Preprints
https://doi.org/10.5194/nhess-2021-220
https://doi.org/10.5194/nhess-2021-220

  02 Aug 2021

02 Aug 2021

Review status: this preprint is currently under review for the journal NHESS.

Investigating the interaction of waves and river discharge during compound flooding at Breede Estuary, South Africa

Sunna Kupfer1, Sara Santamaria-Aguilar1, Lara van Niekerk2,3, Melanie Lück-Vogel2,4, and Athanasios Vafeidis1 Sunna Kupfer et al.
  • 1Coastal Risks and Sea-Level Rise Research Group, Department of Geography, Christian-Albrechts University, Kiel, Germany
  • 2Coastal Systems Research Group, Council for Scientific and Industrial Research CSIR, Stellenbosch, South Africa
  • 3Institute for Coastal and Marine Research, Nelson Mandela University, PO Box 77000, Port Elizabeth, 6031, South Africa
  • 4Department for Geography and Environmental Studies, Stellenbosch University, Stellenbosch, South Africa

Abstract. Recent studies have drawn special attention to the significant dependencies between flood drivers and the occurrence of compound flood events in coastal areas. This study investigates compound flooding from tides, river discharge (Q) and specifically waves using a hydrodynamic model at Breede Estuary, South Africa. We quantify vertical and horizontal differences in flood characteristics caused by driver interaction, and assess the contribution of waves. Therefore, we compare flood characteristics resulting from compound flood scenarios to those in which single drivers are omitted. We find that flood characteristics are more sensitive to Q than to waves, particularly when the latter only coincide with high spring tides. When interacting with Q, however, the contribution of waves is high, causing 10–12 % larger flood extents and 45–85 cm higher water depths, as waves caused backwater effects and raised water levels inside the lower reaches of the estuary. With higher wave intensity, the first flooding began up to 12 hours earlier. Our findings provide insights on compound flooding in terms of flood magnitude and timing at a South African estuary and demonstrate the need to account for the effects of compound events, including waves, in future flood impact assessments of open South African estuaries.

Sunna Kupfer et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on nhess-2021-220', Anonymous Referee #1, 30 Aug 2021
  • RC2: 'Comment on nhess-2021-220', Anonymous Referee #2, 07 Sep 2021

Sunna Kupfer et al.

Sunna Kupfer et al.

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Short summary
In coastal regions, flooding can occur from combined tides, storms, river discharge and waves. Wave’s effects are commonly neglected when assessing flooding, although these may strongly contribute to extreme water levels. We find that waves combined with tides and river discharge at Breede Estuary, South Africa, increased flood extent and depth, and caused earlier flooding, than when waves were neglected. This highlights the need to consider all major flood drivers in future flood assessments.
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