Sub-Bankfull Flow Frequency versus Magnitude of Flood Events in Outlining Effective Discharges. Case Study: Trotuș River (Romania)

Abstract

Effective discharge, which represents the flow, or range of flows, that transport the most sediment over the long-term, was determined based on the mean daily flow discharge and mean daily suspended sediment discharge recorded between 1994 and 2014 at four gauging stations along the Trotuș River. This study proposes an efficient method for the estimation of effective discharge based on observed values of the suspended sediment load. By employing this method the suspended sediment load is no longer either under-or overestimated as in the cases when the assessment is based on sediment rating curves. The assessment on effective discharge was performed at two distinct levels: for the entire data series during the investigated time spans and, subsequently, for flows less than the bankfull discharge. The effectiveness curves of the suspended sediment transport characteristics revealed highly multimodal characteristics with many peaks, indicating ample ranges for the effective discharges. The main effective discharge corresponded to large flood events, which are typical for the upper end of the discharge range, whereas the secondary effective discharges corresponded to sub-bankfull flows, which are more frequent. The changes that occurred in the channel bed are reflected by the temporal variations in the effective discharge.Abstract: Effective discharge, which represents the flow, or range of flows, that transport the most sediment over the long-term, was determined based on the mean daily flow discharge and mean daily suspended sediment discharge recorded between 1994 and 2014 at four gauging stations along the Trotu Abstract: Effective discharge, which represents the flow, or range of flows, that transport the most sediment over the long-term, was determined based on the mean daily flow discharge and mean daily suspended sediment discharge recorded between 1994 and 2014 at four gauging stations along the Trotuș River. This study proposes an efficient method for the estimation of effective discharge based on observed values of the suspended sediment load. By employing this method the suspended sediment load is no longer either under-or overestimated as in the cases when the assessment is based on sediment rating curves. The assessment on effective discharge was performed at two distinct levels: for the entire data series during the investigated time spans and, subsequently, for flows less than the bankfull discharge. The effectiveness curves of the suspended sediment transport characteristics revealed highly multimodal characteristics with many peaks, indicating ample ranges for the effective discharges. The main effective discharge corresponded to large flood events, which are typical for the upper end of the discharge range, whereas the secondary effective discharges corresponded to sub-bankfull flows, which are more frequent. The changes that occurred in the channel bed are reflected by the temporal variations in the effective discharge.River. This study proposes an efficient method for the estimation of effective d...

Increasing flood magnitude, an effect of climate change or natural climate variability?

“…The climate is typical of Carpathian mountainous areas with mean annual precipitation of 720 mm, ranging from 580 in the lowlands to 1150 mm at high altitudes. The share of the precipitation related to the total sum of maximum amounts recorded in 24 hours above 100 l/m 2 has been rising continually: 8.3% between 1941 and 1960; 30.8% between 1961 and 1980; 47.5% between 1980 and 2000; and 67.7% after 2000 (Dumitriu, 2014(Dumitriu, , 2018. As regards the land cover/land use, forests, pasturelands and meadows are prevalent in the higher regions of the upper and mid courses, whereas in the lowlands corresponding to the lower course agricultural lands and pastures are dominant (Dumitriu, 2016(Dumitriu, , 2018.…”

“…The share of the precipitation related to the total sum of maximum amounts recorded in 24 hours above 100 l/m 2 has been rising continually: 8.3% between 1941 and 1960; 30.8% between 1961 and 1980; 47.5% between 1980 and 2000; and 67.7% after 2000 (Dumitriu, 2014(Dumitriu, , 2018. As regards the land cover/land use, forests, pasturelands and meadows are prevalent in the higher regions of the upper and mid courses, whereas in the lowlands corresponding to the lower course agricultural lands and pastures are dominant (Dumitriu, 2016(Dumitriu, , 2018. The multi-year average flow (reported to 2020) was 0.88 m 3 /s at the Lunca de Sus station (in the upper course); 17 m 3 /s at the Târgu Ocna station (in the middle course, at the contact between the Carpathian and subcarpathian areas) and 35 m 3 /s at the Vrânceni station (in the lower course, located at the contact between the subcarpathian and plateau areas).…”

Effects of Floods on Channel Morphology in an Ephemeral Drainage: Case Study in the Semi-Arid Brazilian Northeast / Efeitos Das Cheias Sobre a Morfologia Do Canal Em Uma Drenagem Efêmera: Caso De Estudo No Semiárido Nordestino

“…As vazões oscilaram abaixo do nível de margens plenas (bankfull) identificado pelo topo da barra de pontal, indicada pela seta amarela na figura 5B. Estas vazões menos expressivas são definidas como sub-bankfull e sua importância na modelagem dos leitos móveis dos canais é destacada por vários autores (BOURKE, 2002;TILLINGHAST et al, 2011, DUMITRIU, 2018CONESA-GARCÍA et al, 2022).…”

“…As mudanças substanciais na morfologia dos rios das regiões secas, que consistem em expressivas taxas de erosão e deposição, são impostas pelas grandes e infrequentes vazões dominantes geradas pelas chuvas concentradas. Por outro lado, as vazões menores e mais frequentes provocadas por precipitações menos expressivas (sub-dominantes ou sub-bankfull) retrabalham continuamente os depósitos das grandes cheias (BOURKE, 2002;TILLINGHAST et al, 2011;DUMITRIU, 2018;CONESA-GARCÍA et al, 2022).…”

Causes of spatio-temporal variations in the flood event efficiency index

“…The hydrology of Trotuș river basin is regulated by the pluvio-nival regime, with spring flooding occurring typically in April-May as a result of snowmelt, high precipitation or the overlapping of both. In June, July and occasionally extending to August, summer floods can occur as a result of abundant precipitation, reaching very high amplitudes, as was the case with the floods of June-July 2005 (Dumitriu, 2018). Based on the average suspended sediment yield (263 t/km2/an) Trotuș drainage basin ranks among the category of rivers with moderate rates from the Romanian Carpathian area (Dumitriu et al, 2017).…”