Indicative 2D roughness values

Table of 2D roughness values

Flood risk modellers can use the following 2D roughness vales as a starting point to assess, review and update their data when developing a hydraulic flood risk model

Descriptive group

Feature codes from OS MasterMap (Multi-coloured Manual)

n value

Roads, tracks and paths 10119 (Manmade)
10172 (Tarmac)
10183 (Pavement)
0.02
General surfaces 10054 (Step) 0.025
Roads, tracks and paths 10123 (Tarmac or dirt tracks) 0.025
General surfaces 10056 (Grass, parkland) 0.03
Structures 10185 (Roadside structure) 0.03
Water

10089 (Inland)

10210 (Tidal water)

0.035
Land (unclassified) 10217 (Industrial yards, car parks) 0.035
(Default value) (9999) 0.035
General surface 10053 (Residential yards)
10093 (Unspecified)
0.04
Heritage and antiquities, land 10096 (Landform, historic interest, slope) 0.04
Water, land 10099 (Cliff)
10203 (Foreshore)
0.04
Structures 10193 (Pylon) 0.04
Rail 10167 (Unspecified) 0.05
Natural Environment (Coniferous / Non-coniferous Trees) 10111 (Heavy woodland and forest) 0.1
Land; Heritage and Antiquities 10076 (Unspecified) 0.5
Building 10021 (Unspecified)
10062 (Glasshouse)
0.5 (to be confirmed. Lower values to be considered for 'porous buildings')
Structures 10187 (Generally on top of buildings)

0.5 (to be confirmed. Lower values to be considered for 'porous buildings')

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