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8/10/2019 SWRO.ppt [Compatibility Mode]

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Surface Water Run-Off


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Sur 1 - Management of surface water run-off

from developments

To design surface water drainage for housing developments which

avoid, reduce and delay the discharge of rainfall to watercoursesand public sewers using SuDS techniques. This will protect

receiving waters from pollution and minimise the risk of flooding

and other environmental damage in watercourses


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Assessor approach

Not expected to know

detail of this

Engineer AQP who does

this, knows SuDS

Collating evidence

Assessor needs basic

understanding of

principles only


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RAIN

Stream or sewer

Infiltration

Surface water run off

Green field site /

pervious ground

Sur 1 - Principles

This is our concern


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Stream or sewer

Infiltration

No infiltration

Surface water run off now

greater as rain hitting house

cannot infiltrate

Sur 1 - PrinciplesRAIN

Green field site /

pervious ground


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Stream or sewer

Sur 1 Possible Solutions Sustainable drainage systems

(SuDs)

Soakaway

Water goes here

instead of running off

Run off is reduced to

pre development level

Permeable

Paving


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Stream or sewer

Sur 1 Possible Solutions Rainwater used in the

dwelling

Water goes here

instead of running off

Run off is reduced to

pre development level


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SuDS Management Train

Source control (where the rain falls) Soakaways

Porous/permeable paving

Roof water directed to garden

Rainwater harvesting

Green roofs

Small swales and ponds

Underground attenuation storage

Site control (where the rain runs to) Swales

Infiltration/detention basins

Larger soakaways

Regional control Balancing ponds and wetlands

(to encourage a holistic thought process)


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Criteria Overview

Use SuDS management train:

1 Peak rate of run off No greater than pre developed site

2A Volume of run off

Additional volume entirely reduced Infiltration or other SuDS techniques non holding back solutions

Must do this first, only move to 2B if 2A cannot be satisfied

2B Volume of run off Reduce run off rate to limiting discharge

Must do 2A first where possible

Underground attenuation tanks possible solution here (not in 2A)


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Not acceptable in meeting 2A

Source control (where the rain falls) Soakaways

Porous/permeable paving

Roof water directed to garden

Rainwater harvesting

Green roofs

Small swales and ponds

Underground attenuation storage

Site control (where the rain runs to) Swales (in some cases)

Infiltration/detention basins

Larger soakaways

Regional control Balancing ponds and wetlands

Anything that

holds water

back will not

comply with 2A

egHolding ponds

Oversized

pipes with

hydrobreak


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Details

Mandatory requirements

Appoint appropriately qualified person Professional or team of professionals

Knowledge of

Sites SWRO needs/opportunities SuDS based solutions (+ experience)

Champion SuDS

Provide robust hydraulic design calculations

Report, flood risk assessment and drawings Covering all mandatory requirements

In accordance with Development and Flood Risk: a practice guidecompanion to PPS25

Concessions for smaller sites (see guidance)


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Peak Rate Run-off no greater Than pre development

For 1 year and 100 year return period

events

5 l/s discharge is allowed Post development At a discharge point

To avoid blockage

These rules do not apply If no increase in impermeable areaor

If criteria 2A (next) cannot satisfied

Sur 1 Mandatory criteria 1- Peak run off rate


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If run off volume post

development greater than pre

(2A) Additional volume (m3) must

be prevented from leaving site by SuDS techniquiqes

Holding back soluitons do not comply

100 year 6 hour event

If no increase in impermeable,

criteria met by default

Sur 1 Mandatory criteria 2 - Volume of run off

SuDs


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Unless A cannot be satisfied Responsibility of AQP to decide this

Full justification must be provided

In which case: (2B) the peak rate of run off must be reduced to:

1 year peak flow rate

Mean annual flood flow rate (Qbar)

2 l/s/ha (max)

No single discharge point need be < 5 l/s


} whichever is higher= limiting discharge


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3. Designing for local drainage system failure

Demonstrate that the flooding of the property would not

occur in the event of the above

Caused by Extreme rainfall

Lack of maintenance



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Fag packet maths Poor sitedrainage

Good site

drainage

Site size/increase in impermeable?

Pre dev peak run off rate 1 / 100 yr

1 Required post dev run off rate 1 / 100 yr

2A Peak flow rate increase due to dev. (pre

SuDs) 1 / 100 yr

Additional volume to be dealt with by non holding

back method (for 100 year event of 6 hours)

Equivalent to cube tank side length of

2B 1 yr peak pre dev. flow rate

Qbar

2 l / s / ha

Limiting discharge rate

Rate difference from 100 peak rate post dev.

Additional volume to be dealt with in any way

Equivalent to cube tank side length of

2 ha / yes 2 ha / yes

5 / 10 l/s

5 / 8 l/s

4 l/s

3.51 m

2x60x60x6 =

43,200 litres

5 l/s

4 l/s

4.76 m

108,000

5 l/s

22 / 44 l/s

4x60x60x6 =

86,400 litres

4 / 8 l/s

20 / 40 l/s

4.42 m

20 l/s

18 l/s

4 l/s

20 l/s

24 l/s

518,400

8.03 m

20 / 40 l/s

4 l/s


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Information required

A lot of detail Confirmation of qualifications of consultant

Report

Permeable/impermeable areas pre and post development

If impermeable increased, report to include:

Permeability characteristics of site

Inc infiltration tests where appropriate

Peak run off rate calculations for 1 and 100 year events

Methods used to reduce run off

Volume of run off pre, post development and with proposed

mitigation etc

FRA

Drawings


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Special Cases

Min flow rate set by sewerage undertaker

Max flow rate set by sewerage undertaker

Derelict sites

Planning Consent pre dates Code requirement

Infrastructure in place before Code requirement enforced Planning Authority require a non-compliant flow rate

Supplementary guidance for Wales will be published byWAG

For further guidance visit www.breeam.org/cshguidance
http://www.breeam.org/cshguidancehttp://www.breeam.org/cshguidance


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Assessing tips?

Assessment methodology Useful step by step approach

Calculation procedures Key publications

Allowance for climate change (post development only) Greenfield sites

calc method varies with size


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Using SUDS to improve quality of rainwater discharged or

protect quality of receiving water

Ensure no discharge for rainfall up to 5mm (1 credit) and/or

Run off from all hard surfaces receive treatment (1 credit) In accordance with SuDS Manual

To minimise pollution

2 CreditsSur 1 Optional credits


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Default Cases Rainwater discharges directly into a tidal estuary or the sea

Credits withheld if development goes against EnvironmentAgency recommendations

Sur 1 - Management of surface water run-off

from developments


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Examples


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Remineder of main requirements

There are two key aspects to consider when

assessing surface water run-off in the Code:

Peak rate of runoff

Ensuring the peak rate of runoff is no greater

post development than it was pre development

Volume of runoff

Ensure that the post development volume ofrunoff is no greater than it was predevelopment


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Watercourse or sewer

Possible solutions to deal with volume where

infiltration techniques are possible on site

Soakaway

Water infiltratesrather than

running off

Run-off is reduced to

pre-development level

Green roof

Permeable

Paving


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Watercourse or sewerSome of the run-off

infiltrates rather than

running off

Volume of run-off reduced

with some residual run-offremaining

Swales

Holding Pond

Possible solutions where additional volumes ofrunoff are low, but infiltration rates are alsovery low (such as sites with clay soils),

therefore only partial infiltration is possible


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Example

Site has the following characteristics

Low permeability on the site pre development (therefore small

additional volume of runoff caused by the new development)

Additional Volume of Run-off: 6m3

Fairly large open space

Surface SuDS were the cheapest option and were able to deal with all

the additional volumes of runoffPost development volume of runoff was no greater than pre development

WetlandSwale


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Watercourse or sewer

Possible solutions where infiltration is not at all

feasible on the site

Water collected in the Rainwate

Harvesting System rather than r

Runoff is reduced to



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Watercourse or

sewer

Possible solutions can be a combination of both

infiltration and Rainwater Harvesting where there is alarge volume to attenuate

Water collected in the

harvesting system rather

than running off

Runoff is reduced to


Permeable

Paving


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Example

Greenfield site pre development

Additional Volume of Run-off: 25m3

Holding Pond/Swales were not an option due to size of site

RWH tank installed below the small communal garden

Permeable paving installed on the front drives

Permeable Paving

Communal Rainwater Harvesting Tank

Communal garden

Additional Volume entirelyreduced.

RWH tank sized to also

deal with holding back

the run off for the 100 year

event before then being

discharged at the Limiting

Discharge rate

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