Optimizing Upstream Approach Wall to Navigation Lock in Narrow Rivers

Abstract

In canalized rivers, navigation locks are often located aside a weir. River currents are concentrated towards the weir. As a result, vessels face transverse currents and significant outdraft forces when approaching the lock. An approach wall can be designed to isolate the final lock approach from the weir flow. This wall can be a plain wall, or a perforated wall. Depending on the design of the wall openings, the flow velocity gradient and the outdraft forces are translated upstream at the entrance of the port. At this location, the vessel keeps a larger velocity and can manoeuvre more easily. An optimal design of the opening should minimize the transverse forces acting on the vessel. Guidelines for the design of such approach walls may be found in the literature but cover only large and very large rivers. The present contribution focuses on narrower rivers, with a low ratio between port entrance width and river width.

1 Introduction

The current developments of fluvial transport in Europe often imply the navigation of larger vessels on existing waterways. Larger vessels contribute to reduce transport costs and to a larger modal share of the waterborne transport. These developments may rely on local improvement of the waterways, like local enlargement, but also on improved manoeuvrability of modern vessels. New larger locks are often needed to accommodate those larger vessels. These new locks must be built on confined space as existing amenities have to be maintained both on the river (weir, hydropower plant, etc.) and on land (roads, railways, urbanization, etc.). Accordingly, designing the approaches for these new locks results from a compromise between the extension on the approach area and the remaining space for river flow.

A lock approach can be defined as the necessary navigational area to support a safe entry and exit of the lock chamber (PIANC 2019). The vessels in the lock approaches are protected from the flow in the river and can manoeuvre safely to the lock. Entering the upstream lock approach is usually the most critical phase. River currents are concentrated towards the weir and downbound vessels face significant cross currents while they should slow down. Sufficient width and length of the approach area is required to enable safe navigation. Various recommendations and guidelines on the require size can be found in the literature (see PIANC 2019 for a synthesis).

Designing openings (ports) in the guard wall may also reduce cross currents at the port entrance (PIANC 2015). Different parameters influencing the flow pattern and the intensity of cross currents at the approach entrance can be identified: (1) the ratio between approach zone entrance width and the whole river width; (2) the ratio of the wall openings area and the approach entrance cross-sectional area; (3) the shape of the openings (rounded, sharp, on the whole depth, etc.); and (4) the plan form of the river upstream of the lock and weir.

Apart from case studies specific to a given site, only limited general guidelines for the design of guard wall can be found in the literature. Stockstill et al. (2004, 2005) conducted a systematic investigation of different guard wall geometries: solid wall, multi-cell wall and floating wall. Two test series covered approach width equal to 0.18 and 0.33 times the total river width. For the narrower approach and the multi-cell wall, the optimal ratio between the total wall opening area and the approach entrance area was found to be equal to 0.9. For the wider approach, the optimal value of this ratio dropped to 0.5 and larger transverse forces were observed.

Bousmar et al. (2010, 2014) and Swartenbroekx and Bousmar (2018) conducted specific case studies on narrower rivers where larger locks were planned. The ratio of approach width to total river width grew up to 0.4 in those cases. The optimal values found by Stockstill et al. for wall opening design were no more appropriate for these extreme cases. Specific studies concluded that lower opening ratio were required to minimize transverse forces.

These latest studies also highlighted the need for systematic studies on the design of guard wall for wider approach in narrower rivers. This paper presents preliminary results of such study, covering width ratio from 0.50 to 0.20. A guard wall with circular shaped openings similar to the multi-cell design by Stockstill et al. was tested. The area of the openings was systematically investigated, using physical and numerical modelling, to determine the optimal ratio.

2 Definition of the Investigated Layout

An idealized upstream lock approach is defined, in a straight channel configuration. Figure 1 defines the main dimensions of this layout, in both model and prototype values. The prototype dimensions are fixed considering a ECMT Class Va vessel of 110 m × 11.40 m. The model scale is 1:33.3. The approach width Wa is around 5 times the vessel width. It may correspond to either an approach for a single lock (navigation axis B, located in the middle of the approach) or an approach for a pair of locks (navigation axis A and C, located at first and last quarter). Flow depth in both approach and river is fixed to H = 4.5 m (150 mm in the model).

Fig. 1.

Layout of the investigated upstream approach. Dimensions in meters. Physical model values [prototype values in brackets].

Different ratio of the approach width to the river width are investigated. This ratio is defined as

$$AP= \frac{Wa.H}{Wr.H}$$

(1)

where Wa = 1.7 m is the approach width, Wr is the river width, and H = 150 mm is the flow depth. Different values of the river width are tested, as listed in Table 1, to cover AP values in the range 0.50 to 0.20. The upstream discharge Q is adjusted the obtain a constant upstream specific discharge and velocity.

Table 1. Value of the width ratio AP investigated.

For each AP value, different guard walls are tested. The wall thickness in model dimensions equals 50 mm. In all tests, the 2.2 m part closest to the lock is kept as a solid wall. The upstream 3.3 m part is fitted with rounded openings. In all cases, the size of the openings is kept constant to 50 mm, flanked by two 50 mm diameter half circles, on the whole flow depth. The adjustment parameter is the number of openings, to obtain different values of the ratio M between the opening area and the approach entrance area:

$$M= \frac{\sum {L}_{i}.H}{(Wa-Wg).H}$$

(2)

where Li = 50 mm is the opening width, and Wg = 50 mm is the wall thickness. The investigated values of M are summarized in Table 2. The different tested configurations will be referred by APxxxMyyy where xxx is the AP value in percent and yyy the M value in percent.

Table 2. Value of the opening ratio M investigated.

3 Physical Model

The physical model used for this investigation is illustrated on Fig. 2. The whole configuration is set in a flat bottom basin. The left bank can be translated to obtain different AP configurations. Upstream discharge is controlled through an electromagnetic flowmeter and downstream water level is adjusted with a flap gate. Water level is controlled with two ultrasonic distance probes located up- and downstream.

Velocities are recorded with an electromagnetic velocity probe. An automated displacement carriage moves the probe on a grid with 200 mm longitudinal intervals and 100 mm transverse intervals. Preliminary investigations confirmed that a measurement duration of 30 s per station was sufficient and that measurement at a depth of 80 mm was representative of the depth-averaged velocity.

In the present study, only configurations AP050M000, M033, M050, M100, AP040M033 and M100 are tested. Measurements are used for calibration and validation of the numerical model.

Fig. 2.

General view of the physical model, configuration AP040. Upstream and downstream views.

4 Numerical Model

Depth-averaged numerical modelling of the flow is performed using Telemac2D software. The layout was modelled with the same dimensions as the physical model to facilitate comparisons. The unstructured mesh covers an area extending 8.5 times the approach width Wa in the upstream direction from the approach entrance and 6 times this width in the downstream direction, so that flow in the approach is not influenced by the boundary conditions. The mesh was refined in the wall opening area (with cell size around 10 mm), and in the approach entrance (cell size around 30 mm). As the openings extend on the whole flow depth, no other specific treatment is required. A fixed specific discharge is imposed upstream and a constant water level downstream. Bed roughness is modelled using Manning’ equation, with n = 0.013. Turbulence is modelled using either a depth-averaged k-epsilon model, or a constant eddy viscosity ν_t = 10^–5 m²/s. Figure 3 shows a typical velocity field. The numerical model was eventually validated by comparison with physical model results (see Fig. 4).

Fig. 3.

Numerical model results, configuration AP050M033 (blue: low velocities, orange: large velocities).

Fig. 4.

Configuration AP050M033. Numerical and experimental results at cross section X = 18 m, located at the downstream end of the opened part of the guard wall. Longitudinal (left) and transverse (right) velocities.

5 Results

5.1 Velocities

The global flow pattern is depicted on Fig. 3. At the entrance of the lock approach, the flow converges towards the contracted river and the weir. Transverse velocities are observed on a distance extending from upstream the approach entrance to inside the approach. A velocity gradient area develops in diagonal from the right bank to the guard wall edge. Inside the approach area, only low velocities are observed, with a smooth recirculation zone extending on the whole area. In the river channel, due to the converging flow, maximum velocities are observed along the left bank and a recirculation area develops along the solid part of the guard wall.

The velocities in and upstream the approach area are analyzed in more details by extracting the longitudinal and transverse velocity components along the navigation axis identified on Fig. 1. Figure 5 shows the longitudinal velocities along navigation axis C (close to the bank) and A (close to the wall) for configuration AP050 and various M values. Vertical black lines indicate the extension of the openings in the wall. The velocity gradient is clearly observed from full velocity in the upstream river to quasi-zero in the approach zone. When comparing axis C and A, it is observed that the flow penetrates farther in the approach zone close to the wall, as highlighted on the global flow pattern on Fig. 3. It is also clear that the flow penetrates more the approach zone when the guard wall permeability increases with larger M values. With M100, the flow gradient is almost similar to the pattern observed with a plain wall (M000) just shifted 3.3 m downstream. Lastly, the recirculation zone in the quiet area of the approach zone (X > 18 m) can be identified with positive longitudinal velocities along the wall and negative values along the bank.

The benefit of the wall openings is more clearly highlighted by the analysis of transverse velocities plotted on Fig. 6. The transverse velocities corresponding to the flow contraction develop from upstream the approach zone to inside the zone. Transverse velocities along the wall (axis A) are almost three time larger than along the bank (axis C). Further analysis will therefore focus on axis A. The largest velocities are observed for the solid wall M000 and for the highest permeability M100. The velocity profiles are similar and just shifted 3.3 m as highlighted for the longitudinal velocities. This shows that M100 wall is quasi transparent to the transverse flow and can be expected to have no impact in terms of transverse forces reduction. For intermediate permeabilities, transverse velocity maximum decreases and the flow extends on a longer distance. Minimum values are obtained for M033 and M024 cases, with a (double) peak shifting progressively from inside the approach to upstream the approach.

Fig. 5.

Configuration AP050. Longitudinal velocity along navigation axis C (left) and A (right).

Fig. 6.

Configuration AP050. Transverse velocity along navigation axis C (left) and A (right).

5.2 Forces and Momentum

For an improved analysis of the impact of the guard wall configuration on the vessel navigation, forces and momentum acting on the vessel are estimated from the flow pattern. As a first approximation, the transverse force is estimated from the unperturbed flow distribution. Transverse force is estimated as proportional to the square of local transverse velocity, integrated along the vessel hull length (drafted at 3.00 m). Figure 7 shows the evolution of the force for successive positions of the vessel (given as the center of the vessel) along navigation axis A. Yaw momentum is estimated similarly, considering the center of the vessel as rotation center.

Figure 7 shows the transverse force profiles along axis A for configurations AP025 and AP050. Conclusions drawn from the analysis of velocity profiles are confirmed here. The vessel is submitted to large transverse forces during its transit along navigation axis, from upstream to inside the approach. Largest values are observed for solid wall configuration M000 and large permeability configuration M100, just shifted by the length of the opened part of the wall. Configuration M100 has accordingly too many openings to be efficient. For intermediate configurations (M060 to M015), smaller maximum forces are observed. The vessel is also submitted to the transverse currents on a slightly longer distance. When reducing the permeability, the force peak progressively moves from inside to upstream the approach. Lastly, when comparing AP050 to AP025 configurations, it appears that transverse forces for the narrower channel AP050 are around 30% larger than for AP025, due to the stronger flow contraction.

Quite similar conclusions can be drawn from the analysis of yaw momentum profile as shown on Fig. 8. In the case of momentum, two peak are observed. A negative peak occurs when the vessel enters the transverse current zone, and the bow is drifted towards the river. A positive peak appears when the vessel leaves the transverse current zone, and the stern is now drifted toward the river. Again, largest momentum values are observed for case M000 and M100, shifted by 3.3 m. Intermediate values are observed for intermediate permeabilities (M060 to M015), with the peak location moving progressively from inside to upstream the approach. And again, peak values for configuration AP050 are around 30% larger than for AP025.

Peak values of transverse forces and yaw momentum are summarized on Fig. 9 for all the configurations investigated. This graph shows that the optimal wall configuration is globally M033. For the narrower channel AP050, a smaller permeability M024 could be an alternative considering maximum force and maximum momentum. For the larger channel AP025 and AP020, a larger permeability M050 could also be considered regarding minimum momentum.

Fig. 7.

Transverse force along navigation axis A. Configuration AP025 (left) and AP050 (right).

Fig. 8.

Yaw momentum along navigation axis A. Configuration AP025 (left) and AP050 (right).

Fig. 9.

Maximum transverse force (left) and maximum and minimum yaw momentum (right) as a function of M, along navigation axis A.

Fig. 10.

Integral of yaw momentum along navigation axis A.

As the analysis of minimum and maximum yaw momentum values can lead to undefined results, a further analysis is performed. The integral of the yaw momentum supported by the vessel is calculated along its trajectory on the navigation axis, considering in a first approximation a constant sailing velocity. This integral is supposed to represent the overall steering effort along the entering course of the vessel, to compensate alternatively negative and positive yaw momentum. Results plotted on Fig. 10 confirm that configuration M033 is optimal, with M024 as an alternative for the narrower channel AP050 and M050 as an alternative for the largest channel AP020.

5.3 Discussion

Comparison with previous results by Stockstill et al. (2004, 2005) confirms the effectiveness of permeable guard wall. The need for lower permeability for narrower channels is also confirmed but smaller permeability values are obtained in this study compared to Stockstill et al. For AP020, the present study suggests a permeability of 33 to 50%, while Stockstill suggested 90%. For lower AP033, the present study recommends also a permeability of 33%, instead of 50%. These differences could be due to the different wall configurations. In the present study, openings are present on the whole wall depth, while in Stockstill et al. a beam forced the water to contract on the river bottom. Also, the exact AP value for Stockstill et al. study differs as their river channel was deeper than their approach channel.

Another point of interest to be further analyzed is the flow contraction and the associated recirculation zone in the river channel downstream the approach. This contraction can be severe for the narrowest channel and generate a significant head loss. This head loss may impact the water surface profile upstream and increase flood risk for surrounding areas. An alternative to round shaped openings in the wall is inclined straight openings. The latter will force the orientation of the flow in the river channel and may significantly reduce or even eliminate the recirculation zone (see e.g. Bousmar et al. 2010). Preliminary results depicted on Fig. 11 for AP050M033 configuration with inclined openings at an angle of 45° show than velocity pattern and forces on the vessel in the approach are expected to be quite similar, while the contraction in the river is significantly reduced.

Fig. 11.

Configuration AP050M033, navigation axis A. Comparison between round shaped and 45° inclined openings. Transverse velocity, transverse force and yaw momentum.

6 Conclusions

The proper design of an upstream lock approach and guard wall guarantee a safe and easy entrance of the vessels to the lock. The present study focused on narrow rivers on which larger vessels are nowadays allowed as a results of waterborne transport development. Larger vessels and larger lock imply larger flow contraction upstream the lock, with approach width to river width ratio sometimes up to 50%. Existing guidelines for the design of guard walls did not cover such ratio.

Systematic tests were performed on physical and numerical models, covering width ratio from 20 to 50% and opening ratio from 0 to 100%, with round shaped openings. Results show the effectiveness of a proper design of the wall openings. When openings are too large, the flow behaves as if there was no wall. When reducing the opening, the peak velocity and associated transverse force and yaw momentum decreases and is shifted upstream. If the openings are too much reduced, the peak grow again and moves upstream the approach zone, as if the wall was a solid wall. For all the configurations tested, a permeability of 33% seemed to be optimal. A smaller permeability may be convenient for the narrowest channel (AP050) and a slightly larger permeability could be used for the largest channel (AP020).