The ecology of Lagos Lagoon V. Some physical properties of lagoon deposits

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Abstract
This investigation is an attempt to discover the ways in which differences in composition of natural sand deposits may limit the distribution of burrowing animals by affecting first the volume of water a given weight of sand will contain, and secondly, the freedom with which water will flow between the grains. The geometrical arrangement of systems of closely packed spheres is considered. Measurements of porosity have been made first on different sand fractions prepared by sieving, then on various mixtures of these fractions and, finally, on a number of natural sands of known composition. The relation between porosity and composition in sands is discussed. The extent of the capillary lift in various grades of sand has been studied and the instability of quicksands associated with the volume of capillary water held against gravity relative to the water required to saturate the sand. From differences observed in the rate of evaporation of water from sands, an estimate is made of the percentage of water held by surface forces in the finer interstices of different sand fractions. A method is described for determining the rate of drainage of water through a standard sand column as a measure of permeability. The effect of different quantities of fine sand and silt on the permeability of natural sand deposits has been studied. It is shown that the depth at which blackening occurs in tropical lagoon sands is directly related to the rate of drainage, and provides a convenient method whereby permeability can be estimated in the field. A new scale of sieves is recommended for the analysis of sand samples. It is noted that the occurrence of lancelets in the lagoon sand deposits is related to the stability of the sand and to its permeability

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