Chris Bradley

Simulation of the annual water table dynamics of a floodplain wetland, Narborough Bog, UK

Abstract This paper describes the simulation of a succession of annual hydro-periods describing water table variations in a British floodplain wetland, Narborough Bog. This is achieved through the application of a model, developed using MODFLOW, which requires inputs of weekly precipitation, evapotranspiration and river stage. The model reproduces the observed temporal variation in the wetland water tables illustrating the range in water table conditions experienced by the wetland, their relationship to hydrometeorological conditions, and the seasonal and annual variability in the wetland water budget. The model also estimates defuse water fluxes such as seepage flows to and from the adjacent river. The model simulations demonstrate the importance of successful representation of the stratigraphy of alluvial and organic deposits within the wetland. Model performance is evaluated for 2 years: 1991 and 1992 during a period of field instrumentation. The model is then used to reproduce the wetland hydroperiod for further 6 years: 1979, 1980, 1982, 1983, 1987 and 1988 when local data for precipitation, evapotranspiration and river stage are available. The results indicate how the wetland responds to a combination of hydrological events and emphasise the importance of precipitation—both weekly totals and its annual distribution. The model simulations reveal the importance of the water storage function of the wetland and indicate the varying relationship of the wetland to the lowland river: at low flows the river represents a base level to which the wetland water table grades. Conversely, for small periods of time wetland water tables are elevated by the effects of a marginal silt–clay levee adjacent to the riverbank. Conditions of influent seepage from river to wetland were experienced only on isolated occasions. Such results demonstrate the benefits of floodplain wetlands in moderating extremes of river flow and demonstrating the need for rivers and floodplain wetlands to remain closely integrated.

Transient modelling of water-table variation in a floodplain wetland, Narborough Bog, Leicestershire

This paper describes the application of a transient three-dimensional groundwater model to simulate water flux through a floodplain wetland, Narborough Bog, in Central England. The results of a programme of water-table monitoring are summarised to indicate the extent of water-table fluctuation, and experiments to determine hydraulic flow parameters are detailed. Hydraulic conductivities of deposits are highly variable; observations were in the range 1.16 × 10−3–4.63 × 10−2 cm s−1 for herbaceous peat, 1.16 10−4 cm s−1 for wood peat, and 9.14 × 10−6–3.36 × 10−8 cm s−1 for silty clay. A three-layer groundwater model of the wetland is developed using MODFLOW from these initial values, and with stratigraphic data obtained by hand augering. The model is calibrated using data for two periods of recharge from precipitation by adjusting specific yield and vertical hydraulic conductivity within prescribed limits. The accuracy of the model is then assessed by comparing daily model predictions of water-table response to potential evapotranspiration at specific monitoring points. Model, results demonstrate the importance of variation in hydraulic characteristics with depth in wetlands, thus emphasizing the dependence of water-table response upon water-table position, and preceding moisture conditions. The resulting water-table configuration reflects the balance between horizontal and lateral water flow through the wetland, and its relationship to river stage. The success of the stratigraphic representation is considered and its general applicability evaluated. Finally, the use of the calibrated model for local hydrological management is considered, and the effects on the wetland water budget of varying river stage by specified amounts are examined.

Hydrological monitoring and surveillance for wetland conservation and management; a UK perspective

Abstract Wetlands are under threat from a variety of human-induced changes to their hydrology. To alleviate these threats, and restore wetlands degraded by past human activity or to enhance biodiversity, an understanding of wetland hydrology and hydrogeology, water level management and monitoring of change has become essential. However, many wetland managers have an inadequate knowledge to design effective hydrological programmes or to commission investigations that may be appropriate to the goal of conserving or restoring a specific wetland. The purpose of this paper is to analyse the hydrological and hydrogeological data required for surveillance and to monitor change to provide a basis for on-site management and assessment of the consequences of activities within the catchment area of the wetland on wetland hydrology and hence ultimately ecology. It is timely given international efforts under the Ramsar convention in the area of environmental risk assessment and standardisation of monitoring and surveillance. It highlights what hydrological and hydrogeological information can be extracted from differing types of monitoring. This will permit wetland managers to make rational decisions concerning requirements for instrumentation and wetland monitoring and the type of investigation to commission when hydrological studies are needed for consideration of on-site management alternatives in the face of a potential hydrological threat. The paper illustrates that a range of instrumentation, techniques and methods exist for monitoring and developing an understanding of a wetland sites hydrology. Choice of the most appropriate method is dependent upon the purpose to which the data is to be put, financial resources, availability of manual labour for recording and the characteristics of the wetland site.

Challenges of river basin management: Current status of, and prospects for, the River Danube from a river engineering perspective.

In the Danube River Basin multiple pressures affect the river system as a consequence of river engineering works, altering both the river hydrodynamics and morphodynamics. The main objective of this paper is to identify the effects of hydropower development, flood protection and engineering works for navigation on the Danube and to examine specific impacts of these developments on sediment transport and river morphology. Whereas impoundments are characterised by deposition and an excess of sediment with remobilisation of fine sediments during severe floods, the remaining five free flowing sections of the Danube are experiencing river bed erosion of the order of several centimetres per year. Besides the effect of interruption of the sediment continuum, river bed degradation is caused by an increase in the sediment transport capacity following an increase in slope, a reduction of river bed width due to canalisation, prohibition of bank erosion by riprap or regressive erosion following base level lowering by flood protection measures and sediment dredging. As a consequence, the groundwater table is lowered, side-arms are disconnected, instream structures are lost and habitat quality deteriorates affecting the ecological status of valuable floodplains. The lack of sediments, together with cutting off meanders, leads also to erosion of the bed of main arms in the Danube Delta and coastal erosion. This paper details the causes and effects of river engineering measures and hydromorphological changes for the Danube. It highlights the importance of adopting a basin-wide holistic approach to river management and demonstrates that past management in the basin has been characterised by a lack of integration. To-date insufficient attention has been paid to the wide-ranging impacts of river engineering works throughout the basin: from the basin headwaters to the Danube Delta, on the Black Sea coast. This highlights the importance of new initiatives that seek to advance knowledge exchange and knowledge transfer within the basin to reach the goal of integrated basin management.

Saturated and unsaturated flow dynamics in a floodplain wetland

This paper describes initial work modelling unsaturated processes within floodplain wetlands. The limitations of solely considering the dynamics of the wetland water table and hence saturated processes are examined. Water table, precipitation and evapotranspiration data from Narborough Bog, a degraded floodplain wetland of 10 ha in Leicestershire, were used in the development of a one-dimensional saturated–unsaturated flow model using UNSAT1. Hydraulic parameters were derived from a combination of field and laboratory analysis of wetland deposits previously undertaken to develop a saturated flow model of the wetland. Additional parameters describing unsaturated flow were derived by a mathematical approximation based upon the soil moisture characteristic of representative wetland sediments. The resulting model is used to explore the interaction between water table position, vertical soil moisture fluxes and evapotranspiration. Model simulations are completed over a 25-day period, and used to indicate the relationship of moisture content to key hydraulic properties of soil profiles typical of the wetland and illustrate the way in which this varies laterally along a transect. The results highlight the need to consider the importance of correct description of the wetland substrate and, notwithstanding difficulties in describing unsaturated flow parameters, there is a considerable need to improve the description of near-surface wetland water fluxes and their expansion to two dimensions. Copyright

To what extent can portable fluorescence spectroscopy be used in the real-time assessment of microbial water quality?

The intrinsic fluorescence of aquatic organic matter emitted at 350 nm when excited at 280 nm correlates widely with water quality parameters such as biochemical oxygen demand. Hence, in sewage-impacted rivers and groundwater, it might be expected that fluorescence at these wavelengths will also correlate with the microbial water quality. In this paper we use a portable fluorimeter to assess the relationship between fluorescence intensity at this wavelength pair and Escherichia coli enumeration in contrasting river catchments of poor water quality: in KwaZulu-Natal, S. Africa and the West Midlands, UK. Across all catchments we demonstrate a log correlation (r = 0.74) between fluorescence intensity and E. coli over a seven-log range in E. coli enumerations on non-perturbed (unfiltered) samples. Within specific catchments, the relationship between fluorescence intensity and E. coli is more variable, demonstrating the importance of catchment-specific interference. Our research demonstrates the potential of using a portable fluorimeter as an initial screening tool for indicative microbial water quality, and one that is ideally suited to simple pollution scenarios such as assessing the impact of faecal contamination in river or groundwater at specific sites.

DYNAMICS OF WATER MOVEMENT AND TRENDS IN DISSOLVED CARBON IN A HEADWATER WETLAND IN A PERMEABLE CATCHMENT

We examined trends in the movement and source of water in a headwater wetland in North Shropshire, UK. Six piezometer nests along two transects were monitored over an 18 month period, and flownets were derived to estimate the rate and direction of water movement through the wetland and the interaction between precipitation and groundwater discharge. Individual water sources are identified using stable isotopes and seasonal differences in the composition of wetland soil-water are described. Variations in dissolved inorganic and organic carbon (DIC and DOC) were measured in water samples collected from discrete points in the wetland and the adjacent river and were interpreted using the hydrological data. The results suggest that end-members for DOC and DIC can be identified across the range of sampling sites: a groundwater spring (GS) had the lowest DOC and high DIC (DOC = 5.6 ± 4.5 mg/l; DIC = 36.7 ± 4.6 mg/l); a shallow well (WS) had the highest DOC and DIC (DOC = 32.5 ± 18.7 mg/l; DIC = 61.9 ± 18.9 mg/l); while surface-water (WSW) had the lowest DIC (20.6 ± 12.1 mg/l). Water fluxes between the wetland and river are estimated using the Dupuit-Forcheimer approximation to highlight the degree to which some headwater wetlands may act as a carbon source to ecosystems downstream. These wetlands are potentially a significant pool of C but are particularly sensitive to future changes in groundwater levels.

Continuous field estimation of dissolved organic carbon concentration and biochemical oxygen demand using dual-wavelength fluorescence, turbidity and temperature

Dissolved organic matter (DOM) quality and quantity is not measured routinely in-situ limiting our ability to quantify DOM process dynamics. This is problematic given legislative obligations to determine event based variability, however, recent advances in field deployable optical sensing technology provide the opportunity to address this problem. In this paper we outline a new approach for in-situ quantification of DOM quantity (Dissolved Organic Carbon: DOC) and a component of quality (Biochemical Oxygen Demand: BOD) using a multi-wavelength, through-flow fluorescence sensor. The sensor measured tryptophan-like (Peak T) and humic-like (Peak C) fluorescence, alongside water temperature and turbidity. Laboratory derived coefficients were developed to compensate for thermal quenching and turbidity interference (i.e. light attenuation and scattering). Field tests were undertaken on an urban river with ageing wastewater and stormwater infrastructure (Bourn Brook; Birmingham, UK). Sensor output was validated against laboratory determinations of DOC and BOD collected by discrete grab sampling during baseflow and stormflow conditions. Data driven regression models were then compared to laboratory correction methods. A combination of temperature and turbidity compensated Peak T and Peak C was found to be a good predictor of DOC concentration (R2 = 0.92). Conversely, using temperature and turbidity correction coefficients provided low predictive power for BOD (R2 = 0.46 and R2 = 0.51, for Peak C and T respectively). For this study system, turbidity appeared to be a reasonable proxy for BOD, R2 = 0.86. However, a linear mixed effect model with temperature compensated Peak T and turbidity provided a robust BOD prediction (R2 = 0.95). These findings indicate that with careful initial calibration, multi-wavelength fluorescence, coupled with turbidity and temperature provides a feasible proxy for continuous, in-situ measurement of DOC concentration and BOD. This approach represents a cost effective monitoring solution, particularly when compared to UV- absorbance sensors and DOC analysers, and could be readily adopted for research and industrial applications.This article is protected by copyright. All rights reserved.

Hydrological Assessment of Star Carr and the Hertford Catchment, Yorkshire, UK

Abstract The survival of one of Europes most important Mesolithic sites, Star Carr, has been threatened owing to recent changes in its hydrology and associated changes in groundwater geochemistry. Before this study it was unclear what was controlling these changes, but possible causes were changes in precipitation regime (notably the frequency of drought), changes in groundwater abstraction, recent agricultural drainage or a combination of these factors. This paper evaluates the hydrology of Star Carr and its environs within the River Hertford sub-catchment of the River Derwent. Available hydrological data (precipitation, evapotranspiration, river flow and groundwater levels) were collated and used to characterise the River Hertford catchment and provide a hydrological and hydrogeological context for the site. The data were augmented by the insertion of 12 dipwells into the site which were used for both site monitoring and model testing. The monitoring included water abstraction for isotopic analyses (δ2H v δ18O). Two modelling approaches were undertaken: firstly, calculation of the Palmer Hydrological Drought Index (PHDI); and secondly, development of a three layer, finite difference groundwater model of the site using Modflow. The results strongly suggest that hydrological changes at Star Carr have not been caused by changes in precipitation patterns, although the water flowing through the site is largely of meteoric (rainfall) origin. Groundwater abstraction has also not had any impact upon the site. However, results from the groundwater model suggest that the dominant factor has been the insertion of under-drainage, which has lowered the water-table by >0.5 m into the archaeologically sensitive zone. The study reported here shows how high the sensitivity of a small wetland can be to local land management practices, and, significantly, illustrates how a hydrological assessment of wetland archaeological sites can be made even without a history of on-site monitoring.

Simulation modelling of variable patterns of water movement through a cracking clay soil

. This paper describes the development and application of a simple empirical model describing differences in water movement through a cracking clay soil at Brimstone Farm, Wiltshire, UK. An extended data set comprising readings of soil water tension has been collected from an area of 9 m2 instrumented with 4 nests each of 3 tensiometers. The cracks are responsible for considerable differences both in water pathway and flow magnitude. Variations in water flow suggested by changes in soil-water tension are described by a model developed using ‘ModelMaker’ and applied separately to each profile nest. The model envisages water flow to occur through three soil layers, and to be partitioned into matrix and macropore flow components. Water is lost via drainage to clay tile drains at 60 cm depth. Water flow between layers is described as a function of the hydraulic gradient using Darcys Law, with additional drainage from structural voids within the soil. Differences in the effective hydraulic conductivity describing slow and rapid flow components equate to macro and matrix flow for each tensiometer profile. The results illustrate heterogeneous patterns of flow through a soil block and demonstrate that a comparatively simple model is able to represent satisfactorily water flow dynamics through a cracking clay soil.