Terrestrial structure-from-motion: spatial error analysis of roughness and morphology

Structure-from-Motion (SfM) photogrammetry is rapidly becoming a key tool for morphological characterisation and change detection of the earth surface. This paper demonstrates the use of Terrestrial Structure-from-Motion (TSfM) photogrammetry to acquire morphology and roughness data at the reach-scale in an upland gravel-bed river. We quantify 1) spatially-distributed error in TSfM derived Digital Elevation Models (DEMs) and 2) identify differences in roughness populations acquired from TSfM photogrammetry versus TLS. We identify an association between local topographic variation and error in the TSfM DEM. On flatter surfaces (e.g. bar and terrace surfaces), the difference between the TSfM and TLS DEMs are generally less than ±0.1 m. However, in areas of high topographic variability (>0.4 m) such as berm or terrace edges, differences between the TSfM and TLS DEMs can be up to ±1 m. Our results suggest that grain roughness estimates from the TSfM point cloud generate values twice those derived from the TLS point cloud on coarse berm areas, and up to four-fold those derived from the TLS point cloud over finer gravel bar surfaces. This finding has implications when using SfM data to derive roughness metrics for hydrodynamic modelling. Despite the use of standard filtering procedures, noise pertains in the SfM DEM and the time required for its reduction might partially outweigh the survey efficiency using SfM. Therefore, caution is needed when SfM surveys are employed for the assessment of surface roughness at a reach-scale

Using geophysical subsurface data for the reconstruction of valley-scale spatio-temporal floodplain evolution: implications for upland river restoration

The use of analogues of previous river styles is highly significant for successful river restoration, yet some existing techniques available to assist practitioners are still not widely applied. We explore the use of Ground Penetrating Radar (GPR), to explore past river styles in an upland river valley in the UK, and explore the potential of the approach to reconstruct former channel pattern. Post-glacial evolution of upland floodplains has been influenced by temporal changes in vegetation, sediment supply and hydrological regime. Channel-floodplain morphodynamics over the Holocene were conditioned by glacial deposits, lateral interaction with slope processes and fluvial sediment reworking, changes in flow and sediment supply regimes driven by climatic change, and more recently direct and indirect anthropogenic activities, e.g. deforestation, floodplain land use and channel modification. Current drives towards river restoration often use floodplain topography as a guide to appraise such a planform state, however, reconstruction of former channel state is often restricted to surface features visible on historic maps and aerial photographs. This research focuses upon the floodplain of the upper Swindale Beck, Lake District, UK, which was recently restored to a planform design based on the recent meander pattern visible in floodplain topography. We show the potential of GPR to reconstruct a wider array of past channel pattern and evolution at a site characterised by largely aggradational conditions and consistent sediment supply from glacial deposits at the valley head. Analysis of GPR data from 40 intersecting GPR survey lines revealed several stratigraphic units, including gravel braidplains, berms, chutes and bars, several levels of larger channels and their layered fill as well as backwater deposits. These were interpreted as braided systems, dynamic wandering planform and single-thread meandering systems with spatial transitions conditioned by tributaries and valley slope. Optically Stimulated Luminescence (OSL) dates in combination with GIS analysis of valley slope, channel gradient and local valley floor aspect allowed the interpretation of individual evolutionary stages of river and floodplain development at Swindale over at least the last millennium and provides links to processes in the wider environment including the role of alluvial fans in supplying sediment and forcing channel migration. Such information can be particularly valuable for restoration projects to aid design of channel dimensions, planform configuration, channel gradient, substrate characteristics and connection with tributaries. While restoration generally aims to resemble a more natural reference state, specific targets may seek to improve a particular set of functionalities (e.g., ecological, flood and sediment management, recreational) which should be resilient to the consequences of ongoing climatic changes and should be achieved sustainably (e.g. locally sourced gravel). Here, GPR-based floodplain analysis provides a non-invasive approach to understand possible evolutionary trajectories and to appraise a wider range of restoration options and sustainable resources

Climate-change driven increased flood magnitudes and frequency in the British uplands: geomorphologically informed scientific underpinning for upland flood-risk management

Upland river systems in the UK are predicted to be prone to the effects of increased flood magnitudes and frequency, driven by climate change. It is clear from recent events that some headwater catchments can be very sensitive to large floods, activating the full sediment system, with implications for flood risk management further down the catchment. We provide a 15-year record of detailed morphological change on a 500-m reach of upland gravel-bed river, focusing upon the geomorphic response to an extreme event in 2007, and the recovery in the decade following. Through novel application of 2D hydrodynamic modelling we evaluate the different energy states of pre- and post-flood morphologies of the river reach, exploring how energy state adjusts with recovery following the event. Following the 2007 flood, morphological adjustments resulted in changes to the shear stress population over the reach, most likely as a direct result of morphological changes, and resulting in higher shear stresses. Although the proportion of shear stresses in excess of those experienced using the pre-flood DEM varied over the recovery period, they remained substantially in excess of those experienced pre-2007, suggesting that there is still potential for enhanced bedload transport and morphological adjustment within the reach. Although volumetric change calculated from DEM differencing does indicate a reduction in erosion and deposition volumes in the decade following the flood, we argue that the system still has not recovered to the pre-flood situation. We further argue that Thinhope Burn, and other similarly impacted catchments in upland environments, may not recover under the wet climatic phase currently being experienced. Hence systems like Thinhope Burn will continue to deliver large volumes of sediment further down river catchments, providing new challenges for flood risk management into the future

Morphological dynamics of upland headwater streams in the southern North Island of New Zealand

Short-term channel dynamics of mountain stream reaches in the southern North Island of New Zealand were assessed over two successive 3-month periods using morphological budgeting. Response to floods varies between reaches, even when the catchments were located close to each other and had similar characteristics. The reaches on the Central Volcanic Plateau experienced least morphological change, while streams with steep catchments and migrating planform in the Tararua and Ruahine Ranges showed frequent channel adjustments. Channel response is conditioned by intrinsic variables rendering reaches responsive or robust to the effects of floods, and this is likely to reflect the degree of connectivity between slopes and channels, and reaches

Linking disturbance and stream invertebrate communities: how best to measure bed stability

Substrate stability is a key determinant of stream invertebrate community composition, but its measurement can be problematic. Stream ecologists often use different approaches and techniques to quantify bed stability, and this variability makes comparison among studies difficult. We examined the link between 6 reach-scale measures of substrate stability and invertebrate community metrics in 12 New Zealand mountain streams. The strength of the link varied with the method used to define substrate stability. We used morphological budgeting to measure spatial patterns and volumes of scour and fill. We found that as erosion of sediments increased, invertebrate diversity declined exponentially. In particular, increases in the volume of scour reduced taxonomic richness, whereas deposition of coarse sediments was less relevant for invertebrate communities. Overall, the distance travelled by in-situ-marked tracer stones was most strongly linked with all invertebrate community metrics, whereas the bottom component of the Pfankuch Index related very well to diversity. Both metrics showed near-linear declines in diversity with decreasing stability. In contrast, the link between invertebrate communities and the proportion of bed area affected by entrainment was weak. Therefore, we propose tracer-based indices and the Pfankuch bottom component as the most suitable measures for research involving invertebrate-substrate-stability relationships. Measures derived from in-situ-marked tracer stones reflected only entrainment and transport of particles. In contrast, the bottom component of the Pfankuch Index encompassed the widest range of bed-stability characteristics but is prone to observer bias. An objective method that combines the efficiency of the Pfankuch Index with the characteristics measured using tracer stones could serve as a powerful explanatory tool in stream ecology. © 2011 The North American Benthological Society

A macroinvertebrate index to assess stream-bed stability

Biotic indices based on community composition and calculated from sensitivity scores assigned to individual taxa are commonly used as indicators for ecological integrity of fluvial ecosystems. Macroinvertebrate indices can assess water quality but invertebrate community composition also responds to other environmental factors including stream bed disturbance. This study presents a biotic community index that assesses stream bed stability in stony riffles. This Macroinvertebrate Index of Bed Stability is calibrated on transport and entrainment of in situ-marked tracer stones in 46 streams in New Zealand’s North Island, representing a wide range of substrate stability. Scores were investigated for 67 common invertebrate taxa using Indicator Species Analysis based on taxa abundance at varying levels of substrate stability. The resulting site score, weighted by taxa abundance, improved a predictive model of bed stability, generated with model trees, when added to the pool of habitat variables and explained 69% of the variation in bed stability. Site scores were strongly correlated with measured bed stability at the development sites, but not at eight independent validation sites, suggesting the need for further testing on a larger dataset including streams in other regions of New Zealand, and overseas. </jats:p