Impacts of river engineering on river channel behaviour : implications for managing downstream flood risk

Although knowledge of sediment transport has improved over the last 25 years, our understanding of bedload transfer and sediment delivery is still based on a limited set of observations or on models that make assumptions on hydraulic and sediment transport processes. This study utilises repeat lidar survey data of the River Caldew above the City of Carlisle in the UK to investigate the balance of erosion and deposition associated with channel switching from an engineered and managed single thread channel to a naturalising incipient wandering system. Over the 11-year survey period (four bankfull flood events) around 271,000 m3 of sediment were delivered to the river and floodplain and 197,000 m3 eroded suggesting that storage rates of around 7000 m3/annum occurred. The balance of erosion and deposition is influenced by channelisation with very restricted overbank sedimentation and only limited local and transient in-channel bar deposition along the engineered reach (8000 m3 eroded). This contrasts with the activity of the naturalising reach downstream where a developing wandering channel system is acting to store coarse sediment in-stream as large bar complexes and the associated upstream aggrading plane bed reaches and overbank as splay deposits (87,000 m3 stored). Such behavior suggests that naturalisation of channelised systems upstream of flood vulnerable urban areas can have a significant impact on sediment induced flooding downstream. This conclusion must, however, be moderated in the light of the relatively small volumes of material needed to instigate local aggradation in over-capacity urban channels

An evaluation DEM accuracy acquired using a small unmanned aerial vehicle across a riverine environment

Fluvial systems offer a challenging and varied environment for topographic survey, displaying a rapidly varying morphology, diversevegetation assemblage and varying degree of submergence. Traditionally theodolite or GPS based systems have been used to capture cross-section and break of slope based data which has subsequently been interpolated to generate a topographic surface. Advances in survey technology has resulted in an improved ability to capture larger volumes of data with infrared terrestrial and aerial LiDAR systems capturing high-density (<0.02m) data across terrestrial surfaces but instruments are expensive and cumbersome and fail to survey through water. The rise of Structure from Motion (SfM) photogrammetry, coupled with unmanned aerial vehicles (UAVs), has potential to rapidly record information needed to derive elevation data at reach scale with sub decimetre density. The approach has the additional advantage over LiDAR of seeing through clear water to capture bed detail, whilst also generating orthorectified photographic mosaics of the survey reach. However, the accuracy of the data has received comparatively little attention. Here we present a survey protocol for UAV deployment and provide a reach scale comparison between a Terrestrial LiDAR Survey (TLS) and SfM UAV survey on the River Sprint near Kendal in England.. Comparative analysis of elevation data between TLS and SfM suggest comparable accuracy and precision across terrestrial surfaces with error lowest over solid surfaces, increasing with vegetation complexity. Submerged SfM data captured bed levels generally to within ±0.2 with only a weak relationship recorded between error and flow depth

Drone based quantification of channel response to an extreme flood for a piedmont stream

The influence of extreme floods on the form and functioning of upland systems has concentrated on the erosive impact of these flows. They are seen to be highly competent with coarse sediment transport rates limited by upstream supply and moderated by the ‘blanketing’ effect of an armour layer. This study investigates the effect of extreme events on the upland sediment cascade subjected to a recent extreme rainfall-induced flood event. The drone-based survey generated orthophotography and a DEM surface, which was compared with historic LiDAR data. This allowed erosion and deposition to be quantified and the surface micro-variation used to characterise stable and mobile sediment. The idealised model of sediment residence time increasing downstream is questioned by the findings of this study as relatively little coarse bedload sediment appears to have been transferred downstream in favour of initial local channel erosion (moderated by legacy large sediment), mid-reach palaeo-channel reactivation, sub-channel infilling and downstream deposition of the majority of mobilised sediment across berm and bar surfaces within the active inset channel margins. Channel margin erosion was largely limited to fine sediment stripping moderated by the re-exposure of post-glacial sediment. Only a weak relationship was found between local channel slope and deposition, with storage linked more to the presence of inset berm and bar areas within the inset active channel. Downstream fining of sediment is apparent as is a strong contrast between coarser active sub-channels and finer bar and berm areas

Meltwater temperature in streams draining Alpine glaciers

Water temperature is of considerable importance with respect to lotic habitats. Water temperature influences physical, chemical and biological conditions within river environments and is, therefore, a key determiner of the health of a river. Climate change is significantly impacting lotic environments, through changes to hydrology, biodiversity and species distribution. Effects of climate change are greatest at high elevation and biota in and around glacier-fed rivers is likely, therefore, to be at great risk. How climate change influences the hydrology will have great impact on river water temperature and glacier-fed rivers in Alpine environments are extremely sensitive to climatic change. This paper assesses five rivers: Four glacier-fed rivers (36.9 - 82% percentage glacierisation) located in the Swiss Alps, and one located in an ice-free catchment in the Bernese Oberland, Switzerland. The aim was to assess the impact of basin characteristics on river water temperature. A distinct paradoxical relationship was identified whereby water temperature in some glacier-fed rivers was reduced during the time of highest incoming shortwave radiation receipts and high air temperature. Whether a summer cooling effect presented itself in all glacier-fed rivers within this study was researched. The key findings were that the identified summer cooling effect was not present in all rivers, despite percentage glacierisation. Percentage glacier cover has often been reported as they key determiner of water temperature in such rivers. More important was the stream dimensions, notably stream surface area. Understanding the controlling factors that influence water temperature of glacier-fed rivers will help river managers and planners in knowing how climate change will impact fisheries downstream of glaciers over the coming decades. This may allow plans to be introduced to try and mitigate warmer water temperature that will result, in some glacier-fed rivers, as the climate warms

Recent changes to floodplain character and functionality in England

Regime analysis suggests that temperate alluvial watercourses overtop their banks on average once every 1.5 years transferring water and sediment across the valley floor to form floodplains helping maintain a strong hydrological connection between in-channel and overbank form and process. Flooding also causes erosion, sediment transfer and deposition creating a variety of floodplain morphologic units and functional connectivity with the main river. The result is a morphologically and ecologically varied wetland dominated ecotone where diversity is sustained by the action and flooding and shallow groundwater processes. Floodplains are, however, sensitive to disruption and many have been significantly degraded since the Bronze Age as a result of activities that alter flooding and groundwater processes and manage vegetation communities. The current (2015) floodplain condition and trends of change since 1990, for England are presented here using land use data for 1990, 2000, 2007 and 2015. Floodplain system degradation has been found to be both widespread and severe across the whole of the country. The 1990 data set showed that intensive agriculture occupied around 38% of floodplain zones expanding to 53% by 2000 before slowing slightly to covering 62% in 2007. Between 2007 and 2015 the coverage remained relatively static (64%) with some suggestion that arable areas were being transformed to pasture. Wetland areas in the form of fen, marsh, swamp and bog have been devastated with the data sets indicating that these fundamental floodplain units have been all but lost. Upland and lowland areas are both severely impacted with a near ubiquitous loss of natural floodplain functioning. Despite this some 31% of rivers in England are classified as good or better under the European Water Framework Directive classification system calling into question the UK WFD status classification process

Radiometric calibration of a dual-wavelength terrestrial laser scanner using neural networks

The Salford Advanced Laser Canopy Analyser (SALCA) is a unique dual-wavelength full-waveform terrestrial laser scanner (TLS) designed to measure forest canopies. This paper has two principle objectives, first to present the detailed analysis of the radiometric properties of the SALCA instrument, and second, to propose a novel method to calibrate the recorded intensity to apparent reflectance using a neural network approach. The results demonstrate the complexity of the radiometric response to range, reflectance, and laser temperature and show that neural networks can accurately estimate apparent reflectance for both wavelengths (root mean square error (RMSE) of 0.072 and 0.069 for the 1063 nm and 1545 nm wavelengths respectively). The trained network can then be used to calibrate full hemispherical scans in a forest environment, providing new opportunities for quantitative data analysis

Geomorphological effectiveness and maintenance of a riffle-pool sequence

Riffle-pool sequences in gravel-bed rivers provide the template for a number ol fish andinvertebrate habitats and their morphological complexities create spatial hydraulicvariation over the flow regime that is logistically impossible to measure completely inthe field. The mechanisms responsible for riffle-pool maintenance are unclear anddespite contributions from many researchers, no conclusive explanation has beendeveloped. There is much debate over the popular velocity reversal hypothesis, where itis generally acknowledged that at low flows the maximum bed shear stress in thechannel occurs across the riffle, but at higher less frequent flows areas of maximum bedshear stress can switch to the pool. Sediment interactions control fluvial forms andprocesses, as such specific analysis of spatial hydraulic patterns in boundary shear stressare required to aid understanding of riffle-pool system behaviour and gain the insight into the transport capacity that can scour sediment deposited in the predominandy lowenergy, lower competence pools and thereby provide a mechanism for maintenance olriffle-pool morphology. Terrestrial LiDAR captures reach scale topographical data toyield a 0.02 m digital elevation model and provides input to three-dimensionalcomputation fluid dynamics software (CFD), where the spatial distribution and area!extent of bed shear stress, surface flow velocities and potential sediment entrainmentover the discharge range are investigated for a 188 m gravel-bed reach of KingsdaleBeck, UK. With increasing discharge, spatial distributions of shear stress are revealed,which, until now, have been largely overlooked using previous cross-sectional analysis,highlighting the influence of pool tails as discharge rises. Results show some agreementwith past literature on reversal with higher shear stress zones associated with riffles. Asdischarge increases pools are shown to become more dominant, however the existenceof multiple reversals shows that while reversals do occur, they may be short-lived andmarginal in magnitude, providing inherent problems with current theory. The overalltemporal maintenance of the riffle-pool sequence is more explicitly linked with the longterm flow regime, with the flows responsible for moving pool sediment closely confinedto near bankfull discharge. Results indicate that potential to entrain sediment frompools occurs only from 85% bankfull. Discharges less than 85% bankfull are suggestedto be responsible for maintaining the riffle-pool morphology through flow routing andbed shear stress reversal, however, they are not capable of potentially entrainingsediment from the deepest pools. Combined CFD simulations with temporal dischargedominance derived from triangulated rainfall and the Revitalised FSR/FEH rainfallrunoffmethodology, highlight the role of rarer more extreme flows in terms ofexplaining sediment entrainment from pools. At high flow Kingsdale Beck creates acompetent sub channel, where a continuous sinuous channel with higher levels of bedshear stress develop with embryonic lateral bars, increasing potential sedimententrainment following the line of thalweg. As flow velocity decreases conventional rifflepoolhydromorphology re-establishes and provides a new proposal for the maintenanceof a riffle-pool sequence

Evidence of non-contiguous flood driven coarse sediment transfer and implications for sediment management

We investigate gravel redistribution and morphological response of three headwater streams following the Storm Desmond floods of December 2015. Reactive management of the watercourses following the flooding concentrated on removal of gravel and clearing of vegetation, perceived as having been significant causes of local flooding. Aerial LIDAR and ortho-photography were employed to critically assess the location, type and magnitude of sediment mobilization, using an sUAV to capture imagery of lake sediment fans across the area. Whilst gravel was mobilized during the flood the volumes involved do not appear to be as high as anticipated, with little detectable change in lake fan deposits in the sink zone of the three study streams. Re-exposure of relict gravels on floodplains through stripping of surface vegetation and soils, gave the false impression of fresh deposition. Reactivation of wandering channel zones appear to have acted as buffers to large-scale sediment movement rather than acting as supply zones. Sediment accumulation through towns and villages was harder to quantify due to the rapid clean-up operation, however, the authors suggest that this may not be as large as assumed and the widespread dredging is likely to have caused more problems than it has solved as the bed of many watercourses is now highly susceptible to mobilisation following mechanical disruption of the previously strongly armoured surface