The Regional Laws on ‘Hydraulic and Hydrologic Invariance’

In the last decades, Italian regions approved separate rules to manage urban stormwater and to control urbanization. Regione Lombardia, the region where Brescia is located, very recently approved rules to account for the interaction between the two issues, briefly summarized in the so-called ‘hydraulic and hydrologic invariance’ concept (BURL, 2017). Because of some shortcomings in their application procedures, improvements were also approved shortly after their first publication (BURL, 2018, 2019). The ‘hydraulic invariance’ ensures that the peak flow of the surface runoff event is maintained at the pre-urbanization level, while the ‘hydrologic invariance’ preserves the runoff event volume.

Nature-based solutions are here suggested for stormwater management in urban areas, together with multi-purpose solutions. Besides, the water storage volume needed to satisfy the invariance requirement can be reduced by the total void volume of the infiltration devices, but not by the infiltrated water volume. A simplified design procedure, based on the critical event approach, can be applied if the transformation area has a limited extension (smaller than 300 m2), while the minimum required storage volume can be further reduced in the case that infiltration devices are used and properly sized. New ‘invariance rules’ apply to listed urban/building transformations, not including a few restoring measures because of their potential limited impact on receiving water bodies. On the one hand, financial support mechanisms can be adopted by municipalities for transformation projects affecting areas classified as ‘urban and land restoration areas’ in the current urban plan. On the other hand, in some cases ‘invariance’ storage volume requirements can be simply monetized.

Trend Analysis in Rainfall Characteristics in Brescia

For many years, precipitation has been observed in a few monitoring sites in Brescia and its surroundings. So far, the longest time series of heavy rainfall observations at sub-daily resolution has been recorded by the meteorological station installed in the courtyard of a local high-school named ‘Istituto Pastori’. For this analysis, data recorded at Istituto Pastori since 1949 were integrated for the last two decades with heavy precipitation data recorded at the meteorological station installed on the roof of one of the university buildings (a few kilometers north of the Pastori rain gauge).

The statistical analysis of the heavy precipitation time series suggests that no significant trend is detectable for yearly maxima of heavy rainfall for sub-daily durations.

Nevertheless, plots show a weak negative trend for durations higher than 1 hour and a weak positive trend for durations equal to or lower than 1 hour (Figures 7.2 and 7.3). Heavy precipitation intensity and frequency of occurrence may though change even in the near future, and the precautionary principle invoked by the European directives mentioned above empowers actions aimed at increasing the resilience of urban settlements with respect to the potential change.

As far as the climate of the future is concerned, the output of regional climate models (RCMs) provided by CORDEX for Europe (https://euro-cordex.net/) was considered as a valuable source of information to check the projected monthly precipitation change. The EURO-CORDEX ensemble is based on the Representative Concentration Pathway (RCP) scenarios, which are the most recent climate modeling and

Time series of heavy precipitation with a duration of 3 and 24 hours recorded in Brescia. Trend lines show weak negative slopes

Figure 7.2 Time series of heavy precipitation with a duration of 3 and 24 hours recorded in Brescia. Trend lines show weak negative slopes.

impact modeling methods described in the latest IPCC Assessment Report (AR5). RCPs define the pathways of the additional radiative forcing caused by anthropogenic activity until the end of the 21st century (the value in 1750 is considered as a reference). One modeling chain (Global Climate Model + RCM) in the European spatial domain EUR-lli was selected for this study:

  • • Global Climate Model (GCM): ICHEC-EC-EARTH
  • • Regional Climate Model (RCM): RCA4.

Figure 7.4 shows the position of the RCM grid points (in yellow) in the study area and its surroundings, while the red arrow indicates the relevant point belonging to the study area. RCP 2.6, RCP 4.5, RCP 8.5 (from 2041 to 2060), and the historical climate scenario (from 1981 to 2005) were analyzed.

Figure 7.5 shows the precipitation regime in the current and the future climate for the selected grid point based on CORDEX data. Finally, Table 7.2 lists the value of the multiplicative factor К expressing the precipitation change between the current and future climate for each month. This multiplicative correction factor could be used to build new precipitation data series by multiplying the original data by this factor, in agreement with the procedure used, for example, in Grossi et al. (2013). Then the new precipitation time series can be the input of mathematical models simulating the behavior of nature-based solutions in the future climate.

The position of RCM grid points in Brescia and its surroundings (base map from Google Maps, accessed November 2019)

Figure 7.4 The position of RCM grid points in Brescia and its surroundings (base map from Google Maps, accessed November 2019).

Precipitation regime for the historical and climate change scenarios in Brescia

Figure 7.5 Precipitation regime for the historical and climate change scenarios in Brescia.

Table 7.2 Monthly correction factor К for rainfall depth in Brescia

К coefficient

Month

RCP 2.6 scenario

RCP 4.5 scenario

RCP 8.5 scenario

January

1.40

1.02

0.90

February

1.33

1.30

1.46

March

0.92

1.07

1.31

April

0.99

1.02

0.86

May

1.24

1.03

1.08

June

1.18

0.79

0.69

July

0.79

0.79

0.64

August

1.26

0.42

0.87

September

0.88

0.79

1.18

October

0.87

1.03

1.27

November

0.98

0.83

1.28

December

0.85

0.93

1.14

On the other hand, in the Italian engineering practice, hydraulic devices are usually sized according to the critical or design storm approach, where the features of the single design storm are derived from the depth-duration-frequency (DDF) curves, and the critical storm is assumed to have a rectangular shape. For a given Г-уеаг return period, just to mention some of the used design procedures, the size of a combined sewer channel is assigned based on the Г-уеаг peak runoff, while sewer tanks have to cope with the Г-уеаг runoff volume.

Adaptation of hydraulic devices to climate change requires then that the DDF curves are modified according to the precipitation features in the future climate change scenarios. This is the approach followed for example in Canada (Hassanzadeh et al., 2019; Nguyen & Nguyen, 2018; Nguyen 2019). The main limit of event-based design approaches is though the lack of information about the sequence of dry and wet days and the occurrence of subsequent storms. If a second storm occurs when the storage capacities are not yet restored, its effect can be stronger than expected when it is considered separately. Continuous simulation, instead of event-based simulation, of the hydraulic behavior of the device to be designed is therefore recommended to ensure its more robust sizing.

An alternative approach can be the semi-probabilistic approach first introduced by Guo and Adams (1998a, b, 1999) and then modified into a simplified version by Bacchi et al. (2008). In the semi-probabilistic approach, a stochastic rainfall model is adopted to represent the rainfall process. A deterministic rainfall-runoff model is then coupled with the rainfall model to derive the probability distribution of some characteristics of the outflowing hydrograph. To investigate the effects of potential climate change on the efficiency of the urban drainage system, with respect to the design volume of a water retention tank with a given return period, a procedure based on the semi-probabilistic approach was used to analyze the potential effects on some fictitious basins located in Brescia (Grossi & Bacchi, 2008). Four climate change scenarios were defined: (A) increased storm intensity - same annual volume (storm duration decreased by 20%), (B) increased storm volume - same annual volume (storm volume increased by 20%), (C) increased annual volume - decreased storm duration (storm volume increased by 10%, storm duration decreased by 20%), (D) decreased annual volume - decreased storm duration (the number of storms per year and storm duration decreased by 20%). In summary, the effects of the four climate change scenarios on water retention storage are listed in Table 7.3.

Summary and Conclusions

Climate change affects the water cycle and the dynamics of the hydrological processes all over the globe, but its effects strongly depend on the local geomorpho-climatic features. Land-use change also affects the water-climate interactions in several ways, making urban areas less resilient to climate change effects. The nature-based approach is being promoted in several international, national, and regional frameworks as an opportunity to restore natural ecosystem services and reduce the vulnerability of urban areas.

A variety of nature-based solutions can be considered, but the level of the benefits they can provide depends on the local geomorpho-climatic features, as well as on the adopted maintenance procedures. Therefore, a detailed local analysis of the climatic trends and future expected scenarios has to be carried out to ensure a robust design of these solutions, while maintenance plans need to be co-developed with all the stakeholders, including civil society, to ensure their long-lasting effects. Regione Lombardia is one of the Italian regions already applying the hydraulic and hydrological

Table 7.3 Water retention storage (m3) for a fictitious drainage basin located in

Brescia (area 100 ha, time of concentration 0.4 h, outflow discharge I m3/s, runoff coefficient 0.4, initial abstraction 0); 'control’ means ‘present climate scenario’ (Grossi & Bacchi, 2008)

Return period

Control

Scenario A

Scenario 8

Scenario C

Scenario D

10 years

22,615

23,407

27,092

26,197

22,381

SO years

30,018

30,811

35,976

34,340

29,784

invariance concept through the recent regional laws. Climate change effects on the town of Brescia, besides on the precipitation regime, might not be as strong as those on other European and Italian regions; nevertheless, nature-based solutions can increase its resilience while mitigating the effects of urbanization.

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