Understanding the effects of resolving nautical bottlenecks on the Danube: a KPI-based conceptual framework

“Fuel consumption” measured in litres per 100 km is a major KPI, as resolving nautical bottlenecks leads to fuel consumption decreases and, thus, monetary savings and an increase of the net benefit. Several researchers conclude that low water depths result in higher fuel consumption. Hekkenberg et al. [21] argues that low water depths require a significantly higher amount of power to reach a given speed, therefore resulting in a significant increase in fuel consumption. Hekkenberg [20] adds that, besides low fairway depths, limited fairway widths increase fuel consumption even further, as the vessel acts like a blockage that is moved through the river and that, therefore, even more fuel is needed to propel a vessel at a given speed. Meißner et al. [33] relate higher fuel consumption to higher transport costs. To save fuel, vessels have to sail at a lower speed [38], which influences sailing times and, thus, the total transport time of goods. Higher fuel consumption also leads to an increase in GHG emissions and, thus, in external costs [8, 17, 22, 36]. Transport-related GHG emissions have a substantial negative impact on the environment and must be reduced. A 90% reduction is a major goal of the European climate policy until 2050 [13]. “Transport emissions” is another relevant KPI related to nautical bottlenecks and is therefore included in our KPI framework.

The rehabilitation of nautical bottlenecks influences transport duration directly and may result in an economic benefit, thus we include “transport duration” measured in hours as a KPI. Low water depth restricts the speed of a vessel and leads to an increase in sailing times [38, 40]. According to Hekkenberg et al. [21] the maximum sailing speed of a conventional cargo vessel is determined by the actual water depth. A significant amount of power and fuel is needed to reach the maximum speed. Therefore, experienced captains hardly sail faster than 70% of the maximum speed, which leads to longer transport times. Haselbauer et al. [19] agree that captains usually lower the speed of the vessel at already known low water sections, i.e., nautical bottlenecks. Du et al. [10] add that traversing hazardous sections may take longer when the water depth is low. However, long waiting times can additionally contribute to an increase in overall transport duration [9, 26]. Increased transport costs are directly related to an increased transport duration [33], as longer transport time translates into less cargo being able to be transported overall, which leads to less transport supply and additional trips [9].

The KPI “transport supply” measured in tonnes consists of the actual loading capacity of vessels, the number of (additional) trips that could be carried out within a given timeframe, and the fleet composition, i.e., vessel’s size in operation. Loading capacity is usually measured in tonnes and consists of a vessel’s consumables and the payload [48]. Nautical bottlenecks influence the loading capacity of inland vessels [33] and the transport supply in the market itself [26]. Loading capacities are usually hardly affected above a critical water level. Below this, certain loading capacities decrease accordingly and imply restrictions for inland waterway vessels [27]. For the Mackenzie River, which was highlighted in the research of Du et al. [10], this critical water level is at 4.2 m. Riquelme-Solar et al. [38] indicate a similar value (i.e., 4.3 m) in their research. Above this value, inland vessels can be fully loaded on the Rhine, while they lose 85 tonnes of capacity per 10 cm loading draft. However, these values differ between vessels, as vessels can respond highly differently to changes in waterway conditions, despite carrying the same amount of cargo [21]. Thus, fleet composition plays an important role when it comes to transport supply. Li et al. [29] propose that carriers should focus on smaller ships, which fit better with the actual waterway parameters. Jonkeren et al. [26], Schweighofer [40] and Riquelme-Solar et al. [38] agree that lighter and wider vessels can provide up to 20% more loading capacity at low water depths and highlight that these vessels are performing less efficiently in normal water conditions. Christodoulou et al. [9] discuss the bearing capacities of ships related to different vessel size categories. A large vessel loses more capacity in low water conditions than a smaller vessel. However, larger ships imply lower capital cost per tonne of cargo and, therefore, have a major cost advantage over smaller ships in sufficient water conditions [20]. In practice, a significant trend towards larger vessels is observable [49]. Larger vessels lead to a greater dependency of the loading capacity on the water levels, leading to a greater influence of the water levels on the transport price [44]. Haselbauer et al. [19] agree that transport costs vary based on the fleet composition and therefore propose the calculation of transport costs for any investment strategy referring to fairway conditions on the basis of fleet composition. The vessels used on the Danube can be loaded on average with 55 to 60% of their initial capacity, whereas shallow water sections restrict the capacity further, dropping the loading capacity to only 40% [23]. As a result of the restricted capacity, additional trips frequently need to be carried out [9]. There are two main reasons why additional trips have to be carried out at low fairway levels. Christodoulou et al. [9] argue that low water depth leads to decreasing sailing speeds and, thus, decreases the overall transport supply and requires additional trips. Hekkenberg [20] agrees that the decreased speed (i.e. 70% of maximum speed) limits the number of trips a vessel can make and, thus, limits the amount of cargo a vessel can carry in a given amount of time. Another reason for additional trips is the reduced loading capacity per vessels due to low water depths [9, 33]. According to Meißner et al. [33] the situation regarding additional trips is accentuated particularly in periods of extreme or long-lasting low water. Then the vessel supply is limited compared to the transport demand, which leads to the need of shifting cargo to other transport modes.

The vessel loading draft measured in metres is directly related to fairway depths [10, 33], [1]. According to van Dorsser et al. [48] a vessel’s loading draft correlates with the vessel’s capacity, thus, if the loading draft increases, the capacity increases alike. Vessel draft is the underwater depth of a ship (Sheng et al.), which can vary depending on the weight loaded. In fact, the maximum draft of a vessel cannot be utilised up to a specific water level, leading to a limitation in transport supply [9]. According to Hoffmann et al. [23] and Haselbauer et al. [19], on the Danube a vessel loses between seven and 14 tonnes of capacity with each centimetre of decreased loading draft. In contrast, Riquelme-Solar et al. [38] state that the loading capacity decreases approximately 85 tonnes per 10 cm of lost loading draft. Schweighofer [40] provides an example: A large motor cargo vessel with a permitted draft of two meters is able to carry around 1200 tonnes, which represents only 40% of its maximum loading capacity. The larger the vessel and, related to that, the larger its maximum loading draft, the stronger the effect of low water levels on capacity [49]. As water levels vary on the course of the waterway [5], the most shallow river stretch on any given route will limit the maximum loading draft and, therefore, the maximum loading capacity [19].

Transport costs measured in monetary terms vary with a change in transport supply [28] and, thus, of water levels. Consequently, transport prices rise the same [38], often in a one-to-one relationship with the rise in transport costs [25]. In some specific cases, e.g., the tanker market, the increase of transport prices may lead to a decrease in demand for inland navigation services, as customers are rarely willing to pay increased prices [49]. In general, transport costs are higher for larger vessels in low water conditions than for smaller ones, as larger vessels lose more capacity than smaller vessels [33]. Haselbauer et al. [19] agree that transport costs largely depend on the fleet composition. Transport costs generally rise in low water conditions, not only due to a decrease in loading capacity, but also because of the increase in fuel consumption that occurs when a vessel sails on low water levels [20]. Jonkeren et al. [26] point out that stakeholders of the inland waterway sector can contribute to reducing the effects of low water depths, e.g., the increase in transport costs, by taking adequate adaption measures. According to Riquelme-Solar et al. [38], focusing on smaller vessels could be such an adaption measure,another would be to make already withdrawn barges operational again, which helps to maintain the total carrying capacity of the inland waterway despite low water levels [26]. Sys et al. [44] highlight a major difference between temporary low water periods and permanent nautical bottlenecks. While the rise in transport costs due to low water periods can be compensated for by charging a low water surcharge, low water surcharges are not used in the case of nautical bottlenecks. In the latter case, the increased costs have to be covered mainly by the shipping companies. Jonkeren et al. [26] found that freight prices on the Rhine can increase up to 75% per tonne in low water periods, which may affect the IWT market. Market-related KPIs constitute a separate cluster and are discussed in the section below.

“Transport demand” measured in tonnes is a major KPI when analysing nautical bottlenecks. In Europe, the IWT market is characterised as strongly competitive, since there are many carriers on the market offering similar services at similar prices [27]. The loading capacity of vessels decreases due to low water levels, which, on the one hand reduces the effective transport supply in the IWT market, and, on the other hand is responsible for higher costs in IWT [26]. As the IWT market is strongly competitive, the increase in costs is directly transferred to the shippers, who have to cover higher freight prices, which in turn leads to a decrease in transport demand due to low water levels [49]. Insufficient fairway depth impedes both the demand and the supply of the IWT market. The growth of market shares in IWT depends on substantial improvements in the transport mode, e.g., regarding the higher availability of the waterway, which leads to an increased utilisation and lower costs of IWT. Both utilisation and transport demand rises if the fairway conditions and the water levels are satisfactory throughout the year [23], which is essential to ensure the competitiveness of IWT [19].

"Modal share," measured as the percentage of IWT compared to other transport modes is an essential key performance indicator (KPI), which allows for the quantification of competitiveness. Its value may change through the elimination of nautical bottlenecks. According to Hekkenberg [20], the interaction between vessel and waterway must be considered regarding the competitiveness of IWT, as the majority of an inland vessel’s cost is influenced by the properties of the waterways. If the water level is high enough, the largest vessels incur the lowest costs per tonne transported regarding factors, such as personnel or capital costs. At low fairway depths, however, a large vessel loses the most loading capacity, which is a disadvantage for competitiveness. For navigation on low water levels, smaller vessels are more suitable and more economically viable. This leads to a dilemma, as vessel owners have to decide on a vessel size that is least disadvantageous to the competitiveness of IWT [38]. Higher prices in IWT may force shippers to transport their goods by road or rail, thus decreasing the modal share of inland navigation, while increasing the modal share of road and rail [25]. However, this effect is rather insignificant in the short run, with a modal shift rate to road and rail of less than 10% according to Jonkeren et al. [26]. Riquelme-Solar et al. [38] agree that the loss of transport demand due to modal shift may be limited, as the loading capacity of inland navigation is far higher than the loading capacity of road and rail. Furthermore, the nature and quantity of goods transported using IWT, e.g., bulk goods, impede a modal shift to other transport modes [5] and experienced shippers using IWT may have sufficient knowledge to plan in advance for low water periods, which occur seasonally, but not every year to the same degree [26]. Nevertheless, the cross-elasticity, i.e., the degree to which IWT can be substituted with another transport mode, is, according to Beuthe et al. [4], highly dependent on the goods transported. While for some goods the cross-elasticity is high, e.g., petroleum, for other product groups, e.g., fertilisers, it is low. Hoffmann et al. [23] state that inland navigation is not competitive, if the average loading capacity drops under 50%, which may be the case when water levels are low. Therefore, sufficient fairway parameters on the entire waterways are crucial to guarantee the competitiveness of inland navigation [19].

Throughput (at a specific geographic location) is included in our KPI framework and measured as tonnes passing a specific geographic location. Due to the linear structure of inland waterways, each bottleneck may limit the utilisation of the entire transport fleet. The throughput at a bottlenecks’ location is therefore highly significant. The higher the throughput at a specific bottlenecks’ location, the more utilisation is limited. If the throughput is high, the effect of resolving a nautical bottleneck is much higher than if the throughput at the bottleneck is low [23]. Another relevant aspect is that bottlenecks that are geographically close to each other generally influence each other. If several bottleneck locations are geographically close to each other, the rehabilitation of a single bottleneck is hardly sufficient to generate positive effects on the utilisation of the waterway. For this, the other surrounding bottlenecks must be removed, as the shallowest section limits the utilisation of the entire river stretch [19].

“Fairway depth” measured in metres is one of the most relevant KPIs with regard to nautical bottlenecks, as the actual fairway depth is decisive in determining the criticality of a bottleneck [23]. The smaller the fairway depth, the less a ship can be loaded. Thus, shallow fairway depth reduces loading capacity, which leads to lowered transport supply and negatively impacts economic viability [9, 19]. Therefore, to calculate the monetary benefits of resolving nautical bottlenecks, the fairway depth is an indispensable parameter, as all other KPIs are linked to it.

The thematic analysis proves that various KPIs are affected by nautical bottlenecks. Nautical bottlenecks have such a substantial impact on the net benefit of inland navigation that Meißner et al. [33] propose a monthly to seasonal forecast framework for water levels. To create the conceptual framework based on the KPIs identified above, we extracted the main assumptions from the thematic analysis. The main assumptions for each KPI are illustrated in Table 7, representing the foundation for developing the conceptual framework.

The assumptions for each KPI are interconnected and thus show interdependencies between them. Figure 3 illustrates the synthesis of these assumptions in a conceptual framework based on the identified KPIs, supporting the understanding of the effects of removing nautical bottlenecks and highlighting the interdependencies between the KPIs.

The main goal of resolving nautical bottlenecks is the increase of fairway depth. After successful maintenance or rehabilitation works, fairway depth should be at least at 2.5 m on the Danube. With an increased fairway depth, the vessel draft increases, thus the inland vessels are able to uptake heavier loads, which can be up to 85 tonnes more cargo for each 10 cm more draft. Also, the overall transport duration decreases, as inland vessels may navigate faster without a nautical bottleneck impeding their trip. The actual improvement in transport time depends on whether the river is free-flowing or regulated. These two measures (i.e., transport duration and vessel draft) are linked to an increased overall transport supply, as each inland vessel has an increased loading capacity and due to the lower transport duration, the total amount of possible trips with the available number of vessels increases. A higher transport supply, thus, leads to lower transport costs, as the vessel can be better utilised leading to a cost reduction per transported tonne. An adequate fairway depth results in decreased fuel costs, as inland vessels require a lower amount of engine power to reach a given speed if the fairway depth is sufficient. Less fuel costs contribute directly to lowering the overall transport costs. Lower transport costs may result in an increase of transport demand and, thus, of modal share. Lower transport costs for shipping companies allow to offer more attractive transport services for customers. An increased modal share will raise the overall IWT volume, i.e., the overall tonnes of cargo transported, on the inland waterway. An enhanced IWT volume, which is also influenced by the actual economic activity, contributes directly to the economic benefit, which is generated out of resolving nautical bottlenecks, as the increase in transported tonnes provides financial benefits for the inland navigation sector.