Adaptation of storm sewer systems to climate change
Authors: Κούρτης Ιωάννης 
Published: March 2022

According to the United Nations (2017), more than the half of the world’s population lives in urban and semi-urban areas. As a result, urban areas are becoming larger, denser and more impervious, and so, floods are becoming more frequent and more devastating than in the past. Projections of climate change, urbanization and growing population are expected to augment the pressure on the environment and on human infrastructure, and moreover, challenge water resources sustainability. As a result, the water cycle is projected to be affected by climate change by influencing precipitation patterns, which is anticipated to lead to an increase/decrease in design rainfall intensities. In water resources engineering and management, Intensity-Duration-Frequency (IDF) curves constitute one of the most commonly adopted tools for planning, design and operation of a wide range of water resources related projects (e.g., dams, storm sewers etc.). As a result, update of Intensity-Duration-Frequency curves considering climate change is essential for the adaptation of water-related structures to climate change. The main aims of the present PhD thesis are: (i) to review the main methods employed for updating IDF curves considering climate change; (ii) to propose general guidelines for updating IDF curves considering future climate projections; (iii) to review adaptation of urban drainage networks to climate change and other drivers (i.e., urbanization, Land Use Land Cover - LULC) along with studies referred to impacts of climate change on rainfall extremes for small durations and on urban drainage systems; (iv) to introduce a novel framework for systematic analysis of urban drainage adaptation strategies considering climate change; (v) to assess the main impacts of climate change on short duration rainfall extremes and urban drainage networks; (vi) to introduce an index for assessing overall reliability of an urban drainage system in terms of hydraulic, temporal and volumetric reliability; (vii) to introduce an index for assessing total adaptive capacity of an urban drainage network; (viii) to incorporate in the design procedure benefits offered by ecosystem services in economic terms; (ix) to assess total uncertainty (i.e., climate change projections, hydrologic-hydraulic modeling; rainfall temporal distribution); and (x) to demonstrate the applicability of the proposed framework. Hydraulic reliability refers to the percent of time during which the pipes of the drainage network are able to convey the produced runoff operating based on a predefined level. Temporal reliability refers to he total amount of time the system remains in the accepted level of performance divided by the total range of time considered. Finally, volumetric reliability refers to the ratio of water volume conveyed safely through the drainage system to the total runoff volume generated from precipitationA wide literature search was conducted in various scientific databases in order to review the main challenges regarding the update of Intensity-Duration-Frequency considering climate change. As a result, specific guidelines for addressing the issue of updating IDF curves under climate change are proposed. Moreover, based on the proposed methodology the IDF curves for the Thissio station, Athens are updated. More than one hundred articles published between 2001 and 2021 have been reviewed, summarized and discussed. The aims of this review was to: (i) identify the state-of-the-art scientific approaches regarding Intensity-Duration-Frequency curves update under climate change projections; (ii) assess whether or not these approaches incorporate uncertainty (i.e., uncertainty related to climate models, statistical downscaling techniques, temporal resolution of data, theoretical distribution selection etc.); (iii) identify the main approaches used for statistical downscaling and/or bias correction and temporal disaggregation of climate model-based rainfall projections; and (iv) identify the theoretical distributions used in developing IDF curves. The results revealed that different researchers and countries have adopted a simplified methodology (unique scaling factor) for assessing the impacts of climate change on future rainfall extremes. Update of IDF curves is mostly based on one future period or on multiple future time windows of 30-year future periods. Researches have also proposed and used a non-stationary approach where parameters of the theoretical distribution are considered time variant. Spatiotemporal disaggregation of future climate projections, especially for studies focusing at the urban basin level, is essential. A wide variety of methods for spatiotemporal disaggregation have been proposed in the literature but there is no general agreement which methods perform best. Overall, the application of spatiotemporal disaggregation must be practiced along with uncertainty analysis as it may give rise to additional uncertainties. The proposed general guidelines regarding the updating of IDF curves under varying climate conditions can be summarized as follows: (i) acquisition of high-resolution observations, at least hourly for at least 20 years; (ii) use of a multi model ensemble incorporating several GCMs, RCMs and climate scenarios; (iii) use more than one statistical downscaling/bias correction techniques; (iv) use more than one temporal disaggregation approaches; (v) uncertainty quantification and estimations and reporting of the confidence intervals (e.g., 95, 90 %) for the current and future IDF curves; (vi) assessment of spatiotemporal downscaling and comparison with observations.The second review focuses on challenges regarding the adaptation of urban drainage networks to climate change by comparing 32 case studies from 29 articles, published between 2003 and 2020. The aim was to: (i) identify the state-of-the-art scientific approaches for adaptation of urban drainage networks to climate change; (ii) assess whether or not these approaches incorporated monetization of the adaptation practices and the associated costs/benefits; and (iii) define a novel approach (Blueprint) for the future development and assessment of urban drainage network adaptation to climate change and other drivers. Our research confirmed that future quantity and quality of urban runoff is not widely addressed in the scientific literature. The Storm Water Management Model is the most widely used software for modeling adaptation options. Solutions such as plans of maintenance and rehabilitation, raising of public awareness, flood forecasting and warning, population relocationand insurance are not widely reflected in the literature. Uncertainties of climate projections and bias correction methods are still significant, and thus, uncertainties of socio-economic scenarios, hydrologic and hydrodynamic models, and adaptation options are still not fully addressed. Finally, environmental cost and benefits associated with the ecosystem services provided by the adaptation options are also not fully addressed.A novel methodological framework for urban drainage adaptation to climate change is introduced and tested in a small urban basin of the combined drainage network of Athens, Greece. The proposed framework is composed of three parts: (i) observed data acquisition, ensemble projections of climate change, bias correction, temporal disaggregation and development of observed and future IDF curves; (ii) hydrologic-hydraulic modeling, sensitivity analysis, calibration and flood mitigation scenarios development; and (iii) selection of best alternative for climate change adaptation of urban drainage, and forward uncertainty propagation analysis. The framework comprises hydrologic, hydraulic and economic indices aiming at quantifying the effect of different alternatives regarding flood hazard mitigation. The evaluated alternatives included both conventional drainage solutions (i.e., sewer enlargement) and low impact development measures (i.e., permeable surfaces). The Storm Water Management Model version 5 (SWMM5) was calibrated and employed. Prior to calibration, sensitivity analysis of SWMM hydrologic-hydraulic and permeable surfaces input parameters was conducted using a global sensitivity analysis method. Two novel indices, the Index for Reliability of Urban Drainage Adaptation (IRUDA) and the Climate Variability Adaptation Index (CVAI), were also introduced. IRUDA assesses the total reliability of an urban drainage network in hydraulic, temporal and volumetric terms. Sizing of flood mitigation measures was undertaken employing construction cost along with IRUDA. CVAI used for alternative selection among different flood mitigation scenarios. CVAI assesses the total adaptive capacity of the drainage network based on hydrological, hydraulic and economic aspects of the proposed adaptation plan. A forward uncertainty propagation scheme was employed on the selected alternative in order to examine input data and parametric uncertainty with respect to the peak discharge at the outlet of the basin. Finally, monetization of ecosystem services offered by green measures was undertaken using CIRIA Benefits Evaluation of SuDS Tool. The results revealed that both low impact development measures and conventional drainage solutions were highly effective especially for storm events with low return periods. However, sewer enlargement along with the permeable surfaces construction was found to be the best alternative based on CVAI. Moreover, the results illustrated that under the non-stationarity assumption, future rainfall intensities and urban flooding risk will increase, and therefore, further flood mitigation measures will be required. Furthermore, the majority of the examined adaptation strategies were found to improve the capacity and reliability of the drainage network. Moreover, the results revealed that permeable pavements perform better for storm events with return period of up to five years. Finally, the results of forward uncertainty propagation analysis revealed that input uncertainty, especially for rainfall depth and rainfall time to peak, has significant impacts on peak discharge at the study system outlet.The impact of climate change on IDF curves and on urban drainage is reported. Current and future IDF curves are developed, for Thissio station in Athens, Greece, using Gumbel and Generalized Extreme Value (GEV) theoretical distributions and the Bernard and Sherman empirical models. The results revealed that by the end of the 21st century, the 1 h rainfall with a return period of 10 years is projected to increase by 26 % under the mean climate scenario. In general, the results revealed that the percent change in future rainfall intensity varies greatly according to the return period, the rainfall duration, the climate model and the scenario examined.The impacts of climate change on the urban drainage network are assessed based on drainage capacity of the drainage network, percent change in total surface runoff, total flood volume, number of flooded nodes and the Expected Annual Damage. Overall, it can be concluded that the design criteria need to be reviewed and also green measures, such as permeable surfaces, must be introduced in the design procedure.