Results

Published)

Chapter 5 Meeting Environmental Flow Requirements in a Changing World using

5.5 Results

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The control timestep (Δt, Figure 5.1b) was set at 1 minute, as this provides a good balance between algorithm performance, which improves with a reduction in Δt, and practicality. Updating the orifice opening at 1 min intervals can be achieved easily in practice by using suitable computer-controlled electrical actuators that are connected to the orifice system to adjust its opening/closing (Cui et al., 2017, Ma et al., 2021).

5.4.3 Performance Assessment

Relative Mean Absolute Error (RMAE) is used to assess the performance of the THC approach. This corresponds to the average absolute difference between the target outflows and the actual outflows from the THC approach, scaled by the mean target outflow, calculated as follows:

Relative Mean Absolute Error (%) =

1

𝑛∑^𝑛_𝑖=1|𝑄_{𝑜𝑢𝑡,𝑡}−𝑄_{𝑡𝑎𝑟𝑔𝑒𝑡,𝑡|}

1

𝑛∑^𝑛_𝑖=1𝑄_{𝑡𝑎𝑟𝑔𝑒𝑡,𝑡}

× 100%

(5.2) where 𝑄_{𝑜𝑢𝑡,𝑡} is the actual outflow achieved by the THC approach at time t, and 𝑄_{𝑡𝑎𝑟𝑔𝑒𝑡,𝑡} is the target outflow at time t.

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5.5.2 Ability of the THC Approach to Adapt Outflow Hydrographs to Changing Inflows

As can be seen in Figure 5.2a, b & c, even though the different worlds have significantly different inflow hydrographs, the THC approach is able to adapt the outflow hydrograph to the environmental flow target with low relative errors of 3.1% (current world), 1.3% (future world 2050) and 1.8% (future world 2090).

Figure 5.3 shows that the degree of orifice opening is changed dynamically throughout each storm event based on measured tank water levels, enabling the desired outflow target hydrograph to be achieved under significantly different inflow conditions. This highlights the ability of the proposed THC approach to achieve any pre-specified environmental flow conditions in a changing world via smart storages provided the storage has a sufficiently large capacity.

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Figure 5.2 Performance of the THC Approach to Achieve Target Hydrographs for Three Worlds (a. Current World, b. Future World 2050 and

c. Future World 2090)

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Table 5.3 Summary of Changes in Inflow Hydrographs Scenario Peak Flow

(% increase*) Volume (% increase*)

Current World 7 25

Future World 2050

39 45

Future World

2090 95 57

*Percentage increase compared to the target environmental flow hydrograph

Figure 5.3 Control Schemes of the THC Approach to Achieve a Target Hydrograph for Three Worlds (Current World, Future World 2050 and Future

World 2090) 5.6 Discussion

5.6.1 Practical Benefits and Implementation

The key contribution of this paper is that it demonstrates the potential of using the THC approach to achieve desired environmental flow hydrographs under a range of inflow conditions corresponding to different future worlds. A key practical benefit is that the THC approach achieves this by using a control scheme that adapts to different inflows with minimal changes to infrastructure.

This provides a potentially cost-effective alternative to traditional passive approaches that would require expensive infrastructure upgrades to adapt to changing future worlds.

With the development of smart technologies, sensors can be easily installed in stormwater storages to measure storage level information in real-time. The required orifice opening percentage to achieve the target hydrograph can then

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be calculated using the orifice equation and adjusted using computer-controlled actuators. Consequently, the THC approach can be easily implemented in practice by using current technology. This is a significant practical benefit compared with predictive real-time control schemes that require rainfall forecast information, which can be highly uncertain (Di Matteo et al., 2019a, Liang et al., 2019).

5.6.2 Future Opportunities to Apply the THC Approach

There is also the opportunity in the future to adapt the THC approach to produce different target outflow hydrographs to achieve multiple stormwater management objectives under a changing climate (Wu et al., 2020, Culley et al., 2021, Bennett et al., 2021), such as water quality improvement (Campisano et al., 2017, Muschalla et al., 2014, Shen et al., 2020), stormwater harvesting (Xu et al., 2018, Xu et al., 2020) and erosion control (Higson and Singer, 2015, Sólyom and Tucker, 2004). For example, storages can be used as retention storage during non-flood events to harvest stormwater, and the stormwater can be used to meet water supply and downstream environmental flow requirements.

In addition, by detaining stormwater in a street-scale storage (rather than on- site tanks) for a reasonable length of time, water quality can be improved significantly for urban receiving waters. The THC approach can also improve the performance of previous RTC schemes that are used to reduce system peak flows. For example, the THC approach can control both the magnitude and time of occurrence of peak flows for flood mitigation, and thus provide extra time for flood preparation and evacuation.

5.6.3 Future Research

Future research can be undertaken to test the practicality and improve the performance of the proposed approach, including 1) testing the effectiveness of the THC approach for a range of environmental flow target hydrographs with different objectives, 2) testing the THC approach on a larger scale case study with multiple storages, 3) testing the impact of the size of control time steps and storm durations on the performance of the THC approach, 4) developing an optimization-simulation framework to identify the required storage volume to achieve target hydrographs under specified rainfall events, and 5) testing the THC approach using physical experiments in the field or laboratory.

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This study introduces and demonstrates the utility of the Target Hydrograph Control (THC) approach to achieving environmental flow target requirements by adapting to a wide range of inflow hydrographs caused by land use and climate changes. It does this by adjusting the orifice opening percentage using real-time measured storage level information and the orifice equation. The effectiveness of the THC approach is demonstrated for three different “future worlds” on a simple example catchment located in Darwin, Australia.

The experimental results demonstrate that the THC approach is able to achieve the target hydrograph effectively with less than 3.1% error for all experiments.

This is despite the inflow hydrograph varying significantly as a result of land use and climate change, with the increase in peak flows ranging from 7% to 95%

and volumes from 25% to 57%.

This study demonstrates that the THC approach has significant potential to adapt to land use and climate change and achieve the desired target hydrographs for environmental flow requirements. By only using storage level information that can be easily measured in real-time, the THC approach is able to account for changes in runoff hydrographs caused by land use and climate change and achieve target hydrographs that match pre-development hydrographs. This could maximize the usage of existing stormwater storages and potentially avoid the need for significant investment in stormwater infrastructures to deal with changes in hydrographs caused by land use and climate change while still being able to meet environmental flow requirements.

Acknowledgments: Ruijie Liang received a scholarship provided by the University of Adelaide.

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