THE INTERPRETATION OF TRANSIENT PORE PRESSURES IN SALT MARSH SEDIMENT

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Abstract
The hydraulic properties of salt marsh sediments can be estimated by analyzing the transient pore pressure response that follows the addition of water to the surface of the sediment. The method is demonstrated for pore pressures observed in a lysimeter containing a 137-L sample of sediment from Belle Isle Marsh in Boston, Massachusetts [USA]. The specific storage, also called differential moisture capacity, of the sediment is determined from the net change in pressure at equilibrium following the addition of a known volume of water, and the hydraulic diffusivity, the ratio of hydraulic conductivity to specific storage, is determined from the time scale over which transient pressure conditions persist. The linearized mass balance equation is the basis for the interpretation of the transient time scale, even though the linear theory is a relatively poor representation of the actual conditions in the sediment. Numerical simulations with a saturated-desaturated flow model show that estimates of hydraulic conductivity are not severely biased by the approximate nature of the linear theory underlying the interpretation of the pressure data. The specific storage of the sediment sample was found to be 5 .times. 10-4 cm-1, and the saturated hydraulic conductivity was 5 .times. 10-4 cm/s. Effects of desaturation on the hydraulic properties were observed below an apparent air-entry threshold of -10 cm. Estimates of the specific storage followed an unexpected trend as conditions in the sediment became more unsaturated, possibly caused by the macroporosity of salt marsh sediments.

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