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Managed realignment (MR) schemes are being implemented to compensate for the degradation of coastal habitats. However, evidence suggests that MR sites have lower biodiversity than anticipated, which has been linked to poor drainage. Despite creek networks playing an important role in enhancing site drainage in natural intertidal environments , there remains a shortage of data on the formation and evolution of creeks within MR sites. This study evaluates creek development at the Medmerry Managed Realignment Site, UK. Creek development is investigated using differential global positioning system (dGPS) data, supported by sedimentological analyses and a high-resolution digital surface model (DSM) derived from images taken using a small unmanned aerial vehicle. Measurements indicated that creeks will develop relatively quickly, but are influenced by differences in the sub-surface sedimentological conditions. A suitable level of agreement was found between the DSM and dGPS measurements, demonstrating the appropriateness of this method to study creek development within intertidal environments at a higher resolution than traditional surveying techniques. These results are used to propose the collapse of sub-surface piping as the primary creek formation mechanism. Findings are discussed in terms of increasing the success of MR schemes and enhancing site design to maximise the ecosystem services provided.
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The evolution of embryonic creek systems in a
recently inundated large open coast managed
realignment site
Jonathan Dale, Heidi M. Burgess, Niall G. Burnside, Paul Kilkie, David J. Nash, and
Andrew B. Cundy
Abstract: Managed realignment (MR) schemes are being implemented to compensate for
the degradation of coastal habitats. However, evidence suggests that MR sites have lower
biodiversity than anticipated, which has been linked to poor drainage. Despite creek net-
works playing an important role in enhancing site drainage in natural intertidal environ-
ments, there remains a shortage of data on the formation and evolution of creeks within
MR sites. This study evaluates creek development at the Medmerry Managed Realignment
Site, UK. Creek development is investigated using differential global positioning system
(dGPS) data, supported by sedimentological analyses and a high-resolution digital surface
model (DSM) derived from images taken using a small unmanned aerial vehicle.
Measurements indicated that creeks will develop relatively quickly, but are influenced by
differences in the sub-surface sedimentological conditions. A suitable level of agreement
was found between the DSM and dGPS measurements, demonstrating the appropriateness
of this method to study creek development within intertidal environments at a higher res-
olution than traditional surveying techniques. These results are used to propose the col-
lapse of sub-surface piping as the primary creek formation mechanism. Findings are
discussed in terms of increasing the success of MR schemes and enhancing site design to
maximise the ecosystem services provided.
Key words: managed realignment, creeks, piping, small-unmanned aircraft system (sUAS),
structure-from-motion (SfM).
Tidal marsh systems are of global significance, occupying approximately 5.1 Mha of the
Earthssurface(Pendleton et al. 2012). Only relatively recently, however, has the wider
importance of these environments, in terms of the range of ecosystem services provided,
been realised (Rotman et al. 2008). These services include protection from coastal flooding
through wave attenuation, wildlife habitat, carbon sequestration, immobilisation of
Received 29 November 2017. Accepted 28 March 2018.
J.Dale,H.M.Burgess,N.G.Burnside,andP.Kilkie.Centre for Aquatic Environments, School of Environment and
Technology, University of Brighton, Brighton BN2 4GJ, UK.
D.J. Nash. Centre for Aquatic Environments, School of Environment and Technology, University of Brighton, Brighton
BN2 4GJ, UK; School of Geography, Archaeology and Environmental Studies, University of the Witwatersrand, Private
Bag 3, Wits 2050, South Africa.
A.B. Cundy. National Oceanography Centre (Southampton), School of Ocean and Earth Science, University of
Southampton, Southampton SO14 3ZH, UK.
Corresponding author: Jonathan Dale (e-mail:
Copyright remains with the author(s) or their institution(s). This work is licensed under a Creative Commons Attribution
4.0 International License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium,
provided the original author(s) and source are credited.
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pollutants, and recreation opportunities (e.g., King and Lester 1995;Costanza et al. 1997;
Moller et al. 2014). The recent recognition of these services comes within a context of the
large-scale loss of tidal marshes in recent decades, through reclamation, drainage, and
coastal squeeze (Doody 2004) or other erosional and degradative processes. These losses,
along with concerns regarding the medium- to long-term integrity of coastal flood defences
(French 2006), has resulted in a shift in the approach to coastal management used by engi-
neers and policy makers, changing to techniques that recognise the importance of tidal
marsh and mudflat systems for coastal protection and ecosystem functioning. Several resto-
ration schemes to replace, or compensate for the loss of, the structural or functional char-
acteristics of degraded, reclaimed or eroded tidal marsh habitat have been implemented,
including replanting schemes or decreasing current velocities using offshore breakwaters
to allow vegetation to become rooted and established (Doody 2008). This paper focuses on
the most popular of these techniques (in Europe and America), managed realignment
(MR): the process of shifting the landsea border, usually by de-embanking defences and
constructing new defences inland (French 2006).
Despite being designed to compensate for intertidal habitat loss, there is growing evi-
dence that many MR sites have lower biodiversity and therefore delivery of ecosystem ser-
vices than expected (e.g., Mazik et al. 2010;Mossman et al. 2012). This may be due to poor
sub-surface hydrological connectivity and differences in drainage pathways within MR sites
(Tempestetal.2015), caused by disturbances and former land use practices impacting on
the sediment structure (Spencer et al. 2017). Even though it has been recognised that poor
sediment drainage, and anoxia caused by stagnant water, may be the cause of poor species
diversity in MR sites (Mossman et al. 2012), there remains a lack of understanding of the
geotechnical, morphological, and sedimentary processes within MR schemes (Esteves
2013). Tidal creeks may play an important role in the evolution and development of MR
sites; in natural tidal marsh and mudflat environments creeks help to regulate site drain-
age, and become more effective at increasing drainage and tidal (and sediment) exchange
as they develop (e.g., Symonds and Collins 2007).
In natural marsh systems, creeks form predominantly due to the intertidal environment
being inefficient at draining water as the tide ebbs. Sheet flow becomes concentrated or dis-
sipated by subtle variations in surface topography, creating a depression. Once formed,
flow becomes focused within the area of the depression resulting in larger bed shear
stresses and increased erosion (Whitehouse et al. 2000). The formation and evolution of
creek networks has commonly been associated with the morphological and ecological evo-
lution of the surrounding intertidal zone (e.g., Kirwan and Murray 2007). Models of initial
creek formation usually consider rapid morphological development (e.g., DAlpaos et al.
2005). In the long term (10100 years), the rate of accretion slows and channels start to infill,
as a result of a lowering in the discharge due to accretion on the surrounding intertidal plat-
form (e.g., Marani et al. 2002;DAlpaos et al. 2006).
Whilst advances have been made in understanding the development of creek networks in
intertidal settings (e.g., Allen 2000;DAlpaos et al. 2005;Kirwan and Murray 2007), there are
relatively few empirical field studies of the initial formation of creek and drainage features
in intertidal environments (Vandenbruwaene et al. 2012); the majority of data on initial creek
evolution and development come from numerical morphodynamic models, probably due
to most intertidal creek networks already being in a state of quasi-equilibrium (e.g., Marani
et al. 2003). MR sites could, therefore, provide an opportunity to study creek formation and
development in a previously non-channelled environment. Currently, little is known of the
formation and development of creek-drainage networks in MR sites, with the majority of
previous studies focusing on creek evolution outside of the realignment area following site
breaching (e.g., Symonds and Collins 2007;Friess et al. 2014). The development of tidal creeks
Dale et al. 17
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networks in MR sites has been suggested to be related to the features of the pre-existing
landscape (French and Stoddart 1992), including the presence of drainage channels and
surface features, such as plough lines (Spencer and Harvey 2012). The timescale for embryonic
creek network development is also influenced by sediment properties (including the
drainage characteristics), tidal energy, and the surface gradient of the intertidal zone
(Cornu and Sadro 2002;Crooks et al. 2002;DAlpaos et al. 2007;Spencer and Harvey 2012).
Consequently, the drainage networks that develop in MR sites can vary considerably between
sites, with any pre-existing terrestrial drainage systems being retained for many years and, in
some cases, remaining as permanent features (e.g., Bowron et al. 2011). DAlpaos et al. (2007)
investigated drainage network density, comparing field data to modelled results, for a con-
structed saltmarsh in Venice Lagoon (Italy), whilst Williams et al. (2002) monitored channel
cross-sectional areas in San Francisco Bay (United States) over a 13 year period. Both of
these studies, however, have relatively low (>2 year) temporal resolution. A high resolution
empirical insight into creek formation in MR sites was provided by Vandenbruwaene et al.
(2012), although for a controlled reduced tidal scheme on the Scheldt estuary (Belgium)
where tidal inundation is reduced and controlled using sluice gates. The extent to which
the findings of Vandenbruwaene et al. (2012) are relevant, and applicable, to sites where tidal
inundation is not controlled and which are subject to natural tidal variability remains
This study aims to investigate the evolution of creek networks within a recently
breached open coast MR site at Medmerry, West Sussex, UK (Fig. 1). The Medmerry
Managed Realignment Site is the largest open coast MR scheme in Europe (at the time of
site breaching), occupying 4.5 km
. Specifically, we utilise an innovative combination of
datasets including surface sediment characteristics, and differential global positioning sys-
tem (dGPS) measurements of the variation in embryonic central creek position and bed
elevation, with a high resolution digital surface model (DSM) produced via structure-from-
motion (SfM) analysis of images taken using a small-unmanned aircraft system (sUAS).
Measurements are assessed to gain field-based knowledge on the formation and evolution
of embryonic creek systems in a recently inundated intertidal environment, and to assess
the suitability of using SfM analysis to examine creek development processes within these
settings. We conclude with a consideration of the implications of our results for future
MR site design.
Materials and methods
Study site
LocatedonthesouthcoastoftheUK(Fig. 1a), in the eastern Solent, the Medmerry
Managed Realignment site was constructed due to concerns over the effectiveness of the
former coastal flood defences, which consisted of a shingle barrier beach managed by the
UK Environment Agency. The shingle bank required constant re-profiling during the winter
to maintain the necessary defence standard to protect the coastal hinterland (Cope 2004).
The Pagham to East Head Coastal Defence Strategy (Environment Agency 2007) concluded
that, beyond the short-term, the existing defences were unable to prevent flooding, and
endorsed MR as the most suitable method of managing the risk from coastal flooding.
In addition to its role as a flood defence scheme, the Medmerry site was also designed to
compensate for tidal marsh and mudflat habitat loss elsewhere in the Solent. Over 80% of
the coastline in this region is designated for its nature conservation interests (Foster et al.
2014). However, 40% of the Solents saltmarsh (about 670 ha) were lost due to erosion
between 1971 and 2001 (Cope et al. 2008). Over the 100 years following breaching at
Medmerry, it is predicted that almost 184 ha of new intertidal habitat will be created within
the site (Pearce et al. 2012).
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The Medmerry site consists of 7 km of new defences, reaching 3 km inland. Site design
included areas of lower elevation (borrow pits) designed to encourage a range of (low eleva-
tion) intertidal habitat, where material was excavated and used to create the new flood
defences, and a series of drainage networks consisting of pre-existing terrestrial channels
(known locally as rifes) and steep-sided channels engineered during site construction. The
shingle barrier beach was breached on 9 September 2013, forming a semi-diurnal, mesoti-
dal estuarine system.
For this study, measurements were taken from the bank of a near-breach former barley
field (Fig. 1b), in front of an infilling borrow pit (Dale et al. 2017), named Site 5 by Burgess
et al. (2016), where embryonic creeks had developed following site breaching. This site
was selected as it allowed for analysis of creek development in a rapidly evolving near-
breach environment. Tidal data, reported by Dale et al. (2017), indicated that the bank
would typically be inundated for 2.5 h by approximately 0.70 m (spring tides) and 0.30 m
(neap tides) of water per tidal cycle (Fig. 2).
Sediment analysis
The morphological development of the study site was assessed between July 2015 and
June 2016 (i.e., 23 years after site breaching). Sediment samples were taken regularly
(monthly) from the same location on the bank at low water, within 10 m of where
Fig. 1. (a) The Medmerry Managed Realignment Site, the location of the study site in the Medmerry site, and the
United Kingdom and regional setting (insets) (background data © Crown Copyright and Database Right 2017.
Ordnance Survey (Digimap Licence)). (b) Orthomosiac aerial image of the study site, captured on 13 July 2016 by a
small-unmanned aircraft system (sUAS), including the edge of the borrow pit (dotted line) and the area where
creeks have developed (dashed square). See text for further discussion.
Dale et al. 19
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embryonic creeks had formed, to assess changes in physical characteristics as the site devel-
oped following tidal inundation. Samples were analysed using standard sedimentological
procedures. The moisture content was measured as a percentage of the dry mass (water con-
tent =wet sediment weight/dry sediment weight ×100) after samples had been oven dried
at 105 °C for 48 h. The organic content of the samples was estimated via a proxy method,
using a 6 h loss on ignition test at 450 °C. A Malvern Instruments Mastersizer Hydro
2000G Laser Diffraction Particle Size Analyser was used to determine both the grain size dis-
tribution and mud (clay +silt) content following hydrogen peroxide treatment to remove
organic matter (which may bind the sediment and result in an underestimation of the clay
fraction) and dispersion with sodium hexametaphosphate.
Creek evolution measurements
Position and elevation measurements were taken of developing creek networks using a
dGPS on four occasions; 8 August 2015, 22 October 2015, 3 March 2016, and 10 June 2016.
Measurements were taken of the creek position, from the centre of the creek, from within
the borrow pit to the abrupt break in the longitudinal profile, known as the nickpoint, which
usually characterises low-order creek networks (Symonds and Collins 2007). Positional data
were supported by dGPS elevation data taken along three perpendicular (cross-profile) trans-
ects crossing the creek networks at the edge of the borrow pit (T1), inland (T2) and at the top
of the embryonic system (T3), to evaluate changes in the width and depth of the creeks over
time. Positional and elevation data were taken using a Leica AS19 GNSS antenna, a Leica Viva
GS10 GPS receiver and a Leica CS15 controller. Raw GPS measurements were imported into
Leica Geo Office (version 8.3). Network Receiver Independent Exchange Format (RINEX) cor-
rection data were obtained from Leica Smart Net UK & Ireland (
rinex-download_148.htm), with the correction applied to the raw data by the Leica software.
Leica Geo Office reported the positional quality (XYZ) for all dGPS points as <0.02 m. All data
were projected using the OSGB1936 coordinate system and datum.
Measurements taken using dGPS are subjective (in terms of measurement location) and
are spatially limited, and therefore may not be the most appropriate method of measuring
Fig. 2. Tidal data, previously reported by Dale et al. (2017), for typical (a) spring tides on 5 August 2015 and (b) neap
tides on 11 August 2015. The dashed line represents the elevation of the bank at the edge of the borrow pit (see text
for description).
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morphological development within complex intertidal environments, especially in terms
of recording important submetre-scale spatial variations and morphological characteris-
tics. sUAS are being used increasingly across a number of scientific disciplines as an alterna-
tive approach to provide high-resolution detailed imagery (e.g., James and Robson 2014;
Tonkin and Midgley 2016;Strong et al. 2017). Images can be used for rapid reconstruction
of surface geometry, providing there is sufficient overlap between images, without the need
for camera position or orientation data through automated photogrammetric techniques
(e.g., James and Robson 2012;Westoby et al. 2012;Javemick et al. 2014;Nolan et al. 2015).
The emerging, low-cost photogrammetric method SfM with Multi-View Stereopsis provides
a rapid method for high resolution topographic reconstruction (Westoby et al. 2012). SfM
approaches have already been used to successfully assess geomorphological processes such
as gully erosion (e.g., Castillo et al. 2012).
sUAS.The sUAS was flown at a target altitude of 20 m above ground level and at 5 m line spac-
ing in consistent weather conditions (temperature, 18 °C; wind speed, 9 mph (14 km/h) NW;
sun with minor cloud cover). Following an initial test flight, aerial images were acquired dur-
ing four separate flights over an hour-long period using a crosshatched flight plan to ensure
maximum overlap (>80%) and a complete coverage of the study site. Images were captured
using a DJI Zenmuse X3, 3-band RGB camera with a focal length of 20 mm.
Seven ground control points were recorded using the dGPS system. Fisheye correction
and camera alignment optimisation were applied to minimise the central domingeffect
reported in previous studies (e.g., James and Robson 2014), and potentially amplified due
to the low flight height. A dense point cloud was produced from optimised camera loca-
tions, using mild-depth filtering to ensure the preservation of small and important detail.
The dense point cloud output was used to generate both the orthomosaic image (Fig. 1b)
and the DSM.
Independent assessment of the sUAS modelling was completed using an additional six
control points, recorded using dGPS, to assess vertical and horizontal error of the DSM. To
assess the effectiveness of the model as a tool for evaluating embryonic creek development,
a further 54 dGPS positional measurements of the embryonic creek systems were captured,
along with 53 elevation measurements from the three cross-profile transects. Root-mean-
square-error (RMSE) and mean-absolute error (MAE) were calculated to validate the DSM
against six independent control points, and as a measure of quality in comparison to the
elevation measurements taken in the transects. Creek position dGPS data were compared
visually to the DSM. All analysis was conducted using ArcGIS 10.2.2.
Surface sediment properties
Moisture content ranged between 39% and 50% and showed little change as the creek sys-
tems developed. Loss on ignition increased slightly, ranging between 3.8% and 5.4% (Fig. 3).
Grain size was relatively coarse, with increased concentrations of fine-grained sediment
during the summer. This is probably due to reduced inputs of sandy sediment as a result
of more quiescent conditions and therefore less erosion of the relict hedgerows surround-
ing the site (recognised by Dale et al. (2017) to be the source of coarse-grained sediment to
this site). Analysis of the sediment in the creek beds, sampled in July 2016 and analysed
using the same procedure as the surface samples, indicates they developed on top of a less
organic layer (loss on ignition, 2.80%) with a higher concentration of fine-grained sediment
=22.50 μm; mud content, 54.59%) compared to the surrounding bank. This lower sedi-
ment unit is probably the former terrestrial surface, matching the observations of
Tempest et al. (2015) from the Orplands Farm Managed Realignment Site (Essex, UK).
Dale et al. 21
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The embryonic creek systems
Two embryonic creek networks developed on the bank over the course of the monitor-
ing period (as shown in Fig. 4). Overall, the eastern creek increased in length by 19.70 m dur-
ing the measurement period. However, minimal landward erosion occurred in the eastern
creek network following the second survey in October 2015, with the creek length increas-
ing from 38.37 to 42.73 m during this period. The western creek continued to develop multi-
ple branches and networks, although the length of the main channel only increased by
4.22 m during the measurement period. Elevation measurements taken in westeast cross
section (see Fig. 4 for locations) demonstrate that around the edge of the borrow pit, T1,
the position of the western creek remained relatively constant over time, with the elevation
of the bed increasing by 15 cm and sediment accreting on the banks (Fig. 5a). In contrast,
the elevation of the eastern creek fluctuated at the edge of the borrow pit, initially eroding
by 8 cm (August to October 2015) then increasing in elevation by 10 cm (October 2015 June
2016) with the channel migrating in a westerly direction.
Inland, across the middle transect T2, the position of the western creek network fluctu-
ated and varied in depth and width, although the elevation of the creek bed increased by
13 cm during the study period (Fig. 5b). In the most recent survey (June 2016), a decrease
Fig. 3. Change in sediment moisture content (n=5), loss on ignition (n=5), median grain size (d
)(n=3), and grain
size distribution (clay, grey dashed line; silt, grey dashed-dotted line; sand, grey dotted line; mud (clay +silt), solid
black line; n=3) during the study period. Vertical dashed lines represent when measurements of the position and
elevation of the embryonic creeks were taken.
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in elevation to the west of the main network was detected due to the headward erosion of a
sub-channel (Fig. 4d). The depth of the eastern creek network increased across the middle
transect by 12 cm between the first and third surveys, before decreasing between the third
and final survey, with channel incision creating a broader channel, and accretion occurring
on the surrounding banks.
At T3, the landwards extremity of the developing creek network (Fig. 5c), the eastern
creek bed eroded by 8 cm and incised significantly over time, resulting in a wide channel
with steep banks. The western channel, which had only extended headward to intersect
transect T3 in the most recent survey, was not detected in elevation measurements.
sUAS-derived DSM
The sUAS survey, on 13 July 2016, acquired 319 images, which were used to produce a
dense point cloud comprised of 4 904 206 matched points. The effective overlap of photo-
graphs was >9 images per point within the study area. The DSM (Fig. 6) had a reported res-
olution of 0.0263 m per pixel, and the resolution of the RGB orthorectified image (Fig. 1b)
was 0.00658 m per pixel. The software reported a total RMSE value of 0.027 m for the final
Fig. 4. dGPS measurements of the position of the embryonic creek networks at the Medmerry Managed
Realignment Site on (a)8August2015,(b) 22 October 2015, (c)3March2016,and(d) 10 June 2016. Measurements
were taken from within the borrow pit to the nickpoint; the abrupt break in the longitudinal profile. Positions
of cross-profile transects (T1, T2, and T3) are marked by dashed lines (see Fig. 5).
Dale et al. 23
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orthophoto (Agisoft 2016). Independent comparisons between the DSM and the six dGPS
measured control points (Table 1) indicated that the difference in position (XY)ranged
between 0.017 and 0.058 m (DSM minus dGPS measurements), with RMSE values of 0.028
and 0.033 m for the xand yvalues, respectively. Vertical differences between the indepen-
dent control points and the DSM varied between 0.022 and 0.038 m. The RMSE value was
0.024 m and the MAE was 0.023 m, within the range of acceptable values for reasonable sur-
face reconstruction reported by Tonkin and Midgley (2016). A visual comparison (Fig. 6) indi-
cated that the DSM closely resembled the dGPS measurements of creek position. The
vertical RMSE between the DSM and dGPS transect measurements was 0.032 m and the
MAE was 0.025 m, ranging from 0.085 to 0.056 m (Table 1), indicating a high degree of con-
fidence in the ability of the sUAS-derived DSM to represent the developing morphological
features in newly inundated intertidal settings.
Embryonic creek evolution
Within the Medmerry Managed Realignment Site, two embryonic creek networks have
formed on the bank surrounding a near-breach infilling borrow pit, extending 28 m inland
(i.e., away from the borrow pit, Fig. 4). As the creeks have formed, the elevation of the bank
Fig. 5. Elevation changes across the bank taken from (a) T1 at the edge of the borrow pit, (b) T2 inland, and (c)T3at
the top of the embryonic creek system on 8 August 2015, 22 October 2015, 3 March 2016, and 10 June 2016 (see Fig. 4
for location). The reported error in all elevation measurements was <±0.02 m.
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has increased rapidly, with coarse inorganic sediment being deposited. In contrast, the
change in bed elevation within the creeks has been considerably lower. The evolution of
the creek networks does not appear to have influenced the physical surface sediment prop-
erties at the study site (Fig. 3), in terms of moisture content, loss on ignition, and grain size.
The formation and evolution of creek networks is generally considered to be caused by
pre-existing topographic irregularities, which concentrate the flow and hence promote ero-
sion (e.g., DAlpaos et al. 2006). MR sites may provide the best opportunity to study these
processes empirically in a previously non-channelled landscape (Vandenbruwaene et al.
2012), whereas in most intertidal marshes creeks have already formed and reached a state
Fig. 6. DSM produced of the study site, with the position of the six independent control points, the creek positions
measured by a dGPS, and the location of sampling points taken in three WE transects across the study area
(see Figs. 4 and 5), indicated.
Table 1. DSM quality in comparison to x,y, and vertical dGPS
measurements of six independent control points and the vertical
measurements from 53 measurements taken from three transects
crossing the study area.
Independent control points Transects
Mean difference (m) 0.016 0.021 0.012 0.009
Maximum difference (m) 0.017 0.006 0.022 0.085
Minimum difference (m) 0.057 0.058 0.038 0.056
RMSE (m) 0.028 0.033 0.024 0.032
MAE (m) 0.023 0.024 0.023 0.025
Dale et al. 25
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of quasi-equilibrium (Marani et al. 2003;Vandenbruwaene et al. 2012). From observations
and measurements of creek formation at Medmerry made in this study, it is proposed that
the creek networks formed due to the collapse of sub-surface tunnels, a phenomenon
known as piping. Creeks have formed in areas where water appeared to be draining from
the bank through sub-surface tunnels that emerged at the surface within the borrow pit
(Fig. 7), first observed in June 2014, and which have subsequently collapsed. The occurrence
of piping has commonly been documented in arid and semi-arid regions with occasional
high intensity rainfall events (e.g., Gutierrez et al. 1997), and within the engineering litera-
ture regarding dam failure (e.g., Liu 2012), and was first observed in saltmarsh environ-
ments by Kesel and Smith (1978).Totheauthorsknowledge this is the first record of
piping in a newly inundated intertidal setting.
Piping occurs when there is a sub-surface flow of water through pores, cracks, root chan-
nels, and other sub-surface (high permeability) features, which flushes out the fine sedi-
ment forming a pipe below the surface (Kesel and Smith 1978). In newly inundated former
terrestrial sites, such as Medmerry, previously free draining sediments are exposed to tidal
cyclicity following de-embankment and become saturated twice a day. As tidal waters
recede faster than the soil can drain, a differential hydraulic head forms at the edges of
the banks and main drainage features (channels and borrow pits). During periods of bank
exposure (i.e., low water), water flows through the bank towards the lower hydraulic head
due to the difference in hydrostatic pressure. The flow of water flushes fine sediments from
the bank, increasing the diameter of the sub-surface pipe, with entrances forming at the
top of the banks due to focussed surface collapse. Eventually the sub-surface pipes collapse
along their length, forming embryonic creeks.
From the observations made at the Medmerry Managed Realignment Site, it remains
unclear whether there is a common difference in head required to generate piping, and fur-
ther analysis of other MR sites (and natural intertidal settings) is required to assess whether
these processes are occurring elsewhere. However, the use of the sUAS orthophoto provides
Fig. 7. Water flowing from the bottom of a pipe (inset) through the bank of the borrow pit in June 2014.
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clear evidence for this process continuing to occur at this site 3 years after site breaching.
From the orthomosaic produced, two locations can be identified where additional pipe net-
works appear to be emerging from the bank (Fig. 8) with areas of pooled percolating water
identifiable on the bank; these have been verified visually on site. One of the systems, to the
west of the embryonic creek networks (Fig. 8a), appears to be fed by a series of small pipe
tops running parallel to the borrow pit edge, and emerges as a single outlet within the bor-
row pit. A larger, more irregularly shaped, pipe top feeds the second of these creek systems
to the east of the creek network (Fig. 8b).
The suitability of sUAS technology for measuring embryonic creek formation
The use of sUASSfM surveys elsewhere (e.g., Westoby et al. 2012;Javemick et al. 2014)
has successfully produced high-resolution models without the spatial limitations (i.e., inter-
polation from individual points) and user bias created by selectively choosing the measur-
ing location associated with dGPS measurements. In addition, the data collection process
for sUASSfM surveys takes the same amount of time as taking dGPS measurements, but
at a much higher resolution (<0.03 m resolution for the DSM in comparison to up to several
metres between individual dGPS points). A comparison of the three transects analysed in
this study (Fig. 9), measured by dGPS and extracted from the DSM, indicates that the dGPS
readings are effective at measuring elevation at a centimetre to metre scale, but miss
smaller topographic variations. This comparison also provides further confidence in the
DSM given the similarity, in the majority of measurements, between the dGPS points and
the modelled elevation.
Some discrepancies do exist between the DSM and dGPS measurements (Fig. 9), possibly
caused by variability associated with the x,yerror or the influence of edge effects and the
parameters used in the DSM construction process. There is also the possibility that errors
Fig. 8. Evidence of piping (a)tothewestand(b) to the east of the embryonic creek networks at the Medmerry
Managed Realignment Site.
Dale et al. 27
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exist in the vertical measurements, caused by measuring the elevation using dGPS in rela-
tively soft unconsolidated sediment. Nonetheless, the use of sUASSfM is likely to mitigate
these errors as ground control points can be taken in more consolidated areas.
Furthermore, through the use of separate control points in model construction and as an
independent check, the quality of the model can be assessed; an option not available using
just dGPS measurements. The DSM produced is also at a higher, more suitable, resolution
than standard remote sensing techniques, such as LiDAR, which have previously been used
in the design and prediction of ecological response to MR schemes (e.g., Blott and Pye 2004;
Millard et al. 2013;Krolik-Root et al. 2015), increasing the likelihood that small (but impor-
tant) changes in elevation will be captured. This technique could, therefore, be utilised to
provide a more detailed understanding of how creek features develop within intertidal
wetland environments.
While the dGPS surveys utilised in this study can be effectively used to monitor metre-
scale (and, in cross sections in Fig. 5, centimetrem-scale) changes in creek morphology,
and show strong concordance with sUASSfM data (Fig. 6), repeated sUASSfM surveys of
this nature would allow for the analysis of accurate volumetric changes across the site.
This would then allow for the net changes in sediment accretion and erosion to be estab-
lished on a spatial scale beyond the capabilities of methods currently being utilised in these
environments (e.g., Ni et al. 2014;Dale et al. 2017), and in intertidal wetland environments
generally. This technique may also allow for advanced and effective drainage network map-
ping considering changes pre- and post-site breaching, over the temporal and spatial scales
Fig. 9. Comparison of (a) T1 at the edge of the borrow pit, (b) T2 inland, and (c) T3 at the top of the embryonic creek
system from the DSM (solid black) and dGPS measurements (dashed grey) (see Fig. 4 for location).
28 Anthropocene Coasts Vol. 1, 2018
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in which embryonic creek formation takes place, to inform coastal engineers and managers
of the influence of different site design processes and the impact of any morphological
features (e.g., borrow pits or terrestrialconstructed drainage channels) existing prior to
site breaching. This would improve the design of MR sites, encouraging creek development,
to enhance the success of future schemes.
Influence of the former land use on creek formation and evolution
Watts et al. (2003) proposed that creeks would only form in soft accreted sediments
exceeding a critical depth of 2030 cm. There has been a large amount of accretion in the
vicinity of the creek networks forming at this site; Dale et al. (2017) reported over 15 cm of
accretion in the borrow pit adjacent to the area monitored here over a 1 year study period
during the second year following site breaching. However, at Medmerry, creek formation
was already occurring due to a difference in hydraulic head and sub-surface drainage, prior
to the level of accretion reaching the critical 2030 cm depth proposed, albeit site specifi-
cally, by Watts et al. (2003).
It was reported by Vandenbruwaene et al. (2012) that creek development was hindered
by a compact clay layer in controlled reduced tidal schemes on the Scheldt (Belgium).
Observations made at Medmerry suggest that a similar process is occurring. Following pipe
collapse, creeks have not been able to incise through an underlying layer of finer-grained
(seemingly more compact) sediment (Fig. 10), compared to the coarse-grained surface sedi-
ment. It is likely that this lower sediment unit is of terrestrial origin, suggesting that creek
development is determined by the relationship between different sub-surface sediment
conditions (and sediment types). It is widely considered that de-embankment should be car-
ried out on areas previously reclaimed for agricultural use (French 2006); this inevitably
means that most MR sites will have a complex sub-surface stratigraphy. Cundyetal.
(2002), for example, found evidence that the terrestrial soil horizon could still be detected
at Pagham harbour (southern UK) almost a 100 years after natural site breaching during a
storm event, whilst Tempest et al. (2015) demonstrated that the former terrestrial horizon
Fig. 10. The two distinct sediment units in which creeks have formed consisting of the (lower) terrestrial sediment
and (upper) sediment deposited following site breaching.
Dale et al. 29
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significantly restricted sub-surface hydrological connectivity at the Orplands Farm MR Site
(Essex, UK).
Rapid creek development in MR sites is considered essential for site drainage, the evolu-
tion of the sediment regime, coastal flood defence, and the ecosystem services provided,
although results in this study suggested that moisture content (indicative of sediment
drainage) in surrounding surface sediments did not decrease as creeks developed. Given
that the majority of MR sites are constructed on areas of the coastal hinterland that have
been used for arable agriculture, it is likely that sediment has been compacted and structur-
ally altered (Dent et al. 1976). Consequently, there is a need for high rates of sediment accre-
tion in MR sites to reduce the influence of the terrestrial sediment unit or to engineer sites
to encourage creek development and growth. For example, sites could be ploughed or engi-
neered to expose uncompacted soils that are more susceptible to creek formation, or
dredged material could be distributed over the terrestrial soil prior to site breaching;
DAlpaos et al. (2007), for example, observed rapid creek evolution in a reconstructed salt-
marsh in Venice Lagoon where dredged material had been used in site regeneration.
The design of drainage networks and borrow pits in MR sites could also be enhanced to
encourage the drainage and formation of a differential hydraulic head to accelerate piping,
and therefore creek development, as observed in this study. This could also incorporate pre-
existing drainage features, such as soakaways and drainage pipes; features that have
received little consideration in the design of MR sites to date. Identification of the processes
and analysis of the parameters influencing embryonic creek development within other MR
sites is required to assess the similarity between sites. Sub-surface pipes are transient fea-
tures, limiting the timeframe available for capturing their influence on embryonic creek
development. Experimental laboratory or numerical modelling studies may, therefore, be
necessary to analyse the influence of different sub-surface sedimentological conditions on
creek formation in an intertidal setting, as have previously been carried out for alternative
environments (e.g., Wang et al. 2016). This would improve the design of MR sites and
encourage creek development following site breaching, thereby increasing the level of
coastal flood defence, providing compensation for habitat losses and degradation, and
enhancing the ecosystem services provided.
There is growing evidence that MR sites have lower biodiversity and delivery of ecosys-
tem services than anticipated (e.g., Mossman et al. 2012), which has been associated with
poor drainage and hydrological connectivity within these sites (e.g., Tempestetal.2015).
Despite intertidal creek networks being important for drainage (and indeed sediment sup-
ply), little is known about the evolution of embryonic creek networks in recently inundated
intertidal environments. It has previously been suggested that creeks would only form pro-
viding there had been a sufficient level of sediment accretion (Watts et al. 2003). However,
analysis of the sedimentological factors influencing creek formation at the Medmerry
Managed Realignment Site, where it is proposed creeks have formed as a result of the col-
lapse of sub-surface pipes, indicates that creeks will form in the inundated terrestrial sedi-
ment, although their subsequent incision and further development is limited. Given this,
the use of dredged material and site landscaping to accelerate creek growth post-site
breaching requires wider consideration. Using these measures to promote creek develop-
ment will enhance the ecosystem services, habitat loss compensation, and level of coastal
flood defence provided by MR sites.
Given the highly dynamic nature of these newly-inundated environments, frequent, and
regular, site visits are required to make these observations, especially given the transient
nature of embryonic creek formation processes. Measurements of embryonic creek growth
30 Anthropocene Coasts Vol. 1, 2018
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indicate that creeks will develop relatively quickly but are influenced by sub-surface sedi-
mentological conditions (such as sediment compaction and consolidation), which influence
hydrological connectivity, and which in many cases will relate to historic environmental
change and the former land use. Higher resolution measurements of embryonic creek
growth are required to capture the onset of creek development and to provide further
insight into the factors controlling creek evolution in newly inundated MR sites; the sUAS
method discussed here would provide such measurements.
The authors would like to thank David Stansbury for his support with GPS measure-
ments; Peter Hughes (RSPB); Magda Grove and Matt Leake (both University of Brighton)
for their assistance with fieldwork; and Callum Firth for his guidance during JD and PKs
studentship. We would also like to thank two anonymous reviewers for their supportive
and constructive comments on an earlier version of the manuscript. Financial support
was provided by the Environment Agency (UK) for JDs studentship and by the School of
Environment and Technology, University of Brighton for PKs studentship.
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... Therefore, newly inundated intertidal environments, such as MR sites, may provide the best 70 opportunity to study initial creek formation and evolution (Dale et al., 2018;71 Vandenbruwaene et al., 2012). 72 ...
... Previous studies that investigated 79 topographic variability in MR sites (e.g. Dale et al., 2018;Lawrence et al., 2018 ) have relied 80 on relatively coarse resolution remote sensing and surveying data, such as differential GPS 81 and LiDAR (Light Detection and Ranging) datasets. In a recent study, Chirol et al., (2018) 82 proposed the use of a semi-automated creek extraction algorithm to monitor creek 83 development within newly inundated intertidal settings, based on LiDAR data. ...
... One solution to these limitations was proposed by Dale et al., (2018) through the use of a 93 ...
Coastal and estuarine wetlands provide a range of important ecosystem services, but are currently being damaged and degraded due to human activities, reduced sediment supply and sea level rise. Managed realignment (MR) is one approach used to compensate for the loss of intertidal habitat, however saltmarshes in MR sites have been recognised to have lower biodiversity than natural environments. This has been associated with differences in the physical functioning including the sediment structure, reduced hydraulic connectivity, and lower topographic variability such as the abundance of intertidal creek networks. Intertidal morphology, including creek networks, play an important role in supporting and regulating saltmarsh environments through the supply of sediment, nutrients and water, and in draining intertidal marshes. However, there is a lack of empirical data on the formation and evolution of topographic features and variability in saltmarsh environments. This is likely to be due to creek networks in natural marshes already being in a state of quasi-equilibrium, making MR sites an ideal environment to investigate creek development. However, traditional remote sensing techniques (such as LiDAR) tend to be relatively expensive, infrequent and at a coarse resolution meaning small, but important (cm-scale), changes are often missed. This study advances the ability to detect these small scale changes by demonstrating the suitability and potential applications of using the emerging photogrammetric method Structure-from-Motion (SfM) on images taken using a small-Unmanned Aerial System (sUAS). Three surveys from a rapidly changing, near-breach site were taken at the Medmerry Managed Realignment Site in July 2016, September 2017 and July 2018. A suitable degree of confidence was found between the modelled surface and independent check points (vertical root-mean-square-errors of 0.0245, 0.0704 and 0.1571 for 2016, 2017 and 2018 respectively). DSMs of Difference (DoD) analysis was performed to evaluate elevation change, with areas experiencing up to 85 cm of accretion between 2016 and 2018. However, when considering the error associated with both surveys, between 2016 and 2017, only 34.39% of the survey area experienced change above the level of detection (LoD). In contrast, 76.97% experienced change greater than the LoD between 2017 and 2018. Stream order analysis classified the creek networks into five orders in 2016 and four orders in 2017 and 2018, with 2016 having a higher abundance (291 in 2016 compared to 117 (2017) and 112 (2018)) and density (0.44 m/m2 in 2016 compared to 0.27 m/m2 in both 2017 and 2018) of creek networks. These results provide an innovative high resolution insight into the evolution of restored intertidal wetlands, and suggest that SfM analysis of images taken using a sUAS can be a useful tool with the potential to be incorporated into studies of MR and natural saltmarsh sites. sUAS analysis can, therefore, advance the management of these environments to ensure the provision of ecosystem services and to protect against future anthropogenic activity, sea level rise and climate change.
... Consequently, these areas are commonly managed and engineered to reduce damage, loss of life, and environmental degradation caused by natural hazards originating from the sea. However, sea-level rise and increased storminess due to global warming (e.g., Sayers et al., 2015) are putting unprecedented pressure on these managed systems, rendering some of them uneconomical and unsustainable to maintain, in which case we adopt either a "no active intervention" or a "managed realignment" strategy (Dale et al., 2018;French, 2006;Turner et al., 2007). ...
... The results illustrate the application of the model which emphasizes the use of Monte-Carlo simulation. The processes and phenomena the model aims to simulate, that is, the formation and evolution of creek networks, are characterized by high levels of uncertainty (Whitehouse et al., 2000;Dale et al., 2018). These uncertainties are accounted for through random headward expansion direction selection in each individual model run and represented as heat maps by way of Monte-Carlo simulation. ...
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Plain Language Summary A large part of the world's low‐lying areas is heavily managed to protect lives, assets, and landscapes from marine‐derived natural hazards. However, climate change is putting unprecedented pressure on these managed environments, forcing the adoption of “no active intervention” or “managed realignment” strategies in areas where “hold the line” options cannot be justified due to financial constraints. This leads to the very likely possibility that unmaintained coastal defenses will begin to fail, raising questions as to the potential impacts on low‐lying regions as well as the options available to mitigate impacts through targeted interventions. Of particular interest is the question of whether it is possible to facilitate the shift of managed coastal wetlands towards intertidal environments in which tidal creeks form and evolve to support salt marshes. Combining the use of a numerical model and multiple model runs, we find that although the evolution of tidal creeks is subject to random disturbances, creek networks are likely to form with a clear tendency towards specific types. Grouped results of multiple, random simulations are useful in identifying interventions that are necessary for developing far‐reaching tidal creek networks. Influence of vegetation is found to be secondary compared to the wetland topography in terms of influencing tidal creek evolution.
... This has been associated with anoxic conditions as a result of poor drainage, caused by differences in the sediment sub-surface due the accretion of sediment over the terrestrial soil which had been disturbed, compacted, and altered by the previous agricultural activity [7,8]. In addition, it has been demonstrated that MR sites have lower topographic and morphological variability in comparison to natural environments [6,9], with terrestrial and agricultural morphological features such as tyre tracks and plough lines remaining visible several years after site breaching [10][11][12]. In natural marsh environments, where the typically shallow gradient controls morphological development, un-vegetated mudflat occupies the area below the elevation of neap high tide water level. ...
... In recent years, unmanned aerial vehicles (UAVs) have become increasingly popular as a technique to measure large (site) scale morphological change in high resolution [15][16][17], facilitated by advancements in UAV technology, compatible sensors, and computer software [18,19]. Within coastal wetland environments, UAV technologies are becoming increasingly relevant for assessments of morphological variability [10,20] and ecological functioning [21,22], replacing lower resolution surveying techniques such as LiDAR [9,23,24]. This study explores the potential to employ UAV technologies as a possible solution in order to assess saltmarsh coverage, and the relationship between vegetation cover and morphology, in MR sites. ...
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Managed realignment (MR) sites are being implemented to compensate for the loss of natural saltmarsh habitat due to sea level rise and anthropogenic pressures. However, MR sites have been recognised to have lower morphological variability and coverage of saltmarsh vegetation than natural saltmarsh sites, which have been linked with the legacy of the historic (terrestrial) land use. This study assesses the relationship between the morphology and vegetation coverage in three separate zones, associated with the legacy of historic reclamation, of a non-engineered MR site. The site was selected due to the phased historical reclamation, and because no pre-breaching landscaping or engineering works were carried out prior to the more recent and contemporary breaching of the site. Four vegetation indices (Excess Green Index, Green Chromatic Coordinate, Green-Red Vegetation Index, and Visible Atmospherically Resistant Index) were calculated from unmanned aerial vehicle imagery; elevation, slope, and curvature surface models were calculated from a digital surface model (DSM) generated from the same imagery captured at the MR site. The imagery and DSM summarised the three zones present within the MR site and the adjacent external natural marsh, and were used to examine the site for areas of differing vegetation cover. Results indicated statistically significant differences between the vegetation indices across the three zones. Statistically significant differences in the vegetation indices were also found between the three zones and the external natural saltmarsh. However, it was only in the zone nearest the breach, and for three of the four indices, that a moderate to strong correlation was found between elevation and the vegetation indices (r = 0.53 to 0.70). This zone was also the lowest in elevation and exhibited the lowest average value for all indices. No relationship was found between the vegetation indices and either the slope or curvature in any of the zones. The approach outlined in this paper provides coastal managers with a relatively low-cost, low-field time method of assessing the areas of vegetation development in MR sites. Moreover, the findings indicate the potential importance of considering the historic morphological and sedimentological changes in the MR sites. By combining data on the areas of saltmarsh colonisation with a consideration of the site’s morphological and reclamation history, the areas likely to support saltmarsh vegetation can be remotely identified in the design of larger engineered MR sites maximising the compensation for the loss of saltmarsh habitat elsewhere.
... Further, financial resource limitations may require a move from 'hold the line' to 'no active intervention' or 'managed realignment' options (Esteves and Williams 2017) for coastal management which can potentially impact back-barrier wetlands and habitats (Rupp-Armstrong and Nicholls 2007;Friess et al. 2014;Brady and Boda 2017). Notable managed realignment examples in the UK include Tollesbury (Garbutt et al. 2006), Freiston (Friess et al. 2014), Hesketh Marsh (Tovey et al. 2009), and Medmerry (Dale et al. 2017(Dale et al. , 2018. ...
... Direct interventions in the back-barrier to steer landscape evolution in the wetland, such as creating creeks and developing the necessary topography, can ultimately compensate for habitat loss induced by seawater intrusion (e.g. Dale et al. 2017Dale et al. , 2018Lawrence et al. 2018). For example, the fast seawater ingress promoted by high connectivity of the drainage network mentioned above could be impeded by construction of embankments that disconnect the network. ...
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Coastal wetland ecosystems and biodiversity are susceptible to changes in salinity brought about by the local effects of climate change, meteorological extremes, coastal evolution and human intervention. This study investigates changes in the salinity of surface water and the associated impacts on back-barrier wetlands as a result of breaching of a barrier beach and under the compound action of different surge heights, accelerated sea-level rise (SLR), river discharge and rainfall. We show that barrier breaching can have significant effects in terms of vegetation die-back even without the occurrence of large storm surges or in the absence of SLR, and that rainfall alone is unlikely to be sufficient to mitigate increased salinity due to direct tidal flushing. Results demonstrate that an increase in sea level corresponding to the RCP8.5 scenario for year 2100 causes a greater impact in terms of reedbed loss than storm surges up to 2 m with no SLR. In mitigation of the consequent changes in wetland ecology, regulation of relatively small and continuous river discharge can be regarded as a strategy for the management of coastal back-barrier wetland habitats and for the maintenance of brackish ecosystems. As such, this study provides a tool for scoping the potential impacts of storms, climate change and alternative management strategies on existing wetland habitats and species.
... Increasingly, MRs are also implemented on open coastlines, where estuarine water level variations are negligible, and increased coastal protection is solely achieved by withinmarsh attenuation (Kiesel et al., 2019). Presumably, the lack of meaningful pre-implementation within-marsh modelling is because modelling the geomorphic development of newly inundated salt marshes, e.g. the evolution of tidal creek networks, is challenging and associated with significant uncertainties (Dale et al., 2018). Meanwhile, field and modelling data indicate that the MR size, as well as the nature of the tidal creek networks, may play a deciding role in whether a salt marsh attenuates or amplifies storm surge water levels (Kiesel et al., 2020;Stark et al., 2016). ...
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Nature-based solutions are increasingly suggested for mitigating coastal flood risks in the face of climate change. Managed realignment (MR), a coastal adaptation strategy that entails the landward realignment of coastal defences to restore coastal habitats (often salt marshes), plays a pivotal role in implementing nature-based solutions in the coastal zone. Across Europe, more than 130 sites have been implemented so far, often to harness their potential to mitigate coastal flood risks while restoring coastal habitats (ABPmer, 2021). However, local communities often oppose MR projects, not only because they are seen as returning hard-won land to the sea but also because their coastal protection function is less trusted than traditional hard engineering techniques. This scepticism has foundation. The proclaimed coastal protection function of MRs is based on a broad body of literature on the protective function of natural salt marshes. However, contrary to natural salt marshes, MRs are often semi-enclosed tidal basins with narrow breaches to the open sea/estuary. Recent studies indicate that MR-internal hydrodynamics may significantly reduce their coastal protection, depending on their engineering design. To successfully implement MR, a much-improved scientific knowledge base is needed, as well as a process for addressing community concerns and genuinely engaging stakeholders in decision-making beyond the usual obligatory consultancy approach. Here, we propose the co-production of scientific knowledge with local communities and stakeholders to optimize the success of coastal nature-based solutions and promote community acceptance.
... With salt marsh now developing along the banks this could be being used as a new habitat by the fish, but as much of this was flat farmland the fish may not be venturing too far into these areas as they may get stranded on the falling tide. As the creek systems continue to develop across these areas (Dale et al., 2018) then there is a possibility that these intertidal areas may become accessible to fish. ...
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This looks at the impact that geomorphological development of a new, large Managed Realignment site has on fish habit and useage. It was presented at the ICE coastal management conference in La Rochelle, France in September 2019. The full published version can be obtained from the Institute of Civil Engineers website:
Field investigation and empirical analysis were used to assess the impact of tidal creeks and tidal flap gates (through which land surface runoff drains into estuaries) on sediment transport in the receiving waters downstream of the creeks and propose a semi-empirical model to assess the impact of water level rise and increased runoff flow, as may be expected under many climate change scenarios. Results show that there is increased suspended sediment concentration (SSC) at low tide within the deeper channel downstream of the tidal flap gate. This is due to the high SSC that is ejected by the creek because of land drainage runoff with SSC in the creek ranging from 47 mg/l to 300 mg/l at spring ebb tide and that in the deeper channel ranging from 22 mg/l to 104 mg/l. This effect was more pronounced at spring tides when the water level in the deeper channel was low. Substantial sediment is supplied by the creek on a spring low tide than a neap low tide due to higher exposure period of intertidal flats for spring low tides. The combination of creeks and tidal flap gates has the highest impact on SSC in the deeper channel at low tide and high surface runoff flow. It is recommended that moving forward on developments around coastal areas, management authorities create more effective and sustainable drainage systems with less impact on estuarine environments to protect aquatic life and promote habitat creation for a sustainable future. Such systems whereby there are numerous drainage flaps surrounding estuaries especially in intertidal flat areas should be transformed into systems with fewer secondary effects on estuarine environments.
There is growing evidence that managed realignment (MR) sites have lower biodiversity than natural saltmarshes, which has been associated with differences in the physical function and morphological evolution following site breaching. This evidence has been derived from MR sites previously used for intensive arable agriculture or modified during site construction. Therefore, the development of these sites may not be representative of the sedimentological evolution that would have otherwise occurred. This paper presents analysis of high spatial resolution digital surface models derived from the images collected using a small-unmanned aerial system from a non-engineered MR site at Cwm Ivy Marsh, Gower Peninsula, Wales. These models are examined alongside a novel combination of high frequency measurements of the rate and patterns of sedimentation, suspended sediment concentration (SSC), and the sub-surface structure and geochemical profiles. Results indicated that although the site became topographically less variable over a four year period, the intertidal morphology developed through an increase in the abundance of higher order creek systems and sediment being deposited at a rate of between 3 and 7 cm/year. The SSC followed an inverse pattern to water depth, with bed elevation increasing and then decreasing during both the flood and ebb tidal phases. Analysis of the sediment subsurface geochemical composition indicated redox profiles similar to natural intertidal environments; evidence of a fluctuating water table was found at a saltmarsh site, in comparison to waterlogging identified at an anoxic mudflat site. These findings provide a new insight to the sedimentological processes in a MR site without the influence of landscaping or site engineering prior to site-breaching, which can be used to inform the design of future MR sites.
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The loss of unimproved grassland has led to species decline in a wide range of taxonomic groups. Agricultural intensification has resulted in fragmented patches of remnant grassland habitat both across Europe and internationally. The monitoring of remnant patches of this habitat is critically important, however, traditional surveying of large, remote landscapes is a notoriously costly and difficult task. The emergence of small-Unmanned Aircraft Systems (sUAS) equipped with low-cost multi-spectral cameras offer an alternative to traditional grassland survey methods, and have the potential to progress and innovate the monitoring and future conservation of this habitat globally. The aim of this article is to investigate the potential of sUAS for rapid detection of threatened unimproved grassland and to test the use of an Enhanced Normalized Difference Vegetation Index (ENDVI). A sUAS aerial survey is undertaken at a site nationally recognised as an important location for fragmented unimproved mesotrophic grassland, within the south east of England, UK. A multispectral camera is used to capture imagery in the visible and near-infrared spectrums, and the ENDVI calculated and its discrimination performance compared to a range of more traditional vegetation indices. In order to validate the results of analysis, ground quadrat surveys were carried out to determine the grassland communities present. Quadrat surveys identified three community types within the site; unimproved grassland, improved grassland and rush pasture. All six vegetation indices tested were able to distinguish between the broad habitat types of grassland and rush pasture; whilst only three could differentiate vegetation at a community level. The Enhanced Normalized Difference Vegetation Index (ENDVI) was the most effective index when differentiating grasslands at the community level. The mechanisms behind the improved performance of the ENDVI are discussed and recommendations are made for areas of future research and study.
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The use of small UAVs (Unmanned Aerial Vehicles) and Structure-from-Motion (SfM) with Multi-View Stereopsis (MVS) for acquiring survey datasets is now commonplace, however, aspects of the SfM-MVS workflow require further validation. This work aims to provide guidance for scientists seeking to adopt this aerial survey method by investigating aerial survey data quality in relation to the application of ground control points (GCPs) at a site of undulating topography (Ennerdale, Lake District, UK). Sixteen digital surface models (DSMs) were produced from a UAV survey using a varying number of GCPs (3-101). These DSMs were compared to 530 dGPS spot heights to calculate vertical error. All DSMs produced reasonable surface reconstructions (vertical root-mean-square-error (RMSE) of <0.2 m), however, an improvement in DSM quality was found where four or more GCPs (up to 101 GCPs) were applied, with errors falling to within the suggested point quality range of the survey equipment used for GCP acquisition (e.g., vertical RMSE of <0.09 m). The influence of a poor GCP distribution was also investigated by producing a DSM using an evenly distributed network of GCPs, and comparing it to a DSM produced using a clustered network of GCPs. The results accord with existing findings, where vertical error was found to increase with distance from the GCP cluster. Specifically vertical error and distance to the nearest GCP followed a strong polynomial trend (R 2 = 0.792). These findings contribute to our understanding of the sources of error when conducting a UAV-SfM survey and provide guidance on the collection of GCPs. Evidence-driven UAV-SfM survey designs are essential for practitioners seeking reproducible, high quality topographic datasets for detecting surface change.
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Airborne photogrammetry is undergoing a renaissance: lower-cost equipment, more powerful software, and simplified methods have significantly lowered the barriers to entry and now allow repeat mapping of cryospheric dynamics at spatial resolutions and temporal frequencies that were previously too expensive to consider. Here we apply these advancements to the measurement of snow depth from manned aircraft. Our main airborne hardware consists of a consumer-grade digital camera directly coupled to a dual-frequency GPS; no inertial motion unit (IMU) or on-board computer is required, such that system hardware and software costs less than USD 30 000, exclusive of aircraft. The photogrammetric processing is done using a commercially available implementation of the structure from motion (SfM) algorithm. The system is simple enough that it can be operated by the pilot without additional assistance and the technique creates directly georeferenced maps without ground control, further reducing overall costs. To map snow depth, we made digital elevation models (DEMs) during snow-free and snow-covered conditions, then subtracted these to create difference DEMs (dDEMs). We assessed the accuracy (real-world geolocation) and precision (repeatability) of our DEMs through comparisons to ground control points and to time series of our own DEMs. We validated these assessments through comparisons to DEMs made by airborne lidar and by a similar photogrammetric system. We empirically determined that our DEMs have a geolocation accuracy of ±30 cm and a repeatability of ±8 cm (both 95 % confidence). We then validated our dDEMs against more than 6000 hand-probed snow depth measurements at 3 separate test areas in Alaska covering a wide-variety of terrain and snow types. These areas ranged from 5 to 40 km2 and had ground sample distances of 6 to 20 cm. We found that depths produced from the dDEMs matched probe depths with a 10 cm standard deviation, and were statistically identical at 95 % confidence. Due to the precision of this technique, other real changes on the ground such as frost heave, vegetative compaction by snow, and even footprints become sources of error in the measurement of thin snow packs (< 20 cm). The ability to directly measure such small changes over entire landscapes eliminates the need to extrapolate limited field measurements. The fact that this mapping can be done at substantially lower costs than current methods may transform the way we approach studying change in the cryosphere.
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The influence of the overlying clay on the progression of piping in the sandy gravel foundation of water-retaining structures is often neglected. In order to study this influence, an experimental investigation was conducted on a laboratory-scale model. It was discovered that the critical hydraulic gradient and the area of the piping tunnel increase when the overlying clay thickens. With a thicker clay layer, erosion of the sandy gravel below the clay layer occurs later, but, once the erosion starts, the erosion rate is very high and the average velocity of water seeping through the cross-section of the sandy gravel increases rapidly due to the low deformability of the thick clay layer. Furthermore, it was found that the progression of piping is a complicated and iterative process involving erosion of fine particles, clogging of pores, and flushing of the clogged pores. Two types of erosion have been identified in the progression of piping: one causes the tunnel to advance upstream, and the other increases the depth of the tunnel. The results show that the overlying clay is an important factor when evaluating piping in sandy gravel foundations of water-retaining structures.
Conference Paper
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In September 2013, the largest Open Coast Managed Realignment scheme in Europe was breached in the southern UK and Medmerry Nature Reserve created. Focusing specifically on the newly created intertidal areas, this paper presents observations and a sample of initial data of the intertidal sediment processes and geomorphological changes occurring within the new managed realignment site. Data shows that all of the sites being monitored are in a state of accretion with the source of sediment coming from both local e.g. through bank failure and distal e.g. off site sources. From the observations suggestions are made as to possible future improvements to both channel and borrow pit design.
Saltmarshes are being lost or degraded as a result of human activity resulting in loss of critical ecosystem services including the provision of wild species diversity, water quality regulation and flood regulation. To compensate, saltmarshes are being restored or re-created, usually driven by legislative requirements for increased habitat diversity, flood regulation and sustainable coastal defense. Yet, there is increasing evidence that restoration may not deliver anticipated ecosystem services; this is frequently attributed to poor drainage and sediment anoxia. However, physical sediment characteristics, hydrology and the sediment geochemical environment are rarely examined in restoration schemes, despite such factors being critical for plant succession.
Managed Realignment (MR) schemes are considered by many coastal managers and engineers to be a preferable method of coastal flood defence and compensating for habitat loss, by creating new areas of intertidal saltmarsh and mudflat habitat. Monitoring of MR sites has tended to focus on short term ecological factors, resulting in a shortage of high frequency, high resolution long term measurements of the evolution of the sediment erosion, transportation, deposition and consolidation cycle (ETDC) in newly breached sites. This is particularly true of analysis of the formation and preservation of sedimentary rhythmites and evaluations of sedimentation rates (and their variability) in newly inundated intertidal environments. This study provides an evaluation of sedimentation rhythms and hydrodynamics from two contrasting sites within the Medmerry Managed Realignment scheme, the largest open coast realignment in Europe (at the time of site inundation). Bed sediment altimeter data highlighted different sedimentation patterns at the two sites; near constant deposition of sediment occurred near the breach resulting in 15.2 cm of sediment being accreted over the one year monitoring period, whereas periodic accretion and erosion of sediment occurred inland leading to 2.7 cm of net accretion. Differences in the relationship between suspended sediment concentrations and site hydrodynamics were observed on a semi-diurnal to annual scale. This study highlights the need for further consideration of the sedimentation processes in MR schemes in order to enhance the design and construction of these sites. Advancements in the understanding of these processes will increase the success of MR schemes in terms of the evolution of the sediment regime and the ecosystem services provided, particularly as they are more widely accepted as a form of coastal flood defence and intertidal habitat creation method.