Urban Flood Modelling

Transcription

1 Urban Flood Modelling Adrian J Saul Pennine Water Group Department of Civil and Structural Engineering University of Sheffield a.j.saul@sheffield.ac.uk 1

2 Need for Integrated Urban Drainage within an integrated catchment

3 house and curtilage THE PUBLIC HOUSE THE PUBLIC HOUSE THE PUBLIC HOUSE THE PUBLIC HOUSE THE PUBLIC HOUSE THE PUBLIC HOUSE THE PUBLIC HOUSE Intra Urban Area Pluvial flooding caused by overloaded building drainage curtilage flooding due to domestic drain overload by rainfall and saturated ground Local area local area flooding caused mainly by pluvial drain overload, overland flows, ponding on roads, watercourse spills and inadequate sewers Urban area flooding from pluvial upstream effects, including surface flood waves and overloaded sewers plus culverted and other watercoursesspilling or backing up urban area Peri-urban flooding from rivers backing up from rural areas and also from upstream discharges and overland flood waves Peri-urban area

4 URBAN FLOODING Types of flooding Pluvial flooding Flooding due to asset performance Fluvial flooding Co incident flooding Groundwater flooding Future change considerations Changes in Rainfall Urbanisation and urban creep Impact of asset deterioration and renewal Emerging and new technologies Changes in Groundwater level and Infiltration Changes in local flood pathways and urban form

5 Developing an initial understanding of the problem Flood mechanisms and interactions between different urban drainage systems; Scale of the flooding (e.g. localised, town wide or river catchment wide); Frequency of the flooding; Consequence of the flooding (e.g. degree of nuisance, cost). Basis of modelling is an extremely accurate urban surface DEM/DTM

6 The research challenge is to develop a generalised tool to deal with the interactions of any above ground flows and their interaction with the below ground drainage system 6

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9 Types of Data OS Mastermap LiDAR Flown by plane or helicopter Ground drive overs yes Topographic/GPS surveys Historical data videos, photographs, flood levels

10 Typical DEM and drainage system Ri ve ra i re Meters Legend Study Area 2 Manhole Pipe River Elevation 110 m 80 m

11 DTM Enhancement 1. Major System Delineated Ponds 2. Minor System Sewer network Sinks & Exits Pond catchment Sub-catchment Delineation Connecting Paths Approximate Geometry Reduced pond Catchment Undrained Areas Out of Catch Sewered areas 1D Surface Network (Nodes & Links) R-R model parameters Sewer Network (Manhole & Pipes) R-R Model parameters Interactions Minor-major Model (SIPSON) 1D surface path way + 1D sewer network

12 Flows in and out of the sewer system and subsequent overland flow 1D sewer network model prior to flood Sewer model has to be coupled with overland flow model when flooding occurs. 1D/1D or 1D/2D 12

13 Surface storages and sewer interactions Exit direction Terrain Exit point Lowest point Pipe Manhole 13

14 Flow directions alternate during an event Surface runoff enters drainage system through gulleys and manholes. Sewer flows surcharge from the manhole. Surface overland flow 1D or 2D H H H a) b) c) 14

15 Modelling urban overland flow Cascade of natural retention ponds 15

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17 Determination of surface pathways Connectivity analysis by rolling ball, bouncing ball or sliding ball algorithms Pond to pond Pond to sewer Sewer to pond Sewer to sewer Out of catchment pond pond manholes pond 17

18 Need accurate cross sections of flow paths 18

19 Need very accurate definition of streets Manhole Terrain Sewer Pipe Overflow from one street to another Lowest pt 19

20 Buildings 20

21 Need building coverage ratio α Δx ( α ) 1 Δx y x α = A b A Computational grid Building α Δy ( α ) 1 Δy A b A Conveyance width for flux in the y direction Conveyance width for flux in the x direction 21

22 Need building alignment y a b c x d e f Computational grid Building 22

23 Need conveyance reduction factors ( ) 1 β y Δx i,j+ 1 2 β y Δx i,j+ 1 2 y x β x Δy i 1,j 2 ( ) 1 β Δ x y ( 1 ) i 1,j 2 [ iδx,jδy] β x Δy i + 1,j 2 β x Δy i + 1,j 2 Δy ( ) 1 β y Δx i,j 1 2 Computational grid Building Δx 23

24 Problems with bridges

25 Complex bridges Here is an example map of Spaghetti Junction We can highlight road features Aggregate road polygons Buffer aggregated polygons Intersect analysis to identify possible bridge boundaries

26 DEM/DTM requires very detailed topography of catchment surface grid resolution 1m x 1m, vertical accuracy 50mm. Need to distinguish cover types and infiltration capacity runoff. Need exact position of gulleys and manholes, kerb heights, drop kerbs, walls, fences, permeable structures, etc

27 FRMRC FRMRC has developed new urban interactive model Models trialled in 3 UKWIR funded case studies These have highlighted the need for a joint 1D/1D and 1D/2D approaches The detail of the catchment surface is the critical factor and enhanced DEM s are the way forward

28 Conceptualisation of integrated urban drainage model includes rivers and coasts 1D model of drainage system and local 2D models of surface pathways River 1D model of drainage system and surface pathways 1D model of drainage system 1D model of drainage system and local 2D model of floodplain Simple modelling of local problems within guidelines derived from integrated urban drainage modelling

29 What we require Flood risk maps Location and depth of flooding Damage and vulnerability assessment

30 Acknowledgement and thanks The research reported in this presentation was conducted as part of the Flood Risk Management Research Consortium with support from the: Engineering and Physical Sciences Research Council Department of Environment, Food and Rural Affairs/Environment Agency Joint Research Programme United Kingdom Water Industry Research Office of Public Works Dublin Northern Ireland Rivers Agency Data were provided by the EA and the Ordnance Survey.