The study is based upon the definition of surface drainage performance in terms of the depth, extent, duration, and frequency of flooding, in contrast to the more conventional focus upon flooding frequency alone. This broader definition was intended to provide an answer to slum-dwellers' questions about "what happens when it floods?" as well as "how often will we flood?"

The study team collected and analysed data in three slum catchments in Indore, Madhya Pradesh in India, where DFID has made substantial investments in a slum improvement programme. The three catchments varied in topography and their drainage design and included:

Bhagirathpura, a steep improved catchment, which combined piped drainage with surface routing of flow.

Pardeshipura, a flat unimproved catchment, using conventional surface channels for drainage.

Motilal ki Chal, a flat improved catchment which combined piped drainage with surface routing of flow.

To explore the issues of drainage performance, data were gathered over two monsoons in these catchments on rainfall, conduit flow and level, solids levels and their particle size distributions, and the dimensions and elevations of the engineered drainage systems. Models for these systems were then developed, calibrated and verified using Wallingford Software's WALLRUS, SPIDA, and HydroWorks. A large number of simulations were then performed to explore the effects of changes in solids levels, construction practice, and design approach upon the performance of the drainage systems, as defined by the depth, area, duration and frequency of flooding.

Drawing on the experience of this analysis and the field work in gathering data, a manual is in final preparation entitled xyz to be published by Intermediate Technology Publications.

Hydraulic modelling software can be used to predict the depth of flooding with reasonable accuracy. In Motilal ki Chal, where the most detailed modelling studies were performed, maximum flood depths above ground level were predicted to within 0.1 meter in three out of four flooding events.

The effect of solids and maintenance upon performance. Solids in drains (and therefore maintenance) have a significant impact upon the frequency of flooding, but not on its depth once flooding occurs. The effects of solids upon the extent of flooding, and its duration depends upon the local design and topography. In particular, where systems have "positive drainage" (i.e. have consistent downward slopes out of the catchment) the effect of solids upon duration of flooding is far less than where the pipes or channels of the drainage network are the only way out.

Solids are large and ubiquitous. Sampling from the open channel drain in ardeshipura established a particle size distribution between one and two orders of magnitude larger than those encountered in the drains of the industrialised world. This appears attributable to issues of limited solid waste management, with particular difficulties attributable to construction debris and temporary road repair materials, in addition to domestic solid waste which finds its way into the drain where services are inadequate.

Surface routing of flow should be explicitly considered in design. Given widespread occurrence of large solids deposits, and a significant effect of solids upon the frequency of flooding, it follows that flow above ground will be relatively frequent. It therefore seems wise to design the surface routing explicitly, as was done in both Bhagirathpura and Motilal ki Chal "road as drain" systems to minimise the effects of solids upon performance. Modelling work in Motilal ki Chal has established some of the tradeoffs possible in reducing dependence upon the piped network through establishing suitable road levels and kerbs. This work has vindicated the "road as drain" approach adopted in the Indore Slum Improvement projects, while elaborating further some of the drainage issues that need to be more clearly considered in design.

Kolsky, P.J., D. Butler, and T. Sihorwala (1993). Performance Based Evaluation of Urban Drainage in Developing Countries, Proceedings of the Sixth International Conference on Urban Storm Drainage, Niagara Falls, Canada, September 1993.

Kolsky, P.J., J.N. Parkinson, D. Butler, (1996). Third World Surface Water Drainage: The Effects of Solids Upon Performance, Chapter 11 in Low-Cost Sewerage ed by D.D. Mara,( pp. 189-214.) Wiley: Chichester.

Heywood, G.M., P.J. Kolsky, D. Butler, (1997). Modelling Drainage Performance in an Indian Catchment, Journal of the Chartered Institution of Water and Environmental Management, 11:1 (pp. 31-38).

Kolsky, P.J., J.N. Parkinson, D. Butler, T.A. Sihorwala (1996). Drainage without Drains? Performance Studies in India and their Implications, Proceedings of the Seventh International

Conference on Urban Storm Drainage, pp 521-526. SuG-Verlagsgesellschaft: Hannover.

Kolsky, P.J. Surface Drainage: Low-Cost Evaluation Techniques to Improve Performance. Intermediate Technology Publications: London. (forthcoming).

Tel: 0171 927 2628
Fax: 0171 927 7843
Email: p.kolsky@lshtm.ac.uk