Doing It Right: Some Thoughts on Stormwater Management and Modeling

Chris Brooker, cba Specialist Engineers, Fourways, South Africa

ABSTRACT

We are all here because we enjoy modelling stormwater, or some other water system that generally incorporates free surface flow, but we often lose sight of what we are truly trying to achieve, and we may lose sight of the practicalities surrounding our investigation or modelling. I would like to talk about some of my experiences in the practical aspects of stormwater management and modelling. My address, containing numerous slides of real world examples, will cover some aspects of the process that we follow to achieve a practicable, sustainable and maintainable stormwater management system.

The basic tenets of stormwater management; that it is a space management problem and that water flows downhill without respect for political or most structural boundaries (ASCE, 1992) are the starting point for any project of this type.

We must understand the objectives of our investigation, for example in flood calculation are we interested in flood mapping to identify hazards or bio-refuges, water quality management, river morphology and habitat deterioration, minor system design, street hazard identification etc.

Selection of the correct modelling tool is essential. Is the system so simple that we can make a quick (Ir)Rational Method calculation, is PCSWMM appropriate, is a design storm sufficient or do we have to build a continuous model, do we need a detailed ground water model, is 2D modelling necessary, is 1D modelling (e.g. HEC-RAS) appropriate or would PCSWMM be better. A model of a dam break on a steep mountain stream will be used as an example.

What can go wrong if you get the planning or detailing wrong, choose inappropriate materials, do not make allowance for unexpected events (ice blockages in Johannesburg), or do not maintain the system? But what can be achieved with simple interventions, especially in wetlands and urban river rehabilitation?

Consistency of approach and penetration, at least at city level, are important. There is a notable contrast in the standards applied to private developers compared to those applied to the statutory authorities, and the standards applied from community to community, for example the contrast in Seattle between Bainbridge Island and the City itself.   


Permanent link: