Doctoral thesis: How can a smart city manage excessive rainwater?
Man cannot win against nature, but a stormwater drainage system that can “predict and prevent” in which area the system will become surcharged during rainfall is one of the keys to saving city streets from flooding.
Nils Kändler, who defended his doctoral thesis on this topic, notes that pipeline surcharge can lead to pluvial flood or activation of combined sewer overflow, both of which will have negative consequences on the urban and living environment. The townspeople will, of course, suffer from this situation as well.
Urbanisation, climate change, and deterioration of infrastructure will all affect urban stormwater services in the coming decades. If so far the problems have been addressed by enlarging the underground grey facilities, i.e., installing new pipelines and detention tanks and enlarging the capacity of existing systems, then in the long term this approach is not reasonable.
Why is the existing system not enough?
Due to the large extent of the aforementioned challenges, this method is prone to failure because the systems cannot be adapted to the changing circumstances in the conventional way. Therefore, new smarter solutions that also comprise above-ground facilities and allow automatic control of the inflow to the urban drainage system (UDS) are needed. The use of existing control systems in this way is extremely challenging, as they are centralised in nature and not capable of predicting UDS parameters quickly enough, moreover, it is difficult to apply the control to the whole urban catchment with numerous inlets.
What can we do to change the situation?
The core of Kändler’s doctoral thesis was the development of a decentralised model predictive control algorithm (MPC). The algorithm allows the impact of rainfall event or other disturbance affecting the flow in the pipelines to be predicted and the actuators to be adjusted just in time to capture the peak flow. This reduces the risk of flooding downstream. What is more, decentralised operation makes the control points independent and less vulnerable to major failures.
Compared to other similar algorithms, Kändler’s algorithm has an important advantage: it utilises the dynamic inner model of the actuator but needs no simultaneous modelling of UDS or computationally costly optimisation routines. The decentralized MPC created in this doctoral thesis is universal and robust, allowing the implementation of any type of inlet control in an urban catchment.
“Smart” pipelines in two Estonian towns
Ivar Annus (photo below), Tenured Assistant Professor at the TalTech Department of Civil Engineering and Architecture and the supervisor of the doctoral thesis, explains that in the case of increasing and more intense rainfall events, it is not possible or viable to continue with the current control solutions for urban stormwater systems which provide for the rapid channelling of water from streets to underground collectors as their capacity is limited. “The doctoral thesis investigated, among other things, how much, where, and how long water should be stored on the street to ensure the proper operation of the system in the entire catchment area. To this end, in addition to the control algorithm, a method was developed to map the areas in the urban environment that can and should (due to the operation of the system) be flooded in a controlled manner during heavy rainfall events,” Annus explains.
While there is no urgent need to invest in the proposed decentralised solution, then according to Annus, for the implementation of the system, a digital revolution in the water sector is required to create digital twins of existing systems and to equip critical gullies with sensors that are used to control the entire system. “The first smart stormwater system solutions have been constructed in Haapsalu and Rakvere as part of our NOAH project,” Annus reveals. In the framework of the ongoing DEPART project, it is planned to actually make the developed control algorithm work in one Estonian town within the next three years.
How was the result achieved?
The algorithm was successfully tested in three pilot areas to control adjustable gullies, real-time controlled manholes and street storage units. The results showed that the solution was capable of reducing the peak flow by more than 50%, keeping the inflow to UDS below the pre-set threshold level and reducing the number of flood nodes by 30%.
It does the job quickly and properly. And is affordable!
The algorithm is also computationally faster than most of the solutions available. For example, the calculation of the settings of 18 decentralised orifices took only 0.11 seconds. The developed solution is also economically feasible both in terms of investments and maintenance costs, being in average 20% more affordable than traditional methods for flood risk reduction. Therefore, the developed smart stormwater system is a feasible, robust and efficient method for increasing the resilience of urban areas.
More cooperation is needed
Although the control solutions of UDS presented in the thesis were tested on three pilots, they were designed to be universal, i.e., applicable in any urban area facing the increasing risks of overload of stormwater systems. It is also important to note that above-ground structures, like existing permeable surfaces, are also part of UDS. Typically, in such areas, the water utility operates the underground UDS while the landowner is responsible for maintaining above-ground spaces, i.e., stormwater catchments. However, such division of responsibilities hinders the implementation of both smart and environmentally sound stormwater solutions, as innovative and efficient solutions require comprehensive management to ensure long-term operation of the system. The doctoral thesis clearly demonstrates that cooperation between these two parties can have a beneficial impact on the management of urban stormwater run-off.