An Intermediate Step in the
Good engineering is all about knowing when and how to ask the right questions before diving into technical analyses. Drainage engineers who are experienced in modelling storm sewer systems (for drainage collection and conveyance) know that ‘problems’ fall within a narrow range. A lesson learned through experience is that one need not model every section of pipe.
Many storm sewer systems operate without serious problems for many years. Yet many engineering studies recommend plans for pipe replacement and upsizing that would cost multi-millions of dollars, money that local governments do not have. Reliance on complex computer models and inflexible application of design standards may be having unintended consequences.
In 2012, recognition of the need to look at drainage analysis differently resulted in the BC Ministry of Environment support and federal funding for the Partnership for Water Sustainability to develop the Drainage Infrastructure Screening Tool to add to the practitioner toolbox.
The tool is an intermediate step in the assessment process, not a replacement for detailed analyses. The added value it provides is the capability to look at how land use densification and climate change would affect storm sewer systems.
One Tool, Two Types of Assessment:
Local governments can now consider both climate change and land use change at the same time, and with the same tool.
The Drainage Infrastructure Screening Tool makes it easy for local governments to check and verify the relative impact of a changing climate on conveyance capacity. The resilience of a system depends on the capacity a system has now and how drastic future climate change might be.
The tool also makes it is easy to assess the relative significance of changes in land use, in particular densification.
Ask the Right Questions:
The purpose of the Drainage Infrastructure Screening Tool is to answer this fundamental question: is there a problem that needs detailed analysis? The initial objective is to undertake drainage system capacity analysis without the need for intensive and expensive modelling of the storm sewer system. To this end, four questions provide a framework for screening and decision-making:
- What is the existing level of drainage service within the community?
- What will be the effect of climate change?
- What will be the effect of redevelopment?
- What will be the effect of climate change on redevelopment?
A guiding principle is to provide an equal Level-of-Service or access to the drainage system for all properties within a drainage catchment or watershed.
Level-of-Service (LOS) Methodology:
Level-of-Service is the integrator for everything that local governments do. What level of service does a community wish to provide, and what level can it afford? Everyone will have to make level-of-service choices.
The methodology embedded in the Drainage Infrastructure Screening Tool provides the means to quickly and efficiently identify weak links in a storm sewer system; and then budget affordable solutions. The methodology is founded on relationships that are universal.
The screening tool examines the pipe system that is tributary to a drainage outlet or outfall. Assessing catchments one-by-one keeps the analysis logical, simple and manageable. Every pipe within each catchment is evaluated by examining the INSTALLED PIPE CAPACITY. The ratio establishes a level-of-service.
The Level-of-Service approach is inexpensive and provides relevant information for capital planning. It does this without the detailed and expensive simulation of the drainage system. The process establishes existing system capacity and then identifies those parts that do not meet this standard. These can be prioritized and entered into a capital plan.
- Step One – For each section of pipe, estimate:
- Catchment area and capacity (litres per second/ha)
- Actual Level of Service
- Design Discharge
- Step Two – Compare design discharge to installed pipe capacity (i.e. the ratio)
- Step Three – Identify problem areas (i.e. Qcapacity < Qdesign)
- Step Four – Modelling is optional
Try the Drainage Infrastructure Screening Tool
Register for a FREE trial and get access to all modeling features.