Prolonged droughts induced by climate change are a significant threat to inland shipping on Dutch rivers, particularly on the river Waal, a branch of the river Rhine. The anticipated shift to a more rain-dominated river, due to reduced meltwater from Alpine glaciers, may aggravate extreme low flow conditions, threatening the Waal's navigability. To improve the navigability in a rain-dominated river system, the Dutch government decided the transformation of the river Meuse into a confined river with seven weir-lock complexes. This study aims to compare the discharge regimes of both rivers to examine whether the river Waal turns into a rain-dominated system similar to the river Meuse, which may justify weir-lock complexes in the Waal as a potential solution. Analyzing 100 years of discharge data at Lobith, using the Pardé coefficient, shows that the characteristics of the Waal's runoff regime are shifting from a mixed-river towards a more rain-dominated river. Future climate projections indicate more pronounced extremes in runoff regimes, emphasizing the changing nature of the river Waal. While becoming more rain-dominated, the Waal's discharge regime is not expected to match the Meuse's before 2085.

Prolonged periods of drought affect river discharges and cause water levels and available water depth to drop for extended periods of time. Low water depth has a major impact on the loading capacity of inland ships, and as a consequence on the transport capacity of the overall waterborne supply chain. Individual ship owners have detailed knowledge on how much the draught of their ship and the associated cargo weight should be reduced to adapt to low water. These parameters are even adjusted as a function of environmental circumstances (e.g. composition of the riverbed) and type of cargo. This detailed knowledge is, however, not accessible at an aggregated level to assess the effects on the overall transport capacity of an inland waterway transport network. Based on a range of field observations and information collected from individual ships, this article introduces a general model to define the effect of low water constraints on the deadweight and payload of inland ships, for which only the type, length, and beam of the vessel serve as mandatory input. Availability of a general model of the capacity reducing effect of lowered water depth is important for the design and operation of robust transport chains on the one hand, and for the optimisation of fairway maintenance and long-term infrastructure development on the other.

The summer and autumn of 2018 showed the negative drawback of both low-flow conditions and bed degradation over the last century in the Dutch Rhine. This resulted in record-breaking low water levels, extreme low navigation depth and subsequently nautical problems. The Rhine’s long-term bed degradation is the response to river training of the last centuries focused on improvement of navigation and flood protection. Over the past hundred years the river bed of the Upper Dutch Rhine branches degraded 1 to 1.5 m, while a current trend of 1 to 2 cm per year is observed (Blom, 2016). The ongoing bed degradation is problematic since it induces (i) a reduction of navigation depths due to the existence of non-erodible layers, (ii) lowering of ground water levels and dehydration of nature, (iii) lowering of coverage rates of infrastructure (e.g. cables in subsoil, bridges and groynes) and (iv) a gradual shift in discharge distribution at the bifurcation points. As climate change will increase the inter-annual variability of the Rhine’s discharge pattern, low-flow conditions are likely to occur more often, reinforcing the abovementioned impacts on nature and navigation (Sperna Weiland et al.,