Climate change and human influences on sediment fluxes and the sediment budget of an urban delta: the example of the lower Rhine—Meuse delta distributary network

Abstract

Deltas require sufficient sediment to maintain their land area and elevation in the face of relative sea-level rise. Understanding sediment budgets can help in managing and assessing delta resilience under future conditions. Here, we make a sediment budget for the distributary channel network of the Rhine–Meuse delta (RMD), the Netherlands, home to the Port of Rotterdam. We predict the future budget and distribution of suspended sediment to indicate the possible future state of the delta in 2050 and 2085. The influence of climate and anthropogenic effects on the fluvial and coastal boundaries was calculated for climate change scenarios, and the effects of future dredging on the budget were related to port development and accommodation of larger ships in inland ports. Suspended sediment rating curves and a 1D flow model were used to estimate the distribution of suspended sediment and projected erosion and sedimentation trends for branches. We forecast a negative sediment budget (net annual loss of sediment) for the delta as a whole, varying from −8 to −16 Mt/year in 2050 and −11 to −25 Mt/year by 2085, depending on the climate scenario and accumulated error. This sediment is unfavourably distributed: most will accrete in the northern part of the system and must consequently be removed by dredging for navigation. Meanwhile, vulnerable intertidal ecosystems will receive insufficient sediment to keep up with sea-level rise, and some channels will erode, endangering bank protection. Despite increased coastal import of sediment by estuarine processes and increased river sediment supply, extensive dredging for port development will cause a sediment deficit in the future.

Effects of Sea‐Level Rise on Dredging and Dredged Estuary Morphology

“…The sediment budget was calculated as the sediment flux from rivers minus the accommodation created by SLR. These sediment budgets were then combined with reported maintenance dredging values for various systems (Anthony et al., 2019; Arnaud‐Fassetta, 2003; Cox, Huismans, et al., 2021; Frey & Coe, 2020; Frihy et al., 2015; Habersack et al., 2016; Jordan et al., 2019; Kemp et al., 2014; Liu & Zhang, 2019; Marineau & Wright, 2015; Rudra, 2014; Smets et al., 1997; van Dijk et al., 2021; van Maren et al., 2015; Weilbeer, 2014; Wu et al., 2018) (see Data Set S1 for full overview).…”

“…Estuaries are commonly identified as hotspots for climate risk (Hill et al., 2020) because they are uniquely threatened by both sea‐level rise (SLR) and river basin‐wide climate changes (e.g., glacial melt, temperature variation, changes to discharge) (Wong et al., 2014). They are also under pressure from dredging and sand mining (Bendixen et al., 2019; Cox, Dunn, et al., 2021) undertaken to create deeper shipping fairways for navigation, which has the added effect of removing the necessary sediment to maintain elevation and build/perpetuate estuary morphology. It is still unclear how the morphology and hydrodynamics of such dredged systems will respond to SLR and how this response will differ compared to natural, undredged estuaries.…”

“…Dredging activities can be broadly split into two categories: (a) capital dredging events, which deepen channels permanently, and (b) maintenance dredging, which is the periodic process of keeping channels at a specific depth. Typically, systems undergo capital dredging every 10-20 years, while maintenance dredging typically occurs year-round, particularly in systems that are home to large ports (Cox, Dunn, et al, 2021;Cox, Huismans, et al, 2021;van Dijk et al, 2021). The dredged material can be either completely removed from the system (sold to market or dumped offshore) or reused and relocated within the system.…”

Significant Human Modification of the Lower Arkansas River Sediment Budget

“…For example, barges accounted for the transport of ∼184 Mt of raw materials on the Mississippi River in 2016 (e.g., Wetzstein et al., 2021). Dredging of navigation channels is needed on many rivers to mitigate sediment deposition to meet minimum navigation depths (e.g., Cox et al., 2021; Pinter & Heine, 2005; USACE, 2005). Sand and gravel (S&G) mining is a large and growing global industry (e.g., Bendixen et al., 2019; Hackney et al., 2020), with a threefold increase in sand demand worldwide over the last two decades (UNEP, 2019).…”

Building and raising land: meandering rivers and filling estuaries

“…This culminated in deeper channels (Sherwood et al, 1990;Ahlhorn et al, 2012;Nnafie et al, 2019), which may be aggravated by dredging campaigns (e.g. Cox et al, 2021).…”