The impact of urban groundwater upon surface water - eTheses ...

4.7.3 Grain size analyses by the Hazen and Shepherd methods
MONITORING NETWORKS AND METHODS
The hydraulic conductivity of unconsolidated sediments is directly related to the packing of
the particles and the void spaces between them which are a function of the grain size
distribution. As median grain size increases so will permeability. An increase in the standard
deviation indicates a more poorly sorted and less permeable sample. Coarse sediments are
more sensitive to the degree of sorting than fine sediments, and unimodal samples have
greater permeability than bimodal samples (Fetter, 1994).
The riverbed sediments of the Tame comprise unconsolidated sand and gravel which are
easily subjected to grain size analyses. Samples were selected from the material obtained
during riverbed coring. The samples were dried before being passed through a standard series
of sieves and the weight of each fraction recorded. A grain size distribution was then
constructed by plotting grain size versus percent finer by weight and subjected to the analyses
outlined below.
The uniformity coefficient is the ratio of the grain size that is 60% finer by weight to the grain
size that is 10% finer by weight. This was calculated for each sample to give a measure of the
degree of sorting.
Cu=d60/d10 Cu6 = poorly sorted
The Hazen method (Hazen, 1911) may be used to estimate the hydraulic conductivity for
sandy sediments from the grain size distribution curve. The method is appropriate when the
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