My main area of expertise is in flood frequency estimation, and in assessments of event rarity.
There is considerable value in local knowledge. It helps directly in understanding a flooding problem and in gathering and analysing information to come up with a flood frequency estimate. Wider local knowledge also adds credibility as an expert witness. I would expect to get to know an area well, before drafting proofs of evidence.
It was fascinating to gain up-close experience of democracy and PPS25 in action at a Planning Appeal in Surrey in Nov 2007 and Feb 2008. The technical arguments were themselves instructive and (potentially) far-reaching.
Please note that I charge my time at a higher rate for jobs related to disputes (see charge rates).
The British Hydrological Society publishes a Register of Consultants in Hydrology but accepts no responsibility for the accuracy of its contents nor endorses the suitability of those listed for any task.
Other possibilities are to seek a consulting firm based locally that specialises in water or expert-witness work, or find a drainage engineer who formerly worked for a relevant agency. Local knowledge and a long-term perspective can be particularly helpful in understanding and assessing specific flooding problems.