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, due the intermittent activity of these fluvial systems. Human pressure changes the fluvial environment and so enhances the effects of natural dynamics. Local human-induced modifications can be latent over long periods of time. These changes can be studied after the flood event, to quantify their effects and detect which are most harmful. In this paper we study flash-flood effects at two sites in Spain and compare the results before and after a~flood event. Erosion is associated with areas where there have been more anthropogenic changes in floodplains and channels. Deposition is related to erosional processes in the watershed and to the tributaries. Disruption of river channel patterns changes connectivity and scouring appears due to energy excess. This excess tends to concentrate at weak points downstream produced by anthropic disturbances. Riparian vegetation is an energy sink and reaches with more cover show less erosion than those with deforestation. Infrastructures perpendicular to the direction of flow increase stream power, but peaks of erosion on the floodplain appear displaced downstream. It is important to detect human changes by analysis of hydraulic variables before the occurrence of an extraordinary event in order to anticipate catastrophic consequences resulting from inappropriate fluvial management.

Dynamics of ephemeral streams at the foot of degraded catchments in northern Ethiopia

“…Smaller floods rework bed material but their transport capacity my not be enough to incise the streambed but only capable to redistribute the sediment across the channel bottom (Hooke, ). This conclusion is confirmed by the channel shrinking produced by intermediate flows in the Hara River but it is in contradiction with what is reported by Ortega et al () whose hydraulic modeling shows that channel narrowing and deepening is associated with low flood flows, whereas large floods result in an opposite pattern of width‐to‐depth ratios. Similar results are reported also by Patton and Schumm () and Bull ().…”

“…In the Hara catchment, unfortunately, only daily rainfall data are available, but, if we consider the maximum daily precipitation data of the seven rain gauges reported in Table and we use the correlation equation for northern Ethiopia of Figure we obtain hourly intensities ranging from 32 to 140 mm/hr. These values are large enough to generate high flash floods and a conspicuous sediment supply (Ortega et al, ), especially in the Hara catchment, where Asfaha, Frankl, Haile, and Nyssen () report a high density of linear erosion features.…”

“…Moreover, according to Gaeuman, Schmidt, and Wilcock (), qualitative models for predicting the direction of channel response to changes in discharge and sediment supply return ambiguous predictions for many channel parameters if the characteristics of bed material and sediment load particle size are not considered. The study of Gaeuman et al (), however, does not consider the resisting effect offered by the riparian vegetation and its role in strengthening bank material as, instead, emphasized by Ortega et al (). Vegetation is very difficult to quantify and to model in a flume.…”

“…Channel avulsion, bifurcation and lateral mobility are common processes (Billi, ; Demissie, Frankl, Haile, & Nyssen, ; Demissie, Nyssen, Billi, Haile, Vaneetvelde, & Frankl, 2018). Furthermore, in the graben escarpment, catchment rehabilitation measures (mainly bush reforestation) have been successfully implemented (Asfaha, Frankl, Haile, Zenebe, & Nyssen, ; Frankl, Poesen, Haile, Deckers, & Nyssen, ) and provide a good opportunity to investigate the response of ephemeral streams to changes in sediment supply, that is, a topic that has been poorly explored in the literature (Boix‐Fayos, Barbera, Lopez‐Bermidez, & Castillo, ; Ortega, Razola, & Garzón, ). Moreover, according to Gaeuman, Schmidt, and Wilcock (), qualitative models for predicting the direction of channel response to changes in discharge and sediment supply return ambiguous predictions for many channel parameters if the characteristics of bed material and sediment load particle size are not considered.…”

Assessment of flash flood susceptibility potential in Moldavian Plain (Romania)

“…Furthermore, anthropogenic pressure on the land and spatial expansion processes for both rural and urban settlements in areas exposed to flood risk, lead to flood vulnerability increase for the population living in these particular regions (Jodar‐Abellan, Valdes‐Abellan, Pla, & Gomariz‐Castillo, 2019; Ortega, Razola, & Garzón, 2014).…”