Bottom dynamics in lakes

Abstract

A proper understanding of the bottom dynamic conditions (erosion, transportation, accumulation) in lakes is essential in most sedimentological contexts. Fine cohesive materials generally dominate the open water areas, whereas coarser deposits (sand, gravel) dominate shallow regions where erosion and transportation of fine materials prevail. At present, there is no physical model available which describes the linkage between the energy content of the water-mass and the capacity for sediment entrainment in open water areas. Water-mass energy depends on, e.g. wind direction, duration, velocity, fetch, and the presence of a thermocline. Entrainment depends on, e.g. density, compaction, water and organic content of the sediments and the number and type of bottom fauna.

Four different methods are used to determine bottom dynamics, two aresite typical and two arelake typical.Site andlake typical methods each include one method based on collected field data and one based on theoretical data. One method, the cone apparatus, is presented for the first time. It consists of two cones, one of which has a narrow angle and the other a wide angle, which are zero adjusted at the sediment surface before being released to penetrate the sediments. The differential cone penetration, refered to as the penetration ratio, is used to indicate the degree of surficial sediment compaction. This simple, inexpensive instrument provides quantitative data on physical sediment characteristics which may be related to bottom dynamic conditions.