Laboratory observations of velocity and density fields in the entrance of a harbor on a stratified tidal river

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Delft University of Technology

Department of Civil Engineering

Hydraulic and GeotechnicalEngineering Division HydromechanicsSection

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_{Laboratory observations of velocity and}

density fields in the entrance of a harbor on a stratified tidal river.

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E.J. Langendoen1 M. Kare1se2

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report no. 1-90

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1 Hydrau1ic and Geotechnica1 Engineering Division, Dept. of Civil

Eng., Delft University of Techno1ogy.

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2 Delft Hydrau1ics.

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The work reported here was funded by Rijkswaterstaat (Department of Public Works in the Netherlands) and the Netherlands Technology Foundation (STW)

and executed in cooperation with Delft Hydraulics. The authors wish to thank Messrs. D. Hoogendam,

e.p.

Koree, E.M.L. van Velzen and F .M. de Vreede from Delft Hydraulics for their contributions during the measurements.

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Abstract

Detailed measurements are presented of velocity and density fields in the entrance of a model harbor on a stratified tidal river. Three geometries of the harbor entrance were examined, (1) a harbor with its length axis perpendicular to the river and an entrance width of 1 m, (2) as (1) but with an entrance width of 0.5 m, and (3) a harbor with its length axis at an angle of 45 degrees with the length axis of the river and an entrance width of 0.78 m. Specifically, measured densities and velocities, the gradient Richardson numbers and horizontal flow patterns at various levels in the harbor entrance during high tide, flood slack tide, low tide and ebb slack tide are discussed.

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1. Introduction

2. Experimenta1 procedure 2.1 Physica1 model 2.2 Instrumentation 2.3 Measuring-programme 3. Resu1ts 3.1 Introduction 3.2 Experiment HO 3.3 Experiment 10 3.4 Experiment Il 3.5 Experiment 12

4. Summary and conc1usions

References Contents 1 2 2 4 6 8 8 8 10 14 18 21 23

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1 -1. Introduction

Many harbors in the world suffer from siltation of their basins and in many cases removal of the deposited sediment leads to high costs. This siltation results from a net transport of sediment into the harbor caused by the water motion in the harbor entrance. The water motion is very complex and of a three-dimensional nature. Three main mechanisms of exchange of water through the harbor entrance can be distinguished: (1) exchange in consequence of water flowing along the mouth of the harbor and the resulting gyres in the harbor entrance , (2) a net transport caused by withdrawal and discharge of water, respectively from and into the harbor basin, or by variations in the water level of the adjacent water body (e.g. sea, estuary or river) , and (3) exchange in consequence of a density difference (generally related to differences in salinity) between water in the harbor and in the adjacent water body.

Mechanisms (1) and (2) are being examined in a physical model in the Laboratory of Fluid Mechanics, Department of Civil Engineering of the Delft University of Technology. The influence of mechanism (3) on the exchange between harbor and adjacent water body is examined, simultaneously with mechanisms (1) and (2), in the Tidal Flume of Delft Hydraulics. Only the latter study is discussed in this report. The research can be seen as an extension of previous work in the former Tidal Flume where only the velocity distribution at the transition from harbor to flume has been measured [7].

The main goal of the present research was to generate a data set by which the three-dimensional numerical model "Trisula" can be tested. Furthermore, our insight into the interactions between the three mechanisms was to be enlarged.

The experimental procedure is described in section 2 and the measurements are discussed in section 3. Finally some conclusions follow in section 4.

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2. Experimental procedure

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2.1 Physical model

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In this study the influence of the geometry of the harbor entrance on the velocity and density fields, due to an oscillatory flow -- representing the

tide -- in an adjacent river, is examined. The Tidal Flume of Delft

Hydraulics is weIl suited to study the various mechanisms mentioned in the

Introduction, that determine the flow pattern and the density field in the

harbor entrance. A sketch of the Tidal Flume is shown in figure 2.1. The

two major sections of the Tidal Flume are a basin wi th a surface area of 120 m2, representing a sea, and a f1ume with a width of 1 mand a 1ength of

130 m, representing a river. Reference [2] gives an extensive description of the Tidal Flume. The harbor entrance has been situated at 24.5 m from the transition from sea to flume.

A tidal current can be generated in the flume by introducing a time

-varying water level elevation in the basin controled by a cylinder shaped overflow weir. The sea water density is monitored by a brine injection system. The vertical density distribution in the model sea is regulated by a fresh water skimmer and a jet-mixing system. The upstream river boundary

(x -130 m) has faci1ities for a constant fresh water discharge and a tida1 discharge. The tidal discharge is utilized to increase artificially the

"virtual" length of the flume. A numerical model computes the boundary

conditions for the required "virtual" length of the flume.

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The dimensions of the experimental model are based on the tidal system

Rotterdam Waterway--Botlek Harbor. The Rotterdam Waterway, near the Botlek _'_

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Harbor, has a width of 600 mand a depth of 15 m. The mean water depth in the flume was 0.2 m. This leads to the following scales: length scale ni- 600, depth scale nh- 75, velocity scale ~ -

fiS -

8.66, and time sca1e

n.r-69.3. As aresuIt, the model period is 650 s for a diurna1 tide. The

Botlek Harbor has a storage area of 3.5 km2, resulting in a storage area of the harbor in the experimenta1 model of 10 m2•

The flow rate in the flume is generated by introducing a sinusoida1 water level elevation with an amplitude of 0.025 m at the transition from sea to flume (x - 0). The river discharge in the Rotterdam Waterway is about

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1700 m3

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re sul ting in a fresh water discharge of 0.0044 m3

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s (discharge scale

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390,000) at the end of the flume (x -130 m). The water velocity of the fresh water discharge is about 0.022 mis and the maximumwater velocity of the tidal current about 0.30 mis. The Estuarine Richardson Number, defined by

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equals 0.09. Thus the flow in the flume is partially mixed [4]. Here 6p is the density difference between sea water and fresh water (20 kg/m3), g the acceleration due to gravity, h the water depth (0.2 m), Qf the fresh water discharge (0.0044 m3/s) , p the density of the water, A a characteristic value of the cross-section of the flume (0.2 m2), and ~ the rms water velocity of the tidal current (0.21 mis).

Elements to increase the roughness were placed at the bottom of the flume to: (1) reduce salt intrusion, (2) increase mixing, (3) realize a stabie quasi-steady current, and (4) reduce the adaptation time of the salinity and velocity distributions in the tidal current [3]. The conical elements (height 4 cm, bottom diameter 5 cm and top diameter 3.5 cm respectively) were distributed according to a lozenge-shaped pattern. The distance between two opposite elements was 0.25 m. The elements were placed in the region where the salt intrusion occurred, except for the harbor itself and a section of the flume in front of the harbor (from x - 21 m to x - 28 m). The Chezy-coefficient in the rough part of the flume was 25 m\/s

and in the smooth part 70 m%/s.

As a resu1t, a partia1ly mixed tida1 current was generated with a salt intrusion .extending fr om x - 24 m at ebb slack tide to x - 65 m at flood slack tide in the f1ume. Consequent1y, the harbor entrance is situated

near minimumsalt intrusion.

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Various geometries of the harbor entrance were examined in a basin of 3.6 x 2.76 m2 with a horizontal bottom and vertica1 s idewalls (see fig. 2.2). Three alternatives were chosen (see fig. 2.3), (1) a harbor with its leng th axis perpendicu1ar to the f1ume and an entrance width of 1 m, (2) as (1) but with an entrance width of 0.5 m, and (3) a harbor with its length axis

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at an angle of 45 degrees with the length axis of the flume and an entrance

width of 0.78 m. The geometry of the harbor entrance was obtained by fixing

two vertical walls with a length of 1.75 m. Alternative (1) was studied

both with and without a difference in salinity between flume water and sea

water. Alternatives (2) and (3) were studied with a difference in salinity

between flume water and sea water only.

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2.2 Instrumentation

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The following quantities have been measured at various locations in the

harbor entrance and at four transects in the flume: salinity, temperature,

water velocity , and water elevation. The transects were situated at x=20

m, x - 22 m, x - 27 mand x -29 m, that is at distances of two and four

meters from the harbor entrance . The measured data in these transects provide the boundary conditions for the numerical model.

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The instruments used, were:

- a VAZO (fixed salinity probe) and a TEMP (temperature meter) to measure

the conductivity and the temperature at a single point. The density follows from the conductivity and the temperature.

- two VEZO's (vertical salinity reader) and four TEMP's to measure the

conductivity and temperature. A VEZO can read the conductivity in an

entire vertical at intervals of 0.023 m. Two TEMP's were used with a VEZO to account for differences in temperature in the water column.

- three EFM's (Electro-magnetic Flow Meter) to measure the two horizontal

velocity components.

- a WVM (water velocity meter) to measure the horizontal water velocity in

the flow direction.

- several WAVO's (water level tracker) to measure the water level

elevation.

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The VAZO is a hollow bar positioned vertically. The conductivity probe

(length 10 mm and inner diameter 1 mm) is situated underneath the bar.

Water is continuously sucked through the probe and the bar with a velocity

of 1 mis inside the probe..The measuring-volume is approximately 0.1 cm3•

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-The measuring-range is from 0 to 10 Slm. The accuracy in density is about 0.25 kg/m3•

The TEMP consists of a probe underneath a rod. The response time is 1 s.

The accuracy is 0.1 degree centigrade.

The VEZO consists of a bar of electrical isolating material on which pairs of ring-shaped electrodes are positioned. The resistance between the two rings in a pair is a measure for the conductivity of the surrounding water. There are 12 probes at an interval of 0.023 m on the bar. The measuring-volume per probe is approximately 2 to 4 cm3. The measuring-range is from 0 to 10 Slm. The accuracy is 1%.

The EFM employs Faraday's lnduction Law for measurement of the velocity of a conductive fluid moving through a magnetic field. This field is generated by a pulsed current through a small coil inside the body of the sensor. Two pairs of diametrically opposed platinum electrodes sense the voltages produced by the flow past the sensor. The sensor has been designed in such a way that these voltages are proportional to the magnitude of the two horizontal velocity components parallel to the planes of the electrodes . The sensor, an ellipsoid (height 11 mm and diameter 33 mm), is connected to a rod (diameter 10 mm) with a maximum immersion length of 85 cm. The size of the measuring-volume is of the order of the size of the probe. The measuring-range is from 0 to 100 cm/s. The accuracy is 1 % of full scale. The sensing element of the WH is a plastic micro propeller fitted in a plastic ring. The ring contains 60 equally spaced holes each with a diameter of 0.5 mm. Displacements of these holes by the rotation of the propeller causes variations in electric resistance between the probe frame and two probe-mounted electrodes . The value and direction of the velocity of the liquid are derived from the frequency and mutual phase relation of the electric signals. The measuring-volume is 1.75 cm3. The measuring-range is from 2.5 to 120 cm/s. The accuracy is 1 %.

The WAVO senses the water level by a needle-type electrode, which vibrates at a fixed frequency (50 Hz) and with a small amplitude. The sensor remains

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at rest as long as the periods during which the needie is in and out the

water are equal. At unequal periods a signal is generated which moves the

sensor in such a direction that ba1ance is recovered. Position dependent

output is obtained from the restoring circuit. The measuring-range is 100

mmo The accuracy is 0.2 mmo

2.3 Measuring-programme

In the harbor entrance the sa1inity (VEZO), temperature and velocity (EFM)

were measured in severa1 verticals, forming a grid with gridsizes of 0.225

mand 0.250 m, see figure 2.4. The two horizontal velocity components were

measured: (a) in the experiment without density differences (salt water

only): at 0.02, 0.04, 0.08, 0.12 and 0.16 m above the bottom, and (b) in the experiments with density differences (fresh and salt water): at 0.02,

0.04,0.06,0.08,0.10,0.12,0.14 and 0.16 m above the bottom. The densities were determined at 0.023, 0.047, 0.070, 0.094, 0.ll7, 0.140,

0.164, 0.187 m above the bottom. For the three measuring-positions nearest to the bottom a TEMP at 0.06 m above the bottom was used and for the other

posi tions a TEMP at 0.12 m above the bottom . The water e1evation in the harbor was measured at 2.5 m from the axis of the f1ume.

In the four transects in the f1ume all quantities were measured in one vertica1 in the axis of the f1ume. The velocity (WVM) and density (VAZO and

TEMP) were measured at 0.02, 0.04, 0.06, 0.08, 0.10, 0.12, 0.14 and 0.16 m

above the bottom. In the experiment without density differences, the water velocity was measured at 0.02, 0.04, 0.08, 0.12 and 0.16 m above the bottom.

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experiment experiment

harbor without inc1uding

density differences density differences

(1) HO 10 (2)

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I1 (3)

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Tab1e 1. Experiments executed

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3. Results 3.1 Introduction

Assuming that the velocity and salinity distributions in the tidal current reproduce well during each tidal cycle, the quantities were measured during one cycle in all measuring-points, except for some salinity verticals measured with the VEZO, which were measured two to four times and averaged over the measured cycles.

The sampling period was 6.5 s, that is one hundredth of the tidal period.

The data was recorded by the data acquisition system [1]. The data then is already averaged over this time interval. Because the data was still a little erratic, it was averaged with a triangu1ar shaped filter with a time side of 5 times the sampling period.

The results of the four experiments are discussed for high tide (denoted as HT), flood slack tide (FST), low tide (LT) and ebb slack tide (EST). These times are chosen because the flow patterns cou1d be interpreted more easily at these times, primarily because one of the three mechanisms of exchange then is absent. For example, at high and low tides the inf1uence of ne t flow (filling or emptying of the basin) through the harbor entrance is nil.

The flow pattern in the harbor entrance then is governed by a gyre, which is generated by the water flowing along the mouth of the harbor, and a density-driven exchange flow. The complete data set is avai1ab1e on floppy disk.

3.2 Experiment HO

Figures 3.1 to 3.4 show the measured veloc ity components in the harbor entrance and the flow pattern in two horizontal planes, at 0.02 mand 0.12 m above the bottom, and the depth averaged flow pattern, at HT, FST, LT and

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-High Tide and Low Tide

During high tide (figs. 3.1.a and b) and low tide (figs. 3.3.a and b) a gyre occurs in the harbor. Near the bottom the flow in the gyre is directed

towards the center of the gyre, and closer to the surface the flow in the gyre is directed from the center of the gyre. At the downstream sidewall of the harbor entrance the water velocity near the bottom is larger than higher in the water column (see verticals l-b and l-c in fig. 3.1.a and verticals 5-b and 5-c in fig. 3.3.a).

These phenomena were also observed in similar experiments in the Laboratory of Fluid Mechanics of the Delft University. However, the shape of the gyre at high tide is different from the shape of the gyre at low tide. The shape of the gyre at high tide is more coherent, while the shape of the gyre at low tide is more irregular. This is probably due to the difference in water depth at high and low tide - the water depth at high tide is 0.05 m larger than that at low tide - and the filling and emptying of the harbor basin preceding high and low tides, respectively. The influence of the filling and emptying of the harbor basin is shown in figures 3.5.a (before high tide) and 3.5.b (before low tide) . Figure 3.5.a shows that the development of a new gyre during flood is similar to that observed at the Delft University [5) (no vertical tide). The development of a new gyre during ebb is quite different (see fig. 3.5.b). It seems that

the development of the gyre is hindered by the emptying of the basin. The development takes longer and the shape of the gyre is irregular.

Flood Slack Tide and Ebb Slack Tide

At both FST and EST the center of the gyre moves towards the flume and even

a little into it. This was also observed at the Delft University (see [5).

The influence of the filling of the harbor basin (figs. 3.4.a and b) on the water velocities in the gyre is an increase in magnitude where the flow in the gyre is directed into the harbor and a decrease in magnitude

where the flow in the gyre is directed towards the flume (compare the profiles of the velocity component v in transects 1 and 5).

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on the water velocities in the gyre is a decrease in magnitude where the flow in the gyre is directed into the harbor and an increase in magnitude where the flow in the gyre is directed towards the flume (compare the profiles of the velocity component v in transects

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3.3 Experiment

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Figures 3.6 to 3.9 show the measured densities and water velocity components, the gradient Richardson number, defined by

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---p[(au/az)2

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(av/az)2]

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and some horizontal flow patterns in the harbor entrance, at HT, FST, LT and EST, respectively. Here u is the water velocity and z the vertical co-ordinate (positive in upward direction).

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High Tide

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It can be seen from the vertical tide in the flume (in front of the harbor), plotted in figures 3.6 to 3.9, that during high tide storage effects can be neglected. Consequently the flow pattern in the harbor entrance then is a combination of an exchange flow and a gyre driven by the flood current.

The profiles of the velocity component v along transect 3, see figure 3.6.b,

show

the presence of an exchange flow in the entrance. Water with a higher density is entering the harbor near the bottom over practically the complete entrance width, while water with a lower density is leaving the harbor near the surface (also see transects d and e in fig. 3.6.a).

The influence of the gyre on the exchange flow can be observed in the density and velocity profiles at vertical l-c. Here the denser water is entering the harbor over the entire water column. The influence of the gyre on the exchange flow (or vice versa) can also be observed in fig. 3.6.d where the horizontal flow patterns at three levels, 4 cm, 10 cm and 14 cm

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-11-above the bottom, are plotted. At 10 cm above the bottom, where the velocity of the exchange flow is approximately zero, a gyre exists that spans the entire width of the harbor entrance. Near the bottom the flow is mainly directed into the harbor and a small gyre exists at the upstream harbor sidewall with its center nearer to the back of the harbor. Near the surface the flow is mainly leaving the harbor and a small gyre exists at the downstream harbor wall with its center nearer to the flume. Thus the exchange flow causes a gyre with an oblique vertical axis. The influence of the gyre on the exchange of water in consequence of the exchange flow is a reduction of the effective entrance width.

The velocity of the exchange flow can be determined from (see [6])

where ~P is density difference between harbor water and flume water, and Pa

the density of harbor water. With ~P - 10 kg/m3, Pa - 1005 kg/m3 and h - 0.225 m, ud equals 0.05 mis. This agrees quite weIl with the measured velocity of the exchange flow (averaged over the depth of either upper or lower layer) in verticals 3-d and 4-d. Because the presence of a gyre in the harbor entrance has hardly any effect on the magnitude of the velocity of the exchange flow, the total exchange of water due to the exchange flow is reduced by the presence of a gyre.

Fig. 3.6. c shows gradient Richardson numbers smaller than 0.25 along transec ts c, d and 3. The flow at these locations in the harbor entrance , consisting of a gyre and a shear layer at the transition from harbor to flume, is turbulent. The flow further into the harbor and in the flume is stably stratified (in the sense that the turbulence has collapsed).

Flood Slack Tide

At FST the emptying of the harbor basin has just begun as can be observed fr om the plotted vertical tide in the river (see figs. 3.7 .a, 3.7.b or 3.7.c).

Fig. 3.7.b shows the presence of an exchange flow (see the profiles of the velocity component v along transects 1, 2, 3 and 4), so the harbor

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water has not been comp1ete1y exchanged with water from the f1ume during the preceding f100d phase. This can be exp1ained by ca1cu1ating the time that is needed to exchange the harbor volume with water from the f1ume.

Using the ca1cu1ated velocity of the exchange flow, that is O.05

mi

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and assuming that the harbor has a virtua1 1ength of 10 mand the density of water in the f1ume remains constant during the f100d phase (which is approximate1y true because the harbor is situated near minimum salt intrusion) , it wi11 take 400 s to exchange the harbor water with water from

the f1ume when the exchange flow enters the harbor over the entire entrance width (1 m). This is too long to exchange all the water in the harbor during the f100d phase.

A1though at FST the water velocity al.origthe mouth of the harbor entrance practica11y vanishes, a remainder of the gyre at high tide is present. As aresult the flow pattern at FST is a combination of a gyre, an exchange flow and a current due to the emptying of the basin. However, the current due to the emptying of the basin can be neg1ected with respect to the exchange flow.

Observations of the density profiles alorig transects 1 and 5 in fig.

3.7.a shows that water from the f1ume is entering the harbor main1y at its sidewa11 near transect 1.

Fig. 3.7.d portrays the flow pattern in the harbor entrance and the exchange mechanism at FST. The gyre that was present at high tide can on1y be recognized near the bottom. Ha1f-way down the water column and near the surface the gyre has disappeared, a1though some circu1ation is present. The flow pattern at 0.14 m above the bottom at the sidewa11 near transect 1

-observe the way the velocity decreases towards the f1ume - together with

the velocity and density profiles aLong transects 1, 2 and 3 show an ob1ique interface between the salt and fresh water.

Fig. 3.7 .c shows gradient Richardson numbers 1arger than 0.25 in all verticals, except near the bottom a10ng transects 1 and 2. The flow then is stab1y stratified.

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Low Tide

At low tide the flow pattern in the harbor entrance is a combination of an

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-exchange flow and a gyre driven by the ebb current in the f1ume. The exchange flow can be recognized from the profiles of the velocity component v aLorig transect 3 in fig. 3.8.b. A1so notab1e is the jump in the u-component between upper and lower 1ayers at the transition from harbor to flume.

The density profiles along transect 5 in fig. 3.8.a show that fresh water mainly enters the harbor near the surface over the entire entrance width, and at its downstream sidewa1l over the entire water column. Denser water is primari1y 1eaving the harbor near the bottom at its upstream section. The density profiles are remarkable near the stagnation point at the downstream sidewa11 of the harbor entrance (examine the density profiles 5-c and 5-d). Near the bottom the density decreases.

Although the flow at the transition from harbor to flume is norma1 to the flume over the entire width of the harbor entrance, the flow pattern in a horizontal p1ane at 0.04 m above the bottom (see fig. 3.8.d) shows a small gyre at the upstream corner of the harbor entrance. Fresh flume water flows towards the bottom at the downstream sidewall of the harbor entrance and meets, close to the bottom, the outgoing exchange flow by which it is guided out of the harbor. This also explains the sudden change in the density distribution from vertica1 5-a to 5-b.

The flow pattern near the surface shows a smal1 gyre at the upstream sidewall of the harbor entrance, which is, as opposed to the gyre near the bottom, driven by the ebb current in the f1ume. Just as during high tide, a gyre spanning the entire entrance width cou1d be expected ha1f-way down the water column where the velocity of the exchange flow is approximately zero. However, the gyre is smaller and is situated more downstream in the harbor entrance . This is probab1y due to the complicated flow pattern near the stagnation point and the preceding emptying of the harbor basin, as in experiment HO.

lt can be conc1uded from fig. 3.8.d that the influence of the gyre at 10w tide on the exchange of water due to the exchange flow is a reduction of the effective entrance width.

Fig. 3.8.c shows a turbulent flow (Ri<0.25) near the stagnation point at the downstream sidewall of the harbor entrance and in the flume. The flow further into the harbor is stably stratified.

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Ebb Slack Tide

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At EST the flow pattern in the harbor entrance is a combination of a gyre,

an exchange flow and a net flow through the harbor entrance due to the

filling of the harbor basin. The exchange flow still exists because the

time to exchange all the water in the harbor, as calculated before, is too

large compared to half the tidal period.

The influence of the filling of the basin should be an increase in

velocity of the exchange flow near the surface and a decrease in velocity

near the bottom. However, the velocity of the net flow is approximately 1.3

cm/s. As aresult this increase or decrease is hardly perceptible.

The very complex flow pattern near the stagnation point at low tide is

still present. Here, fig. 3.9.c shows a turbulent flow in the upper layer

along transect 4 and at the verticals 3-c, 5-d and 5-e. Further into the harbor the flow is stably stratified. The flow patterns in a horizontal plane near the bottom and surface show this complex flow pattern (see for

example the decrease in velocity near the surface in the point on vertical

5-c in fig. 3.9.d). The gyre at low tide in the upper layer has remained at the sidewall of the harbor entrance near transect 1.

At EST the exchange flow seems to be the dominant exchange mechanism.

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Figures 3.10.a and b show measured density profiles as functions of time along two transects (transects d and 3 in figs. 3.6.a to 3.9.a), to

illustrate the changes in the density field in the harbor entrance during a complete cycle (not only HT, FST, LT and EST).

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3.4 Experiment 11

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Figures 3.11 to 3.14 show the measured densities and water velocity components, the gradient Richardson number and some horizontal flow

patterns in the harbor entrance, at HT, FST, LT and EST, respectively. In

figs. 3.11.a to 3.l4.a the density profiles in the verticals 3-d and 5-d

are absent, because this data was lost during the processing of the

measured data by the data acquisition system.

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- 15 -High Tide

At high tide the flow pattern in the harbor entrance is a combination of a gyre, driven by the flood current in the flume, and an exchange flow. The profiles of the velocity component v along transects 2 and 3 in fig. 3.ll.b show the presence of the exchange flow. The velocity of the exchange flow is about as large as in experiment 10.

Denser water is mainly entering the harbor at its downstream section (compare density values near the bottom in transect 1 with respect to

transects 2 to 5), and only from the bottom as far as half-way up the water column. This differs from high tide in experiment 10 where denser water was entering the harbor at its downstream sidewall over the entire water column.

Fig. 3.ll.d shows a gyre near the bottom at the upstream sidewall of the harbor, a gyre almost spanning the width of the harbor half-way down the water column where the velocity of the exchange flow is approximately zero, and a gyre near the surface at the downstream sidewall. The size of the gyre near the bottom is somewhat larger than in experiment 10, because the exchange flow is situated more towards the downstream section of the harbor, as a result of the narrowing of the entrance and consequently a downstream shift of the separation point. As opposed to experiment 10, the gyre near the surface is transporting fresh water, which can be observed in the density profiles in verticals l-a and l-b.

Fig. 3.11.c shows gradient Richardson numbers smaller than 0.25, indicating a turbulent flow, in the interfacial layer at the transition from harbor to flume (transect c) , in the lower layer just inside the

harbor (transect 2 and verticals l-a, l-b and 3-b) and in the upper layer just outside the harbor (vertical l-d and 2-d). Further into the harbor the flow is stably stratified.

Flood Slack Tide

At FST the flow pattern in the harbor entrance is a combination of a gyre, an exchange flow and a current due to the emptying of the harbor bas in. The current due to the emptying of the basin can be neglected with respect to

- 16

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the exchange flow and the gyre.

It should take about twice as long to exchange all the water in the harbor as in experiment 10 because of the narrowing of the entrance. As a result, the water in the harbor is more stratified than at FST in experiment 10. The gradient Richardson number in the harbor entrance is about l.0 in expo 10 (see fig. 3.7.c) and about 2.5 in expo 11 (see fig. 3.l2.c).

Fig. 3.12. d shows that at the sidewall of the harbor entrance near transect 5 the gyre near the bottom is still present. This gyre prevents the exchange flow fr om spreading in transverse direction. Furthermore at the downstream corner of the harbor entrance a new gyre has been generated by the separation of the exchange flow at the jetty in the entrance.

Fig. 3.l2.c shows gradient Richardson numbers larger than 0.25 in all verticals , except near the bottom at the transition from harbor to flume

(transect c). The flow then is stably stratified.

LowTide

At low tide the flow pattern in the harbor entrance is a combination of a gyre, driven by the ebb current in the flume, and an exchange flow.

Figs. 3.13. a and b show that fresh water enters the harbor primarily along the downstream sidewall of the harbor entrance , from the water surface as far as half-way down the water column. Furthermore, the density

near the bottom in vertical 5-a is smaller than in vertical s-b. This means that, just as in experiment 10, fresher water flows downwards near the downstream sidewall of the harbor entrance . However, in this case this occurs somewhat further to the back of the harbor. The horizontal flow pattern near the bottom (see fig. 3.l3.d) confirms this.

The horizontal flow pattern at 0.10 m above the bottom shows a gyre

spanning almost the entire width of the harbor. About the same flow pattern exists near the surface. The shape of the gyre at 0.10 mabove the bottom is more coherent than in experiment 10, which is probably due to the downstream shift of the separation point. Furthermore the water velocities in the harbor are much smaller than in experiment 10 (compare fig. 3.8. d with 3.l3.d).

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- 17

-The gyre in the upper 1ayer provides for an extra narrowing of the exchange flow, in addition to the narrowing due to the reduction in the entrance width. Near the bottom denser water is 1eaving the harbor 1ike a jet in consequence of the narrowing of the entrance.

As in experiment 10 the upper 1ayer is slipping over the lower 1ayer at the transition from harbor to river (see fig. 3.13.b).

Fig. 3.13.c shows a turbulent flow (Ri<0.25) near the stagnation point at the downstream sidewa11 of the harbor entrance and in the f1ume. The flow further into the harbor is stab1y stratified.

Ebb Slack Tide

At EST the flow pattern in the harbor entrance is a combination of a gyre, an exchange flow and a current due to the fil1ing of the harbor basin. The current due to the fil1ing of the basin is approximately 2.5 cm/s. The influence of this current on the exchange flow is a decrease in velocity of the exchange flow near the bottom and an increase in velocity of the exchange flow near the surface. This inf1uence is observable in the profiles of the velocity component v at the transition from harbor to river

in fig. 3.14.b.

Fig. 3.14.d shows that the exchange flow is entering the harbor with velocities that are much 1arger than in experiment 10. Fig. 3.14.c shows that the flow here is turbulent. A gyre develops in the upper layer behind each jetty. Further into the harbor the flow is stably stratified.

Figures 3.l5.a and b show measured density profiles as functions of time along

two

transects, to illustrate the changes in the density field in the harbor entrance during a complete cycle (not only HT, FST, LT and EST). A transect at the transition from harbor to river (transect c, including verticals l-b and 5-b) and a transeet along the length axis of the harbor

- 18

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3.5 Experiment 12

Figures 3.16 to 3.19 show the measured densities and velocity components, the gradient Richardson number and some horizontal flow patterns in the harbor entrance, at HT, FST, LT and EST respectively.

High Tide

At high tide the flow pattern in the harbor entrance is a combination of an exchange flow and a gyre driven by the flood current. The velocity of the

exchange flow is approximately as large as in experiments 10 and 11 (see

fig. 3.l6.b). The measured densities near the bottom, both at the

downstream and at the upstream sidewall of the harbor entrance, differ on1y

little in magnitude (see fig. 3.16.a).

A gyre in the lower layer is absent (see fig. 3 .16 .d) owing to the

small angle between the harbor and the flume. The salt water therefore

flows unhindered into the harbor bas in. As observed in experiment 10 (see

fig. 3.6.d), I1 (see fig. 3.11.d) and 12, the flow direction near the

bottom makes an angle from about -155 to -160 degrees with respect to the

length axis of the flume at the transition from harbor to f1ume. For

experiments 10 and 11 this sufficed to drive a gyre in the harbor entrance.

However, in experiment 12 the flow direction is almost parallel to the

length axis of the harbor entrance.

Fig. 3.16.d shows a small gyre at the downstream corner of the harbor

entrance both near the surface and half-way down the water column.

Fig. 3.16.c shows a turbulent exchange flow (Ri< 0.25), viz. the lower

layer in the harbor entrance and the upper layer in the flume.

Flood Slack Tide

At FST the flow pattern in the harbor entrance is a combination of an exchange flow, a gyre and a current due the emptying of the basin. The

current due to the emptying of the basin can be neglected with respect to the exchange flow.

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- 19

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The average density in the harbor (see fig. 3.17.a) is much 1arger than in experiment 11, and about as large as in experiment 10. This means that, a1though the entrance width in experiment 12 is smaller than experiment 10, the exchange of water between harbor and f1ume is about the same. Thus the absence of a gyre in the lower 1ayer in the harbor entrance does not reduce the exchange of water due to the exchange flow.

Fig. 3.17.d shows that the gyre near the surface has disappeared, and the size of the gyre ha1f-way down the water column has decreased.

Fig. 3.17.c shows gradient Richardson numbers 1arger than 0.25 in all verticals, except near the bottom. The flow is stab1y stratified.

It shou1d be noted that the resu1ts obtained with the EFM' s, as wi11 be seen hereafter, gave incorrect va1ues from low tide unti1 ebb slack tide near the downstream corner of the harbor entrance. This cou1d be caused by interna1 waves in the mixing 1ayer or a re1ative large vertica1 velocity component.

Low Tide

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At low tide the flow pattern in the harbor entrance is a combination of a gyre, driven by the ebb current, and an exchange flow. Fresh water main1y enters the harbor at its downstream sidewa11. The same phenomena are visib1e near the stagnation point at the downstream corner of the entrance as in the experiments 10 and 11 (see figs. 3.18.a, band d).

Figs. 3.18.band d show that the water veLoc Lt Les in the harbor are much smaller than in experiments 10 and 11. Fig. 3.l8.d shows a transverse flow half-way down the water column and near the surface in the harbor entrance.

Fig. 3.18.c shows gradient Richardson numbers smaller than 0.25 near the stagnation point and in the flume in almost the entire vertical. The flow is turbulent in the upper layer at the transition from harbor to f1ume (except the part near the stagnation point). Further into the harbor the flow is stably stratified.

20

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Ebb Slack Tide

At EST the flow pattern in the harbor basin is a combination of a gyre, an

exchange flow and a current due to the filling of the harbor basin. The

velocity of the current due to the filling of the basin is approximately 2 cmjs. The influence of the filling of the basin on the exchange flow cannot

be observed in the profiles of the velocity components (see fig. 3.l9.b).

Fresh water flows around the corner of the entrance near transect 2

into the harbor like a jet (see fig. 3.19.d), as opposed to the denser water, the outflow of which near the bottom is more uniformly distributed.

Fig. 3.l9.c shows that the flow in the flume is turbulent (Ri<0.25). Also the flow into the harbor entrance in the upper layer is turbulent

(transects c and d). Further into the harbor the flow is stably stratified.

Figures 3.20.a and b show measured density profiles as functions of time along two transects, (1) at the transition from harbor to river (transect

c, including vertical 5-b) and (2) parallel to length axis of the harbor (verticals 4-a, 3-b, 4-b, 3-c and 4-d). to illustrate the changes in the density field in the harbor entrance during a complete cycle (not only HT,

FST, LT and EST).

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- 21

-4. Summary and conc1usions

The research that has been discussed in this report is a part of an on-going study on the si1tation of tida1 harbors. The study deals with the water motion in the harbor entrance, which causes the si1tation. As yet,

too 1itt1e is known about this comp1icated time-dependent water motion. The data obtained during the research will be used to calibrate the 3-D numerical model Trisuia, so that it can be used as a t.ooL to predict the water motion in a harbor entrance. With the present knowledge of the transport of cohesive sediments, a better prediction of the siltation of a harbor entrance will then be possible.

Experiments with density differences have been performed in the Tida1 Flume of Delft Hydraulics. This study is an extension of the research in the Laboratory of Fluid Mechanics of the Delft Universi ty of Technology, where the water motion without differences in density is being examined.

The water motion in the harbor entrance is a combination of a gyre driven by the water flowing along the mouth of the harbor, net flow through the entrance and an exchange flow driven by density differences between harbor and river water.

Measurements of the time-dependent velocity and density fields were made in three model harbors, two harbors with their length axis perpendicular to the length axis of the flume (one with a smaller entrance width) and the third with its length axis at an angle of 45 degrees to the length axis of the flume. It can be concluded that:

- compared to results obtained at the Delft University of Technology without a vertical tide, a vertical tide has little influence on the flow patterns in the harbor entrance , which are due to an oscillatory flow (without density differences) along the mouth of a harbor. The main difference is the way the gyre develops after flood slack tide and ebb slack tide.

- exchange flow driven by a density difference between harbor and flume water is the dominant exchange mechanism for all three model harbors. - at low tide the velocity and density profiles in the entrance show that

water near the stagnation point at the downstream sidewall of the harbor entrance flows toward the bottom in all three model harbors. This is probably the reason of the higher water velocity near the bottom around

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high and low tides in the experiment without density differences (HO). - the presence of a gyre during ebb and flood reduces the total exchange of

water due to the exchange flow by reducing the effective entrance width of the harbor.

- during flood slack tide and ebb slack tide the gradient Richardson number in the harbor entrance is larger than 0.25 for all three model harbors. The flow then is stably stratified. During high tide and low tide the flow is turbulent mainly in the harbor entrance for all three model harbors (Ri

<

0.25) .

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- 23 -References

[1] Delft Hydrau1ics, "Informatienota betreffende het regelsysteem voor de getijgoot.", Z 0017, maart 1986.

[2] Delft Hydraulics, "Beschrijving van het flowschema t. b. v.

getijgoot.", versie 2, Z 0017, maart 1986.

de

[3] Delft Hydraulics, "Orienterend onderzoek naar de getij beweging en de zoutverdeling in de getijgoot", verslag 1S4, 1989.

[4] Fischer, H.B., List, E.J., Koh, R.C.Y., 1mberger, J., and Brooks, N.H.,

"Mixing in 1n1and and Coasta1 Waters.", Academic Press, 1979.

[5] Langendoen, E.J., "Laboratory observations and calculations of the depth averaged flow patterns in a square harbor on a tida1 river.",

report no. 8-89, Delft University of Technology.

[6] Massie, W.W., "Coastal Engineering.", Vol. I, Delft University of Technology, 1979.

[7] Roe1fzema, A., and van Os, A.G., "Effect of harbours on salt intrusion in estuaries.", publ. no. 204, October 1978, Delft Hydraulics.

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nivers

i

ty

of

T

echnology

and Delft

H

ydrau

l

ics

t

=

4

61. 5

5

F

ig

. 3.4

.

a

0.00 0.25 ,

,

2

\, , J, I I , 0.00

+-

J 0.25

3

0.00 0.25

4

0.00 0.25

5

.

..._ --0.00 -.10 \ \ I , , , , '-_ I I J \ ., .05 -.10

V

e

lo

c

ity

prof

il

e

s

.

(

E

x

p.

H

O

)

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

,

-I

_/

,/' I r

'

/

/1

\

f

'\

~

J

1

/

\

₁

1

I

\

-\

J

0.02 m above the bed

, , I _I

\

r

/ <,en

-

_I

I

E CD 0 \ , -.

\

₁

0 11

\

1

\

.

_\

1

1

\

\

-\

1

0.12 m above the bed

\

-I

_I

J

r

/ _<,

l

_I

1\

I

--i

₁

_'. ~

I

_\

\

_i

\

"\

-\

1

depth averaged

Flow

patterns

in the

x-y-plane.

(Exp.

HO

)

t =

461. 5

s

---_____,\ t = 487.5 s

J

I

\

I

\

/ t

=

520.0 s

<

<<:<-

~

1

I

\

~

~ I

\

t

=

585.0 s

1

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

\

D

ept

h

a

v

eraged

f

low patterns

.

(

E

x

p.

H

O

)

r

;1\

I

Delft

U

n

i

versity of Technology

a

nd D

e

l

ft Hydrau

li

cs

i

I

I

I

~

r

I

~

___

~

/

/7

,

--:

r

i

I

i

I

/

I

i

I

t = 162.5 5

I

~~ (J) <,

I

r---E

i

0 I ,/

,

0

I

/

-

I

r

I

I

/' /'

I

t = 195.0 5

I

I

/ ",

J

I

I

_t

I

'\

/

j

I

t = 260.0 5

I

Depth

a

ver

a

g

ed

flo

w

p

at

t

e

rn

s

.

(

E

x

p

. HO)

I

D

e

lft Univ

e

r

s

ity

o

f T

ec

h

nology and De

lft Hy

drau

l

ics

Fig

.

3

.

5.b

I

I

I

I

I

I

I

I

I

I

I

I

I

0.225 c -; O.200+---I---___".,---I---+ ~ T a. QJ '0 0.175

vertical tide in river

profile a

0.25 0.00 +---'--+ 0.25 j_

2

0.00 +----'-- -+-0.25

3

0.00 -+-_ _'___+ 0.25

4

o.00+----'--'-'-- -t-0.25

5

0.0

b

20.0 0.0

Densit

y

profi

l

es

.

(

Exp

.

lOl

c

20.0 0.0

d

20.0 0.0

-

=

d

e

n

s

ity

- 1

000 k

g

/m

3

D

e

lft Univ

ers

ity

o

f T

ec

hn

o

l

ogy

a

nd De

lf

t

Hy

dr

a

u

li

cs

I abc d e 1 2 3 4 5

e

20.0 0.0 20.0 [kg!m3] t =

65

.

0

5

Fig

.

3.6.a

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

0.225 0.175 vertical tide in river c :.: O.200+-+--____"~----I--- +-.._, Cl. QJ "Cl a b I ede u 1 2 3 4 5 .10 [m/sl

I

I

-'-' '-' CJ U1 c:: ro '---'-'

p

ro

fil

e

a

b , , I ) ,

'.-"

c

, ,

"

V

el

o

city

pro

fil

es.

(E

xp.

1

0)

d , ,

"

, (

,

I , I I \ .10 -.20

-

=

U, ----. = V

e

t

if-7

~ ' ,,' / , , , : I o00

--P-=-=---tL

--+

0.25 0.25

3

... I , 0.00

+-

~

.

.

~--+

0.25

4

,, I , i I .15 -.10 .10 -.15

D

e

lft

U

n

i

vers

i

ty of

T

ech

n

olog

y

a

n

d De

l

ft

H

y

d

ra

u

lics

, I I < , I {

I

o.oo+-~--+ 0.25

5

I

"

, , I , i r I 0.00

+---l--+

-.10 .10 -.25

t

=

65

.

0

s

Fi

g. 3

.6.b

I

o

225

\

.:

a b c d e E

I

1 c ₂ .c: 0.200

"V

3 ...., _{t T} Cl. ~ (lJ D 5 0.175 vertical tide in river .._, '-' Q.) (.fJ

pro

f

de a

b

c

_c

_d

_e

ro c., ..., 0.20

r

+ + + + + + + + +

1

++ ++ ++ ++ ++ + + + + +

I

+ + + + + + + + + + + + + 0.00 _I I _I 0.20 + + + + + + + + + +

2

++ ++ + + ++ ++ + + + + + + + + + + + + + + + + + + + 0.00 0.20 + + + + + + + + + +

3

++ ++ + + ++ ++ + + + + + + + + + + + + + + + + + + 0.00 0.20 + + + + + + + + + +

4

++ + + + + ++ ++ + + + + + + + + + + + + + + + + + + 0.00 0.20 + + + + + + + + + +

5

+ + + ++ + + + + + + + + + + + + + + 0.00 0.01 0.1 I 10 0.01 0.1 I 10 0.01 0.1 I la 0.01 0.1 I 10 0.01 0.1 1 la

G

r

adie

nt

R

i

chardson

numb

e

r.

(

E

x

p

.

10

)

t

=

₆₅

_.

₀

_s

D

e

lft Univer

s

ity of T

e

chn

o

l

og

y

and

D

e

lft Hy

dra

uli

cs

F

ig

.

3

.

6

.

c

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

~~

~

k

~

~

~I

i

V//

~

1

/

_I

/

1

'

I

/ 1 t/

\

\

j

,

I 1

0.04 m above the bed

<

~~ UJ .;::: <E--~~~ <,E C"\J ~

V

I \ 1 0 ft I1

J

-,

-... /

1

/

\

\ , /

0.10 m above the bed

~~ ~

\

\

\

-V

,

r

\

i

--

i

r

r

\

j\

_-

\

\

r

0.14 m above the bed

Flow p

a

tt

e

rn

s

in th

e

x

-y-p

l

a

n

e

.

(

E

x

p

. 1

0

)

t =

65

.

0

s