Classification of Gravel Component Lake Beaches in the South Island
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Classification of Gravel Component Lake Beaches in the South Island, New Zealand By Margaret Winter A thesis submitted to the University of Otago in fulfilment of the requirements of a Masters of Science degree in Geography. University of Otago 2012 ‘There are only two types of waves you need to know about in life, those that’ll kill you and those that won’t.’ G.L. Winter i Abstract Lake beaches with a gravel component (sediments > 2 mm in b-axis diameter) have received very little attention in the coastal geomorphology literature. This research is based on 19 lakes in the South Island of New Zealand that were surveyed and sampled in summer, 2010 to 2011, and winter, 2011. Methods included lakeshore surveys, sediment sieving, and lake wave hindcasting using the ACES model. Results show that three types of lake beach were present. The beach types were pure gravel beaches (all sediments > 2 mm diameter), mixed sand and gravel (MSG) beaches (mixed sediments down the profile), and composite gravel beaches (pure gravel upper beach with an often shallower sand/silt lower profile). MSG beaches sometimes lacked sand in the upper shore and changed to a more mixed sand and gravel sediment in the lower shore. Two of the seven composite beaches studied displayed mixed sediment in the upper shore instead of pure gravel. Linear discriminant analysis returned a good discrimination rate between lake beach types (66.7 % of beach correctly predicted), though only sorting was a useful discriminator (average grain size, sorting, beach width, storm berm elevation, number of berms, and Iribarren number were tested). This compared poorly with a previous classification scheme developed for oceanic gravel component beaches. Lacustrine gravel component beaches were generally narrower (pure gravel: 0.41 – 18.67 m, MSG: 1.09 – 31.02 m, and composite: 0.62 – 94.66 m, though most were between 1- 5 m wide) and steeper than the oceanic beaches. No relationship could be found between average grain size and beach slope for any lake beach types. The lake beaches did not demonstrate any cuspate morphology or beach steps, however armouring was observed in some cases at the shoreline, regardless of lake beach type. Other results from this study found that the number of berms on the profiles were significantly different between summer and winter (p value = 0.008). When the lakes used for hydroelectric power generation were removed however, this variable no longer showed any difference, suggesting the influence of lake level ii changes of the lake beaches. None of the other measured profile features were significantly different between summer and winter. Hindcasted wave data for the lake beaches found that two thirds received Hmo > 0.5 m, and Tp > 3 s. This suggests the limited wave energy available on lakes to rework sediments, though measured wave data is required to validate these findings. Finally, a morphodynamic model is presented for lacustrine beaches with a gravel component, which is placed within the wider conceptual framework of two previously well accepted morphodynamic models. iii Acknowledgements I’d firstly like to thank my supervisor, Dr. Wayne Stephenson, for all the help and guidance he has provided me, and for the useful feedback he gave during my write up. Thanks to Nigel and Dave for arranging my field equipment, and putting up with the daily roar of the sieve shakers when I was in the lab. I would also like to acknowledge the Ngāi Tahu Kā Pūtea Grant and the Geography Department Hugh Kidd Field Research Grant for providing this project with some financial support. Thanks to Oamaru New World for my car. Secondly, I want to very gratefully thank my field assistants (Sam, Rachel, Clare and my Mum) for spending many boring hours surveying the lake shores, and for accompanying me on the long drives between field sites. Special thanks to my Mum and family for being so wonderful during the crash. To my Aunty Lynn, and Phyllis and Richard Bruce, I greatly appreciate you allowing me to stay with you during my field work. These recuperation times were crucial for maintaining my sanity. I’d like to also thank the various people and cats who distracted me from my degree throughout these past three years, making my quality of life significantly better because of it. In particular, my friends Tabi, Marie and Blake were great for cheering me up with random conversations and baking. Thanks to my office friends from 5.S.6. and 5.S.4, the members of the Kaikorai Metropolitan Brass Band, and the Antiques Roadshow for being generally entertaining. Thanks to my family for being nice. Finally, there are two people who need extra special acknowledgement. My Mum, who was an amazing University role model for me when I was younger, and who has always provided me with unwavering support in everything I do. Thanks too for sometimes reminding me of Dad. And my wonderful partner, Sam, who not only spent six weeks on the road with me during my first field trip, but also proof-read the entire first draft of my thesis. I can’t tell you how grateful I am for all your help, encouragement, and wonderful sense of humour. You have been my rock. Thank- you both so much, I couldn’t have done all of this without either of you. For my Mum and Dad. iv Table of Contents Abstract ........................................................................................................... ii Acknowledgements ........................................................................................ iv Table of Contents ............................................................................................ v List of Figures ................................................................................................ ix List of Tables ............................................................................................... xiii Chapter 1 Introduction .......................................................................................... 1 1.1 Research objectives ............................................................................... 5 1.2 Thesis outline ......................................................................................... 6 Chapter 2 Classification and Morphodynamics of Gravel Component Beaches .................................................................................................. 7 2.1 Terminology .......................................................................................... 8 2.2 Past classifications of beach type .......................................................... 9 2.2.1 Lake beach classifications.................................................... 24 2.3 Morphodynamics of gravel component beaches ................................. 28 2.3.1 Material factors .................................................................... 29 Grain size ...................................................................... 29 Sorting .......................................................................... 33 2.3.2 Morphology.......................................................................... 37 Beach slope................................................................... 38 Beach width .................................................................. 42 Number and elevation of berms ................................... 44 The beach step .............................................................. 48 2.3.3 Lacustrine Beaches .............................................................. 49 2.4 Research objectives ............................................................................. 51 2.5 Conclusion ........................................................................................... 53 Chapter 3 Methods ............................................................................................... 55 v 3.1 Site selection ........................................................................................ 55 3.2 Field methods ....................................................................................... 60 3.3 Variable calculations ........................................................................... 66 3.3.1 Beach width ......................................................................... 66 3.3.2 Number of berms and storm berm height ............................ 67 3.3.3 Grain size ............................................................................. 67 Sieving methods ........................................................... 68 Large clast measurement .............................................. 68 Summer clast measurement ................................... 69 Winter clast measurement ...................................... 73 3.3.4 Sorting .................................................................................. 74 3.3.5 Iribarren number .................................................................. 74 Beach Slope .................................................................. 75 Wave data: ACES ......................................................... 76 Observed windspeed..................................................... 77 Wind direction .............................................................. 79 Elevation, latitude, duration of the observed wind and wind observation type .................................................. 79 Air-sea temperature difference ..................................... 80 Duration of the final wind ............................................ 80 Radial inputs ................................................................