Flovial Processes in Dryland Rivers
Chapters (1)
... The work is the result of a combination of different techniques (Brown, 1999): a study of aerial photographs and satellite images; analysis of forms and deposits in the field and collection of samples; analyses in the laboratory. Fluvial processes are interpreted according to methods described in the important literature concerning dryland rivers (Graf, 1988;Tooth, 2000). ...
... Our observations thus revealed a pattern of behaviour, which, during the major part of the Holocene and all of the period of the formation of T1, was completely different in the case of this drainage channel, when compared to that of the Pulvar. The way this tributary operated is thus typical of a dryland river (Graf, 1988;Powell et al., 2007). However, during the Medieval period, the morphogenesis was similar to that observed on the plain, in terrace T2. ...
... From a hydro-morphological viewpoint, the silty deposits, homogenous and relatively well sorted, were deposited by a perennial watercourse (Graf, 1988;Tooth, 2000). This observation was true only for the Pulvar, and not its tributaries, which appear to have been mostly intermittent throughout the Holocene. ...
Located in the middle basin of the Pulvar river (Fars, Iran), Pasargadae was founded around 550 BCE in the early days of the Achaemenid Empire. Its territory is dotted with remains of imposing hydraulic facilities (dams, dikes, canals), some of which date to that very period. The purposes and functions of these structures, built to exploit surface water, located in today's landscape (a deeply incised valley with temporary watercourses), raise questions and problems for which geomorphological studies provide major elements for consideration. Erosion of the Pleistocene glacis by the Pulvar and its tributaries caused several phases of alluvial deposition during the Holocene. They can be seen today in the Pasargadae region by examining the remains of three well-developed step-like terraces. These sedimentary units are primary archives for the reconstruction of river dynamics, and thus allow a better estimation of water availability. Since 2016, within the framework of a joint Iranian-French archaeological mission, several geo-archaeological campaigns have been carried out. Sediment analyses, C-14 dating and OSL dating of sediments collected in the alluvial formations, as well as in archaeological contexts, have been made to accurately reconstruct the variations over time in the Pulvar regime, as well as the past regional waterscape. From the beginning of the Holocene to the onset of the 1st millennium BCE, the Pulvar catchment area was marked by an aggradation phase, which led to the valley being filled with alluvial deposits. This sedimentary unit provided vast areas of arable land where cultivation could be enhanced by irrigation. The creation of water inlets in the Pulvar and its tributaries was at that time facilitated by riverbeds less incised than present-day ones. Ancient hydraulic systems reflect these geomorphological conditions, since they were built to manage higher water levels, as well as stronger flows, when compared to irrigation facilities developed in modern times.
... Por isso, a principal característica hidrográfica do ambiente semiárido é o caráter intermitente e efémero dos rios, a descontinuidade dos fluxos. Essa característica está diretamente relacionada com a precipitação na bacia hidrográfica, onde o fluxo de água superficial desaparece durante os longos períodos de estiagem (GRAF, 1988;BIGARELLA, SUGUIO e BECKER, 1979). Desse modo, os processos em ambientes secos, semiáridos e subúmidos apresentam características processuais diferenciadas em relação aos ambientes úmidos, o que irá resultar em diferentes formas e estrutura no sistema (SOUZA e ALMEIDA, 2015). ...
... Esses dois eventos de perturbação hidrológica exercem forte influência na organização e no funcionamento dos rios, visto que o padrão para esses ambientes é quando os canais estão secos. Sendo assim, quando o canal apresenta fluxo está no evento de cheia, pois normalmente as respostas de aumento do fluxo nos momentos de precipitações costumam ser mais rápidas do que em regiões úmidas (MALTCHIK, 2012;GRAF, 1988;FRYIRS e BRIERLEY, 2013). ...
A pesquisa analisou a estabilidade lateral em ambientes fluviais do Semiárido paraibano na bacia Riacho do Tigre. O foco principal foi estudar a capacidade de ajuste lateral e sua relação com a energia do fluxo, através da identificação da composição do material das margens por meio de teste de textura e consistência, sendo também realizadas análise da cobertura vegetal e da distribuição da energia. Dos oitos trechos avaliados, dois foram classificados como estáveis; quatro, instáveis; e dois com estabilidade intermediária. A partir dos resultados gerados foi possível concluir que a vegetação desempenha um papel fundamental na estabilidade das margens, pois mesmo nas margens compostas de materiais pouco coesos não foram identificados indicios de processos erosivos devido à resistência exercida pela cobertura vegetal.
... Nas Terras Secas há a predominância dos rios efêmeros e intermitentes, havendo fluxo de água nos canais apenas durante as precipitações ou estação chuvosa, respectivamente (BIGARELLA; SUGUIO; BECKER, 1979). Estes serão os momentos em que o escoamento superficial irá "abastecer" o canal fluvial, e, deste modo, para entender a dinâmica fluvial é necessário compreender a relação chuva/escoamento (GRAF, 1988), como também analisar a relação magnitude/frequência dos eventos chuva/escoamento, os quais irão controlar as vazões dos canais. O entendimento atual aponta para a importância hidrológica e sedimentológica do processo do escoamento superficial nos chamados ambientes "runoff-dominated geomorphic system" (BRACKEN; CROKE, 2007), tal qual os ambientes semiáridos e subúmidos do Nordeste brasileiro. ...
... Para analisar a relação entre precipitação e escoamento é necessário avaliar o estado da bacia, porque, "se houver uma precipitação sobre a bacia previamente supersaturada, o escoamento será totalmente superficial, não havendo mesmo evaporação se a umidade do ar estiver saturada" (BIGARELLA; SUGUIO;BECKER, 1979, p. 9). A saturação do solo está relacionada com a capacidade de infiltração do mesmo, que é colocada como uma das características que apresentam forte sensitividade, com valores flutuantes relacionados ao uso e à conservação do solo, como também a alterações naturais das características do solo ou da cobertura superficial (LOVELL; JOHNSTON, 2009;GRAF, 1988). A partir dessas mudanças o volume do escoamento superficial é mutável e vai depender do estado do sistema (Burt, 2001), assim o estudo sobre a capacidade de acúmulo de água no solo torna-se essencial não só por ser o fator-chave para a mensuração da saturação e do escoamento, como também por estar diretamente relacionado com o crescimento das plantas e o tipo de vegetação da área (RAMOS; CASANOVAS, 2006). ...
Resumo: No Brasil ainda há pouca informação sobre o escoamento em pequenas bacias semiáridas, em especial para bacias que apresentam variações altimétricas relevantes, visto o pouco detalhamento dos dados climáticos. Pensando nisso, o presente trabalho analisou a relação precipitação/escoamento a partir do método do Balanço Hídrico Sequencial Diário, da Bacia do Riacho do Saco, Sertão Central pernambucano. A bacia apresenta relevo complexo com diferença altimétrica de cerca de 700 metros entre a foz (409m) e as cabeceiras (superior a 1.100m), o que reflete diretamente nas características climáticas, em especial na variação da precipitação. Deste modo, o Balanço Hídrico Sequencial Diário foi realizado utilizando dados de quatro PCDs, mapeamento de cobertura da terra e pedológico. Essas informações foram integradas gerando 24 zonas homogêneas de precipitação/solo/cobertura, sendo realizada a análise para cada uma delas. Ao avaliar os resultados, nota-se a importância da zona altimétrica da PCD Triunfo, que, apesar de representar apenas 25,6% da área da bacia, é responsável por mais de 37% do volume de água captado pela bacia, alcançando 46,31% do volume escoado em toda a bacia, sendo assim a principal fonte de captação de água para a bacia. É necessário lembrar que a alta porcentagem de escoamento está diretamente ligada ao uso agrícola indiscriminado e que altas taxas de escoamento levam a perda de água e sedimento para a bacia, e, deste modo, torna-se clara a necessidade do aumento dos níveis de infiltração para que a água seja retida e movimente-se como fluxo de base, prolongando o tempo de permanência na paisagem.
Palavras-chave: Balanço Hídrico Sequencial Diário. Semiárido; CAD máximo.
EVALUATION OF THE EFFECTIVENESS OF PROTECTED AREAS: CONCEPTS, METHODS AND CHALLENGERUNOFF AND WATER BALANCE IN DRYLANDS WITH COMPLEX TOPOGRAPHY – SACO CREEK WATERSHED – PERNAMBUCO
Abstract: In Brazil, there is still lack of data on the runoff small semi-arid basins, especially for basins that present significant altimetric variations, given the lack of detail in the climatic dat. Accordingly this research examined the relationship rain/runoff through Daily Sequential Water Balance methodology, of Saco Creek Watershed, Pernambuco central dryland. The watershed presents a complex relief with altimetric variation of 700 meters between the river mouth (409m) and the headwaters (above 1.100m), that directly reflect in the climatic characteristics, principally in the precipitation variation. The daily water balance was performed using four PCD, landcover map and soil map. These three data was integrated creating 24 uniform zones of precipitation/soil/landcover, and the analysis was undertaken to each one of them. Looking to the results, it is clear the importance of PCD Triunfo altimetric zone, where despite representing only 25,6% of the watershed it represents for more than 37% of water volume collect by watershed, 46,31% of all basin runoff. This area is the main collect water source from watershed. It is necessary to remember that the high runoff percentage is directly tied to widespread agricultural use, and the high runoff rates lead to loss of water and sediment. Therefore, it is clear the necessity to increase the infiltration levels order for the water be retained and move as base flow, prolonging the residence time in the landscape.
Keywords: Daily Sequential Water Balance. Semiarid. Field Capacity.
ESCORRENTÍA SUPERFICIAL Y BALANCE HÍDRICO EN AMBIENTES SECOS CON TOPOGRAFÍA COMPLEJA - CUENCA DEL RIACHO DO SACO - PERNAMBUCO
Rsumen: En Brasil, todavía hay poca información sobre la escorrentía en pequeñas cuencas semiáridas, especialmente para las cuencas que presentan variaciones altimétricas relevantes, dada la falta de datos climáticos detallados.Pensando en esto, el presente estudio analizó la relación lluvia / escorrentía con basis en el método del balance hídrico. secuencial diario de la cuenca del Riacho do Saco en la región del Central Sertão, Pernambuco . La cuenca presenta un relieve complejo con una diferencia altimétrica de aproximadamente 700 metros entre la desembocadura (409m) y las cabeceras (más de 1100m), que se refleja directamente en las características climáticas, especialmente en la variación de la precipitación. Por lo tanto, el balance hídrico diario se realizó utilizando datos de cuatro PCD, mapeo de la cobertura del suelo y datos pedológicos. Esta información se integró generando 24 zonas homogéneas de precipitación / suelo / cobertura, y se realizó el análisis para cada una de ellas. Al evaluar los resultados, se observa la importancia de la altimetría del PCD Triunfo, que a pesar de representar solo el 25.6% del área de la cuenca, es responsable de más del 37% del volumen de agua recolectada por la cuenca, alcanzando el 46.31% del volumen drenado en toda la cuenca, por lo que es la principal fuente de captación de agua para la cuenca. Es necesario recordar que el alto porcentaje de escorrentía está directamente relacionado con el uso agrícola indiscriminado, y que las altas tasas de escorrentía conducen a la pérdida de agua y sedimento a la cuenca, por lo que se hace evidente la necesidad de aumentar los niveles de infiltración para que el agua sea retenida y se mueva como un flujo de base, prolongando su tiempo de permanencia en el paisaje.
Palabras clave: Balance hídrico secuencial diário; semiárido; CAD máximo.
... Schumm and Lichty (1965) discussed the role of time and space in determining erosion rates and processes, highlighting that erosion rates and mechanisms are affected by various factors that operate at different temporal and spatial scales. Graf and Lecce (1988) also claimed that over geological time scales, the sediment delivery ratio (SDR) must equal 1. However, this theory has been challenged because the millennial-scale evidence of small SDR values far below 1 has been found at the watershed scale in Belgium (Rommens et al., 2005;Notebaert et al., 2009;Verstraeten et al., 2009), Germany (Houben, 2008;Houben, 2012;Wolf and Faust, 2013), and France (Macaire et al., 1997;Macaire et al., 2002). ...
Water erosion, a notorious major threat to food security and ecosystem sustainability, is strongly conditioned by spatial and temporal scale effects. This paper systematically reviews the scale issues in runoff and sediment delivery (SIRSD) as a research field by integrating the traditional review approach and bibliometric analysis. This review summarises SIRSD's roots and the scale effect on runoff and sediment delivery. Then, we provide quantitative insights into the SIRSD domain's development history, thematic structure, geographic distribution, international cooperation, and methodologies. Findings show that: i) SIRSD arises from the gap between the non-linearity of runoff and sediment delivery across scales and our ability to measure it. Point-based and short-term measurements cannot capture the non-linearities from the spatio-temporal heterogeneities and cross-scale interactions of factors or processes. ii) Previous literature provides evidence that the spatial scaling of specific runoff (r), soil erosion (SE), sediment yield (SSY), or sediment delivery ratio (SDR) with drainage area (A) or slope length (L) exhibits contrasting patterns due to distinct mechanisms. Infiltration-excess and saturation-excess processes account for inverse and positive r-A relations, respectively. Interrill-erosion and rill-erosion cause inverse and positive SE-L relations. Hillslope-erosion and channel/bank-erosion explain inverse and positive SSY-A relations. Downstream increasing deposition and additional sediment inputs drive inverse and positive SDR-A relations. These scaling relationships can be nonlinear or complex due to spatial heterogeneities in land use, vegetation, topography, climate, lithology, and soil characteristics. Hence, applying an empirical scaling equation developed from the region with distinct environmental contexts is not recommended. Furthermore, the existing scaling patterns or equations may require updating given global climate and land use change. iii) SIRSD is a complex and multidisciplinary issue investigated by scientists from 93 countries since 1928. International research has substantially facilitated the understanding of SIRSD; still, more collaboration should focus on less-developed countries with high soil and water loss risks and urgent conservation needs, such as those in Africa and South America under cropland expansion. iv) Scale mismatch and scale break have discredited large-scale erosion and sediment assessments. Incorporating gully and bank erosion into modelling, extending the scale range of the L factor, and expanding the sediment scaling scope from watershed to slope may make a difference. Therefore, more research with nested design incorporating multiple scales is necessary for cross-scale analysis and scalable modelling. Addressing global climate change requires improving real-time urban flood forecasting and integrating downscaled climate models with hydrology, erosion, and sediment transport models.
... A further point to note is that, while flows in ephemeral rivers always carry very high concentrations of suspended sediment (Graf & Lecce, 1988;Powell et al., 1996), maximal concentrations occur during the rising limb of the hydrograph and especially in the bore front (Alexandrov et al., 2009), modestly increasing the unit weight of the water column and increasing time-averaged shear stress and possibly also increasing bedload transport capacity (see also Liébault et al., 2016). Conversely, high sediment concentrations can dampen turbulence and TKE (e.g., Villaret & Davies, 1995), although our current study showed that TKE in the bore was comparatively high (see Figures 4a, 4c, and 4e). ...
Evidence of the effect of unsteady flow on bedload sediment transport is meagre, particularly in desert rivers, despite its scientific, environmental and engineering importance. Our study of the phenomenon was conducted in Nahal Eshtemoa, Israel, an ephemeral river located in a semi-arid area and subjected to flash floods. During four years, we monitored bedload transport and contemporaneous hydraulics, focusing on two intervals during which flow changed most rapidly: (i) the arrival of flood bores over dry river beds and (ii) the propagation of subsequent bores over flowing water. Monitoring involved deployment of plate microphones, video cameras, surface velocity radar and a 3D velocimeter, enabling synchronized and unique data acquisition at high temporal resolution. We hypothesized that turbulence would be considerably elevated during the arrival of a flood bore and that this would induce transient entrainment of bedload. Our results demonstrate that bedload is indeed transported by the bore front and ceases soon after the arrival of most bores, even though water depth continues to rise. Bores arriving on a dry river bed usually generate higher bedload fluxes than bores advancing over flowing water. Elevated values of shear stress, turbulent kinetic energy and water-surface slope occur as a bore impinges but then decrease. This is likely to be the explanation for high transient rates of bedload transport. Once the threshold depth previously determined as inducing continuous bedload motion is attained (≈40 cm), transport rates rise considerably relative to those associated with the bores.
... Todavia, a Canais intermitentes e efêmeros tendem a evoluir lateralmente, alargando o vale ou migrando o seu leito. As migrações decorrem do trabalho fluvial de ajuste às flutuações nas descargas em função da variação de entrada de fluxos e sedimentos (GRAF, 1988). Os fluxos esporádicos em canais de drenagem efêmera são responsáveis pela ocorrência de feições no leito que expressam a intensa sedimentação como barras e leques (TOOTH, 2000;HASSAN, 2005;STORZ-PERETZ e LARRONE, 2013), ou ainda, podem expressar perdas na transmissão de fluxo (GORE et al., 2000;TOOTH et al., 2014). ...
Em setores específicos da Bahia, modelos paleoclimáticos desenvolvidos nas duas últimas décadas contribuíram para elucidar questões relacionadas às variações nas condições de umidade durante o Quaternário. Da mesma forma, dinâmicas geomorfológicas pretéritas, investigadas em diferentes contextos do estado, corroboraram para o avanço nas discussões sobre a evolução do relevo, apontadas de maneira genérica na literatura clássica. Nesse artigo, objetivou-se avaliar a contribuição de pesquisas desenvolvidas no setor continental baiano, no sentido de compreender a morfogênese regional quaternária, no contexto dos grandes compartimentos do relevo. A sistematização e espacialização dos proxies utilizados em literaturas anteriores contribuíram para a identificação de três períodos conhecidos na escala do Quaternário, que demonstraram a relação entre períodos úmidos/secos e as respostas do relevo às diferentes condições de umidade. Da mesma forma, quatro gaps temporais foram identificados, os quais necessitam de futuras investigações que visem a complementar as informações sobre o setor pesquisado.
... The behavior and morphology of these discontinuities are not geographically homogeneous, varying according to the morphoclimatic regions of the globe. Especially for tropical semi-arid environments, river connectivity is essentially controlled by the extreme temporal variability of flow, followed by the concentrated and irregular rainfall regimes (Graf, 1988;Souza et al., 2016). ...
This article aims at identifying and describing short-term connectivity scenarios in a semi-arid watershed in northeastern Brazil. The emergence of different decadal connectivity patterns is discussed, addressing the role of recent climatic inputs in triggering cut-and-fill erosive-depositional patterns within a valley bottom. A confluence stretch between a tributary and the stem channel was selected in order to analyze changes in connectivity and dysconnectivity patterns in response to climatic inputs from 2004 to 2014. The climatic behavior was assessed using the Rainfall Anomaly Index, which established dry and wet years, hence highlighting the relationship between connected and unconnected channel phases and a decennial precipitation dynamic. The years of 2004, 2008 and 2011 stand out as the rainiest and with the highest RAI values, in contrast to 2012, with a total rainfall of less than 600 mm and a RAI value below -2. The discrepancy between the 2012 rainfall total and the 2013/2014 biennium points to the relevance of this period for the reworking of alluvial deposits and connectivity resumption in the watershed. In the ephemeral drainages, connectivity is dramatically interrupted every dry season, creating fluvial disconnection. Dysconnectivity is characteristic of tropical semi-arid fluvial environments, but at landscape scale it may attest to the longevity of dry conditions, hence shall be addressed at time intervals ranging from a few days to thousands and tens of thousands of years.
... The hydrological dynamic during the onset of the Holocene in TB is defined by a short-lived neutral phase, with hydrological conditions similar to those experienced during the Bølling-Allerød, which is followed throughout the Greenlandian and Northgrippian by nonregular oscillations between dry and degraded (highly resilient) phases, allowing for the alternating deposition of larger-diameter particles (sands and gravels) by channelling wetland water flow and increasing the energy of the system (Fig. 2). Holocene sediment accumulation rates show a slight increase compared with the Pleistocene that is correlated with this energy increase in conjunction with the reduction in tree cover (Fig. 4), which generated an increase in the erosive rate (Graf, 1988). ...
During the last glacial termination, the climate system experienced intense global variations whose causes and impacts are not fully defined, particularly for low latitudes. The northwestern Mexico Sky Islands present a climate‐sensitive ideal setting to record palaeoecological and palaeoclimatic changes due to their physiographic complexity and location in the ecotone between temperate and tropical ecosystems. High‐resolution pollen analysis and a detailed sedimentological study were conducted at the Ciénega Tonibabi tropical thorn scrub site. The 15 540–0 cal a bp nearly continuous record shows that the North Atlantic Ocean did have a cold and humid climatic influence during the glacial stages of the end of the Pleistocene, including a sharp pulse during the Younger Dryas. However, a shift to the Pacific Ocean influence occurred during the Holocene, which led to the development of the El Niño conditions prevailing today. Colder and warmer phases follow one another with higher or lower winter precipitation, including a sharp Bølling–Allerød and development and intensification of the North American monsoon. They are reflected in hydrological changes as well as in the advances, retreats and intermingling of coniferous forests and tropical thorn scrub.
... Nas regiões consideradas áridas e semiáridas há predominância dos rios intermitentes e efêmeros, onde há fluxo de água nos canais apenas durante e por horas ou dias após eventos pluviais intensos, ou ao longo da estação considerada chuvosa, sendo momentos em que o escoamento superficial irá "abastecer" os canais fluviais e, desta forma, faz-se necessário compreender que a dinâmica fluvial é diretamente proporcional à relação chuva/escoamento, como também à relação magnitude/frequência dos eventos chuva/escoamento, os quais irão controlar as vazões dos canais (GRAF, 1988;SOUZA e CORRÊA, 2019). ...
A dinâmica morfológica em ambientes tropicais semiáridos reflete a ação de condicionantes responsáveis pela modelagem do relevo, tendo nas ações eólica e hídrica os principais fatores para evolução morfodinâmica. As áreas semiáridas são consideradas ecossistemas frágeis, com cobertura da vegetação escassa, em que os processos erosivos ocorrem de forma rápida e severa durante e após curtos episódios de precipitação pluvial, sendo os solos susceptíveis à erosão. Ademais, “gatilhos” desencadeados por formas de usos antrópicos podem levar a intensificação e ampliação da incidência de processos erosivos. Como resultado deste artigo, de caráter teórico, e embasado em levantamento bibliográfico e observações de campo nos estados de Pernambuco e Paraíba, objetivou-se reconhecer a dinâmica superficial que condiciona a modelagem do relevo no semiárido nordestino, considerando como eventos pluviais intensos contribuem para a morfodinâmica das paisagens sobre os domínios fluviais e interfluviais, assim como as inserções antrópicas influenciam a intensificação desta dinâmica.
... Precipitation: (semi)arid regions experience greater temporal and spatial variations of precipitation than those in humid environments (Graf and Lecce, 1988). In most cases, it is difficult to quantify these variations since they differ between different regions and also because of the low instrumentation density compared to humid areas (Liebmann et al., 2012). ...
... Water temperature is always high, with values greater than 23.5 °C, being the lower values observed between June and semi-arid lands, flooding is mostly associated with short flow events that vary in intensity, frequency and magnitude. In accordance to these criteria, Graf (1988) classified flooding in dry rivers as being seasonal, with multiple peaks, with a single peak and flash flooding. The type of flooding is the result of local and regional precipitation in a given hydrological cycle, flash flooding occurring in smaller river systems and seasonal flooding associated to larger basins. ...
Aquatic systems in semi-arid Brazil include natural shallow lakes, artificial reservoirs and intermittent streams and rivers. These systems are distinctive features in the semi-arid landscape and comprise a range of associated systems functioning as an ever-changing mosaic of dry/wet patches. Lakes and reservoirs in semi-arid Brazil are subject to important periods of water shortages, whereas rivers and streams are characterized as highly variable and driven by the extremes of water flow and its absence. Within this view a catchment-scale approach must be used to create a holistic model to conceptualize and comprehend these aquatic systems, since the aquatic environment types in the semi-arid region of Brazil incorporate broader aspects within the catchment scale such as geomorphology, vegetation, climate and land use. This paper summarizes some of the information on the aquatic systems of the Brazilian semi-arid region and shows the importance of limnological studies in this region. It also attempts to establish perspectives for future research considering the catchment as a scale for surveying biological processes and limnological characteristics of the various aquatic systems. It is presented information on their overall structure and functioning, as well as characteristics of some biological communities, such as phytoplankton, periphyton, aquatic macrophytes, benthic invertebrates and fish. The importance of the understanding of eutrophication in reservoirs and the role of the dry phase in streams is emphasized, and information on possible actions of planning and management to improve water quality of reservoirs are presented. Keywords: arid and semi-arid zones, aquatic biodiversity, reservoirs and intermittent rivers.
... High-magnitude flash floods produce major morphological changes and transport of bed material, including LW, in ephemeral and intermittent streams of the European Mediterranean region (Graf, 1988;Hooke, 2016;Šilhán et al., 2018). As a result, LW can be transported downstream and stored outside the channel margins in semiconfined or unconfined valley settings during these events ( Figure 10). ...
The majority of research on instream large wood (LW) focuses on perennial streams in temperate zones, and much less is known about wood distribution and its driving factors in other environments, such as semiarid bioclimatic regions. In this study, we sought to predict LW retention by assessing reach‐scale variables to infer the influence of external factors on the presence of LW in 37 independent reaches of ephemeral and intermittent streams draining a mountain range in a Mediterranean semiarid environment (Lefka Ori Mts., Crete, Greece). In addition, we focused on LW pieces that had ever been mobilized by fluvial transport to find links between wood mobility and the external reach‐scale characteristics of this environment. Markedly low average LW frequency (6.5 LW/100 m) and volume (9.06 m3/ha) were observed in the studied reaches in comparison with streams draining other bioclimatic regions. Only one complex general linear model was found to predict LW volume (including valley confinement and basal area of coniferous trees) and five explanatory variables (elevation as a proxy of annual precipitation, valley type, valley confinement index, basal area of coniferous trees and basal area of all trees) were significant in separated models predicting LW volume. Only two variables (valley floor width and the basal area of broadleaf trees) were significant in separated models for prediction of LW frequency. The mobilized LW in the studied streams tends to be stored as individual, dispersed pieces rather than in jams, likely due to overall low LW abundance which effectively prevent clustering of wood.
Ephemeral alluvial streams pose globally significant flood hazards to human habitation in drylands, but sparse data for these regions limit understanding of the character and impacts of extreme flooding. In this study, we document decadal changes in dryland ephemeral channel patterns at two sites in the lower Colorado River Basin (southwestern United States) that were ravaged by extraordinary flash floods in the 1970s: Bronco Creek, Arizona (1971), and Eldorado Canyon, Nevada (1974). We refer to these two floods as ‘fluviomorphic erasure events’, because they produced blank slates for the channels that were gradually moulded by more frequent but much smaller flood events. We studied georectified aerial photos that span ~60 years at each site to show that both study sites recovered to their pre‐flood condition after ~25 years. We employ channel network metrics: stream‐link area (SLA), geometric braiding index and junction‐node density. Each metric decreased during the short‐duration extreme flood erasure events. Subsequently, a fluviomorphic trajectory at a decadal tempo returned the channels to pre‐flood values. The SLA decreased at rates of 3.6%–4.1% per year in the decade following the floods. The extreme flood events decreased the pre‐flood geometric braiding index at the two sites by 56%–68%, and it took 15–24 years for this index to recover to pre‐flood values. In contrast, it took 30–35 years for the channels to recover to a uniform pre‐flood channel form, as indicated by the spatial distribution of bars and junction nodes. Our results document baseline examples of ephemeral stream channel evolution trajectories, as future climatic change will likely accelerate increases in the magnitudes and frequencies of extreme floods and geomorphic erasure events.
Endorheic river basins and their terminal lakes are highly sensitive to climate change and human activities. Based on chemical and pollen indicators, lake level, and erosion/accumulation rates of rivers, we explore the phasing of the evolution of the river system in the Hexi Corridor during the Holocene. The results suggest that climate change dominated the evolution of the river system during the early-Mid-Holocene. Entering the historical period, humans began to have an impact on runoff, water resources, and lake evolution, and since 1000 BP, anthropogenic perturbations recorded by regional proxies increased and humans dominated the migrations of river . In addition, we discuss the widespread erosion of rivers in the global endorheic zone and the impact of human activities in this context and found the timing of human influence on river evolution is not the same in different regions.
Previous syntheses of the hydrologic, geomorphic and biotic functions of floodplains have largely integrated research conducted along perennial stream networks, despite a growing body of literature on analogous functional floodplain zones along ephemeral streams. Although variable inundation characteristic of ephemeral streams complicates the delineation between channel and floodplain, ephemeral stream floodplains are distinct landforms that are hypothesized to host similar ecosystem functions as perennial stream floodplains, including the attenuation of downstream fluxes and provision of habitat to diverse flora and fauna. Here, we review the literature on dryland ephemeral stream floodplains in order to identify: (1) common definitions and styles; (2) unique hydrologic, geomorphic and biotic functions separate from uplands or channels; (3) current and future anthropogenic and natural stressors to such functions; and (4) knowledge gaps regarding research on ephemeral stream floodplains. Although active floodplain definitions follow criteria commonly used in perennial stream corridors, the extent of ephemeral stream floodplains is typically defined by less frequent floods. A set of floodplain styles emerges from the literature dependent on confinement and the presence of channelized streamflow. Functions related to attenuation and storage are typically concentrated in unconfined, channelled floodplains. The temporary storage of sediment and subsurface water in ephemeral stream floodplains makes them disproportionately important for biogeochemical cycling and hosts to richer, denser and more diverse vegetation communities compared with surrounding uplands. Many functions of ephemeral stream floodplains are also found in perennial counterparts, but variable flow regimes and sediment loads in ephemeral streams can potentially impact rates and magnitudes of comparable processes and functions. Future research needs to incorporate a holistic view of the ephemeral stream corridor—including channels and floodplains—to better understand current and future fluvial processes and cascading impacts in drylands globally.
We develop a robust and simple rule‐based algorithm to autonomously simulate alluvial fan deposition and evolution under continuously developing landscape conditions without prescribing deposition locations or imposing topographic constraints. Augmented with this algorithm, landscape evolution models are capable of dynamically detecting locations of potential fan deposition by statistical measures of surface topography and fluvial dynamics, then depositing fan sediments where and when the developed conditions require. To assess the method's efficacy in depositing sediment at a mountain‐valley transition zone characterized by a transport surface that permits unobstructed exit of sediment and water, a hypothetical scenario is created that involves a frontal, normal fault. It is followed by a series of sensitivity analyses to ascertain the influence of parameters affecting fan deposition and secondary processes. Uplift ( u ) and precipitation significantly impact fan morphological characteristics, which are within the range of real‐world fans. Higher rates of each cause the notable expansion of the fan area except in cases of exceptionally high precipitation rates. Fan area has a power‐law relationship with most of the tested parameters, , where is erodibility (lithology), and are fluvial parameters, and is catchment area (~0.9). This study is the first showcasing fan power‐law relationships using numerical modelling. While fan area increases with precipitation, there exists a threshold beyond which fan area diminishes, and the formation of fans ceases altogether. The algorithm provides a basis for improving mechanistic understanding of fans by offering a robust platform for testing process dominance and scaling. The results demonstrate its applicability for landscape evolution simulation over a long time and broad spatial scales. We also investigate the hydrological significance of including autonomously generated alluvial fans in coupled landscape evolution—hydrology models that focus on groundwater as well as surface water hydrology.
River corridors along non‐perennial stream networks provide diverse physical and ecological functions that are thought to be related to the spatial and temporal variability of geomorphic units, also known as geomorphic heterogeneity. While studies on the characteristics and drivers of geomorphic heterogeneity have been developed in perennial streams, similar studies in ephemeral streams are lacking. Given the ubiquity of non‐perennial streams globally, we aim to answer questions regarding the magnitude and drivers of geomorphic heterogeneity in ephemeral river corridors as well as how geomorphic unit assemblages reflect processes related to flood disturbance. Geomorphic units were mapped in 30 unconfined river corridors within six non‐perennial watersheds in Utah and Arizona, USA. Landscape heterogeneity metrics—Shannon's Diversity Index, Shannon's Evenness Index, and patch density—were used to quantify geomorphic heterogeneity within each reach. Additionally, variables that potentially constrain or drive heterogeneity were quantified, including floodplain shape, grain size, large wood abundance, and proxies for flood disturbance. While heterogeneity positively correlated with metrics for morphology and disturbance, statistical models suggest that morphologic context, particularly river corridor width, was a more important predictor for estimating geomorphic heterogeneity. Still, geomorphic units reflected aggradation processes indicative of a range of flood energies, suggesting a strong tie between heterogeneity and disturbance. Results suggest that geomorphic heterogeneity may be resilient to changes in flood disturbance frequency or magnitude, but future studies investigating long‐term temporal heterogeneity are needed.
Accurate groundwater management requires the estimation and propagation of rainfall-surface runoff. Rainfall is the principal source of groundwater recharge. Consequently, it is essential to monitor precipitation and runoff to assess water availability. Developing hydrometeorological data is necessary to design water resources and artificial recharge in Tamellalt. This study has two objectives: first, to estimate this region’s runoff storm water using the storm water management model (SWMM) with the soil conservation service curve number (SCS-CN) method and second, to use the Iber code to create runoff flow maps based on runoff depth using the 2D shallow Saint–Venant’s water model for finite volume. It shows that the rainfall is instantly infiltrated. Iber simulation runoff is due only to the exterior zones. This is confirmed by surface lithology. Terrain permeability is characterized by mean or high capacity. The extreme northern east zone shows a water depth exceeding 11 m. The eastern zone exceeds 15 m of water depth. The western submergence region has an approximate 3-m depression and 0.8-m water depth. Finally, the western subsurface zone of the south is characterized by a 1.8-m depression and a 0.4-m water depth. The extreme northern east and eastern zones can be considered favorite artificial recharge zones because of their high permeability and greatest water depth, considered natural water storage on the plain surface.
Vegetation can have a particularly strong effect on hydraulics and processes in dryland, ephemeral channels, where plants often occupy the channel bed. To understand the hydraulics, feedback effects and ecological dynamics for use in modelling and management, data on flow effects and vegetation dynamics are needed. Evidence from sites in southeast Spain, monitored for decades, is analysed here in relation to an extreme flood in September 2012 to identify thresholds for damage and destruction of plants, to assess rates of recovery, and to quantify effects of varying vegetation density and height on channel hydraulics. Repeated quadrat measurements provide data on vegetation cover, health and heights of plants, and were analysed in relation to measured flow stage and cross-section topographic surveys before, after and since the 2012 flood. Much of the vegetation was destroyed in the flood event, including the dominant Retama, resetting the vegetation. Threshold levels of shear stress for mortality and removal of the plants have been calculated. Rates of recovery have varied spatially, with little regrowth in main channels or on elevated floodplains, but strong growth occurring on bars. The different degrees of vegetation cover and height are calculated to have a very large effect on the flow hydraulics. The results support the proposal that dense vegetation cover can be highly effective in channel management by slowing flow, reducing erosion and increasing sedimentation.
This chapter deals with some of the fluvial landscapes that occur in desert areas. The concentration is upon features that are particularly characteristic of drylands compared to more humid areas. These include areas of internal (endhoreic) drainage, various types of piedmont landform (alluvial fans and pediments), areas of incision (badlands and arroyos), areas moulded by groundwater sapping, tiger bush patterns created by sheetfloods, and debris flows.KeywordsEndorheic drainagePedimentsAlluvial fansBadlandsArroyosTiger bushGroundwater sappingDebris flows
The Dabaan is a Middle Eocene to Early Miocene sedimentary basin. It stretches from the plateau margin to the Gulf of Aden and is drained by two main ephemeral streams: the Byoguure and the Kalajab. The climate ranges from semiarid in the headwaters to hyper-arid in the middle and lower reaches. Both rivers have an ephemeral flow regime as they are dry for long periods, and water flows only in response to occasional, very intense rainfalls. Field investigations were carried out to investigate channel morphology and sediment organisation in order to improve our knowledge of dryland rivers hydrology and sediment transport processes. The main morpho-sedimentary units of the streambed were identified and measured. Sediment samples were collected, and detailed observations on bed material sediment organisation, the occurrence of bedforms and sedimentary structures were carried out. The results indicate that, despite the braided channel morphology, the bars do not originate by depositional processes and downstream migration, as in permanent rivers. The bars and the channels, in fact, have no sedimentary structures but horizontal lamination, which implies flows with Froude numbers around one and the formation of a plane bed. During extreme floods, the upper part of the streambed is supposed to move en masse, and the bars are formed by scouring processes during the receding flood flow. Other, much less common bedforms are described. Boulder crescent scours and a new bedform, the cuspate sand waves, are found to be formed in flashy streams and, therefore, are considered as diagnostic of arid environments.
Biogeomorphological research blossomed during the second half of the twentieth century, partly in response to some big, unanswered questions about the role of vegetation in fluvial geomorphology, but also as technical advances allowed more detailed study of the complex interactions between biota and earth surface processes. Formal recognition of biogeomorphology (also known as ecogeomorphology) as a sub-field of geomorphology came in the late 1980s, building on several foundational pieces of research. Key foci of interest for biogeomorphological research up until the end of the 20th century were quantifying the impact of vegetation on erosion and the geomorphological roles of individual animal species, as well as understanding the human influences on biogeomorphic systems.
Dryland stream channels adjusting to urbanization are difficult to study without some indication of the extent and frequency of specific hydrological events. Investigations of changing channels in Fountain Hills, Arizona, urbanized since 1970, can benefit from knowledge gained from a storm event of over 55 mm rainfall in January 2016, equivalent to ~25% of annual precipitation. Morphological flow capacity was reached with this rainfall event, which had a recurrence interval of 1.4 years for a 24‐hour period and 3.6 years for a three‐day duration. This storm is typical of events that occur in less than half the years of record, occurring between 0 and 3 times each year in the 26 years of record available. Analysis of this event assists in the calibration of measured cross sections, providing a useful benchmark for space‐time substitution in dryland environments where flow is ephemeral. Correspondence of the recurrence interval of the storm to somewhat frequent “bankfull” channel‐forming events, despite signs of flashiness, suggests a tight coupling between rainfall and runoff and a tempered response to urbanization effects in Fountain Hills—achievable through decades of proactive planning and adaptive wash management in a master‐planned community.
The imprint of geologic, climatic and anthropogenic memory as controls on geomorphic river diversity is assessed for twelve River Styles in the Sabarmati Catchment. Geologic controls are the primary determinant of river character and behavior in the hinterland and pediment landscape units, where headwater streams transition to partly confined valleys and gorges with distinct structural lineations in a rectangular drainage network. Climate memory imprints entrenched rivers that make up alluvial fan and alluvial plain landscape units of the mid-lower catchment, shaped by phases of sediment aggradation and subsequent incision in response to enhanced monsoonal rains around 10 ka. Terraces constrain channels within confined and partly confined valleys with occasional and discontinuous floodplains respectively. Laterally unconfined channels are only found in the lower parts of the catchment, immediately upstream of the estuary/delta. Limited space for adjustment, impacts of flow regulation and ephemeral conditions restrict the range of contemporary river morphodynamics, but ridges and swales and abandoned channels on floodplains indicate more dynamic conditions in the past. Impacts of anthropogenic memory are most pronounced in the stopbank-controlled, barrage- and dam-impacted reaches, especially in the cities of Gandhinagar and Ahmedabad. An explanation of controls upon geomorphic river diversity, including an assessment of the role of stream power, presents a coherent platform to develop geomorphologically-informed approaches to river management.
Information on past dryland environments is commonly derived from geomorphological landforms and sediments (‘geoproxies’). The Okavango Delta in the middle Kalahari, Botswana, has been subject to a long history of arid‐humid transitions but its potentially rich archive of fluvial geoproxies is largely untapped. Previous palaeoenvironmental studies in the Delta region have focused mainly on aeolian dunes, lacustrine beach ridges, and rare pollen sequences in surrounding locations, and the Delta’s channel dynamics have remained poorly constrained, both chronologically and in quantitative palaeohydrological terms. Focusing on the Delta’s Xugana region, we present the first optically stimulated luminescence (OSL) ages for palaeochannel fills and scroll bars, revealing significantly enhanced fluvial activity in the ~7–4 ka interval. Along 120–150 m wide, sand‐bed, sinuous palaeochannels, lateral migration and bend cutoffs were driven by palaeodischarges up to ~350–450 m3 s‐1, at least nine times those of the present‐day channels. These palaeochannels represent the last major phase of meander activity in the middle and lower Delta. The palaeodischarges imply significantly higher rainfall over the catchment in the mid Holocene, possibly resulting from a southwards shift of the African tropical rainbelt. Over the last few thousand years, diminished fluvial activity in the Xugana region has continued; <20 m wide channels with discharges typically <40 m3 s‐1 have been more laterally stable, albeit subject to local avulsions. These findings provide the ‘missing link’ between evidence for past intervals of enhanced rainfall in the Delta’s tributary catchments in the northwest and west and the filling of large lacustrine basins to the southeast and east. The findings contribute to improved understanding of changing Kalahari hydroclimates and support growing evidence indicating that the mid to late Holocene was a time of significant fluvial transformation across many of the world’s drylands.
Humans have long augmented the water storage available in nature by constructing dams—barriers to the flow of water—in streambeds to create reservoirs. Broadly, the function of a reservoir determines whether storage of water is temporary or indefinite, e.g., flood-control reservoirs are kept empty while water-supply reservoirs are kept full. Most reservoirs are single-purpose, though multi-purpose reservoirs have been increasingly built in the twentieth century. For water-supply reservoirs, the key parameter is yield, whose evaluation requires a long period of flow records for the impounded river(s). Dependable yield is that draft which can be maintained constantly without failure throughout the time history of reservoir storage. Use of dependable yield as a basis for safe allocation therefore assumes the historical drought is the worst possible, an assumption that is flawed if the measurement record is too short, or if hydroclimate changes. A reservoir is a replacement of a segment of the river with a watercourse that is very different, a larger, more quiescent water body with different water quality and capable of stratification. All reservoirs act as sediment traps and will eventually silt up, unless special actions are taken to manage sediment. Reservoirs significantly alter the hydrology of the river downstream from the dam, and can affect its water quality and its ecosystem. With increases in population and agriculture, water demands will increase, exacerbated by climate changes. Skillful management will be needed to temper conflicts that arise over how reservoirs will be operated under situations of incompatible objectives.
The impact of large dams primarily built in the 1950s and 1960s has been extensively studied; more recent dam construction often occurs alongside habitat restoration activities, which have the potential to mitigate impacts. Jordanelle Dam on the Provo River, constructed in 1992, is economically and ecologically important to the region. This study aims to elucidate the effects of the dam on the sediment distribution and mobility of the river to inform future management of the system. Due to decreased flows, gravels (D16, D50) downstream of the dam are generally larger and subsurface grains are significantly coarser (p < 0.05), than those upstream of the dam. Further, the flow required to mobilize sediment is larger downstream of the dam and occurs rarely, leading to less mobile sediment and a more stable channel. The reduced mobility of sediment below the dam has decreased channel complexity. Much of the mobile sediment below the dam may be supplied from an unaltered river reach, rather than upstream mobilization. This sediment is vital to trout habitat, an important aspect of the recreation economy of the region. Monitoring changes in grain size distribution below the dam is necessary to prevent the loss of this valuable resource through diminished spawning habitat.
The advent of 2D hydraulic modelling has improved our understanding of flood hydraulics, thresholds, and dynamic effects on floodplain geomorphology and riparian vegetation at the morphological‐unit scale. Hydraulic concepts of bed shear stress, stream power maxima, and energy (cumulative stream power) have been used to characterise floods and define their geomorphic effectiveness. These hydraulic concepts were developed in the context of reach‐averaged, 1D hydraulic analyses, but their application to 2D model results is problematic due to differences in the treatment of energy losses in 1D and 2D analyses. Here we present methods for estimating total and boundary resistance from 2D modelling of an extreme flood on a subtropical river. Hydraulic model results are correlated with observations of the flood impacts on floodplain geomorphology and the riparian vegetation to identify thresholds and to compute variants of flood energy. Comparison of LiDAR data in 2011 and 2014 shows that the 2011 flood produced 2‐4 m of erosion on floodplain bars that were previously forested or grass‐covered. Deposition on flood levees, dunes and chute bars was up to 3.4 m thick. Various hydraulic metrics were trialled as candidates for thresholds of vegetation disturbance. The accuracy of thresholds using metrics extracted at the flood peak, i.e. boundary resistance and stream power maxima, was similar to that using energy as a threshold. Disturbance to forest and grass on vegetated bars was associated with stream powers of >834 W/m2 and unit flows of >26 m2/s, respectively. Correlation of the hydraulic metrics with erosion and deposition depths showed no substantial improvement in using flood energy compared to metrics extracted at the flood peak for describing erosion and deposition. The extent of vegetation disturbances and morphological adjustments were limited for this extreme flood, and further 2D studies are needed to compare disturbance thresholds across different environments.
In-depth understanding of the core content of LDPC code aligned cvqkd, and in-depth study of SEC protocol-based coordination process and multidimensional data coordination. Aiming at the problem of long code length and low coordination rate caused by multiple iterations of decoding algorithm in coordination process, a heterogeneous OpenCL platform is proposed to facilitate heterogeneous computing and differential code. The platforms involved are CPU / GPU and CPU / FPGA. Then, the IFAS model uses the uncorrected radar precipitation data to simulate the runoff process, and there are many errors in the simulation results. The IFAS model combined with the corrected precipitation radar data can simulate the discharge process of the upper reaches of the X river with high accuracy. This shows that the use of the data has a basin specific practicability for correcting the radar precipitation data in measuring the plain rainfall near the small river basin; the corrected precipitation radar data can be used as plain rainfall data in IFAS model; finally, this paper studies the influence of sports fatigue on basketball players' free throw, and provides a lot of theoretical basis for exploring the biomechanical mechanism of sports fatigue in free throw and avoidance mechanism. The results show that the bending angles of the wrist and knee do not change before and after fatigue, but the angular velocity of the knee changes significantly. During the picking process of wrist and fingers, the flexion angle of hip joint changed significantly before and after fatigue. At the same stage, the bending angle or angular velocity of shoulder joint did not change significantly. Based on the heterogeneous computing of runoff simulation, combined with relevant materials and research, this paper has a good effect on the development of athletes’ fatigue detection technology.
GIS technology was first introduced by Canadian surveyors. This technology was primarily used to digitize paper-based maps so that data can be analyzed and used on a computer that is convenient for distribution, storage, and use of information. Since GIS technology has been put forward, academic circles have been very interested. The growth of computer technology has also contributed to the development of GIS technology. GIS technology has now become a sophisticated topic that combines classification, storage, analysis, and performance of spatial information. Three-dimensional spatial knowledge can be realized by GIS technology. Spatial knowledge is often unique in real life, offering security for people’s clothes, food, housing, and transport. GIS technology also has great advantages in the processing of spatial information which makes GIS technology the most common application technology for spatial information processing and has been widely used in geological surveying, construction, and transport. In addition, the GIS framework can also be used to analyze atmospheric data. According to the results, the water content of the atmosphere can be measured in order to forecast precipitation and weather conditions. With the rapid advancement of computer technologies, GIS technology can be combined with the programming of databases. However, the progress of science and technology contradicts the principle of harmonious development between man and nature. Global warming caused by excessive carbon dioxide pollution is probably the most serious environmental issue.
Much progress has been made in recent years towards a set of recognition criteria for river discharge variability in river channel deposits, and thus sedimentary proxies for precipitation variability. Despite this progress, there is currently no consensus on how different styles of discharge variability are reflected in river sedimentary records, and whether variable-discharge river records from different climate types can be distinguished. Herein, river discharge and precipitation variability in the Paleogene is investigated using associations between river channel and floodplain deposits across the Paleocene–Eocene boundary from the Paleocene upper Nacimiento Formation and the early Eocene San Jose Formation in the San Juan Basin, New Mexico, USA.
The succession is identified as deposits of variable-discharge river systems based on shared channel-deposit characteristics with modern and ancient variable-discharge river systems and the proposed facies models, in addition to alternations of poorly drained and well-drained floodplain deposits and/or slickensides indicating alternating wet–dry cycles. A long-term stratigraphic trend toward increasingly well-drained floodplain deposits is also observed and hypothesized to indicate successively more arid conditions from the Paleocene into the early Eocene. Comparisons with modern rivers from various climate zones suggest a long-term shift from a monsoonal climate in the Paleocene, to a fluctuating subhumid climate, ultimately leading to semiarid to arid conditions in the early Eocene. These observations suggest that floodplain deposits may be a better indicator of ambient climate, whereas channel deposits are records for frequency and magnitude of high-intensity precipitation events. Therefore, the existing facies models for variable-discharge rivers that consider only channel facies may not capture critical information needed to make accurate interpretations of paleoclimatic conditions. This study also adds to a growing body of evidence from geologic records of mid-latitude Paleogene river systems suggesting increases in the magnitude or variability of river discharge coinciding with established climate perturbations.
From March 29 to April 08, 2017, the city of Comodoro Rivadavia (Chubut province, central Patagonia) had experienced the worst floods due to rainfalls that reached up to 399.4 mm. This study investigates the response of channel reaches in a semiarid environment, analyzing the range of erosional and depositional processes observed in ungauged catchments. Seven ephemeral rivers cover approximately 1546 km2 and are characterized by high slopes, that favored high runoff velocity and gully development during the storm. Heavy rains generated intense rain splash erosion and overland flow, providing high-concentration of suspended particles to the streams, with the additional incorporation of pebble-sized clastic components associated with natural and anthropogenic sources. Due to unusual discharges, aggradation and widening of the channels occurred in the higher-order streams, flooding large urban areas. Medium to fine sand-sized, transient deltas up to 500 m of radius were built in the coastal zone during the storm. The extreme rainfall event revealed a lack of planning at the catchment scale. It is reflected in a substantial reduction of the plant cover, anthropogenically-induced changes in channel shape and route of sediments, urban development in low and flat zones with flooding risk, and under-sized human-made waterways within the city.
Background
The flash flood still constitutes one of the major natural meteorological disasters harmfully threatening local communities, that creates life losses and destroying infrastructures. The severity and magnitude of disasters always reflected from the size of impacts. Most of the conventional research models related to flooding vulnerability are focusing on hydro-meteorological and morphometric measurements. It, however, requires quick estimate of the flood losses and assess the severity using reliable information. An automated zonal change detection model applied, using two high-resolution satellite images dated 2009 and 2011 coupled with LU/LC GIS layer, on western El-Arish City, downstream of Wadi El-Arish basin. The model enabled to estimate the severity of a past flood incident in 2010.
Results
The model calculated the total changes based on the before and after satellite images based on pixel-by-pixel comparison. The estimated direct-damages nearly 32,951 m ² of the total mapped LU/LC classes; (e.g., 11,407 m ² as 3.17% of the cultivated lands; 6031 m ² as 7.22% of the built-up areas and 4040 m ² as 3.62% of the paved roads network). The estimated cost of losses, in 2010 economic prices for the selected three LU/LC classes, is nearly 25 million USD, for the cultivation fruits and olives trees, ~ 4 million USD for built-up areas and ~ 1 million USD for paved roads network.
Conclusion
The disasters’ damage and loss estimation process takes many detailed data, longtime, and costed as well. The applied model accelerates the disaster risk mapping that provides an informative support for loss estimation. Therefore, decision-makers and professionals need to apply this model for quick the disaster risks management and recovery.
Management of desert grasslands requires rapid, low technology, coarse assessment methods that provide a triage-like prioritization for the manager. Such approaches necessitate the ability to quickly and effectively identify coarse-scale plant communities that provide guidance for this prioritization. Complex, computer intensive digital image classification of Landsat TM data, while marginally successful, requires time, equipment, and expertise not always available in such environments. This study identifies landform boundaries in the Armendaris Ranch, New Mexico by visual inspection of Landsat-7 Enhanced Thematic Mapper imagery and topographic maps using traditional photoreconnaissance techniques. Employing predetermined hierarchical landform classifications, it was possible to map plant communities using ecological relationships that exist between the general physiographic and vegetation settings in the area and representative geomorphic landform-mapping units. The authors’ field work verified the plant community map using a random walk approach and visual inspection. This synthetic expert opinion-based approach proved successful and is repeatable in other arid rangeland settings.
Management of desert grasslands requires rapid, low technology, coarse assessment methods that provide a triage-like prioritization for the manager. Such approaches necessitate the ability to quickly and effectively identify coarse-scale plant communities that provide guidance for this prioritization. Complex, computer intensive digital image classification of Landsat TM data, while marginally successful, requires time, equipment, and expertise not always available in such environments. This study identifies landform boundaries in the Armendaris Ranch, New Mexico by visual inspection of Landsat-7 Enhanced Thematic Mapper imagery and topographic maps using traditional photoreconnaissance techniques. Employing predetermined hierarchical landform classifications, it was possible to map plant communities using ecological relationships that exist between the general physiographic and vegetation settings in the area and representative geomorphic landform-mapping units. The authors’ field work verified the plant community map using a random walk approach and visual inspection. This synthetic expert opinion-based approach proved successful and is repeatable in other arid rangeland settings.
The monitoring of bedload flux under flash flood conditions has been successfully achieved since 1992 using slot samplers in the semiarid Nahal Eshtemoa. In the present study, a surrogate bedload monitoring technique ‐ the Japanese plate microphone ‐ has been deployed and calibrated against data from the slot samplers. Since a slot sampler has a sensitivity threshold that becomes especially important when transport rates are low, different averaging periods should be considered for high and low fluxes. In order to overcome the deficiencies of time‐based aggregation used hitherto, we have developed a new method involving mass aggregation and commensurably variable intervals, thereby enabling a more accurate analysis and optimizing the bedload sampler's capabilities. The data derived with this new method has then been utilized to calibrate the Japanese plate microphone. The Eshtemoa is an ephemeral gravel bed channel with a high proportion of fine gravel (< 0.02 m); for these conditions, acoustic sensors have not been calibrated as yet. Two multiple linear regression models incorporating the effect of median bedload grain size on pulse rate have been established to predict bedload flux and cumulative transported bedload mass. The coefficients in these models are statistically significant. Good predictions are obtained for bedload flux (adj. r² = 0.83) and for cumulative bedload mass (adj. r² = 0.98) during flood recession. Overall, the multiple linear regression models, used in conjunction with the mass aggregation method of estimating bedload flux, suggest that field calibration of acoustic devices is feasible under these conditions for ca. 90% of the duration of bedload transport.
Desert precipitations are rare but torrential, and ephemeral streams are ungauged, by which synthetic hydrograph methods become essential for flood risk assessment. This study quantifies the impact of urbanization on generating flash floods in desert watersheds. A definition of flash flood was presented in terms of the relative strength of partial storm intensity to storm design frequency for existing stormwater drainage systems. A large desert catchment was chosen to generate synthetic surface runoff hydrograph records using HEC-HMS model. The catchment characteristics were extracted using GIS tools and digital elevation data. Intensity-duration-frequency curves were used for the catchment to simulate hypothetical storms of 24-h duration for different return periods. A sensitivity analysis was performed for curve number parameter to simulate urbanization level scenarios. The results show that it is possible to employ simple models to synthesize the impact of urbanization on desert flash floods with a drastic change in hydrograph peak discharge, time to peak, and runoff volume. The former hydrograph characteristic is found to be the most sensitive, with an increase by 316% in response to increase in curve number by 10%. This finding implies that a small variation in land cover due to urbanization may significantly increase the flash flood strength and potential damage to infrastructure and human life.
The southern Kachchh Mainland (SKM) of western India, situated in a seismically active
intraplate region with a considerable Quaternary landscape, provides an opportunity to
reconstruct the roles of climate and tectonics. Employing geomorphology, detailed sedimentology
supported by geochemistry and optical dating, we integrate the fluvial to
fluvio-marine records to evaluate the potential of dryland environments in archiving the
palaeo-events. The climate reconstructions suggest that the fluvial systems have
responded to the variations in monsoonal strength through widespread aggradation
during 57 until 41 ka, corresponding to the later part of the MIS-3. Following this, a relative
weakening of monsoon and onset of aridity is observed between 28 and 11 ka,
with short spells of enhanced monsoon. The monsoonal conditions again strengthened
during 11 to 6 ka, reaching its maximum during the Early Holocene optima, which triggered
a sea-level rise during the Middle Holocene period, that is, 6 to 3 ka. Following
this, the sequences incised in response to the relative sea-level fall to the present level
in post 3 ka. We have also employed this OSL ages to reconstruct the phases of aggradation
and incision of valley fill sediments/estuarine tidal terraces from the SKM during
the last 57 ka. The present study illustrates and contributes to the advancement in
understanding of complex processes and their influence on climatic/tectonic signals in
dryland landscape as well as their decoupling.
KEYWORDS
aggradation-incision cycle, dryland valley fill, OSL chronology, southern Kachchh Mainland,
western India
Plant community responses to biocontrol of invasive plants are understudied, despite the strong influence of the composition of replacement vegetation on ecosystem functions and services. We studied the vegetation response to a folivore beetle (Diorhabda genus, Coleoptera) that has been introduced along southwestern US river valleys to control the invasion of non-native shrubs in the genus Tamarix (Tamaricaceae). We collected detailed plant compositional and environmental data during four different surveys over 7 years (2010-2017), including two surveys prior to when substantial beetle-induced dieback occurred in summer 2012, along the lower Virgin River, Nevada. The study river was of special interest because it is one of only a few largely unregulated rivers in the region, and a large flood of 40-year return period occurred between the first and second surveys, allowing us to study the combined effects of fluvial processes, which typically drive riparian plant community assembly, and biocontrol. Vegetation trajectories differed as a function of the dominant geomorphological process. Tamarix cover declined an average of 75% and was replaced by the native shrub Pluchea sericea as the new dominant species in the floodplain, especially where sediment deposition predominated. Following deposition, and especially erosion, opportunistic native herbs, Tamarix seedlings, and noxious weeds colonized the understory layer but did not increase in cover over time. Stands of the native shrub Salix exigua, a desirable replacement species following Tamarix control, only increased slightly and remained subordinate in the floodplain. Overall, our results showed that, by successfully controlling the target non-native plant, a biocontrol agent can substantially modify the replacement plant communities in a riparian system, but that fluvial processes also strongly influence the resulting communities.
An outcrop dataset from the early Eocene Sunnyside Delta Interval of the Green River Formation in the Uinta Basin, Utah, USA, documents alluvial channel lithosomes. The abundance of Froude supercritical‐flow sedimentary structures, together with an abundance of high‐deposition‐rate sedimentary structures, in‐channel bioturbation and pedogenic modification, in‐channel muds and thick soft‐clast conglomerates, identify these lithosomes as deposits of variable‐discharge rivers. These recognition criteria are part of an emerging facies model for variable‐discharge rivers. This facies model, however, yet lacks robust recognition criteria for macro‐scale or bar‐scale stratal patterns of variable‐discharge rivers. This study presents a dataset that corroborates some known stratal patterns and provides examples of hitherto unknown bar‐scale stratal patterns of variable‐discharge rivers, including: (i) low‐angle downstream‐accretion sets that may form as washed‐out sheets in high sediment supply conditions or downstream of hydraulic jumps; (ii) high‐angle upstream‐accretion sets that imply deposition from systematically upstream‐migrating channel‐scale hydraulic jumps (cyclic steps); (iii) concave‐up, upward‐flattening high‐angle downstream‐accretion sets that are consistent with aggradation in channel‐scale hydraulic‐jump scours; (iv) upstream‐accretion and lateral‐accretion sets that may be linked to high‐magnitude flood reworking of point bars; and (v) aggradation or vertical‐accretion sets of ambiguous origin. These unconventional stratal patterns are compared to the established bar strata, such as those formed by point bars and braid bars and a discussion is provided on formative conditions for the here documented unconventional strata. This work highlights a need for further studies on the effect of discharge variability on bar formation and on the link between river morphology and bar types.
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.
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.
Purpose
Floods and eolian activities are the dominant external agents to shape the topographic forms in ephemeral desert streams of drylands. So far, few studies have discussed the modern processes of eolian–fluvial interactions. To bridge this gap, we studied the modern interactions of eolian and fluvial process in a desert ephemeral river, the Maobula Gully in Inner Mongolia, which exhibits typical eolian–fluvial interactions.
Materials and methods
Multisource data such as integrated particle size data, hydrological data from the Tugerige Hydrological Station, high-spatial-resolution satellite images, and an eolian sediment saltation emission model were integrated to analyze the effects of eolian and fluvial delivery to the sediment on the riverbed, the eolian sediment feeding rate to the gully, the transport of sediment in flood events, and the interactions between eolian and fluvial processes.
Results and discussion
The desert reach of the Maobula Gully is a replacement reach between coarse sediment from the upper reaches and eolian sediment from the Kubuqi Desert. The annual eolian sediment feeding into the gully exhibited a significant decreasing trend. The eolian sediment into the gully increases the available sediment and the bed roughness, affecting the transport of sediment during floods. The sediment concentration and yields in flood events are mainly decided by the discharge and water yield, respectively. Through a comparison of the channel forms between 1970 and 2013, a recovery mechanism in the Maobula Gully was identified, which involves the equilibrium state between abrupt flood erosion and continuous dune migration.
Conclusions
This study analyzed the modern processes of eolian and fluvial processes and their interactions in a typical ephemeral desert stream named the Maobula Gully, and some interesting results were found. We believe that the methodology and results could provide references and evidence for understanding the mechanisms of fluvial and eolian interactions in other ephemeral desert streams around the world.
Catchment-scale hydrological studies on drylands are lacking because of the scarcity of consistent data: observations are often available at the plot scale, but their relevance for the catchment scale remains unclear. A database of 24 years of stream gauge discharge and homogeneous high-resolution radar data over the eastern Mediterranean allows us to describe the properties of floods over catchments spanning from desert to Mediterranean climates, and we note that the data set is mostly of moderate intensity floods. Comparing two climatic regions, desert and Mediterranean, we can better identify specific rainfall-runoff properties. Despite the large differences in rainfall forcing between the two regions, the resulting unit peak discharges and runoff coefficients are comparable. Rain depth and antecedent conditions are the most important properties to shape flood response in Mediterranean areas. In deserts, instead, storm core properties display a strong correlation with unit peak discharge and, to a lesser extent, with runoff coefficient. In this region, an inverse correlation with mean catchment annual precipitation suggests also a strong influence of local surface properties. Preliminary analyses suggest that floods in catchments with wet headwater and dry lower section are more similar to desert catchments, with a strong influence of storm core properties on runoff generation.
The evolution of alluvial fans on the narrow coastal plain of the Atacama Desert in northern Chile is so far poorly investigated. Therefore, a detailed geomorphological map at a scale of 1:7500 of a coastal alluvial fan complex at Guanillos (21°58′S, 70°10.5′W) is provided as a first step to understand the fan’s morphogenesis. Geomorphological mapping was based on a digital elevation model with a resolution of 2 m generated from Pleiades-1 tri-stereo satellite imagery, derived terrain parameters, and on-site field mapping. The resultant map is used to characterize and categorize the overall morphology of the alluvial fan complex. In particular, linear features constructed by primary alluvial fan processes can be differentiated successfully from those developed by secondary processes. Furthermore, the advanced evolutionary state of the fan complex is revealed. We introduce a morphostratigraphic model comprising the fan’s prograde evolution, dissection, and successive abandonment of surface generations.
The purpose of this study is to characterize torrential floods in the plain of Saïdia, caused by water discharges of the rivers (oueds) passing through Ouled Mansor plateau, by determining the following parameters: hydrographs, watershed properties, and hazards maps. Hydrographs of torrential floods are elaborated for different return periods (5,10, 20, 50, and 100 years), based on an hourly disaggregation of rainfall and on the construction of a local Digital Elevation Model (DEM) in the city of Saïdia, using the Storm Water Management Model (SWMM). The watersheds are extracted from the SRTM digital elevation model (DEM) (30 m resolution), while Iber code is used to elaborate hazard maps based on the 2D Saint-Venant shallow water model using finite volumes. The hydrographs maxima of the different oueds for different return periods show good correlation with the values recorded by the Agence du Bassin Hydraulique de Moulouya (ABHM). Overall, the obtained hazard maps are consistent with the flooded areas derived by the ABHM. The results show that there are three longitudinally variable runoff areas. For a better illustration of the aforementioned zones, three cross-sectional profiles A-A, B-B, C-C were carried out. The water accumulation is mainly due to the presence of two depressions that have been highlighted by other works. The maximum height of the runoff is in the central area of Saïdia, which is a city developed for tourism. Thus, the realization of a hydraulic structure to deviate the water towards the Moulouya River is necessary. The design of the structure is based on the hydrographs obtained in this work. The parameters obtained play an important role in defining flood susceptibility zones in the study region. The results are a key input for the definition of flood mitigation measures in the area and are a useful tool for future urban planning and development of the city of Saïdia.
A flash flood on 28th September, 2012, rose to a peak discharge of 2357 m3 s−1 from zero within one hour in the ephemeral Nogalte channel in SE Spain. Channel morphology and sediment sizes were measured at existing monitored sites before and after the flood and peak flow hydraulics calculated from surveyed floodmarks and cross-sections. Maximum peak sediment fluxes were calculated as ~600 kg s−1 m−1, exceeding maximum published, measured dryland channel values by 10 times and common perennial stream fluxes by 100 times. These high fluxes fit the established simple bedload flux - shear stress relations for dryland channels very well, but now extended over a much wider data range. The high sediment fluxes are corroborated by deposits at >1 m height in a channel-side tank, with 90 mm diameter sediment carried in suspension, by transport of large blocks and by massive net aggradation as extensive, structureless channel bars. Very high sediment supply and rapid hydrograph rise and recession produced the conditions for these exceptional sediment dynamics. The results demonstrate the extreme sediment loads that may occur in dryland flash floods and have major implications for catchment and channel management.
Eleven possible definitions of ?bank-full? have been used by various investigators. The active floodplain is the most meaningful bank-full level to the fluvial geomorphologist, whereas the banks of the valley flat are the most important to engineers. Comparison of 16 ways of determining bank-full discharge suggests that bank-full discharge at gaged sites should be obtained from the station's rating curve, where bank-full gage height is determined from a longitudinal profile of the floodplain along the entire reach. At ungaged sites, bank-full discharge can be estimated from the empirical equation of this study or from the Gauckler-Manning equation. In the latter case the resistance coefficient n should be estimated at the field site for bank-full flow; a measured low-flow n should not be used. Bank-full discharge does not have a common recurrence frequency among the rivers studied, and the discharge corresponding to the 1.5-year recurrence interval in most cases does not represent the bank-full discharge.
A stochastic model for generation of synthetic data on basins of 100 mi or less is developed using data from the Walnut Gulch Experimental Watershed in south-eastern Arizona. Variables describing the intermittent and independent runoff events are: start of runoff season, number of runoff events per season, time interval between events, beginning time of runoff event, volume of runoff and peak discharge. Each of these variables is generated from its own probability distribution. The means and standard deviations of the various distributions form the set of parameters that define the stochastic model. Some parameters are expressed as functions of drainage area and some are assumed constant for the range of basin areas used in the study. The means of the runoff variables vary with basin area while the standard deviations appear to be independent of basin area. By describing the variation of the model parameters with basin area, a model for a specific basin is developed into a model of a general basin for runoff events.