Setting a discharge rate into a combined sewer

Discharging surface water to a combined sewer is the bottom of the hierarchy, and the rate you get is set by the sewerage undertaker's view of its own network rather than by your hydraulic calculation. On a brownfield site with an existing connection, the usual starting position is that the undertaker wants less water than currently arrives, not the same amount. That changes the drainage strategy from a question of attenuation volume into a question of how much existing impermeable area you can disconnect.

Settle it before the layout is fixed. A scheme that assumes a connection at the existing rate and then has to find storage for a 60 per cent reduction loses either a plot or a basement.

Why a combined sewer changes the argument

A combined sewer carries foul and surface water in one pipe to a treatment works, with overflows to watercourses when the capacity is exceeded. Every litre of rainwater added to it is a litre that may end up in a spill, and it is a litre pumped and treated in dry weather terms. Undertakers have spent a decade reducing surface water inputs to these systems, and a planning application asking to add one starts from behind.

The right to connect under section 106 of the Water Industry Act 1991 is real but it is not unconditional. The undertaker may refuse where the mode of construction or condition of the drain is inadequate, or where the connection would be prejudicial to its sewerage system, and capacity in a combined system is exactly the ground on which that refusal is framed.

Two consequences follow. First, the connection point and rate are a commercial and technical negotiation with the undertaker's developer services team, carried out in parallel with the planning application rather than after it. Second, the planning authority's drainage condition and the undertaker's connection offer are two separate instruments that must be made to agree.

The hierarchy you have to work through first

Part H3 of the Building Regulations sets the order: an adequate soakaway or other infiltration system, or where that is not reasonably practicable a watercourse, or where that is not reasonably practicable a sewer. The national policy expectation and the non-statutory technical standards for sustainable drainage push the same way, and the lead local flood authority, as statutory consultee on surface water for major development, will test whether the higher options were properly ruled out.

"Not reasonably practicable" needs evidence, not assertion.

• For infiltration, infiltration testing to a recognised method at the depths and locations proposed, with groundwater levels, and the contamination position where the site history requires it.
• For a watercourse, the distance, the ownership of the intervening land, the availability of an easement, the consents needed from the lead local flood authority or the Environment Agency, and the receiving bank's condition.
• For a separate surface water sewer or a highway drain, the asset records and the capacity response.

Only then does the combined sewer become the answer, and the drainage strategy has to show that sequence with the evidence attached. Where infiltration is partially viable, a hybrid arrangement with infiltration for the lightly trafficked areas and a controlled connection for the rest is usually better received than an all-or-nothing case.

What the undertaker offers and on what basis

The rate offered tends to come from one of three positions: a reduction on the existing measured or calculated rate from the site, a figure derived from the greenfield runoff rate for the site area, or a nominal flow the undertaker considers the network can absorb.

The greenfield calculation depends heavily on the rainfall dataset and the method used, and that choice is contestable, which is why the question of which rainfall dataset to cite is worth settling before you submit a number. Present the calculation with its inputs so that the reviewer can reproduce it.

There is also a practical floor. A flow control device passing a very small rate blocks, and many authorities and undertakers work to a minimum controlled discharge of around 5 litres per second unless a device capable of passing debris at a lower rate is specified and its maintenance secured. On a small site that floor can exceed the greenfield rate, in which case say so plainly and agree the departure rather than designing a control that will not survive its first autumn. The behaviour of such a device when partly obstructed is worth modelling, as modelling a partially blocked flow control sets out.

Removing existing flows as an offset

This is where most combined sewer negotiations are actually resolved. If the site currently drains 2,400 m² of roof and yard to the combined sewer, disconnecting part of that area creates headroom for the proposed connection.

The arrangements undertakers use include:

1. Flow for flow, where the new connection is permitted at the rate removed elsewhere.
2. Betterment, where more flow has to be removed than is added, expressed either as a percentage reduction on the existing rate or as a multiple of the new flow.
3. Off-site disconnection, where the developer funds removal of surface water from a third-party area, such as a school roof or a length of highway, in exchange for the connection.

Each needs evidence of the existing connected area. That means a measured survey of impermeable surfaces, the asset records, and where connections are uncertain, dye testing or a camera survey. Do not assume that a yard which looks drained is connected to the combined system, and do not assume that a roof downpipe discharging to a gully reaches the sewer at all. Several sites turn out to have less connected area than the drawings suggest, which weakens the offset case and needs to be known early.

Off-site disconnection deals are slow and involve a third-party landowner, so programme them as a twelve-month item rather than a condition discharge.

Phasing occupation against a network upgrade

Where the undertaker has an improvement scheme programmed, the practical answer is often to tie occupation to it. The mechanisms are:

• A Grampian condition restricting occupation of specified units until the undertaker confirms that capacity is available.
• A requisition of a public sewer under section 98 of the Water Industry Act 1991, where the developer funds works the undertaker then provides.
• Phased connection agreements, with an interim restricted rate and a higher rate once the upgrade is complete.

Grampian conditions on sewer capacity are commonly imposed and commonly accepted at appeal where there is a reasonable prospect of the works happening. Where there is not, the condition is vulnerable, and the honest answer may be that the site cannot come forward at the proposed density until the network changes. Say that to the client in month one.

Evidence to put in the drainage strategy

The Drainage Strategy should contain, in this order:

1. The existing site drainage, with the measured impermeable areas, the connection points and the evidence for each.
2. The hierarchy assessment, with the infiltration test results and the reasons each higher option was ruled out.
3. The pre-development and post-development runoff calculations, with the rainfall dataset, the method, the climate change allowance and the urban creep allowance stated.
4. The correspondence with the undertaker, including the capacity enquiry response and any offered rate.
5. The proposed control, the attenuation volume, the exceedance routing and the maintenance regime with the responsible party named.
6. The residual risk and what happens when the control blocks.

Where the site is in a flood zone or exceeds a hectare, this sits alongside a Flood Risk Assessment and the two must use the same levels, the same climate allowances and the same exceedance routes. Reviewers check that first.

Where the condition and the connection offer conflict

A drainage condition specifying a rate the undertaker will not accept is a trap. So is one requiring "a scheme to be agreed" with no rate at all, because it gives the undertaker no reason to move and leaves you discharging a condition against a moving target.

Ask for condition wording that ties the two together: approval of a surface water drainage scheme limiting discharge to the combined sewer to a rate agreed in writing with the sewerage undertaker and not exceeding a stated maximum, with the connection agreement or offer submitted as part of the discharge. That gives the authority its certainty and leaves room for the undertaker's figure.

And get the capacity enquiry in before the application is submitted. The response takes weeks, it is the single most useful document in the drainage pack, and officers increasingly ask for it at validation.

For the rest of the drainage and flood evidence base, enter your postcode on PlanScope to see what the site is likely to need. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.