STAGE 4: DETAILED DESIGN

▪ initial health and safety risk assessment (as part of CDM process)

▪ plan and elevation drawings of the proposed scheme

▪ an operation and maintenance plan (that includes waste disposal).

A more formal list of requirements might be set by a drainage approving body at outline planning stage.

An example list of requirements for reporting is set out in Appendix B (Section B.1.2, Table B.2).

7.7.1 Test hydraulic performance of scheme

The scheme should be tested hydraulically to identify the worst case hydraulic condition for each component for all design return periods. This should be undertaken using either design storms or time series rainfall (TSR) analysis.

The hydraulic performance of the system should then be optimised to make sure that all storage and conveyance areas are used as effectively as possible during design rainfall events. Drainage simulation models can be used where appropriate.

Consideration should be given to the extent to which the SuDS components should function during flood events affecting the receiving surface waters.

Account should also be taken of any overland flow routes across the site from external areas or other external flood sources. Such flows will either need to be routed around the site or conveyed across the site, either in exceedance routes or within the SuDS components themselves. The likely rates and volumes of these extra flows will need to be accounted for in the system design so that people and property on the site are not put at risk.

Exceedance routes should be evaluated and designed where required, ensuring appropriate levels of freeboard between extreme flood levels and building floor levels

Guidance on rainfall characteristics is provided in Section 24.2.3

Guidance on Interception, attenuation and Long-Term Storage design, conveyance and exceedance route sizing is presented in Sections 24.8 to 24.12.

Guidance on infiltration design is provided in Chapter 25.

Guidance on the design of RWH systems is provided in Chapter 11.

Guidance on design of inlets, outlets and flow control systems is provided in Chapter 28.

7.7.2 Check scheme meets design criteria and agreed standards

Checks should be made that the final scheme meets all of the design criteria established for the site (Section 7.6.2). At this stage, it is likely that the main checks will be that the system meets the agreed standards for quantity and quality, that:

▪ Interception is delivered for all hard areas

▪ peak rates of runoff for low return period events are adequately controlled

▪ peak rates of runoff for high return period events (including appropriate climate change and urban creep factors) are adequately controlled

▪ volumes of runoff for high return period events are adequately controlled

▪ high return period events (including appropriate climate change and urban creep factors) do not pose an unacceptable risk to people or property, as a result of the development

▪ the flow velocities and depths for regular events allow effective treatment to be delivered by components for which treatment performance is assumed.

The performance of the system with respect to the remaining water quantity and water quality criteria, and the amenity and biodiversity criteria is unlikely to change materially at this stage in the process.

However if, because of non-compliance with the above standards, a decision is made that a component should change fundamentally (eg a surface component has to become a subsurface component), then a review of performance to the full suite of criteria will be required.

Indicators are likely to be a useful way of assessing the performance of the scheme to agreed criteria and where these have been established early in the design process – they can potentially form a framework

for the assessment process (eg “the proportion of the scheme that is on or near the surface” is likely to be a useful indicator of the extent to which the scheme might deliver a number of the water quality, amenity and biodiversity criteria).

Example indicators for water quantity, water quality, amenity and biodiversity criteria are presented in Chapters 3–6.

7.7.3 Refine SuDS component sizing and flow controls

Where the agreed standards (described in Section 7.8.2) are not met satisfactorily, the design will need to be revisited, and amendments made to component sizing and/or flow controls. The hydraulic testing will then need to be undertaken again. This process will usually occur iteratively until a satisfactory solution has been identified.

Guidance on Interception, attenuation and Long-Term Storage design, conveyance and exceedance route sizing is presented in Sections 24.8 to 24.12.

Guidance on infiltration design is provided in Chapter 25.

Guidance on the sizing of RWH systems is provided in Chapter 11.

Guidance on treatment design is presented in Chapter 26.

Guidance on sizing of inlets, outlets and flow control systems is provided in Chapter 28.

7.7.4 Finalise design

The final design should be refined, taking costs and benefits into account, together with any health and safety risk assessment (required as part of CDM), constructability and maintainability considerations.

Individual SuDS components should be finalised and detailed following the guidance within the technical component chapters of this manual and/or appropriate manufacturer literature.

Specifications will need to be prepared for all the materials used in the design, and for the construction and landscaping works, together with full construction method statements and Maintenance Plans.

Community education and engagement strategies for the completed system should be developed. In some cases these will evolve from community input provided during earlier design stages.

A design statement should be prepared, which includes a description of each of the system criteria and standards, and the approaches through which these criteria and standards have been met.

Guidance on component design is provided in Chapters 11–23.

Guidance on design of inlets, outlets and flow control systems is provided in Chapter 28.

Guidance on construction is provided in Chapter 31.

Guidance on maintenance of SuDS is provided in Chapter 32.

Guidance on waste management is provided in Chapter 33.

Guidance on community environmental learning is provided in Section 5.2.7. Guidance on community engagement is provided in Chapter 34.

7.7.5 Detailed design: Reporting

At this stage, reporting should include the final detailed design and specification for the SuDS scheme sufficient to support a full planning application, including:

▪ infiltration and geotechnical test results and evaluation

▪ design methods used (eg for greenfield or previously developed runoff rate assessments) and justification

▪ full calculations for the overall scheme and individual components

▪ justification of any non-compliance to national or local standards

▪ detailed design drawings

▪ materials specifications

▪ landscape specifications

▪ construction method statement

▪ scheme Maintenance Plan (including costs)

▪ final design statement.

A more formal list of requirements might be set by a drainage approving body at full planning application stage.

An example list of requirements for reporting is set out in Appendix B (Section B.1.3, Table B.3).