This study focuses upon flow hydraulics, sediment transport and riffle-pool maintenance on the River Rede, Northumberland, UK. Analysis of bed structure indicate pools to be coarser than riffles, suggesting these to be zones of maximum tractive force at high flow. Tractive force reversal can be demonstrated using a combination of velocity, shear stress and gravel tracer data, and is therefore advocated as a mechanism for maintaining the riffle-pool form. Three dimensional flow structures are likely to increase the likelihood of reversal in pools situated on bends, which may not always be detected using one-dimensional measures of flow hydraulics. Magnetic tracing and basket trapping techniques were used to provide an insight into rates of movement, accumulation, initial motion criteria and routing, of sand. Sand is transported selectively and is mobilised at between 11-22 Nm⁻². Deposition of sub 2mm material is prevalent on morphological high points (bars/riffle margins), although greatest quantities were routed through morphological lows. Freeze core evidence shows limited intragravel storage. Gravel tracer movements showed evidence of size selective entrainment overall, however hiding effects were also found to be evident at two scales; 30-50mm and 110-140mm (for riffles) and 20- 90mm and 11O-140mm for pool. Slope exponents for log-log relations between scaled grain size (D/D₅₀) versus dimensionless shear stress (Өc) of ≈-0.9 suggest that hiding strongly influences sediment transport. Stream power estimates from pgQs demonstrate a higher threshold for motion for gravel in pools (132 Wm⁻²) compared with riffles (127 Wm⁻²). Differences in initial motion criteria (8e) between riffles and pools were found to be significant (p<0.05), indicating pool sediments to be less mobile than riffle, despite pool sediments being less compact. Reduced mobility of pool bedload sediment results from clasts being sheltered by immobile lag gravel found in the pool. It appears therefore that mobility differences between riffles and pools, related to bed structure, does not explain riffle-pool maintenance on the Rede. Scaled travel distance (L/L₅₀s) for tracers in the reach as a whole showed a convex-up relationship with scaled grain size (D/D₅₀s), demonstrating that for tracer grains progressively coarser than the surrounding D₅₀ surface grains, travel distance drops off rapidly, whereas grains progressively finer than the surrounding clasts, travel further but at a less rapid rate. Furthermore, virtual velocity (V*) of tracer grains showed a positive dependence upon D/D₅₀s. Gravel tracer movement provided important insights into riffle-pool maintenance. Transfer of material through the Rede riffle-pool sequence appeared to be influenced by flow magnitude and duration. For low magnitude high frequency flows below 25% bankfull, intra-unit movement was found to predominate. Medium magnitude and frequency flows (up to 50% bankfull) appeared capable of inter-unit transport; scour from pool troughs and deposition on pool exit slopes I riffle heads, movement of material from riffles to bar edges and from bar to bar. For higher magnitude low frequency flows up to bankfull, there was less scour from pools, and a dominance of bar-to-bar sediment transfer. Limited evidence of sediment routing and deposition in pools suggest these to be scour / sediment source zones only, with supply originating from the bed and outer bank. These data demonstrate the importance of different flow magnitude and frequency in creating / maintaining different areas of the riffle-pool structure.

The plants and animals that inhabit river channels may act as zoogeomorphic agents affecting the nature and rates of sediment recruitment, transport and deposition. The impact of benthic-feeding fish, which disturb bed material sediments during their search for food, has received little attention, even though benthic feeding species are widespread in rivers and may collectively expend significant amounts of energy foraging across the bed. A series of experiments were conducted to investigate the impacts of benthic feeding fish on the structure and composition of gravel-bed river sediments, and the implications for bed material transport. An ex-situ experiment was conducted to investigate the impact of a benthic feeding fish (European Barbel Barbus barbus) on particle displacements, bed sediment structures, gravel entrainment and transport fluxes. In a laboratory flume, changes in bed surface topography were measured and grain displacements examined when an imbricated, water-worked bed of 5.6-16 mm gravels was exposed to feeding juvenile Barbel. For substrates that had been exposed to feeding fish and control substrates which had not, grain entrainment rates and bedload fluxes were measured under a moderate transport regime. On average, approximately 37% of the substrate, by area, was modified by foraging fish during a four-hour treatment period, resulting in increased microtopographic roughness and reduced particle imbrication. Structural changes caused by fish increased bed load flux by 60% under entrainment flows, whilst on average the total number of grains transported during the entrainment phase was 82% higher from substrates that had been disturbed by Barbel. An ex-situ experiment utilising Barbel and Chub Leuciscus cephalus extended this initial study by considering the role of fish size and species as controls of sediment disturbance by foraging. Increasing the size of Barbel had a significant effect on measured disturbance and bedload transport. Specifically, the area of disturbed substrate, foraging depth, microtopographic roughness and sediment structure all increased as functions of fish size, as did bedload flux and total transported mass. In a comparison of the foraging effects of like-sized Barbel and Chub 8-10 in length, Barbel foraged a larger area of the riverbed and had a greater impact on microtopographic roughness and sediment structure. Foraging by both species was associated with increased sediment transport, but the bed load flux after foraging by Barbel was 150% higher than that following foraging by Chub and the total transported mass of sediment was 98% greater. An in-situ experiment quantified the effects of foraging fish, primarily Cyprinids (specifically Barbel and Chub), on gravel-river bed sediment structures, surface grain-size distributions, sediment transport fluxes and grain entrainment in the River Idle, Nottinghamshire, UK. This was achieved by installing large experimental sediment trays seeded with food at typical densities. The experiments yielded data about 1) topographic and structural differences between pre- and post-feeding substrates using DEMs interpolated from laser scans, 2) modifications to surface and sub-surface grain-size distributions as a function of fish foraging and 3) differences in sediment entrainment from water-worked substrates exposed to feeding fish and control substrates, without fish. Small sections of the substrate trays were recovered in tact from the field and for substrates that had been exposed to feeding fish and control substrates which had not, grain entrainment rates and bedload fluxes were measured under a moderate transport regime in the laboratory. On average, approximately 74% of the substrate, by area, was modified by foraging fish during a twelve-hour period, resulting in increased microtopographic roughness and substrate coarsening which had significant implications for bed material transport during the steady entrainment flow. Together, results from these experiments indicate that by increasing surface microtopography, modifying the composition of fluvial substrates and undoing the naturally stable structures produced by water working, foraging can influence sediment transport dynamics, predominately by increasing the mobility of river bed materials. The implication of this result is that by influencing the quantity of available, transportable sediment and entrainment thresholds, benthic feeding may affect sediment transport fluxes in gravel-bed rivers. In addition, three discrete studies were performed alongside the core experiments described above. A quantitative examination of habitat conditions favoured by feeding Barbel was conducted in the River Idle (Nottinghamshire, UK) which served to supplement existing literature pertaining to Barbel ecology, and inform experimental design during the core experiments. Two further studies considered the potential importance of foraging as a zoogeomorphic activity in terms of spatial extent, at a variety of scales, thereby extending core experiments to larger spatial scales in-situ.

Yes / This work presents new water surface elevation data including evidence of the spatial correlation of water surface waves generated in shallow water flows over a gravel bed without appreciable bed forms. Careful laboratory experiments have shown that these water surface waves are not well-known gravity or capillary waves but are caused by a different physical phenomenon. In the flow conditions studied, the shear present in shallow flows generates flow structures, which rise and impact on the water-air interface. It is shown that the spatial correlation function observed for these water surface waves can be approximated by the following analytical expression W(rho) = e(-rho 2/2 sigma w2)COS(2 pi L-0(-1)rho). The proposed approximation depends on the spatial correlation radius, sigma(w), characteristic spatial period, L-0, and spatial lag, . This approximation holds for all the hydraulic conditions examined in this study. It is shown that L-0 relates to the depth-averaged flow velocity and carries information on the shape of the vertical velocity profile and bed roughness. It is also shown that sigma(w) is related to the hydraulic roughness and the flow Reynolds number.

Au cours du XXe siècle, les lits de nombreuses rivières ont été sujets à l'installation de végétation alluviale. Dans le cas des rivières aménagées, cette tendance est souvent associée à des altérations géomorphologiques directes (extractions de granulats, endiguements, etc.) ainsi qu'à des modifications anthropiques de leur régime hydrologique et sédimentaire conduisant à une stabilisation du lit qui permet l'installation de la végétation. Cette végétation augmente le risque d'inondation en diminuant les vitesses d'écoulement et en augmentant les niveaux d'eau en crue. Par ailleurs la biodiversité est dégradée par la diminution des habitats pionniers caractéristiques de ces environnements. Manipuler artificiellement le régime hydrologique d'une manière qui pourrait limiter l'installation de végétation sur les bancs est une option considérée par les gestionnaires. Dans ce contexte, ce projet de thèse a pour objectif de comprendre les impacts des crues d'amplitude variable sur la destruction de végétation, et d'identifier les mécanismes associés. Le site d'étude sur lequel cette thèse se focalise est l'Isère en Combe de Savoie, une rivière à galets très aménagée des Alpes françaises.Dans le cadre de cette étude, la destruction de végétation a été étudiée à l'échelle du tronçon à partir d'une analyse des données hydrologiques, des photos aériennes, et des données topographiques disponibles pour la période 1996-2015. À l'échelle du banc, un suivi de terrain avant et après les événements hydrologiques marquants entre avril 2014 et septembre 2015 nous a permis d'étudier l'action des crues sur la mobilité sédimentaire et sur la végétation. Ces observations ont été complétées par une modélisation numérique bidimensionnelle de l'écoulement en crue.À l'échelle du tronçon (20 km), nous avons trouvé une corrélation très forte entre les volumes d'eau ayant transité dans le chenal sur une période donnée, et la destruction de végétation associée au cours de la période 1996-2015. Les débits associés à des temps de retour infra-annuels semblent permettre la destruction de végétation. Le mécanisme de destruction le plus efficace que l'on observe à cette échelle est l'érosion latérale ; les mécanismes prenant place à la surface des bancs sont très minoritaires. Cependant, les surfaces détruites sont modestes ; 3,4 % de la surface végétalisée est détruite annuellement en moyenne. À l'échelle du banc, la période de suivi de terrain a couvert une série de crues fréquentes (temps de retour < 1 an) et une crue de temps de retour 10 ans. Seule cette crue a partiellement détruit la végétation pionnière sur les bancs suivis. La destruction de végétation ligneuse jeune a eu lieu par le biais de quatre mécanismes : 1) déracinement par érosion de surface supérieure à 20 cm, 2) enfouissement sous une couche de sédiment grossiers supérieure à 30 cm, 3) déracinement par une combinaison d'érosion et de dépôt, et 4) érosion latérale en marge des bancs. La destruction de végétation est toujours associée à une mobilité sédimentaire importante.Ces résultats montrent qu'une crue très importante est nécessaire pour détruire la végétation par la mobilisation de la surface des bancs sur ce site. Par contraste, les débits forts mais non exceptionnels (temps de retour infra-annuel) sont en mesure de détruire la végétation par érosion latérale. Dans le cas de l'Isère en Combe de Savoie, il semble que l'utilisation de crues artificielles ne peut pas seule permettre de maintenir la largeur inter-digues libre de végétation. Pour la suite, on propose de s'intéresser à la destruction de végétation dans le contexte de la dynamique des bancs alternés plus ou moins végétalisés, en prenant en compte les apports et le transport des sédiments en plus de l'hydrologie. / Many rivers worldwide have seen vegetation establish within their beds throughout the 20th century. In the case of managed rivers, this trend is usually linked to direct geomorphological alterations (sediment mining, diking, etc.) as well as anthropic alterations of flow regime and sediment supply. These pressures have stabilized river beds, allowing vegetation to establish permanently. This vegetation increases the risk of flooding by decreasing flow velocities and increasing water levels. In addition, the associated reduction in availability of pioneer habitats characteristic of these stabilized environments typically degrades biodiversity. Managing hydrology in a way that would limit vegetation establishment on bars presents an interesting management option. In this context, our study was aimed at understanding the impacts of floods of varying magnitude on vegetation removal, as well as identifying and quantifying the underlying mechanisms. This work focused on the Isère River, a heavily managed gravel bed river located in the western French Alps.Vegetation removal was studied at the reach scale using hydrological data, aerial photographs, and topographic data available between 1996 and 2015. At the bar scale, field monitoring before and after floods from april 2014 to september 2015 allowed us to document the impact of floods on sediment mobility and vegetation. A 2D numerical model was used to document fine scale hydraulics.At the reach scale, we found a strong correlation between water volume flowing through the river channel and the amount of vegetation removal. Discharges with return intervals of less than one year seem to have an impact on vegetation removal. The main mechanism observed from aerial photographs was lateral erosion; surface processes were negligible in comparison. However, global vegetation removal was modest: since 1996, on average 3,4 % of vegetated area was removed annually. At the bar scale, our study period permitted monitoring of a series of high frequency floods (return interval < 1 year) and a 10-year food event. Only the largest flood partially removed pioneer vegetation from bars. Young vegetation removal occurred through four different mechanisms: 1) uprooting by surface scour > 20 cm, 2) burial under a thick layer of coarse sediments > 30 cm, 3) uprooting by a combination of surface scour and sediment deposition resulting in no net topographic change, and 4) lateral erosion of bars. Vegetation removal was always associated with significant sediment mobility.We conclude that on the Isere River a very important flood is required to remove vegetation by mobilizing bar surfaces. In contrast, high but not exceptional flows (return interval < 1 an) are capable of removing vegetation through lateral erosion. However, artificial floods alone are unlikely to maintain the full width of the channelized bed of the Isere River free of vegetation. In the future, vegetation removal needs to be studied in the context of alternate bar dynamics with or without vegetation. It seems necessary to consider sediment transport as well as hydrology to understand the overall dynamics of the bed.

xviii, 128 p. : ill. (some col.) / This dissertation investigates the potential impacts of canals and small dams on gravel-bed rivers and methods for documenting those impacts. First, I evaluate the potential for mapping channel depths along the McKenzie River, OR, using 10 cm resolution optical aerial imagery with a hydraulically-assisted bathymetry (HAB-2) model. Results demonstrate that channel depths can be accurately mapped in many areas, with some imagery limitations. The HAB-2 model works well in the majority of the river (R 2 =0.89) when comparing modeled to observed depths, but not in areas of shadow, surface turbulence, or depths >1.5 m. Next, I analyze the relative effects of a small dam and two diversion canals on sediment distribution along bars of the lower McKenzie River. The typical pattern of downstream fining is disrupted at each feature and several tributaries, particularly in the "reduced water reaches" below canal outtakes. Most modeled discharge values necessary to mobilize bar sediments fall at or below the 2-year flood return interval, with the remaining at or below the 5-year flood return interval, generally reflecting the D 50 values at each bar (20-115 mm). The third analysis investigates the potential to document geomorphic impacts of small dams in Oregon at ecoregion extents using air photos and publically available data sets. This analysis highlights data disparity with respect to the collecting agency's mission and the difficulty of using remote sensing for small dams. Though the imagery was not useful in evaluating small dam impacts due to resolution and feature size, the data were useful in mapping the small dam distribution across Oregon and each ecoregion. Sixty-one percent of Oregon land is located in the catchment of at least one small dam and the greatest number of dams per area is in the Willamette Valley ecoregion. Overall, this research suggests that, while the application of these techniques must be improved, our ability to observe, study, and understand rivers is enhanced by remote sensing advancements and the combined use of these methods in river restoration and management. This dissertation includes previously published and co-authored material. / Committee in charge: W. Andrew Marcus, Chairperson; Patricia F. McDowell, Member; Dan G. Gavin, Member; David W. Hulse, Outside Member