GEOMORPHOLOGICAL EFFECTS OF CATASTROPHIC FLOODING ON A SMALL SUBTROPICAL ESTUARY

The Mgeni Estuary is situated on the east coast of South Africa. Tidal salinity changes commonly extend 2.5 km upstream from the mouth. The subtropical climatic regime causes seasonal flooding, the geomorphological effects of which are increased by a steep river gradient. A 120‐150 year flood event during September 1987, with an estimated peak discharge of 10800 cumecs, resulted in the erosion of 1.86 × 106 m3 of sediment from the estuary. The normal high tide volume of the estuary (0.35 × 106 m3) increased to 1.85 × 106 m3 after the flood. The post‐flood intertidal volume (tidal prism) of the estuary was 0.43 × 106 m3 compared to 0.19 × 106 m3 before the event. The nature and extent of erosion in the estuary was controlled by the composition of the estuary banks and bed material. Gravel and sand substrates were preferentially eroded from the estuary bed before the cohesive bank materials were undercut in sections of the estuary. Mud and fine sand were deposited on overbank areas when the flood waned and a lag of coarse sand and gravel was produced on the estuary bed. Approximately 46 per cent of the bedload sediment supplied to the estuary since 1917 was retained in the estuary until the 1987 flood. The remainder was transported through the system and into the Indian Ocean. Results show that catastrophic floods play an important role in sedimentation in small estuaries. Copyright © 1990 John Wiley & Sons, Ltd