hydrodynamics of floodplain wetlands in a chalk catchment: the river lambourn, uk

Hydrodynamics of floodplain wetlands in a chalk

catchment: The River Lambourn, UK

T.R. Grapes, C. Bradley*, G.E. Petts

School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK

Received 10 May 2005; revised 23 May 2005

Abstract

Variations in floodplain channel water levels and valley floor groundwater levels (measured in piezometers and boreholes)

are examined at selected points along the course of the River Lambourn, a chalk river in southern England. A local alluvial

gravel aquifer in the valley bottom is associated with numerous small wetlands that extend over much of the river’s perennial

profile. Variations in hydraulic gradient between local borehole levels and/or floodplain channel water levels are described for

three sites in the seasonal section of the channel at Bockhampton, East Garston and West Shefford. The results indicate that

observed groundwater levels are closely associated with flows from discrete springs at the margins of the channel and

floodplain. However, as the floodplain widens and the alluvial gravel aquifer increases in size, the gravel aquifer accounts for a

substantial down-valley component of groundwater flow with a diffuse vertical water flux. In the lower catchment, the exchange

of flows between the gravel aquifer and the river enables some attenuation of floodplain water-table variability, providing a

stable hydrological regime for valley-bottom wetlands. Catchment controls upon the local, valley-bottom, wetland regime are

demonstrated with the application of a simple groundwater model developed using MODFLOW. The model is used to simulate

groundwater discharge to the river in the upper and lower catchment, in addition to the water level regime at selected points in

the valley bottom in the lower catchment. The results demonstrate the importance of taking catchment-scale water flow into

account when managing isolated wetlands in a permeable catchment.

q 2005 Elsevier B.V. All rights reserved.

Keywords: Floodplain wetland; Wetland hydrology; MODFLOW; Groundwater hydrology

1. Introduction

Wetlands are characterised by considerable

differences in terms of their water budget and

pattern of water-table variation, which reflect

0022-1694/$ - see front matter q 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.jhydrol.2005.07.028

* Corresponding author. Tel.: C44 121 414 8097; fax: C44 121

414 5528.

E-mail address: [email protected] (C. Bradley).

the spatial and temporal variations in water flows

to and from the wetland, as well as the extent of

water redistribution through the wetland (Mitsch and

Gosselink, 2000). There is a clear need to define

water budget components more accurately and to

identify controls on water redistribution in order to

improve our understanding of floodplain wetland

hydrology. Derivation of the wetland water budget

requires identification and characterisation of

Journal of Hydrology 320 (2006) 324–341

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T.R. Grapes et al. / Journal of Hydrology 320 (2006) 324–341 325

the discrete water inflows and outflows, and together

these determine water availability (e.g. Gilman,

1994) and wetland hydrochemistry (Wassen et al.,

2002), which enable different wetland types to be

distinguished, ranging from acid fen/bog fed solely

by precipitation to a calcareous fen where ground-

water discharge occurs. Meanwhile, the dynamics of

water-table variation may reflect hydrometeorologi-

cal conditions (Bradley, 2002) with certain distinct

patterns indicating the predominance of one par-

ticular water source (Winter, 1999). For many

wetlands, groundwater represents an important

component of the water budget, albeit one that is

difficult to quantify accurately, especially given the

spatial and temporal variability of groundwater

inputs that a wetland may receive (e.g. Schot and

Wassen, 1993). To assess this component, a number

of studies have used groundwater models to estimate

sub-surface seepage (e.g. Gilvear et al., 1993; Hunt

et al., 1996; Weng et al., 2003). Successful model

applications require a good understanding of local

stratigraphy and regional hydrogeology and although

model verification may be problematic, the results

provide more robust estimates of groundwater

seepage than simply taking the residual of the

water balance equation.

Studies of groundwater–wetland interaction have,

for obvious reasons, largely focussed on discrete

wetlands, whose boundaries can be readily identified

in the field. However, even within such wetlands the

groundwater contribution may vary considerably

across the wetland area, as illustrated by Grieve

et al. (1995) on the Insh Marshes, located in the

valley of the River Spey in the Scottish Highlands.

Grieve et al. determined that groundwater discharge

represented the principal water source at the lateral

midpoint of the wetland (halfway between the river

channel and the valley margin), but groundwater

was far less significant at the edges of the wetland.

Moreover, studies in peat wetlands have demon-

strated that local variations in stratigraphy may lead

to significant variability in the direction of ground-

water flows (e.g. Drexler et al., 1999), which has

implications for the density of monitoring points

required to characterise subsurface flows adequately.

These difficulties are compounded when considering

less discrete wetlands, such as those found on the

floodplains of lowland rivers (Haslam, 2003) or

occurring as mires in shallow valleys. In these

environments, wetlands can extend over consider-

able longitudinal distances, following, for example,

a spring or seepage line in an area of groundwater

discharge. These wetlands are potentially vulnerable

to the impacts of groundwater abstraction, and in the

case of floodplain mires, to recurrent low flows, and

it is only possible to understand their hydrology in

the context of the regional or catchment ground-

water system. This is particularly important for

wetlands in permeable catchments such as those

underlain by chalk. Haria et al. (2003), for example,

have highlighted the relative lack of work on

shallow chalk groundwater systems, although they

are of evident importance in sustaining chalk

wetlands.

This paper considers the degree to which data

from a regional groundwater model, together with

local hydrological data, may be used to determine

spatial and temporal controls on wetlands along the

floodplain of the River Lambourn in central

southern England. The research is also applicable

more widely, to chalk stream ecosystems generally;

these being distinctive, groundwater-dependent,

lowland rivers found in North West Europe,

which are characterised by their stable discharge

regime and high nutrient concentration (Berrie,

1992).

The aims of the paper are:

† to identify the relationship between groundwater

levels and spring discharges in the catchment

headwaters;

† to investigate the hydrological controls on flood-

plain wetland water tables in a chalk valley;

† to illustrate the use of the results of catchment-

scale groundwater modelling to determine the

hydrodynamics of ‘local’ floodplain wetlands.

2. Study area: the River Lambourn, Berkshire, UK

This study was undertaken in the catchment of the

River Lambourn; a chalk stream that drains 234 km2 of

the West Berkshire Downs, a range of low hills in

central southern England at the western extremity of

the London Basin (Fig. 1). The seasonal head of

Fig. 1. The Lambourn Catchment, showing the location of sites mentioned in the text.

T.R. Grapes et al. / Journal of Hydrology 320 (2006) 324–341326

the River Lambourn is located at Lynch Wood springs

(Fig. 1), with the perennial source being located at

West Shefford, fromwhere the river flows south east to

the River Kennet, a major tributary of the River

Thames. The landscape of the Lambourn catchment

comprises rolling, grass-covered hills, dissected by a

dendritic network of dry valleys, and underlain by the

Chalk Formation, a fine grained white limestone of

Upper Cretaceous age, which is c. 200 m thick and

outcrops (beneath a thin soil cover) over much of the

catchment. The local geological structure is straight-

forward, with the Chalk dipping at an angle of c. 0.68 to

the south east (Bradford, 2002), but there is some

evidence to suggest that the line of the Lambourn

valley may be fault-controlled (Grapes, 2004). In the

lower catchment, the Chalk is unconformably overlain

by sediments of Eocene age. Quaternary deposits in the

catchment consist of ‘clay with flints’ (comprising the

detrital remnants from dissolution of chalk, which

mantle the elevated areas of the catchment) and valley

floor deposits comprising between 1 and 5 mof alluvial

gravel overlain by silt and clay with occasional peat

lenses also present (Grapes, 2004).

The River Lambourn has been relatively unaf-

fected by groundwater abstraction and is characterised

by a semi-natural flow regime with peak flows

coinciding with maximum winter groundwater levels.

The mean annual flow was 1.69 m3/s over the period


1962–2000 (NRFA, 2001), with a base flow index of

0.96 (Gustard et al., 1992). The river was notified as a

Site of Special Scientific Interest1 in 1995 as an

example of a lowland chalk river; and it displays

classic winterbourne behaviour in that an upper

section of the river is non-perennial, but carries

significant flow only during the winter months as the

drainage net varies seasonally in relation to ground-

water levels. Several sites along the floodplain of the

River Lambourn (together with that of the River

Kennet in to which it drains) have been notified

separately as SSSIs, including Boxford Water

Meadows and Easton Farm Meadow (Fig. 1). Both

sites comprise small meadows/flood pastures with

calcareous alluvial gley soils. Easton Meadow

exemplifies a Cynosurus cristatus–Caltha palustris

plant community, with Boxford also having a Carex

acutiformis swamp and fen community. Both sites

would traditionally have been managed, with con-

trolled flooding in spring each year to encourage

vegetation growth. This type of habitat is becoming

increasingly rare as many sites in southern England

have been affected by groundwater abstraction,

despite recognition that these wetlands have a

demonstrable positive influence on catchment water

quality (Prior and Johnes, 2002).

An extensive instrumentation network was

installed in the Lambourn and other surrounding

catchments between 1962 and 1974 as part of the

West Berkshire Groundwater Scheme (Owen, 1981),

in which groundwater abstraction was intended to

augment river flows. The augmentation system,

comprising large abstraction boreholes and associated

pipelines, has only been used very occasionally since

its completion in 1976. Gauging weirs were con-

structed at East Shefford, Welford and Shaw; and on

the Winterbourne Stream at Bagnor (Fig. 1), and a

total of 43 abstraction boreholes and 67 observation

boreholes were drilled in the Lambourn and adjacent

Pang catchments between 1967 and 1974.

Analysis of discharge records from the four fixed

gauges in the catchment, supplemented by current-

meter surveys along 12 reaches, has revealed that

1 A Site of Special Scientific Interest (SSSI) is designated by

English Nature to provide legal protection for an area of recognised

conservation interest.

accretion along the River Lambourn displays a

marked spatial pattern. At a catchment-scale, mean

flow accretion is c. 0.09 m3/s/km, but high rates of

accretion are found in both upstream and downstream

channel sections, which are often associated with

intersections between the river valley and perpen-

dicular dry valleys (Grapes et al., in press). Generally,

the dynamics of groundwater flow accretion to the

river are affected by catchment topography, by the

existence of local (i.e. valley-floor) controls on

groundwater flow, and by seasonal changes in the

drainage net.

3. Riparian hydraulic gradients

Data from three sites along the main river:

Bockhampton, East Garston and West Shefford

(Fig. 1) are examined to determine the relationship

between local groundwater levels, river stage and

floodplain spring discharges. At these points,

measurements of channel-proximal groundwater

levels have been undertaken over the period 1993–

2001.

3.1. Bockhampton

The measurement site at Bockhampton is situated

1.6 km downstream of the seasonal source of the

River Lambourn at Lynch Wood springs, at a point

where the floodplain is c. 150 m wide. The site

comprises a 50 m deep observation borehole (refer-

ence number SU37/60), cased to a depth of 12 m,

which is located 20 m from the river channel. The

relationship between groundwater levels and periods

of discharge at Lynch Wood springs is shown in

Fig. 2A. The relative timing of seasonal increases in

groundwater level at this site demonstrates that these

are not simply a function of channel seepage, but that

they reflect catchment groundwater levels. The

maximum groundwater level observed over the period

is c. 121 m O.D. (metres above Ordnance Datum;

equivalent to sea level). At this groundwater level, the

hydraulic gradient (and the resultant groundwater

discharge) towards the river channel is sufficiently

large as to prevent any further increase in the local

water table. Low groundwater levels in 1990/1991,

1992/1993 and 1997 reflect low winter recharge, and

Fig. 2. (A) Comparative graphs showing the coincidence of flows at Lynch Wood and their relationship to groundwater levels at Bockhampton

(SU 37/60), dashed lines indicate the range (A: maximum to minimum; 119.3/120.3 m OD) of groundwater levels at which flow either

commences or ceases at Lynch Wood springs; (B) The relationship between the hydraulic gradient at Bockhampton and spring discharge at

Lynch Wood; numbered points are referred to in the text.


are clearly associated with prolonged cessation of

spring flow from Lynch Wood.

The relationship between groundwater levels at

Bockhampton and spring flows are shown in more

detail in Fig. 2B, which gives the hydraulic gradient

between the borehole and the adjacent river channel

from October 1998 to October 2001, and the measured

discharge at Lynch Wood springs between April 1999


and October 2001. The hydraulic gradient varies

between K0.014 and 0.047, with a mean value of

0.023, while the spring discharge varies between 0

and 0.69 m3/s, with a mean value of 0.17 m3/s. The

similarities in the two data series indicate a common

dependence upon regional groundwater levels,

although there are some significant differences. For

example, the short period of zero discharge at Lynch

Wood springs occurring after October 1999 (point 1)

is associated with a period when the local ground-

water level at Bockhampton fell below the bed of the

adjacent channel, thereby producing a negative

gradient. Meanwhile, the proportionally higher dis-

charge from Lynch Wood springs in January 2001

(point 2) was due to unusually high early winter

recharge. This was not reflected to the same degree in

the Bockhampton gradient time series, as the peak

groundwater level was constrained by the stage in the

adjacent channel, which produced only a subdued

response to the high discharge due to the wide and

shallow morphology of the local channel. However,

once the peak discharge from Lynch Wood springs

had receded (point 3) the two parameters displayed a

very similar pattern of recession. Overall, the

consistent correlation between the discharge at

Lynch Wood springs and the lateral gradient at

Bockhampton indicates the potential for predicting

the onset, cessation and magnitude of groundwater

discharges to an adjacent river channel or floodplain

wetland, based upon a record of local groundwater

levels. To estimate the magnitude of such ground-

water discharges requires a key additional component;

the local hydraulic conductivity, and this was

calculated for the Bockhampton site using flow

accretion data.

Increases in river flow along a 2 km section of the

River Lambourn adjacent to the Bockhampton bore-

hole enable valley floor hydraulic conductivity to be

determined using Darcy’s Law, from:

Q ZKKiA

where Q is reach accretion (m3/s/km), K is hydraulic

conductivity between borehole and channel (m/s); i is

hydraulic gradient and A is the area of flow exchange

between aquifer and channel (m2/km of channel). The

latter, A, is taken to represent the average wetted

perimeter, which is approximately 5.7 m multiplied

by the channel length. Determination of the relation-

ship between local hydraulic gradient and flow

accretion, for 44 distinct periods of time when current

meter gauging was undertaken, yielded an estimate

for hydraulic conductivity of 3.8!10K4 m/s (equiv-

alent to 33 m/day). This lumped parameter represents

an estimate of valley floor hydraulic conductivity,

which here includes the river bed, limited alluvial

deposits and the upper active layer of the chalk

aquifer, and compares to a catchment average value

for the chalk aquifer of 25 m/day derived from

pumping tests (based upon a transmissivity value of

1490 m2/day and an effective aquifer thickness of

60 m; Brettell, 1971).

3.2. East Garston

Measurements at East Garston elucidate the

relationship between local groundwater levels, river

channel bed levels and headwater spring discharges

at a site approximately 7.5 km below the seasonal

source of the Lambourn, in a reach which is

characterised by low magnitude river flow accretion

(Grapes et al., in press). In this area, the floodplain is

c. 150 m wide and the river channel displays very

little incision. Floodplain water levels have been

monitored since 1992 in two narrow diameter

tubewells (TW22/39 and SU37/21) which have

been installed to a depth of c. 4 m at two points

approximately 500 m apart, but both within 5 m of

the river channel. They are constructed from 50 mm

diameter galvanised steel pipes that are perforated

over their basal 100 mm length. Observed floodplain

groundwater levels for the two tubewells are given in

Fig. 3A and these indicate a close relationship with

the occurrence of discharge from Lynch Wood

springs. Water levels in the two tubewells varied

by as much as 2.5 m, with seasonal minimum water

levels coinciding with the cessation of spring flows,

which normally lasts around 3 months every year,

although it did not occur in 1993/1994 or 2000/2001.

In Fig. 3A, line BL1 at 111.65 m O.D. represents the

bed level of the river channel adjacent to the

upstream site (TW22/39), which is 1.37 m below

the tubewell datum elevation of 113.02 m O.D.

Meanwhile, line BL2, at 110.35 m O.D., represents

the bed level at the downstream site (SU37/21),

Fig. 3. (A) The relationship between groundwater levels at East Garston (tubewells SU37/21 and TW22/39) and the occurrence offlows at Lynch

Wood.BL1 is the approximate river-bed elevation near TW22/39 andBL2 the river-bed elevation near SU37/21; (B) Scatter plot showing how the

lateral hydraulic gradient between the River Lambourn and the two boreholes varies in proportion to calculated reach accretion.



2.1 m below the datum elevation of the tubewell at

112.45 m O.D.

The relationship between local (positive) hydraulic

gradients for the two piezometers, and the measured

increase in flow across the adjacent channel section, is

shown in Fig. 3B. The full hydraulic gradient dataset

for SU37/21 (including both positive and negative

values) varies between K0.18 and 0.11, with a mean

value of 0.03. The full gradient dataset for TW22/39

ranges from K0.57 to 0.17, with a mean value of

K0.01. For comparison, over the period November

1992 to January 2001 the longitudinal gradient

between the two sites varied from 0.0017 to 0.0027,

with a mean value of 0.0023. These values show that

locally lateral gradients, perpendicular to the channel,

are approximately two orders of magnitude greater

than the longitudinal gradient, along the axis of the

channel/valley.

A total of 55 paired (upstream and downstream)

readings of discharge from current meter surveys on

an adjacent 2.3 km channel section enable local

valley floor hydraulic conductivity to be estimated

separately for the two tubewell locations,

giving values of 1.7!10K4 m/s (15 m/d) and

2.9!10K4 m/s (25 m/d) for the upstream and

downstream sites respectively. Alluvial deposits

are thin or absent in this area, as weathered chalk

is visible in the bed and banks of the river at several

locations. Hence, the calculated values of hydraulic

conductivity will mainly reflect the character of the

shallow chalk aquifer and the occurrence of any

armouring of the thin gravels on the river bed.

Numerous springs occur in the vicinity of the

village, indicating that locally the shallow chalk

Fig. 4. Schematic cross-section across the Lambourn valley at West Sh

relationship of valley floor sedimentology to local and regional groundwa

may exhibit sub-karstic properties, including

enhanced fissuring with high anisotropy.

3.3. West Shefford

Groundwater and floodplain channel water tables

were recorded over the period 1998–2001 at a number

of points across the Lambourn valley at West

Shefford, located 9.4 km downstream from the

seasonal source of the River Lambourn. Measure-

ments included river stage, chalk groundwater levels

(both deep and shallow) and the alluvial water table.

The relative positions of the monitoring points and

cross-sectional stratigraphy are given in Fig. 4 with

water flow directions inferred from local groundwater

level measurements. The groundwater observation

borehole (reference SU37/1) is cased to a depth of

12.5 m and extends to a depth of 100.6 m, with a

datum elevation of 106.7 m O.D. Along this cross-

section the chalk has been eroded to a depth of 5–

5.5 m and infilled with c. 4 m of gravel that is itself

overlain by a thin calcareous peaty soil (c. 1 m thick).

At piezometer B this layer comprised 0.3 m of brown

silty soil, 0.25 m of brown clayey silt and 0.45 m of

dark wood peat. The River Lambourn appears to be in

close hydraulic contact with the underlying alluvial

gravels and the floodplain alluvial water table lies just

below the ground surface.

Groundwater flow paths shown in Fig. 4 indicate

that the inferred direction of groundwater flow in both

the chalk and alluvial gravel is principally down-

valley. However, groundwater levels indicate that

there is some local flow from the chalk to the gravel,

and current meter surveys identify that there are also

efford showing the location of measuring points and the inferred

ter flows.


floodplain areas characterised by significant ground-

water discharge (for example the spring-fed channel

on the north east margin of the cross section). The

relationships between different flow systems in the

chalk and gravel aquifers are likely to be very

important in determining the potential for significant

floodplain wetlands to persist on the valley floor. The

gravel layer appears to vary considerably in lateral

extent and thickness, being absent less than 200 m

downstream, where weathered chalk bedrock is

visible in the river bed. Gravel exposed during

installation of piezometer B appeared to be largely a

coarse flint gravel, similar to that observed in the river

bed, but a pit excavated 30 m further downstream

uncovered 1 m of wood peat overlying cobbles of flint

and sarsen (Tertiary silcrete) in a putty chalk matrix,

showing that the lithology of the ‘gravel’ layer also

displayed significant local variability.

Observed water levels from points identified in the

cross-sectional transect (Fig. 4) are given in Fig. 5A.

The results indicate the degree to which both the

lateral and vertical controls on groundwater flows,

discussed above, are important in determining water

movement across the floodplain. The highest water

levels were consistently found in piezometer A,

situated 80 m from the river channel, which represents

shallow groundwater levels in the chalk on the valley-

side above the floodplain. The three monitoring points

close to the channel; namely the deep observation

borehole, piezometer B, and the river stage itself,

indicate that there is a high upward hydraulic gradient

beneath the floodplain, confirming that this is an area

of groundwater discharge. Deep groundwater levels

are consistently higher than the floodplain alluvial

water table observed in piezometer B, with the

exception of a short period in October and November

2000, when intense precipitation produced a rapid

increase in the floodplain alluvial water level.

The relationship between piezometer B and the

other water levels indicates how floodplain alluvial

levels, deep groundwater levels and river stage may

vary through time. Piezometer B was installed by

augering through c. 1 m of silt/clay soil and wood peat,

and its base lies immediately above a coarse gravel

layer at 105.5 m O.D. The bed of the River Lambourn,

within 15 m of the piezometer, is at 105.2 m O.D. and

consists of a similar coarse flint gravel to that found at

the piezometer base. Despite the proximity of

the piezometer to the river, water levels in the

piezometer are consistently 0.2–0.4 m higher than

river stage. This may indicate a degree of armouring or

cementation of the river bed, which reduces its

permeability and partially isolates the channel from

upwelling groundwater. One consequence of this is the

focusing of groundwater discharge at discrete flood-

plain springs (such as the one shown in the floodplain

cross-section, Fig. 4), which are found at several

locations along the valley of the Lambourn.

Valley floor hydraulic conductivity, with respect to

the three groundwater monitoring points, was esti-

mated across theWest Shefford section using a total of

54 flow accretion measurements along the adjacent

1.9 km long channel reach. The relationship between

reach accretion and the hydraulic gradient for each of

the three points is shown in Fig. 5B. The estimates of

valley floor hydraulic conductivity obtained were:

4.5!10K3 m/s (390 m/d) between piezometer A and

the river; 6.0!10K4 m/s (52 m/d) between piezometer

B and the river; and 1.6!10K3 m/s (140 m/d) between

the deep observation borehole and the river. The latter

value is likely to include elements of both horizontal

and vertical hydraulic conductivity. Significantly

however, the regression lines shown in Fig. 5B fail to

pass through the origin: the lines representing both the

observation borehole and piezometer A intercept the

‘gradient’ axis at c. 0.015 suggesting that the chalk

aquifer discharges to a higher point than the river

channel at the site, which is likely to represent a spring

at the floodplain margin. Conversely, piezometer B

intersects the ‘gradient’ axis at c. K0.009, indicating

that discharge from this point is to a lower elevation

than the river nearby, suggesting that the direction of

seepage through the gravel layer is down the axis of the

river valley. This indicates that the longitudinal

gradient at this site may be proportionally greater

(relative to the local lateral gradient) than at East

Garston, but the absence of an appropriate monitoring

point, located a short distance upstream or downstream

of the West Shefford sites, prevented a definitive

assessment.

4. Numerical modelling

The results presented in the previous section

indicate a variable relationship between floodplain

Fig. 5. (A) Variations in groundwater and river levels at West Shefford over the period June 1999 to October 2001; (B) lateral hydraulic

gradients from Piezometers A and B and the groundwater observation borehole (SU37/1) to the river and their relationship to reach accretion.



channel levels, groundwater levels and patterns of

river flow accretion. This reflects the channel bed

gradient (macro-scale longitudinal profile), channel

and floodplain width, and the continuity of the alluvial

gravel aquifer, as well as the lithological and

structural characteristics of the chalk aquifer. Given

the difficulty in obtaining field data to investigate

these factors, a groundwater model of the Lambourn

catchment was developed to investigate patterns of

river flow accretion and the variability in floodplain

groundwater levels (which provide the main con-

straints affecting riparian valley floor wetlands in the

catchment). The model builds upon several other

models that have previously been developed for the

Lambourn catchment, at both catchment and regional

scales (Oakes and Pontin, 1976; Morel, 1980; Rushton

et al., 1989; Bullock et al., 1995). These studies, with

the exception of Bullock et al. (1995), were designed

to investigate issues relating to groundwater resource

management in conjunction with the West Berkshire

Groundwater Scheme. The models were generally

transient, with a timescale varying from 6 months to

25 years and with grid cell size of generally 1 km2. As

a result of using a relatively coarse mesh size, these

models fail to adequately represent the variation in

Fig. 6. Configuration of the area of the groundwater model and its active

detail based on digital spatial data licensed from the Institute of Hydrology,

Landranger co-ordinates for square SU, in metres; A–D are boundary con

groundwater levels close to the river channel, which is

important in determining the location of valley-floor

wetlands, fed either by point or diffuse groundwater

discharge.

The model described here was developed using

Groundwater Vistas v2, incorporating pre- and post-

processors for MODFLOW (McDonald and Har-

baugh, 1988). The aim of the modelling work was to

investigate subsurface processes and parameter

configurations that control the pattern of groundwater

discharge through the Lambourn catchment by:

1. investigating the accuracy of catchment-scale

parameters by simulating surface-water discharge

from the catchment;

2. using the model to investigate local-scale control

on flow accretion and variations in local ground-

water levels adjacent to the river in the lower

catchment.

The extent of the model simulation area and its

relationship to the Lambourn catchment is illustrated

in Fig. 6. The active model domain was defined by a

series of no-flow cells along the model boundaries,

representing: A—the regional groundwater divide

domain, superimposed upon catchment topography. (Topographic

qIH.qCrown copyright (0186A). Axis scales are Ordnance Survey

ditions described in the text.).

Fig. 7. Spatial variation in hydraulic conductivity across the

Lambourn Catchment, which was envisaged to vary from 10 m/d in

the interfluves to 100 m/d in the river valley, but with locally high

conductivities of 180 m/d in areas indicated by the dense shading.

Table 1

Characteristics of model developed for the Lambourn catchment and short justification

Parameter Value Notes

Model dimensions 700 columns!400 rows;

grid cell size: 50 m!50 m

Model orientated at 378 East of grid North so that the river valley axis is parallel

to the long axis of the simulation

Layer(s) Single layer Unconfined

External boundary con-

ditions

No flows

Internal boundary con-

ditions

Drain cells for seasonal

water courses; river cells

for perennial channel

Hydraulic conductivity Ranging from 180 to

10 m/d

Adopting the transmissivity distribution used by Owen (1981); and with a

constant saturated thickness of 60 m

River bed conductance 250 m/d Parameter based upon a river, 5 m wide, with river-bed sediments, 0.2 m deep,

and a hydraulic conductivity of the river-bed sediments of 10 m/d

Storage coefficient Ranging from 0.35 to 2.

65%

Calculated from Transmissivity (T) using the equation: SZ10K4.4T0.7; derived

by MacDonald and Allen (2001)

Recharge Modified weekly MOR-

ECS EP (excess precipi-

tation)

Starting Head levels Mean groundwater level

derived from data from

w100 boreholes

Stress period; and time

step length

1 month; with 2 week time

step

Calibration period 1978–1983 Limited by the extent of input/calibration datasets.

MODFLOW solver pack-

age

Pre-conditioned Conju-

gate Gradient version 2

(PCG2)

Selected as most reliable solver based upon tests using trial models. Supported

by Osiensky and Williams (1997)


along the crest of the chalk escarpment; B—an

inferred groundwater flow line following the ‘ground-

water’ divide between the Lambourn catchment and

the adjacent catchment of the River Pang to the north

east; C—the edge of the confined zone of the chalk

aquifer (from Owen, 1981) where the Chalk is

overlain by Eocene deposits in the Kennet valley;

and D—the approximate groundwater divide between

the Lambourn catchment and the adjacent catchment

of the Aldbourne located to the south west. The model

comprised a single layer of constant saturated

thickness and a regular mesh of 50 m!50 m grid

cells was used throughout the model domain. A

summary of model parameters is given in Table 1.

Hydraulic conductivity generally varies over

several orders of magnitude in chalk catchments.

Development of the numerical model relied heavily

upon the spatial distribution of transmissivities for the

Lambourn catchment that were derived by Owen

(1981), and used subsequently by Rushton et al.

(1989); Allen et al. (1997). A constant saturated

thickness of 60 m was used to estimate hydraulic

conductivity from the transmissivity data, with the

resulting pattern of hydraulic conductivities shown in

Fig. 7, varying from 10 m/day in the interfluves to

over 100 m/day in the river valley, with a small zone


of particularly elevated values being located upstream

of Bockhampton.

The River Lambourn was represented in two ways

in the model, depending upon whether the reaches

were perennial or seasonal. Perennial reaches were

denoted using MODFLOW’s river cells, which

estimate subsurface flow through the river bed between

the river and the aquifer as a function of the hydraulic

gradient and the ‘river bed conductance’, which is

calculated fromvalues for channelwidth, bed sediment

thickness and bed sediment hydraulic conductivity.

Constant values for these three parameters were used

for all channel cells, comprising a channel width of

5 m, a bed sediment hydraulic conductivity of

10 m/day and a bed sediment thickness of 0.2 m.

Using river cells the model allows seepage to occur in

either direction between the river and aquifer,

depending upon the direction of the hydraulic gradient,

enabling both gaining and losing reaches to be

represented. MODFLOW drain cells were used to

represent groundwater flows to the non-perennial

section of the river, and point source discharges from

discrete springs along the valley-sides. Drain cells are

activated once groundwater levels rise above their

invert elevation, allowing water to be removed from

the model. The discharge is based upon the difference

between aquifer head at the drain cell and the drain-bed

elevation, multiplied by the ‘conductance’ term

(described above). If the groundwater level falls

below the specified height of the drain, then discharge

to the drain ceases. The accuracy with which both river

and drain cells are able to describe the exchange of

water between groundwater and surface water depends

largely upon the precision with which drain bed and

river bed elevations are specified in the model. As a

result, river bed heights and the elevations of several

springs were surveyed into Ordnance Datum at points

including Lynch Wood, East Garston, and West

Shefford

Recharge was estimated from weekly MORECS

(Meteorological Office Rainfall and Evaporation

Calculation System) excess precipitation (EP) data

(Hough and Jones, 1997), which revealed that effective

recharge took place almost exclusively in winter

months, between October and March. In order to

provide a representative recharge input to the model,

the EP dataset was modified in two ways. Firstly, the

magnitude of the weekly EP values was increased by

60% to account for (i) the recognised, long-term deficit

of 50% when comparing EP to catchment discharge,

and (ii) an estimated additional 10% deficit due to

unrecorded groundwater losses from the catchment,

for example, by flows beneath the lowest gauging

station. Secondly, the timing of the EP dataset was

modified by introducing a temporal delay, to represent

the time taken for recharge to pass through the

unsaturated zone. The delay was estimated by time

series analysis of the relationship between EP and

changes in mean catchment groundwater level, which

identified amean lag of 1 month between the two series

(Grapes, 2004). This mean lag was assumed to

represent the travel time through the mean unsaturated

zone thickness of 37 m for the catchment, and thus the

recharge delay was spatially weighted to account for

the variability in unsaturated zone thickness across the

catchment.

4.1. Model calibration

The model was initially calibrated in a steady-state

form by assessing its output relative to mean

catchment conditions. For this purpose, starting

heads were derived from mean groundwater levels

recorded in the network of c.100 boreholes across the

catchment, while a fixed value of recharge of 1 mm/d

was used, calculated from the long-term mean

catchment discharge of 1.72 m3/s. Model predictions

of groundwater losses arising from seepage to the

river at points corresponding to three reaches

immediately upstream of gauges at East Shefford,

Welford and Shaw were then compared with field

data. Fig. 8A indicates that the model was able to

reproduce mean flows at these points in the catchment

within G5% of recorded values. Investigation of the

residual surface showing the difference between mean

groundwater levels observed in the borehole network

and the model predictions, indicated that the majority

of the model area, and particularly the river valleys,

had residuals that were within G2 m of the observed

level.

The model was converted to a transient state

through the addition of storage data and a time

varying recharge input (described previously), and

then calibrated by assessing the ability of the model to

simulate the annual variation in catchment discharge.

Initially a single hydrological year (1978/1979) was

Fig. 8. (A) Steady-state calibration results based upon varying the hydraulic conductivity, and reproducing patterns of reach-based accretion

(top); (B) Recorded and simulated discharge at Shaw gauging station for the 1978–1983 calibration period.


simulated, but subsequently a 5-year period of

simulation (October 1978–September 1983) was

undertaken, and results from this are given in

Fig. 8B, which shows that catchment discharge was

generally well simulated, both in terms of timing and

magnitude of flow maxima and minima.

4.2. Modelling results: groundwater and floodplain

water levels

To assess the performance of the model in

simulating groundwater levels, the records from a

number of boreholes were compared to the simulated

pattern of water levels from corresponding points in

the model, with comparative results for two boreholes

(whose locations are shown in Fig. 1) being given in

Fig. 9. Borehole SU37/60 is the Bockhampton

observation borehole, discussed in Section 3.1;

while SU37/36 (Fig. 9A) is located close to the centre

of the catchment between Lambourn and West

Shefford. In the upper catchment near Bockhampton,

Fig. 9A indicates that the model simulates changes in

water level fairly well for years of average recharge

(i.e. 1978–1980), although it is less satisfactory in

Fig. 9. A comparison between observed groundwater levels and simulated ‘local’ or valley bottom water levels at two boreholes with locations

given in Fig. 1: A: SU37/60 and B: SU37/36.


1980–1981 when it predicts a fall of 2 m in the

groundwater level, and in 1982 when it over-predicts

the amount of recharge, although the timing of

maxima and minima are generally well simulated. In

the middle of the catchment, near borehole SU37/36,

the model is much better in reproducing the

fluctuations in groundwater level, although observed

water levels are consistently c. 1 m below the model

predictions. As groundwater flows are affected to a

large extent by high hydraulic conductivities within

the valley-floor, which were identified by pumping

tests, further improvement of the modelled water

levels at specific points within the catchment will only

be possible if the extent of this area can be better

defined. For example, it is possible that local

constraints on groundwater flow along the axis of

the valley, such as where the valley narrows, may

account for consistent differences between the

observed and modelled water levels. However, the

model results clearly demonstrate the importance of

considering the interaction between regional ground-

water flow and river seepage in explaining variations

in the groundwater contribution to valley-bottom

wetlands—whether reflecting the discharge of springs


in the upper catchment or on valley side, or diffuse

seepage through upwelling groundwater within the

floodplain.

5. Discussion

The implications of the work described here are

that water flow pathways can vary significantly over

comparatively small distances. This is likely to have

important ramifications for the management of

riparian ecosystems in that water and nutrient

availability will reflect patterns of surface-water–

groundwater interaction at a variety of scales,

particularly where an alluvial aquifer is present.

There are also likely to be clear trends in groundwater

seepage along the river profile, characterised by a

fairly simple relationship between inflows to head-

water wetlands and regional groundwater levels, but

with more complicated flow paths occurring where a

significant floodplain exists. Occurrence of these flow

dynamics, which are driven by catchment-scale

hydraulic gradients, need to be recognised, if

vulnerable floodplain wetlands in the lower catchment

(such as Boxford) are to be conserved.

Whilst modelling simulations may help in an initial

assessment of the hydrology of specific sites,

subsequent field investigation and model refinement

are likely to be needed. This is particularly important

in chalk catchments given the commonly recognised

difficulty in simulating groundwater flow where

hydraulic conductivities vary in a non-linear pattern

with depth (Rushton et al., 1989). This is generally

attributed to rapidly increasing permeability as

groundwater levels rise into areas where there is a

greater frequency of fractures extending through the

chalk matrix. However, this paper demonstrates that

in the river valley a laterally extensive alluvial gravel

aquifer enables two distinct scales of sub-surface flow

through the catchment. Firstly, catchment-scale flow

occurs through the regional chalk aquifer, with

seasonal discharge to the river channel and floodplain

channels as groundwater levels rise in the autumn and

winter; and secondly, local (and laterally discontinu-

ous) flow occurs through the alluvial aquifer in the

valley bottom. Water levels in the latter are closely

associated with river flows, but there are likely to be

local areas characterised by upwelling groundwater

where the floodplain and alluvial aquifer are narrow,

whilst downward groundwater movement is probable

in areas where the floodplain widens (Larkin and

Sharp, 1992; Woessner, 2000). Water levels in the

alluvial aquifer are likely to be further moderated by

diffuse seepage from the chalk aquifer, and may

become increasingly important down the long profile

as the gravel thickness (and storage volume)

increases. Given the complexity of the groundwater

flow system, the limited success of a one-layer

numerical model in replicating groundwater level

variations in deep boreholes (cased to between 10 and

20 m) is not surprising. However, of more interest is

the potential to utilise the results and limitations of the

model to (i) improve our understanding of water flow

pathways through a chalk catchment; and (ii) evaluate

how catchment-scale models, developed for water

resources management, may be used to advance our

understanding of sensitive and vulnerable wetland

ecosystems that depend upon continued groundwater

discharge.

As far as the first point is concerned, there are a

number of outstanding research questions regarding

water flow pathways through the chalk. In the upper

catchment, floodplain wetlands depend upon spring

discharge. The magnitude and/or timing of such

discharges derived from saturated groundwater flow

can be readily related to the seasonal variation in

groundwater levels, but there is some evidence to

suggest that these discharges are partly dependent

upon lateral water movement in the unsaturated zone.

Such flows reflect the occurrence of planar features in

the chalk aquifer that possess low vertical per-

meability, including hard grounds (indurated horizons

formed during quiescent periods of chalk sedimen-

tation), flint bands and marl bands. These features are

particularly important in areas with high topographic

gradients, such as valley sides, where they may be

partly responsible for the local occurrence of springs

at the floodplain margin.

Generally, however, this paper indicates the need

for further work including both hydrological and

geomorphological approaches to the investigation of

water flow paths within alluvial aquifers that are

closely associated with a major aquifer. This is

supported by a recent review identifying the need

for improved characterisation of interfaces, such as

between a bedrock and alluvial aquifer, or between an


aquifer and a surface-water body (Sophocleous,

2002). In this context, improved understanding of

the role of alluvial aquifers in determining the

hydrological regime of valley bottom wetlands is

urgently needed. For example, in the Lambourn

catchment, groundwater flow through the alluvial

aquifer would account for the limited variation in

wetland water tables observed in the lower catchment,

and at Boxford (Fig. 1), 5 km downstream of East

Shefford, Prior and Johnes (2002) found that water

tables varied between a winter (i.e. January and

February) minimum depth of 0.2 m; to a summer

(August and September) maximum of 0.4 m below the

surface. The maintenance of this stable water level

regime could be affected considerably by groundwater

abstraction, which would reduce spring discharge in

the upper catchment, and seepage flows from the

chalk to the alluvial aquifer. This would contribute to

reductions in discharge as well as lowering floodplain

water tables, which could increase nutrient avail-

ability through enhanced decomposition of dewatered

organic deposits.

Further work on floodplain wetlands in per-

meable catchments also requires careful consider-

ation of how catchment-scale groundwater models

can be used to investigate specific floodplain

wetlands. This might include a phased approach,

in which a regional model is used to assess general

fluxes, followed by more focussed or nested

modelling where additional details are required. In

this latter type of approach it is critical that

representative boundary conditions are used, to

avoid significant errors in output. Important issues

remain to be considered in detail, including the

resolution of field data needed to verify flow

exchange between alluvial and bedrock aquifers,

and the relationship of both deep and shallow water

levels to river stage. Clearly both are important if

we are to improve our understanding of the

hydrological controls on the narrow, discontinuous

but widespread valley bottom wetlands of chalk

catchments.

Acknowledgements

This research was carried out under a NERC/-

CASE funded studentship (GT/04/98/FS/17) in

association with the Centre for Ecology and Hydrol-

ogy at Wallingford; Mr R.B. Bradford was the CASE

supervisor at CEH. We are grateful for the comments

of two anonymous reviewers on an earlier version of

this paper.

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