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Wave Generated Currents

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Wave Generated Currents UNIVER S ITY OF QUEENS LA ND DEPARTMENT OF CIVIL ENGINEERING "'�"' ,;t,. .c B U L L E T I M M o. 7 - MOVE N I FRY, TA .;1 1 .B83 l N0,7 2 WAVE GENERATED CURRENTS SOME OB SERVATIONS MADE INFIXED BED HYD RAULIC MODEL S M. R. GOURLAY, B.Sc., B.E.(Syd.), M.E. Lecturer in Civil Engineering I TA I · 688 No.'7 � CURRENT BULLETINS Engineering Economics - A commentary on Hawken's "Economy of Purchase": J. H. Lavery. 2 Analysis by Computer - Prestressed concrete sections in flexure : J. L. Meek. 3 Analysis by Computer - Nonbraced multistorey building frames: J. L. Meek and E. W. Karamisheff. 4 Brittle Fracture of Steel - Performance of NDl B and SAA Al structural steels: C O'Connor. 5 Buckling in Steel Structures-1. The use of a characteristic imperfect shape and its application to the buckling of an isolated column : C. O'Connor. 6 Buckling in Steel Structures-2. The use of a characteristic imperfect shape in the design of determinate plane trusses against buckling in their plane: C. O'Connor. Copies of these bulletins are available on application to- Secretary, Department of Civil Engineering, University of Queensland, Brisbane, Q. WAVE GENERATED CURRENTS SOME OBSERVATIONS MADE IN FIXED BED HYDRAULIC MODELS by M.R. GOURLAY, B.Sc., B.E.(Syd), M.E. Lecturer in Civil Engineering This bulletin is a revision of a paper to be published in the Proceedings of the Second Australasian Conference on Hydraulics and Fluid Mechanics at the University of Auckland, New Zealand, 6th to 11th December, 1965. CONTENTS Page ABSTRACT 1 INTRODUCTION 3 TYPES OF WAVE GENERATED CURRENT 3 THE MODEL INVESTIGATIONS 4 MODEL OBSERVATIONS OF CURRENTS 5 PREVIOUS INVESTIGATIONS 16 DISCUSSION OF RESU LTS 17 CONCLUSION 19 ACKNOWLEDGEMENT 19 REFERENCES 20 ABSTRACT Following a brief summary of the different types of currents associated with waves, some observations of wave generated currents made during three fixed bed wave model investigations are described. The data presented show the influence of wave direction, wave period, wave height and changes in breakwater geometry on the form and magnitude of the currents. The trends from these model results are compared with the model and field observations and theoretical analyses of other investigators and their general agreement shown. In par ticular variations of wave height along a shore and the location of breaker zones are found to be important factors in the production of wave currents. 3 INTRODUCTION It is well known that the form of a sedimentary coastline is determined by the wave climate to which it is exposed. Sand is transported along a coast as littoral drift, the direction of transport depending upon the direction of the waves within the breaker zone. Most coastlines are held at various points by fixed rocky headlands which cause modifications to the form of the beach. On a coastline undisturbed by man equilibrium conditions generally exist, with the amount of sand transported into a given zone being balanced by the amount being removed from it. However, this equilibrium may be disturbed when an engineering structure is built on such a coast. Sand is moved along a coast by currents generated by waves. An engineering structure such as a breakwater will produce local modifications to these wave generated currents which will result in accretion or erosion at certain points. It is the purpose of this bulletin to describe some observations of wave generated currents made in three separate fixed bed hydraulic models and the effect of proposed harbour works and other factors on the magnitude and form of the currents. As the measure­ ments were made by rather crude methods, and as the currents were not the primary phenomenon being measured in each case, the results are of qualitative significance only. However, as the author is unaware of any comparable published information of model observations of wave current patterns, it will be of interest to compare them with what is already known from field observations and more idealised model studies. TYPES OF WAVE GENERATED CURRENT There are several forms of current associated with ocean waves which have been described in the literature (l) . Briefly the following types of current may occur on a coast. 1. BEACH DRIFTING Strictly speaking this is not a current, but a mechanism for sediment transport. When waves break at an angle to the shoreline such that the uprush has a longshore component, then sand will undergo a sawtooth motion, being moved up the beach with a longshore component and falling back along the line of greatest slope under the action of gravity during the backwash. This action appears to predominate under the fztiOH of the relatively flat swell waves associated with the so called summer profile an 3) . 2. LONGSHORE CURRENT When waves break at an angle to the shore, the resultant transformation of the oscillatory wave into a translatory wave results in a movement of water parallel to the shore within the breaker zone. This current will result in the transport of the sand put into suspension in the br ker zone, particularly when steep waves break (� t obliquely on an offshore bar . In thl5 case quite a significant current may form within the trough landward of the bar < ) . 4 3. UNDERTOW By this term is meant a seaward return current along the bottom. Its reality has been subject to debate, but it is possible that it may be identified with the seaward bottom return current which is developed during heavy seas with an onshore wind. The latter sets up a landward surface current due to direct shear stress, which tends to pile water up along the shore. A seaward current then develops along the bottom to compensate for this and recent model investigations(5) show that this is a significant factor in beach erosion. 4. RIP CURRENTS The water transported shorewards by the breakers must be balanced by some form of return current and this appears to occur primarily in the form of rip currents rather than undertow. A rip is a narrow seaward current generall� located at a point of low wave height and spaced at intervals along a beach(6, 7, and 9). They generally coincide with breaks in the offshore bar and are fed by inshore feeder currents from either direction parallel to the beach. Thus they may cause local reversals of the longshore current. 5. WAVE CURRENTS OR LATERAL EXPANSION CURRENTS The refraction of waves by offshore bottom topography or their diffraction around islands or headlands may result in local variations in breaker height along a coast. Under these conditions the local mean water level inshore of the breakers will tend to increase at the higher wave height zones due to the transport of increased volumes of water landward. This will result in a lateral expansion current parallel to the shore towards the zones of lower wave height accompanied by a rip current at the latter point to return the water seawards. Indeed it is probable that it is this type of mechanism which causes rip currents(8) in which case the latter may be considered as part of the lateral expansion current system or vice versa. The local reversal of littoral current in the lee of an obstacle is also primarily a lateral expansion current(lO). 6. MASS TRANSPORT CURRENTS It has been shown theoretically(ll) and experimentally(12) that for waves of finite height there is a net landward movement of water particles along the bottom and at the water surface with a compensating seaward movement at mid-depth. These currents are known as mass transport currents and tend to move sand along the bed from offshore towards the breaker zone under swell or offshore wind conditions. However, if the sand is thrown up into suspension by large waves the tendency is to remove it seawards(5 and 13). THE MODEL INVESTIGATIONS The observations of wave generated currents presented in this bulletin were made during three fixed bed model investigations carried out at the University of Queensland. All three models concerned the design of small boat harbours on coast­ lines subjected to heavy wave and swell action. Further, the proposed structures were to be located within the breaker zone in each case. The currents observed included longshore currents, rip currents and lateral expansion currents. 5 Two of the models, Moffat Beach(14) and Mooloolaba (in progress at the time of writing, April 1965) were built for the purpose of designing a harbour for a shore based pilot service for the port of Brisbane as well as providing facilities for the expansion of the fishing industry and general recreational activities. Both models were built to an undistorted scale of 1 in 120. The third model was an exploratory one for determining the best site for a fishing harbour behind the short coral reef on the southern shore of Norfolk Island(15). The scales of the latter model were 1 in 200 horizontally and 1 in 100 vertically and the waves were reproduced to the vertical scale for both height and deepwater wave length. Currents were assumed to follow the normal Froude law relations, i. e. V = L2 for the undistorted models r and V = D21 for the distorted scale model. * r r r In all three situations the harbour sites were exposed and subject to continuous swell action, while storm waves were largely a function of the weather patterns as land masses were too far away to limit the fetch. Consequently, in each case tests were made with several offshore wave heights for a given wave direction and period, the directions being selected having regard to refraction effects and meteorological observations and the periods from limited visual observations (subsequently confirmed for Moffat Beach and Mooloolaba by recordings).
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