Articles & Case Studies

Using InfoWorks for very large models in Utrecht

Posted: Tuesday 22nd August 2006

In the Netherlands each municipality has a regulatory requirement to reduce the combined sewer overflows by at least 50 percent, and the reduction has to be proved not by measurements, but by model calculations. To monitor this, the water board tests the calculated annual overflow volume and sometimes the extreme overflow volume with a return period of one and two years. For this calculation the use of rain series of at least ten years is compulsory.

The city of Utrecht, the fourth largest in the Netherlands, is a very experienced user of network modeling. Two recent projects that have been conducted within the regulatory guidelines have stretched the limits of conventional modeling in terms of the run times of the simulations.

Both projects have used InfoWorks as their platform, and they address:

Time series calculations

Time series calculations from a model – calculations and results based on the simulation of extended periods – have considerable advantages in assessing combined sewer overflows (CSOs) and addressing their reduction. The frequency and volumes of CSOs, both total annual volume and extreme volumes of each incident, depend not only on the discharge capacity of the system, but also on its storage capacity and the pump capacity to the treatment plant. The reason why the storage capacity of a system is important in very flat regions such as the Netherlands is that the overflow frequency of a sewer system depends on the availability of storage capacity. Therefore the time that is needed to empty a sewer system is important. An accurate calculation of the filling and emptying of a sewer system is an essential part of estimating potential CSOs.

The component parts necessary for running the model are a good model of the sewer system, estimates of dry weather flow (domestic, based on population, and commercial wastewater), and the specification of surface run off.

The InfoWorks model used contained 19.404 nodes and 21.091 conduits, with 227 overflows and 232 pumps. The dry weather flow was calculated from accurate population data of 233.589 inhabitants generating with 2.814 m3/h domestic wastewater, and commercial wastewater of 1418,8 m3/h. The run-off surface was an area of 1.482 ha contributing to the combined system en 123,9 ha to the improved separate system.

The rain series selected, the De Bilt Series, covered ten years from 1955 to 1964 in fifteen minute periods – clearly a huge computation load for a model of this number of nodes, links, and pumps. The results file occupied 6 Gbytes. InfoWorks contains a statistics template for the results, and overflows occurring within 24 hours of each other were combined to a single event. A separate program was built to identify the extreme events within the results.

Presenter Erwin Rebergen commented “we are very happy that InfoWorks makes it possible for us to simulate 10 years in 5 minute intervals for a 20,000 node network. This is one of the biggest hydraulic simulation in the Netherlands.”

Real Time Control

The Real Time Control project in Utrecht is designed to explore the control regime required to improve the hydraulic conditions, using control structures such as pumps to achieve this. The modeling of RTC requires software that can represent the control rules of each structure, which InfoWorks allows the user to specify.

In Utrecht, the total sewer system is divided into districts (sub-sewer systems), and wastewater is transported between them by pumps. The system contains 22 such districts, and the overall objective of the system management, as defined by Local Authority regulations, is to try to keep water in the sewer system as much as possible, thereby minimizing the combined sewer overflows. The frequency and volumes of these overflows depend on two factors - the storage and pump capacity of the system and the ability to optimize the storage capacity of the different districts by devising the best operating regime for the pumps. The scope for optimization is based on the ability to compensate for differences in rainfall between different locations within the city. A control system for the main sewer pumps has been in existence in Utrecht since 1988.

The simulation, based on a real storm event, used the RTC capability of InfoWorks to examine how different RTC scenarios can regulate the amount of water that is being pumped between the districts velocity and depth of flow in the network. Each scenario has three elements: the regulators involved, the other components of the system whose states are used as triggers to change the regulators, and the rules that specify when the settings of the regulators are changed. The model contained 192 rules for the 54 regulators defined.

The conclusions from the study are that the InfoWorks model will be used into the future to improve the RTC strategy, increasing the understanding of the impact of different rules and combinations of rules, and enabling the capacity of the system to be optimized.

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