Surface sediments, cores and seismic reflection profiling delineate sedimentary environments and processes of sedimentation in Lake Tekapo. Sedimentation is dominated by the Godley River which forms an extensive delta in the northern third of the lake. Delta growth accounts for 55% of annual sediment deposition. In winter sandy muds are deposited at the top of the delta slope, where they may move under gravity as a surficial slide. Oversteepening of the upper slope also generates deep seated failures. The entire 20 km2 of delta slope is subjected to rotational slumping which episodically reworks large volumes of sediment. Down the delta slope sedimentation rates decrease, surface sediments get finer and varves become better developed.
In the lake basin sediments are parallel bedded varves, which contain typical winter‐summer annual cycles as well as minor, non‐annual flood varves. Annual varve thickness and semi‐annual varve frequency are determined by variations in the discharge of the Godley River. Sedimentation in the basin accounts for 40% of the budget and sedimentation rates decrease with distance from the delta, except at the distal end of the basin, where turbid underflows are stopped by the rising lake floor. Beyond the basin, sedimentation rates decrease abruptly. Coriolis deflection of inflowing river water increases sedimentation rates down the eastern shore. The remaining 5% of the sediment is deposited on the lateral slopes and shoulders where sediments form a thin muddy veneer over basement, which occasionally slumps to the basin floor.
Inflow into Lake Tekapo is dominated by the glacially-fed headwater Godley River. Measurements of water temperature and transmissivity at periods of seasonal maxima and minima provide data on dispersion of river water entering the lake. During spring, lake waters warming from isothermal winter conditions receive turbid cold meltwater which interflows or underflows downslope to the deepest basin to pond against the rising lake floor. Waters stratify weakly in summer, and turbid inflowing water interflows. In winter, near isothermal lake water receives cold clear water underflowing to the deepest basin. In all seasons inflowing water is deflected towards the eastern side of the lake by Coriolis force. Diurnal changes in inflow across the Godley delta in spring are complex, with interflow and overflow influenced by heating of waters flowing over wide, braided river channels. In winter, underflows are strongest in early morning when inflows are coldest, and they weaken through the day as river waters warm.
The traditional practice to assess accuracy in lidar data involves calculating RMSEz (root mean square error of the vertical component). Accuracy assessment of lidar point clouds in full 3D (three dimension) is not routinely performed. The main challenge in assessing accuracy in full 3D is how to identify a conjugate point of a ground-surveyed checkpoint in the lidar point cloud with the smallest possible uncertainty value. Relatively coarse point-spacing in airborne lidar data makes it challenging to determine a conjugate point accurately. As a result, a substantial unwanted error is added to the inherent positional uncertainty of the lidar data. Unless we keep this additional error small enough, the 3D accuracy assessment result will not properly represent the inherent uncertainty. We call this added error “external uncertainty,” which is associated with conjugate point identification. This research developed a general external uncertainty model using three-plane intersections and accounts for several factors (sensor precision, feature dimension, and point density). This method can be used for lidar point cloud data from a wide range of sensor qualities, point densities, and sizes of the features of interest. The external uncertainty model was derived as a semi-analytical function that takes the number of points on a plane as an input. It is a normalized general function that can be scaled by smooth surface precision (SSP) of a lidar system. This general uncertainty model provides a quantitative guideline on the required conditions for the conjugate point based on the geometric features. Applications of the external uncertainty model were demonstrated using various lidar point cloud data from the U.S. Geological Survey (USGS) 3D Elevation Program (3DEP) library to determine the valid conditions for a conjugate point from three-plane modeling.
Lake Wakatipu occupies an overdeepened glacial trough dammed by former terminal moraine. The steep subaerial slopes above the lake continue steeply below the surface of the water to a plane floor that is horizontal in the central and most of the southern sections and slopes longitudinally in the northern section. Below the general level of the sediment surface, a system of current channels has been developed, extending from the delta of the Dart and Rees Rivers to the flatfloored sedimentary basin in the central and southern section. Active deposition of sediment from density currents is taking place; the action of such currents is discussed.
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