Recognizing that water distribution system flushing practices and procedures vary greatly among utilities, a proactive flushing program is an effective way to improve water quality and assess system maintenance needs.  Computer hydraulic models can play an important role in this effort.

Conventional

Many utilities coordinate their system flushing with routine hydrant exercise and maintenance activities.  When system hydrants are inspected and exercised, they are also opened to flush sediment from the distribution system pipeline network until the water clears. This is commonly referred to as a "conventional" hydrant flushing program and is often performed once or twice per year.  Depending on the hydrant flow rate, pipeline size and pipeline network characteristics (looped or branched) supplying the hydrant, this type of system flushing may or may not be effective in discharging sediment that has accumulated in the system.

Directional

Some utilities perform "directional" flushing, (also called uni-directional flushing) which involves closing system valves prior to opening hydrants to control the flow path to the open hydrant, increase pipeline flushing velocities and improve the flushing effectiveness.  However, planning and executing a system-wide directional flushing program requires significantly more time and labor than a conventional program.  Therefore, it is more efficient to only perform directional flushing in areas of the system where conventional flushing is not adequate to achieve the flushing program objectives.

Model Impact

Water distribution system computer models can be used to assess and improve the effectiveness of conventional flushing programs. Using the model, flow rates for each hydrant are simulated to predict pipeline flow velocities. Model output is then analyzed to identify pipeline segments that do not achieve the target flushing velocity. The target flushing velocity can vary from system to system and may depend on the flushing program objectives. The length of pipeline segments that do not achieve the desired flushing velocity compared to the total length of pipeline in the system provides an estimate of the efficiency of the conventional hydrant flushing program.

Additional Benefits

The model can also be used to develop specific procedures for improving the flushing effectiveness by increasing the flow velocity in areas of the system that would not otherwise achieve the desired flushing velocity. For example, the model can predict how opening additional hydrants and/or strategically closing valves could increase flow velocities in certain pipes. Directional or other special flushing procedures can be implemented based on the model results, to improve flushing effectiveness for specific areas of the system. The primary advantage of this system flushing approach is that the utility only performs more costly flushing techniques in areas of the system where they are required to achieve flushing velocity goals.