Ew Zealand Oceanog Aphi.C .Institute Memoir No. 15
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ISSN 0083-7903, 15 (Print) ISSN 2538-1016; 15 (Online) • NEW Z 'ALAND , EPARTMENT OF SCIEN'lll1C AND INDUSTRIAL RESEARCH ew Zealand Oceanog aphi.c .Institute Memoir No. 15 Bay-head Sa d Beaches of Ba .. ks Pen·nsula,I New Zealand 1974 NEW ZEALAND DEPARTMENT OF SCIENTIFIC AND INDUSTRIAL RESEARCH New Zealand Oceanographic Institute Memoir No. 15 Bay-head Sand Beaches of Banks Peninsula, New Zealand P.R. DINGWALL W ellingt.011 1974 Price: $2 This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ FOREWORD The area of greatest mobility of Recent marine sediments lies in the beach and wave zones, and the same forces which produce this mobility can make observations and measurements difficult. Until recently there has been little fundamental work on beach processes in New Zealand, though many of our coastal developments are highly susceptible to damage by erosion or accretion of beach sediments. The author presents in this memoir results from a general study of a group of Canterbury beaches. Here the geographic features minimise the number of variables in the hydrological environment and thus allow a more ready consideration of factors in the process of beach development. This manuscript has been technically edited and prepared for publication by Dr D. A. Burns, N.Z. Oceanographic Institute. J. W. BRODIE, Director, New Zealand Oceanographic Institute, Wellington Manuscript received May 1967 Revised manuscript received February 1973 Edited by Q. W. Ruscoe, Information Service, DSIR ISSN 0083-7903 Crown Copyright 1974 © A.. R. SHEARER, GOVERNMENT PRINTER, WELLINGTON, NEW ZEALAND-1974 This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ CONTENTS page Wind-generated currents Abstract • • • • . 5 Littoral currents Tidal currents Introduction and Ackno\vledgments • • 6 Waves • • • • • • 43 Area studied .. • • 7 Winds . • • 46 Banks Peninsula Characteristics of beachsands The bay-head sand beaches Textural characteristics . 47 Geological setting . • • 8 Grain size Rocks of Banks Peninsula Sorting Sediments of the continental shelf Skewness Shelf morphology Kurtosis Previous investigations . • • • • 14 Roundness Purpose of investigation . 14 Mineralogy . 47 Methods of study . • • 14 Carbonate content Grain size Heavy minerals Roundness Magnetite Carbonate content Quartz Mineralogy Classification of beach sands • • 51 Supply of sand fTom source areas 52 Part I BEACH MORPHOLOGY • • • • Transpon of sediments to beaches Classification of beach profiles . 16 • • Petrological origin of sediments . 54 Relationship between beach gradient and grain size .. 19 The role of loe.ss Sorting of beach sands . 19 • • Grey\\-acke-derived sediments Sorting normal to the shore Banks Peninsula Yolcanics Sorting parallel to the shore Origin of beach groups .. 55 Seasonal variation in grain size • • • • Beaches of Group I in Sorting cusps Beaches of Group JI Sand lamination Beaches of Group 111 Disc:ussion Changes in beach morphology • • . 24 Conclusions • • • • • • • • 56 Long-term changes .. • • .. 24 References . Short-term changes .. • • • • 26 • • • • . • • • • 57 Seasonal changes Sequence of changes in successive profiles Changes associated with storms Appendices . Changes related to wa\e approach • • . 58 IA Formulae and verbal scales for sedimentary grain Changes associated with tidal regimes size parameters Changes during tidal cycles lB Grain size distribution in each beach sample, Discussion Summer 1966 Mobility of beach material . 38 • • IC Grain size distribution in samples from mid-tide Variables affecting mobility and gradient zone, May 1966 Part II CHARACTERISTICS, TRANSPORT, AND ID Grain size distribution in samples from mid-tide ORIGIN OF BEACH SEDii\JENTS zone, Winter 1966 Grain size distribution in san1ples from cusps Mechanisms of transport .. 43 IE II Scale of roundness (Powers 1953) Currents • • 43 Coastal currents Ill Wave observations TABLES page I Properties of bays and beaches .. • • 9 2 Control grid for measurement of sand levels . 15 7 Sorting of beach sediments parallel to shore . 23 3 Classification of beaches according to profiles 16 8 Seasonal change in grain size . 23 4 Mean grain size and sorting of upper and lower 9 Mean grain size and sorting in cusp horns and cusp foreshore sands . 22 bays . 23 5 Mean grain size and sorting of backshore and 10 Mean grain size and sorting of laminated beach foreshore sands . 22 sand at Tumbledo\vn Bay . 24 6 Mean grain size, sorting and skewness of beach 11 Seasonal changes in upper and lower foreshore and dune sands .. • • . 23 slopes 29 This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ FIGURES page 24 Monthly variation in berm widths .. 34 6 1 Location of bay-head sand beaches .. • • 25 Effects of storm waves on beach profiles at Taylors 2 General views of beaches investigated .. • • 10 Mistake and Hickory Bay . 35 3 Submarine relief and sediments offshore from 26 Short-term lateral shifts of sand at Goughs and Banks Peninsula . .. • • 12 Tumbledown Bays . .. .. 36 4 A Selected continental shelf profiles .. 13 27 Neap-tide berm at Le Bons Bay . 37 B Longitudinal bay profiles 28 Neap-tide berm at Goughs Bay . .. 37 C Transverse bay profiles 29 Changes in beach profiles during tidal regimes 38 5 Beach profile at Taylors Mistake . 17 30 A Changes in sand levels during single tidal 6 Beach profile at Goughs Bay .. 17 cycles at Okains Bay . .. 39 7 Beach profileat Okains Bay . 18 B Changes in sand levels during single tidal 8 Beach profile at Raupo Bay . 18 cycles at Goughs Bay 9 Ridge and runnel at Le Bons Bay exposed during 31 Sweep zone profiles . .. 41 low tide . 20 32 Comparison of sweep zones . 42 10 Spilling breakers marking the position of the ridge 33 (A) Observed wave periods and (B) direction of at Le Bons Bay during high tide . 20 approach of swell; annual wind distribution at 11 Beach profile at Peraki Bay . 21 (C) Harewood and (D) Akaroa Lighthouse 44 12 Beach profile at Flea Bay . 21 34 Refraction pattern for south-easterly swell . 45 13 Relationship between foreshore slope and grain 35 Refraction pattern for north-easterly swell . 46 size at mid-tide zone . 22 36 Grain size parameters for sand from the mid-tide 14 Lamination of beach sand at Ikoraki Bay 25 zone of each beach 48 15 Cusp development at lkoraki Bay .. 25 37 Cumulative curves of distribution for samples from 16 Accretionary series of beach ridges at Okains Bay .. 27 the mid-tide zone . 49 17 Seasonal changes in beach profiles . 28 38 Comparison of grain size parameters of backshore, 18 Summer beach profile at Goughs Bay .. 29 foreshore, and nearshore bottom sands . 50 19 Winter beach profile at Goughs Bay .. 30 39 Relationship between mean grain size and sorting 20 Vertical scarp at Le Boos Bay . 31 at the mid-tide zone . 51 21 Beach foundation exposed by winter wave action 40 Relationship between mean grain size and skewness at Peraki Bay . 31 at the mid-tide zone . 52 22 Summer beach profile at Jkoraki Bay .. 32 41 Distribution of heavy minerals, magnetite, and 23 Winter beach profile at lkoraki Bay .. • • 32 quartz in mid-tide beach samples . 53 4 This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ BY NC ND Bay-head Sand Beaches of Banks Peninsula, New Zealand By P. R. DING\\' AI,I,* Department of Geograph)l, Canterbury University New Zealand Oceanographic Institute Nlemoir �o. 15. 63 pp. 1974. ISSN 0083-7903 • Abstract Analysis of the physical and mineralogical attributes of sands on 13 bay-head sand beaches of Banks Peninsula indicates that the beaches consist of medium sand derived from the bedrock of the pen insula and fine sand from greywacke-derived sediments mantling the adjacent continental shelf, and a contribution from the cover of loess on the peninsula. This indication is strongly supported by an evaluation of the capability of waves and currents in the vicinity of the peninsula to transport material. The beaches are arranged into three categories according to the origin of their sands: (l) those consisting predominantly of medium-grained heavy mineral sands derived largely from weathering of the peninsula's basic igneous rocks; (2) those consisting predominantly of fine, or very fine, light mineral sands derived from offshore sediments by constructive wave action; (3) those whose sands contain significant contributions from both the peninsula's bedrock and the detrital material on the shelf. On individual beaches the relative abundance of material derived from each of the two primary sources depends on the degree of exposure of the beach to wave action, which controls the strength of water movements in the bay head. The results of beach surveying and sediments sampling indicate that the interplay between the beach deposit and hydrological forces in the bay head is analogous to that on exposed coastlines. Variation in sediment characters show correlations with differences in beach morphology. Gently sloping, smooth profiles on fine sands contrast with conspicuous berms and moderately steep beach faces on medium-grained sands. Sediment properties vary slightly, both across and along the beach as a result of selective sorting by wave and tidal action. The beaches change in response to differing wave and tide regimes. For example, large winter waves smooth out summer beach profiles by cutting back berms and redepositing the sediment on the lower foreshore. Similarly spring tides destroy small neap tide berms. Measurements of sand level changes during single tidal cycles show that beach material is being continually resorted.