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USGS1981

Large-scale bedforms in the Platte River downstream from Grand Island, Nebraska; structure, process, and relationship to channel narrowing

The Platte River channel in Nebraska, especially downstream from Grand Island, is characterized by large, periodic, and geometrically distinct bedforms called macroforms. Macroforms have dimensions commensurate with the width and depth of the channel and are emergent at all but the highest flow stages. The encroachment of vegetation on macroforms and their consequent stabilization since the large-scale development of irrigation in the Platte River basin is the major cause of the reduction in channel width upstream from Grand Island. From simple geometrical considerations of macroform shape, an equation is developed to predict the depth and duration of flow required to erode the stoss sides of the macroforms to remove new vegetal growth each year. The methods developed in this report to eliminate vegetal growth on macroforms may provide a useful management tool for controlling width of the Platte River channels. Although further testing is required to establish the validity of these methods at actual stream sites, a sample calculation for the Silver Creek reach shows good agreement between the flow conditions predicted by the methods developed in this report and actual flow conditions. (USGS)

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USGS1982

A field study of littoral processes in Estero Bay, California

Estero Bay, which lies on the central California coast, has rocky headlands at both ends and sandy beaches within it. The shoreline of the bay has adjusted to be in equilibrium with the predominant wave climate, which is from the northwest. Because of its present shoreline configuration, the net southward littoral transport found along much of the California coast does not occur within Estero Bay. Instead, the sand primarily moves on- and offshore with a reversing longshore component. This sand transport pattern produces a littoral cell within Estero Bay even though there is no submarine canyon in the area. The primary sand sinks for this cell appear to be the sand spit south of Morro Rock and the entrance to Morro Bay itself, although this opinion was not experimentally verified. Field work during one summer (1978) and the following winter (1979) produced baseline data on the profile of and grain-size distribution across the littoral zone. In the offshore part of the littoral zone we also studied ripple size and type, internal structure, depth of erosion, and mineralogy. Although these data, which were collected along nine transects spaced 2 km apart, are inadequate to yield transport and energy rates, they indicate a northward decrease in wave energy within Estero Bay and a mixing of the sediments in the offshore. Box core and rod height data from grid points in seven meters of water showed that on the order of a meter of erosion occurred in the central part of the bay between the two sampling periods. Offshore, the data were incomplete, but at one station, in 17 m of water, at least 20 cm of erosion occurred.

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