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The Environment Agency issued this report in November 2009 on the size of the 6th September 2008 event
Hydrological Summary of the September 2008
Flood Event Return Period in Morpeth
The size and nature of the flood event in Morpeth in September 2008 makes it difficult to assess the actual size of the event. Many gauging stations got outflanked by the flood flow so estimates have to be made based on the information available. However, there is a lot of information that can be used to make a determination of the size of the flood event and this information can be used to produce runoff models and flood hydrographs with some degree of certainty.
While still an estimate, a confidence level of ±10% has been achieved. The information used to determine the size of the September 2008 flood includes:
- Recorded 15 minute stage data at Mitford, Middleton, Hartburn and Nunnykirk gauging stations
- Wrack Mark surveyed levels at Mitford Gauging Station and elsewhere
- 15minute rainfall data at Wallington, Harwood and Font Reservoir
- Storage rainfall data at Cockle Park, Morpeth
- Photographic Record of Critical Flooding Thresholds at various times on rising limb of event in Morpeth
- Photographs of 1963 flood
- 1963 & Provisional 2008 Flood Event Outlines in Morpeth
- Threshold Data within Morpeth
- Post event channel change survey
A combined modelling approach has been taken to produce a flow estimate that is most consistent with all the data available. This includes:
- A new ISIS model of the Mitford Gauging Station to produce a rating curve which best accounts for bypassing flow and possible drowning of the weir
- Use of the existing ISIS routing model to attempt to recreate the observed hydrograph at Mitford based on rainfall and utilising the observed records at upstream gauging stations
- Use of the existing HEC-RAS model of the Wansbeck through Morpeth to assess flow rates in Morpeth
Using all of the available data, and particularly using the observed flood levels through Morpeth, clearly suggests a flow of around 345 m3/s, which is assumed to imply a flow of around 335 m3/s at Mitford. With the current growth curve this flow therefore has a return period of 115 years.
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