Quantification and Validation of a Sediment Budget for the Lower Hutt River, Wellington, New Zealand

A sediment budget for the Hutt River was developed to assess the volume of sediment that needs to be removed to maintain hydraulic efficiency of the channel, and the potential environmental effects of sediment extraction along the river and at its mouth. The transport of both suspended sediment and bedload was calculated using the characteristics of the available sediment and flow records from Taita Gorge. There are no significant inputs of either sediment or water downstream of this location. The volume of suspended load was estimated using a sediment rating curve. Bedload transport was estimated using a range of theoretical equations relating sediment transport to flow and channel parameters, and the characteristics of the material forming the river bed and banks. The sediment transport models were validated using the cumulative change in sediment volume determined from repeated cross-section surveys, and the volume of sediment extracted from the river. Analysis of the sediment budget indicates that the average annual sediment transport, including suspended and bedload, downstream of Taita Gorge is approximately 104,000 m /year. The calculated average sediment transport rate compares favourably to that derived from the analysis of channel cross-sections and sediment extraction records (approximately 88,000 m /year). The difference is from material deposited downstream of the analysed cross-sections (and beyond the river mouth) which was not quantified, and also from errors in the cross-section data analysis and the records of sediment extraction volumes. Only 8% of the material transported downstream is bedload; the remaining 82% is suspended sediment. These estimates are consistent with other New Zealand data. The total volume of suspended sediment and bedload transported exceeds that removed for flood protection works, and extracted at the river mouth. The excess sediment contributes to aggradation of the river bed and harbour floor. Sediment transport varies considerably over time and is controlled largely by the number, magnitude, and duration of floods. The annual rate of sediment transport since 1987 has ranged from 75,000 to 139,000 m . Lower than average rates since 2004 reflect the lack of significant floods over this period. The calibrated sediment budget for the lower Hutt River is valuable for guiding river management and flood mitigation works, and assessing the potential environmental effects of human activities such as sediment extraction at the river mouth.