What's the weight of all that rain?
Discussion
smg916 said:
The 50mm/hr figure is what is typically quoted (for fag packet calculatoins) as rainfall intensity for a 1 in 30 year 'design' rainfall event; which is the level of protection that the water company regulator requires the industry to provide protection from internal property flooding against. The only trouble is that 1 in 30 year events seem to be happening every few years...
I'm not sure I follow you - could you clarify what you mean? Are you saying that water companies/sewerage undertakers are required to provide a standard of protection for a 1:30 rainfall event in terms of the design of the drainage system around a house or surface water drains, or are you talking about the design standards for drainage systems that are required to be met before a sewerage undertaker will adopt them? Aren't these design standards required by the Building Regs rather than OFWAT?Lotus 50 said:
I'm not sure I follow you - could you clarify what you mean? Are you saying that water companies/sewerage undertakers are required to provide a standard of protection for a 1:30 rainfall event in terms of the design of the drainage system around a house or surface water drains, or are you talking about the design standards for drainage systems that are required to be met before a sewerage undertaker will adopt them? Aren't these design standards required by the Building Regs rather than OFWAT?
Sorry, I'll try to clarify; I was talking more specifically about resolving hydraulic inadequacy in a catchment that causes flooding during rainfall events; this typically is in an existing public sewer owned by the water company that has problems due to being on a combined system (surface and foul) that may have become overloaded over time, typically due to development. Properties affected by this type of flooding are resolved by instigating a capital project, which in agreement with OFWAT, is required to provide protection of a minimum standard. Usually the catchment would be hydraulically modelled, but for fag packet calculations rainfall figures, such as those quoted earlier can be used. You're correct that in recent years, new developments have had to be designed to meet certain standards, so not to impact on downstream catchments, by designing to 'sewers for adoption' among others... Cheers.Timsta said:
Reading the page linked above, why didn't the rain drop discussed in the link max it's speed out at terminal velocity (120'ish mph isn't it?) In the article they're talking about the rain drop hitting the ground at 450mph?smg916 said:
Lotus 50 said:
I'm not sure I follow you - could you clarify what you mean? Are you saying that water companies/sewerage undertakers are required to provide a standard of protection for a 1:30 rainfall event in terms of the design of the drainage system around a house or surface water drains, or are you talking about the design standards for drainage systems that are required to be met before a sewerage undertaker will adopt them? Aren't these design standards required by the Building Regs rather than OFWAT?
Sorry, I'll try to clarify; I was talking more specifically about resolving hydraulic inadequacy in a catchment that causes flooding during rainfall events; this typically is in an existing public sewer owned by the water company that has problems due to being on a combined system (surface and foul) that may have become overloaded over time, typically due to development. Properties affected by this type of flooding are resolved by instigating a capital project, which in agreement with OFWAT, is required to provide protection of a minimum standard. Usually the catchment would be hydraulically modelled, but for fag packet calculations rainfall figures, such as those quoted earlier can be used. You're correct that in recent years, new developments have had to be designed to meet certain standards, so not to impact on downstream catchments, by designing to 'sewers for adoption' among others... Cheers.As someone who designs buildings... roof outlets (gutters and downpipes) and site run-off are easily calculated.
Generally for a roof in the UK the upper limit for a 1:100year storm is about 0.052litres/sec/sq.m. (that's a factored number that allows for risk of any gutter overflow into a building, and reflects risk of teh rainfall profile across teh UK: more in NW scotland, less in essex). That allows you to derive total drain requirements.
But on any given site, the local 'adopted' mains may also have a limit on rate it can accept surface water runoff (roof and rest of site surface drainage) - I had this a couple of years ago, 1.5Ha site, total runoff limit - of 5litres a second(tiny!) As a result, you end up creating (burying) storage volume and a hydrobrake - basically , you create a void on site that is potentialy filled by the worst-possible case of rainfall, and fit a passive device (hydrobrakes v simple and v clever!) that limits the rate at which this intermediate storage can disharge the temporary stored water into the public drainage away from the site.
The truth is, the upper World maxima sustained rainfall rates flatlines at about 80mm/hr of total rainfall.
That's 800 tonnes per hectare per hour. Maybe on Borneo; but even for my 1.5Ha site in Bristol, we ended up with 600 cu metres = 600 tonnes storage, which is a full 50% of that value, as a result.
Edited by Huff on Friday 18th September 22:59
AshVX220 said:
Timsta said:
Reading the page linked above, why didn't the rain drop discussed in the link max it's speed out at terminal velocity (120'ish mph isn't it?) In the article they're talking about the rain drop hitting the ground at 450mph?V6Pushfit said:
Timsta said:
Isn't 120mph the TV for skydivers? I don't think the rain drop reaches its terminal velocity.
I think air pressure has something to do with it, it can slow things down. Doesn't a feather take the same time to drop as a brick in a vacuum? V6Pushfit said:
I think air pressure has something to do with it, it can slow things down. Doesn't a feather take the same time to drop as a brick in a vacuum?
Yup...https://www.youtube.com/watch?v=5C5_dOEyAfk
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