Trends in hydraulics laboratory research in the Netherlands

T R E N D S I N H Y D R A U L I C S L A B O R A T O R Y R E S E A R C H I N T H E N E T H E R L A N D S

B Y I R . H . J . S C H O E M A K E R A N D C O - W O R K E R S HYDRAULICS LABORATORY DELFT

S C O P E A N D A I M S I N M O D E L T E C H N I Q U E S B Y I R . H , J . S C H O E M A K E R

T h e impetus that was given by the great endeavour f o r c i v i l engineering works i n the Netherlands i n the time between the two w o r l d wars resulted also i n the establishment of the D e l f t Hydraulics L a b o r a t o r y .

A t that time the h y d r a u l i c model as a tool i n design had already proved its usefulness i n various countries of Europe. Thus the general develop-ment i n scope and size o f t h e works to be designed was necessarily coupled w i t h the development o f scientific means, among w h i c h hydraulics got its workshop, viz. the laboratory.

T h i s type of laboratory may p r i m a r i l y be looked upon as the place where the pure science of fluid mechanics and physics is adapted to engineering needs.

These needs, however, are always ahead of w h a t science offers and is understood b y engineers; moreover, there exists a difference i n character (and perhaps also i n alertness) between the scientific and the engineering approach to problems, so that this difference has to be bridged.

I n engineering practice lack of basic understanding of n a t u r a l processes is many times compensated by means of the more or less condensed experience i n the past. T h e slow process of gaining experience does n o t meet the present-day requirements of the technique so t h a t also this process has to be speeded up.

T h e position between science and technique is c o m m o n for m a n y labor-atories i n the w o r l d . T h e well-developed i n t e r n a t i o n a l exchange of know-ledge and experience gives the o p p o r t u n i t y to p r o f i t f r o m the results o f research o f one another, so that differences between the laboratories arise m a i n l y f r o m the k i n d of problems submitted f o r solution.

The m a i n task o f a hydraulic laboratory, established f o r practical pur-poses, Hes i n the adaptation o f t h e existing achievements o f science to the requirements of engineering and to b u i l d up a completeness b y means of theoretical and experimental research as far as a sound scientific back-ground o f engineering requires.

Where basic knowledge fails the laboratory has to provide the means of gaining experience i n a short time.

I n the Netherlands the task is also accomphshed i n connection w i t h the graduate and post-graduate engineering education; the m a i n emphasis remains on the consultative aspect i n connection w i t h the hydraulics i n projects typical f o r the country.

These projects range f r o m the largest f o r l a n d reclamation, coastal protec-t i o n and sprotec-trucprotec-tures f o r i n l a n d and seagoing navigaprotec-tion protec-to protec-the smallesprotec-t for industrial and domestic water circulation. Wherever h y d r a u l i c prob-lems may be expected as such, or i n connection w i t h questions o f material transport, constant or transient loads on structures, the hydrauhcs labor-atory has a task. O f course, i n special cases w h i c h involve also problems of another k i n d t h a n those arising i n hydrodynamics, co-operation is arranged w i t h institutes specialized i n the p e r t a i n i n g fields.

T h e task already mentioned covers the field around the established tech-nique o f solving problems by means of scale models. As these models can never be designed, or give results, w i t h o u t an understanding of the dynamic background, the results o f research i n any branch o f fluid mechanics is o f importance.

I n a d d i t i o n to the broad held of experience already gained i n hydraulics itself, the mostly experimental knowledge i n the heterogeneous collection of phenomena cafled turbulence is also available. H y d r a u l i c s thus profits f r o m the achievements o f aerodynamics and i n some case, use is even made o f airflow as analogous to the flow of water.

was m a i n l y attained i n aerodynamics lies i n the great difficulties o f con-structing the necessary equipment f o r the instantaneous measurement of the spadally fluctuating velocities o f flow i n water. T h e counterpart o f the hot wire anemometer is not yet available for routine measurements. O n l y the small propellor type current meter has been developed to such an extent that some aspects of turbulence i n water can be analysed. B u t because of the dimension of the instrument, details of the structure still escape the observations. A great drawback i n the study of i n t e r n a l details of flow i n water is that other means of observation, e.g., visual and photo-graphical, are too time-consuming for practical purposes.

Together w i t h the achievements i n aerodynamics the great advances i n electronics are useful for hydraulics. Beside the present possibilities o f constructing pickups for water level variations, pressure fluctuations, etc., the recording of observations i n a reproduceable f o r m is available so that the p e r f o r m i n g o f one or another type o f analysis of observations can be facilitated by means of computers or electronic devices.

T h e general outcome of the improvements i n instrumentation is not that the experiments give results quicker than before but, on the contrary, require more time and attention. This seemingly paradoxal trend i n hydraulic experimentation can be considered as an advance. The inves-tigator can d r a w more and more i n f o r m a t i o n f r o m the model and gets more and more insight i n t o the possibilities and l i m i t a t i o n of his exper-iments.

Experiments, where turbulence is the object o f observation i n its context w i t h scour o f stream beds or vibrations of structures, can only i n rare cases be carried out by means of two dimensional models. T h e turbulence i n its d y n a m i c a l properties being essentially three dimensional is w r o n g l y influenced by g u i d i n g walls existing i n models and not i n prototype. T h e distortion of the d y n a m i c a l effects by walls i n the model thus make con-clusions d r a w n f r o m such experiments unreliable. This fact was already k n o w n to experienced investigators before the measuring equipment for turbulence measurements was available, b u t now i t g r a d u a l l y becomes possible to provide for the criteria to determine how far a model can be reduced and schematized.

T h e same holds good for the design of models i n w h i c h , beside the turbulence, the patterns of secondary flow have an accumulative

ef-feet, i n tlie course of time, on the bottom configuration i n loose beds. T h e g r o w i n g of understanding of the dynamics of these flow phenomena may be seen as a synthesis between experience i n hydraulics and analyses of the d y n a m i c a l relationships i n boundary layers and wakes i n fluid mechanics. Also here the hmitations of the possible simpliflcation of models become visible.

I n the multiphase flows (grains of sohd material or air bubbles i n water) the investigation i n hydraulics has to provide many of its o w n means and methods f o r analysis. T h e most d i f f i c u l t and most frequently encountered p r o b l e m is the determination of sediment transportation i n n a t u r a l streams. T h e inhomogenity and anisotropy of the m o v i n g mixtures pose problems i n analysis w h i c h require special ways of attack.

T h e available measuring instruments give only a general picture o f the correlation of the stream characteristics and the amount o f sediments transported. Because o f the stochastic element i n transport too many accurate observations, both i n time and i n location, are required. New methods o f measurements, e.g., by means of coloured or radioactive tracers, require extensive research on techniques o f application and inter-pretation of results.

Evidence has already been obtained that the empirical relationships between sediment l o a d and flow characteristics are v a l i d only i n per-manencies. Even slow fluctuations i n flow, as almost always encountered i n nature, are follovs'ed by the load o f transported material w i t h a non-negligible timelag. A l t h o u g h this p r o b l e m is one o f t h e oldest i n hydrauhc engineering, i t cannot be considered as even approximately solved. O f similar nature is the p r o b l e m o f local scour. As the time scale o f the models i n w h i c h the scour is reproduced is of importance f o r b i g projects, continued research is indispensable on its mechanism i n connection w i t h the perpetually changing flow characteristics w i t h the details of turbulence and secondary currents.

T h i s trend i n hydrauhc research to paying attention to the details has had its influence on the general set-up o f models. N o t only the choice o f the scales but also o f t h e location o f both the u p and downstream hmits is made on gradually i m p r o v i n g grounds.

T h e i n i t i a l turbulence, f o r example, i n w i n d tunnels, already well dis-cerned as a disturbing factor i n the proper interpretation o f the

measure-ments, appears also i n the hydraulies of t u r b u l e n t a g i t a t i o n of structures and i n experiments on local scour as a recognized disturbing factor.

Also the technique of using models of currents w i t h density stratification requires a critical hydrodynamic analysis.

T h e numerous problems connected w i t h the intrusion of salt water i n the Netherlands, both as a nuisance f o r agriculture and as one o f the agents i n silt deposition i n harbours, led to a programme f o r the i m -provement of the model technique i n w h i c h the density stratification is reproduced.

B o t h silt suspensions and actual salt solutions were used to simulate sea water. T h e former, being better tractable as a fluid i n the model, h a d the great disadvantage that measurements o f densities d u r i n g the experi-ments by means of simple devices appeared to be cumbersome. Because of the necessity of m a n y measurements, the salt solution is u p t i l l now the best means of reproducing the sea water.

F o r experiments w i t h equal importance o n the dynamics due to density stratification and the m i x i n g process, a systematic research on the two interconnected aspects has been set up i n order to determine the admis-sible range o f scales f o r models.

I n the study of current patterns near harbour entrances and i n cooling-water circuits, o f hawser forces i n locks and o f discharge coeflacients the remarkable differences i n the results obtained w i t h homogeneous water and density difference were sufficient reason f o r a perfection o f this model technique.

T h e experiments w i t h b o t h density stratification and transport of solid materials are still too cumbersome f o r routine purposes.

T h e items of research mentioned so far concern m a i n l y the scale and boundary effects to be avoided or correctly estimated. A n o t h e r p o i n t o f interest is the proper reproduction of the circumstances i n nature, i n -c l u d i n g the eflFe-ct of statisti-cal variations.

I n the design of structures exposed to wave attack i t is already a t r a d i t i o n a l practice to simulate i n the models the stochastic element of w i n d -generated waves because i n most cases the estimate o f risks i n extreme circumstances is decisive f o r the final design.

statis-tics o f occurrence o f storms i n strengtii, direction and fetch i n com-b i n a t i o n w i t h the phases of the tides, a detailed analysis of the waves is necessary.

T h e well-developed mathematical theory on the statistics o f time series is widely used f o r the analysis of wave observations. U n f o r t u n a t e l y the u n d e r l y i n g hypotheses of statistical r a n d o m processes and o f ergodic prop-erties show some lacunae w h i c h make estimates of risks i n the extremes d o u b t f u l . H e r e i n lies the necessary field o f research concerning the h y d r o -dynamic limits i n waves o f extreme magnitide and the evaluation o f the elements o f sequences and coincidences that are i m p o r t a n t f o r design. T h e requirement of proper reproduction i n the models o f waves o f the total variations of circumstances is again a stimulus f o r the i m p r o v e m e n t o f t h e technique of observations i n nature. As the statistical and dynamic evaluations require not only accurate observations but also observations covering sufficient intervals of dme, the amount o f material to be col-lected requires a well-organized system f o r the deduction o f the char-acterising parameters.

Thus the present research o n waves includes the techniques of observa-tion, the basic mathematical theories and the numerical treatment i n computers. This research going beyond the limitations o f hydrauhcs is carried out thanks to the collaboration w i t h mathematicians, designers of instruments and the services i n charge w i t h the observations i n nature. T h e m a i n task o f t h e laboratory remains the study o f t h e po.ssible mech-anism o f destruction and to provide quantitatively the loads of i m p a c t o n structures, w i t h the i n c o m i n g wave being w e l l reproduced.

A p a r t f r o m the interests f o r research on structure design, knowledge o f the regime o f waves combined w i t h current is the starting-point i n inves-tigations o n the penetration of waves i n harbours. Experiments w i t h harbours are, moreover, used to study the layout of the entrance o n the possible dangers for navigation under extreme circumstances.

I n this aspect ship model technique and hydraulic design go together. T h e avoiding of possible scale effects i n the steering devices of ship models, however, requires bigger models t h a n is needed f o r pure h y d r a u l i c en-gineering, so that the trend i n this model technique points t o w a r d special facilities.

I t is self-evident that these model techniques, backed by the knowledge of waves and currents, are apt f o r the solution o f special problems such

as loading and u n l o a d i n g i n the open sea and f o r structures f o r construc-t i o n and exploraconstruc-tion work.

T h e use of mathematics and computers has already been mentioned as an indispensable aid. T h e r a p i d developments i n this field give reason to investigate whether some hydraulic problems solved hitherto only by means o f models can be solved entirely-or partially by means o f computer techniques. Unsteady flow i n netvs'orks o f canals and flow i n two h o r i z o n -t a l dimensions under -the influence of -the -topography are already, i n theory, flt f o r treatment i n computers. I t is only a matter of the t r a i n i n g of the programmers and the availability of computers.

T h e use o f computers is already n o r m a l practice i n statistical and analyt-ical evaluation of data and the design of models w i t h movable bed according to the o p t i m a l set of scales f o r dimensions and current velocities combined w i t h the choice o f material f o r the bed i n the model.

I n a d d i t i o n to the p r i m a r y a i m of proper hydraulic design the element of h y d r o d y n a m i c a l research may be discerned i n the foregoing remarks. A n i m p o r t a n t group of investigations by means of models, however, is possible only w i t h the elements o f basic knowledge at hand, viz., the models used for the study of processes of f o r m a t i o n of shores, estuaries and river beds. T h e complex of the circumstances i n w h i c h these f o r m a -tions develop m_ust be simulated schematically in order to k n o w the coiu'se ol' the processes and to study the possibilities and the effects i n the long r u n , of h u m a n interference by means of structures or dredging operations. A simulation of the varying circumstances of waves, t i d a l currents and river discharges i n every detail is impracticable, moreover, as the h y d r o -dynamics of simultaneous action o f t h e m a n y agents is only a p p r o x i m a t e l y k n o w n , i n many cases the only means o f obtaining models w h i c h re-produce on the average the processes i n nature, is the r e p r o d u c t i o n o f that part of the process k n o w n f r o m successive hydrographic maps, com-pleted w i t h estimates o f t h e relative importance of currents and waves. As the time scale of these processes has only a remote connection w i t h the h y d r o d y n a m i c time scale, the possible schematization o f t h e variations i n circumstances has to be sought b y sound j u d g m e n t . Once such a model works properly, i t provides a means of gaining experience i n a relatively short time w i t h the various designs to be investigated.

T h e items of research mentioned i n this outhne have been chosen to demonstrate the technique o f using models considered as a necessary supplement to the theoretical approach to the problems.

M a n y see models as a k i n d o f a complicated analogue computer, and w a n t to have these computers as reliable as possible. T h e efforts of research thus become merely an effort to create better and more p o w e r f u l computers.

I n one aspect the hydraulic model has, indeed, something i n c o m m o n w i t h a computer i n the ordinary sense o f the w o r d : bad operation, poor p r o g r a m m i n g and unreliable data can never give reliable results.

For the hydraulic engineer, however, the model is j u s t as real as the objects o f his design: the efforts of research to i m p r o v e the operation of models is f o r h i m basically the same as the improvement o f his under-standing of hydrodynamics.

I N S T R U M E N T A L A I D S F O R H Y D R A U L I C M O D E L S T U D I E S B Y I R . J . v. D . W E L

Introduction

Since the start o f the laboratory there has been a need f o r a variety of instruments f o r the measurement o f several quantities, such as wave heights, water levels, velocities etc. A t first the instruments were very simple, b u t when the need arose f o r greater reliability and accuracy and the possibility o f recording the signals, better instruments and methods were developed or are i n development. For example, the wave height metering started w i t h the measurement o f the water level w i t h a scale. T h e next method used was a floating ping-pong ball w i t h a needle w h i c h scratched the wave height on a blacked glass plate w h i c h was moved along the needle along a horizontal line. T h e improvements i n the elec-tronic measuring techniques then made i t possible to design an elecelec-tronic instrument w h i c h measures the variations i n the electrical resistance between t w o parallel stretched wires. Nowadays, when sometimes two or more wave height meters are used simultaneously f o r the measurement

o f correlation functions, i t seems that this method again needs some i m -provement i n order to get better stability i n c a l i b r a t i o n and linearity. T h e development of new instruments, data h a n d l i n g systems and control systems design is done by the I n s t r u m e n t a t i o n Department o f the L a b o r -atory w h i c h has gradually come into being and w h i c h is still expanding. T o avoid an accumulation of developments i n the D e p a r t m e n t some o f them are p u t out to contractors, while at other times i t is possible to w o r k i n co-operation w i t h a contractor who develops that part of an instrument i n the design o f w h i c h he is an expert.

Birds-eye view of Control Systems and Instruments

Several specially-designed tidal control systems are i n use now i n the L a b o r a t o r y . T h e simple ones are controlled by a feeler and a cam o n w h i c h the t i d a l curve is cut, whilst the more complicated ones are m u l t i -point control systems w h i c h are programmed by p l u g boards or punched tape. Special care had to be taken i n order to avoid instability or inter-acting of the control systems. Especially i n the Delta model instability o f the control systems is d i f f i c u l t to be avoided completely i n some situations. For the measurement and recording o f water levels, velocities and dis-charges i n open water flows i n tidal models special instruments have been designed. T h e possibilities of speeding u p the data h a n d l i n g o f measure-ments i n t i d a l models are now being studied.

For the measurements of wave heights the above-mentioned wave height-meter is used while an analyser f o r the automatic analysis o f t h e a m p l i t u d e distribution f u n c t i o n w i t h neglect of the small waves has j u s t been developed.

For the measurement o f waves i n situ o n a lake capacitive-type trans-ducers have been developed f o r the evaluating o f correlation functions, the wave heights being recorded on a multi-channel magnetic tape re-corder. The measurements of sea waves can be done by an Ospos wave meter w h i c h measures the pressure fluctuations, caused b y the wave m o t i o n , as close to the surface as possible. The signal is recorded i n the instrument.

A contractor is developing a floating wave height meter f o r the measure-ment o f sea wave heights. T h e floating buoy w h i c h is smoothly anchored contains an acceleration meter w h i c h detects only the vertical accelera-tions. This signal is converted i n an F M signal, modulated o n a carrier

PHOTO 2 Preveldi (measurement of pressure, velocity and direction)

radio wave and broadcast to tlie shore where i t v/ill be p u t on punched tape. T h e hrst item is under test now.

V e l o c i t y measurements are made by a propeller current meter w i t h a diameter of 15 m m (see PHOTO 1). Because the meter is specially designed for the measurement of t u r b u l e n t velocities, as m a n y electrical impulses as possible have to be generated by the meter. This is done b y a small r i n g w i t h 60 holes mounted on the circumference o f the propeller and two electrodes mounted i n the holder, and giving signals w i t h a 90° phase shift, recorded on magnetic tape. W i t h these signals detection o f the direction of rotation is possible. A special apparatus has been designed for evaluating the standard deviation of the t u r b u l e n t fluctuations of the velocity.

For the measurement of velocity and direction o f the current near the b o t t o m under heavy conditions (sand i n suspension) i n situ, a c u r r e n t meter w i t h a cuprotor and a vane has been constructed (see PHOTO 2 ) .

T h e r o t a t i o n of the rotor and the vane is coupled magnetically to a l a m p and photo cell device and a low torque potentio meter. T h e instrument can be mounted on a support w h i c h is placed on the sea b o t t o m . A pressure sensitive device is mounted i n the underside of the instrument f o r the measurement o f pressure fluctuations near the b o t t o m . T h e signals are fed to a floating buoy w i t h a modulator and a transmitter w h i c h broadcasts the signal to the shore.

For the study o f t h e m i x i n g process at the interface between salt and fresh water, a conductivity probe has been designed w i t h a short response period (3 m/sec). T h e water of w h i c h the salinity must be measured is sucked by a siphon through a tube w i t h an inner diameter o f 0.5 m m and a length of 5 m m . T h e conductivity o f t h e volume of this tube is measured w i t h a conductivity meter. Due to the h i g h velocity of the water i n the tube (1.5 m/sec) the time of response is very short. For the measurement of salinity distributions the probe is moved up and d o w n at a constant speed o f 5 cm/sec, a speed at w h i c h the time lag is stiU acceptable.

I N V E S T I G A T I O N S O N S T R U C T U R E S F O R F L O W C O N T R O L BY I R . J . E . PRINS

I n the design of structures f o r flow control two m a j o r aspects stand o u t : the structure must withstand the h y d r a u l i c forces, and the hydrauhc eff'ects on the surroundings must not interfere w i t h the proper operation or stability o f t h e structure.

I n the hydraulic research m u c h attention has been given f r o m the very beginning to these aspects, viz., the determination o f pressure distribu-tions and total forces on gates or valves, and the study o n downstream scour or culvert abrasion.

Both subjects are very complex.

First o f all, the structures are very diverse i n type (gates w i t h u n d e r f f o w and overffow, submerged gates, h i g h and low head valves, free surface and closed conduit systems) and the capacities vary widely i n absolute values and i n the range o f operation.

Secondly, there is not a straightforward approach i n relating the h y d r a u l i c conditions to the dynamic loads, nor to the scour or abrasion.

I n considering the model investigations for the determination o f t h e forces on gates and such-like, technical limitations have for a long time restricted the results to average values, leaving undefined the magnitude of v i b r a -tions i n the prototype. Later on sensitive pressure pick-ups were able to give recordings of the pressure fluctuations along the periphery o f the gate due to turbulence. A l t h o u g h this provided qualitative data enabling comparative studies, the interaction between turbulence and gate move-ments, i.e., vibrations, could not be predicted. Finally, the technique developed i n the hydraulic research sector d u r i n g the last decade, o f constructing the model of the gate geometrically and elastically similar to the prototype structure according to Froude L a w , has largely solved the problem, but still raises questions on the effects of scaling d o w n the turbulence and the reproduction o f damping.

L o o k i n g into the studies of scour and abrasion, generally the interpreta-t i o n of interpreta-the model resulinterpreta-ts m a i n l y relies on empiricism. For interpreta-the process o f scouring, the p r i n c i p a l question is the ultimate state of the erosion. I t has been established that average velocity, intensity of turbulence and applied bed material are i m p o r t a n t factors as to the f o r m and develop-ment o f the scour hole, and that due to the eff'ect of vortices i n the zone of separation o f the m a i n ffow and the side eddies a three-dimensional model is indispensable. Fundamental research carried out i n the last few years has refined the empirical approach i n many respects; nevertheless, the urgent necessity remains to pursue a physically sound mathematical description of the scouring process using determinative parameters f o r the flow and the bed material. This w i l l by no means replace model investigations, but w i l l provide a basis for justified scale deduction and i n t e r p r e t a t i o n o f the results.

I n view o f t h e farreaching possibilities emanating f r o m the use o f d y n a m -ically similar models, this technique w i l l be given f u r t h e r consideration later. T h e aspect o f the local scour w i l l be discussed more extensively i n another c o n t r i b u t i o n to this all-round review.

I n its most simple f o r m the dynamic behaviour of a gate structure can be considered as a mass-spring system, the gate and added water being

the mass and the Hfting device being the spring. Assuming that the gate is exposed to f l o w i n g water, i t w i l l react on the exerted forces. W h e n the model is dimensioned according to Froude L a w , w h i c h is the practice i n free surface problems, the mass-spring system i n the model is b o u n d to the f o l l o w i n g conditions:

- the mass of the gate has to be reduced by the length scale to the t h i r d power and

- the n a t u r a l frequency of the system has to be reduced inversely propor-tional to the square root o f t h e length scale (^/i^*''"').

T h e first c o n d i t i o n satisfies the need for equal reduction o f the mass of the gate and the v i r t u a l mass of water playing a part i n the dynamics o f the system. T h e second condition takes care of a similar location o f the n a t u r a l frequency o f t h e system i n the range of frequencies o f t h e exciting forces i n the flowing water. T h e conditions together lead to equal relative deflections for the prototype and the model.

For the exact prediction of the prototype movement f r o m the model still two other factors are determinative, i.e., the reproduction of d a m p i n g and turbulence.

T h e d a m p i n g is a most troublesome matter, because rather undefined magnitudes are often introduced i n the f o r m of seals and guides or bear-ings of the l i f t i n g device. T h e magnitude of the internal d a m p i n g o f the prototype structure is generally very small, i n contrast w i t h the value obtained i n the model. Moreover, the d a m p i n g of the system by the surrounding water due to viscosity efiects and surface waves is, f o r an otherwise freely suspended system, determinative i n itself O n the latter factors experiments on scale effects have been carried out, b u t up to now no conclusive results have been obtained. O n theoretical grounds the model may give too large a d a m p i n g .

The reproduction o f the frequency spectrum of the turbulence has i n general been f o u n d satisfactory. T h e loss of the very h i g h frequencies i n the spectrum cannot be of any significance. W h a t is i m p o r t a n t is the way of generation of the frequencies for w h i c h the structure shows resonance tendencies. I n many cases violent vibrations are due to the i n t e r a c t i o n between the gate movement and the pulsating forces, imposing on the pulsation the n a t u r a l frequency o f the system (feed-back). For the l o w frequency range, i.e., macro-turbulence, viscosity effects and exactness of the model w i l l be of m i n o r i m p o r t a n c e ; for the higher frequencies,

how-•J

Examples of recordings of the vibration of the model of a sluice gate treated as a simple mass-spring system. Mass 34 x 10'kg. Spring constant 55 x 10" N / m . Natural frequency i n dry 6.4 H z .

a. Record with discharge of 0.7 m^secm and waves that overtop the gate. Elevation of gate 0.06 m .

b. Record with discharge of 2.7 m'/secm. No waves. Elevation of gate 0.50 m .

N

r

Vr

Examples of recordings of the vibration of the vizor-gate at Hagestein. Operational conditions: Elevation of gate in middle 1.10 m ; water level upstream N . A . P . - j - 2.3 m , downstream N.A.P. -|- 0.7 m . Mass of gate approx. 200 x 10' kg.

a. Record of prototype, measured at point of suspension i n horizontal direction perpendicular to the main flow direction. The record shows a voluntary movement of the gate, which by times is accompanied by two natural frequencies both due to flexm'al deformation (0.5 and 0.8 H z ) . A t this direction and location torsional deformation is nearly not noticeable.

b. Record of model, measured horizontally i n radial direction i n the middle of the upper side of the gate. The record shows a voluntary movement of the gate, which by times is accompanied by two natural frequencies, one due to flexural deforma-tion (0.8 Hz) and one due to torsional deformadeforma-tion (1.2 H z . ) . A t this direcdeforma-tion and location the flexural deformation with natural frequency of 0.5 Hz. is not noticeable.

PHOTO 3 Elastically similar model of the gates and supporting beam of Haringvliet Sluice. .Scale of model «i = 20. The model is provided with strain gauges

ever, w h i c h are likely to be bound to small-sized parts of the structure, viscosity effects or a geometrically inaccurate reproduction o f the p a r t i n question can suppress or prevent the spontaneous reaction o f t h e system. I t is evident, i n view of this, that also the i n i t i a l turbulence upstream o f the gate requires an exact reproduction.

T h e simple mass-spring system as described above no longer suffices when the gate itself is subject to flexural and torsional vibrations, or w h e n the construction is such that i m p o r t a n t parts may get i n resonance by the pulsating forces o f the flowing water. I n this case, elastical similarity o f the structure itself and the parts i n question is required.

Next to the two conditions f o r the reduction o f mass («i^) and n a t u r a l frequency now related to any n a t u r a l frequency, axial, flexural and torsional, a t h i r d condition is necessary, h o l d i n g that the mass has to be distributed correctly i n view o f the axial, flexural and torsional v i b r a t i o n characteristics.

M o d e r n materials have facilitated the realization of the technique of elastically similar models. By the use o f the hardened polyvinylchloride " t r o v i d u r " , w h i c h has the practical advantage that i t can be welded, the above given conditions f o r the model can be met at length scales ni> 11. T h e condition f o r the time scale of the n a t u r a l frequency f o r the axial v i b r a t i o n gives H E = K I - K ^ , i n w h i c h H E = i?steei/-Etrovidur s 60 {E is modulus of elasticity), where iii can be chosen w i t h the restriction that IIQ < 5.6, since gsteei/etroviaur = 5.6 { Q is density).

W h e n IIQ < 5.6 - w h i c h is common practice as m is generally larger t h a n 11 - additional mass has to be introduced to the t r o v i d u r mass already present. This can be done locally under maintenance o f the proper distribution as to the correct mass polar moment o f inertia. Scale consid-eration also shows that the trovidur is strong enough to compete w i t h prototype («„ = HE).

I n cases where the study includes also the supporting structure of the gate, the elastical properties of this structure have to be reproduced as well. As the forces are those o f t h e water, transmitted t h r o u g h the gate, the mass and n a t u r a l frequencies of this structure again have to be reduced according to Froude L a w . W h e n reproducing a concrete structure w i t h t r o v i d u r the length scale is bound to ni < 7, w h i c h does not conflict w i t h the existing condition for the reproduction o f the steel structure ( K I > 11).

W h e n the assembly o f t h e gate and its supporting structure is elastically similarly reproduced, the f u l l response of this system to externally imposed loads can be detected. I n this respect i t may be of interest to recall that for models o f this k i n d the u n i t elongation i n a point o f t h e model is equal to that i n the same point o f t h e prototype, i.e., w i t h a straingauge exactly the same intensity o f the signal is measured i n model and prototype.

I n the foregoing the erratically pulsating forces by the flowing water - at r a n d o m excitation - have been taken to illustrate the need for a f u l l dynamic performance o f the model. A method to calculate f r o m these pressure fluctuations the behaviour o f the system, viz., the forced a n d resonance oscillations, is not feasible i n hydrodynamics, since the reper-cussion o f t h e gate movement on the nature of excitation is u n p r e d i c t a b l e : i t may cause a change f r o m a random excitation to a clear frequency-b o u n d excitation!

There is another phenomenon asking for the use of elastical similarity, i.e., the response o f a structure to impact forces of h y d r o d y n a m i c a l o r i g i n . I n this case calculation can be carried through only when the i m p a c t force can be actually handled for mathematical operation. For the cases thought of here, namely, impact forces caused by surface waves, this is h a r d l y possible and a hydrauhc model is needed. This is a l l the more true because the practical cases are those w i t h w i n d waves, and the approach should be made statistically.

Summarizing, i t can be said that the use o f models w i t h geometrical and dynamical similarity enables the designer to judge the ability o f his design and provides the hydraulic engineer w i t h a tool to approach the problems of vibrations and i m p a c t loads i n a most extensive way. A t the same time, i t puts on the hydraulic engineer the responsibility o f not a p p l y i n g a refined model technique w i t h o u t v e r i f y i n g the exactness of the h y d r a u l i c conditions to be fed into the model and the exactness o f t h e i n t e r p r e t a t i o n that can be given to the results.

R I V E R S T U D I E S B Y I R . M . DE VRIES

T h e development of river models d u r i n g the past few decades is closely related to two i m p o r t a n t factors: on the one hand, the theoretical and the experimental b u i l d i n g up o f transport equations f o r sand b y currents, and, on the other hand, the technique f o r measuring the transported quantities o f sand i n nature. I t is clear that the stage o f t h e river models w i l l always lag behind the results i n those two fields. I n fact, the meas-u r i n g techniqmeas-ue i n natmeas-ure is the determining factor.

W o r k on these three i m p o r t a n t factors i n river-hydraulics-transport-formulae, transport instruments and transport models can be traced back i n the history of the Laboratory. O f course, the work i n these fields is closely related, and the solutions o f practical problems at any time have to be based on the best knowledge available at that very moment. A t present, there are still m a n y questions w a i t i n g to be answered i n order to obtain a better insight into the phenomena and greater accuracy i n predictions.

M a n y river models have been b u i l t f o r the Rijkswaterstaat, and transport instruments have been developed i n close co-operation w i t h its special services.

T h e measurement of suspended load, rather simply compared w i t h that of bed load, could be cff'ccted b y the construction o f t h e " D e l f t - B o t t l e " . T h e transport of sand close to the bed surface is d i f i i c u l t to measure w i t h instruments. I n the 1930's the " B . T . M . A r n h e m " was developed and calibrated i n Z ü r i c h . T h e later calibration some years ago, w h e n the large high-discharge-flume of the de Voorst L a b o r a t o r y was available, showed f a i r agreement w i t h the Z ü r i c h data. T h e spread i n the measure-ments, however, is still large, p a r t l y due to the phenomenon and p a r t l y due to this type o f instrument of the basket type. N e w developments are under way to overcome the difficulties o f t h e " B . T . M . A r n h e m " and to study the statistics of the variations w i t h time o f the local bed load i n order to get the o p t i m a l result i n accuracy for the instruments.

T h e measurements o f sand-transport w i t h radioactive, and ffuorescent tracers m i g h t be of great help. But i t is clear that m u c h w o r k on the quantitative interpretation o f these investigations w i l l have to be done

before answers along this quite different hne can be obtained f o r the transport of sand i n rivers.

Those tracer techniques have already been carried out i n the Netherlands for the coastal waters where the influence of ddes and waves makes the transport p r o b l e m m u c h more complicated. Tracer experiments w i t h a special view to quantitative results f o r rivers are under way.

T h e development o f bed load formulae is being carried out by a great many investigators i n d i f f e r e n t countries. The large number o f f o r m u l a e is already an i n d i c a t i o n about the small accuracy of sand transport calculations. I n this connection, the work of F R I J L I N K can be mentioned. By comparing the formulae of M E Y E R - P E T E R and M Ü L L E R w i t h that of E I N S T E I N and K A L I N S K E , he f o u n d that these formulae had basically the same f o r m and that the differences were small compared w i t h the meas-u r i n g data. T h e d i f f i c meas-u l t comeas-upling of water-modon and sand transport by the bed roughness was part of the L a b o r a t o r y research. Recently the non-steady character of the phenomenon i n nature became a subject f o r theoretical and experimental studies. Similar to the flow on a fixed bed, where backwater-curves were studied even though roughness formulae still have small accuracy because of their nature, non-steady sand-motion is also being studied, although there are still many questions about the steady conditions.

T h e river models show the development of the measuring technique and the description o f the transport formulae. T h e attitude remained the same: a mobile-bed model can never be used as a computer b u t only as a tool f o r the engineer to take conclusions out o f tendencies noticed i n prototype and model. The scalerules were developed on " f l o w p a r a -meter" and "transport-para-meter" as soon i t was noticed t h a t the scales of water velocity and bed material had to be chosen i n such a way that the transport scale was independent o f time and place i n the model. G r a d u a l l y the concept o f the " i d e a l velocity scale" was developed and upto now this basic concept has remained the same, although refinements due to the growing knowledge o f the phenomenon have been made. Recently the determination of the o p t i m a l model-scales has led to a programme f o r a d i g i t a l computer. I t is clear that this does not give a solution f o r physical questions, b u t the method can be used to determine

the scales i n such a way that experimental corrections d u r i n g calibration tests can easily be carried out.

A special chapter is f o r m e d by the problems w h i c h are studied by river models. Mostly these are concerned w i t h the demands o f navigation. W i d e entrances to river harbours, canals and locks ask f o r special arrange-ments i n order to give enough guidance to the m a i n stream. I n different cases egg-shaped entrances were constructed to guide the m a i n stream by the eddy i n the entrance. T h e crossing o f t h e Amsterdam-Rhine Canal and the River L e k got one " e g g " on both sides, and special attention was given to the construction so as to get i n the river at the crossing the same depth of water as i n the canal, w h i c h was more t h a n the depth i n the undisturbed river.

Suitably constructed screens at the entrances make i t possible to diminish the sanding-up of the entrance. This principle was carried out for dif-ferent entrances.

A large group o f river studies is connected w i t h the canalization o f the L o w e r Rhine. T h e b i f u r c a t i o n o f t h e Rhine near Pannerden was studied i n the D e l f t L a b o r a t o r y and the b i f u r c a t i o n at Westervoort is under study i n the De Voorst Laboratory. T h e d i f f i c u l t y of those b i f u r c a t i o n studies is that a compromise for the model scales has to be made as three different rivers, each w i t h its o w n characteristics, have to be simulated by the model. T h e latter case forms a special study due to the combined use of a computer and a mobile-bed model.

A step-by-step procedure w i t h a hydraulic model and a computer is used for the f o l l o w i n g reason. T h e artificial changes i n the water d i s t r i b u d o n at the b i f u r c a t i o n w i l l influence the bed o f t h e three branches over a large distance. B u t the length o f a river w h i c h can be simulated i n any mobile-bed model is restricted. T h e influence o f the river-mobile-bed outside the model area is taken i n t o account i n this case via the calculated boundary condi-tions of the model.

I n a d d i t i o n to those studies of bifurcations several model investigations were carried out of parts o f the River R h i n e where new movable weirs have to be b u i l t . A g a i n navigation was here the d e t e r m i n i n g factor. I n regard to the scales of the river models, the f o l l o w i n g trend can be observed. T h e first river models were relatively small, h a v i n g large scale-factors for the length. Later on, when the De Voorst open-air laboratory was established larger models could be constructed as more space was

available. Therefore more detail-information about phenomena could be obtained. T h e modern trend is that the scale-factors can become still larger, leading to smaller models w i t h larger time-scales i n order to o b t a i n the model results quicker. T h e difference f r o m the original models o f some decades ago is that the experience and the knowledge obtained has led to the situation that nowadays m u c h more detailed problems can be investigated and more accurate predictions can be made.

M O D E L I N V E S T I G A T I O N S O N L O C A L S C O U R B Y I R . A . P A A P E

T h e application of movable bed models i n studies on sediment transport has importance i n many practical problems. T h e problems met i n scaling d o w n the bed materials have led to special model techniques, while extensive research has been carried out by many investigators on the p r o b l e m of the interpretation of test results. T h e methods used f o r inves-tigations into the various aspects o f sediment transport show great dif-ferences, b u t this review w i l l deal only w i t h the investigations connected w i t h the process of local scour downstream o f structures (evacuating sluices, ship locks, weirs, structures f o r enclosing estuaries, etc.).

T h e difficulties w h i c h are met i n scaling d o w n the bed material need some f u r t h e r explanation. I n many problems investigated only external forces (gravity) and inertia forces are considered. This leads to the Froude model law. T h e inffuence o f f r i c t i o n forces caused by the viscosity of the water is then neglected. H o w far this is justified depends o n the ratio between inertia forces and f r i c t i o n forces, w h i c h can be expressed as the Reynolds number Re = vljv i n w h i c h v and / are respectively a character-istic velocity and length, and v is the kinematic viscosity o f the water. T h e influence of viscosity increases as the Re number decreases.

D e f i n i n g the length scale o f a model

and a p p l y i n g the Froude model law, the Reynolds number is i n the model a factor A'^i'^' smaller than i n prototype.

T h e behaviour of sediment grains on the b o t t o m is determined b y the f l o w conditions close to the w a l l . I n this region the velocities decrease w i t h decreasing distance to the w a l l , whereas the influence of the viscosity increases. W i t h respect to the flow around the grains, the Reynolds number can be deflned, f o r instance, by Re* = v*djv, i n w h i c h d is the diameter of the grains and v* is the shear stress velocity =

Vthe b o t t o m shear stress/density o f t h e water.

W h e n the behaviour of gravel is considered, for instance w i t h ( / = 4 • 10^2 v* = 0.25 m/sec and Re* = 8000, i n a model w i t h Ni = 25, the Reynolds number is Re* = 64. A l t h o u g h there is an influence o f viscosity, i n this case a model i n w h i c h the bed material is reproduced on length scale, can provide satisfactory results.

I n many practical problems i n the Netherlands, the conditions are: d= 10-4 to 2 - 1 0 - 3 m and v* = 10-2 to 4 - 1 0 - 2 m/sec. So Re* is o f t h e order o f magnitude: Re* = 0.8 to 65.

T h e f r i c t i o n forces are now so i m p o r t a n t compared w i t h the i n e r t i a forces that a reproduction of the bed material on length scale, thus decreasing Re, is no more acceptable.

T h e application o f the Froude model law is necessary f o r the reproduction of the characteristics of the m a i n flow and orfly i n some special cases can the velocities i n the model be exaggerated. Hence f o r the r e p r o d u c t i o n of the scouring process i n m a n y cases other ways had to be f o u n d . A method w h i c h is generally applied, f r o m the first tests that were carried out i n this field, is the application of bed materials w i t h densities smaller t h a n those i n prototype. T h e diameter of the grains i n the model can be the same as or even greater t h a n i n prototype (thus increasing the Re*-n u m b e r ) , whereas the critical velocities are reduced because o f t h e small efliective weight of the grains.

I n the Hydraulics L a b o r a t o r y at D e l f t , f o r instance, as early as 1929 the scouring downstream o f t h e evacuation sluices i n the A f s l u i t d i j k (Zuyder Sea Enclosing D a m ) was investigated, a p p l y i n g i n the model grains o f pumice-stone. Another example, w i t h grains of 1-2 m m , is shown on photo 6 ( M l 8 5 , 1940).

P H O T O 6

A l t h o u g h application of these materials has the advantage that the critical velocities (for i n i t i a t i o n of motion) are reduced and that i n the m o d e l the bed material can be transported, the difficulties encountered i n the interpretation o f the test results still exist. I n practice the p r o b l e m is to determine the required b o t t o m protection i n order to ensure the stability of the structure w i t h m i n i m u m costs.

T o consider these problems and the progress made i n t r y i n g to solve t h e m , the scouring process is characterized b y :

~ the geometrical shape of the scour hole; - the ultimate depth, and

- the development of the erosion as a f u n c t i o n of time.

For the moment i t is assumed that the flow conditions are reproduced correctly. I f i n the model the velocities are great compared w i t h the critical velocities f o r i n i t i a t i o n of motion, i t can be understood that the transport of sediment is distributed over the area considered i n the same

way i n model and i n prototype. Consequently i n this case i t m a y be expected that the geometrical shape of the scouring hole is f a i r l y w e l l reproduced. T h i s is the basis for a design procedure i n w h i c h the required b o t t o m protection is determined by means of comparative tests. T h e erosion that w i l l occur for different b o t t o m protections can be qualita-tively compared. I t is clear that when the ultimate depth or the develop-ment of the erosion as a f u n c t i o n of time are not k n o w n , the solution w h i c h is chosen may be too expensive or may possibly provide insufficient safety.

A l t h o u g h new materials have become available ( i n the Hydraulics L a b o r -atory, for instance, bakelite and polystyrene), the reproduction of fine sand i n such a way that the scale of the critical shear stress velocity is the same as the scale of the actual shear stress velocities is i n most cases still impossible. T h e procedure of p e r f o r m i n g comparative tests as des-cribed above has been applied i n most of the investigations, b u t i t is not necessary to deal w i t h these tests separately here. T h e i n f o r m a t i o n ob-tained is useful, although the results have to be interpreted w i t h due caution.

T h e ultimate depth of the scour hole can be derived f r o m model inves-tigations w h e n the i n i t i a t i o n of motion is reproduced correctly. As already stated, i n case of fine sand i n prototype this is mostly impossible, as the effective weight of the bed material i n the model must be so small that lor practical reasons the performance of the tests becomes too d i f f i c u l t . I t is generally only applicable for rather coarse materials i n nature. Some-times a better a p p r o x i m a t i o n can be obtained by exaggerating the veloc-ities i n the model, w h i c h means that the velocveloc-ities applied i n the model are greater t h a n those corresponding w i t h the Froude model l a w . This, however, may introduce deviations i n the reproduction o f the flow char-acteristics (flow pattern), especially i n three dimensional models. These problems need special attention.

W h e n the ultimate depth is reached, the geometrical shape o f the scour hole w i l l be reasonably the same i n model and i n prototype. Deviations are possible w h e n d u r i n g the process a considerable suspended load occurs. I n a d d i t i o n to the critical velocity, the settling velocity o f the material also has to be taken into account.

b o t t o m protection, because tire time w h i c h is required to reach this situa-t i o n is u n k n o w n and may exceed situa-thasitua-t w h i c h is of insitua-teressitua-t situa-to situa-the designer. I n that case, actually only i n f o r m a d o n about the development o f the scour hole w i t h time can provide the required data. This is a very complex problem, however, w h i c h i n fact involves the determination o f the time-scale of the model.

I n the Netherlands there are at present very i m p o r t a n t problems w i t h respect to erosion downstream o f structures. A m o n g these can be men-tioned the erosion w h i c h occurs d u r i n g the b u i l d i n g o f t h e enclosure dams i n the various estuaries and the erosion downstream of the H a r i n g v l i e t evacuation sluice. I n a l l these cases the b o t t o m consists o f non-cohesive fine sand w i t h a mean diameter of 0.1 to 0.2 m m . T h e b o t t o m protec-tions for these works are very expensive. For an economical design a better knowledge about the scouring process, i n c l u d i n g i n f o r m a t i o n about the time-scale o f a model, is imperative.

T h i s made the Rijkswaterstaat initiate a f u n d a m e n t a l study to be carried out by the D e l f t Hydraulics Laboratory.

T h e general a i m of this research work is:

a. T o estabhsh relationships between the flow conditions, the properties of the bed materials and the sediment transport. For practical p r o b lems a model w i l l always remain necessary due to the complex b o u n d -ary conditions;

b. T o v e r i f y i n systematic tests the similarity of the scouring i n model and i n prototype, and to determine the time-scale o f the m o d e l . I n practice these studies have to be carried out i n close connection w i t h each other.

T h e study mentioned under a. implies that the flow conditions must be k n o w n . For a complete description of the flow i t is not sufficient to use only mean values for the velocities. Especially i n the region downstream of structures the flow can be very turbulent and b i g eddies may occur w h i c h certainly w i l l have an influence on the erosion. Measurements o f turbulence characteristics i n water, however, have always raised m a n y problems. W i t h respect to this, i m p o r t a n t progress has been made w i t h the development of a small plastic propeller-type current meter used i n combination w i t h recording o n magnetic tape. F r o m continuous records o f t h e velocities obtained i n this way, the turbulence characteristics w h i c h

are of interest can be derived w i t l i the aid of a computer (see also the c o n t r i b u t i o n i n this book i n " I n s t r u m e n t a l aids for hydrauhc model studies").

F r o m these investigations an i m p o r t a n t influence of the turbulence inten-sity on the scouring process was f o u n d .

A unique relationship between the flow characteristics and the sediment transport could up t i l l now not be established. T h e phenomenon is very complex, and analogous processes i n physics could not yet be f o u n d . Systematic tests w i t h variable bed materials and flow conditions con-firmed the statement that w h e n there is a considerable transport i n the model, the development of the scouring hole is similar for d i f f e r e n t mate-rials and diflferent values o f the length scale and velocity scale of the model. U n d e r these conditions for a given situation a time-scale can be defined w h i c h is constant d u r i n g the process.

T h e time scale Nf is a f u n c t i o n of the length scale Ni (already defined) the velocity scale A^„ and the properties of the bed m a t e r i a l i n prototype and i n model, f o r simphcity expressed as Nmut- T h i s f u n c t i o n is o f t h e f o r m :

T h e values o f 7,n, n and p depend on certain properties o f the bed ma terial and the turbulence intensity of the flow. T h e determination o f these ex-ponents is one of the m a i n items being investigated at present.

Consideration may now be given to the progress made i n respect of the present practical problems.

T h e Veersche Gat estuary was closed i n A p r i l 1961. T h e investigations on the scour to be expected were carried out i n three-dimensional models i n the open air department De Voorst and a two-dimensional model i n

the laboratory at D e l f t .

The great advantage o f the models i n De Voorst is that large areas i n prototype can be reproduced on a reasonable scale. T h e importance of a large scale has been amply explained. These possibihties and the use of bakelite as a bed material have increased the a p p l i c a b i l i t y o f the test results considerably. A better insight into the scouring process has i m

-proved the interpretation o f the test results, while a well-founded use o f distorted and undistorted models can also be made.

A l t h o u g h for the scouring models of the Veersche Gat the determination o f t h e time-scale o f t h e model was still questionable, the tests, i n combina-t i o n w i combina-t h invescombina-tigacombina-tions i n a combina-two-dimensional model a p p l y i n g bakelicombina-te and polystyrene, led to satisfactory results.

T h e remarks on current investigations w i l l be restricted to the tests con-cerning the erosion downstream of the H a r i n g v l i e t evacuation sluice. A l t h o u g h this structure is intended to have a long lifetime, also i n this case the erosion as a f u n c t i o n of time must be k n o w n . T h e final d e p t h of the scour hole, i f i t can be defined, depends on large discharges w i t h a relative small frequency of occurrence.

Besides the depth o f the erosion the angle of i n c l i n a t i o n of the upstream slope of the scour hole is also i m p o r t a n t , especially as layers o f loose sand occur.

Also i n this case the investigations are being carried out i n b o t h a three-dimensional and two-three-dimensional model.

PHOTO 7

Two-dimensional model of Haringvliet evacuation sluice. Bed material: polystyrene

I t has been f o u n d that because o f t h e great w i d t h o f t h e sluice, the results of the two-dimensional model are representative f o r a great part o f the total w i d t h . T h e bed materials applied are bakelite and polystyrene. One of the first problems investigated was the d e t e r m i n a t i o n o f the op-t i m u m shape o f op-the b o op-t op-t o m proop-tecop-tion i n respecop-t of op-the sop-tabiliop-ty o f op-the

structure [e.g., a horizontal protection or some type of inclined protection i n order to arrive at a "scour hole" w i t h hxed slope).

I n a d d i t i o n to the m a i n shape, also the influence o f t h e length and rough-ness o f the protection was investigated, and i t has been f o u n d that the latter has a considerable influence on the erosion. For the various solutions measurements of velocities and turbulence intensities were made, and these sustained the conclusions d r a w n f r o m the scouring tests.

I t is expected that w i t h the current systematic tests the time-scale can be estimated w i t h i n reasonable limits.

T o summarise: f r o m these models and investigations i m p o r t a n t data required f o r an economical design have already been obtained.

P R O B L E M S C O N N E C T E D W I T H F L O W S D U E T O D I F F E R E N C E S I N D E N S I T Y

B Y I R . G . A B R A H A M

F l u i d motions i n a gravitational field w h i c h are originated or influenced by variations i n density w i t h i n the fluid or by differences i n density of the fluids involved are characterized by the term flows due to differences i n density. Examples are given i n T A B L E 1.

T h e present activities of the D e l f t Hydraulics L a b o r a t o r y are concerned w i t h the f o l l o w i n g problems mentioned i n this table:

1. intrusion o f salt water t h r o u g h navigation locks; 2. siltation o f harbours, situated along t i d a l rivers; and 3. j e t diffusion i n connection w i t h waste disposal. These phenomena w i l l be discussed here.

Salt water intrusion is one of the m a j o r problems of the water management o f t h e Netherlands. This intrusion is p r i m a r i l y caused by the i n t r u -sion of salt water i n t o t i d a l rivers, the seepage o f salt g r o u n d water, and the salt intrusion t h r o u g h navigation locks situated along the coast. T h e problem o f the increasing salinity o f ground water and surface water i n the Netherlands is described i n the summaries o f the papers presented

T A B L E 1 Examples o f flows due to differences i n density

Cause of differ- Characterizing Examples of flows Significance for en-ences i n density quality of fluids due to the differen-ences gineering purposes

involved i n density considered

1. Difference i n fluids a. non-miscible b. miscible 2. Same fluids, difference i n salt content 3. Same fluids, difference i n temperature 4. Same fluids, difference i n concentration of solid matter 5. Same fluids, difference i n air content no mixing at interface mixing at interface mixing at interface heat transfer at water surface, interface, and bottom settling of par-ticles ascending of bubbles of air

spreading of oil over water flow phenomena i n estuaries, exchange phenomena in navigation in locks along coast, jet diffusion spreading of warmed cooling water over colder water

density flows through reservoirs

flows induced by releasing bubbles of air

salt water intrusion i n rivers, siltation, nav-igability of harbour entrances

salt water intrusion through locks, mooring forces due to exchange flow

waste disposal i n marine environment how to avoid short circuiting of cooling water

siltation

preventing freezing of water at water surface, reduction of intrusion of salt water through locks

i n the S i x t h T e c h n i c a l M e e t i n g o f the C o m m i t t e e f o r H y d r o l o g i c a l Research T . N . O . , J a n u a r y 18, 1950, a n d at the 18th I n t e r n a t i o n a l N a v i g a -t i o n Congress, R o m e , 1953, Sec-tion I I , C o m m u n i c a -t i o n 3, p p . 185-218. T h e construction o f the Zuiderzee Works b r o u g h t i m p o r t a n t advantages f o r the water management, as described by P r o f THIJSSE ( T i j d s c h r i f t K o n i n k l i j k Nederlands A a r d r i j k s k u n d i g Genootschap, Tweede reeks, 52, 1935, pp. 481-507 i n D u t c h ) . O n one side, i t was possible to create a f r e s h water reservoir to supply fresh water to the N o r t h e r n p a r t o f t h e c o u n t r y i n seasons o f d r o u g h t ; on the other side, the sea water was

kept f r o m penetrating far inland. The Delta Works w i l l b r i n g similar advantages f o r the South Western p a r t of the country.

Advantages o f a different order of magnitude, b u t nevertheless i m p o r t a n t ones, can be obtained by controUing the intrusion of salt water t h r o u g h navigation locks, w h i c h is caused by the fact that d u r i n g each locking cyclus a volume o f sea water is exchanged w i t h a volume of fresh water. T h e sea water w h i c h has entered i n t o the body of fresh water on the i n l a n d side o f the locks must be expelled f r o m i t . The present desalting processes (such as flushing w i t h fresh water) cause a loss o f fresh water, w h i c h may not be available i n periods of drought. Therefore research has been concentrated on the foUowing subjects:

1. h i n d e r i n g the intrusion of salt water i n the lock chamber; 2. i m p r o v i n g the efficiency o f the de-salting processes.

T h e most significant results of these studies are those of the investigation about pneumatic barriers, obtained by releasing bubbles of air f r o m perforated pipes placed on the b o t t o m of the lock i n a direction per-pendicular to its axis. A n extensive study b o t h i n a hydraulic model and i n the field showed that pneumatic barriers are an economically feasible method o f reducing the i n t r u s i o n of salt water t h r o u g h navigation locks. I t is intended to apply pneumatic barriers i n d i i ï e r e n t locks situated along the coast o f t h e Netherlands. Some results o f t h e pneumatic barrierstudy, carried out by the Service f o r Water Management and the A r r o n -dissement "het Noorclzeekanaal" of Rijkswaterstaat and by the D e l f t Hydraulics L a b o r a t o r y are presented i n Publication N o . 28 of the D e l f t Hydraulics Laboratory, August, 1962.

A t present the possibility is being studied of reducing salt i n t r u s i o n i n canals b y deepening a part o f t h e b o t t o m adjacent to the navigation locks and p r o v i d i n g a drainage culvert. T h e deep section is intended to act as a reservoir to trap salt water penetrating f r o m the locks, while the culvert is intended as a means o f sluicing out the salt water at a small steady discharge. This study is performed i n models of different scales, so as to obtain an insight into the scale rules w h i c h have to be applied. T h e largest flume used f o r this study has the foUowing dimensions: length

100 m , w i d t h 3 m , water depth 1 m . T h e conductivity probes especially designed f o r this study make i t possible to study the m i x i n g processes w h i c h are brought about when salt water is i n t r u d i n g into a stagnant

body of fresh water (see I n s t r u m e n t a l Aids for H y d r a u l i c M o d e l Studies, by J . V . D . W E L ) .

A considerable amount of money is spent each year on the maintenance dredging of the harbours of the city o f R o t t e r d a m , w h i c h are situated along the Rotterdamse Waterweg (or N e w W a t e r w a y ) , a t i d a l river w h i c h connects R o t t e r d a m w i t h the N o r t h Sea. I n this tidal river fresh and salt water come i n contact w i t h each other, and due to t i d a l action the salinity o f the river water varies w i t h time at the entrances o f the harboui-s. T h e salinity of the water varies less r a p i d l y i n the harbours t h a n i t does i n the river, as the v a r i a t i o n of the salinity of the water i n the harbour is brought about by exchange of water between the harbours and the river. Such an exchange of water may be a result of:

1. exchange flows, due to differences i n density between the water i n the harbours and the river;

2. v a r i a t i o n of the water level i n the harbour due to tidal a c t i o n ; and 3. horizontal exchange of m o m e n t u m between the water o f t h e h a r b o u r

and the water of the river.

For most o f t h e harbours along the Rotterdamse Waterweg the q u a n t i t y of water exchanged clue to differences i n density exceeds the quantities exchanged as a result o f the other phenomena. As the exchange flows are a mechanism by w h i c h solid matter can be brought into the harbours, the eff'ect o f t h e flows induced by differences i n density must be considered i n studying the siltation problem.

T h e m a i n questions to be answered i n studying the siltation are:

1. w h a t maintenance dredging w i l l be required for new harbours?, and 2. w h a t reduction of the maintenance dredging w i l l be obtained b y

ap-p l i c a t i o n of technical means, as, e.g., ap-pneumatic barriers at the en-trance o f the harbours?

I f on one side the relationship between the v a r i a t i o n of the salinity o f the river water at the entrance o f the (projected) harbour and the ex-change flows induced is k n o w n , and on the other side the relationship between the rate of siltation and the q u a n t i t y of water exchanged between river and harbour, then i t seems possible to predict the maintenance dredging f o r new harbours. A n extensive programme of field measure-ments is being carried out by the Service of Pubhc Works, C i t y o f

Rotter-d a m i n orRotter-der to f i n Rotter-d tiiese relationships anRotter-d to obtain an insight i n t o the complex siltation mechanism.

T h e field data collected w i l l provide a basis to calibrate hydraulic models to study how the exchange flows due to differences i n density (and thus the rate o f siltation) can be influenced by technical means at the entrance of the harbours. U n t i l now the eiïect of such technical means has been studied only i n hydraulic models under schematized conditions.

One of these studies was concerned w i t h the application o f pneumatic barriers to reduce siltation. T h e results obtained by this model investiga-t i o n gave a sinvestiga-timulus investiga-to invesinvestiga-tigainvestiga-te investiga-the applicainvestiga-tion of pneumainvestiga-tic barriers i n navigation locks on a prototype scale. W i t h i n a short time a field study of the effect o f pneumatic barriers on the siltation o f harbours w i l l be initiated b y the city o f R o t t e r d a m .

Submarine o u t f a l l disposal of domestic and industrial sewage is a method of disposal of steadily g r o w i n g importance. T h e submarine o u t f a l l at Scheveningen, one o f the earliest m a j o r submarine outfalls along the coast o f the Netherlands, was constructed i n 1 9 3 4 on the advice o f a committee of w h i c h P r o f THIJSSE was C h a i r m a n .

I n the direct surroundings o f the oudet the flow f r o m an ocean o u t f a l l is essentially that of a submerged j e t . Therefore a study o f the h y d r o -dynamics o f such jets is needed to evaluate the d i l u t i o n o f the sewage flow. A theoretical study of this k i n d was recently made at the D e l f t Hydraulics L a b o r a t o r y under the direct guidance o f P r o f THIJSSE and P r o f V A N SPIEGEL (Publication N o . 2 9 of the D e l f t H y d r a u l i c s Labor-atory, J u l y , 1 9 6 3 ) . T h e study gives the relationship between the d i l u t i o n and the determinative circumstances f o r jets issuing vertically upwards i n t o lighter or heavier ambient fluid, whose density varies a r b i t r a r i l y w i t h height and for jets issuing horizontally i n t o homogeneous ambient fluid of different density.

T h i s study is restricted to the case of stagnant ambient fluid, while the influence of the free water surface is not considered. For this reason a model study about the influence o f the free water surface i n case of stagnant ambient fluid was initiated recently at the D e l f t Hydrauhcs L a b o r a t o r y . I t is hoped to expand this experimental study to the case of jets issuing i n t o flowing water.