Keywords

1 Introduction

The Krammer locks (see Fig. 1) are set of shipping locks in the Netherlands between the fresh water lake Volkerak (Volkerak Zoommeer) and the saltwater Eastern Scheldt estuary. They form part of the Dutch Deltaworks in the South-West Delta of the Netherlands and have been operational since the 1980s. They currently use a salt-freshwater separation system incorporating salt water substitution (van der Kuur 1985).

The complex consists of two commercial shipping locks capable of accommodating class CEMT VIb vessels and two smaller recreational shipping locks. The target water level in the lake Volkerak-Zoom is around NAP+0.0 m, with a normal variation of NAP ± 0.25 m, where NAP is the Normaal Amsterdam Peil or approximately Mean Sea Level. On the Eastern Scheldt side, the water levels are tide dependent, for which a maximum operational water level was set to NAP +2.75 m and a minimum operational water level of NAP −2.50 m.

The commercial locks are 280 m long between the gates and 26 m wide. The available width between the fender is 24.1 m. The lock floor is at a depth of NAP −6.25 m. This floor is perforated to allow the substitution of salt water for freshwater during lockage. The salt water is then transferred to a chamber below the lock which has a floor at depth NAP −11.0 m. The density difference across the lock complex can reach as high as 20 kg/m3.

The concept of salt water substitution was developed in the 1960s for the lock at Dunkirk and has come to be known as the ‘Dunkirk system’. This system has been optimized and applied at several locations, such as the Krammer and Kreekrak in the Netherlands. The improved system of the large locks at Krammer in the Netherlands, is considered the state of the art for this type of locks (PIANC 2021).

In the Dunkirk system, the saltwater is introduced and withdrawn vertically through the perforated lock floor while the fresh water is supplied and evacuated in horizontal transversal direction through wall openings, located high in the lock chamber walls. The openings in the walls and on the lock floor are distributed along the entire length of the lock to reduce mooring forces.

Fig. 1.
figure 1

(Source Rijkswaterstaat).

Aerial view of the Krammer locks complex. The approach harbor on the Eastern Scheldt side is shown to the left and lake Volkerak-Zoom to the right.

Full size image

Owing to tidal variations in the waterlevel on the sea side relative to the lake side, different locking cycles are possible, as illustrated in Fig. 2. The first illustrated situation is when a ship has entered the lock from the sea side. Once the salt water gate is closed (phase 1), the lock is levelled via the lock floor to or from a salt water basin (phase 2 - levelling), depending on the tidal phase. Afterwards, the boundary plane between salt and fresh water in the lock chamber is lowered (phase 3 - exchanging) by simultaneously withdrawing salt water through the lock floor and introducing fresh water through the wall openings. This operation is ended once the boundary plane is below the perforated lock floor (phase 4). The lock gates can then be opened without a density current being initiated as the density is equal across the gate.

The overall time required for a lockage is larger (more than twice as long) than in conventional locks with levelling valves in the lock gates. While levelling through the lock floor is more efficient (lower hawser forces of vessels), a lot of time is lost by exchanging salt and fresh water. This additional time is governed by the allowable mixing of salt and fresh water on the one hand and by allowable hawser forces on the other hand.

At the large Krammer locks, the lateral wall openings are in direct contact with the fresh water basin through short ducts. The Krammer lock complex is designed in such a way that the fresh water basin enfolds the lock chamber entirely, see Fig. 1. This is an important improvement of the original Dunkirk system, because this ensures an even distribution of the fresh water entering through the lock chamber walls.

The actual lock floor is located below the perforated floor. The space in between the perforated and the actual lock floor is connected to several salt water culverts in the middle of the lock chamber. The required salt water is supplied or withdrawn through these culverts. The supply through the culverts can be either gravitational or mechanical (by means of pumps). In the case of the large Krammer locks, several water saving basins have been constructed next to the lock complex in which salt and/or brackish water are stored. The basins and the outer salt water basin are interconnected with culverts that are equipped with valves and pumps to allow different filling and emptying scenarios.

The salt – fresh separation system is very effective with reductions up to 90% to 98% of the salt load on the fresh water basin. The system leads to an additional fresh water loss of up to 30% to 80%, depending on the tidal cycle.

The main disadvantages of the Krammer system are the high initial capital expenditure and ongoing operating expenditure, the additional time required for locking, the transversal forces on the ships, the necessity of pumping and the complicated control system.

Fig. 2.
figure 2

Schematic representation of the current (Dunkirk) system of salt-freshwater separation.

Full size image

2 The Innovative Fresh-Salt Separation System (IZZS)

Rijkswaterstaat plan to replace the salt-freshwater separation system at the Krammer locks with a new innovative system, the Innovatieve Zout-zoet Scheiding in Dutch (IZZS). The motivation for considering a change in the system are long lockage times and high maintenance costs of the current system. The basis of the new system is the installation of a bubble screen (Van der Burgh) to allow the lock gates to be opened directly after levelling. The lock-exchange flow, as a result of density currents, is reduced by the bubble screen. In this way, the exchange of salt and freshwater through the perforated floor of the lock is skipped and the total lockage time is shorter. In addition, the valves along the length of both lock chambers can be decommissioned. The bubble screen does not have the same effectiveness as the Dunkirk system so in addition a flushing discharge though the locks is needed. An option of an additional water screen has also been investigated but will not be used in the final system (see Fig. 3 for a schematization of all components). The adapted lockage cycle of the IZZS is given in Fig. 4.

Fig. 3.
figure 3

Schematic representation of the options which have been investigated for the Krammer locks, including bubble screen, water screen, flushing discharge and raised lock sill. (From Keetels et al. 2011)

Full size image

The Innovative Salt-Fresh Separation system has been designed by a series of research projects. In 2009 the first investigations were made for the system. Laboratory tests and Computational Fluid Dynamics (CFD) simulations were made of bubble screens followed by a field measurements at the Stevin locks in the Netherlands (Keetels et al. 2010). These tested a new design of a bubble screen to improve the effectiveness and robustness when compared to older designs (Abraham van der Burgh, van der Kuur). These first investigations were made for a possible system at the Volkerak locks, at the inland end of lake Volkerak-Zoom owing to plans (since put on hold by the Dutch government) to make the lake Volkerak-Zoom a saltwater body. As the two lock complexes are rather similar, the first designs of the system could be easily transferred to the Krammer locks. Subsequently, in 2014 a pilot field campaign was made at the smaller Krammer recreational locks. The hydraulic model WANDA-Locks was then developed for the Krammer system, having been validated against the Stevin lock measurements (Deltares 2010). This model was used from 2015 until the present to study effect of the IZZS on the salt load into lake Volkerak-Zoom and to weigh up all the options of operation of the new system for many of the different functions of the lock complex.

2.1 Functions of the Krammer Locks

The Krammer locks perform many functions in the water system of the Dutch South West Delta;

  • Safe and fast shipping; the lock complex provides a shipping route between the Scheldt and the Rhine.

  • Flood safety; excess water from upstream is mostly discharged via the Haringvliet but the Krammer locks also have a flushing function in extreme cases.

  • Water management; management of water levels in lake Volkerak-Zoom is provided together with Bathse Flushing Sluice

  • Fish migration; an additional fish migration route through the new flushing sluice at the Krammer locks is part of the future renovation.

  • Salt management; the Krammer locks are the principle point of salt intrusion into lake Volkerak-Zoom and mitigation is therefore most effective at this location.

2.1.1 Restrictions on Water Usage

In many parts of the Netherlands (and elsewhere (Mausshardt and Singleton 1995) saltwater intrusion at locks is effectively mitigated through flushing of the locks. However, the Krammer locks have several restrictions on its water usage which limit this option.

  • Water availability owing to drought or agriculture; lake Volkerak-Zoom acts as an important freshwater provision for agriculture in Brabant and the South West Delta. It is maintained by freshwater discharges from the Hollands Diep and the rivers of Brabant. In times of reduced discharges restrictions are in place in on the discharges available for flushing of the locks.

  • Saltwater ecology in the Eastern Scheldt; near the Krammer locks on the Eastern Scheldt side there are ecologically valuable saltwater habitats and a number of aquaculture areas for mussel and oyster farming. Too much freshwater discharge through the locks would adversely affect these areas. It is therefore limited to average daily discharges of 9 m3/s in the growing season. Outside the growing season the discharges can be up to 3 times more.

  • Nautical safety; flushing through the locks can create currents in the approach harbours which have an effect on the maneuverability of vessels entering (and exiting) the locks. Rijkswaterstaat maintains guidelines on limiting longitudinal and transverse velocities in the fairway.

  • Energy usage for pumping; in order to minimize energy use at the locks flushing of water is performed as much as possible under the gravitational head available at low water on the Eastern Scheldt.

Fig. 4.
figure 4

Schematic representation of the proposed Innovative Salt-Freshwater separation system.

Full size image

These multiple functions and numerous restrictions on water use make the design of a salt-fresh separation system and its planned operation complicated. The model instrument developed in this project has been used to balance all the conflicting requirements of the operation. Some of the results of this modelling are highlighted in the following section.

3 Hydraulic Modelling of the Krammer System

3.1 WANDA-Locks Model

WANDA (see reference) is a one-dimensional computational program developed by Deltares. The origin of the program is found in calculations on flow through pipeline systems. Various hydraulic components, such as pumps and valves, are included in its library. The WANDA-Locks library (see reference) allows the determination of the salt mass in the reservoirs. The formulae used are based on Deltares (2010). The lock chamber is a component included in the WANDA-Locks component library. Approach harbours can be modelled using the boundary condition for water level and salinity. The lock chamber and approach harbour are connected via valves that can comprise both the levelling system and the doors.

In case of advective flow the computation can be thought of as bookkeeping: the incoming discharge has a known salinity and can thus be taken as a load of salt being added to the reservoir. In case of baroclinic flow the hydrodynamic formulae simulate the density current using a fourth component in the WANDA-Locks library. These formulae are extended to include the impact of mitigating measures such as water and/or bubble screens by defining a so-called salt transmission factor, see Weiler et al. (2015).

Fig. 5.
figure 5

The schematization of lake Volkerak-Zoom in the WANDA-Locks model The Krammer locks are modelled in detail and provide fluxes and concentrations into the box at the west (left) side.

Full size image

The model also schematises lake Volkerak-Zoom as different basins of water (see Fig. 5) which exchange salt through diffusion. The diffusion coefficients are an input of the model and have been calibrated and validated against measurements in the Eastern Scheldt and lake Volkerak-Zoom, see Fig. 6. The calibration was made against measurements at a number of locations (seen in Fig. 5) for a relatively dry year (2003). Only the January to June months were used for the calibration and subsequently the model was validated against the remaining months of the year (July to December).

Fig. 6.
figure 6

A comparison of the predicted concentrations from the WANDA-Locks model and measured values in 2003. The model is calibrated on the data January to June and validated on the data July to December.

Full size image

3.2 Effect of Lockage Operations

Previous work (Weiler et al. 2015) has shown the effect of gate open times (the amount of time that the gate is open for vessel sailing in and out of the lock) on salt intrusion through lock-exchange. The lock-exchange component of WANDA-Locks models the lock-exchange as a time dependent transfer of salt from lock chamber to approach harbour (or vice versa). The traffic handling is managed via a control module (van der Ven 2015).

For the Krammer locks the gate open times are not of importance for the current Dunkirk system as the gates are only opened after the saltwater in the lock chamber has been exchanged with fresh water, so there is no density difference over the lock gate. For the IZZS, the gates are opened directly after levelling and the bubble screens are operated. Their effectiveness is dependent on the gate open times (see Fig. 7).

Fig. 7.
figure 7

WANDA-Locks simulations of the IZZS for a representative dry year (2003) and a representative normal year (2009)Footnote

The simulation for this representative normal year was performed at an early stage in the project. Some of the aspects of the operation used for this simulation are not fully compatible with those used for the definitive simulations of the dry year. The normal year is included therefore only as a rough reference level.

. The effect of gate open times and fish migration measures at the locks is also included. The dashed black line is the threshold value for salinity maintained by Rijkswaterstaat.

Full size image

3.3 Effect of Sea Level Rise (SLR)

Simulations have been run of the base case scenario with additional Sea Level Rise (SLR). A maximum 80 cm SLR has been simulated (see Fig. 8). In the base case a representative dry year (based on 2003) was simulated. In this case the lower basin, which stores the salt lockage prisms during high tide, is emptied to the Eastern Scheldt during low tide under gravity. As the sea level rises, the tidal window for emptying becomes shorter and smaller and the lower basin cannot be fully emptied. Some lockage prisms must then be discharged onto lake Volkerak-Zoom, causing salt intrusion. The threshold value for allowable salt concentration in lake Volkerak-Zoom is reached after only 20 cm of sea level rise.

To mitigate this, it is possible to install a pump in the culvert leading from the lower basin to the Eastern Scheldt to assist (when necessary) with emptying the lower basin. When this is done (see Fig. 9) the salt intrusion can again be controlled within the threshold value until a higher level of sea level rise. At that point the capacity of the pump is reached, and other factors in the operation make control of salt concentration within the limits difficult (see flushing discharge below). This required only 1 pump in the 4 available culverts.

Fig. 8.
figure 8

WANDA-Locks simulations of the IZZS for a representative dry year including the effect of SLR. The dashed black line is the threshold value for salinity maintained by Rijkswaterstaat.

Full size image
Fig. 9.
figure 9

WANDA-Locks simulations of the IZZS for a representative dry year including the effect of SLR and an additional pump from the lower storage basin (Laagbekken) to the Eastern Scheldt. The dashed black line is the threshold value for salinity maintained by Rijkswaterstaat.

Full size image

An additional factor of the IZZS is the flushing discharge through the lock chambers. This is added when one of the lock gates is open for vessels sailing in and out. This flushing is done under gravity and without pumps. For reason of nautical safety, the flushing discharge is only included when it does not make maneuvering of the vessels unsafe (see next section). There are also limits based on water management (see Sect. 2.1.1). Finally, there are the limits based on the available head of the inactive (closed) lock gate at the moment of flushing. For flushing to the Eastern Scheldt, this head is reduced (on average) by sea level rise. Also, the available time window for flushing is reduced. In order to maintain the same amount of freshwater usage the instantaneous flushing discharge is increased as the low tide window shortens (see Table 1). In this way the average water usage for flushing is maintained as long as possible. When the lock chamber cannot be used for flushing, an additional flushing sluice between the lock chambers is used. This flushing sluice is will also be used for fish migration. At a certain level of SLR the capacity of the flushing sluice is reached. The salt intrusion increases as a result (see Fig. 8).

Table 1. Increase in the required instantaneous flushing discharge owing to SLR.
Full size table

3.4 Safe Flushing Discharge for Vessels Entering and Exiting the Lock

Although an important aspect of the IZZS is the simultaneous flushing of the lock chamber with the bubble screen operations, there remain some moments in the lockage cycle flushing will not be used. The reasons for this are threefold: there is not always an available hydraulic head for flushing under gravity; it may not be nautically safe to flush; and, the sailing speed of vessels may be reduced too much by flushing.

The first point above is the case during sailing in and out at the Eastern Scheldt side during high water. During low water vessels sailing out of the lock should not be adversely affected by a flushing discharge in the direction of movement, however vessels sailing into the lock will also be sailing into the flushing discharge. This will reduce their speed and when the blockage of the vessel is large (such as when entering the lock and when the under keel clearance is lower at lower water levels) it has been decided to impose a maximum flushing discharge which is dependent on the water level (and available under keel clearance), see Table 1. This is the third point from above. Also, for the largest vessels it is recommended to limit the flushing discharge in all cases to 10 m3/s. This is equal to the maximum flushing discharge during the dry season, which is also limited for reasons of water management in lake Volkerak-Zoom.

When the gate is open in the lake Volkerak-Zoom side flushing will be used even at high water. The flushing discharge is stored in the Laagbekken (lower basin), which is kept at the lower water level to allow for a hydraulic head between the lake and this basin. At low water the chamber can be flushed directly into the Eastern Scheldt. For outgoing vessels, sailing into the flushing discharge there should always be sufficient under keel clearance available at this side (lake water levels at NAP 0.0 m) for the flushing discharge to not cause excessive delays, except for the largest vessels for which the maximum discharge of 10 m3/s is used. For ingoing vessels the flushing discharge could cause an unsafe stopping procedure for ships in the lock. Therefore, the flushing will be stopped before the ships enter the lock. The moment at which the flushing discharge will be stopped has been chosen to allow for the longest flushing time. Vessels will be allowed to exit the lock and the incoming vessels will be allowed to approach the lock during flushing. The discharge will be stopped as the incoming vessels reach the approach harbour (Table 2).

Table 2. Maximum flushing discharge while vessels are entering the lock from the Eastern Scheldt.
Full size table

3.5 Effect of Fish Migration Sluice on the Salt Intrusion

The IZZS envisions an addition of a flushing sluice between the lock chambers. This will have multiple functions: ; for salt management it will be used as an extra flushing discharge next to that through the lock chambers and it will also be used for fish migration; and, to increase the discharge capacity of the entire complex for extreme river discharge situations for which lake Volkerak-Zoom is used as storage basin for the Meuse and Rhine rivers.

The sluice gate will be kept open at for a short period after the end of low water to allow additional fish migration as the current reverses. This may allow some salt intrusion if salt water is present near the opening. The WANDA-Locks model does not account for possible density currents in the opening during this operation but the volumes of water exchange from the fish friendly operation are sufficiently small not to warrant additional mitigation measures (see Fig. 7).

4 Conclusions

For the Krammer locks the multiple functions and restrictions on fresh water usage complicate the design of an alternative salt-fresh separation system. The Innovative Salt-Fresh Separation System (IZZS) has been designed, combining bubble screens at both lock heads with an intermittent flushing discharge through the locks, using a combination of laboratory experiments, field measurements and CFD simulations. In addition, a hydraulic model of the lake Volkerak-Zoom system which accurately predicts the salt intrusion at the locks and the effect of operation and mitigation measures there, has been essential in weighing all the different and myriad functions and requirements. The model developed here, a box model with WANDA-Locks, can capture the complexity of the operation and give confidence that the planned operation will keep the fresh water availability in lake Volkerak-Zoom within the required limits of salinity for the foreseeable future.