Integration of shelf evolution and river basin models to simulate Holocene sediment dynamics of the Humber Estuary during periods of sea-level change and variations in catchment sediment supply.

Abstract

Three modelling elements and sedimentary evidence provide an understanding of sediment characteristics, river basin processes, tidal regimes and sea-level changes to explain sediment supply to the Humber Estuary through the Holocene (the last 10,000 years). An upscaled cellular catchment model simulates water and sediment fluxes from river basins, illustrating significant variations in response to climate change, especially precipitation and vegetation changes, principally deforestation. Much of the sediment mobilised remains in stores within the catchment and only a small fraction reaches the Humber tidal system. An empirical model helps to explain sediment erosion, transport and deposition from the offshore and coastal zones through the Holocene and sea-level rise caused the transgression of the continental shelf of the North Sea. Comparison with the sediment fill of the lowlands around of the Humber estuary, that represent the extent of the estuary during the Holocene, demonstrates that most of the fill (approximately 95-98%) was derived from non-fluvial sources. A shelf evolution model, with reconstructions of sea level, palaeogeography and palaeobathymetry for 1,000 year time steps through the Holocene predicts significant changes in tidal regimes, first over wide areas of the coast as the transgression of the continental shelf progresses. The most significant changes occur with the inner reaches of the palaeo-estuaries, especially those of the Humber and the Fenland. Throughout the mid-Holocene they are characterised by significantly lower tidal ranges (MWHST approximately 2.5 m less than present) and low tidal currents. The simulated patterns of tidal currents concur with the transport of fine grain sediment from the coastal zone into the estuaries. The major tidal range changes revise estimates of late Holocene and ongoing relative sea and land level changes.

Full text

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The Science of the Total Environment 314–316 (2003) 737–754
0048-9697/03/$ - see front matter ? 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0048-9697(03)00081-0
Integration of shelf evolution and river basin models to simulate
Holocene sediment dynamics of the Humber Estuary during
periods of sea-level change and variations in catchment sediment
supply
Ian Shennan *, Tom Coulthard , Roger Flather , Ben Horton , Mark Macklin , John Rees ,
Matt Wrighta
a,
bcdbd
Environmental Research Centre, Department of Geography, University of Durham, Durham, DH1 3LE UK
Institute of Geography and Earth Sciences, University of Wales, Aberystwyth, SY23 3DB UK
Proudman Oceanographic Laboratory, Bidston Observatory, Birkenhead, CH43 7RA, UK
British Geological Survey, Keyworth, NG12 5GG, UK
a
b
c
d
Accepted 2 January 2003
Abstract
Three modelling elements and sedimentary evidence provide an understanding of sediment characteristics, river
basin processes, tidal regimes and sea-level changes to explain sediment supply to the Humber Estuary through the
Holocene (the last 10 000 years). An upscaled cellular catchment model simulates water and sediment fluxes from
river basins, illustrating significant variations in response to climate change, especially precipitation and vegetation
changes, principally deforestation. Much of the sediment mobilised remains in stores within the catchment and only
a small fraction reaches the Humber tidal system. An empirical model helps to explain sediment erosion, transport
and deposition from the offshore and coastal zones through the Holocene and sea-level rise caused the transgression
of the continental shelf of the North Sea. Comparison with the sediment fill of the lowlands around of the Humber
estuary, that represent the extent of the estuary during the Holocene, demonstrates that most of the fill (;95–98%)
was derived from non-fluvial sources. A shelf evolution model, with reconstructions of sea level, palaeogeography
and palaeobathymetry for 1000 year time steps through the Holocene predicts significant changes in tidal regimes,
first over wide areas of the coast as the transgression of the continental shelf progresses. The most significant changes
occur with the inner reaches of the palaeo-estuaries, especially those of the Humber and the Fenland. Throughout the
mid-Holocene they are characterised by significantly lower tidal ranges (MWHST ;2.5 m less than present) and
low tidal currents. The simulated patterns of tidal currents concur with the transport of fine grain sediment from the
coastal zone into the estuaries. The major tidal range changes revise estimates of late Holocene and ongoing relative
sea and land level changes.
? 2003 Elsevier Science B.V. All rights reserved.
Keywords: Land-Ocean Evolution Perspective Study; Holocene; River basin; Sea-level change; Models
*Corresponding author. Tel.: q44-191-334-1934; fax: q44-191-334-1800.
E-mail address: ian.shennan@durham.ac.uk (I. Shennan).

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1. Introduction
The Land-Ocean Evolution Perspective Study
(LOEPS) of the Land-Ocean Interaction Study
(LOIS) NERC Thematic Programme aimed to
describe the evolution of coastal systems during
the Holocene (the past 10 000 years) in response
to changes in relative sea level (RSL) and the
impact of human activities. The major outcomes
of LOEPS, described by Shennan and Andrews
(2000), included the first reasonable estimates of
Holocene sediment budgets in the (Yorkshire)
Ouse River system and the Humber Estuary of
northern England and of the western North Sea
coast. The integrated modelling phase of LOIS
brings together this evidence and understanding of
sediment characteristics and river basin processes
with new research on tidal regimes and sea-level
changes to develop linked models to help explain
sediment supply to the Humber Estuary through
the Holocene. The modelled sediment supply to
the estuary (and theoretical sediment storage)
could be compared with the high-resolution strati-
graphy identified within the estuary as a result of
the LOIS investigations (Rees et al., 2000). The
first aim of the modelling was thus to simulate
sediment erosion and deposition during periods of
sea-level rise, changes in flood regime and land-
use change through the Holocene. The second aim
was to investigate RSL changes during the Holo-
cene as a major driving force in controlling sedi-
mentation at the coast and within estuaries. Critical
to understanding RSL change and sedimentation
is the potential of tidal regime being significantly
different in the past compared with the present.
Any significant changes in tidal regimes impact
both upon the quantification of land and sea-level
movements and the transport and deposition of
different sediments.
To undertake the modelling, a Shelf Evolution
Model (a nested hierarchy of models, highest
resolution approximately 1.2 km=1.2 km in the
western North Sea, northern limit at 548309N and
eastern limit at 18209E, Shennan et al., 2000a) and
an upscaled cellular catchment model (Coulthard
and Macklin, 2001) are integrated with data and
outputs from other LOIS projects (Andrews et al.,
2000a,b; Brew et al., 2000; Horton et al., 2000;
Macklin et al., 2000; Metcalfe et al., 2000; Rees
et al., 2000; Shennan et al., 2000a,b) to address
the range of changes in sediment flux at a range
of timescales. In the following sections we first
describe model development and integration and
then present the major results.
2. River basin model
To simulate sediment fluxes from the river
catchments draining into the northern part of the
Humber Estuary, a cellular landscape evolution
model is used. The model, called the Cellular
Automaton Evolutionary Slope And River model
(CAESAR; Coulthard and Macklin, 2001; Coul-
thard et al., 2000, 2002), represents river catch-
ments with a mesh of regular square grid cells.
Every cell has values for elevation, water dis-
charge, vegetation cover and grain size distribu-
tion. During each model iteration, these values are
altered according to rules that calculate catchment
hydrology, fluvial erosion and deposition and slope
processes. Using hourly rainfall data, a modifica-
tion of TOPMODEL (Beven and Kirkby, 1979) is
used to determine runoff, which is then routed
using a multiple flow algorithm that also calculates
flow depth and velocities. These are then used to
calculate fluvial erosion and deposition between
cells for 11 separate grainsizes according to the
Einstein-Brown expression (Einstein, 1950). The
stream bed is represented through 11 active layers
(Hoey and Ferguson, 1994) which allow the sim-
ulation of armouring effects and the evolution of
a simple alluvial stratigraphy. Mass movement
occurs when a slope threshold is exceeded, soil
creep is also incorporated and a simple linear
vegetation growth model allows a turf mat to
develop. Changes in elevation resulting from flu-
vial erosion, mass movement and soil creep are
updated simultaneously, and a variable time step
is used to restrict net erosion to 10% of the local
slope preventing computational instability. As all
these processes are operating within the same
regular grid, feedback between them is automati-
cally integrated, allowing inputs of rainfall, vege-
tation cover and topography to drive landscape
evolution.

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Table 1
Characteristics of tributary catchments
RiverSwaleUreNidd Wharfe
Modelled drainage area (km )
Max relief (m)
Main channel length (km)
2
383
514
62
646
564
82
281
560
49
697
546
97
To model the Holocene riverine sediment input
into the Humber Estuary, four major tributaries of
the Yorkshire Ouse, the Rivers Swale, Ure, Nidd
and Wharfe, were modelled in detail and the results
upscaled to represent the whole catchment. The
topography of these catchments was taken from a
50 m Ordnance Survey digital elevation model
(DEM) and details of the simulated catchments
are shown in Table 1.
To drive the Holocene river simulations, climate
and land cover inputs are required. The climate
input is derived from a combination of two proxy
wetness indices from peat bogs at Bolton Fell
Moss (Barber et al., 1994) from 6300 cal. BP to
present, and from Northern Scotland for the early
Holocene (Anderson et al., 1998). This combined
high resolution sequence dating back to 9000 cal.
BP is sampled at 50 year intervals and normalised
to values between 0.75 and 2.25 to create a rainfall
index (Fig. 1 top panel). A 10-year hourly rainfall
record (1985–1995) is repeated five times to
create a 50-year sequence which is then multiplied
by the rainfall index, in turn creating a 9000-year
hourly rainfall reconstruction. Changes in land
cover in the Yorkshire Ouse are poorly documented
and local palynological records (Tinsley, 1975;
Smith, 1986) are used to produce a land cover
index ranging from 2 (forested) to 0.5 (grassland),
see Fig. 1a. This index is used to simulate changes
in land cover by altering hydrological parameters
within CAESAR that control the magnitude and
duration of flood hydrographs for a given storm
event. To simulate future climate change the River
Swale was modelled on the basis of a 10% increase
in rainfall between 2000 and 2100. During simu-
lations, continuous sediment discharge data are
recorded and at 50 year intervals, grid elevation
and grainsize data are saved, from which detailed
topographic, run-off and
records of the catchments can be produced. Impor-
sediment discharge
tantly, this provides water and sediment discharge
data which can be fed into the shelf evolution
model.
3. Shelf evolution model
The first stage of the shelf evolution modelling
is to calculate the volumes of sediments derived
from cliff erosion, shoreface erosion and tidal
scour from different parts of the western North
Sea (initially at 1000-year intervals) during Holo-
cene post-glacial sea level change. The modelling
is designed to determine the annual sediment
budget to the nearshore zone along different sec-
tions of the coast. The nearshore zone is here
taken to represent that in which sediments remain
effectively mobile and may be transported and re-
deposited along other parts of the coast, including
estuaries and sandbanks, or may be exported out
of the region. Here the results of the modelling
could be compared with stratigraphical evidence
regarding the source of materials in the estuary
fill, based on geochemical and mineralogical
grounds (Rees et al., 2000). The boundaries of
selected onshore areas (Fig. 2 and Table 2) were
extended offshore from the cliff (or beach, where
absent) to below the position of RSL at the start
of the Holocene to create eight offshore areas. The
boundaries of these are complex but define dis-
tinctive offshore physiographic areas, the shape of
which has little impact on the results of the
modelling. Efforts were made to ensure that the
distinctive boundaries of Holocene features, such
as Sand Hole, Inner Silver Pit, Docking Shoal and
the Sand were contained in separate offshore areas
thereby allowing calculation of the impact that the
formation of these had on local sediment budgets.
The modelling is based on work undertaken by
the British Geological Survey on onshore to off-
shore profiles of the English coast from Dorset
through to Northumberland (Evans, in press).
Although the forms of these profiles (Fig. 3) are
very variable, some patterns can be readily drawn
out of the data. Of particular note is the evidence
that profile gradients within most coastal areas
appear to change consistently at set depths below
a datum, regardless of geology. On the western
North Sea coast, for instance, it may be observed

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Fig. 1. Land cover and precipitation indices used to drive the Holocene simulation, log sediment discharges for the four modelled
catchments and the frequency of British dated alluvial units (after Macklin, 1999).
that a steep ramp occurs between the shore and
contour at approximately 13 m below MHWST
(mean high water of spring tides), that a shallower
ramp occurs between this bathymetric contour and
a water depth of approximately 35 m below
MHWST, and that below this gradients again
steepen (Fig. 4). The shorewards steep ramp
immediately seawards of the shore is the shoreface,
which is being actively scoured by wave action,
and the depth of the base of this, the closure depth,

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Fig. 2. Location of areas modelled in stage 1 of the shelf evo-
lution model.
Table 2
Characteristics of onshore and offshore areas
Coastal area Offshore area Coast length (km)
Mean cliff height (m)
Northumberland
Tees Estuary
Yorkshire Moors
Holderness
Humber Estuary
Lincolnshire Marsh
Wash-Fens
Norfolk
Off South Shields
Geordie Trough
Cook
Spurn
Sand Hole
Protector Overfalls
Inner Silver Pit
Docking Shoal
47.6
34.6
105.6
75.0
167.5
73.4
94.6
511.8
20
–
80
12
–
–
–
(Hunstanton only)
is defined by effective wave base. Evans (in press)
suggests that the shallow-gradient ramp below this
was cut by migration of the shoreface landwards
during the Holocene. The steeper gradients sea-
ward of the shoreface migration surface broadly
reflects the bathymetric profile prior to the migra-
tion (Fig. 4).
Identification of these features allow construc-
tion of a simple model in which rates of sediment
erosion can be calculated in a systematic manner
across the western North Sea during the Holocene.
It is possible to make a reasonable reconstruction
of the pre-erosion (end-Pleistocene) topography of
the region at the start of the Holocene by extrap-
olating the land surface between the top of the
present cliff (where present) or from the base of
the Holocene deposits in areas where coastal Holo-
cene deposits have accreted, and the base of the
shoreface-migration-surface (here taken at y5.62
m and y55 m below MHWST at the northern and
southern edges of the region. With knowledge of
the RSL curve of an area it is possible to predict
the altitude (relative to OD) of the base of the
shoreface in that area at any time during the
Holocene. More significantly, if the geology of the
shoreface and the shoreface–migration-surface is
broadly homogenous it is likely that the distance
from the shore to the closure depth is likely to
have remained relatively stable. Thus shoreline
position through the Holocene may be estimated.
On the basis of these observations it is possible to
systematically build a model of a Pleistocene
surface (bathymetry and topography), and to sim-
ulate erosion of this during the Holocene marine
transgression (Rees and Newsham, in preparation).
The topography of the onshore area is modelled
from data at 5 m contour intervals on Ordnance
Survey 1:50 000 scale maps. Bathymetric data
were obtained from UK Hydrographic Office
(UKHO) sources. Borehole data from onshore
areas of Holocene, stored in the National Geosci-
ence Resources Centre, Keyworth, were used to
determine the base of the Holocene deposits in