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David Sear

University of Southampton, School of Geography, Faculty Member

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I am a fluvial geomorphologist with interests in the interaction between geomorphology adn ecology, particularly salmonids, but also trees (wood in rivers) and riparian habitats. I also research on river restoration, sediment transport and morphodynamics. I use a range of approaches from field, flume to numerical modelling.

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Books by David Sear

River Restoration: Managing the uncertainty in restoring physical habitat

River Restoration: Managing the uncertainty in restoring physical habitat

Papers by David Sear

Exploring the Capability of Natural Flood Management Approaches in Groundwater-Dominated Chalk Streams

Water

This study aims to address the gap in the Natural Flood Management (NFM) evidence base concerning... more This study aims to address the gap in the Natural Flood Management (NFM) evidence base concerning its implementation potential in groundwater-dominated catchments. We generated a typology of 198 chalk catchments using redundancy analysis and hierarchical clustering. Three catchment typologies were identified: (1) large catchments, (2) headwater catchments with permeable soils, and (3) catchments with impermeable soils and surfaces (urban and suburban land uses). The literature suggests that natural flood management application is most effective for catchments <20 km2, reducing the likelihood of significant flood mitigation in large catchments. The relatively lower proportion of surface runoff and higher recharge in permeable catchments diminishes natural flood management’s likely efficacy. Impermeable catchments are most suited to natural flood management due to a wide variety of flow pathways, making the full suite of natural flood management interventions applicable. Detailed g...

X-ray computed tomography reveals that grain protrusion controls critical shear stress for entrainment of fluvial gravels

Geology, 2019

The critical shear stress (τc) for grain entrainment is a poorly constrained control on bedload t... more The critical shear stress (τc) for grain entrainment is a poorly constrained control on bedload transport rates in rivers. Direct calculations of τc have been hindered by the inability to measure the geometry of in situ grains; i.e., the shape and location of each grain relative to surrounding grains and the bed surface. We present the first complete suite of three-dimensional (3-D) grain geometry parameters for 1055 water-worked grains, and use these to parameterize a new 3-D grain entrainment model and hence estimate τc. The 3-D data were collected using X-ray computed tomography scanning of sediment samples extracted from a prototype scale flume experiment. We find that (1) parameters including pivot angle and proportional grain exposure do not vary systematically with relative grain size; (2) τc is primarily controlled by grain protrusion, not pivot angle; and (3) larger grains experience larger forces as a result of projecting higher into the flow profile, producing equal mobil...

The influence of geomorphology on large wood dynamics in a low gradient headwater stream

Water Resources Research, 2014

Where a licence is displayed above, please note the terms and conditions of the licence govern yo... more Where a licence is displayed above, please note the terms and conditions of the licence govern your use of this document. When citing, please reference the published version. Take down policy While the University of Birmingham exercises care and attention in making items available there are rare occasions when an item has been uploaded in error or has been deemed to be commercially or otherwise sensitive.

The effects of river restoration on catchment scale flood risk and flood hydrology

Earth Surface Processes and Landforms, 2016

The effects of river restoration on catchment scale flood risk and flood hydrology.

Spatial variations in surface sediment structure in riffle-pool sequences: a preliminary test of the Differential Sediment Entrainment Hypothesis (DSEH)

Earth Surface Processes and Landforms, 2012

Riffle-pool sequences are maintained through the preferential entrainment of sediment grains from... more Riffle-pool sequences are maintained through the preferential entrainment of sediment grains from pools rather than riffles. This preferential entrainment has been attributed to a reversal in the magnitude of velocity and shear stress under high flows; however the Differential Sediment Entrainment Hypothesis (DSEH) postulates that differential entrainment can instead result from spatial sedimentological contrasts. Here we use a novel suite of in-situ grain-scale field measurements from a riffle-pool sequence to parameterise a physically-based model of grain entrainment. Field measurements include pivoting angles, lift forces and high resolution DEMs acquired using Terrestrial Laser Scanning, from which particle exposure, protrusion and surface roughness were derived. The entrainment model results show that grains in pools have a lower critical entrainment shear stress than grains in either pool exits or riffles. This is because pool grains have looser packing, hence greater exposure and lower pivoting angles. Conversely, riffle and pool exit grains have denser packing, lower exposure and higher pivoting angles. A cohesive matrix further stabilises pool exit grains. The resulting predictions of critical entrainment shear stress for grains in different subunits are compared with spatial patterns of bed shear stress derived from a 2D Computational Fluid Dynamics (CFD) model of the reach. The CFD model predicts that, under bankfull conditions, pools experience lower shear stresses than riffles and pool exits. However, the difference in sediment entrainment shear stress is sufficiently large that sediment in pools is still more likely to be entrained than sediment in pool exits or riffles, resulting in differential entrainment under bankfull flows. Significantly, this differential entrainment does not require a reversal in flow velocities or shear stress, suggesting that sedimentological contrasts alone may be sufficient for the maintenance of riffle-pool sequences. This finding has implications 3 for the prediction of sediment transport and the morphological evolution of gravel-bed rivers.

Process-based Principles for Restoring River Ecosystems

BioScience, 2010

Process-based restoration aims to reestablish normative rates and magnitudes of physical, chemica... more Process-based restoration aims to reestablish normative rates and magnitudes of physical, chemical, and biological processes that sustain river and floodplain ecosystems. Ecosystem conditions at any site are governed by hierarchical regional, watershed, and reach-scale processes controlling hydrologic and sediment regimes; floodplain and aquatic habitat dynamics; and riparian and aquatic biota. We outline and illustrate four process-based principles that ensure river restoration will be guided toward sustainable actions: (1) restoration actions should address the root causes of degradation, (2) actions must be consistent with the physical and biological potential of the site, (3) actions should be at a scale commensurate with environmental problems, and (4) actions should have clearly articulated expected outcomes for ecosystem dynamics. Applying these principles will help avoid common pitfalls in river restoration, such as creating habitat types that are outside of a site's natural potential, attempting to build static habitats in dynamic environments, or constructing habitat features that are ultimately overwhelmed by unconsidered system drivers.

Untangling the controls on bedload transport in a wood‐loaded river with RFID tracers and linear mixed modelling

Earth Surface Processes and Landforms

Nichols, M. H., (2004) A radio frequency identification system for monitoring coarse sediment par... more

Annotated Bibliography from A restatement of the natural science evidence concerning catchment-based 'natural' flood management in the UK

A restatement of the natural science evidence concerning catchment-based "natural" floo... more

The Hyporheic Handbook

The views and statements expressed in this report are those of the author alone. The views or sta... more The views and statements expressed in this report are those of the author alone. The views or statements expressed in this publication do not necessarily represent the views of the Environment Agency and the Environment Agency cannot accept any responsibility for such views or statements. This report is printed on Cyclus Print, a 100% recycled stock, which is 100% post consumer waste and is totally chlorine free. Water used is treated and in most cases returned to source in better condition than removed.

Are equilibrium multichannel networks predictable? The case of the regulated Indus River, Pakistan

Geomorphology, 2018

Arguably, the current planform behaviour of the Indus River is broadly predictable. Between Chash... more Arguably, the current planform behaviour of the Indus River is broadly predictable. Between Chashma and Taunsa, Pakistan, the Indus is a 264-km-long multiple-channel reach. Remote sensing imagery, encompassing major floods in 2007 and 2010, shows that the Indus has a minimum of two and a maximum of nine channels, with on average four active channels during the dry season and five during the annual monsoon. Thus, the network structure, if not detailed planform, remains stable even for the record 2010 flood (27,100 m 3 s −1 ; recurrence interval N 100 years). Bankline recession is negligible for discharges less than a peak annual discharge of 6000 m 3 s −1 (~80% of mean annual flood). The Maximum Flow Efficiency (MFE) principle demonstrates that the channel network is insensitive to the monsoon floods, which typically peak at 13,200 m 3 s −1. Rather, the network is in near-equilibrium with the mean annual flood (7530 m 3 s −1). The MFE principle indicates that stable networks have three to four channels, thus the observed stability in the number of active channels accords with the presence of a near-equilibrium reach-scale channel network. Insensitivity to the annual hydrological cycle demonstrates that the timescale for network adjustment is much longer than the timescale of the monsoon hydrograph, with the annual excess water being stored on floodplains rather than being conveyed in an enlarged channel network. The analysis explains the lack of significant channel adjustment following the largest flood in 40 years and the extensive Indus flooding experienced on an annual basis, with its substantial impacts on the populace and agricultural production.

Section I Introduction: The Nature and Signifi cance of Uncertainty in River Restoration

... 105 NJ Clifford, MC Acreman and DJ Booker 8 Uncertainty Surrounding the Ecological Targets an... more

The Significance of Ecohydraulic and Geomorphic Uncertainties in River Restoration

1.1 The Quantifiable Continuum of Uncertainty. Notice that once uncertainties are acknowledged as... more 1.1 The Quantifiable Continuum of Uncertainty. Notice that once uncertainties are acknowledged as unquantified uncertainties, increased knowledge about the uncertainties will determine their position on the continuum.. .. .. 11 1.2 Typology for sources and degree of uncertainty. Adapted from Van Asselt's (2000) proposed typology for uncertainties in integrated assessment.. .. 12 1.3 Some potential relationships between knowledge and uncertainty through time. Contrary to the argument of the positivist, no unique inverse relationship between uncertainty and knowledge exists. See Section ref{TYPOLOGYSEC 15 1.6 Five Philosophical Attitudes Towards Uncertainty. The Venn diagram is meant to illustrate the overlap between contemporary attitudes towards uncertainty. Note that ignoring uncertainty, shares no overlap with contemporary attitudes towards uncertainty.. 1.8 Cope With Uncertainty Strategy. The reductionist view of coping with individual sources of uncertainty. The end result of this strategy is always to cope with uncertainty. In terms of river management and restoration applications, applying the precautionary principle allows decisions to be made in the face of uncertainty and adaptive management allows iterative 2.1 Premise of Reducible Ignorance in Embracing Uncertainty Framework Context. The limited knowledge within the restoration community about uncertainties is highlighted here as reducible ignorance, which could be reduced through further research and education. The grayed out areas are the linkages to the rest of the 'embracing uncertainty' framework (see Figure 1.

Sources of sediment-bound organic matter infiltrating spawning gravels during the incubation and emergence life stages of salmonids

Agriculture, Ecosystems & Environment, 2014

The biodegradation of organic matter ingressing spawning gravels in rivers exerts an oxygen deman... more The biodegradation of organic matter ingressing spawning gravels in rivers exerts an oxygen demand which is believed to contribute to detrimental impacts on aquatic ecology including salmonids. Catchment management strategies therefore require reliable information on the key sources of sediment-bound organic matter. Accordingly, a novel source fingerprinting procedure based on analyses of bulk stable 13 C and 15 N isotope values and organic molecular structures detected using near infrared reflectance (NIR) spectroscopy was tested for assessing the primary sources of sediment-bound organic matter infiltrating artificial Atlantic salmon spawning redds in five rivers across England and Wales. Statistically-verified source fingerprints were identified using a combination of the Kruskal-Wallis Htest, principal component analysis and GA-driven discriminant function analysis. Interstitial sediment samples were obtained from artificial redds using retrievable basket traps inserted at the start of the salmonid spawning season and extracted subsequently in conjunction with critical juvenile phases (eyeing, hatch, emergence, late spawning) of fish development associated with incubation and emergence. Over the duration of these four basket extractions, the overall relative frequency-weighted average median source contributions to the interstitial sediment-bound organic matter sampled in the study rivers ranged between 26% (full uncertainty range 0-100%) and 44% (full uncertainty range 0-100%) for farm yard manures/slurries, 11% (full uncertainty range 0-75%) and 48% (full uncertainty range 0-99%) for damaged road verges, 16% (full uncertainty range (0-78%) and 52% (full uncertainty range (0-100%) for decaying instream vegetation and 4% (full uncertainty range 0-31%) and 10% (full uncertainty range (0-44%) for human septic waste. The results of mass conservation tests suggest that the procedure combining bulk 13 C and 15 N isotope values and NIR spectroscopy data on organic molecular structures is sensitive to the risks of significant non-conservative tracer behaviour in the fluvial environment and will therefore not necessarily work at all in-channel sites in all catchments. 2014 Elsevier B.V. All rights reserved.

The Scope of Uncertainties in River Restoration

River Restoration

hope that it is not debilitating for the project at hand, or accept the uncertainty and use it as... more hope that it is not debilitating for the project at hand, or accept the uncertainty and use it as a feature of the research.' In this chapter the rich topic of uncertainty is presented in a broader, more generic context. This foundation is intended to help separate the sources and types of uncertainties that the various authors in this book present, and meanwhile unravel some of the ambiguities surrounding uncertainty in river restoration. As cautioned earlier, potentially signifi cant uncertainties are rarely recognised, much less explicitly dealt with in river restoration. Hence, a lexicon and typology for uncertainty is outlined fi rstly in this chapter. This is done to dispel the notion of a certain world with certain outcomes within the broader scope of types and sources of uncertainty. A return specifi cally to river restoration then follows to identify types of uncertainties using the above-mentioned typology. The tremendous diversity of river restoration in the context of uncertainties arising from restoration motives, notions and approaches are considered. A case will be made that a basic strategy for dealing with uncertainty is needed by the river restoration community to allow both the community and individual investigators or practitioners to: • explore the potential signifi cance (both in terms of unforeseen consequences and welcome surprises) or insignifi cance of uncertainties; • effectively communicate uncertainties; • eventually make adaptive, but transparent, decisions in the face of uncertainty.

The Impact of Fine Sediment on Macro-Invertebrates

River Research and Applications, 2011

The sustainable use of water resources requires clear guidelines for the management of diffuse po... more The sustainable use of water resources requires clear guidelines for the management of diffuse pollution inputs to rivers. Without informed guidelines, management decisions are unlikely to deliver cost-effective improvements in the quality of rivers as required by current water policy. Here, we review the evidence available for deriving improved guidelines on the loading of fine sediment to rivers based on the impact on macro-invertebrates. The relationship between macro-invertebrates and fine sediments is poorly defined. Studies of the impacts of fine sediment on macro-invertebrates have been undertaken at various scales, which has an influence on the range of responses displayed and the reliability of the results obtained; results obtained from investigations at smaller scales may not manifest at the scale required to manage rivers and vice versa. Many of the identified effects of increased loading of fine sediment on macro-invertebrates occur as a consequence of deposition on the river bed, yet many current management guidelines are based on suspended sediment targets. On this basis, existing water quality guidelines for sediment management are unlikely to be appropriate.

The Relationship Between Fine Sediment and Macrophytes in Rivers

River Research and Applications, 2011

The interplay between erosion and deposition are fundamental characteristics of river basins. The... more The interplay between erosion and deposition are fundamental characteristics of river basins. These processes result in the delivery, retention and conveyance of sediment through river systems. Although the delivery of sediment to rivers is a natural phenomenon, in recent years there has been increasing concern about the enhancement of sediment loadings as a result of anthropogenic activities. The presence of macrophytes in river channels tends to increase the retention of fine sediment leading to changes in bed composition. However, a complex relationship exists between macrophytes and fine sediment: macrophytes affect the conveyance of fine sediment and are, in turn, affected by the sediment loading. This review deals with these two reciprocal effects and, in particular, summarizes the available evidence base on the impact of fine sediment on macrophytes. Increased inputs of fine sediment appear to have both direct and indirect impacts on the macrophyte community, altering light availability, and the structure and quality of the river bed. The nature of these impacts depends largely on the rate of deposition and the nature of the material deposited. Changes in macrophyte community composition may ensue where the depositing material is more nutrient rich than the natural river bed. Many of the changes in macrophyte flora that occur with increased fine sediment inputs are likely to closely parallel those that occur with increased dissolved nutrient availability. If attempts to manage nutrient inputs to rivers are to achieve their goals, it is critical that fine sediment-associated nutrient dynamics and transfers are considered.

Linking geomorphic changes to salmonid habitat at a scale relevant to fish

River Research and Applications, 2010

The influence of geomorphic change on ecohydraulics has traditionally been difficult to quantify.... more The influence of geomorphic change on ecohydraulics has traditionally been difficult to quantify. With recent improvements in surveying technology, high-resolution, repeat and topographic surveys have become a common tool for estimating fluvial sediment budgets and documenting spatial patterns of net erosion and net deposition. Using a case study from a spawning habitat rehabilitation (SHR) project on California's Mokelumne River, some new DEM-differencing analytical tools and ecohydraulic models were used to test whether hypotheses about pool-riffle maintenance mechanisms used in designing SHR projects were producing self-sustaining spawning habitat when subjected to competent flows. Following peak flows associated with the spring snow-melt, a total of 999.6 m 3 of erosion and 810.1 m 3 of deposition were recorded throughout the study area, with a net loss of 196.2 m 3. Using an ecohydraulic spawning habitat suitability model to segregate the sediment budget, over 53% of the area in which gravel was placed in a 2005 SHR retained the same habitat quality characteristics, and 22% improved. The response to the flood was generically characterized by shallow deposition associated with areas of divergent flow over riffles and scour associated with areas of convergent flow in pools. Areas where habitat remained stable generally experienced only lowmagnitude elevation changes, and accounted for only 19.5% of the total volumetric change. Areas where habitat quality degraded (primarily pool exit slopes) were dominated by larger magnitude erosion and made up 46% of the total volumetric change. By contrast, areas where habitat quality improved (primarily constructed riffle) accounted for 34.5% the total volumetric change, and were dominated by shallow, low magnitude deposition. The results support hypotheses about pool-riffle maintenance mechanisms used to design the rehabilitation projects, while also highlighting some simple but powerful techniques for linking ecohydraulic and geomorphic field monitoring data at a salmon-relevant spatial scale.

Groundwater dominated rivers

Hydrological Processes, 1999

This paper explores the signi®cance of groundwater dominance in the surface water system through ... more This paper explores the signi®cance of groundwater dominance in the surface water system through a combination of review and an exposition of the general hydrology, ecology and geomorphology of rivers draining the main UK aquifers. Groundwater dominance is shown to vary according to the nature of the aquifer lithology, the mechanism of groundwater:surface water interaction and the scale at which one examines this interaction. Using data derived from a range of studies including the UK Environment Agency River Habitat Survey and the UK Institute of Freshwater Ecology RIVPACS invertebrate database it is shown that the nature of the aquifer and mode of in¯uent discharge strongly control the hydrological and ecological characteristics of the environment but that a speci®c groundwater ecology or hydrogeomorphology is masked by the overriding controls exerted by aquifer geology and catchment topography. Despite this, it is clear that river systems dominated by groundwater¯ows have speci®c hydrological characteristics and management issues that require holistic, multidisciplinary approaches that recognise the signi®cance of groundwater and the nature of the interaction with the surface water environment.