Emergencies: Preventing, Preparing, and Responding

BENTLEY
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Population health is a primary concern of water utilities

Population health is a primary concern of water utilities, whether water demands are typical (daily demands) or an out-of-the-ordinary event occurs and threatens the continuous, safe supply of potable water. Water utilities must be prepared to respond to emergencies before they occur, and this is where hydraulic modeling can be particularly useful.

Water utilities’ operations departments typically rely on a supervisory control and data acquisition (SCADA) system to monitor the water network, as SCADA data provides a snapshot of what is currently occurring in the water system. However, relying uniquely on SCADA data is limited because it cannot interpolate between measured points or see what will happen in the future.

An integrated SCADA-hydraulic model can forecast what will happen in the water system when an out-of-the-ordinary event occurs. It can not only provide an accurate and clear understanding of how the current system behaves, but it can also simulate various alternatives to identify the optimum emergency response.

The task of preparing for and responding to emergencies is not an easy one, so more and more utilities are taking this approach of leveraging the strengths of both SCADA and hydraulic modeling to make better emergency response plans. The difficulty in preparing emergency response plans is that the response will differ depending on the type of emergency that ensues. The only common denominator with responding to various types of emergencies is that utilities will try to minimize disruption to customers.

Most water utilities already have a hydraulic model of their system that they use for planning and design. Starting from that point, constructing a real-time model for use in operations is not difficult.

Identifying Vulnerable Assets to Limit their Failure

Before preparing for an emergency response plan, it is good maintenance practice to prioritize pipe and asset renewals to mitigate the risk of infrastructure failure (unplanned by definition). Hydraulic modeling will help find the pipes and facilities that are the most vulnerable.

A criticality analysis in a water modeling software helps modelers identify the most critical elements: the user can shut down individual segments of the water system and determine the

impact on system performance like the flow, volume, or pressure shortfalls, for each segment outage. Barwon Water, the largest regional water provider in Victoria, Australia, conducted this type of analysis. The organization wanted to optimize asset management decisions and improve customer service by limiting the number of customers affected by planned and unplanned supply interruptions. Barwon Water’s integrated GIS-WaterGEMS model improved accuracy by identifying critical mains and avoiding manual identification of critical pipes.

Criticality analysis results can then be fed into a hydraulic modeling capability that optimizes the replacement and rehabilitation of distribution network pipes. For example, comprehensive pipe renewal capabilities help engineers identify pipes for renewal by ranking the worst-performing pipes based on their year of installation, materials of construction, pipe break history, and hydraulic performance under various constraints. Through this ranking process, hydraulic modeling helps ensure long-term system reliability by decreasing pipe breaks, leakage, lost revenues, and outages, and ensuring adequate capacity. With a proactive pipe renewal program, infrastructure failure is less likely; however, because the water system cannot be 100 percent reliable, water utilities with a renewal program still need to be prepared to manage emergencies.

Managing Shutdowns

Shutdowns can happen for various reasons, from maintenance issues to unplanned events. A shutdown does not always mean taking a single pipe out of service. Instead, a shutdown removes a distribution system segment, which is bounded by valves, from service. Modeling a shutdown should take into consideration the exact locations of valves.

Hatch Mott MacDonald managed a shutdown in this way. It performed hydraulic analyses for DC Water in Washington, D.C., U.S.A. to identify alternative supply sources and forecast potential outages before removing a main transmission artery for rehabilitation. Criticality analyses allowed Hatch Mott MacDonald to identify the scenarios that could result in acceptable service levels and enabling DC Water to develop emergency response and contingency plans.

Responding to a Fire

Depending on the size of the fire and the capacity of the distribution system, fire demands can seriously impact the performance of the system. Limitations in pumping and piping capacity can impair firefighters. However, there are some modifications in system operation than can increase fire flows. The most obvious change is overriding normal control settings and turning on additional pumps. Depending on piping capacity, turning on an additional pump can produce a significant increase in flow in most cases. The hydraulic model can calculate the change in flow before turning on the second pump.

Providing additional water sources by opening connections with neighboring district metered areas (DMAs), pressure zones, or neighboring water systems may significantly improve fire flows. The model can give a quantitative estimate of the additional flow available for the fire from these sources.

Preventing water from leaving the pressure zone with the fire by temporarily turning off pumps

to higher zones or closing pressure reducing valves (PRVs) to lower zones can make more water available for the fire if those zones have storage, but it is best to use the model to check on the impacts before turning off pumps.

Salt Lake City, Utah, U.S.A. followed this procedure when evaluating its distribution system to meet current fire flow service requirements. Using water distribution network analysis software, the city determined which pipes needed replacing and identified where to place the control valves and pressure zones. With this analysis, the city helped boost the available fire flow and provide higher pressure for many customers.

Restoring Power

Power outages can severely impact the pumping capacity of a water system. Depending on the spatial extent of the outage and the expected duration, operators can take a variety of steps to minimize the adverse impacts. Given the amount of water in storage at the start of the outage and the capacity of backup power, the operator can use the hydraulic model to estimate when the supply will run out and which portion of the distribution system will be affected first.

If the power outage is isolated to a fraction of the pumping stations in the system, modifications in the operations of the other pump stations can minimize the adverse impacts. In general, the operator wants to increase pumping toward the pressure zone without power and decrease pumping away from the pressure zone without power. The model can be used to estimate the efficacy of this type of strategy.

If the outage is long enough that it causes tanks to drain, more drastic measures may be taken, such as turning on backup wells, moving in portable generators, or opening interconnections with neighboring utilities. The effectiveness of these measures can be evaluated with the model. If they will not prevent loss of pressure, the model can indicate the neighborhoods that will lose pressure and areas where water tankers or bottled water can be deployed for the most benefit.

Sabesp used this type of approach when its Jardim da Conquista pump station lost power. Approximately 240,000 people in Sao Paulo, Brazil remained without water overnight. To solve this problem, Sabesp simulated the hydraulic conditions with a model and defined a cost-effective solution. The analysis software helped the organization test new pump setups and determine when the pumps would fill the newly constructed reservoir.

Responding to a Contamination

When planning a response to a contamination, it is crucial that water utilities visualize where contaminated water may travel and assess this contamination-spread from various sources. The impacts of contamination can be minimized if the system is flushed by opening hydrants, but it is important to flush at the location of the contaminant plume. If the wrong hydrant is flushed, it can discharge good water and spread the plume. If a reasonable estimate of the initial position of the contaminant plume is available, the model can identify where it will be located later to tell the operator which hydrants to flow.

When a serious contamination occurs, hydraulic models created in Bentley’s hydraulic modeling applications can often be exported as an “iModel” for easy visualization of problematic areas on a tablet. This capability enables operators to quickly identify affected customers in nearly real-time and to attain useful information calculated by the model, such as pressure or concentration, while in the field.

Summary

During emergency situations or in emergency planning, operators need to make informed decision on how to respond. They can rely on experience, along with some guesswork, or they can use their hydraulic model to more precisely estimate the impact of various response measures. The hydraulic model essentially gives distribution system operators X-ray vision into what is currently occurring inside the pipes and the ability to reasonably predict what will happen based on possible solution options. The integration of modeling and system operation is now the current and optimal solution.

Emergencies: Preventing, Preparing, and Responding

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