Wave effects on submarine groundwater seepage measurement

Variation of sea-surface and water pressure above the sea bed induces temporal variation of submarine groundwater discharge over time scales ranging from seconds to years. Hydrodynamic theory and measurements suggest that wave-induced exchange between a permeable sediment bed and overlying water column is significant but conventional seepage-meter studies have focussed mainly on tidal dynamics and ignored waves. At Cockburn Sound in Western Australia we measured wave-induced flow reversals through a seepage meter of amplitude 60cmd-1; however, it was unclear whether the measured flows were real seepage or, partly or wholly, an artifact of wave action on the seepage meter. A numerical model of seepage patterns beneath a vented benthic chamber demonstrated an observer effect introduced by the chamber and not previously identified. Placing a chamber on the sediment bed disturbed the pressure field and changed both the pattern and magnitude of the wave-induced flow. A separate analysis of benthic-chamber movements under the action of shallow surface waves established that micron-scale movements of the chamber at the wave frequency were sufficient to produce apparent seepage amplitudes of O(1–100) cm d−1. We concluded that wave action is a key control on bed seepage and should not be neglected without justification in direct-measurement studies of marine bed discharge. A systematic error during each wave cycle can accumulate to a significant measurement error if the wave cycle error is large or if wave-induced flow is the dominant component of the seepage. In the latter case, the error could potentially be misinterpreted as a steady seepage component.