David Sear | University of Southampton (original) (raw)

Papers by David Sear

Water and Environment Journal, Feb 1, 1998

This paper reviews the contribution that can be made to land-drainage and flood-defence practice ... more This paper reviews the contribution that can be made to land-drainage and flood-defence practice by consideration of the sediment transport system of river catchments. It also outlines the problems posed by not monitoring sediment transport over wider and longer time and space scales than are conventionally adopted. The contribution made by geomorphology to this aspect of river management is identified, and examples of its application to flood-defence problems are given.

Quaternary Science Reviews, Apr 1, 2022

Hydroclimate on 'Uvea (Wallis et Futuna) is controlled by rainfall associated with the South Paci... more Hydroclimate on 'Uvea (Wallis et Futuna) is controlled by rainfall associated with the South Pacific Convergence Zone (SPCZ), the southern hemisphere's largest precipitation feature. To extend the short observational precipitation record, the hydrogen isotopic composition of the algal lipid biomarker dinosterol (d 2 Hdinosterol) was measured in sediment cores from two volcanic crater lakes on 'Uvea. The modern lakes differ morphologically and chemically but both contain freshwater within the photic zone, support phytoplankton communities inclusive of dinosterol-producing dinoflagellates, and experience identical climate conditions. d 2 Hdinosterol values track lake water isotope ratios, ultimately controlled in the tropics by precipitation amount and evaporative enrichment. However, in 88-m-deep Lac Lalolalo a steadily decreasing trend in sedimentary d 2 Hdinosterol values from −227 ‰ around year 988 CE to modern values as low as −303 ‰, suggests this lake's evolution from an active volcanic setting to the present system strongly influenced d 2 Hdinosterol values. Although current hydrology and water isotope systematics may now reflect precipitation and evaporation in this lake, the interaction between these processes and large changes in basin morphology, geochemistry, and hydrology obstruct the recovery of a climate signal from Lac Lalolalo's sedimentary d 2 Hdinosterol records. This work emphasizes the importance of site replication and the use of complementary climate reconstruction tools, especially when using molecular proxies that may be sensitive to more than one environmental parameter. Contrary to its neighbor, duplicate d 2 Hdinosterol records from 23-m-deep Lac Lanutavake varied between −277 ‰ and −297 ‰ and indicate slightly drier conditions during the timeperiod known as the Medieval Climate Anomaly (MCA, 950-1250 CE). The d 2 Hdinosterol signal in Lac Lanutavake was muted compared to published records from 'Upolu (Samoa) and Efate (Vanuatu) indicating that 'Uvea's location is not as sensitive to precipitation variability at sites farther from the SPCZ central axis. Lithogenic runoff proxies combined with d 2 Hdinosterol support the interpretation of a relatively dry MCA on 'Uvea, 'Upolu, and Efate, potentially due to less intense precipitation, a contracted, or a more zonally oriented SPCZ. 2013)). Lipid biomarker and runoff paleoprecipitation proxies stored in sedimentary archives from lakes (Konecky et al., 2013; Sear et al., 2020) and marine settings (Tierney et al., 2010; Toomey et al., 2016) in and near the SPCZ offer an opportunity to extend the length of records in this region and quantify the longer timescale variability in precipitation. Theoretical explanations for observed modern SPCZ precipitation variability invoke ocean-atmosphere forcings (Brown et al. 2020). However, recent attempts to synthesize Pacific-basin paleoproxy data (e.g. Atwood et al., 2021; Higley et al., 2018; Sear et al., 2020) indicate the patterns and mechanisms of tropical precipitation variability are complex and/or that some paleohydrology reconstructions contain unknown biases. Additional records from the SPCZ region are needed to replicate existing paleoreconstructions and to clarify how and why the SPCZ varied in the past. Tropical hydroclimate reconstructions using lipid biomarker hydrogen isotope ratios rely on the link between fluxes of water though the hydrologic cycle and resulting changes in water isotopic composition (Sachse et al., 2012). In the maritime tropics, the isotopic composition of precipitation largely follows precipitation rates where higher regional precipitation rates result in isotopically depleted rain (Aggarwal et al., 2016; Conroy et al., 2013; Dansgaard, 1964; Kurita, 2013). Surface waters, including lakes that accumulate valuable sedimentary archives, primarily reflect precipitation isotope ratios; imprinted on top of the precipitation signal are the effects of evaporation and non-rain contributions (i.e., stream, groundwater, hydrothermal, or seawater) (Bowen, 2010; Bowen et al., 2019). Phytoplankton lipids, biomolecules that make up cellular membranes and energy stores, have hydrogen isotope (2 H/ 1 H) ratios that are set by their environmental water (Englebrecht and Sachs, 2005; Maloney et al., 2019; Zhang and Sachs, 2007), and the accumulation and preservation of these lipids in lake sediment offers a tool for reconstructing past hydrology. The dinoflagellate lipid biomarker dinosterol (4a, 23, 24-trimethyl-5a-cholest-22E-en-3β-ol) was targeted for this study since it is highly source-specific and produced by only a limited number of taxa (Volkman, 2005; Volkman et al., 1998, 1993). The hydrogen isotopic composition of this lipid biomarker is expressed as δ 2 Hdinosterol (δ 2 H = [(2 H/ 1 H)sample/(2 H/ 1 H)VSMOW]-1, VSMOW is Vienna Standard Mean Ocean Water). The δ 2 Hdinosterol proxy has enhanced our understanding of late Holocene climate in brackish lakes in Palau (Richey and Sachs, 2016; Sachs et al., 2018; Smittenberg et al., 2011), brackish (Nelson and Sachs, 2016) and freshwater (Atwood and Sachs, 2014) lakes in the Galápagos Islands, and a freshwater lake on Washington Island in the Northern Line Islands (Sachs et al., 2021). An earlier study established that δ 2 Hdinosterol values in modern lake sediments throughout the SPCZ region are well correlated with spatial variability in precipitation rates (Maloney et al., 2019), offering an opportunity to make quantitative estimates of precipitation changes in this region; an approach that was recently applied to freshwater SPCZ lakes in Samoa and Vanuatu (Sear et al., 2020) and explored in more detail here. This work presents new δ 2 Hdinosterol records (Table 1) from two lakes on 'Uvea (Wallis et Futuna) in the western tropical Pacific to understand how central SPCZ precipitation varied during the late Holocene. Duplicate cores from each lake allow for intra-and inter-lake comparisons and provide important lessons on how unique lake environments influence δ 2 Hdinosterol values. We also present records of lithogenic runoff proxies from 'Uvea, Lake Emaotul (Efate, Vanuatu), and higher resolution records from those published from Lake Lanoto'o ('Upolu, Samoa) (Gosling et al., 2020), that complement the δ 2 Hdinosterol records in the same cores, aiding interpretations of precipitation change within the SPCZ.

Aquatic Conservation-marine and Freshwater Ecosystems, 2009

1. The assessment of links between ecology and physical habitat has become a major issue in river... more 1. The assessment of links between ecology and physical habitat has become a major issue in river research and management. Key drivers include concerns about the conservation implications of human modifications (e.g. abstraction, climate change) and the explicit need to understand the ecological importance of hydromorphology as prescribed by the EU's Water Framework Directive. Efforts are focusing on the need to develop 'ecohydromorphology' at the interface between ecology, hydrology and fluvial geomorphology. Here, the scope of this emerging field is defined, some research and development issues are suggested, and a path for development is sketched out. 2. In the short term, major research priorities are to use existing literature or data better to identify patterns among organisms, ecological functions and river hydromorphological character. Another early priority is to identify model systems or organisms to act as research foci. In the medium term, the investigation of patternprocesses linkages, spatial structuring, scaling relationships and system dynamics will advance mechanistic understanding. The effects of climate change, abstraction and river regulation, eco-hydromorphic resistance/ resilience, and responses to environmental disturbances are likely to be management priorities. Large-scale catchment projects, in both rural and urban locations, should be promoted to concentrate collaborative efforts, to attract financial support and to raise the profile of eco-hydromorphology. 3. Eco-hydromorphological expertise is currently fragmented across the main contributory disciplines (ecology, hydrology, geomorphology, flood risk management, civil engineering), potentially restricting research and development. This is paradoxical given the shared vision across these fields for effective river management based on good science with social impact. A range of approaches is advocated to build sufficient, integrated capacity that will deliver science of real management value over the coming decades.

Sear, DA, Grieg, SM and Carling, PA (2004) Fine sediment accumulation in spawning gravels and the... more Sear, DA, Grieg, SM and Carling, PA (2004) Fine sediment accumulation in spawning gravels and the effects on interstitial flow. In, de la Diego García, JL and Pilar, VM (eds.) Fifth International Symposium on Ecohydraulics. Madrid, Spain, International Association of Hydraulic Engineering and Research, Universidad Politécnica de Madrid,, 808-812. ... Full text not available from this repository. ... RDF+N-Triples, RDF+N3, RDF+XML, Browse.

Hydrobiologia, Aug 1, 2005

A refinement to the conductiometric standpipe method for determining interstitial flow velocities... more A refinement to the conductiometric standpipe method for determining interstitial flow velocities is described. Three modifications to the original calibration are presented: (i) development of calibration curves for gravels of varying permeability; (ii) statistical validation of a practicable field run time; and (iii) integration of zero velocity flow data to the calibration procedure. These modifications are shown to improve the conductiometric probe's ability to delineate interstitial flow velocities considered critical to salmonid incubation success. Field deployment of the probe highlighted its practical application for determining interstitial flow velocities in salmonid spawning gravels.

International Journal of Geographical Information Science, Jul 1, 1997

The modelling of environmental processes based on catchment elevation surfaces is well-establishe... more The modelling of environmental processes based on catchment elevation surfaces is well-established, but the use of TIN and grid surface models as three-dimensional representations of river channel topography at the reach scale is much less common. In this paper, surface modelling facilities in ARC/INFO GIS have been used to model the geometry of seven short sections of gravel-bed river channels in upland Britain from field surveys taken in 1976 and 1994. The methods used in converting field survey data and ...

River Research and Applications, 2017

The NERC and CEH trademarks and logos ('the Trademarks') are registered trademarks of NERC in the... more The NERC and CEH trademarks and logos ('the Trademarks') are registered trademarks of NERC in the UK and other countries, and may not be used without the prior written consent of the Trademark owner.

Science of The Total Environment, 2016

Understanding the controls on deposited fine sediment in the streams of agricultural catchments.

Aquatic Conservation-marine and Freshwater Ecosystems, 1998

Skip to Main Content. ...

Science of The Total Environment, Jul 1, 2003

Rivers integrate the impacts of change in atmospheric and terrestrial systems; they then deliver ... more Rivers integrate the impacts of change in atmospheric and terrestrial systems; they then deliver these to the coast. En route geomorphological processes create dynamic and diverse habitats, both in-stream and in riparianyfloodplain ecotones. The dynamics of channel change conflict with human resource development, the outcome is that many river and riparian environments have been significantly modified, complicating the interpretation of change. Collection of geomorphological data on both form and process has to date been overwhelmingly an academic pursuit; standard measurement networks and long-term monitoring have, as a result been largely absent-as in the Environmental Change Network (ECN), despite the emerging requirements of legislation such as the EU Water Framework Directive. In this paper, we utilise a unique set of repeat channel surveys and long-term bed-load sediment yields to provide guidance on both definitions of change and those variables and survey techniques which might form the basis, in future, of improved national-scale monitoring. The Environment Agency's River Habitat Surveys suggest the basis for channel typologies that could structure a sampling framework and rationalise the variables to be monitored. We also point to the value of more detailed geomorphological procedures in use at the catchmentyproject scale-Catchment Baseline Surveys and Fluvial Audits-as a standardised basis for monitoring the detail of change in the fluvial sediment system. A perfect opportunity to lay foundations for such monitoring activity has been provided in England and Wales by the winter floods of 2000y2001.

Aquatic Conservation-marine and Freshwater Ecosystems, Jul 1, 1998

2. The River Habitat Survey (RHS) database contains much geomorphological information which has b... more 2. The River Habitat Survey (RHS) database contains much geomorphological information which has been statistically analysed for those semi-natural sites surveyed in 1994. Additional geomorphological data have been gathered in an attempt to make the resulting typology &amp;#x27;dynamic&amp;#x27;, ie capable of predicting channel stability, an important component of the information needed for sustainable river management. ... 3. The outcome of TWINSPAN and redundancy analyses is disappointing in statistical terms and does not, therefore, constitute an objective taxonomy. However, ...

River Research and Applications, Dec 1, 2004

Pool-riffle installation is increasingly becoming the standard form of river habitat enhancement ... more Pool-riffle installation is increasingly becoming the standard form of river habitat enhancement undertaken, largely for the benefit of fisheries. This study documents the effect of riffle installation on the morphological and hydraulic diversity of a low gradient engineered river. Despite their prevalence there have to date been few published studies of the impacts of these features on channel hydraulics, despite concerns as to their potential impact on flood levels. In this paper the impacts of the installation of gravel bedforms on water surface elevations and flow resistance are considered. The performance of the rifflepool sequences is assessed against a set of criteria derived from the scientific literature. The analysis reveals that the gravel bedforms do display the hydraulic functionality associated with natural pool-riffle sequences. At bankfull discharge, water surface elevation is not significantly increased over those existing prior to installation, and physical habitat is shown to be more diverse following rehabilitation. The stability and appropriate classification of the gravel bedforms created in the scheme are discussed, together with the implications for floodplain and river rehabilitation in general.

Regulated rivers, Dec 1, 1994

Earth Surface Processes and Landforms, Nov 1, 1995

This paper addresses the role that fluvial geomorphology might play in the management of sediment... more This paper addresses the role that fluvial geomorphology might play in the management of sediment-related river maintenance in the U.K. Sediment-related river maintenance refers to the operational requirement of river management authorities to remove deposits of sediment or protect river boundaries from erosion, where these compromise the flood defence levels of service. Using data collected as part of a National Rivers Authority (NRA) Research and Development Project it is possible to identify the geomorphic causes of problems, and engineering responses to sediment-related river maintenance (SRRM) in England and Wales. The Project identified the management problem as widespread and often treated in isolation from the causative processes. Geomorphological guidance is shown to be both relevant and complementary to conventional engineering practice through its ability to identify the cause of a SRRM problem. A methodology for conducting a geomorphological survey, or 'fluvial audit', is presented, which synthesizes historical data on the catchment land-use and channel network, with contemporary morphological maps to present a statement of the location and type of sediment supply, transport and storage within the river basin under scrutiny. The application of geomorphology to two contrasting SRRM problems is explored using case studies from two catchments: the River Sence, a fine sediment system, and the Shelf Brook, a coarse sediment system.

Sear, DA, Thorne, CR and Newson, MD (2004) Guidebook of applied fluvial geomorphology : Defra/Env... more Sear, DA, Thorne, CR and Newson, MD (2004) Guidebook of applied fluvial geomorphology : Defra/Environment Agency Flood and Coastal Defence R&D Programme, London, Defra Flood Management Division, 256pp. (R&D Technical Report FD1914). ... Full text not available from this repository. ... Official URL: http://www.defra.gov.uk/science/project_data/Docum... ... RDF+N-Triples, RDF+N3, RDF+XML, Browse.

Anthropocene Coasts

Small island developing States, such as those in the Pacific, are often prone to multiple hazards... more Small island developing States, such as those in the Pacific, are often prone to multiple hazards that have potential to result in disaster and / or restrict development. Hazard data can be limited in resolution or omitted in or near SIDS’ coasts, but a growing and improved range of datasets are becoming available. Through an analysis of approximately 100 policy documents on hazards and disaster risk management in Pacific island nations, we found: limited information on hazards and how they manifest to disasters at local levels, thus not fully connecting drivers and subsequent risk; at times a non-specific multi-hazard approach prompting the need to address more specific hazards; and restricted temporal and spatial scales of analysis that potentially limit continuity of actions where mitigation methods evolve. These limitations suggest that appropriate and timely high resolution hazard data is needed from the top-down to underpin the design and development of local disaster risk man...

The Holocene

Historical salmon catch records suggest that climatic variability, and more recently human exploi... more Historical salmon catch records suggest that climatic variability, and more recently human exploitation, control patterns of abundance in Atlantic salmon populations. We present the first long-term (2000-year) reconstruction of Atlantic Salmon population variations based on a Marine Derived Nutrient (MDN) lake sediment record. Our record is constructed from nitrogen isotopes (δ15N) measured from a lake sediment core, which we compare with an escapement index (EI) derived from historic net catch data on major Scottish salmon rivers. We used an isotope mixing model to demonstrate that the N isotope values are likely enriched with MDN and demonstrate that Loch Insh sediments are enriched compared with a control site (Loch Vaa) that has never had exposure to salmon. We demonstrate that current adult spawner returns are around half that of historic values prior to major human exploitation. Before the onset of widespread human exploitation and habitat degradation, large fluctuations in sa...

Journal of Archaeological Science, 2022