Chapter Twenty – Monitoring Braided River Change Using Terrestrial Laser Scanning and Optical Bathymetric Mapping

The availability of new geomatics technologies and methods has transformed the investigation of sediment transport rates using the morphological approach. Terrestrial laser scanning (TLS), in particular, has transformative potential for mapping river change in braided rivers since much of the bed is sub-aerially exposed at low flows. TLS offers the opportunity to collect high-precision and high-accuracy data over large spatial extents, at temporal frequencies commensurate with individual flood events. When coupled with a suitable bathymetric mapping technique, high-resolution digital elevation models (DEMs) can be developed for both dry and wet areas of the braidplain. This chapter presents results from the ReesScan Project, where a methodology has been developed to apply TLS and empirical–optical bathymetric mapping to monitor the evolution of the braided Rees River, New Zealand, through a sequence of competent flood events. A reach-scale sediment budget is calculated and subjected to a statistical method to estimate a threshold level of detection to distinguish spurious changes in the DEM of difference. Tests at all the confidence levels considered (84%–99%) show that the reach is aggrading, with deposition volumes exceeding erosion volumes by 9% for the 84% confidence interval. This equated to a mean aggradation rate of 11 mm. The DEM of difference is rich in detail and, even when subjected to stringent statistical testing, subtle geomorphological changes are evident. The results show that the coupling of TLS with empirical–optical bathymetric mapping is a compelling approach for quantifying topographic change in unprecedented detail.