MATERIALES Y MÉTODOS

Now we have drawn the network, it is time to go over the data we will need. As you will know from the tutorial, when double-clicking on an object a dialog box opens and shows its properties. In this box, you can read some things and modify others but cannot modify all of them.

This is the complete dialog box for a junction, with all its properties.

Those which are shown inside the red square (above), with the exception of description or tag, can be modified, but will acquire an automatic value when the object is drawn. These properties are self explanatory and the tag has little use In black (underneath), is calculated data which is read-only. Any required data comes with an asterisk which in the case of a junction would be elevation and name.

Focus on the parameters in the centre: 1. Elevation 2. Base demand 3. Demand Pattern 4. Demand Categories 5. Emitter coefficient 6. Initial quality 7. Source quality

These parameters will be covered in the following sections. The main part, the demand (points 2, 3 and 4), are covered in the following chapter.

Elevation

The elevation is simply the height of a given point. Because we work with relative height, it makes no difference if the heights refer to sea level or to a stone where you may have forgotten your hat. The reference point which is used to compare elevation points is called the datum. Normally, for example with topographic surveys, this will be the base of a tank or a borehole. You can use any datum but it must remain fixed and visible. The datum determines the height and elevation for subsequent points.

Imagine you do not know the height above sea level for the following three points in a network (they have been put in a column as a reference).If you decide that the datum is going to be theborehole and you assign it an elevation of 0 meters, you will have:

Object Relative height (Height above sea level)

Datum Borehole 0 meters 500 m

Tank 67 meters 567m

Spring 1 22 meters 522m

If you used the tank as the datum, you would have:

Object Relative height (Height above sea level)

Datum Tank 0 meters 567 m

Borehole - 67 meters 500m

Spring 1 - 45 meters 522m

The most accurate way to determine heights is by doing a topographic survey. Never use a barometer or a conventional GPS in order to determine heights. These devices have an error of ± 10m at best, which is the whole pressure range of a well balanced system.

Another alternative is to use a contour map if available.

In some places, you can work with free digital elevation models with a reasonable precision. For example, with the free program Google Earth (http://earth.google.com), navigate until you can match the UTM coordinate of a certain point in the navigator and read its height.

If you need the elevation of numerous points, you may require a program to digitalize them. Click in the Google Earth Map and it will write down the coordinate of the points and their heights in a file. You can find one at www.zonums.com/zge_toolbox.html and see how it works as in the image below:

If you use these, be cautious and check the accuracy in your area. In some places and moments, you may get surprises, like the sea level is at 17m:

It is human nature to forget one or more of the points. Oversights will have a value of 0. Repeat the search in ‘query’:

Watch out with “tricky” elevations

Determining which elevation you should use may be tricky. Read the following paragraph carefully, because sometimes it is not just as simple as determining the height of a certain point.

Within a model, not all house connections are represented. For example, 25 houses between junctions A and B are omitted in the sketch, and their demand is distributed between points A and B. One of those houses, C, is located much higher than either of the end junctions. Because the height has been introduced in points A and B, there is a risk that problems with lack of pressure in point C remain hidden due to its greater height. The model will only provide pressure values for A and B.

The picture below shows a group of houses positioned on the top of a hill. These houses are supplied from a main along the road. Although the model gives a pressure of 1 bar, o 10 meters, because the house is located 15 m above the road, its real pressure is 10m-15m= -5 m.

What should we do then?

 If there are just a few connections in this situation, draw a junction for each of them

 If there are many, put some sample junctions at greater heights to warn of pressure problems occurring in connections located at higher points.

When determining heights, it is not necessary to determine the exact height of a pipe. Use ground level as an approximation, the only error being the pipe’s depth which is less than a meter.

The opposite problem, a connection in a low point, is less common. The sections on demand allocation and skeletization explain this further.

Be careful with high pipe points

Following the same reasoning, if the pipe has a point, A, which is higher than the water pressure line, it will not be able to reach point B.

In order to prevent this happening, although problematic, place a junction with no demand in the highest point of the pipe. Once the water starts moving, the hydraulic gradient will not be horizontal any more. It is more difficult to evaluate the pressure in point A if there is no junction. Except for some particular and exceptional cases, the minimum pressure of these points must be 10 meters in order to avoid pressure problems.

In cases where the pipe cannot be re-routed and would need pumps or elevated tanks in order to meet the requirement. In these circumstances, if the pressure in point A is a least 5 meters, it is not worth adding complexity to the network by adding a pumping station or an elevated tank.

Emitter Coefficient

This application is to model leaks and pressure-dependant demands… but it is unlikely that you will simulate those for new networks.

Initial Quality

This is usually left blank. It would be used when modeling if you were required to start with a value for water quality, for example, 0.6 ppm of chlorine. Its main purpose is to avoid long waits before the chlorine of the source spreads into the system.

Source Quality

In junctions where water enters the network, this parameter is used to describe its chlorine concentration. For example, you can model a drop chlorine injector with a discharge of 0.001 l/s and a source qualityof 100 ppm. In Chapter 5 chlorination will be described in detail.