PERFORMANCE EVALUATION OF AN URBAN DRAINAGE SYSTEM

Abstract

Urban drainage systems are essential for mitigating flooding, preventing water pollution, and safeguarding public health in urban areas. They efficiently manage stormwater runoff, channeling excess rainwater away from streets and buildings to prevent inundation and damage. However, the accelerating forces of urbanization and climate change are now pushing these systems toward unsustainability. Urbanization leads to increased impervious surfaces, as cities expand with concrete and asphalt, reducing natural water infiltration. This surge in runoff taxes drainage systems beyond their capacity, resulting in more frequent and severe flooding. Additionally, climate change intensifies rainfall patterns, causing extreme weather events that further strain these systems. Aging infrastructure compounds the issue, as outdated systems struggle to cope with heightened demands. Vulnerable communities often bear the brunt, facing increased health risks due to contaminated floodwaters and experiencing property damage that perpetuates cycles of poverty. Businesses and local economies suffer from disrupted operations and losses inflicted by flood-related damages. The strain on public resources for emergency response and recovery undermines city budgets and diverts funding from essential services, impacting education, healthcare, and infrastructure development. Keeping these in mind, this study tries to carry out a performance evaluation of an urban drainage system. The study proposes a methodological framework for the assessment of the urban drainage system using SWMM. For the drainage network of a part of Gurugram City of Haryana, model has been build up in SWMMv5.1 for carrying out the analysis. Sensitivity analysis is conducted to overcome the problem of lack of in depth data to perform model calibration and validation. Besides, a comparison of three different catchment width estimation approaches, a critical SWMM parameter, is also discussed briefly. The first objective of the study is to evaluate different methodologies of estimating subcatchment width and Time of Concentration for SWMM model setup. This objective has two sub-objectives i.e., a) Determine the various sub-catchment and conduit properties for SWMM model setup, critically focussing on sub-catchment width. b) Estimate design storm and Time of Concentration (ToC).For the drainage network model setup in SWMMv5.1 various sub-catchment properties, conduit properties are required to be estimated. Among these sub-catchment properties, subcatchment width which is a critical input parameter to SWMM is discussed in detail. The width of the sub-catchment can be elaborated as the ratio of the area of the sub-catchment to the longest overland flow path length that water can travel. Variation of width tends to affect the peak of hydrograph and runoff volume. Researchers have used different methods to estimate the width of sub-catchment. In the present study, three different methods were used to derive sub-catchment width and compared for their effect on runoff volume as well as on hydrograph peak. Further, sensitivity analysis is conducted to overcome the problem of lack of in-depth data to perform model calibration and validation. For evaluating the Urban drainage system, to obtain the maximal flow from the system, the storm duration must be at least equal to the ToC. The duration of a storm should not be longer than ToC as longer durations of storms have statistically lower intensities. Therefore, ToC is an important parameter in rainfall-runoff simulation for designing and evaluating an urban drainage system (UDS). There are several lumped and distributed methods available in the literature for estimating the ToC. However, these methods lead to significantly varied values. Therefore, it is imperative to choose an appropriate and best-suited method for estimating the ToC. This study analyses eight lumped approach-based and two distributed approach-based methods for estimating ToC in an urban area of Gurugram. Considering ToC obtained by Natural Resource Conservation Service (NRCS) method as the 'true' value, the Carter method among lumped methods and the SWDM method between the distributed methods result in ToC values in agreement with the NRCS method. Further, to study the impact of underestimation or overestimation of ToC on drainage, the system is evaluated in terms of variation in flood volume, duration, peak discharge, and the time to peak for different ToC values. The simulations were carried-out employing SWMM, and it was found that flood volume increases by 4.25 times and the duration by 7.25 times if the ToC is increased from 0.19 h to 6.14 h. The results infer that ToC estimation methods significantly impact the design and performance of an Urban Drainage Infrastructure. For simulating the response of drainage system, half-hourly rainfall data available from GPM-IMERG for the period of year 2000 to year 2019 was utilized to estimate the design storm of suitable duration. Prior to this, frequency analysis of the rainfall data to estimate the design storm was performed by identifying the best-fit distribution among three widely used distributions i.e., Gumbel, Generalized Extreme Value, and Log-Pearson Type III. Two robust goodness of fit tests namely, Anderson-Darling and Kolmogorov–Smirnov were employed for identification of the best-fit distribution. The second objective of the study is to determine the functional and structural resilience of an urban drainage system. In view of emanating threats, where extreme events occur very frequently, and their extent is difficult to determine, it is now realized that a drainage system should be designed not only to be reliable but also resilient to unexpected circumstances. Furthermore, reliability based approaches could not be used for structural failure evaluation as causes and mechanism of failure are usually undetermined and complex to quantify. Resilience ensures that a given system efficiently provides uninterrupted service to society both during unexpected and normal loading conditions. Resilience can be defined as “The degree to which the system minimizes the level of service failure magnitude and duration over its design life when subject to exceptional conditions." Functional failure is a function of climate change (i.e., change in precipitation) and urbanization, and their effect must be assessed to study the functional failure scenarios. Functional failures are triggered by external threats, whereas internal threats cause structural failures. In an urban drainage system, malfunctioning of single or multiple components may cause structural failures, which further leads to the inefficacy of the failed component to perform the designated chore in full or in part. Structural failures (also denoted as component or system failures) can be broadly categorized as blockage of inlets or sewers, structural damage of a pipe, sensor or pump failure, and chronic stresses such as bed load sediment deposition, asset aging/decay, and sewer collapse. The study prposes a framework, through a case study, for resilience-based evaluation of drainage system under the impact of functional and structural failure modes. Future based scenarios were developed by altering the rainfall and urban growth to analyse the functional and structural resilience of drainage system in various situations. For the studied UDS, a total of 22 vulnerable nodes were identified through the structural resilience approach, and the functional resilience approach revealed that urbanization has more pronounced effects on UDS than climate change. The third objective of the study is to model and evaluate the enhancement in resilience of an urban drainage system with implementation of Low Impact Development techniques.Conventional strategies to alleviate flooding, such as augmenting the capacity of drainage systems, are incompatible with the principle of sustainable development. In this regard, Low Impact Development (LID) techniques have emerged as a promising and sustainable approach to manage storm water. LID pertains to systems and practices that either utilize or imitate natural processes of infiltration, filtering, storing, evaporating, and detaining runoff to decrease the quantity and enhance the quality of stormwater runoff. The present study elaborates the role of LIDs in functional and structural resilience enhancement of the drainage system. Storm water management model (SWMM) is utilized to model the drainage system along with the LIDs. The present study examines two LID practices, namely Green Roofs and Rain Gardens. To showcase the role of LIDs in enhancing drainage system resilience, a case study was conducted in Gurugram, India, a city frequently impacted by flooding. To effectively integrate the green roofs and rain gardens in the runoff management, the sub - catchments with substantial impervious area and high runoff volume were designated as the potential locations for LIDs’ application. To determine the potential sub catchments, a Potential Sub-catchment Index (PSI) was computed for each sub catchment using the percentage imperviousness of sub-catchment and the total runoff volume. To evaluate the effectiveness of LID measure in enhancing the functional resilience, the LID performance Index (LPI) was computed. The main concern in capacity augmentation of existing drainage system with LID techniques is the cost of LID implementation. In the present study, the effectiveness of LIDs was quantified in terms of Benefit-Cost Ratio (BCR), using the present value of costs (PVC) and the present value of benefits (PVB). It helps in determining the cost-effectiveness of LID combination or specific LID practices. The Life Cycle Cost analysis approach was used to estimate the PVC considering all the costs associated with project., whereas PVB infers the effectiveness of the LIDs implementation in flood mitigation using the reduction rates of flood volume, outflow volume, and peak flow. The study determined the optimal percentage of LIDs to incorporate into the urban drainage system through the creation of various scenarios that considered the impacts of urbanization, climate change, and the cost of implementing LIDs. Results indicated that a system incorporating 10% of LIDs with a Benefit-Cost ratio of 2.05 was the most suitable scenario for the case area. The study also identifies the barriers to the implementation of LIDs in developing countries like India, categorizing them into Planning, Implementation, and Maintenance stage barriers. The study provides a comprehensive understanding of naturebased solutions for the effective management of urban drainage infrastructure, offering valuable insights for urban planners, design engineers, and policy makers to protect cities from flood hazards.