RESIDENCE TIME OF SAND COMPOSING THE BEACHES AND BARS OF OUTER CAPE COD

John M. Zeigler, Sherwood D. Tuttle, Graham S. Giese, Herman J. Tasha

Abstract


When a grain of sand is delivered to the sea by erosion it begins a journey being transported along beaches or bars, or offshore by waves and currents. The history of this journey can be extremely complex for the grain might spend a few seconds in one place and many years trapped in another before being released again for travel. It is therefore important at the outset to place some limitations upon our study. Firstly, our time scale is limited to the past seventy years, the time over which data were gathered. Secondly, the geographical position is limited to a strip of the east coast of Cape Cod 29,400 yards in length (Figure 1), extending seaward to where water depth is about forty feet. After a grain of sediment leaves this area we are no longer concerned with it. We define residence time as the average number of years a grain of eroded sediment is likely to spend in this prescribed area before it is transported elsewhere. We will further try to show that sediment takes a preferred path, some of it moving along the beach and some along the bars. It makes no difference to us if specific environments share grains; that is to say, some material will be on the beach one day and on the bar the next. In the end those grains which tend to be more stable in the beach environment will spend more time there and will travel with a characteristic velocity which is different from the velocity of those grains which are in hydrodynamic equilibrium on bars. The method used to compute residence time involves volume stability. We measured the volume of the beaches and bars in the definition area and assumed that these volumes have not changed within the time limits of our study. We also measured the yearly addition of sediment to the area. In-as-much as there is neither gain nor loss of the average volume of sand -built features, i.e., the beaches and bars, sand must be moving out of the study area at the same rate it is being introduced. Therefore, the residence time in years is the average volume of a beach or bar divided by the yearly volume of sediment added by erosion to the beach or bar.

Keywords


residence time of sand; Cape Cod; sediment transport

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