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 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.

Relationships between Sediment Transport and Various Hydrological and Hydraulic Characteristics of Flood Events on Trotuș River (Romania)

“…Type II hysteresis loops account for a significant share of the total number of D type floods (25% or 13 flood events), therefore confirming that this type is specific to moderate floods with limited flow discharge, low sediment yield and high SSC [73,75,77]. The prevalence of type I hysteresis loops is in full compliance with recent channel changes observed on Trotuș River, namely the tendency of degradation [33,34].…”

“…(Figure 7). The values of the b exponent are above 1 in all the cases (ranging between 1.79 and 3.07), which reflects the size of bed material, indicating a gravel-bed channel [34].…”

“…In the case of Romanian rivers, the studies assessing the sediment transport during flood events [32,33,34] are rather scarce, whereas the papers tackling climatic and hydrological conditions prompting the occurrence of flood events or associated risk phenomena are more numerous [35 -38].…”