Classification of Gravel Component Lake Beaches in the South Island, New Zealand

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 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.