Catchment Planning – What Level of Detail is Appropriate When Modelling Future Development Scenarios
Stormwater Conference
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Introduction
Proper management of stormwater is greatly aided in catchments where a stormwater catchment management plan has been developed. As part of developing this plan, analysis of both stormwater quantity and quality is required. This analysis varies greatly between Councils and catchments sometimes involving just broad-brush calculations and in other instances involving much more detailed modelling.
Even where modelling is undertaken the level of detail also varies considerably. This paper focuses on flood modelling (stormwater quantity) for Tauranga City Council for both existing and future scenarios. It examines different techniques and levels of detail applied in different catchments. It examines which techniques are appropriate where, and when it is better to use more or less detail. In particular it focuses on the issues surrounding modelling future development scenarios, where the level of detail of the future development can vary enormously.
Background
Commonly when modelling a catchment, two base scenarios are included in the analysis. The first is the existing scenario, i.e. modelling the catchment in its current state. The other is a future scenario which usually includes allowances for climate change and sea level rise. The future scenario also usually allows for further development of the catchment. A term often referred to is Maximum Probable Development or MPD.
Modelling the existing development situation is useful for defining the current flooding in the catchment. Flood maps produced from such modelling are an invaluable tool for engineers and planners when granting approval or consents for work within the catchment. Data from the flood maps produced also inform LIM reports for properties within that catchment.
For these reasons therefore it is important that this modelling is as accurate as possible. While not always entirely true, it generally follows that the more detail put into a model the better it will represent the real world giving more accurate results. (Care however needs to be taken that the model is able to deal with this additional detail. An example where this is not the case is using rain on grid for steep urban catchments. Although rain on grid may seem like it should give better results than lumping flow directly into the pipe network, experience has shown that in steep catchments