One of the most common questions I get from modelers who are consultants for land development projects is, “We want to model our development but we don’t know what to set as the pressure at the location where we take water from the city/authority/wholesale supplier.”

If the system expansion is tying into a large tank, then the head at that tank provides a good boundary condition for the land development project. However, in most cases, the connection is far from a known, fixed hydraulic grade and the head today may or may not be a good indicator of the head in the future.

Some engineers simply assume they know the head (pressure) at the connection. This is dangerous. Is that head going to be there in the future? Or will it drop during a fire?

The best solution is to have the supplier perform the modeling or to provide information on the connection head for a variety of scenarios using the supplier’s model. However, this is not always the way that the procedures are set up.

Usually, the engineer for the developer can get the results of a fire hydrants flow test and can represent the dependence of head on the flow provided using a fictitious reservoir at the connection point and a fictitious pump such that the head vs. flow relationship at the connection point is maintained. This works all right if the hydrant flow test is representative of the future. We discuss how this is done on pages 328-333 of Advanced Water Distribution Modeling and Management and we cover it in our modeling training classes.

However, when there are multiple connection points, the logic of this procedure breaks down unless hydrants are opened at each connection point simultaneously, which is not the way that typical hydrant flow tests are run.

If this doesn’t work and there is considerable risk in modeling the connection, it may be necessary to build some type of skeletal model back into the supply system. I realize that this is not the kind of work that developers want to pay for, but sometimes there may not be any way around it.

It is also good practice for the modeler working for the developer to talk with the engineers/modelers for the supplier. Even though they won’t guarantee the head at the connection point, they can provide the developer’s engineer with insights as to the head. For example, the supplier may be planning a new tank or transmission main near the connection such that the service will be much more reliable within one year, or there is a better connection point than where the engineer was originally planning to connect.

As with any other engineering work, the best approach boils down to the engineer’s judgment as to what are reasonable assumptions. The risks range from excessive cost for overly conservative assumptions to inadequate pressures and flows from optimistic assumptions.