Report No FR0488
GUIDE TO THE DESIGN OF COMBINED SEWER OVERFLOW STRUCTURES
SECTION 1 - INTRODUCTION
1.1 PURPOSE OF THE GUIDE
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.
1.3 RELATIONSHIP WITH OTHER DOCUMENTS
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.
1.4 SUMMARY OF DESIGN PROCEDURE
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.
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