Impact on downstream systems
The rapid transfer of exceedance flow over the surface can have a significant and damaging impact on downstream receptor systems. The situation is exacerbated when such systems themselves are subjected locally to the effects of an extreme event at the same time, and this can impose significant additional liabilities on stakeholders.
Detailed advice on assessing the impact on downstream systems and developing mitigation measures is given in the drainage exceedance guidance (C635). However a few vital points are worth noting.
It is important to understand the interaction between the upstream system conveying the flow, and the downstream receptor system. As well as considering the peak rate of runoff and the flood volume, the timing of the peak relative to that in the receptor system is essential. For example, where a small upstream area discharges into a large river system, the actual impact may be small, not because the rate of exceedance flow is small, but because the maximum value occurs ahead of the peak in the receiving river. It may pass downstream without detriment and in such cases it may be detrimental to provide storage attenuation if this leads to the peak flows occurring at around the same time.
The downstream system can also prevent the exceedance flow from freely discharging, increasing the risk of upstream flooding. For example, when discharging to coastal areas, tide levels may affect the performance of surface flood pathways. An extreme event coinciding with a high tide may not drain as effectively as one occurring at the time of a low tide. In such cases a joint probability analysis may be necessary. Outfalls from surface flood pathways may require agreements/consents from the owners of receiving watercourse, riparian owners and/or environmental regulators.
Early planning of such consents or agreements will greatly assist in land (re)development. Exceedance flows may convey large quantities of sediments, pollutants washed of surface areas, and other pollutants discharged from wastewater collection systems.
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