Sediment pathways in San Francisco South Bay

Abstract

This study aims for a better understanding of the sediment pathways in San Francisco South Bay (South Bay). Many issues relevant to the Bay Area community such as shipping, recreational and commercial fishing, habitat restoration, human health, and environmental water quality are reliant on understanding sediment pathways (McKee et al. [2006]). Existing theories suggest that the Sacramento-San Joaquin Delta deliver sediment to South Bay during periods of high river flows. Different hydrodynamic forces, such as the tide, wind and baroclinic flows, redistribute these sediments around South Bay.

In this research, trends are analysed using a new set of data (WY2015-WY2017) and sediment pathways in time and space are investigated using a 3D numerical model. In this model, the pathways of three sediment classes with different settling velocities are traced from their source throughout the Bay Area. No such study has yet been undertaken in the South Bay Area. There is a high Delta sediment input to South Bay during periods of high river flow. The local tributaries contribute only marginally to the suspended sediment concentration of South Bay. With a decreasing sediment load from the Delta, the importance of the local tributaries as a sediment source for South Bay could increase.

In this research, three different types of sediment exchange between Central Bay and South Bay are observed. The first type is observed during periods of low river flows (Q < 800m3/s). A seaward directed residual flow is found in the channel at Bay Bridge. Sediments are slowly transported out of South Bay. The second type is observed after a period of moderate river flow (Q > 800m3/s). A pulsed sediment flux from the Delta increases the turbidity of Central Bay, resulting in a horizontal spatial variation in SSC from Central Bay to South Bay. The diffusive character of the tide transports the sediments slowly from the turbid Central Bay to the relatively clearer South Bay. The third sediment exchange type is observed after an extreme Delta river flow (QDel ta È 5000m3/s)). The extreme Delta flow refreshes a large part of Central Bay and South Bay. The salinity of Central Bay increases slowly, resulting in a substantially more saline Central Bay than South Bay for a couple of months. The resulting baroclinic flow transports the sediments from Central Bay into South Bay through the landward directed bottom current.

Two dominant pathways with opposite transport direction characterise the sediment transport in South Bay. One pathway is located in the channel and is directed southward during periods of high river runoff. The second pathway is located on the extensive east flat and is directed northward during periods of high river runoff. The transport in the channel is dominant during the wet period, resulting in a net transport landward. Besides these two dominant pathways, four re-circulations cells are found, facilitating the exchange between the channel and the shoals.

The next step in gaining more insight into the sediment physics is a model studywhich includes bed-interaction with the suspended sediments. A study like this could confirmthe hypotheses opted in this research.