Topographical change caused by moderate and small floods in a gravel bed ephemeral river – a depth-averaged morphodynamic simulation approach

Abstract

Abstract. In ephemeral rivers, channel morphology represents a snapshot at the end of a succession of geomorphic changes caused by floods. In most cases, the channel shape and bedform migration during different phases of a flood hydrograph cannot be identified from field evidence. This paper analyses the timing of riverbed erosion and deposition of a gravel bed ephemeral river channel (Rambla de la Viuda, Spain) during consecutive and moderate-(March 2013) and low-magnitude (May 2013) discharge events, by applying a morphodynamic model (Delft3D) calibrated with pre-and post-event surveys by RTK-GPS points and mobile laser scanning. The study reach is mainly depositional and all bedload sediment supplied from adjacent upstream areas is trapped in the study segment forming gravel lobes. Therefore, estimates of total bedload sediment mass balance can be obtained from pre-and post-field survey for each flood event. The spatially varying grain size data and transport equations were the most important factors for model calibration, in addition to flow discharge. The channel acted as a braided channel during the lower flows of the two discharge events, but when bars were submerged in the high discharges of May 2013, the high fluid forces followed a meandering river planform. The model results showed that erosion and deposition were in total greater during the long-lasting receding phase than during the rising phase of the flood hydrographs. In the case of the moderate-magnitude discharge event, deposition and erosion peaks were predicted to occur at the beginning of the hydrograph, whereas deposition dominated throughout the event. Conversely, the low-magnitude discharge event only experienced the peak of channel changes after the discharge peak. Thus, both type of discharge events highlight the importance of receding phase for this type of gravel bed ephemeral river channel.

A customised approach to determining the geomorphic effectiveness of small flood events in a regulated river

“…The traditional view that geomorphic change requires large floods means that the geomorphic outcomes of small floods are often overlooked by river managers (Knighton, ; Lotsari et al, ). Yet, some channel forming benefit has been shown in the Snowy River system from the return of small floods.…”

“…This demonstrates that small floods can be incorporated within a geomorphic effectiveness framework and that general geomorphic effectiveness models that try to capture complex non‐linear responses are at a coarser scale than we have demonstrated. Some work, however, suggests that channels are formed by moderate and small floods in gravel‐bed river channels (Calle et al, ; Hooke, ; Lotsari et al, ) and, also, formed and maintained by a range of flows (Segura & Pitlick, ).…”

Extreme Floods in Small Mediterranean Catchments: Long-Term Response to Climate Variability and Change

“…Assigned Manning's n values were 0.03 for the valley floor and 0.04 for the valley margins. These n values were chosen according to our previous works in Rambla de la Viuda that used the Limerinos equation [47] for D 50 and D 84 grain sizes [20,48]. The hydraulic model was calibrated using field survey of high water marks of the December 2007 flood (20 m 3 s −1 ).…”

Three‐dimensional reconstruction of fluvial surface sedimentology and topography using personal mobile laser scanning

“…Examples of high‐end systems being used to acquire topographic data for geomorphological investigations are now emerging (e.g. Kukko et al, ; Flener et al, ; Calle et al, ; Lotsari et al, , ; Brooks et al, ). High‐end mobile systems have also been applied to map surface sedimentology; for example, Wang et al .…”