The City of Lima is the capital of Peru, with a rapidly growing population currently pegged at about six and a half million. Water supply is provided by a combination of groundwater and treated surface water taken from the Rimac River that flows from the Andes. Contrary to any preconceived notions you might have about these snow capped mountains however, water on the Pacific side is scarce to the point where the entire western coastal region from Chile to Ecuador is a desert.

Rainfall in Lima is virtually unknown and winter is characterized by an occasional shower of soft rain that is little more than condensation within the ever present fog created by the cold Humboldt current making its way up from the Antarctic. The foothills surrounding the city are completely barren and one can be thankful for small mercies as there are no prevailing winds to whip up the layer of dust that substitutes for vegetation. All in all, it is a rather dismal picture but one that doesn't do justice to the city as a place where Delcan was more than happy to spend the past year designing a trunk watermain system to supply a developing area known as Cono Norte or the north "cone" for its geographical shape on the map.

Groundwater is the only option for supply to the city's fringes because every drop of water from the Rimac is already spoken for. The entire Rimac flow is diverted to La Atarjea water treatment plant as it enters the city, leaving a bone dry river bed downstream until it mysteriously begins to flow again before it enters the ocean. One suspects that raw water quality is a little poorer than that upstream of the plant ­ enough said! As the city grows therefore, the pressure on groundwater increases and the water table is falling rapidly, leaving well pumps high and dry. In areas close to the Pacific, there is evidence of saltwater intrusion.

Lima's water authority, SEDAPAL, with assistance from various international lending agencies, has embarked on a programme to augment its surface supply through diversions high in the Andes with the ultimate goal of supplying more of the city from La Atarjea. This is where Delcan entered the picture on a $1.2M project funded by the World Bank.

Delcan's task was to model the north end of the city, assuming a gravity supply from Atarjea via an extension of an existing 1600 millimetre diameter watermain. The model is shown schematically in Figure 1. The design flow from Atarjea was 11.6 cubic metres per second or 220 million gallons a day, a substantial flow by any standard. This work led to the detailed design of 34 kilometres of watermain ranging in size from 1600 to 350 millimetre diameter, threaded through a densely populated urban environment to serve Los Olivos in the north "cone" and Callao to the south.

The design population for Los Olivos was 750,000 with a corresponding water demand of 2.3 cubic metres per second or 43.5 million gallons a day. The region is currently supplied by fifty-seven wells drawing from a declining resource and demand outstrips supply by an ever-widening margin. Despite the shortages, people continue to flood into the area and the city has been unable to stem the tide. The best they can do is ensure that each new development has its own system of wells and an elevated "head" reservoir paid for by the developer. The economics are perhaps questionable, but the general feeling is that they will at least have the tools to ration whatever water is available.

Unfortunately, this policy was created without a master plan to address the eventual connection of Los Olivos and Callao to La Atarjea's gravity supply. This left Delcan with the unenviable task of designing a gravity system to supply an area with at least twenty-four tanks, all operating at different top water levels as shown on Figure 2. Does the expression, a silk purse from a sow's ear spring to mind?

The design objective was to divide Los Olivos into a number of manageable sectors with control valve connections to the Atarjea trunk to be remotely operated based on a rationing schedule for the whole city. The sectorization process was essentially a visual exercise taking into consideration population, existing interconnections between adjacent developments, natural boundaries formed by roads or other geographical features, distribution of elevated storage and operating water levels.

Using EPANET, each sector was modelled to determine the best location for its connection to the Atarjea trunk. Extended period simulations were used to ensure that the tanks would work in a "floating" mode despite differences in level. The results were surprisingly good but we have been around long enough to realize that comparisons between theoretical and actual operation are unlikely to be flattering. Whatever the outcome however, SEDAPAL has been given an opportunity to approach the integration of groundwater and surface supplies from a new perspective; one that should result in significant economies and improved operation for the future.

On a lighter note, one of the realities of working in a developing area such as Los Olivos, is the increased likelihood of meeting entrepeneurs whose economic outlook makes survey equipment appear attractive for re-sale. This is not a trivial concern but in one instance at least, negotiations resulted in a settlement leaving Delcan with the total station equipment and our intrepid entrepreneur with the spare tire off the truck. On another occasion we were less successful. For the most part however, the community recognized the benefits of the project and went out of its way to be helpful.