Flood Risk Analysis for Proposed Development

The planning application for a proposed development in Rosanna was rejected after it was deemed to exacerbate an existing overland flow and would likely increase stormwater flows into the neighbouring properties. This may lead to potential flood damage onsite and to adjoining properties. Therefore, this development could become a larger issue if not investigated and mitigated.

The client brought this issue to the attention of Morphum for independent advice on the effect that this proposed development would have.

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The Analysis

Morphum’s team proposed undertaking a hydrological analysis and 2D hydraulic modelling (using HEC-RAS) for both pre- and post-development scenarios. The process began with reviewing all available documentation regarding flooding impacts to ensure all concerns would be addressed throughout the analysis itself.

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Hydrological (catchment) analysis was undertaken first. This would determine the catchment and sub-catchment delineation in the affected area. Understanding the contributing catchment will help us understand how rainfall will behave in this area.

With the catchment understood, the 2D hydraulic models were developed, creating a simulation of flow based on the topography and land use of the area. This model ran based on pre-development and post-development scenarios to assess change in overland flow and flood risk across the area. As part of this, Morphum worked alongside the client to develop a practical design solution to meet requirements that would allow for the construction while mitigating and reducing flooding impacts.

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Results

The modelling results were presented in a technical memorandum to the client, which showed that with the changes supported by Morphum Environmental to the design, the potential flood depth and velocity post-development are not expected to increase the extent of flooding for the neighbouring properties for the 1% Annual Exceedance Probability (AEP) critical event.

Given the assessed findings that the post-development scenario does not post risk to people, vehicles or buildings, the client was able to resubmit for approval hat the development would have minimal impact on both the subject site and neighbouring sites.

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