Wave Generated Currents

Total Page:16

File Type:pdf, Size:1020Kb

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).
Recommended publications
  • Rip Current Observations Via Marine Radar
    Rip Current Observations via Marine Radar Merrick C. Haller, M.ASCE1; David Honegger2; and Patricio A. Catalan3 Abstract: New remote sensing observations that demonstrate the presence of rip current plumes in X-band radar images are presented. The observations collected on the Outer Banks (Duck, North Carolina) show a regular sequence of low-tide, low-energy, morphologically driven rip currents over a 10-day period. The remote sensing data were corroborated by in situ current measurements that showed depth-averaged rip current velocities were 20e40 cm=s whereas significant wave heights were Hs 5 0:5e1 m. Somewhat surprisingly, these low-energy rips have a surface signature that sometimes extends several surf zone widths from shore and persists for periods of several hours, which is in contrast with recent rip current observations obtained with Lagrangian drifters. These remote sensing observations provide a more synoptic picture of the rip current flow field and allow the identification of several rip events that were not captured by the in situ sensors and times of alongshore deflection of the rip flow outside the surf zone. These data also contain a rip outbreak event where four separate rips were imaged over a 1-km stretch of coast. For potential comparisons of the rip current signature across other radar platforms, an example of a simply calibrated radar image is also given. Finally, in situ observations of the vertical structure of the rip current flow are given, and a threshold offshore wind stress (.0:02 m=s2)isfoundto preclude the rip current imaging. DOI: 10.1061/(ASCE)WW.1943-5460.0000229.
    [Show full text]
  • Assessing Long-Term Changes in the Beach Width of Reef Islands Based on Temporally Fragmented Remote Sensing Data
    Remote Sens. 2014, 6, 6961-6987; doi:10.3390/rs6086961 OPEN ACCESS remote sensing ISSN 2072-4292 www.mdpi.com/journal/remotesensing Article Assessing Long-Term Changes in the Beach Width of Reef Islands Based on Temporally Fragmented Remote Sensing Data Thomas Mann 1,* and Hildegard Westphal 1,2 1 Leibniz Center for Tropical Marine Ecology, Fahrenheitstrasse 6, D-28359 Bremen, Germany; E-Mail: [email protected] 2 Department of Geosciences, University of Bremen, D-28359 Bremen, Germany * Author to whom correspondence should be addressed; E-Mail: [email protected]; Tel.: +49-421-2380-0132; Fax: +49-421-2380-030. Received: 30 May 2014; in revised form: 7 July 2014 / Accepted: 18 July 2014 / Published: 25 July 2014 Abstract: Atoll islands are subject to a variety of processes that influence their geomorphological development. Analysis of historical shoreline changes using remotely sensed images has become an efficient approach to both quantify past changes and estimate future island response. However, the detection of long-term changes in beach width is challenging mainly for two reasons: first, data availability is limited for many remote Pacific islands. Second, beach environments are highly dynamic and strongly influenced by seasonal or episodic shoreline oscillations. Consequently, remote-sensing studies on beach morphodynamics of atoll islands deal with dynamic features covered by a low sampling frequency. Here we present a study of beach dynamics for nine islands on Takú Atoll, Papua New Guinea, over a seven-decade period. A considerable chronological gap between aerial photographs and satellite images was addressed by applying a new method that reweighted positions of the beach limit by identifying “outlier” shoreline positions.
    [Show full text]
  • What Are Rip Currents? Types of Rips Are Rips Dangerous
    What are Rip Currents? Rips are strong, narrow currents that flow from the shoreline seaward past the breaking waves. They exist as a way of getting water carried to the beach by breaking waves back out to sea and are usually confined to deeper channels between shallow sand bars. The bigger the waves, the stronger the rip. Think of them as "rivers of the sea". Types of Rips • Low energy rips are the most common and occur when waves are smaller or haven't changed in a while. They are usually fixed in place and sit in channels between sand bars and don't move much. • High energy or flash rips are bigger and occur when waves have increased suddenly, or during a storm. They tend to move around a bit and flow faster. • Headland and fixed rips are often permanent and occur next to headlands and structures such as groynes and jetties. Low Energy Rip High Energy Rip Are Rips Dangerous? Rips are only dangerous if you don't understand what they are and you are not a good swimmer. They can carry you more than 100 m offshore in less than a minute and are the major cause of surf drownings and rescues in Australia. However, surfers use rips to their advantage to help them swim out through the surf quickly. Page1of2 © 2007 Dr R Brander continued Spotting a Rip Since rips often sit in deeper channels between shallow sand bars, always spend 5-10 minutes looking at the surf zone for consistent darker and "calmer" areas of water that extend offshore between the breaking waves.
    [Show full text]
  • Identification of Rip Current Hazards Using Fluorescent Dye and Unmanned Aerial Vehicle (A Case Study of Drini Beach, Gunungkidul, Indonesia)
    https://doi.org/10.5194/nhess-2021-221 Preprint. Discussion started: 23 August 2021 c Author(s) 2021. CC BY 4.0 License. Identification of Rip Current Hazards Using Fluorescent Dye And Unmanned Aerial Vehicle (A Case Study Of Drini Beach, Gunungkidul, Indonesia) 5 1Hendy Fatchurohman1, Alfiatun Nur Khasanah1, Ahmad Cahyadi2 1Department of Earth Science, Vocational College, Universitas Gadjah Mada. Sekip Utara Jalan Kaliurang, Bulaksumur, Yogyakarta, Indonesia, 55281 2Department of Environmental Geography, Faculty of Geography, Universitas Gadjah Mada. Sekip Utara Jalan Kaliurang, 10 Bulaksumur, Yogyakarta, Indonesia, 55281 Correspondence to: Hendy Fatchurohman ([email protected]) Abstract. Coastal tourism is a leading sector substantially contributing to the regional income of Gunungkidul Regency, 15 Indonesia. However, with more tourists visiting the beach, more lives are threatened by coastal hazards. Rip currents are a channel of powerful, narrow, fast-moving water that can carry floating objects away from the shore, presenting one of the most common coastal hazards to swimmers. Unfortunately, most tourists are unaware of rip currents and their threats and how to avoid them. This study was designed to identify the types and dimensions of rip current in one of the regency’s tourist attractions, Drini Beach. For this purpose, an environmentally friendly fluorescent dye, Uranine, was injected from the 20 shoreline, then the velocity and direction of its movements were observed from aerial video footage captured with a drone. Results showed stationary rip currents with a narrow channel, called a channel rip, with the mean dimensions: 250 m from the shoreline to the head and 10.25 m in width.
    [Show full text]
  • Rip Current Generation and Management Implications in South Florida
    MOJ Ecology & Environmental Sciences Research Article Open Access Rip current generation and management implications in south Florida Abstract Volume 3 Issue 3 - 2018 Miami Beach is one of the most dangerous beaches for rip current drownings in the Stephen B Leatherman United States. There are three principal wave generators that result in rip currents: Department of Earth & Environment, Florida International onshore winds associated with high pressure cells, swell waves produced by offshore University, USA nor’easters, and tropical storms. A logistic regression analysis showed a correlation between rip currents, wave height and month of the year even under low wave energy Correspondence: Stephen B Leatherman, Department of conditions at Miami area beaches. Most rips were found to occur during 15–20 knot Earth & Environment, Florida International University, 11200 onshore winds with 0.6–0.9m significant wave height. Eleven social, physical and SW 8th Street, Florida Miami, Florida 33199, USA, Tel 1305- safety factors make Miami Beach a rip current hotspot and pose a major coastal 3482-00, Email [email protected] management challenge. Received: November 16, 2017 | Published: May 31, 2018 Keywords: rip current forecasting, rip current drowning, rip current hot spots, safety factors, beaches Introduction generated by 0.6 to 1.2 m waves during fair–weather conditions, which seem like perfect beach days, making these rips deceptively dangerous South Florida is a tourism destination that is well known for its to bathers. Another type of rip generator is nor’easters offshore the beaches. Miami Beach is one of the ten most famous beaches in the mid–Atlantic and Northeast coast, which produce large swells (Figure 1 world and boasts of tens of millions of visitors each year.
    [Show full text]
  • Reef Rip Current Generated by Tide and Wave During Summer Season: Field Observation Conducted in Yoshiwara Coast, Ishigakijima, Okinawa, Japan
    Proceedings of the 11th International Coral Reef Symposium, Ft. Lauderdale, Florida, 7-11 July 2008 Session number 15 Reef Rip Current Generated by Tide and Wave during Summer Season: Field Observation Conducted in Yoshiwara Coast, Ishigakijima, Okinawa, Japan M.P. de Leon1, R. Nishi2, F. Kumasaka3, T. Takaesu4, R. Kitamura5, A. Otani6 1) Graduate School of Science and Engineering, Kagoshima University, 1-21-40 Korimoto, Kagoshima, Japan 2) Faculty of Fisheries, Kagoshima University, 4-50-20 Shimoarata, Kagoshima, Japan 3) Japan Hydrographic Association, Tsukiji, Tokyo, Japan 4) Marine Information Science, 11th Japan Coast Guard, Naha, Okinawa, Japan 5) Department of Ocean Civil Engineering, Kagoshima University, 1-21-40 Korimoto, Kagoshima, Japan 6) IO Technic, 2-28-19 Minami, Tsukushino, Machida, Tokyo, Japan Abstract. In 2004 and 2005, five drowning accidents in Ishigakijima, Okinawa were documented by Japan Coast Guard and were found to be caused by a strong offshore current commonly known as “reef current”. This type of current has been identified to be responsible in generating the circulation of water and transportation of sediment in coral reefs. In this paper, however, high-speed offshore current in coral reefs is specifically referred to as “reef rip current”. Considering that the generation mechanism of reef rip current is not revealed yet, the researchers conducted a hydrodynamic study in Yoshiwara Coast, Ishigakijima, Okinawa in two phases (normal and storm conditions) during summer season to determine the location and time of occurrence of the strong offshore current that will significantly address safe utilization of coral reef and carbonate beach thereby reducing drowning accidents.
    [Show full text]
  • Waves and Wind Shape Land
    KEY CONCEPT Waves and wind shape land. BEFORE, you learned NOW, you will learn • Stream systems shape Earth’s • How waves and currents surface shape shorelines •Groundwater creates caverns •How wind shapes land and sinkholes VOCABULARY THINK ABOUT longshore drift p. 587 How did these longshore current p. 587 sandbar p. 588 pillars of rock form? barrier island p. 588 The rock formations in this dune p. 589 photograph stand along the loess p. 590 shoreline near the small town of Port Campbell, Australia. What natural force created these isolated stone pillars? What evidence of this force can you see in the photograph? Waves and currents shape shorelines. NOTE-TAKING STRATEGY The stone pillars, or sea stacks, in the photograph above are a major Remember to organize tourist attraction in Port Campbell National Park. They were formed your notes in a chart or web as you read. by the movement of water. The constant action of waves breaking against the cliffs slowly wore them away, leaving behind pillarlike formations. Waves continue to wear down the pillars and cliffs at the rate of about two centimeters (one inch) a year. In the years to come, the waves will likely wear away the stone pillars completely. The force of waves, powered by wind, can wear away rock and move thousands of tons of sand on beaches. The force of wind itself can change the look of the land. Moving air can pick up sand particles and move them around to build up dunes. Wind can also carry huge amounts of fine sediment thousands of kilometers.
    [Show full text]
  • Earth's Atmosphere
    Earth & Space Science Packet April 20th- May 1st, 2020 Griffin & King This packet contains: 1. Worksheets for Ocean Floor, Ocean Water and Ocean Life, Earth’s Atmosphere 2. All About Waves Article 3. Atmospheric Density Lab What needs to be turned in for a GRADE? ● Worksheets with questions answered (Ocean Floor and Ocean Water/Life is April 20-24; Atmosphere is April 27-May 1) Optional/Enrichment included: ● Labs ● Science Articles If these are completed, we would LOVE for you to share: Send us pictures on remind/email, or tag @TheBurgScience and #Team DCS on Twitter EARTH & SPACE SCIENCE GRIFFIN & KING THE OCEAN FLOOR ___ THE VAST WORLD OCEAN ● Nearly 70 percent of Earth’s surface is covered by the global ocean. ​ ​ ● Oceanography is a science that draws on the methods and knowledge of geology, ​ chemistry, physics, and biology to study all aspects of the world ocean. ● The world ocean can be divided into four main ocean basins- the Pacific Ocean, the ​ ​ ​ ​ ​ ​ Atlantic Ocean, the Indian Ocean, and the Arctic Ocean. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ● The topography of the ocean floor is as diverse as that of continents. ​ ​ ● Bathymetry (bathos=depth, metry=measurement) is the measurement of ocean depths ​ ​ ​ ​ ​ and the charting of the shape or topography of the ocean floor. ● Today’s technology- particular sonar, satellites, and submersibles- allows scientists to ​ ​ study the ocean floor in a more efficient and precise manner than ever before. ● Sonar is an acronym for sound navigation and ranging. Sonar calculates ocean depth by ​ ​ ​ ​ ​ ​ ​ ​ ​ recording the time it takes sound waves to reach the ocean floor and return.
    [Show full text]
  • Aucelli Et Al
    Rip current evidence by hydrodynamic simulations, bathymetric surveys and UAV observation Guido Benassai1, Pietro Aucelli2, Giorgio Budillon2, Massimo De Stefano2, Diana Di Luccio2, Gianluigi Di Paola2, Raffaele Montella2, Luigi Mucerino3, Mario Sica4, and Micla Pennetta5 1Department of Engineering, University of Naples Parthenope 2Department of Science and Technology, University of Naples Parthenope 3Department of Earth, Environment and Life Sciences, University of Genova 4Autorità di Bacino Campania Centrale 5Department of Earth, Environment and Resources Sciences, University of Naples Federico II Correspondence to: Guido Benassai ([email protected]) Abstract. The prediction of the formation, spacing and location of rip currents is a scientific challenge that can be achieved by means of different complementary methods. In this paper the analysis of numerical and experimental data, including RPAS (Remotely Piloted Aircraft Systems) observations, allowed to detect the presence of rip currents and rip channels at the mouth of Sele river, in the Gulf of Salerno, southern Italy. The dataset used to analyze these phenomena consisted of two different 5 bathymetric surveys, a detailed sediment analysis and a set of high-resolution wave numerical simulations, completed with Google EarthTM images and RPAS observations. The grain size trend analysis and the numerical simulations allowed to identify the rip current occurrence, forced by topographically constrained channels incised on the seabed, which were compared with observations. Keywords: rip-currents, hydrodynamic simulations, transport vectors, UAV observation. 10 1 Introduction The monitoring and the forecasting of beach processes become particularly critical along the unstable coastal areas, possibly subjected to coastal vulnerability (Di Paola et al. (2014)), and coastal risk due to sea level rise (Benassai et al.
    [Show full text]
  • 174 Coastal Processes and Hazards I. WATER, WAVES and COASTAL
    Coastal Processes and Hazards I. WATER, WAVES AND COASTAL DYNAMICS A. Beach and coastal areas represent sites of dynamic sedimentation, erosion and re- working of river-borne sediments as they reach coastal regions 1. Dynamic interaction between wind, waves, sedimentation and erosion 2. Coastal areas noted for extremely variable meteorologic conditions 3. Hurricanes and storms profoundly influence coastal morphology a. Wave energy and wind energy during coastal storms exhibit great capability to do work in the form of erosion and transportation of sediment (1) Eg. storm wash-over processes (2) storm surges (3) changes in coastal morphology via erosion and sedimentation 4. Gross coastal configuration primarily a function of plate tectonic history, however relentless wave processes modify coast through erosion and sedimentation B. The Ocean and Wave Activity 1. Beach/coastal sand/sediment profoundly influenced by wave action a. waves provide motion/energy for transportation and erosion of sediment 2. Waves generated by wind shear blowing across ocean surface for long distances a. shear creates orbital rotation of water to form water waves (1) orbital motion = circular to and fro motion, diminishing in intensity at depth from surface (2) Although the wave is passed through the water as a medium; actual water molecules are NOT displaced as the wave is propagated b. Wave form controlled by: (1) wind velocity (2) duration of wind (3) fetch- distance over which wind blows (length across water) (> fetch, > wave amplitude) 3. Wave Morphology a. Wave crest: high upper peak of wave train b. Wave trough: low separating two crests c. Wave Height: vertical distance between crest and trough (1) Avg.
    [Show full text]
  • Narrowneck Artificial Reef: Construction
    Narrowneck Artificial Reef: Construction Our coastline is a dynamic system. Sand travels north via a process called longshore drift. At the same time, there is a cycle of erosion and accretion taking place, driven by wave action. Narrowneck was identified as requiring coastal management works in the mid-1990s in order to retain a more permanent supply of sand on the visible portion of the beach to satisfy tourist and local demand, and to stabilise the coastline. To achieve this objective, the Northern Gold Coast Beach Protection Strategy was established in 1999. Its objectives were to: ♦ Widen the beach and dunes along the Surfers Paradise Esplanade to increase the volume of sand within the storm buffer and provide more open space. The Reef was a key feature of the Northern Gold Coast Beach ♦ Protection Strategy, delivered by a partnership between Gold Coast To improve surf quality and recreational City Council and Griffith University Centre for Coastal Management. amenity at Narrowneck by the construction of Its design was informed by extensive numerical and physical a submerged reef to stabilise the nourished modelling. (Source: GCCC 2011) beaches. Key concepts Longshore drift is the movement of sand and other material by longshore currents in a direction parallel to the beach along the shore. Erosion is the periodic movement of sediments from the visible portion of a beach (during rough weather) to the submerged nearshore region. Sand will subside in one of these nearshore bars or compartments for a time before returning to the visible portion of the Aerial view of Narrowneck Artificial Reef (Source: Skyepics 2011) beach (during calmer periods) through the process of accretion.
    [Show full text]
  • Tides Longshore Drift
    9613 - SC&H KS2 FactSheet - TIDES&WAVES-vB01 03/04/2012 14:12 Page 2 Suffolk Coast and Heaths www.suffolkcoastandheaths.org Coastal Knowledge Longshore drift Waves also transport sand and shingle along the shore, a process known as longshore drift. Waves often hit the shore at & an angle, depending on the direction of the wind. Longshore Tides waves drift has created features like Orford Ness and Landguard Point. Sometimes man tries to slow down longshore drift by building defences called groynes. “Hi I’m Marvin Moon. Did you know I act like a big magnet? Using the “Longshore drift moves sand sideways force of gravity, I pull the oceans towards me as the along the beach in a zig-zag pattern – this can earth spins. You see this as movement of water up sometimes cause sand to pile up on one side of and down a beach or in and out of an estuary. the beach! Look out for this next time These are called tides.” you go to the beach”. Things to do: See how currents Things to do: move sand Tides How tides are made Using the same You need 7 people and 7 When you’ve been at the beach, have you noticed tray and water add a balloons. 1 green balloon is the 1 that the water level changes? drop of ink or paint and earth, 1 white balloon is the blow and watch how the Sometimes it changes so much you may need to move moon, 1 yellow balloon is the ink moves. your deck chairs further up the beach as the tide sun and 4 blue balloons are the See how groynes work comes in! This is all because of the invisible attraction sea.
    [Show full text]