Accounting For Future Developments in Large Scale 2d Modelling – A Different Approach
Annual Conference
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Large scale 2D flood models built to support Integrated Catchment Management Plans (ICMPs) need to account for future alteration to landform as a result of development. This paper examines a new method of taking account of these changes. The Te Awa O Katapaki (TAOK) flood model, built by AECOM for Hamilton City Council with input from Beca and T&T, is presented as a case study.
LiDAR is typically used to generate surfaces for 2D flood models. This works well for existing development cases or maximum probable development cases with only minor development or earthworks proposed in the catchment.
Where major alterations to the catchment landform as a result of development are proposed, these changes may fundamentally alter downstream flows due to the creation or removal of barriers to flow; alteration to catchment storage; or smoothing of previously undulating catchments. Traditionally these changes are simulated through increased curve numbers and roughness values and in some cases, where these are available at the time of modelling, future development surfaces are integrated into the LiDAR surface. This can be impractical or introduce errors where a large number of different developments are proposed within the modelled catchment.
For the TAOK flood model, the approach of adding future development surfaces was initially taken. Due to challenges related to integration of development surfaces into the model, supplementary 1D modelling was carried out in HEC-HMS for those areas with proposed developments. Hydrographs from the HEC-HMS model were then substituted into the 2D model in order to provide a second, simpler and more adaptable, assessment of downstream flows and flooding. This provided a useful and more conservative assessment of flood risk, and allowed Hamilton City Council to make level of service decisions on upstream infrastructure based on a more robust flood assessment. This paper discusses Hamilton City Council and AECOM’s experience using this approach.