A plea to all hydrologic engineers.

[mrpercysnodgrass]

In my ongoing battle with Herefordshire planing department to get permission to build a bridge over the brook in my garden I have just one obstacle left. The drainage department want me to commission a 1D and topographical survey to prove my clear span bridge will not cause flooding to my neighbours! It has been estimated these surveys will cost between seven and ten thousand pounds! I am not going to do these surveys, one because I can't afford them and two it is a complete nonsense to suggest a small clear span bridge would raise the flood water by approximately one meter!
My house abuts a road bridge designed by the well known victorian bridge designer George Jack who designed many large and beautiful bridges around the country. The bridge with its large parapet essentially acts as a dam controlling the flow of flood water. Im sure an engineer such as George Jack would have calculated the volume of water the bridge tunnel would displace into the channel that carries the water past my house and eventually into the Lugg, I have managed to obtain a copy of his original design ( with beautiful hand written aesthetic movement script) unfortunately none of his notes survive.
I am a complete dunce when it come to maths, so (eventually) my question is.....can any of you geniuses calculate the volume of water displaced by the tunnel and can the channel take this water without flooding over the channel?
I should explain, there is a short run of channel for approximately eight meters from the bridge (bottom left on the sketch) then a small waterfall, the channel is then slightly larger (bottom right) The attached photo shows the road bridge behind the small existing foot bridge and waterfall.
Any help on this would be gratefully received.

 

[Sideways]

Not an answer to your question but some comments hoping to be helpful :
The George Jack bridge looks like it might be a "basket handle" arch.
From the proportions it isn't a simple section of a circle so finding some way to describe the shape mathematically would be the key to calculating the cross sectional area under the arch.
I find this academic paper interesting for the way it describes these arches but it doesn't give the area. That will need more googling.
https://www.mdpi.com/2073-8994/11/10/1243
Subjectively, the area under the road bridge looks bigger to me than your rectangular cut.
So conceivably your simple bridge could force water to overflow in extreme flood conditions.
The question then would be how much / for how long / where would the displaced water go and what damage if any along the way.

I have had a property flooded when a farmer failed to keep his drainage clear. The resulting water crossed two intervening properties, into the underfloor void of ours through the air bricks and rose up through the carpets ...
It wasn't as bad as 20 years before when the neighbour on the other side failed to keep a gulley clear and an exceptional rain storm resulted in an overflow onto our property and a tide mark 2 feet high on the back door of the house. That was another big insurance claim.

I have sympathy for your battle with the planners, but they may have a point and your convenience doesn't justify exposing your neighbours to increased risk of serious loss and damage. It is on your shoulders to prove that your plans will not cause harm.

Have you lived in the property for long enough to know what the flood risk is - like 20 years long enough ?

Are there waterways records, newspaper archives about flooding and a depth marker on the bridge that would give any indication of the historical high water under the bridge, as it may never have come close to the top of the arch.
Worse though would be any tide mark on the brick above the top of the arch ? If that's the case, they shouldn't let you build your bridge...

 

[Deadeye]

In my ongoing battle with Herefordshire planing department to get permission to build a bridge over the brook in my garden I have just one obstacle left. The drainage department want me to commission a 1D and topographical survey to prove my clear span bridge will not cause flooding to my neighbours! It has been estimated these surveys will cost between seven and ten thousand pounds! I am not going to do these surveys, one because I can't afford them and two it is a complete nonsense to suggest a small clear span bridge would raise the flood water by approximately one meter!
My house abuts a road bridge designed by the well known victorian bridge designer George Jack who designed many large and beautiful bridges around the country. The bridge with its large parapet essentially acts as a dam controlling the flow of flood water. Im sure an engineer such as George Jack would have calculated the volume of water the bridge tunnel would displace into the channel that carries the water past my house and eventually into the Lugg, I have managed to obtain a copy of his original design ( with beautiful hand written aesthetic movement script) unfortunately none of his notes survive.
I am a complete dunce when it come to maths, so (eventually) my question is.....can any of you geniuses calculate the volume of water displaced by the tunnel and can the channel take this water without flooding over the channel?
I should explain, there is a short run of channel for approximately eight meters from the bridge (bottom left on the sketch) then a small waterfall, the channel is then slightly larger (bottom right) The attached photo shows the road bridge behind the small existing foot bridge and waterfall.
Any help on this would be gratefully received.

Insufficient info. The section looks like it's an elliptical curve, but need to know the foci.
How accurate do you need?

 

[mrpercysnodgrass]

Thank you for your reply Deadeye. it is indeed a load bearing elliptical arch but I don't know how to measure it. I suppose it needs to be fairly accurate but not precise if that makes sense. Is there a way of measuring a space like this? I was thinking I could measure up from the sides to make an oblong, take lots of measurements so that I could divide the oblong into a grid then subtract the two areas either side at the top. Is there an easier way?
Here are two more photos which show it a bit more clearly with one from the other side of the road showing the large parapet that acts as a dam.

 

[Jones]

I've no idea what a 1D survey is but a topographic survey is relatively simple. It is basically accurately mapped contours. In a small area you could do that yourself with a large scale plan from your deeds and a laser level and tape measure. If you employ surveyors count on two guys for a day or half day , so not cheap but not too much. The council quite reasonably want to know that if your bridge did cause damming in flood conditions that the water would not flow onto your neighbours.

 

[paulrbarnard]

Perhaps a simple approach would be to ensure the lowest part of your bridge is at the same height as the top of the arch. The square section below your bridge would have a higher volume than the arch. That needs to be coupled with knowledge that the section you are bridging has never flooded.

 

[MikeJhn]

In my ongoing battle with Herefordshire planing department to get permission to build a bridge over the brook in my garden I have just one obstacle left. The drainage department want me to commission a 1D and topographical survey to prove my clear span bridge will not cause flooding to my neighbours!

If your bridge has a larger flow area than the downstream bridge, isn't that all they need to know? Or am I missing something.

 

[Spectric]

Has this brook ever flooded and if so how high was the water. If your bridge is above this point then it will not contribute to the flooding. Once the level is above the channel it will now spread out.

 

[pip1954]

I would have thought the water fall would have been more of a problem than the bridge as that has restricted the stream by a lot and would contribute to flooding

 

[Lampost46]

Thank you for your reply Deadeye. it is indeed a load bearing elliptical arch but I don't know how to measure it. I suppose it needs to be fairly accurate but not precise if that makes sense. Is there a way of measuring a space like this? I was thinking I could measure up from the sides to make an oblong, take lots of measurements so that I could divide the oblong into a grid then subtract the two areas either side at the top. Is there an easier way?
Here are two more photos which show it a bit more clearly with one from the other side of the road showing the large parapet that acts as a dam.

I know that this suggestion may not assist but it’s an option. Number of friends have part of their property intersect by the river Blackwater. They were not allowed to build permanent bridges. They solved it by using drawbridges or similar types of temporary bridges. No objection by the authorities and a unique selling point and full access to your property.

 

[Pedronicus]

I would have thought the water fall would have been more of a problem than the bridge as that has restricted the stream by a lot and would contribute to flooding

As the water appears to be flowing from upstream towards the OPs property from the road bridge I would have thought that, during periods of very high flow rates, there would be a great chance of flotsam being caught up on the upstream side of said road bridge and causing a rise in levels and local flooding. Perhaps the powers that be don't take that fact into account.

I would also agree that the waterfall probably does not help the flow of water through the bridge during high flow periods.

 

[Deadeye]

Thank you for your reply Deadeye. it is indeed a load bearing elliptical arch but I don't know how to measure it. I suppose it needs to be fairly accurate but not precise if that makes sense. Is there a way of measuring a space like this? I was thinking I could measure up from the sides to make an oblong, take lots of measurements so that I could divide the oblong into a grid then subtract the two areas either side at the top. Is there an easier way?
Here are two more photos which show it a bit more clearly with one from the other side of the road showing the large parapet that acts as a dam.

If it is indeed a pure elliptical curve then I think if you give the x,y coordinates of 5 points along the curve (the two ends and the top being three obvious ones) then you can work out the curve. The area is then trivial. That'd be the mathematical way (and I think wolfram has a solver probably)
Alternatively take a face on photo and overlay a grid of known size. Colour in all squares over half. Accuracy goes up with a smaller mesh, but would get within a few percent very easily. And you could err on caution for your submission.

 

[slimshady]

Is that more flooding risk than the existing footbridge (in white).?

 

[mrpercysnodgrass]

Thank you for all your replies. As you can imagine this is a complicated process, the accepted convention is that clear span bridges do not cause extra flooding and some county councils including Devon but also water authorities in Australia and Canada state this in their published material with wording such as 'if you are to build a bridge over a waterway the best practice to mitigate flooding is to build a clear span bridge'. I commissioned an FRA and my hydrological expert stated this. Im not sure the drainage dept at my council have even bothered to read it!
Between the drainage and Ecology departments, they would have had me commissioning about twenty five thousand pounds worth of surveys!! I have managed to bat them all away and the last one standing is the drainage expert with his demand of a 1D and topographic surveys. Ecology and drainage insisted I apply for OWFDC (ordinary watercourse flood defence consent) This was as onerous as the full application plan, I duly filled in the form and sent it to them, they rejected it within hours saying it was almost the same as my application! Well duh! So I started searching Herefordshire Councils website for more info (not an easy task) I came across a document published by the drainage department, it listed no less than fourteen structures with helpful diagrams on, in and over waterways and stated which structures needed OWFDC, which ones did not and which ones you needed to consult them about. Guess what, a clear span bridge does not need OWFDC! No ifs, no buts, no perhaps you should consult us first, an unequivocal 'NO consent required'. Part of the OWFDC application was to take into account flood risks of your proposed structure so they had obviously taken into account clear span bridges and decided they do not pose a risk to flooding. Needless to say I pointed this out to them and asked them on no less than four messages to state unequivocally that I did not need OWFDC. They will not give me a straight answer. Clearly they have been embarrassed and are now trying to save face.
MikeJhn.
"If your bridge has a larger flow area than the downstream bridge, isn't that all they need to know? Or am I missing something."
I'm not sure what you mean by this Mike!
Spectric, "Has this brook ever flooded"?
The house dates from around 1650 and has never flooded.
Pip 1954. I would have thought the water fall would have been more of a problem than the bridge as that has restricted the stream by a lot and would contribute to flooding
I'm not sure what you mean by this! The waterfall is very small and has little to no effect on the general flow of water.
Lampost46 Thank you for the suggestion, a few neighbours have already suggested that, one even offering to source a British Army bridge (the sort that gets carried into battle on top of a tank!) but these and drawer bridges are clear span bridges!!
Pedronicus "there would be a great chance of flotsam being caught up "
The brook has a very small catchment, there are many trees upstream that have fallen across the brook and have been left, these act as dams slowing the flow and catching any debris, so we get very little flotsam and it has never been a problem.
Slimshady "Is that more flooding risk than the existing footbridge (in white)."
My proposed bridge will mirror the white foot bridge, it will be a little wider and with a larger bed as it needs to carry vehicles but not much difference.
Deadeye. Thank you again, I like the idea of the grid and when I get dressed I will get out there and take some photos and more measurements.

 

[Jameshow]

You need to measure depths say every 4" across the bridge and use Simpsons rule to give you an accurate area of the stone bridge.

If less than yours your ok.

But I expect the council want a professional taking the buck!

 

[mrpercysnodgrass]

James how, You are spot on when you say the council want a professional taking the buck, they have actually said as much.

 

[cowtown_eric]

Historic flood data may not be sufficient nowadays!

We have near Calgary a community that experienced 3 "100 year flood levels" within 5 years.

IE, the "planners perspective" may have changed with our changing environment's more intense weather events!

 

[mrpercysnodgrass]

Historic flood data may not be sufficient nowadays!

We have near Calgary a community that experienced 3 "100 year flood levels" within 5 years.

IE, the "planners perspective" may have changed with our changing environment's more intense weather events!

We have had two 1 in 100 year floods in recent times and neither of these floods has broken the banks of our brook In the EA 2007 review of floods, chairman Sir John Harman said in his forward “Over 55,000 homes and businesses were flooded this summer as we experienced the wettest May to July period in the last 250 years”
In the EA 2020 National Flood and coastal Erosion Risk Management Strategy for England, Emma Howard Boyd, chair of the EA wrote “February 2020 was England’s wettest ever and it was the UK’s fifth wettest winter on record”
The EA has our house in flood zone 3 - the highest risk. I have been asking them repeatedly to put a figure on this level and the will not do so. They cannot because they will then have to admit it is all just guess work based only on a JFLOW survey taken from an aircraft in 1989!
But even if their flood map came true it shows my property and one neighbours property engulfed in flood water. The neighbour is up stream on lower lying land, his property will flood before mine so the whole point of the 1D modelling is to show that my bridge will not cause extra flooding. The EA's made up map shows it makes no difference but the drainage dept would have me spend 7K+ to prove this!

 

[mrpercysnodgrass]

Jameshow I have taken many measurements but not sure what to do with them now! I have looked at YouTube vids on the Simpsons rule but I'm non the wiser. When I said I was a complete dunce when it come to maths I was not being modest, I have the certificate to prove it!

 

[stuckinthemud]

Perhaps if they make it expensive enough you’ll give up and go away?