Lower Macleay Valley Flood Study - Final Flood Study Report
Stage 1 - Lower Macleay Floodplain Risk Management Plan

November 2019

Introduction

Cover of Lower Macleay Valley Flood Study

The Lower Macleay River floodplain occupies an approximately 500km² of coastal floodplain downstream of Kempsey, to the Macleay River’s main outlet at South West Rocks. The floodplain drains an upstream catchment area of 11,500km² and also has several secondary outlets to the ocean at Hat Head, Crescent Head, Ryans Cut and South West Rocks and a hydraulic link to the Maria River and Hastings River to the south, all owing to improved channel linkages constructed during historic flood mitigation schemes.

Kempsey Shire Council engaged Jacobs to undertake a catchment-scale flood study of the Lower Macleay floodplain, downstream of Kempsey. Numerous flooding studies and assessments have been conducted for the Lower Macleay River in the past, with the recent studies including the Kempsey CBD Floodplain Risk Management Study in 2017. The recent studies are fragmented in their study extents.

This current study provides an expansive floodplain-wide assessment of flooding conditions according to the latest floodplain management guidance and policy. The purpose of this study is to undertake investigation to assess mainstream flood behaviour in the broader Lower Macleay floodplain including villages and urban areas.

Key objectives of this study are to:

  • Review available catchment, topographic, flood and drainage infrastructure data; flood study reports, overland flow path studies and associated hydrologic and hydraulic models;
  • Undertake hydrologic analysis of upstream catchment inflows to define the river inflow characteristics for historic and design flood events and establish hydrologic models to estimate flood event inflows for the study area downstream of Kempsey;
  • Establish and calibrate a TUFLOW hydraulic model for the study area to determine flood hydraulic behaviour, with consideration of interactions with flooding in adjacent river catchments and the ocean. There is a particular emphasis on floodplain villages, flood mitigation infrastructure and ocean outlets;
  • Prepare flood mapping including peak flood extents & levels, depths and velocities for all modelled events;
  • Assess the sensitivity of flooding to modelling assumptions;
  • Assess the sensitivity of flooding to climate change, including sea level rise and increased rainfall;
  • Undertake community consultation to communicate the outcomes of this study; and
  • Prepare a report on the above.

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