Yarmouth flythroughs
2013
Channel Coastal Observatory
National Oceanography Centre
European Way
Southampton
SO14 3ZH
Tel: +44 (0)23 8059 8467
Fax: +44 (0)23 8059 6320
e-mail: cco@channelcoast.org
Website: www.channelcoast.org
Document Title:
Reference:
Status:
Date:
Project Name:
Explanation of the visualisations produced for the CCATCH The
Solent Yarmouth flythrough
TR 50
Final
13th May 2013
CCATCH The Solent: Yarmouth flythrough
Fledermaus flythrough - Gareth Grewcock and Dr Samantha Cope
Flood modelling - Dr Matthew Wadey
Project manager – Dr Samantha Cope
Yarmouth flythroughs
Contents
CCATCH The Solent: Yarmouth flythroughs
Channel Coastal Observatory: Yarmouth flythroughs
Contents
1.
2.
Introduction .................................................................................................................. 1
Flooding flythrough ..................................................................................................... 1
2.1 Base data ..................................................................................................................................... 2
2.2
Flooding grids using LISFLOOD-FP software ......................................................................... 4
2.3
Visualisation using Fledermaus software ................................................................................ 4
3.
4.
5.
6.
Erosion mapping.......................................................................................................... 5
Acronyms ..................................................................................................................... 7
Glossary ....................................................................................................................... 7
References ................................................................................................................... 7
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List of Tables & Figures
List of Tables
Table 1: Reference still water levels (including the flood scenario levels)
2
List of Figures
Figure 1: Still water level return periods for Yarmouth for the baseline year of 2008 (source: McMillan
et al., 2011) and the scenario water levels referred to in the flythroughs
2
Figure 2: Gap in the DTM along the embankment between the A3054 and the River Yar boatyard 3
Figure 3: Photograph along the embankment the A3054 and the River Yar boatyard. Looking south
to the River Yar boatyard
3
Figure 4: No Active Intervention (NAI) and With Present Management (WPM) indicative erosion zone
for Yarmouth (shapefiles courtesy of the IOW SMP2)
ii
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1.
Introduction
Introduction
Hampshire County Council is working with 6 other partners on an EU Interreg IVa – 2 seas
cross-border programme project called ‘Coastal Communities 2150 and Beyond’ (CC2150).
Hampshire County Council’s element of the CC2150 project is ‘Coastal Communities
Adapting to Change (CCATCH) – the Solent’. This builds on the work that was carried out
through the ‘CCATCH – the Solent trial’ project which covered the coast from Beaulieu to
Calshot by widening it to the whole of the Solent region.
The CCATCH – the Solent project is focusing on 6 discrete stretches of coast and involves
all sectors of the local community in developing a plan for the future of the coastal area.
These sites may not be the most at risk but reflect a range of the different communities and
issues around the Solent and are:






Beaulieu to Calshot.
Southampton, Upper West Itchen
Netley and Royal Victoria Country Park
Solent Breezes Holiday Park
Hayling Island and Langstone.
Yarmouth, Isle of Wight
As part of CCATCH - the Solent project, the Channel Coastal Observatory were
commissioned by Hampshire County Council to undertake a visualisation of Yarmouth town
to demonstrate to the Yarmouth community how they will be affected under a range of future
flooding scenarios. Erosion maps were also produced using the Isle of Wight Shoreline
Management Plan 2 outputs.
2.
Flooding flythrough
The flythroughs visualise three water level scenarios as specified by the Yarmouth Coastal
Defence Working Group. These are listed in Table 1 and Section 2.3. To understand the
nature of extreme flood events, sea level is decomposed into three additive components:
mean sea level, astronomical tidal level and a residual (often referred to as surge)
component (refer to Pugh, 1987).
Scenarios 2a and 2b are used here, to replicate a similar level of flooding to that observed
during the 10 March 2008. The exact magnitudes of each sea level component at Yarmouth
that day is unknown, although the tide gauges elsewhere in the Solent, and the observed
high water level at Yarmouth, suggest over a metre surge on top of the spring tide.
According to recent return period analysis, Scenarios 2a and 2b are equivalent to
approximately 1 in 20 and 1 in 65 year events respectively (Figure 1). The relatively large
range of probability associated with this 0.1m water level variation highlights the small
differences between ‘extreme event’ magnitudes and their definitions on this part of the
south coast.
The deliverables from the project focus on Scenario 2a, rather than Scenario 2b. This is
because flooding the terrain model (Section 2.1) to the requested Scenario 2a water level
produced an area of flooded hinterland greater than that experienced on the 10th March
2008. Using Scenario 2b would have resulted in even more of an over-estimation of
flooding.
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Yarmouth flythroughs
Flooding flythrough
The addition of 0.3m to Scenario 2b to provide Scenario 3 generates a hypothetical event of
a probability which is implausible at present day mean sea level, and is hence relevant as a
scenario for mean sea-level rise. Scenario 3 is not depicted on Figure 1 as it is out of the
remit of present day return period analysis given the addition of the 0.3m of mean sea level.
Level
Scenario
Reference
Scenario description
mCD
mOD
MLWS
0.80
-1.18
MLWN
1.60
-0.38
MSL
2.20
0.22
Scenario 1
MHWS
3.00
1.02
N/A
HAT
3.10
1.12
Scenario 2a
MHWS + 1.1m storm surge
4.10
2.12
Scenario 2b
HAT + 1.1m storm surge
4.20
2.22
Scenario 3
HAT + 1.1m storm surge + 0.3m SLR
4.50
2.52
N/A
Table 1: Reference still water levels (including the flood scenario levels).
Scenario 2b
Scenario 2a
HAT
Scenario 1 (MHWS)
Figure 1: Still water level return periods for Yarmouth for the baseline year of 2008 (source: McMillan
et al., 2011) and the scenario water levels referred to in the flythroughs.
The flood simulations for all scenarios assumed present-day defences, buildings and
topography.
2.1 Base data
High resolution Lidar data was used to represent the land surface for the flood simulations
and flythroughs. This data was flown by the Environment Agency (EA) in 2008, and provided
via the Southeast Strategic Regional Coastal Monitoring Programme.
Lidar is commonly attributed with a vertical accuracy of +/- 0.15m, although survey reports
provided by the EA suggest the accuracy exceeds +/- 0.10m in most areas. A less quantified
uncertainty is the EA’s data cleaning procedures to generate the filtered ‘digital terrain
model’ (DTM) (omitting vegetation, cars, etc.).
The original Lidar data was collected at 1m resolution, producing a dense grid of points to
represent the land surface of Yarmouth and the surrounding areas. This resolution was
retained for the modelling and visualisation.
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Yarmouth flythroughs
Flooding flythrough
To represent surface features that are relevant to the main routes of floodplain flow, the EA’s
defence/asset survey data set was appended to the DTM. This represented the defence
layout and crest heights (which may be missed by the sampling of the Lidar survey).
Buildings were digitised (using the aerial photography and raw Lidar data) and appended to
the DTM, to benefit both the flood modelling and the quality of the visualisation. This
produced a master DTM.
The embankment between the A3054 and the River Yar boatyard required additional
attention as there appeared to be a gap in the DTM (Figure 2). This let water through the
embankment in the LISFLOOD-FP software (see Section 2.2), thereby flooding the reedbed
area behind the embankment and the River Yar boatyard at a much lower tide than
expected. Following liaison with the Yarmouth Coastal Defence Working Group and a site
visit (Figure 3), it was confirmed that the embankment was indeed continuous. The flooding
predictions were subsequently rectified.
A3054
Reedbeds
Embankment
Gap in DTM
Figure 2: Gap in the DTM along the embankment between the A3054 and the River Yar boatyard
Figure 3: Site visit photograph along the embankment looking north towards the A3054
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Yarmouth flythroughs
Flooding flythrough
2.2 Flooding grids using LISFLOOD-FP software
One of the most simplistic methods to generate a flood map is to combine a still water level
(SWL) with a DTM, to view land above or below the given SWL. However, such methods do
not consider hydraulic connectivity (as a result of defences or other flow diverting features),
or mass conservancy. Therefore, most flood studies prefer to generate a more realistic
approximation of flood extent by using some form of two-dimensional numerical simulation
which accounts for the dynamics of flow over the floodplain surface. This includes the
amount of inflow possible due to the rise and the fall of the tide, and interaction with the
effects of defences and topography. Such models vary hugely in complexity with regard to
the physics that the model attempts to replicate (refer to Neelz et al., (2009) for more
details).
In principle, there are numerous 2D models that would be suitable for this task, although
licensing and set-up costs are typically high. The model used for the Yarmouth flood
simulations was LISFLOOD-FP (Bates and De Roo, 2000, Bates et al., 2010) which was
developed by a team of researchers at the University of Bristol. This is a non-commercial
model, and its availability for this visualisation is due to its application within a flood research
project in the Solent region (refer to Wadey et al., 2012). This model has been tested and
validated, with variants previously used for coastal flood modelling at sites around the UK
(Bates et al., 2005) including the Solent (Wadey et al., 2012).
In reality, whilst the flood scenario simulations provided here would be dominated by the still
water level (tide and storm surge), waves would contribute to additional flooding. However,
methods available for quantifying wave overtopping volumes and coupling these to
inundation modelling can produce uncertain results (e.g. Smith et al., 2012); although this
may be a consideration for future work.
2.3 Visualisation using Fledermaus software
Fledermaus Professional is the industry leading interactive 3D geo-spatial processing and
analysis tool. The outputs from the LISFLOOD-FP software were processed, along with the
master DTM and aerial photography and inserted into the Fledermaus software to visualise
the following three flooding scenarios:



Scenario 1 - MHWS
Scenario 2a – MHWS+1.1m surge
Scenario 3 – HAT+1.1m surge+0.3m SLR (year 2050)
The visualisations focused on the town and Thorley Brook, as requested by the Yarmouth
Coastal Defence Working Group.
Two sets of outputs were produced for the project:


Output 1: Each flood scenario was visualised as a static water level with the camera
moving
Output 2: The water level was visualised rising through each flood scenario with the
camera moving
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Yarmouth flythroughs
3.
Erosion mapping
Shapefiles showing future indicative erosion zones were provided by the Isle of Wight
Council from the Shoreline Management Plan (2010) for the following scenarios:
No Active Intervention (NAI) – the NAI scenario assumes there is no expenditure on
maintaining or improving existing coastal and flood defences; therefore defences will fail at a
time dependent upon their residual life and the condition of the fronting beaches and intertidal areas.
With Present Management (WPM) – the WPM scenario assumes all existing management
practices are continued and defences are maintained to provide a similar level of protection
over the next 100 years to that provided at present.
Both scenarios covered the following epochs:



Erosion up to 2025
Erosion up to 2055
Erosion up to 2105
The NAI and WPM scenarios are depicted in Figure 4. There is no erosion shown in Figure
4 for the With Present Management scenario as the shoreline is defended.
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Erosion mapping
Figure 4: No Active Intervention (NAI) and With Present Management (WPM) indicative erosion zone
for Yarmouth (shapefiles courtesy of the IOW SMP2).
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Acronyms
4. Acronyms
CD – Chart Datum
DTM – Digital Terrain Model
EA – Environment Agency
HAT – Highest Astronomical Tide
Lidar – Light Detection and Ranging
MHWS – Mean High Water Springs
NAI – No Active Intervention
OD – Ordnance Survey
SLR – Sea-Level Rise
SMP – Shoreline Management Plan
SWL – Still Water Level
WPM – With Present Management
5. Glossary
Return period – the probability of a given water level, usually with reference to an annual
probability of occurrence, e.g. a 1 in 200 year water level has a 1/200 = 0.005 probability of
occurring in any given year (although this does not necessarily mean that this level cannot
be exceeded in a given year). The return periods used are referenced to the year 2008.
Regional sea-level rise in the past decade has been approximately 1.7 mm per year (Haigh,
2011) hence the most recently derived return periods do not require significant adjustment.
Water level – the term ‘water level’ and ‘sea level’ in this report refer to the same
phenomenon: the still water level (comprising tide, mean sea level and surge) which does
not incorporate the effects of gravity (wind or swell) waves.
6. References
BATES, P. D. & DE ROO, A. P. J. 2000. A simple raster-based model for flood
inundation simulation. Journal of Hydrology, 236, 54-77.
BATES, P. D., DAWSON, R. J., HALL, J. W., HORRITT, M. S., NICHOLLS, R. J.,
WICKS, J. & MOHAMED AHMED ALI MOHAMED, H. 2005. Simplified twodimensional numerical modelling of coastal flooding and example
applications. Coastal Engineering, 52, 793-810.
BATES, P. D., HORRITT, M. S. & FEWTRELL, T. J. 2010. A simple inertial
formulation of the shallow water equations for efficient two-dimensional flood
inundation modelling. Journal of Hydrology, 387, 33-45.
HAIGH, I. 2011. RE: Quantification of sea-level rise at the Southampton and
Portsmouth tide gauges. Personal communication with Ivan Haigh on 25th
January 2012.
MCMILLAN, A., BATSTONE, C., WORTH, D., TAWN, J. A., HORSBURGH, K. &
LAWLESS, M. 2011. Coastal flood boundary conditions for UK mainland and
islands. Project: SC060064/TR2: Design sea levels.: Published by:
Environment Agency, Bristol, UK.
NÉELZ, S., PENDER, G., BATES, P., FALCONER, R., LIN, B. & WRIGHT, N. G.
2009. Desktop review of 2D hydraulic modelling packages. Science Report:
SC080035. Environment Agency, UK.
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References
PUGH, D. T. 1987. Tides, surges and mean sea-level. A handbook for engineers
and scientists, Wiley, Chichester.
SMITH, R. A. E., BATES, P. D. & HAYES, C. 2012. Evaluation of a coastal flood
inundation model using hard and soft data. Environmental Modelling &
Software, 30, 35-46.
WADEY, M. P., NICHOLLS, R. J. & HUTTON, C. 2012. Coastal Flooding in the
Solent: An Integrated Analysis of Defences and Inundation. Water, 4, 430459.
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References
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