Jilly

'design storm period intensity'

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I'm currently stuck with the 'design storm period intensity' part of the SUDs and am hoping to avoid the need for a 'drainage engineer/consultant'.  

 

I can happily do a drive and garden drainage design using SUDs principles with a dose of common sense,  but the learning curve for jargon and calculations is outside my comfort zone. It seems there are Excell sheets for this but I'm afraid of garbage in garbage out. Has anybody got a Noddy's guide? 

 

 

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If you have any specific questions I might be able to help. There are a couple of online tools to estimate Greenfield runoff if they might help (one is called UKSUDS or something like that).

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  1. No building hereby permitted shall be occupied until details of surface water drainage works have been submitted to and approved in writing by the local planning authority and the surface water drainage works have been completed in accordance with those details. The details submitted must include the results of an assessment of the potential for disposing of surface water by means of a sustainable drainage system. Where a sustainable drainage scheme is to be provided, the submitted details must also:

    i. provide information about the design storm period and intensity, the method employed to delay and control the surface water discharged from the site, including permeable paving and harvesting water butts and the measures taken to prevent pollution of the receiving groundwater and/or surface waters;

    ii. include a timetable for its implementation; and

    iii. provide a management and maintenance plan for the lifetime of the development which shall include the arrangements for adoption by any public authority or statutory undertaker and any other arrangements to secure the operation of the scheme throughout its lifetime.

 

 

This is the line in the planning condition I'm stuck with, it seems to require calculations. If possible I'd really like to try and avoid an underground tank for cost reasons.

 

Thanks!

 

 

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This is a standard term used in calculating flow rates. A SUDs scheme would need attenuation or soakaway tanks - have the sewage company said they will allow a combined outlet to the foul drain or do they have a storm drain ..?

 

You can do this above ground with some very clever water butts

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20 minutes ago, Jilly said:
  1. No building hereby permitted shall be occupied until details of surface water drainage works have been submitted to and approved in writing by the local planning authority and the surface water drainage works have been completed in accordance with those details. The details submitted must include the results of an assessment of the potential for disposing of surface water by means of a sustainable drainage system. Where a sustainable drainage scheme is to be provided, the submitted details must also:

    i. provide information about the design storm period and intensity, the method employed to delay and control the surface water discharged from the site, including permeable paving and harvesting water butts and the measures taken to prevent pollution of the receiving groundwater and/or surface waters;

    ii. include a timetable for its implementation; and

    iii. provide a management and maintenance plan for the lifetime of the development which shall include the arrangements for adoption by any public authority or statutory undertaker and any other arrangements to secure the operation of the scheme throughout its lifetime.

 

 

This is the line in the planning condition I'm stuck with, it seems to require calculations. If possible I'd really like to try and avoid an underground tank for cost reasons.

 

Thanks!

 

 

 

I thought the calculation was in the Building Regs documents.

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Hi Jilly, re the calcs for suds, we had to install one in our build and a company called 'rainwater harvesting ltd -and Ian' produced the calcs for free on the back of a hopeful sale which he ended up with :) 

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As peter alluded to, if you do require a suds system it could be done in many ways, just depends on the amount of water you'd need to attenuate as to how feasible it is to do above ground in your situation. The tank that we ended up burying basically has a big hole for the rainwater to enter in from then a small hole half way up the tank for it to exit through which controls the discharge rate to meet the calcs provided and holds the set amount of water below the exit point back for the attenuated required amount  and remaining capacity for the various 1/100 year events and so on. 

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5 hours ago, Madam1980 said:

various 1/100 year events and so on. 

Those will be the ones that happen every 20 years now.

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As far as I'm aware (and I don't work much in England so take this with a pinch of salt), you will need to attenuate the 1 in 100 year, 6 hour event so that post development runoff equals the greenfield runoff rate. In terms of meeting the planning condition you'd just need to detail aspects of the calculation showing how you come up with the greenfield runoff rates for the site. If the online tool works and allows you to do this (I don't use them so I can't confirm one way or the other) then you can probably just extract the outputs from this. Alternatively, I have heard of a standard 60mm rainfall depth being used for this event but you'd probably need to confirm with your BCO before adopting this for design.

Obviously there are many ways of providing the attenuation, some more aesthetic than others and some have been referred to above.

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A detailed Flood Risk Assessment and Surface Water Drainage/SUDS Strategy was prepared for my site at Outline Planning stage.  As it was done before I purchased the site and poor drainage/flooding is the number 1 bugbear of the neighbours, I have been through it and The SUDS Manual (968 pages !!) with a fine tooth comb.  As you say, the jargon and the methodology is very off-putting for the lay person.

 

A couple of extracts from my report summarise the first steps (as well as the jargon) required:

 

"In order to quantify the existing runoff rate from the site, the methodology outlined within the Institute of Hydrology Report Number 124 (IoH 124) entitled Flood Estimation for Small Catchments, has been adopted. This document together with the guidance stipulated in the Interim Code of Practice for Sustainable Drainage Systems, compiled by the National SUDS Working Group in July 2004, suggests that an estimation of peak runoff rates from areas below 50 ha, and up to 200 ha, can be derived from the calculated mean annual flood flow, QBAR."

 

"The ICPSUDS function within the Microdrainage software Version 2016.1 can be used which implements IoH 124 method with a pro-rata below 50 ha. The SAAR value of 623mm has been determined from the catchment descriptors taken from the FEH CDROM Version 3. The soil value has been determined using the information from the Winter Rain Acceptance Potential (WRAP) map within the Flood Studies Report, 1975, together with Table 6 and equation 12 of the ADAS document entitled Pipe Size Design for Field Drainage, 1980."

 

Thankfully Microdrainage is not required and the online tools at Online tools for sustainable drainage systems (SuDS) (uksuds.com) will allow you to calculate mean annual flood flow (QBAR) for the existing site.  The methodology is based on the premise that the flow rate discharge constraints for storm water runoff from the site are defined by the greenfield runoff rates for the 1 year, 30 year and 100 year return periods. The tool is flexible in allowing the use of three different methods for estimating greenfield runoff rates along with various options for assessment of storage volumes using variations in the design criteria (e.g. minimum design flow rates from the site, requirements for control of runoff volumes for the 1:100yr, 6 hour event).  

 

Note that in Microdrainage a range of storm durations and intensities are modelled (i.e. 15 - 10080 minute for both summer and winter).  For my site, the 1440 minute summer storm was the worst case used for the permeable paving design.

 

Note also that although SUDS attenuation/infiltration devices are usually designed for 1 in 100 year events, the design is checked against an exceedance case which is usually the 1 in 1000 year storm events.

 

Let us know how you get on 😀

 

 

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5 hours ago, jamieled said:

As far as I'm aware (and I don't work much in England so take this with a pinch of salt), you will need to attenuate the 1 in 100 year, 6 hour event so that post development runoff equals the greenfield runoff rate. In terms of meeting the planning condition you'd just need to detail aspects of the calculation showing how you come up with the greenfield runoff rates for the site. If the online tool works and allows you to do this (I don't use them so I can't confirm one way or the other) then you can probably just extract the outputs from this. Alternatively, I have heard of a standard 60mm rainfall depth being used for this event but you'd probably need to confirm with your BCO before adopting this for design.

Obviously there are many ways of providing the attenuation, some more aesthetic than others and some have been referred to above.

 

@Jilly is converting a stable in a paddock, so that should be doable by some means.

 

Nick a bit more land for the site and dig a seasonal pond.

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9 hours ago, Ferdinand said:

 

@Jilly is converting a stable in a paddock, so that should be doable by some means.

 

Nick a bit more land for the site and dig a seasonal pond.

Yes, that was my first thought too.

 

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Yep, that could be a nice solution. It's what I've done for our place. A small pipe limits the outflow of the pond, with a backup higher up and a small overflow. It's not as much work as it sounds and it's starting to green up well.

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can anyone give me a rough cost for getting an engineer to do the following? thanks

 

  • Drainage Design 
  • Scottish Water & SEPA Applications
  • Topographical Survey with Existing Levels
  • Proposed Levels Design

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@Amateur bob the scottish water (clean water connection?) and sepa applications (presumably registration for a treatment tank) can be done yourself with little work, though no doubt an engineer would be happy to charge you.

As for topo survey, it depends on the spec and the size of your plot but I suspect a few hundred quid.

Not sure exactly what you mean by proposed levels but these are usually shown on architects/planning drawings so would be included in their fee?

Drainage design I can't help with cost wise but many on here including me do it themselves.

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19 minutes ago, jamieled said:

@Amateur bob the scottish water (clean water connection?) and sepa applications (presumably registration for a treatment tank) can be done yourself with little work, though no doubt an engineer would be happy to charge you.

As for topo survey, it depends on the spec and the size of your plot but I suspect a few hundred quid.

Not sure exactly what you mean by proposed levels but these are usually shown on architects/planning drawings so would be included in their fee?

Drainage design I can't help with cost wise but many on here including me do it themselves.

the scottish water (clean water connection?) and sepa applications (presumably registration for a treatment tank) can be done yourself with little work,, 

 

is there not an actual fee to be paid for these though?

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Ah yes, sorry. Not sure about Scottish water as I'm on a private supply but others can probably help.

I think the sepa registration is about £160.

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How does this sound for my DIY SuDS attempt? The surface water storage volume estimation tool didn't seem to work, so I've just used common sense, is it correct?!

 

 

 

To quantify the existing runoff rate from the site, the on line tool on uksuds.com was used to calculate the mean annual flood flow, QBAR, using the Institute of Hydrology Report Number 124 (IH 124) methodology. (see Appendix 1). The post development run off rate must be no more than the green field run off rate, generally accepted as a 1 in 100 year, 6 hour event.

 

 

If QBAR 1 in 100 yr = 0.52 l/s

 

Then, surface water storage Volume estimation= 0.52 x 6 hrs x 60min x 60 sec

= 11 232 L

 

 

Rain gardens will be placed adjacent to the permeable driveway and a seasonal pond to attract wildlife and increase biodiversity, in the meadow is proposed with a volume of 15 000 litres (5m length x 3m width x 1m depth) with a bund on the south  and east sides receiving water via a gentle swale which is to be incorporated as part of the garden design.

 

A system of above ground storage (water butts)  will be installed for attenuation of storm water from the roof for reuse on the garden (gravity fed). If these are overwhelmed, water  will be diverted into the seasonal pond. 

 

 

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We are being held to a 1 in a 100 year event plus 40% allowance for climate change!  150+ cu m of crates at that rate.

 

This is becoming one of those things you just have to agree to get all of your paperwork lined up and then just go and build what is realistic and rapidly backfill.  To be fair - I am happy to live with a puddle in the garden once in 100 years instead of burying £10k of plastic in the garden.

 

The whole thing is madness.

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@Jilly I think it's not quite right, I've tried to explain why below.

 

1) The principle is that your post-development drainage rate should not exceed the greenfield runoff rate for design events that are presumably agreed with the BCO. Unless I've misinterpreted your text above, you seem to have calculated the storage volume required using the greenfield runoff rate (i.e. you are trying to store all the greenfield runoff). There are two separate calcs you need - the greenfield and post development. You just need to store enough to that the post-development runoff rate does not exceed the greenfield.

 

2) QBAR is what's termed an index flood, usually with a nominal return period of about 2.3 years, not 100. There's usually a scaling factor applied to QBAR to get you to whatever return period you need such as 30,100. Note at this point you are only getting a peak flow rate.

 

3) You need to consider both the runoff rate and volume. QBAR (or the 100 year peak flow) is a peak flow rate, I don't think it's appropriate to apply it across a 6 hour window as that assumes that the peak flow is occurring constantly for 6 hours. In practice you'd expect something closer to a bell shaped graph of runoff, peaking somewhere in the middle. The pre and post development runoff rates are important because the difference between them tells you what volume of storage you need.

 

4) The 6 hour, 100 year event is referring to a rainfall event. It would be typical to simulate this, along with some losses to try and estimate a runoff curve. Do this for both pre and post development and you should be able to estimate the storage volume. I'd have thought the uksuds tool might do this and will take a look sometime.

 

 

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The site seemed not to be working properly and wouldn't accept figures (but I may have been doing it incorrectly!)

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On 15/02/2021 at 23:18, Ferdinand said:

 

@Jilly is converting a stable in a paddock, so that should be doable by some means.

 

Nick a bit more land for the site and dig a seasonal pond.

A bit behind on this thread, but we went through the whole SuDS nightmare with our build. LPA rejected the first attempt by a drainage engineer which included a hugely expensive concrete chamber (with a ladder to climb down into it - about 2.5m deep and 1.5m wide) with a hydrobrake to limit the output - I was kind of glad the LPA rejected it! Then I went the rainwater harvesting route but LPA still wanted the hydrobrake - pointless. In the end I had to employ new local drainage engineers who 'knew what the LPA wanted' and specified a 'swale', which is in fact the pond @Ferdinandrefers to! This last report and scheme drawing cost about £600 but indeed the LPA accepted it. SuDS was without doubt the most expensive, long-running and craziest of the 15 planning conditions we had to meet. And we live halfway up a hill, with a slightly sloping site and a stream at the bottom! Learning, employ engineers within the LPA area and quiz them in advance about getting SuDS accepted.

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7 hours ago, Oldsteel said:

A bit behind on this thread, but we went through the whole SuDS nightmare with our build. LPA rejected the first attempt by a drainage engineer which included a hugely expensive concrete chamber (with a ladder to climb down into it - about 2.5m deep and 1.5m wide) with a hydrobrake to limit the output - I was kind of glad the LPA rejected it! Then I went the rainwater harvesting route but LPA still wanted the hydrobrake - pointless. In the end I had to employ new local drainage engineers who 'knew what the LPA wanted' and specified a 'swale', which is in fact the pond @Ferdinandrefers to! This last report and scheme drawing cost about £600 but indeed the LPA accepted it. SuDS was without doubt the most expensive, long-running and craziest of the 15 planning conditions we had to meet. And we live halfway up a hill, with a slightly sloping site and a stream at the bottom! Learning, employ engineers within the LPA area and quiz them in advance about getting SuDS accepted.

 

There is a reason that Swale Dale is so floody.

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