Paragraph 13 Surface water drainageI1

England and Wales

(1)

No part of the authorised development is to commence until, after consultation by the undertaker with the relevant planning authority, the lead local flood authority, the local highway authority and the Environment Agency, written details of the surface water drainage system, reflecting the mitigation measures in the environmental statement and including means of pollution control, have been submitted to and approved in writing by the Secretary of State.

(2)

Prior to consultation with the relevant planning authority, the lead local flood authority and the Environment Agency as required by sub-paragraph (1), the undertaker will carry out—

(a)

a CCTV survey of the location and condition of all drainage assets where—

(i)

the existing A303 is to be de-trunked and retained; and

(ii)

the existing A303 drainage is connecting into the proposed drainage network; and

(b)

a topographical and condition survey (i.e. visual inspection) of the extents of the ditches downstream of the proposed outfalls as follows—

(i)

outfall from Pond 1: up to and including culvert at Farm Lane overbridge (SMIS reference: 6245, culvert registration ID: 13795);

(ii)

outfall from Ponds 2 and 3: up to and including culvert at Royal Naval Air Station Yeovilton;

(iii)

outfall from Pond 4: up to culvert north of Frog Lane, West Camel (DRN ID: EAEW1001000000172218); and

(iv)

outfall from Pond 5 and 6: up to Dyke Brook; and

(c)

an assessment of the sustainability of the drainage proposals and of the opportunities to increase sustainable drainage provision.

(3)

The undertaker will make the results of the surveys and assessments undertaken in accordance with this requirement available to the relevant planning authority, the lead local flood authority, the local highway authority and the Environment Agency when undertaking any consultation required by sub-paragraph (1).

(4)

The drainage system must be constructed in accordance with the approved details referred to in sub-paragraph (1) unless after consultation with the relevant planning authority, the local highway authority and the Environment Agency, otherwise agreed in writing by the Secretary of State.

(5)

Runoff from natural catchments must be intercepted to prevent flooding of the carriageway in accordance with HA106/04.

(6)

Highway drainage will be designed in accordance with HD 33/16 Design of Highway Drainage Systems and any subsequent design manuals amending or replacing that. The system as a minimum must achieve—

(a)

no surcharge of the drainage system during the 100% Annual Exceedance Probability (AEP) storm event;

(b)

no flooding from the drainage system during the 20% Annual Exceedance Probability (AEP) storm event;

(c)

design exceedance management during the 1% Annual Exceedance Probability (AEP) storm event;

(d)

an allowance for the effects of climate change by allowing for a 40% increase in rainfall intensity.

(7)

The highway drainage system off-site discharge will be limited, up to and including the 1% annual probability (1 in 100 year event) plus a 40% allowance for climate change, to no greater than the undeveloped rate of run-off as determined by the calculation of Qbar or 2 l/s/ha.

(8)

In this paragraph, “the lead local flood authority” has the same meaning as in the Flood and Water Management Act 2010 M1.

Paragraph 13 Surface water drainageI1

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