From what I gather, they are building an undergound sump which can store flood water until such time as it is safe to release it and pump it further down the river.I saw some very large bore tubes going in.
Apparently this has been done at other flood sites (storforth lane rail bridge for example), and I beleive on the former Trebor factory site.
I could be wrong of course, my wife says I usually am, so please completeley diregard what I have just posted.
Kind regards,
henpecked