Thursday, 5 November 2009
Example tidal inundation model at Winterton-on-Sea, Great Yarmouth, UK using SRTM digital land topography data

Introduction


Location; Environment Agency Flood Zone Map (no allowance for sea level rise due to climate change)

Bathymetry

SRTM (Shuttle Radar Topography Mission) data is available in a 3 arc-second (90.0m grid resolution) for the UK. The vertical accuracy of SRTM data can be several metres (or more!). As such, SRTM data is not recommended for detailed assessment of flooding / inundation at a particular location and has been used in this example only to provide a coarse, initial estimate of likely model extents and an approximation of overland flow mechanisms (please read the Important Information at the bottom of this page).

In this example, the SRTM height data has been assumed to approximately correlate to Ordnance Datum (OD) Newlyn. However, this may not the case. SRTM heights are referenced to the EGM96 vertical datum. As such, SRTM data must be checked and verified for accuracy. If it can be demonstrated that SRTM data closely corresponds to Ordnance Datum, it may be possible in certain circumstances to use the data in conjunction with LiDAR and physical (GPS) topographic surveys in order to greatly extend a model domain only - this is not guaranteed. However, SAR (Synthetic Aperture Radar) data may offer a more generally acceptable solution to extend a model domain [1].

The SRTM data has been downloaded from http://dds.cr.usgs.gov/srtm/


[1] See https://www1.vtrenz.net/imarkownerfiles/ownerassets/868/brochure_IFSAR.pdf for an example comparison of SAR based data (IFSAR from Intermap) with LiDAR; information on the Environment Agency’s LiDAR data can be found here http://www.geomatics-group.co.uk/

The raw data has been converted for use in MIKE FLOOD using 3DEM by Visualization Software LLC and ESRI ArcGIS / MIKE URBAN; in order to transform the horizontal and vertical planar grid resolution to 50.0m.


Bathymetry; transformed 3 arc-second grid SRTM data (50.0m grid resolution)

Assumed Tidal Flood Levels

A quick search of available on-line data[2] suggests that I in 200-year extreme sea levels (storm surge levels) along the east coast could be in the order of 3.0m A.O.D or higher at Lowestoft to 4.6m or higher at Cromer. As such, the 1 on 200-year extreme sea level at Winterton-on-Sea could be in the order of 3.9m A.O.D or higher.

PPS25 requires that a sea level rise of approximately 1.1m is added to the above extreme sea level to consider the long term impacts of climate change over a 100-year design life of building (residential standard).

For the purpose of this example, from the above, the 1 in 200-year tidal flood level plus climate change for Winterton-on-Sea has been taken to be 5.0m A.O.D. This value has been taken to represent the peak of an approximate 3-day period (sinusoidal) storm surge.

Wave action will exacerbate flooding, particularly along the coast, but has been ignored for the purpose of this example. River flow will also increase flooding but has again been ignored for the purpose of this example.

The boundary conditions of the hydrodynamic model are simplistically assumed to be uniformly level. As such, a point time series file has been used throughout (see below). However, in reality, the tidal flood levels will vary over this short stretch of coastline and a profile time series could be more appropriate / accurate.


Tidal boundary conditions; assumed 3-day period 1 in 200-year tide levels plus climate change

[2] 'ESTIMATES OF EXTREME SEA CONDITIONS Final Report SPATIAL ANALYSES FOR THE UK COAST' - Mark J. Dixon and Jonathan A. Tawn (Department of Mathematics and Statistics, Lancaster University, Lancaster LA1 4YF in collaboration with The Proudman Oceanographic Laboratory, Bidston Observatory, Birkenhead, Merseyside L43 7RA) - June 1997; 'Great Yarmouth and Gorleston Strategic Flood Risk Assessment' - Capita Symonds - June 2006; 'Integrated analysis of risks of coastal flooding and cliff erosion under scenarios of long term change' - R. J. Dawson, M. E. Dickson, R. J. Nicholls, J. W. Hall, M. J. A. Walkden, P. K. Stansby, M. Mokrech, J. Richards, J. Zhou, J. Milligan, A. Jordan, S. Pearson, J. Rees, P. D. Bates, S. Koukoulas, A. R. Watkinson - January 2009

Model Parameters

Map projection: WGS_1984_UTM_Zone_31N
Time step interval: 10.0s
Flood and Dry: Drying depth = 0.002 / Flooding depth = 0.003
Initial surface elevation: 0.0m (from file)
Eddy Viscosity[3]: Constant, flux based = 2.0
Resistance: Manning's M = 33.3

[3] Eddy viscosity: The most suitable eddy viscosity formulation for models of this type is often the Smagorinsky model but this formulation may potentially create instabilities in combination with significant flooding and drying. The safe and recommended approach in such applications is to choose the constant eddy viscosity description. Furthermore, the 'flux based' formulation is recommended as practical experience has shown that the velocity based formulation can potentially cause numerical instabilities. A rule of thumb guestimate of the eddy viscosity constant is 0.02 Δx Δy / Δt [m2/s]

Model Results

Approximate 1 in 200-year flood depths (Scale 0.0m to 10.0m)

Approximate 1 in 200-year flood velocities (Scale 0.0m/s to 5.0m/s)


Approximate 1 in 200-year (high risk) flood extents presented in Google Earth
Wednesday, 19 August 2009
Demonstration model of tidal flows around Guernsey and Sark, The Channel Islands, for renewable energy production

Introduction

The Channel Islands have one of the highest resources of tidal flow energy in the United Kingdom. This simple modelling exercise has been undertaken in order to demonstrate the functionality and capability of MIKE 21 to investigate the viability of harnessing the tidal flows around Guernsey and Sark for energy production.





Defining the Hydrodynamic Model
  • A triangular element flexible mesh has been applied over the entire model domain; comprising 38679 Elements & 20405 Nodes
  • An overall time step of 300 seconds has been selected. The duration of the simulation is 7 days (2016 overall time steps). The simulation period covers an arbitrary week from 03/07/2009 – 10/07/2009. as such, higher spring tides have not been considered in this example.
  • The horizontal eddy viscosity type has been set to a velocity based Smagorinsky formulation with a constant value of 0.28
  • The bed resistance type has been set to Manning number and a constant value of 32 m1/3/s applied
  • Coriolis forcing has been set to varying in domain
  • Wind forcing has not been considered

Define model domain; import shoreline data, redistributing vertices to prepare for mesh generation


Increase definition at study area


Create computational mesh; increase definition in study area


Bathymetry data; depth values extracted from MIKE C-MAP for greater accuracy at study area


Bathymetry data; ETOPO1 1-minute grid (Amante, C. and B. W. Eakins, ETOPO1 1 Arc-Minute Global Relief Model: Procedures, Data Sources and Analysis, National Geophysical Data Center, NESDIS, NOAA, U.S. Department of Commerce, Boulder, CO, August 2008)


Interpolated bathymetry; water depths (study area) relative to Chart Datum [1]
[1] Datum info: Chart Datum (CD) is usually equal to LAT in the UK (e.g. see http://www.ordnancesurvey.co.uk/oswebsite/partnerships/research/publications/docs/2003/ICZMAP_GISRUK_full_dm.pdf for height integration at the coastal zone). For a global relief model, like ETOPO2v2, which has 2 arc-minute (~4 km) cell size, the differences between vertical datums are considered to be not significant, so long as they are all near Mean Sea Level (MSL). As such, a height correction should be applied to datasets relative to Chart Datum (see below).

Mean Sea Levels relative to Chart Datum


Adjusted model bathymetry; water depths relative to Mean Sea Level


Assign tidal boundary conditions to East & West boundaries; line time series generated using MIKE 21 Toolbox (prediction based on global tide model data)


West boundary

Model Results


Calibration against UKHO tidal predictions for St Peter Port over the study period; good correlation, parameters are appropriate for the purpose of this modelling exercise


Animation of tidal current velocities; The Channel


Animation of tidal current velocities; Channel Islands


Animation of tidal current velocities; Guernsey & Sark


Introduce turbine structures between Guernsey & Sark; Turbine 1 (North) & Turbine 2 (South). Assumed parameters; Turbine diameter = 16m, Drag coefficient = 0.4


Modelled turbine velocity (m/s); assumed criteria for viability > 1m/s (say 2m/s for spring tides)

The energy available from the turbines can (for example) be estimated as follows: -
P = Cp x 0.5 x ρ x A x V³
where: -
P = power generated (W)
Cp = turbine performance coefficient
ρ = water density (seawater ~ 1025 kg/m³)
A = sweep area of the turbine (m²)
V = flow velocity (m/s)


Modelled turbine force (N)


Consider the effects of renewable energy installations in Alderney Race


Introduce pier structures. Assumed parameters; Height = 100m, Diameter = 25m.

The impact of these pier structures on modelled tidal current velocities is negligible in this example.
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