STAGE 3: OUTLINE DESIGN

The third stage of the SuDS design is the outline design, which should be developed alongside the agreed layout and design of the development, and landscape and building characteristics. Key steps in outline design are shown in Figure 7.11 and described in the following sections.

Outline designs will usually be required where outline planning permission is sought. Where only a full planning permission is sought, the stages involved here will still be required, but should be incorporated into the detailed design stage (Section 7.8).

7.6.1 Size SuDS components at site scale

In order to further develop the SuDS design, estimates will be required for the following:

▪ the greenfield (and/or agreed proportion of previously developed) runoff rates, to which the runoff from the site will need to be controlled

▪ likely runoff rates from the developed sub-catchments (including any climate change and urban creep provisions)

▪ infiltration capacities where infiltration components are proposed

▪ demand for non-potable water where rainwater harvesting components are proposed

▪ the remaining difference in runoff volume between the development runoff volume and the greenfield (or other agreed) runoff volume for a specified large event.

The required attenuation storage volume will be dependent on the increase in runoff rate because of the development, and the design rainfall event characteristics.

This will allow initial sizing calculations to be done of the:

▪ volumes assumed to be harvested for different return periods

▪ volumes assumed to be infiltrated for different return periods

▪ total attenuation storage volumes required for the site for different return periods

▪ extra storage volumes likely to be required for volume control for the 1:100 year event (ie Long- Term Storage).

Figure 7.11 The outline design process

The design process is set out in Figure 7.11. Detailed guidance on each of the steps is presented in Chapters 24 and 25.

The design criteria and standards for water quantity are presented in Chapter 3.

Guidance on methods for estimating greenfield runoff rates is presented in Section 24.3.

Guidance on methods for estimating previously developed site runoff rates is presented in Section 24.5).

Guidance on methods for estimating runoff rates from the developed site surfaces is presented in Section 24.6.

Guidance on designing for climate resilience is provided in Section 3.2.7.

Guidance on climate change factors and urban creep factors is provided in Section 24.7.

Guidance on infiltration design is provided in Chapter 25.

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

Guidance on attenuation storage design is presented in Section 24.9.

Guidance on Long-Term Storage design is presented in Section 24.10.

7.6.2 Develop design at sub-catchment scale

At this stage, the individual SuDS components should be sized, and their designs refined. Any assumptions made at conceptual design stage, such as infiltration capacities, groundwater levels and existing sewerage infrastructure and capacities, should be confirmed, using robust evaluation methods.

Required storage volumes should be distributed between sub-catchments (where appropriate), estimates should be made of required conveyance and exceedance flow rates and checks should be made that proposed treatment components are adequate. Any required flow control components should be defined and scoped. This process is set out in Figure 7.12.

Guidance on methods conveyance system design and exceedance design is provided in Sections 24.11 and 24.12.

Guidance on designing for treatment is provided in Chapter 26.

Guidance on component sizing for water quantity and water quality management is provided in Chapters 11–23.

Guidance on infiltration testing and design is provided in Chapter 25.

Guidance on flow control component design is provided in Chapter 28.

7.6.3 Check design feasibility

The constructability and maintainability of the proposed SuDS scheme should be given full consideration, and initial construction and maintenance strategies developed for consideration by stakeholders.

A preliminary health and safety risk assessment should be developed. SuDS designs will require consideration as part of any CDM risk assessment process.

Any requirements of the drainage approving body and other engaged stakeholders regarding design detailing should be evaluated at this stage.

The costs of the scheme should be given full consideration and agreed with scheme funders, and likely long-term operation and maintenance costs should be approved by the drainage adoption body before final design. Where scheme investment is driven by benefits delivered by the scheme, benefit quantification should also be undertaken.

Guidance on generic SuDS construction requirements is provided in Chapter 31, with component specific detail contained within Chapters 11–23.

Guidance on generic SuDS maintenance requirements is provided in Chapter 32, with component specific detail contained within Chapters 11–23.

Guidance on designing safe surface water management systems is provided in Section 5.2.4.

Guidance on health and safety is provided in Chapter 36, and the health and safety checklist is provided in Appendix B (Section B.3, Table B.5).

Guidance on costing designs is provided in Chapter 35.

7.6.4 Outline design: Reporting

Reporting at this stage should be sufficient to support outline planning applications, and should include preliminary sizing for each component and exceedance flow management route. The outline design statement should establish:

▪ points of discharge of the surface water runoff from the site

▪ the extent to which each of the design criteria (quantity, quality, amenity and biodiversity) will be delivered by the SuDS design, and the impact of any stakeholder engagement undertaken during the design process

▪ statements regarding how each of the criteria will be delivered by the system

▪ a description and evaluation of proposed Interception measures for all impermeable areas

▪ a suitable SuDS Management Train(s) for all sub-catchments that delivers appropriate treatment

▪ infiltration tests (where practical – or other infiltration assessments based on desk studies) and approximate infiltration designs (where relevant)

▪ approximate attenuation and Long-Term Storage volumes and appropriate flow control systems to manage flows and volumes for different return periods, including design exceedance events

▪ 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).