What controls the coarse sediment yield to a Mediterranean delta? The case of the Llobregat River (NE Iberian Peninsula)

Abstract

Abstract. The human pressure upon an alluvial river in the Mediterranean region has changed its riverine and deltaic landscapes. The river has been channelized in the last 70 years while the delta has been retreating for more than a century (a set of data unknown, so far). The paper concentrates on the fluvial component, trying to connect it to the delta evolution. Is the channelization responsible for the delta retreat? We develop a method to compute the actual bed load transport with real information of the past river morphology. The paper compares the computation with very limited measurements, among which are bulk volumes of trapped material at a modern, deep river mouth. The decrease in sediment availability in the last 30 km of the channelized river is deemed responsible for the decrease in the sediment yield to the delta. Moreover, power development and flood frequency should be responsible for a baseline delta retreat during the 20th century. The sediment trapping efficiency of dams is less important than the flow regulation by dams, in the annual sediment yield. Therefore, it is more effective to dismantle channelization than to pass sediment at dams, to provide sand to the beaches.

Basin‐wide hydromorphological analysis of ephemeral streams using machine learning algorithms‡

“…These observed geomorphic alterations therefore indicate a change in the sedimentary continuity and connectivity at this point (Fryirs, 2013; Wohl, 2017). Since sediment load‐carrying capacity is proportional to the alluvial width, bed slope and discharge (Martín‐Vide et al, 2020), the observed decrease in active channel width (Figure 9b) indicates a reduction in sediment load, which, under natural conditions, should tend to increase downstream (Mueller & Pitlick, 2005; Pitlick et al, 2008).…”

Regressió del delta del Llobregat. Efecte de les obres d’enginyeria al riu d’ençà del segle XIX

“…Per últim, s'ha utilitzat el càlcul del volums erosionat a la llera després de la crescuda extraordinària de l'any 2000 obtingut per la comparació de perfils longitudinals (Martín-Vide et al, 2020), i la dada de transport en suspensió del Llobregat (Liquete et al, 2009), per tal de comparar la magnitud del transport sòlid de les grans crescudes respecte al corresponent a un any mitjà i per a validar els valors estimats de capacitat de transport sòlid de fons.…”

“…La crescuda del juny de l'any 2000 amb una punta de 1.500 m 3 /s es va produir just després de la construcció del dic esquerre (segona autopista) l'any 1998. Comparant el perfil longitudinal del riu un cop finalitzades les obres amb una batimetria immediatament posterior a la crescuda s'obté un volum erosionat a la llera de ≈ 125 × 10 3 m 3 /any als trams de la vall (1-3) (Martín-Vide et al, 2020). Tot aquest volum, com a mínim, va ser transmès cap al delta (4-5) i finalment a la costa.…”

Large‐Scale Channel Response to Erosion‐Control Measures

“…Channels respond to such changes through upstream-and downstream-migrating aggradation and incision waves (An et al, 2019;Chowdhury et al, 2023;De Vriend, 2015;Y. Lin et al, 2023;Martín-Vide et al, 2020;Madej & Ozaki, 1996). Depending on the magnitude and spatial extent of the change, channel response may develop over centuries and extend over hundreds of kilometers (e.g., Simon & Rinaldi, 2006;Surian & Rinaldi, 2003;Yang et al, 2018;Ylla Arbós et al, 2021a).…”