Use Cases
In 1994, the Dutch water authority posted the guideline for water companies to evaluate the level of water supply while coping with calamity events. A tentative guideline requirement is that a water system must meet 75% of the original demand for the majority of customers and no large group of customers (2000 resident addresses) should receive less than 75% of their original demand.
The guideline is applicable to all the elements between the source and tap in a water system and is required to find the effect of every element. In order to calculate the water supply level under a calamity event, a hydraulic modeling approach is proposed:
- Take one element at a time out of a model, copying the calamity event of element outage
- Run the model for peak hours of all demand types and also the peak hours of tank filling. The actual demand needs to be modeled as a function of pressure; the supply is considered unaffected if the pressure is above the required pressure threshold
- Evaluate the water supply level for each demand node. If there is less than 2000 resident customers receiving less than 75% of the normal demand, then the requirement is met. Repeat Step 1 to simulate another calamity event. If the requirement is not met, continue with step 4.
- Perform 24 hours pressure dependent demand simulation for the maximum demand day under the calamity even
- Sum up the actual demand for each node over 24 hours
- Check if there is any node where the totalized demand over 24 hours is less than 75% of the maximum day demand; if not, the guideline is met. Otherwise an appropriate system improvement needs to be identified in order to meet the guideline.
UK water companies are required by law to provide water at a pressure that will, under normal circumstances, enable it to reach the top floor of a house. In order to assess if this requirement is satisfied, companies are required to report against a service level corresponding to a pressure head of 10 meters at a flow of 9 liters per minute. In addition, water companies are also required to report the supply reference for unplanned and planned service interruptions.
Both use cases provide some generality for water utilities world wide to evaluate the performance of water systems under emergency and low pressure conditions. An emergency event can be specified as one set of element outages. In order to quantify the water supply level under such an event, the demand must be modeled as a function of nodal pressure. Hydraulic model needs to be enhanced to perform pressure dependent demand simulation and to compute the level of certainty/supply level.