Engineering the Levelling Systems of the Sea Locks in The Netherlands; Taking into Account the Effects of the Density Difference
Ir. W.C.D. Kortlever*, Ir. A.J. van der Hout**, Ir. T. O’Mahoney**, Ir. A. de Loor**
In The Netherlands two deep sea locks are being built, one at the entrance of the North Sea Canal and one at the entrance of the Ghent-Terneuzen Canal to allow larger sea-going vessels to call in at the main ports of Amsterdam and Ghent. At both locations the new lock is built next to the existing deep lock. Principally, the choice of a type of levelling system is determined by the maximum head difference, the required levelling time, main vessel dimensions and mooring configurations. Since these locks maintain the transition between fresh (brackish) and salt water, density currents in the lock during levelling lead to additional hydrodynamic forces on the moored vessel. Therefore, this density effect must be included when engineering the levelling system.
The Existing North Lock and the New Lock at IJmuiden
The North Lock has been built in the nineteen twenties. The lock chamber is 400 m long and 50 m wide. The sill lies 15 m below mean sea level. At IJmuiden the head difference varies between about 4 m and - 1.5 m. As the maximum head difference during mean springtide varies between only 1.6 m and - 0.3 m, differences during normal conditions are relatively small.
The design of the levelling system for this lock was based on the designs of the German sea locks built at that time. A scale model study was carried out in Germany to study the behaviour of several levelling systems considering different culvert lay-outs. In these model tests the density difference was not considered. Based on the test results, a system with short culverts in the lock heads was chosen. Levelling through gate openings was regarded as not feasible, mainly because of the impact on the steel construction of the gate, but also due to the expected flow forces on the moored vessel in the lock.
The new lock chamber which is now under construction is 545 m long and 70 m wide. The sill lies more than 17 m below mean sea level. For the new lock two types of levelling systems have been considered: openings in the lock gates and a system with short culverts in the lock heads. Now, based on the current state of knowledge in gate construction and lock hydraulics, it has been concluded that gate openings are feasible, provided that levelling times may be longer to some degree. Again, an extensive scale model study has been carried out, now including the effects of the density currents during levelling due to the density difference over the lock head. These tests showed that with the more extreme head differences the density currents during levelling produce the highest forces on the vessel. When the system with openings in the lock gates had been chosen for the final design, special valve lifting programs have been derived to keep the forces below the set force limits and to meet the required levelling times. The chosen system consists of 16 rectangular ducts through the gate, each 2.2 m wide and 3 m high and with a valve in the middle of the gate.
The Existing West Lock and the New Lock at Terneuzen
The West Lock has been built in the nineteen sixties. The lock chamber is 335 m long and 40 m wide. The sill lies almost 13 m below mean sea level. The water level on the canal is about 2 m above mean sea level. The head difference varies between 1.5 m and – 4.8 m. Compared to IJmuiden the daily maximum absolute value of the head difference is considerably larger, 4 m versus 1.4 m, corresponding to a mean water level on the canal of 2.1 m and mean low tide of - 1.9 m outside. Taking into account these higher head differences, it was decided to fill and empty the lock through two bottom grids, located at about one quarter and three quarters of the chamber length. By distributing the discharge over these two grids, the resulting translatory waves are significantly reduced, and the corresponding forces as well. This concept was originally worked out without considering the density effects. However, when this system was tested in a scale model, in a later phase also including a density difference, it showed that the density forces did not lead to extra-long levelling times. Not only the translatory waves are significantly reduced, but also the density currents.
The building contract for the new lock has been awarded in the summer of 2017. The lock chamber length is 427 m, the width 55 m and the sill depth 16.5 m. During the conceptual design of the levelling system a comprehensive study has been carried out focusing on the possibility of a through-the-gate-system with 12 circular ducts, based on the recent results for the new lock at IJmuiden. An exploratory scale model study has been carried out to determine the shortest levelling times which could be attained, and to solve the uncertainty regarding the density currents. These model tests indisputably showed that the density forces when filling during low tide, i.e. filling with fresh water, could only be reduced by prolonging the levelling times outside the acceptable range. Therefore, it was decided to specify in the requirements for the tender a levelling system which is more comparable with the system of the existing West Lock. In the requirements two alternatives were specified: the West Lock system with two bottom grids, and the Baalhoek system with four wall grids, two per lock head located opposite each other. In both systems the levelling discharge is distributed over two parts of the lock, thus reducing the hydrodynamic forces due to the density currents. The contractor of the new lock has chosen for the West Lock system. The final scale model tests, to determine the valve lifting programs are planned in May 2018.
*Hydraulic engineer, Rijkswaterstaat, Ministry of Infrastructure and the Environment, Griffioenlaan 2, 3526 LA, Utrecht, The Netherlands
**Researcher / consultant, Department of Hydraulics for Infrastructure and Industry, Hydraulic Engineering, Deltares, Boussineqweg 1, 2629 HV, Delft. The Netherlands