Long-term Water Distribution Design

Water distribution piping has a useful life on the order of a century while pipe sizing decisions are usually based on a planning horizon on the order of 20 years. Because of the substantial economy of scale in pipe sizing and the long useful life, there may be benefits to installing piping with some excess capacity in the short term depending on the expected long term growth. The analysis presented in this paper shows the value in considering long term growth in demand even if the planning horizon for the study is much shorter. While the only decision that needs to be made in the current year is the size of the initial pipe, it is important that the pipe fits into a long term plan for the distribution system but the exact diameter doesn’t make much difference. INTRODUCTION Typically water distribution system planning is done for a planning horizon of about 20 years. However, water distribution piping has a useful life on the order of 100 years. Reliable demand projections are not available for a 100 year planning horizon. Most water distribution design optimization models analyze the system to develop a single design to minimize cost at the end of some shorter planning horizon. In many cases, having a single design flow over the planning horizon is adequate. But in many other cases, real systems do not get installed all at once but gradually evolve over decades. Think of what the distribution system looked like in any community in 1913 and compare that with how it looks in 2013. Real systems (and in particular transmission networks) are continually expanded and upgraded over time. Sequencing of capacity expansion is important and is generally ignored in optimization models which attempt to minimize cost (and hence minimize excess capacity) at some specific point in time. This virtually guarantees that the system will be in a failed state shortly after the end of that planning horizon. Fortunately, there is a great deal of economy of scale in water main installation. A small increase in pipe size results in a small increase in cost but a very large increase in capacity. By looking at longer term planning horizons or multiple planning horizons, the engineer can avoid installing bottlenecks in the distribution which might be optimal in the 20 year planning horizon but troublesome in the longer term. When one includes the issue of uncertainty of future demands (which is often very large), the sizing problem becomes one of “when do we install this big 830 World Environmental and Water Resources Congress 2013: Showcasing the Future © ASCE 2013