Flood Risk Assessments!!!! - HELP!

[JCB123]

I'm trying to write a Flood Risk Assessment using the SFRA and PPS25 (for those of you in the know)

I've got to demonstrate to the Environment Agency the velocity and depth of the water at the site 110m away from the river, should there be overtopping or a breach in the river defence bank.

Anyone done how this equation is worked out, or know anyone that can help?!?!

Thanks all...

JCB

[RickH]

You're talking 2D modelling there. Something along the lines of JFlow or TuFLOW would be the normal way to do it. That way you still account for the conservation of mass and momentum. TuFLOW solves the full two-dimensional, depth averaged, momentum and continuity equations for free-surface flow, also known as the Shallow Water Equations or the Saint Venant Equations.

Have a look here: http://en.wikipedia.org/wiki/Shallow_water_equatio...

RickH

[JCB123]

That's great thanks - but I need to get this done asap and in-house...I've got a report for a site just down the road in front of me, and there are a couple of equations that I can't quite work out, there's the broad-crested weir equation? the theoretical equation for spillway and a continuity equation......

I'm confused!

[10 Pence Short]

Have you thought about asking the Environment Agency?

My other half works in the planning department in a regional Environment Agency office. They certainly don't mind people getting in touch.

[RickH]

See this is the problem. You're now into the realms of 2D modelling. I've asked around here for people who were doing the FRAs before PPS25 and they said the question of rate of inundation was not asked. Now that PPS25 (and SPP7 up here and TAN15 in Wales) is in place, most people are either glossing over that element or are investing in the software to do the calcs. We invested in TuFLOW but have only had the opportunity to deploy it once. As an aside, that project continued with just the inundation extents because we couldn't get reliable base data for the topographic surface.

For England, we've done PPS25 based assessments and done the whole Sequential Test and Flood Action Plan element, but the surface water velocities have not been 'required' as part of the reporting process (yet). Most of our clients have been happy with the inundation extent pre- and post-mitigation should a problem be identified.

What size of development/site are you looking at? Are you being asked to produce velocities at various locations? Could bee that there may not be an answer. I've no experience at the manual calulation side of things (being a geographer in a civils firm), and have only approached this from 1D modelling and topographic comparison. Sorry I can't give more help.

RickH

[Simpo Two]

RickH said:

See this is the problem. You're now into the realms of 2D modelling. I've asked around here for people who were doing the FRAs before PPS25 and they said the question of rate of inundation was not asked. Now that PPS25 (and SPP7 up here and TAN15 in Wales) is in place, most people are either glossing over that element or are investing in the software to do the calcs. We invested in TuFLOW but have only had the opportunity to deploy it once. As an aside, that project continued with just the inundation extents because we couldn't get reliable base data for the topographic surface.

For England, we've done PPS25 based assessments and done the whole Sequential Test and Flood Action Plan element, but the surface water velocities have not been 'required' as part of the reporting process (yet). Most of our clients have been happy with the inundation extent pre- and post-mitigation should a problem be identified.

What size of development/site are you looking at? Are you being asked to produce velocities at various locations? Could bee that there may not be an answer. I've no experience at the manual calulation side of things (being a geographer in a civils firm), and have only approached this from 1D modelling and topographic comparison. Sorry I can't give more help.

RickH

Blimey, how on earth did we manage to build an Empire without all that?

[DrTre]

RickH said:

See this is the problem. You're now into the realms of 2D modelling. I've asked around here for people who were doing the FRAs before PPS25 and they said the question of rate of inundation was not asked. Now that PPS25 (and SPP7 up here and TAN15 in Wales) is in place, most people are either glossing over that element or are investing in the software to do the calcs. We invested in TuFLOW but have only had the opportunity to deploy it once. As an aside, that project continued with just the inundation extents because we couldn't get reliable base data for the topographic surface.

For England, we've done PPS25 based assessments and done the whole Sequential Test and Flood Action Plan element, but the surface water velocities have not been 'required' as part of the reporting process (yet). Most of our clients have been happy with the inundation extent pre- and post-mitigation should a problem be identified.

What size of development/site are you looking at? Are you being asked to produce velocities at various locations? Could bee that there may not be an answer. I've no experience at the manual calulation side of things (being a geographer in a civils firm), and have only approached this from 1D modelling and topographic comparison. Sorry I can't give more help.

RickH

Out of interest, why couldn't you get reliable topo data?
Am glad to see TuFlow has taken off, where I worked a few years ago we were an "early adopter" of it and it seemed to me (not an engineer) to be a much better solution that 1D...seems to have taken off as I now see it all over the place.
Got a feeling you and I are in the same field if you're a geographer...

ETA sorry I can't help OP.

Edited by DrTre on Thursday 22 January 15:51

[Tom_C76]

DrTre said:

RickH said:

See this is the problem. You're now into the realms of 2D modelling. I've asked around here for people who were doing the FRAs before PPS25 and they said the question of rate of inundation was not asked. Now that PPS25 (and SPP7 up here and TAN15 in Wales) is in place, most people are either glossing over that element or are investing in the software to do the calcs. We invested in TuFLOW but have only had the opportunity to deploy it once. As an aside, that project continued with just the inundation extents because we couldn't get reliable base data for the topographic surface.

For England, we've done PPS25 based assessments and done the whole Sequential Test and Flood Action Plan element, but the surface water velocities have not been 'required' as part of the reporting process (yet). Most of our clients have been happy with the inundation extent pre- and post-mitigation should a problem be identified.

What size of development/site are you looking at? Are you being asked to produce velocities at various locations? Could bee that there may not be an answer. I've no experience at the manual calulation side of things (being a geographer in a civils firm), and have only approached this from 1D modelling and topographic comparison. Sorry I can't give more help.

RickH

Out of interest, why couldn't you get reliable topo data?
Am glad to see TuFlow has taken off, where I worked a few years ago we were an "early adopter" of it and it seemed to me (not an engineer) to be a much better solution that 1D...seems to have taken off as I now see it all over the place.
Got a feeling you and I are in the same field if you're a geographer...

ETA sorry I can't help OP.

Edited by DrTre on Thursday 22 January 15:51

Getting meaningful survey data for a site at that distance from a river is pretty impossible, doesn't take that many high or low spots in the intervening 110m stretch to dramatically change the effect of the overtopping. From the equations you've got, the broad crested weir would be appropriate for overtopping of a levee, provided that the crest level is constant for the reach.

We still seem to be able to get FRAs through based on EA predicted flood levels, but that is for less sensitive sites and/or sites close to the edge of the floodplain.

[Dr_Rick]

DrTre said:

Out of interest, why couldn't you get reliable topo data?
Am glad to see TuFlow has taken off, where I worked a few years ago we were an "early adopter" of it and it seemed to me (not an engineer) to be a much better solution that 1D...seems to have taken off as I now see it all over the place.
Got a feeling you and I are in the same field if you're a geographer...

The particular project was for a ngvigable section of the River Forth and the FRA was for a site on the banks. The length of reach involved was some 15km so wuite long really. So we were mixing NextMap (5m posting) data with bathymetric data.

The main problem was the different projections for the datasets; NextMap was in OSDN and the Bathymetry was WGS84. That and the fact that the bathymetric data was depths from a datum point that varied depending on the benchmark that that particular survey set was tied to (benchmarks tying Chart Datum (CD) to ODN are spaced up the rivers and vary in tehir correction factors) so each dataset piece needed individually correcting.

Oh yes, and from the data vendors own mouth came the words - 'the data we've given you does not necessarily give the correct depth in the correct location'. Well what use was that then!! So we went for the conservative approach and used the NetMap for the river (which contained the water surface) and tested the hydrology against that.

As for being a geographer, I'm GIS / remote sensing based and have a (limited) background in hydrology / hydraulics so I'm just here to remind the civils people of the magical geography word CONTEXT. That and to make the fancy pictures and visuals that GIS can do.

Tom_C76 said:

We still seem to be able to get FRAs through based on EA predicted flood levels, but that is for less sensitive sites and/or sites close to the edge of the floodplain.

Same here. EA have been quite flexible in the fact that the whole 2D modelling thing is still developing and not everyone has it. So the inundation rates are quite often overlooked and they're happy to go with the predicted flood levels.

Dr Rick