Investigating hydrological parameters for Nature Based Solutions characterization

Alternative stormwater management approaches for urban watersheds, also called Sustainable urban Drainage Systems (SuDS) or Low Impact Developments (LIDs), are increasingly being adopted with the aims of providing flow management, flood control, water quality improvements and opportunities to harvest stormwater. SuDS structures are typically small, decentralized systems form managing stormwater runoff near the source. These systems interact with the urban hydrological cycle, modifying the evapotranspiration, runoff, and groundwater recharge fluxes. It is challenging to quantify these hydrological changes because of the cost and complexity of modelling multiple SuDS systems in larger scale urban watersheds. Nevertheless, hydrological design of SuDS is commonly achieved by setting rainfall volumetric percentiles from daily rainfall series, but due to the small scale of urban watersheds and quantities involved, design rainfall data at sub-hourly time step are necessary. For these reasons, operation of new modelling and designing tools need to be explored. The research work discussed here is aimed to analyze the ability of a synthetic rainfall generation process to improve SuDS design. Particularly, the temporal disaggregation of daily rainfall records using stochastic methodologies is applied to improve SuDS design parameters and to run a long-term hydrological model to evaluate watershed scale effects of several SuDS scenarios. The first part of this research is aimed to analyze: 1) the ability of the synthetic rainfall generation process to reproduce the main characteristics of the observed rainfall; 2) the hydrologic parameters often used for SuDS design based on the generally available daily rainfall data. Other specifics objectives concern with the evaluation of Minimum Inter-event Time (MIT) and storm volume threshold on rainfall volumetric percentiles, commonly used in SuDS design. The reliability of the stochastic spatial-temporal model RainSim V.3 to reproduce observed key characteristics of rainfall pattern and volumetric percentiles, is also investigated. Observed and simulated continuous rainfall series with v sub-hourly time-step are used to calculate four key characteristics of rainfall and two types of rainfall volumetric percentiles. To separate independent rainstorm events, MIT values of 3, 6, 12, 24, 48 and 72 h and storm volume thresholds of 0.2, 0.5, 1 and 2 mm are considered. Results show that the proposed methodology improves the estimation of the key characteristics of the rainfall events as well as the hydrologic parameters for SuDS design, compared with values directly deduced from the observed rainfall series with daily time-step. Moreover, MITs rainfall volumetric percentiles of total number of rainfall events are very sensitive to MIT and threshold values, while percentiles of total volume of accumulated rainfall series are sensitive only to MIT values. In the second part of the research work discussed here, an approach based on a stochastic temporal disaggregation of daily rainfall data is coupled with a hydrological model implemented at urban watershed scale. The long-term efficiency of several LID scenarios on reducing runoff is tested both with independent flow events approach and with the annual maxima peak flows associated with some return periods over the considered time-period. The evaluation is done using twenty LID scenario characterized by four percentages of impervious area retrofitted with LIDs (25, 50, 75 and 100%), and five LID combinations of Green Roofs, Rain gardens and Cisterns on peak flow reduction. Stochastic temporal disaggregation and generation of 500 years of rainfall data with sub- hourly time-step has been achieved using the rainfall generator RainSim V.3. and coupled with the LID module of the Soil and Water Assessment Tool (SWAT) hydrological model. Results show that combinations of different LID types generally offer higher peak flow reduction when more type of LIDs area is added. Hydrological performances of LID combinations are very sensitive to the intensity of rainfall events as well as percentages of area treated. Watershed scale performance of single LID types may not be proportional to what observed for the single infrastructure, due to synergy processes that occurs between multiple structures.