The impact of the 1996 floods on a tidal sand bank in the Knysna estuary, Western Cape : research in action

We report on the cumulative effect of three flood events during October and November 1996, their impact on the intertidal banks of the Salt River estuary and on a single bank in the Knysna middle estuary (19lsupgol/supg01'E; 34 o 04'S) upstream of the railway bridge.We found that where land-derived, medium-to-fine coversand did not exceed 0.03 m, the invertebrate fauna survived and recovered within two years. Burial to a depth of 0.20 m, also over a pre-existing marine bank, affected the fauna so that only 42% of the original numbers were present after two years. At a site 100 m southwest of the other location, where 0.40-0.50 m of land sediment had accumulated over a bank not previously exposed even at neap low tide, only 6.7% of the pre-flood faunal population was evident. Upogebia africanal/Ig (mud prawn), once common on the bank, was absent; Callianassa krausil/Ig (sand prawn) was the only prawn present.