The influence of macroalgal mats of Ulva intestinalis, growing attached to intertidal sediment, on sediment stability and hydrodynamic field was studied experimentally by investigating the flow structure induced by waves and currents and concomitant bedform dynamics. The experimental investigation was conducted at the open-channel flume at the TES (Total Environment Simulator) facility, University of Hull, UK, equipped with PIV, ADV and ABS probes.  The flume was set up with a bed of fine sand, partially covered by strands of Ulva intestinalis attached to pebbles, collected from the intertidal area of Budle Bay, north-east England.  This study aims to determine whether these algal mats increase flow resistance, promote bed stability and therefore reduce the risk of erosion leading to tidal flooding or to degradation of coastal lagoons. Direct observations and preliminary analysis of the data indicate that macroalgae play a crucial role in bio-stabilizing a sediment bed forced by current and waves.  The presence of Ulva intestinalis tends to inhibit the overall mobility of sediments, thus reducing flow resistance associated with bedforms and the amount of sediment transported. The quantification of these effects through relationships involving a few relevant hydrodynamic, sedimentological and biological parameters bears important practical implications in the management of coastal environments, and is the subject of ongoing research.The findings should therefore be of benefit to coastal engineers and managers who are faced with the problem of devising novel strategies to accommodate tidal inundation and to restore tidal environments