Drainage infrastructure under scrutiny: economic and operational evaluation of sustainable and grey systems based on rainfall time series

Urbanization and associated increase of imperviousness alters the hydrological cycle of urbanizing catchments. Low Impact Development (LID) tools have been developed and applied to mitigate these hydrological impacts. Hydrological models are one way to evaluate the performance of LID tools before their implementation. As these tools represent small-scale hydrological processes, hydrological models used in assessment require a high spatial and temporal resolution in their process descriptions. Both flow and rainfall data at high recording frequencies (e.g. 1 min) are usually not available for large urban catchments and detail in spatial data for surface description has to be complemented through on-site observations. Thus, the assessment of LID performance for large urban areas has to overcome these constraints. Previous studies provide suggestions to overcome the lack of flow data for model calibration through parameter regionalization. Recently presented methods for reductions in spatial resolution while maintaining a detailed surface description provide a feasible way to characterize large urban catchments for LID performance assessment. However, rainfall data at high temporal and spatial resolution remains a key element for hydrological model applications in urban areas. The authors evaluated the impact of spatial and temporal rainfall variability on model performance using the Stormwater Management Model (SWMM) and high-resolution parameterizations of three urban catchments in combination with two rain gauges. While the distance between rain station and catchment did not affect model parameters the authors found a reduction in model efficiency with an increasing rain station distance from the catchment.

»Design Storms«, which are usually derived from point intensity-duration-frequency (i-d-f) relationships, neither reflect the areal variations of the rainfall pattern nor the dynamics of the moving storms. Design storms derived from point i-d-f curves can, no longer be accepted as a reasonable rainfall input for simulation of runoff from large urban catchments. One possible way of improving the rainfall input for runoff simulationes is to develop areal i-d-f relationships and use them for deriving design storms. The raingauge network operating in Lund for the past four years provided the set of good quality data necessary for developing such relationships. This paper describes the procedure of developing point and areal i-d-f relationships and shows the differences between them. Statistically developed factors reducing point values to areal values for areas up to 25 sq.km for different durations and return periods of up to three years are given. Presented relationships will give more realistic design storms in comparison with design storms derived from point i-d-f curves. Observed spatial variations of the maximum rainfall intensities in Lund can probably be explained by the city's influence on the rainfall pattern. Orographic effects, earlier observed in Lund, can not be seen from the data used during this study. Since the used data set includes only short term, high intensity rainstorms, it can be concluded that long and low-intensity rainfalls are mainly responsible for orographic effects. This paper is the first part of a major work aiming at developing a stochastic model simulating a time series of the areal rainfall.

Progressing urbanisation is one of the key causes of environmental degradation. This problem also applies to stormwater management. For this reason, drainage infrastructures should be designed in harmony with nature and the decision for selecting a specific stormwater management system solution must not be taken on an ad-hoc or single-perspective basis. The purpose of this paper is to identify the criteria for selecting the best solution for a problem involving the selection of a stormwater management system, and to present a method that will enable all relevant criteria to be taken into account in the decision-making process. The developed decision problem structure takes into account all criteria related to the construction and operation of stormwater infrastructure, and its individual elements were identified based on the analysis and synthesis of information regarding the principles of stormwater management in Poland. The presented approach will allow for the taking into account of all, often mutually exclusive, criteria determining the choice of the stormwater management system option. This, in turn, will make it possible to significantly simplify the decision-making process. The indicated criteria can form the basis for choosing the most favorable stormwater management system for both large urban catchments and individual facilities. Thanks to the considerable flexibility of the developed decision problem structure, its widespread application can contribute to improving the efficiency of stormwater management systems. An example of the developed model’s application in a decision-making process is presented, concerning the selection of a design variant of a single-family residential building’s stormwater management system in Poland. Four design variants were included in the analysis, and the Analytic Hierarchy Process was used as the tool to select the most favorable option. This study shows that nature-based solutions are the most beneficial decision stormwater management options.

Rainfall has been observed to exhibit large spatial and temporal variation over large urban catchments. Hence, knowledge of distribution of the precipitation in time and space will enable to design better urban rainfall-runoff models and to forecast urban floods to improve flood response and mitigation measures. The present paper deals with the spatial and temporal variation of high intensity monsoon rainfall over 437.71 km2 area under the jurisdiction of the Municipal Corporation of Greater Mumbai (MCGM) with a population of 17.7 million (2001 census). The preliminary analysis presented here is based on the data from the rain gauge network of 26 stations in Mumbai for the July 3–5,2006 event, which resulted in a 3-day rainfall total of 409.8 mm. Based on the analysis of data it is concluded that the single rain gauge at Colaba, which has been taken to be representative of the rainfall in the city overestimates rainfall by 11.5%, and the single rain gauge at Santa Cruz, which has been taken to be representative of the rainfall in the suburbs overestimates rainfall by 16.6%. Thus, single point rainfall data cannot be taken as an indication of rainfall over a large urban catchment. Hence all cities in future should install a detailed rain gauge network for better understanding the rainfall variation in the city. This will improve drainage design and flood rescue measures.

Khulna City Corporation (KCC), located in a low-lying area in southwestern Bangladesh, has been suffering from frequent water logging in the rainy season almost every year. This situation is intensified due to rapid urbanization and the impact of climate change, which have consequently created numerous problems in KCC related to stormwater management issues that require immediate consideration. Therefore, the current study aims to explore stormwater management problems in KCC and find possible countermeasures through different best management practices. Stormwater management must be incorporated into urban design and development, particularly in densely populated urban areas like KCC. In Ward No. 30 of KCC, controlling stormwater is a vital concern due to heavy rainfall, a lack of proper drainage infrastructure, unplanned growth, extensive land cover areas, etc. In the current study, the most widely used Personnel Computer Storm Water Management Model (PCSWMM) software was adopted to evaluate the city's current stormwater management practices and develop a long-term stormwater management strategy. A reconnaissance investigation of the existing states of the soil, drainage, and outlet infrastructures was conducted. The study area (KCC Ward No. 30) was then divided into many small catchments in the ArcGIS platform, and areas of each sub-catchment were identified. To determine the percentage of impervious area and the elevation of the study area, the land use and land cover (LULC) map and the digital elevation model (DEM) were generated using the ArcGIS software. All the aforementioned parameters were then entered into the PCSWMM software, and simulations were performed for various rainfall durations using different best management practices. The PCSWMM simulation provided the results of runoff, infiltration, peak runoff, floods, or surcharges at every drainage node or discharge point in the existing and altered drainage system of the study area due to the occurrence of extreme rainfall. The results indicate that various best management practices, increasing the slope, reducing the roughness of the drainage systems, reducing imperviousness, permitting more infiltrations, and promoting green infrastructure, are highly effective in controlling urban flooding or surcharges in the study area caused by the occurrence of extreme rainfall in the future. The findings of this study are expected to be supportive to policymakers, urban planners, and water managers in implementing and promoting sustainable and climate-resilient urban drainage infrastructures to tackle stormwater management problems in the KCC area. Journal of Engineering Science 15(1), 2024, 107-118

Current topographic survey technology provides high-resolution (HR) datasets for urban environments. Incorporating this HR information in models aiming to provide flood risk assessment is desirable because the flood wave propagation is depending on the urban topographic features, i.e. buildings, bridges and street networks. Conceptual, numerical and practical challenges arise from the application of shallow water models to HR urban flood modeling. For instance, numerical challenges are occurrence of wet-dry fronts, geometric discontinuities in the urban environment and discontinuous solutions, i.e. shock waves. These challenges can be overcome by using a Godunov-type scheme. However, the computational cost of this type of schemes is high, such that HR two-dimensional shallow water simulations with practical relevance have to be run on supercomputers. The porous shallow water model is an alternative approach that aims to reduce computational cost by using a coarse resolution and accounting for unresolved processes by means of the porosity terms. Usually, a speedup between two and three orders of magnitude in comparison to HR simulations can be obtained. This study reports preliminary results of a practical test case concerning pluvial flooding in a district of the city of Nice, France, caused by the intense rainfall event on October 3rd, 2015. HR topography data set on a 1 m resolution is available for the district, whereby street features of infra-metric dimensions have been included. A reference solution is calculated by a HR shallow water model on a 1 m by 1 m structured computational grid. The porous shallow water model is run on a 10 m by 10 m grid and the influence of the drag source term is studied. The model results show a large deviation, which is caused by the poor meshing strategy of the porous shallow water (AP) model. The study also summarizes practical challenges that arise during the application of the AP and HR models to a large urban catchment. The main difficulty is to obtain a good mesh. In smaller scale investigations, the mesh is currently constructed by hand such that the cell edges align with buildings. This approach is not feasible for large scale urban catchments with a large number of buildings. Future steps that have to be taken, such as a strategy for automatic mesh generation, are reported on.

Modelling micropollutant fate in sewer systems – A new systematic approach to support conceptual model construction based on in-sewer hydraulic retention time

The Storm Water Management Model was adapted and calibrated to the Ballona Creek Watershed, a large urban catchment in Southern California. A geographic information system (GIS) was used to process the input data and generate the spatial distribution of precipitation. An optimization procedure using the complex method was incorporated to estimate runoff parameters, and ten storms were used for calibration and validation. The calibrated model predicted the observed outputs with reasonable accuracy. A sensitivity analysis showed the impact of the model parameters, and results were most sensitive to imperviousness and impervious depression storage and least sensitive to Manning roughness for surface flow. Optimized imperviousness was greater than imperviousness predicted from land-use information. The results demonstrate that this methodology of integrating GIS and stormwater model with a constrained optimization technique can be applied to large watersheds.

Because of multiple constraints, e.g. existing drainage systems, little available space and higher costs, Best Management Practices (BMP) for stormwater-runoff in existing urban areas is more difficult to apply than for new urban developments. For a large urban catchment (about 22 km2) with a separate drainage system in Berlin, Germany a combination of decentral (non-structural) and semi-central stormwater-management measures proved to be the best solution. It offers a high effectiveness concerning stormwater treatment at relatively low costs. Modern planning tools such as Geographic Information Systems (GIS) were used to investigate the possibilities of implementing decentral measures in larger areas. Correlations between field surveys and data from the ‘Environmental Information System’ of Berlin shows that even in highly urbanised areas a disconnection of 30% of the impervious area can easily be achieved. The resulting reduction of the discharge makes it possible to convert existing retention tanks to soil filter tanks. The purification efficiency of this combined measures is higher than of a central stormwater settling tank which has been simulated with a pollution load model.

Because of multiple constraints, e.g. existing drainage systems, little available space and higher costs, Best Management Practices (BMP) for stormwater-runoff in existing urban areas is more difficult to apply than for new urban developments. For a large urban catchment (about 22 km2) with a separate drainage system in Berlin, Germany a combination of decentral (non-structural) and semi-central stormwater-management measures proved to be the best solution. It offers a high effectiveness concerning stormwater treatment at relatively low costs. Modern planning tools such as Geographic Information Systems (GIS) were used to investigate the possibilities of implementing decentral measures in larger areas. Correlations between field surveys and data from the ‘Environmental Information System’ of Berlin shows that even in highly urbanised areas a disconnection of 30% of the impervious area can easily be achieved. The resulting reduction of the discharge makes it possible to convert existing retention tanks to soil filter tanks. The purification efficiency of this combined measures is higher than of a central stormwater settling tank which has been simulated with a pollution load model.

Low Impact Development (LID) tools and green infrastructure approaches have been developed and applied to mitigate the urbanization impacts on increasing runoff and pollutant washoff. The present work is the first part of a larger effort to simulate LID scenarios for a large scale urban catchment through up-scaling of high-resolution study catchments using the Stormwater Management Model (SWMM). In this study we present the setup, calibration, validation, and the results of a parameter sensitivity analysis of a high-resolution SWMM model for a highly urbanized small catchment located in Southern Finland. The homogenous subcatchments and associated narrow parameter boundaries, which are allowed by the high spatial resolution, result in insensitivity of SWMM to the fraction of impervious cover. The model optimization, using only the two identified key parameters “depression storage” and “Manning's roughness n for conduit flow”, yielded good performance statistics for both calibration and validation of the model.

An enhanced inundation method for urban flood hazard mapping at the large catchment scale

It is well known that the pumps are the largest consumers of industrial motor energy and account for more than 25% of electricity consumption. The life cycle cost of a pump is the total lifetime cost associated with procurement, installation, operation, maintenance and its disposal. For majority of heavy usage pumps, the lifetime energy and/or maintenance cost will dominate the life cycle costs. Hence a greater understanding of all the cost components making up the total life cycle costs should provide an opportunity to achieve a substantial savings in energy and maintenance costs. This will further enable optimizing pumping system efficiency and improving pump and system reliability. Therefore in this context, the life cycle cost analysis of heavy usage pumps is quite important. This paper focuses on an application of a methodology of determining the life cycle cost of a typical heavy usage multistage centrifugal pump. In this case, all the cost components associated with the pump-set have been determined and classified under different categories. The data with regard to initial investment costs, operation costs, maintenance and repair costs and disposal costs for the pump considered for this case study was collected from the concerned pump manufacturer along with the unit cost of each component, quantity used and their weights. By applying the principles of reliability and maintainability engineering and using the data obtained from the design, manufacturing and maintenance departments, the component-wise values of MTBF (Mean Time Between Failures) and MTTR (Mean Time To Repair) were estimated. The results of the life cycle cost analysis of the specimen pump were compared with the life cycle costs of similar pumps reported in the literature. From this comparison of results, it can be concluded that, the initial cost of the pump is the only a fraction of the total life cycle cost. The operating cost of the pump dominates the life cycle costs especially in case of heavy usage pumps. The maintenance cost varies approximately from 0.6 to 2.5 times the initial cost of the pump. The life cycle cost of the pump varies approximately from 12 to 33 times the initial cost of the pump. The operation and maintenance cost is almost 92 to 97 per cent of the life cycle cost. The detailed analysis carried out in this paper is expected to provide guidelines to the pump manufactures/practicing engineers in selecting a heavy usage multistage centrifugal pump based on the total lifetime cost rather than only on initial price.

Design of urban drainage infrastructure depends on the rainfall pattern and runoff volumes. Under the current impacts of urbanization and industrialization in the Kingdome of Saudi Arabia (KSA), there is a need to review the guidelines for design of drainage infrastructure. In this study, a methodology is developed to investigate the impact of variations in rainfall on the capacity of drainage infrastructure for the city of Madinah (Taibah), KSA. Rainfall data collected from Madinah Municipality was analyzed to determine the normal annual rainfall (average of the annual rainfall for 30 years). Data consistency was checked using double mass curve technique, and the design rainfall (DR) was estimated by Gumbel Extreme Value Distribution. The Mann-Kendall test was run at 5% significance level on rainfall time series over the period 1985–2015; however, no significant trend was observed. Urban drainage schemes are designed for peak flows based on DR. The true values of DR are difficult to estimate due to uncertainties associated with variations in estimation procedures and data limitations. An attempt is made to elaborate the impact of data errors (both systematic and random) on DR to facilitate engineers in selecting safety factor for design of urban drainage infrastructure. The stormwater trunk-sewer was designed to safely convey the runoff for an area around the Masjid-e-Al-Nabawi. Rainfall–runoff modeling was performed by Rational Method to find the peak flow and Nash GIUH. It was found that there is about 20% change in diameter of trunk sewer for change in return period from 2 to 10 years.

The integrated green-gray-blue (IGGB) system is considered to be a new way of stormwater management, and a comprehensive evaluation of the green-gray-blue infrastructure layout mode under different return periods is the key to the implementation decision-making of stormwater management. In this study, a blue-green synergism evaluation model is established to optimize the layout of blue-green infrastructure. An evaluation framework combining the evaluation indicator system and the hydrology model is established. Stormwater storage, peak flow reduction, and life cycle cost are selected as evaluation indicators. On this basis, seven optimal scenarios, including green, blue, gray, green-blue, green-gray, blue-gray, and green-gray-blue, are established. The Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) method is used to analyze these seven scenarios under different return periods. The results indicate that (1) when the drainage infrastructures are arranged in combination, the peak flow reduction is significantly improved compared to that of a single drainage. (2) TOPSIS results show that green-gray and blue-gray perform better when the cost weight is 0-0.35, and green-gray-blue performs best when the cost weight is 0.35-1. (3) The integrated green-gray-blue system has obvious synergistic effects. This study can provide support for planning department workers for the urban stormwater management strategy.