Investigation of flood risk & erosion mitigation on the Rangitata River at Klondyke

Abstract

Risks to operational integrity of the Rangitata Diversion Race (RDR) presented by bank erosion and extreme discharges in the Rangitata River are investigated. Specific investigations are confined to two locations within the Klondyke reach of the river, namely Intake Bend and Klondyke Bend.

A fluvial geomorphological assessment of the Rangitata River, and specifically the Klondyke reach, is carried out to facilitate the selection of appropriate erosion mitigation measures.

Historical flood data for the Rangitata River are analysed and a flood frequency analysis using annual maximum discharge data is carried out. A risk analysis of flood damage to the RDR intake structure and canal at Intake Bend is undertaken using the results of flood frequency analysis and a calibrated MIKE 11 numerical hydraulic model.

A 1:150 scale, moveable bed, physical hydraulic model of the Intake Bend reach is used to investigate and evaluate methods for the mitigation of fluvial erosion of the RDR embankment and sediment admission to the RDR canal. An analysis of the effect of RDR water abstraction on bed aggradation in the river below the RDR intake is undertaken using the river flow-duration curve and semi-empirical sediment transport equations.

Assessment of erosion trends and mitigation options at Klondyke Bend are undertaken primarily through analysis of historical photographs and river discharge data. River morphology trends are inferred from study of river terrace remnants, recent river bank erosion, bar morphology and intervening discharge regimes. A generic analysis of the effects on river bed and water surface profiles from a proposed bend cut-off, to mitigate erosion at Klondyke Bend, is undertaken using a HEC-6 morphological model. Assessment of erosion mitigation options and conceptual design of a recommended option are based on the above data and from documented experience of similar situations on other rivers.