Report No FR0488



Nov 1994



The Guide is concerned with the design of Combined Sewer Overflow (CSO) structures and appurtenances to ensure satisfactory hydraulic performance criteria are achieved and regulatory aesthetic control requirements(') are met. The Guide is intended to complement planning documents such as the Sewerage Rehabilitation Manual (SRM)and the Urban Pollution Management (UPM) Manual which allow the acceptable discharge regime at a CSO to be specified. The Guide picks up from this point and addresses the issues relating to the engineering design of the structure itself to ensure that the specified flow and solids separation functions are achieved effectively.


The Guide is intended to provide practical and useful information to the practitioner. It should not be regarded as prescriptive, but rather seeks to assist the designer to arrive at an effective solution, acceptable to all parties in terms of cost and environmental performance.

The early sections of the document provide general and background information to place the issues to be addressed in context (Sections 1 and 2).

In Section 3, the performance objectives for an acceptable CSO are defined and a step-by-step design procedure proposed whereby these aims can be achieved in a practical manner. The basic principles upon which the design procedure is based are explained in the later parts of this section.

Detailed design considerations for each of the basic types of recommended CSO structure are presented in Section 4, together with some practical considerations for a successful design.

Design of the hydraulic control arrangements is a key issue for any CSO structure and Section 5 is devoted to consideration of this aspect.

The topic of screens, to add a physical solids separation capability to that which can be achieved by the good hydraulic design (as described in earlier sections of this report), is addressed in Section 6.

Post project appraisal is recommended for all new CSOs. The methods by which this can be undertaken are described in Section 7.

Appendices are provided to illustrate how the design procedures set out in the Guide can be applied in practice for each of the major types of CSO structures.


The Guide supersedes ER304E 'A Guide to the Design of Storm Sewer Overflow Structures' which has been recognised as providing guidance for "good engineering design" for CSO structures for some years. The Guide builds upon and updates the previous guidance in the light of improved knowledge resulting from recent research and recent changes in regulatory requirements. The updated design procedures should now be regarded as appropriate guidance for "good engineering design".

The Sewerage Rehabilitation Manual and the Urban Pollution Management Manual are complementary documents which provide the overall planning framework for the upgrading of urban drainage systems within which the detailed design procedures of the Guide fit.


The design procedure assumes that the acceptable spill regime (frequency, volume and polluting loads for any discharge) for the CSO has been predetermined. Thus the inflow, continuation flow and spill flow rates are fixed and, consequently, any storage needs are defined.

The design procedure is therefore concerned only with identifying the optimum type and configuration of CSO structure to meet these constraints and, where appropriate, the provision of supplementary devices, such as screens, to improve the solids separation performance of the installation.

No single preferred type of CSO structure is identified. Four options are considered, all of which are capable of achieving acceptable performance. The final choice is dictated by local site conditions.

The design procedure achieves compliance with the regulatory requirement for "good engineering design" (see Table 3.1) as a minimum standard. Chambers designed in this way will achieve effective hydraulic control and a significant degree 9f solids separation from the spill flow. Guidance is provided to allow the solids separation performance of CSO structures to be further enhanced hydraulically should the designer wish it (although this cannot be quantified relative to the regulatory standards for 6mm and lOmm solids separation).

General guidance on good practice is provided for those circumstances where the use of screens is deemed necessary to achieve compliance with standards for protecting amenity use (6mm and lOmm solids separation). No firm guidance can be given as to the precise nature of those circumstances. This must remain a matter for the judgement of the designer based on an assessment of the risks associated with reliance on stillinghydrodynamic design versus the cost of providing screens.

Copies of the report are available from FWR, price 25.00, less 20% to FWR Members.