Sand Compatibility

It is important to note the compatibility of sediment when undertaking a beach fill project. Sediment is the word geologists use for loose pieces of minerals and rock. And since pieces of rock come in all sizes, geologists have developed a way to classify them.

The chart to the right shows how sediment can be classified by grain size. Most of the names are ones you already use: sand, boulders and clay, for example. The comparative illustration of sand and pebbles on the next page are shown at their actual sizes. Silt and clay are so small, they wouldn't show up here. Cobbles and boulders, on the other hand, are too big to fit on the page.

Though sediment is frequently classified by particle size, it can also be classified by origin. Sediment of inorganic origin is the result of the inland erosion of rocks such as granite, resulting in the shoreline deposition of the mineral sediment components such as quartz and feldspar. Sediment of organic origin is either siliceous or calcareous and comes from the hard skeletons of marine animals such as corals, mollusks and plankton. Rocks, shells and shell fragments are also sediment components.

Sand composition plays a very important role in determining the longevity, quality, and ecological impact of a fill project. Each natural beach has a characteristic sediment composition, which includes attributes such as statistical variations in sand particle size, chemical composition, moisture content, density and color. Correctly matching sediment particle size and overall composition for fill projects has important implications for beach ecology, fill longevity and water turbidity.

• Beach ecology: A number of microscopic invertebrates called meiofauna inhabit the tiny spaces between shoreline sediment. These organisms range in size from 0.1 mm to 1mm, and a shift in sediment size can severely impact meiofauna habitat. On the other end of the size spectrum, a number of studies have shown an increased occurrence of turtle false crawls and a decreased occurrence of turtle nesting on nourished beaches, perhaps due to significant changes in sediment composition and resulting changes in shoreface slope.

• Fill Longevity: Average sediment particle size plays a role in determining the equilibrium profile of a beach, so changes in beach sediment size - coupled with the characteristic physical processes of a given beach such as wave climate, storm frequency and severity - can cause unpredictable changes after the fill. As mentioned in Chapter 1, fine sediment is more likely to move offshore than coarse sediment, so filling with sediment much finer than the native sediment may result in the seaward displacement of the finest sediment particles. Due to habitat concerns, however, it is not reasonable to fill all beaches with coarse sediment simply to ensure that sand will stay on the beach.

• Water turbidity: Sediment that contains high levels of fine sand, silt or clay may not quickly or completely settle in the water column, which can cause high levels of turbidity (opaque and muddy color of water when particles and sediment are stirred up). Turbidity not only affects water quality, but also may significantly impact the behavior of organisms that depend on sight for feeding, reproduction and locomotion.

The range of impacts associated with filling a beach with incompatible sediment highlights the importance of seeking adequate quantities of well-suited sand for any beach fill project.