Unveiling Stage Zero conditions in the New Forest National Park: A drone-based Structure-from-Motion Photogrammetry and LiDAR approach for reconstructing an anastomosing wet woodland at the Avon Water.

In recent years there have been significant technological advances in Unmanned Aerial Vehicles and in particular the use of drones. This, combined with the development of structure-from-motion (SfM) technology have provided river restoration practitioners with a new tool that can be used to give an affordable, repeatable and objective assessment of river restoration projects. In this study, SfM photogrammetry is utilised on a section of the Avon Water, a small watercourse in the New Forest National Park. It was subjected to extensive Victorian-aged straightening and channelisation which have left the SSSI in an unfavourable ecological and morphological condition. Much of this is now being reversed by river restoration projects led by the New Forest Higher Level Stewardship Scheme. Whilst historic maps reveal the rivers former alignment, the straightening pre-dates these maps in places. Furthermore, even where the former route is shown, it is likely that anthropogenic modification began much earlier, and these maps may not represent the natural course of the river. High-resolution orthophotos and 3D photogrammetric models of the site are created that reveal a palaeolandscape consisting of a mosaic of anabranching channels. These are interpreted to represent a former anastomosing wet woodland and ‘Stage Zero’ of the river system. It is inferred that early human habitation and associated land clearance within the catchment altered hydrological and hydrogeological conditions, reducing floodplain connectivity and promoting single-threaded planform configurations. This was then followed by the most recent Victorian-aged straightening. The palaeolandscape revealed by the study provides a template for future Stage Zero river restoration in the New Forest and elsewhere, demonstrating the capabilities of low-cost, UAV‑derived photogrammetry in river restoration research.