mathematician needed

It probably isn't the clear channel that will cause trouble it is what happens when a log or two jams across and impedes the flow.

Bulk fast moving water is scary stuff that can easily knock you off your feet and you need to be aware of where every manhole cover is in any flooded section. The lids can and do lift off in a flood.

I sometimes get a fountain at the bottom of my drive from the land drains and it has been know to lift the lid off!

What tends to happen is that the overtopping of the sharp boundary creates a much wider but slower moving shallow flow.

Trying to put some bounds on how much peak flow that can accommodate before it gets to the threshold of the house might be worthwhile.

Where is the evidence they haven't?

It is around 50% of the time that the met office temperature and windspeed at a given airbase equipped with thermometers and anemometers is > 2°C out s up to 20mph out.

It is in short sketchy stuff.

The forecast of wind power versus the actuality is likewise not very good even 10 hours out.

In reality it is best to study the weather charts, which due to more data stations are way better than they were in the 1950s, and do your won forecast based on local knowledge, incoming weather systems and a bit of weather nous.

The met office has its agenda. Its data is OK, mainly. Its forecast are not.

Well my thanks to GB. I'll try the Manning equasions.

I'm 1/4 way up a hill. The banks are well over 2.5m high so there's no flood problem expected, the upstream bend is around the other side of the hill 300m away, so no problem either, and I'm not expecting 9m long tree-trunks to jam across the gap. There's adequate flooding plains below me in the fields too. I'm not worried.

This is just a tick-box exercise for the insurers, planning department and other local council officials, etc. The Environment Agency have provided the 41.2 cu.m/sec figure as a maximum, so all I need to prove is that max depth will be below, say, 2m.

Indeed. There will be too many weather-related variables upstream of the location to have much hope of a reliable prediction. Solving Navier-Stokes is not the main problem here, it's the inputs & boundary conditions. And probably something simpler than NS, if calibrated to the watershed, would suffice.

Often, however, on a large scale numerical prediction can be useful. I recall, when in Queensland, hearing how various flood-water maxima were predicted to move downriver, and did, as the rains changed and the water travelled over several days (even to a week, maybe). This sort of thing is quite typical in river systems -- depending on their length.

#Paul

They are perfectly amenable to numerical integration, but you just have to take more care about how you do it.

#Paul

I have the flood maps for my area issued in 2016 by the Environment Agency. I can see the expected flood plains. There's one above. one below and one opposite but I'm in the clear. My 2.5m bank is higher than the opposite, downhill one, anyway. Both the upper and lower corners beyond my strip will flood onto fields. I just want to prove I'm safe, like "I've got a 100% margin"

----------------------------- They need to be interviewing Ecologists. They're looking at the whole biosphere, the big picture. Doctors, epidemiologists, and politicians are just looking a piece of the picture, whichever suits them! There's nothing "normal" about adding one billion people every 12 years!

The mistake was made 50 years ago when the people didn't listen to the scientists who called for Zero Population Growth! That's what brought about all the crises! If we had stayed at 4 billion, we wouldn't have climate change, the refugee crisis, decimation of wildlife, & environmental degradation!

I'm completing my DIY-Wiki post now, got a solution using the Manning equations (thanks M.B.) of 0.9m depth at full flow, so I've got 1.1m freeboard, absolute;y no problem!

Good result!

------------------------------ We're adding one billion every 12 years, and will continue for some time! Many countries have an aging population, with fewer young workers to support the system. Just because you can suppress communicable diseases doesn't mean there can't be any negative consequences to where the costs outweigh the benefits! We had to back up with DDT, leaded gasoline, CFC's, Thalidomide, etc etc

Will the insurers actually take any notice of your calculation or will they just ask the question "is the properties with x metres from a river" and adjust the premium or excess accordingly? They may not even ask that question as they may already have a data base of likely flood areas and where insurance cost may/will be higher.

Is the supplied maximum figure for the once in 10 to 100 year event or just the maximum from previous monitoring?

Doesn't it very much depend on where you live, on a large land mass or a soggy little island surrounded by sea? :)