Effects of drought on the traffic capacity of the river Waal and the occurrence of congestion

Abstract

This
research focuses on the impact of extreme low river discharges, meaning
discharges below 1200 m2/s at Lobith. In 2018 extreme low river discharges in
the river Rhine led to congestions in the main Dutch part, called the river
Waal. The river Waal is an important river for inland navigation, but during
low discharges the vessel draught reduces and consequently the transported
cargo volume per shipment reduces. To compensate the loss of transport
volume, the total number of shipments increases, leading to an increased
traffic intensity on the river Waal.  The
purpose of this study was to investigate the effects of extreme low discharges
on the traffic flow and traffic capacity in the river Waal. The study consisted
of two elements: a study combining fleet data and hydraulic information and a
traffic simulation study. During this research IVS90 data was used as the
source for inland waterway transport data. Based on literature and previous
river Waal studies the river section between the Pannerdensche Kop and the
Maas-Waal canal was selected as the river section to investigate in more
detail. Multi-beam measurements in combination with water level data were used
to generate cross-sectional profiles in order to carry out the simulations. The
cross-sectional profiles were highly variable. From these cross-sectional
profiles the navigable river width was determined. It was found that a river
depth of 2.80 m was no longer available at all cross-sections from a discharge
of 900 m2/s and lower. Therefore, the navigable width was determined at a river
depth of 2.0 m for the discharges 1020, 900, 800, 700 and 600 m2/s. Also, it
was found that with reducing discharge the navigable width of the
cross-sections reduced.   The fleet
composition was determined in detail for four weeks representing the drought of
2018. These four weeks in 2018 represented weeks with average discharges around
1020(2x), 800 and 700 m2/s. The river Waal fleet composition was determined
based on the Rijkswaterstaat vessel classification system (RWS-class) and
categorised in three groups: coupled units (all RWS-classes with index C),
push-tow units (all RWS-classes with index B) and motorised vessels (all
RWS-classes with index M). It was found that the number of passages by coupled
units and push-tow units was effected largely during the drought of 2018. The
number of passages by push-tow units reduced significantly from October 2018 as
the discharge dropped below 1500 m2/s and the number of passages remained low
until the discharge raise above the 1020 m2/s at the end of 2018. The number of
passages by coupled units increased already before the discharge reached the
1020 m2/s limit and continued to increase throughout the period of drought. The
number of passages by coupled units started to decline only after the discharge
rose above the 1020 m2/s again. Even though the daily average number of
passages increased during the drought of 2018, the total transported cargo
volume per day decreased. There was a strong relationship between discharges
below 1200 m2/s and the transported cargo volume per day.  Within this study special attention was given to the occurrence of
congestion in the river Waal. The occurrence of congestion was
investigated using the traffic simulation model SIMDAS. As indicators for
congestion a fluency and safety limit of 8% was used to evaluate the simulated
traffic, as well as simulations of the traffic flow. As safety parameter the
penetration of the safety margin of a vessel in percentage of the total number
of vessel interactions was used. While the percentage of the number of vessels
that need to reduce their speed fully during their passages was used as the
fluency parameter. With SIMDAS also the impact of increased traffic intensity
and reduced navigable width were analyzed. The simulation results showed that
the reduced navigable width had more impact on the delay time, the fluency
parameter and the safety parameter than the increase of the daily intensity.
During the simulations large congestions occurred for discharges of 800 m2/s
and lower, but small harmonically moving congestions already occurred for a
discharge of 1020 m2/s. Harmonically moving congestions, meaning seven or more
vessels traveling behind one larger or slower vessel while awaiting room to
overtake, were observed in the traffic simulations. Permanent congestions with
ten or more vessels were observed in the simulation of the river width with a 800
m2/s discharge. The data analysis and the traffic simulations clearly showed
the effects of the extreme low discharges on the traffic flow and traffic
capacity. The conclusion of this study is that the traffic capacity of the
river Waal is at its limit at discharges of 800 m2/s and lower.  This study made also clear the need for
correct fleet data and river bed levels. The limited available fleet data
reduced the accuracy of the results. The river bed should be monitored
regularly in order to know the actual water depth particularly during low
discharges. Furthermore, it is recommended that highly variable river profiles
are implemented in the traffic simulation model SIMDAS to improve the
simulation of the vessel's sailing trajectories. Also, the validation of the
vessel trajectories in SIMDAS with AIS data is recommended in order to evaluate
the traffic intensity on the traffic lanes in the river.