Hard sand refers to unconsolidated substrate containing elements that confound the single beam acoustic sounder output causing the signal to appear either harder or rougher than would be expected from sand. There are several factors that lead to a substrate being classified as hard. These are defined as modifiers in the habitat classification and includes large grain size, shell matter (either whole shells or shell grit) or biological material.
Examples of modifiers that are included in this hard sand category include: Physical: Coarse sand/gravel, compacted sand, rippled sand, shell or shell grit in sediment; Biological: Burrows, seawhips, holothurians
Hard sand is common in and around seagrass beds indicating the possible presence of rhizoidal mats of the seagrass and associated organisms in the substrata. It is also common on the seaward side of reefs indicating the presence of shells, detritus and organisms whose origin is dependent on the nearby reefs. The extensive areas of shelly substrate in high current areas in the D'Entrecasteaux Channel were characterised by a hard sand signature. On more exposed shores sand is harder closer to shore, due to coarser grade sands produced by the sorting action of waves.