Recent human impacts and change in dynamics and morphology of ephemeral rivers

Abstract

Abstract. Ephemeral streams induce flash-flood events, which cause dramatic morphological changes and impacts on population, mainly because they are intermittent and less predictable. Human pressures on the basin modify load and discharge relationships, inducing dormant instability on the fluvial system that will manifest abruptly during flood events. The flash-flood response of two ephemeral streams affected by load supply modification due to land use changes is discussed in a combination of geomorphic and hydraulic approaches. During the Rivillas flash flood, intensive clearing on the basin led to high rates of sediment flowing into an artificially straightened and inefficient channel. The stream evolved from a sinuous single channel into a shallow braiding occupying the entire width of the valley floor. Misfits and unsteady channel conditions increased velocity, stream power and sediment entrainment capacity and considerably magnified flood damage. Resulting morphosedimentary features revealed a close relationship with the valley floor postflood hydraulic model, and pre-event awareness would have made it possible to predict risk-sensitive areas. In the second case, the Azohía stream, modelling of current pre-flood channel conditions make it possible to determine channel narrowing and entrenchment in the lower alluvial fan stretch. Abandonment of intensive agriculture, basin reforestation and urbanization diminish load contribution and trigger channel incision. This induces an increase in slope and velocity in the bankfull channel, producing renewed erosive energy and thus activating upstream propagation of incision and bank undermining. The absence of water-spreading dynamics on the alluvial fan in favour of confinement in a single channel produces an unstable dynamic in the system, also offering a false sense of stability, as long as no large magnitude floods occur. When modelling flood-prone areas and analysing hydraulic variables, it is important to detect possible anthropic disturbances that may affect basin load budgets in order to anticipate catastrophic consequences resulting from inappropriate fluvial management before the occurrence of an extraordinary event.

River network evolution and fluvial process responses to human activity in a hyper-arid environment – Case of the Tarim River in Northwest China

“…With the accelerated expansion of human activities from temperate and humid environments into arid environments, many dryland rivers have already been strongly influenced by, or are under significant threat from, various human interventions such as land reclamation and flow regulation, which often have more significant potential to disturb hydrological (e.g., Jacobson et al, 1995;Schick, 1995;Tooth, 2000a;Mansur and Nurkamil, 2010;McDonald et al, 2013) and fluvial processes (e.g., Graf, 1978Graf, , 1979Ortega et al, 2014). Related research has mainly been carried out in recent decades, for example, the changing channel patterns of the Gila River in Arizona, in the southwestern United States of America, resulting from flow regulation and the introduction of Tamarix, an exotic plant with more suitability than local vegetation (Graf, 1978(Graf, , 1979(Graf, , 1988; the flow regime and hydrologic change of the Murray and the Barwon-Darling Rivers (Maheshwari et al, 1995;Thoms and Sheldon, 2000); the adjusted channel morphology of ephemeral streams due to urbanization in southwest America (Chin and Gregory, 2001;Coleman et al, 2005); and changes in the dynamics and morphology of ephemeral rivers in Mediterranean regions Bledsoe, 2011, 2013).…”

Towards an increase of flash flood geomorphic effects due to gravel mining and ground subsidence in Nogalte stream (Murcia, SE Spain)

“…In contrast, the 2012 floodwaters reached this area with high flow depth values (> 1.5 m. see Fig. 6c), and the water velocity may have reached 1 m s −1 , as suggested by the resulting scours and bars compared with similar features studied in other flood events (Ortega et al, 2014). In this case, instead of the flow stagnating, the floodwaters continued downstream, unable to retain such a high flood volume on the broad, flat endorheic area (Fig.…”

“…A flood model proposed by Conesa-García and Alvarez (2003) even considered that, due to its more concave profile, the vulnerabil- ity to flooding was greater towards the southern edge of the fan axis. The only reference found to previous flood propagation downstream from the endorheic area is the 1948 flood in which 240 mm rainfall in only 5 h was recorded in Puerto Lumbreras, resulting in a flash flood that affected an area of 7900 ha (Perez and Gil, 2012). The flooded area was smaller than the 116 km 2 of the 2012 event, and affected mainly the lower part of the depression, as the floodwater arrived mostly from the Lorca fan through La Tiata channel, not from the Nogalte Rambla.…”

Sobre la evolución como ciencia de la geomorfología dinámica en ambientes fluviales

“…En Valencia y Murcia hay valiosas aportaciones al estudio de las ramblas mediterráneas (Mateu, 2000;Segura y Sanchis, 2013;Conesa y García Lorenzo, 2009) y en Andalucía a la dinámica fluvial asociada a elementos históricos y usos del suelo (Guerrero y Baena, 2002;García Martínez et al, 2009). Desde el ámbito de la geología destacan los trabajos sobre ríos en roca (Ortega y Garzón, 2009;Ortega y Durán, 2010;Ortega et al, 2014), las paleocrecidas (Benito y Thorndycraft, 2005), así como los análisis y cartografías geomorfológicas aplicadas a la gestión de riesgos desde organismos como el IGME (Díez Herrero et al, 2008Lastra et al, 2008), el INDUROT (Sánchez Martínez y Lastra, 2011) y la universidad (Uribelarrea, 2008). Por último, el grupo que desarrolla su investigación en Aragón, País Vasco y Galicia está trabajando en clasificación fluvial, diagnóstico hidromorfológico y aplicaciones a la restauración fluvial y a la gestión de riesgos (Ollero, 2010;Ollero et al, 2011Ollero et al, , 2014Ibisate et al, 2013;Ibisate, 2014;Horacio, 2012Horacio, , 2014.…”