MIKE by DHI UK and Ireland Blog

Example tidal inundation model at Sutton Harbour, Plymouth using standard structures available within MIKE 21 Flexible Mesh

Introduction

The Sutton Harbour area of Plymouth is defended against tidal flooding primarily by a system of lock gates at the entrance to the inner harbour. It is understood that the defended level afforded by the lock gates is 4.03m A.O.D (although a number of openings in the parapet wall of the southern harbour wall may slightly compromise the level of defence). This example investigates the suitability of the existing defences to accommodate a 1 in 200-year tidal event plus an allowance for climate change.

Additional notes: It is assumed that all surface water outfalls into the inner harbour have been bypassed; hence, flooding from urban drainage has been ignored. No assessment of wave height has been included within this example. The aerial photograph above was obtained from the Channel Coastal Observatory Data Catalogue (see below).

LIDAR data, for use in the model, was obtained from the Channel Coastal Observatory Data Catalogue here http://www.channelcoast.org/. Downloading data and reports from the Channel Coastal Observatory Data Catalogue is currently free of charge (registration is required).

Additional notes: CCO LIDAR data was verified by comparison with EA LIDAR data and augmented by additional depth values.

Model Development

The model extent was determined by considering the assumed 1 in 200-year tide level plus an allowance for climate change (see below); hence, the domain has been terminated at the 6.0m A.O.D contour.


The model mesh has been constructed from both triangular and quadrangular elements. Quadrangular elements have been used to describe areas of uniform directional flow (e.g. flow through narrow streets, structures, etc). The finest mesh (above) has a maximum area of approximately 5.0m2.

Additional notes: it is recommended that a minimum of 5 transverse elements are employed to accurately describe flows in a 'channel'. As such, the above mesh is a little coarse in the very narrow streets.

Generated model bathymetry.

Additional notes: Buildings have been coarsely removed from the mesh by examination of the LIDAR data and aerial photos (and by memory) only. This may have resulted in some artificially raised areas that could impact flows. It is recommended that time is taken to refine roads, paved areas, raised planters, etc in conjunction with highly detailed spacial information (e.g. OS MasterMap).

A note on roughness: In urban areas, when omitting building footprints from the model mesh using the standard default Land (zero normal velocity) boundary condition, the building elevations will have a 'zero' roughness (i.e. a 'glass wall' normal to the 2D model surface - this is because, although there is no flow allowed perpendicular to the land boundary, flow is allowed parallel to the land boundary). As such, thought must be given to the Manning's roughness between buildings, particularly where flood waters are predicted to be deep and fast flowing. As an example, the following schematic simply adopts the procedure for flow in a channel section with composite roughness as described in Chow VT (1959) 'Open-Channel Hydraulics': -

A global Manning's 'n' of 0.031 has been used in the model.

However, a detailed roughness map would be more appropriate. Alternatively, where a building is likely to exert a frictional effect on passing flows, then the alternative Land (zero velocity) boundary should be used; as this allows no flow either perpendicular or parallel to the boundary – which is exactly the boundary condition used at the ground interface as it is assumed that velocity at the bed level is zero due to frictional effects. The effect of friction (i.e. zero flow) will then be applied at the land boundary, as it is at the ground boundary, thus allowing the walls of buildings to remove energy from the local flowfield.

Assumed Tidal Flood Levels

The 1 in 200-year extreme tidal still water level has been taken to be 3.68m A.O.D (i.e. equal to the levels for Devonport, Plymouth); from the Environment Agency South West Region (2003) 'Report on Regional Extreme Tide Levels' (prepared by Royal Haskoning).

The cumulative sea level rise, due to the long term effects of climate change, has been taken to be 1.04m; from Table B.1 of PPS25.

As such, the maximum 1 in 200-year tidal flood level including an allowance for climate change has been taken as 4.72m A.O.D.

For use as a boundary condition, a three peak tidal sequence was generated using data from the National Tidal Sea Level Facility (NTSLF) websites at http://www.pol.ac.uk/ntslf/ (POL) and http://www.bodc.ac.uk/data/online_delivery/ntslf/ (BODC). A fairly small, 5-year, data set of tidal extremes (including surge effects) for a tide gauge at Devonport was reviewed to establish a suitable maximum tide. More detailed 15-minute data values were then used to construct a tidal sequence comprising the high tides preceding and following the maximum tide. Finally, the tide values were uniformly scaled so that the maximum (second) peak in the sequence equalled 4.72m A.O.D (see above). The minimum tide level was set to 0.28m A.O.D being the minimum retained water level within the inner harbour.

Additional notes: The data were supplied by the British Oceanographic Data Centre as part of the function of the National Tidal & Sea Level Facility, hosted by the Proudman Oceanographic Laboratory and funded by the Environment Agency and the Natural Environment Research Council.

Structure Arrangement

The standard structure arrangement is presented above.

For flood prevention the lock gates will close at 2.48m A.O.D on a rising tide and will not recommence operation until 2.38m A.O.D on a falling tide.

Additional notes: For simplicity, only the outer lock gates have been considered (the inner gates have been ignored in the model). As weirs 2, 4 and 6 describe flow through openings in the parapet wall, it is perhaps more appropriate to define these structures as open culverts (also a standard MIKE 21 FM structure).

Model Scenarios

Scenario 1

Inundation by overtopping of existing defences with harbour lock gates in operation; including dynamic flood hazard assessment to FD2320 (in the vicinity of 'the most important building in Plymouth')

Additional notes: see http://evidence.environment-agency.gov.uk/FCERM/Libraries/FCERM_Project_Documents/FD2321_7400_PR_pdf.sflb.ashx for the current Environment Agency guidance on hazard ratings.

Scenario 2

Inundation as a result of harbour lock gates failing and remaining open (residual risk)

Model Results

Scenario 1 Results

Apparent maximum 1 in 200-year flood water levels (m A.O.D); after first peak (upper left), after second peak (upper right), after third peak (bottom)



Apparent maximum 1 in 200-year flood water levels (m A.O.D)



Dynamic hazard mapping; water depth, m (upper left), current speed, m/s (upper right), FD2320 debris factor, DF (bottom left), FD2320 hazard rating, HR (bottom right)

Hazard map output (maximum hazard rating); predominantly Danger for most (includes the general public)

Scenario 2 Results


Apparent maximum 1 in 200-year flood water levels (m A.O.D); after first peak (upper left), after second peak (upper right), after third peak (bottom)




Apparent maximum 1 in 200-year flood water levels (m A.O.D)

Results Comparison

Point time series comparison of 1 in 200-year flood water levels (inset displays output point for comparison)
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