MODES OF SEDIMENT BEHAVIOR AND SELECTION OF HARBOR DESIGN AND MAINTENANCE TECHNIQUES FOR MINIMUM SHOALING IN ESTUARIES

Abstract

Recent field and laboratory studies on San Francisco Bay sediment have revealed modes of sediment transport and shoaling processes there. These studies have been partially described in reports (Krone et al., 1959> i960) and articles (Einstein and Krone, 1961a, b) and sufficient information has now been accumulated to encourage qualitative recommendations for selecting harbor designs and maintenance programs causing minimum shoaling in this bay. San Francisco Bay is a transitional region between the nearly steady uniform fresh-water flows in contiguous rivers and the marine environment. In this region unsteady quasiperiodic flows and water depths prevail, and salinities range from very low values in the rivers to that of sea water. Water density variations due to salinity differences complicate hydraulic conditions resulting from the transition from river to tide-induced flows, particularly in channels, to the extent that flows in opposite directions are found near the channel bottom and near the water surface. Large areas exist in the Bay with MLLW depths less than six feet. In these areas wind and tide-induced currents having low flow velocities, and wave action are the important hydraulic conditions. The sediments transported into the Bay generally range from fine sand downward in size, and compose the "wash loads" of entering streams that have found conditions in the Bay favoring deposition. Sediments involved in shoaling problems are generally composed of more than half clay minerals. These minerals are a heterogeneous mixture but are predominately montmorillonite and illite. Organic matter content of deposits within the Bay are of the order of .02 by dry weight. Similar estuarial conditions and clayey sediments to those in San Francisco Bay are believed to occur widely in other areas, with variations in detail of configuration, flows, salinities, wave action, and sediment composition and loads. Such details are more or less subject to modification, however, and are the principal interest of those concerned with harbor design. This paper first presents descriptions of the factors and physical processes important to sediment transport and shoaling in an estuarial environment as bases of later general recommendations for harbor design and maintenance. These descriptions are based on the San Francisco Bay studies, with supplementary information from other estuarial regions when known.