Analysis of reach-scale elevation distribution in braided rivers: Definition of a new morphologic indicator and estimation of mean quantities

This work has been carried out within the SMART Joint Doctorate (Science forthe MAnagement of Rivers and theirTidal systems) funded with the support of the Erasmus Mundus programme of the European Union. Data of the Rees River were derived as part of UKNatural Environment Research Council grant (NE/G005427/1) awarded to PI Brasington, along with further support from the NERC Geophysical Equipmen tFacility (Loan 892) and Leverhulme Trust IAF2014-03

Extracting an accurate river network: Stream burning re-revisited

Extracting river networks that are both accurate and topologically connected is important for applications that involve correct routing of material, for example water and sediment, through such networks. We combined water and sediment extraction using radar and multispectral imagery from Sentinel-1 and Sentinel-2 to create both water and sediment masks over a range of study areas. These were then used to condition topographic Digital Elevation Models (DEMs) by lowering the elevation of pixels with both water and sediment present, in a process known as stream burning. We examined how stream burning could improve accuracy of extracted networks and identified the most effective method of burning for optimal results. We find deeper burning depths improved accuracy, with diminishing returns: we suggest burning 40 to 50 meters. We find sediment burning improves accuracy in humid and temperate landscapes, but arid landscapes should be burned using only water pixels. We find accuracy of extracted networks is significantly better on the COP30 global topographic dataset compared to the NASADEM dataset, mainly due to the time of collection. The AW3D30 DEM and FABDEM datasets have accuracies just below that of the COP30 DEM

The Blurred Line Between Form and Process: A Comparison of Stream Channel Classification Frameworks

Stream classification provides a means to understand the diversity and distribution of channels and floodplains that occur across a landscape while identifying links between geomorphic form and process. Accordingly, stream classification is frequently employed as a watershed planning, management, and restoration tool. At the same time, there has been intense debate and criticism of particular frameworks, on the grounds that these frameworks classify stream reaches based largely on their physical form, rather than direct measurements of their component hydrogeomorphic processes. Despite this debate surrounding stream classifications, and their ongoing use in watershed management, direct comparisons of channel classification frameworks are rare. Here we implement four stream classification frameworks and explore the degree to which each make inferences about hydrogeomorphic process from channel form within the Middle Fork John Day Basin, a watershed of high conservation interest within the Columbia River Basin, U.S.A. We compare the results of the River Styles Framework, Natural Channel Classification, Rosgen Classification System, and a channel form-based statistical classification at 33 field-monitored sites. We found that the four frameworks consistently classified reach types into similar groups based on each reach or segment’s dominant hydrogeomorphic elements. Where classified channel types diverged, differences could be attributed to the (a) spatial scale of input data used, (b) the requisite metrics and their order in completing a framework’s decision tree and/or, (c) whether the framework attempts to classify current or historic channel form. Divergence in framework agreement was also observed at reaches where channel planform was decoupled from valley setting. Overall, the relative agreement between frameworks indicates that criticism of individual classifications for their use of form in grouping stream channels may be overstated. These form-based criticisms may also ignore the geomorphic tenet that channel form reflects formative hydrogeomorphic processes across a given landscape

The Blurred Line Between Form and Process: A Comparison of Stream Channel Classification Frameworks

Stream classification provides a means to understand the diversity and distribution of channels and floodplains that occur across a landscape while identifying links between geomorphic form and process. Accordingly, stream classification is frequently employed as a watershed planning, management, and restoration tool. At the same time, there has been intense debate and criticism of particular frameworks, on the grounds that these frameworks classify stream reaches based largely on their physical form, rather than direct measurements of their component hydrogeomorphic processes. Despite this debate surrounding stream classifications, and their ongoing use in watershed management, direct comparisons of channel classification frameworks are rare. Here we implement four stream classification frameworks and explore the degree to which each make inferences about hydrogeomorphic process from channel form within the Middle Fork John Day Basin, a watershed of high conservation interest within the Columbia River Basin, U.S.A. We compare the results of the River Styles Framework, Natural Channel Classification, Rosgen Classification System, and a channel form-based statistical classification at 33 field-monitored sites. We found that the four frameworks consistently classified reach types into similar groups based on each reach or segment’s dominant hydrogeomorphic elements. Where classified channel types diverged, differences could be attributed to the (a) spatial scale of input data used, (b) the requisite metrics and their order in completing a framework’s decision tree and/or, (c) whether the framework attempts to classify current or historic channel form. Divergence in framework agreement was also observed at reaches where channel planform was decoupled from valley setting. Overall, the relative agreement between frameworks indicates that criticism of individual classifications for their use of form in grouping stream channels may be overstated. These form-based criticisms may also ignore the geomorphic tenet that channel form reflects formative hydrogeomorphic processes across a given landscape

A combined field, laboratory and numerical study of the forces applied to, and the potential for removal of, bar top vegetation in a braided river

Vegetation can have an important role in controlling channel planform, through its effects on channel roughness, and root-reinforcement of bank and bar materials. Along the Platte River in central Nebraska, USA, The Platte River Recovery Implementation Program (PRRIP) has been tasked with managing the planform of the river to benefit endangered species. To investigate the potential use of planned Short Duration High Flow events (SDHFs) to manage bar vegetation, this study combined several approaches to determine whether flows of up to 227 m3s-1 through the central Platte River, could remove cottonwood, Phragmites and reed canarygrass stands of various ages and densities from in-channel bars. First, fieldwork was carried out to measure the uprooting resistance, and resistance to bending for each species. Second, a set of flume experiments was carried out to measure the forces exerted on the three species of interest under different flow conditions. Finally, a numerical study compared drag forces (driving) measured in the flume study, with uprooting forces (resisting) measured in the field, was carried out for each species to determine the likelihood of plant removal by SDHFs. Results showed that plants with more than a year of root growth, likely cannot be removed through drag and local scour alone, even at the 100-year recurrence interval discharge. At most, a few cottonwood seedlings could be removed from bars through drag, scour and undercutting, where rooting depths are still small. The results presented here help us further understand the positive feedbacks that lead to the creation of permanent, vegetated bars rather than mobile braided channels. As such, the findings could help inform management decisions for other braided rivers, and the combined field, flume and modelling techniques used in this study could be applied to other fluvial systems where vegetation and planform dynamics are of interest

Groundwater erosion of coastal gullies along the Canterbury coast (New Zealand): a rapid and episodic process controlled by rainfall intensity and substrate variability

Gully formation has been associated to groundwater seepage in unconsolidated sand- to gravel-sizedsediments. Our understanding of gully evolution by groundwater seepage mostly relies on experiments and nu-merical simulations, and these rarely take into consideration contrasts in lithology and permeability. In addition,process-based observations and detailed instrumental analyses are rare. As a result, we have a poor understandingof the temporal scale of gully formation by groundwater seepage and the influence of geological heterogeneityon their formation. This is particularly the case for coastal gullies, where the role of groundwater in their for-mation and evolution has rarely been assessed. We address these knowledge gaps along the Canterbury coastof the South Island (New Zealand) by integrating field observations, luminescence dating, multi-temporal un-occupied aerial vehicle and satellite data, time domain electromagnetic data and slope stability modelling. Weshow that gully formation is a key process shaping the sandy gravel cliffs of the Canterbury coastline. It is anepisodic process associated to groundwater flow that occurs once every 227 d on average, when rainfall intensi-ties exceed 40 mm d−1. The majority of the gullies in a study area southeast (SE) of Ashburton have undergoneerosion, predominantly by elongation, during the last 11 years, with the most recent episode occurring 3 yearsago. Gullies longer than 200 m are relict features formed by higher groundwater flow and surface erosion>2 kaago. Gullies can form at rates of up to 30 m d−1via two processes, namely the formation of alcoves and tunnelsby groundwater seepage, followed by retrogressive slope failure due to undermining and a decrease in shearstrength driven by excess pore pressure development. The location of gullies is determined by the occurrenceof hydraulically conductive zones, such as relict braided river channels and possibly tunnels, and of sand lensesexposed across sandy gravel cliffs. We also show that the gully planform shape is generally geometrically similarat consecutive stages of evolution. These outcomes will facilitate the reconstruction and prediction of a prevalenterosive process and overlooked geohazard along the Canterbury coastline

River planform, soil stratigraphy and the temporal and palaeoenvironmental significance of terraced valley fill deposits in upland Scotland, with specific reference to Glen Feshie, south-west Cairngorms

River terraces form prominent features of the valley floor morphology of many upland valleys in Scotland. Little is known, however, about valley floor landform development in many of these valleys. Previous studies have generally explained the terraces as the outwash products of meltwaters from the most recent cold periods. Detailed investigation of a major valley in the Scottish Highlands challenges, for at least one site, this well established concept and proposes the occurrence of at least three phases of Holocene terrace development.The correlation of terrace surfaces has traditionally been based upon the construction of height-range diagrams. An alternative approach to terrace correlation and dating is developed in this study using data from Glen Feshie, south-west Cairngorms. Terrace fragments are numerically classified and objectively grouped using quantitative soil-stratigraphic data. Principal Components Analysis and a hierarchical clustering technique numerically define five soil-stratigraphic units and place these on a relative time scale. Various methods of absolute dating control permit association of these units with five phases of terrace development. These are placed at 13,000, 10,000, 3,600, 1,000, 80, radiocarbon years BP.Comparison of palaeochannel networks preserved on the terrace surfaces suggests that these phases of terrace development have been associated with changes in channel pattern morphology. A unified approach to analysis of channel pattern morphology is developed and from this a new technique for palaeohydrological interpretation of gravel-bed streams. A segment density index is developed which allows total sinuosity to be predicted from just a part of the braided channel network. Application of these techniques to the Glen Feshie terraces demonstrates a trend for an overall decrease in discharge from the oldest terrace surfaces to the present day.Assessment of these landform changes within the context of known environmental fluctuations in the Cairngorms suggests that the early-mid Holocene was a period of relative landscape stability while the late Holocene was characterised by increasing instability. These changes may have been associated with the changes in river behaviour. However, spatial variation in the depth of the fill/bedrock interface may produce a discontinuous river response to changing environmental conditions