fluvial systems. Fowlers Creek, Australia, is an ephemeral dryland mud-aggregate river whose sediments provide insights into the dynamics of mud-aggregate floodplains, the origin of massive mudrocks from and depositional environments, and the nature of planar bedding. Fowlers Creek's flow conditions were inferred from relationships between landforms and the sediment texture, bedforms, and sedimentary structures remaining after flow ceases. Floodplain muds, consisting of fine sand and sand-size mud aggregates, are distributed over the floodplain in suspension. As flow decelerates they are deposited as bedload. The shallow depth, high sediment load, and low aggregate particle density promote flow conditions ranging from high in the lower flow regime to upper flow regime, producing coexisting ripples, scours, flat beds, and clay layers. With time, visible signs of aggregate structure are lost, leaving a massive cohesive mud; consolidation is not achieved by burial. Aggregates reappear when muds reenter fluvial transport. In unchanneled reaches, sheetflows deposit sediment with a pervasive horizontal fabric. Channel sediments (coarse sands and gravels) are a minor component of Fowlers Creek's deposits. Widespread lower-flow-regime conditions produce planar beds and 2-D dunes, usually deposited without internal stratification or in horizontal laminae. Lower-flow-regime planar bedding is also observed in fine silty sands. Near the close of flow, rapid shallowing may move channel conditions from lower to upper flow regime, or from lower to higher positions in the lower flow regime bedform stability spectrum, leading to unusual bedform associations. In the rock record sediments from a river like Fowlers Creek would be characterized by structureless gravelly sands (channel facies), massive red mudstones (floodplain facies), and sediments with horizontal fabric but poorly expressed bedding (sheetflow facies).
... The term "fl ood" is commonly, and sometimes mistakenly, mixed with terms used to describe fl ow behavior (e.g., mud fl ood). Flooding has a geomorphic context, and the term fl ooding may be defi ned as: the result of fl ow exceeding channel bankfull capacity through overfl ow (Graf, 1988). Flooding in the alluvialfan environment is similar to fl ooding in the riverine environment-only in both cases, water "accumulates" in a channel as a result of rainfall and runoff from overland fl ow. ...
Debris flows are one of the natural hazards present in the southern Sierra Nevada of California. Historic debris flow activity is documented at a USDA Forest Service research facility in the Kings River drainage related to a 1937 storm event. No subsequent study of this phenomenon was undertaken until 1982. Observations of debris flows over the succeeding 10 years offer an initial assessment of the physical geomorphology of debris flows in this area. This information provides a starting point for future efforts to avoid or limit the effect of this natural hazard.
... The comparative planforms (Fig. 6) of the Mahi river channel in the study reach indicate that the channel has become narrower over time. The high width/depth ratio and low sinuosity for the mid-late Holocene channel are indicative of greater bedload transport (Graf, 1988). The cross-sectional area was significantly larger thus with higher channel capacity. ...
This paper attempts to quantify contemporary and palaeo-discharges and changes in the hydrologic regime through the mid–late Holocene in the alluvial reach of the arid Mahi River basin in western India. The occurrence of terraces and pointbars high above active river levels and change in the width/depth ratio can be regarded as geomorphic responses to changes in discharge. Discharge estimates are made based on the channel dimensions and established empirical relations for the three types of channels: mid–late Holocene, historic (the channel that deposited extensive pointbars above the present-day average flow level) and the present ones. The bankfull discharge of the mid–late Holocene channel was ∼55000 m3 s−1 and that of the historic channel was ∼9500 m3 s−1, some ∼25 times and ∼5 times greater than that of the present river (2000 m3 s−1), respectively. Since the mid–late Holocene, the channel form has changed from wide, large-amplitude meanders to smaller meanders, and decreases in the width/depth ratio, unit stream power and the bed shear stresses have occurred. It can be inferred that there has been a trend of decreasing precipitation since the mid–late Holocene.
The development of river channels and related fluvial landforms has probably been studied more than any other aspect of geomorphology in arid and semi-arid areas. This is due largely to the originality, forcefulness, and intellectual background of American geomorphologists developed during and sustained long after the exploration of the American West. The strong sense of field observation and experiment coupled with a deep historical perspective, both derived from a geological training, led to some of the most formative contributions to the subject. So much so perhaps that, despite the major endeavours by Europeans in denudation chronology, in climatic and glacial geomorphology, and later in hillslope and channel geomorphology, the conceptual basis of the subject at the end of the 20th century still bears a heavy imprint from the American source. In the Old World, only the Israelis, under A. Schick, have been able to maintain such a strong contribution to desert fluvial geomorphology.
Wadis emerging from the southwestern Sinai Mountains (Egypt)westwards to the Gulf of Suez are filled by >40 mthick late Pleistocene sediments, which have been subsequently incised to bedrock after the Last Glacial Maximum (LGM). Sedimentation and erosion resulted from changes in the basin's hydrological conditions caused by climate variations. Sediment characteristics indicate distinct processes ranging from high to low energy flow regimes. Airborne material is important as a sediment source. The fills are associated with alluvial fans at wadi mouths at the mountain fronts. Each alluvial fan is associated and physically correlated with the respective sediment fill in its contributingwadi. The alluvial fans have steep gradients and are only a few kilometers long or wide. The alluvial fans converge as they emerge from the adjacent valleys. According to optically stimulated luminescence dating, the initial sediment has an age of ∼45 ka and the sedimentation ends ∼19 ka, i.e., happened mainly during marine isotope stage (MIS) 3 and early MIS 2 formation and initial incision sometime during LGM. As the delivery of sediments in such a hyper-arid environment is by extreme floods, this study indicates an interval of intense fluvial activity, probably related to increased frequency of extreme floods in Southern Sinai. This potentially indicates a paleoclimatic change in this hyper-arid environment.
CSIRO Division of Wildlife and Ecology, Canberra DISCLAIMER: This study was commissioned in 1995 to investigate the natural heritage values of the Lake Eyre Basin in South Australia. At that time the possibility of pursuing World Heritage Listing for the Lake Eyre region was being considered, but this did not proceed. There are no current plans to progress a WH nomination for the Lake Eyre region. The views and opinions expressed in this publication are those of the authors and do not necessarily reflect those of the Lake Eyre Basin Ministerial Forum and its sub committees; the Australian Government; or the Queensland, South Australia and Northern Territory governments. While reasonable efforts have been made to ensure that the contents of this publication are factually correct, the above parties do not accept responsibility for any errors, accuracy or completeness of the contents, and shall not be liable for any loss or damage that may be occasioned directly or indirectly through the use of, or reliance on, the contents of this publication.
The distribution of riparian vegetation in relation to channel morphology is poorly understood in canyon rivers, which are characterized by in-channel fluvial sediment deposits rather than flood plains. This study focuses on vegetation and sandbar characteristics in two reaches of the lower Little Colorado River canyon in Arizona–one reach with ephemeral flow from the watershed, and another with perennial baseflow from a spring. Both reaches have been colonized by the exotic Tamarix chinensis, a riparian species known for its geomorphic influence on river channels. On the basis of a sampling of 18 bars, results show that vegetation frequency and density is significantly greater in the perennial study reach. However, sandbar morphology variables do not differ between reaches, despite a significantly narrower and deeper ephemeral channel. Hydraulic calculations of flood depths and Pearson correlations between bar and vegetation variables indicate reach-specific biogeomorphic relationships. In the ephemeral reach, higher bars are less affected by flood inundation, support older vegetation, and may be more stable habitat for vegetation. In the wider perennial reach where bars are lower and more expansive, vegetation patterns relate to bar size, Tamarix being most common on the largest bars. Overall results suggest that (1) vegetation variation relates to baseflow hydrology, (2) bar formation relates to high discharge events, and (3) vegetation patterns respond to, rather than influence, sandbar form in this canyon riparian system.
Palaeogeographic interpretation of the sedimentary rock record depends on correct recognition from the preserved evidence of the processes responsible for transporting and depositing the sediment. This in turn depends on robust knowledge transfer from previous workers, and the successful exchange of ideas between workers requires consistent use of a well-defined vocabulary. We have identified serious breakdowns in all these interpretation steps in the case of terrestrial unconfined flow and its deposits, and these failures are leading to unreliable environmental and climatic interpretation. This is significant because such alluvial deposits commonly form a majority of the rock record of continental environments.
Highlights
► We analysed the effect of extreme events on the morphological changes in the Quelite River and its floodplain. ► We examine the changes related to the river dynamics, specifically, on meanders and bars; migration and removal. ► The effect on the railway infrastructure was clear after the hydrological, hydraulical and morphological analysis. ► It is necessary to consider the system as part of the River Basin, looking at spatial-temporal and short-long time actions.
Floodplains are unique ecosystems because of their Linear form, the sometimes extreme dynamism of their geomorphology and because they process large fluxes of energy and materials from upstream areas. This article focuses on the importance of hydrological inputs to floodplains through 1) their influence on the arrangement of landforms and vegetation communities and 2) the connections between flooding regimes and the regeneration and turnover time of floodplain vegetation. Many researchers have demonstrated close links between the arrangement of vegetation communities and sedimentary landform types, elevation, soil characteristics, tolerance to flooding and availability of soil moisture. It is suggested that plants on floodplains are found along a combined gradient of available moisture and oxygen which can be viewed simultaneously as a flooding frequency gradient and a complex soil moisture gradient. Discussion of experimental work on floodplains demonstrates the importance of these gradients to a range of floodplain species in different environments. The relationships between these environmental gradients and the apparent high level of overlap between planform patterns of landforms and vegetation communities on floodplains are related to lag times in different parts of vegetation communities. mood regimes greatly influence the availability of areas suitable for vegetation regeneration from year to year and the age structure of floodplain communities over decadal time frames. Biotic factors also influence biogeomorphological relationships on floodplains and range from sediment-trapping by vegetation to the impacts of beaver and grazing animals on floodplain hydrology and vegetation. Restoration of floodplains is high on the agenda in many countries and it is argued that, for sustainable results, restoration of hydrological pathways is essential. Planned flood releases below darns in several African countries have had varied success rates but the development of models for managing flows to achieve different restoration targets is the start of an integrated approach to restoring complex floodplain ecosystems.
The Abarkoh Basin is situated in central part of the NW-SE-trending Gavkhoni-Sirjan depression of Central Iran. The studied fluvial megafan represents a Quaternary fan, which is located in the western part of this intermontane, extensional basin. This fail covers an area of 940 km(2) and with a gentle slope (about 0.5 degrees) over a 45 km-length terminates in a playa take in the centre of the Abarkoh Basin. Its catchment is the Abadeh Basin (> 2000 km(2)), which is a NW-SE-trending intermontane basin and mainly floored with the Quaternary alluvial-fan sediments. The studied megafan was built by stream-dominated processes, as revealed by the presence of thick-bedded, clast-supported gravels-sandy gravels (gravelly-sandy conglomerates) in the proximal-fan areas that grade into thick marls with sinuous, lenticular channel-filled, grain-supported, imbricated, pebbly conglomerates in the proximal-medial fail regions. Thick calcareous marls and lenticular, single channel-fill sandstone-conglomerate deposits characterize the distal part of the fan and pass laterally into sediments of the playa lake fringe. The aim of this paper is to present an example of a fluvial megafan in in arid-semi-arid setting, to highlight the flash-flood, sheet flood-channelized sedimentation in megafans, and to stress their importance in water resources in and lands. Fluvial fans are frequently identified in the rock record. The fluvial megafan of the Abarkoh Basin represents one of only a few described, large-scale, stream-dominated, Quaternary megafans in arid-semi-arid settings.
On pediment, alluvial slopes and plains in semi-arid environments small washes constitute potential flood hazards. Population expansion into these geologically less suitable areas represents a major engineering concern. In this study, four flood hazard zones were identified based on the hydrogeomorphic analysis of an area in central Arizona, USA. The flood hazard zones depicted as map units provide a basis for flood management plans which are highly beneficial to engineers and urban planners. As they are based on common geomorphic indicators it is considered they are applicable to other semi-arid environments.
n the arid zone, loosely defined as having rainfall below 300 mm/year, life survives at the limits of its capacity for adaptation. Survival of both plants and animals requires them to be specialists and species must have the capacity to cope with, or avoid, stress resulting from lack of moisture in their environment. Behavioural adap- tations of animals and physiological adaptations of plants and animals able to survive in the arid zone are therefore of special interest. The study of their physiology raises general issues about the limits of adaptation of which species are capable. Although areas subject to drought, the arid zone and semi-arid zones, comprise between 30 and 45 per cent of the Earth's landmass, they have been relatively little studied. Nevertheless there are several web links to organisations working in the arid zone, and at the end of this chapter web addresses are given for the Royal Botanic Gardens, Kew, the Convention on Biological Diversity and the Arid Lands Informa- tion Centre, Arizona. For an introduction to the arid zone, see Heathcote (1983) and the Action Plan resulting from the UN Conference on Desertification (UN, 1977); a number of research papers have also been produced by Unesco on specific topics. A recent arid zone resource study in Jordan sets out problems typical of the field (Dutton, Clark and Battikhi, 1988). Top tip You must be thoroughly conversant with the use of all equipment before the expedition. Heat, dust, sand and being bumped around in vehicles are not conducive to the efficient working of delicate instruments. You must know what is likely to go wrong with your equipment, have spare parts and back-up repair facilities that will get the equipment back into the field in time for you to gather sufficient data.
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