Lake waves and gravel beach variation, Loch Lomond Scotland. Keywords: beach variation; gravel beaches; lake beaches; waves; sediment budget; restricted fetch. Beaches respond morphologically to changes in wave conditions, water level and sediment supply. As coastal sediment stores, which reflect the terrestrial hydrological process balance, beaches are very sensitive to environmental change. However, controls on beach variability are not yet fully understood. In comparison to sand or mixed sand/gravel beaches, gravel beach response to environmental changes is muted. Low-energy gravel beach adjustments to external controls remain poorly understood. This research investigates gravel beach variation within a relatively low energy upland lake environment in mid-high latitudes (Loch Lomond). To assess the nature of the lake wave climate, waves were recorded throughout 1994 and statistical and spectral analysis performed. The wave climate is distinctive, characterised by small amplitude, high frequency waves and periods of calm. Water levels fluctuate and showed clear seasonal trends with bi-annual periods of rapid rise/fall. Water levels are a fundamental control on beach variability, both in rates of fluctuation and in that they provide the underlying control on the effects of waves. The research was largely field-based and gravel beach variability was examined with respect to morphological and sedimentological change and sediment budgets for two beaches were calculated. Fluvial sediment delivery was modelled from peak monthly stage. Cliff recession and beach morphology were surveyed to show highly variable longshore beach characteristics which are closely related to beach elevation, exposure and sediment supply. Sections of beach represented by individual profiles type showed profile types may persist from month to month. Sedimentology was examined for the sub-aerial and sub-aqueous beach into the offshore. Cross beach offshore fining was observed, with a clear abrupt limit to coarse sediment in the nearshore. Fluvial discharge exerts a significant control on beach development as it affects sediment entrainment and delivery, distribution and storage within the beach. Water level is also significant in sediment redistribution. The deltas are major sediment stores within the beach sediment budgets often for long time periods (years). At high water levels deltaic sediments are often below wave base and are therefore not entrained and transported. This limits sediment availability for beach morphological readjustment and shore defence. This research is important for the understanding of sediment-poor, low energy beach behaviour and response to changing environmental conditions. The research has implications for modelling lake gravel beach sediment transport and storage mechanisms. It also highlights the need for appropriate management strategies for lake coastal environments.
Abstract The nature and environmental significance of variations in sedimentological parameters of a restricted-fetch lake coastal system (Milarrochy, Loch Lomond) are described in the context of climate change. Grain size characteristics of the beach, streams, cliffs and nearshore/offshore sedimentary environments are distinct, with variability being related to sediment supply, beach altitude and process conditions. Beach grain sizes range from silt to cobble, and are predominantly rounded discs and blades. There is an overall trend of offshore fining with increasing depth, beyond the mixed surf zone and clear limit of coarse sediment (coarser than -1 phi), resulting from dominant shore-normal process trends. Seasonal trends of water level fluctuation, sediment discharge and transfer from river to beach are important controls on local scale variability. The broader significance is that local physiography, sediment characteristics and supply strongly influence beach sedimentology and morphological response, even in the context of larger scale climate change. The record of sedimentological variability detailed here is significant for beach management projects and a better understanding of lake sedimentary facies within the Quaternary.
The variability of two lake gravel beaches (Loch Lomond, Scotland) is described in the context of changing environmental conditions. Waves, water levels and wind orientation and velocity were recorded for 1 year. The wave climate comprises small amplitude, high frequency, short-period waves and periods of calm (mean frequency = 0.92 secs; mean significant wave height = 0.08 m). Incident wind-wave direction was an important influence on beach morphology. Water levels fluctuate and there is a general increase in level. Wave activity combined with high water levels affects the upper beach and backshore although here, the highest waves were recorded at low water levels which minimised wave damage. Cliff erosion rates were recorded. Beach morphological changes showed highly variable profile types which were closely related to beach elevation, exposure and sediment supply. Sections of beach represented by individual profile types showed re-occurring profile types and different sections of beach responded differently to similar conditions. Specific characteristics e.g. narrow beach form, steep foreshores, scarps, beach ridge formation and preservation were noted and a morphological classification for coarse clastic beach profile types was devised. Beach planform is complex and antecedent morphology can persist for some 10 years. Sediment movement is restricted to individual bays which behave as sediment cells. Recommendations for shore management plans which are closely linked to the geomorphological and sedimentological conditions at specific sites within the context of the whole shoreline are given. These have wider application for restricted-fetch coarse clastic beach sites.
