Impacts of land developments and land use changes on urban stormwater management

With the rapid urbanization happening around the world, the nature of the natural hydrological cycle has been changed and it causes many adverse effects like urban flooding, erosion and degradation of water quality in urban areas. Due to the increasing population, urbanization will continue rapidly and this increases impervious lands which generate more runoff. Anthropogenic climate change has influenced the strength of storm events and reduced the recurrent intervals. Current urban stormwater management systems are becoming increasingly lacking with rapidly increasing demands and climatic effects. Groundwater has been found as a key factor in creating inadequacy in urban drainage to carry stormwater runoff in catchments having a shallow groundwater table. Water sensitive urban design (WSUD) and modifications to urban stormwater management systems (USWMSs) according to the best management practices (BMP) should be implemented after systematic analysis to overcome the situation.This study has focused on assessing urban land development activities and changing patterns of land use in urban areas as the main anthropogenic stress on urban hydrology. In addition, the adaptation to natural phenomenon such as climate change has been studied. A numerical hydrological model was used to analyse the behaviour of catchments and their characteristics. Urban flood identification and prevention was one of the major concerns of this study. Several urban stormwater drainage systems have been assessed under three case studies.The stormwater drainage system of Canning Vale Central catchment, which is one of the urban catchments in Western Australia, has been assessed by using numerical modelling in case study number one. The model was developed by using existing mapped data and data collected from an ongoing telemetric observation system and several field visits. Surface runoff has been routed by using different modelling techniques such as hydrological surface runoff and two-dimensional (2D) surface runoff modelling. Groundwater has been treated as a critical issue during the modelling. The effects of land use changes and their sensitivity to the USWMS have been assessed. Necessary recommendations to improve the USWMS and mitigate localised flood issues have been given. Flood vulnerability maps have been developed to identify the critical areas where there is the potential to be flooded under different Average Recurrent Interval (ARI) events. These flood vulnerability maps will be used by the local authorities to develop recommendations and guidelines for future developments of infrastructure during land development and subdivision works.The urban ungauged catchment of Victoria Park in Western Australia has been assessed by using a 2D surface runoff routing model. The catchment has built flood storage areas (stormwater basins) and the inadequacy of them in protecting against recent storm events has caused local concern. The area has been developed rapidly in recent decades and land use has been changed to more impervious surfaces than was expected at the time the basins were designed. These changes to the land use—together with anthropogenic climate change—has caused runoff from rapid storms to exceed the basin top water level. The catchment‘s existing stormwater basins‘ capacities were assessed against different ARI events during case study number two. Flood vulnerability maps and water level contours have been developed to identify the possible inundations and flood depths of basins and surrounding areas.The overall study is based on hydrological modelling of different USWMSs and urban hydrology. Land use change was considered as the main anthropogenic stress upon urban hydrological catchments. Factors such as encountering groundwater in stormwater drainage have been analysed to support the study. Recommendations based on WSUD and BMPs have been given to mitigate the adverse effects of urban land use changes to urban stormwater management

Comparing Hydrology and Hydraulics Surface Routing Approaches in Modeling an Urban Catchment

Urban hydrological environment is difficult to assess with its complex verity of components by using simple hydrological models which are not fully capable enough to represents the exact urban catchment’s features. The urban catchments comprising with various infrastructures having low permeability and low roughness coefficients have increased their runoff volume, while decreasing the time of concentration. This has been caused urban flooding threatening the urban environments in many aspects. This study uses two of numerical modeling approaches to represent the urban catchment, and assess the flood risk of an urban catchment; hydrological surface routing approach and hydraulic 2D surface routing approach. Both approaches were combined with 1D hydraulic stormwater drainage network. XPSWMM was used as the modeling tool. Comparison of results of two approaches shows that the both approaches are suitable to represent urban catchment’s hydrological behavior, but the results of hydrological surface routing are more close to observation data. When representing the flood inundation areas, it is better to use the hydraulic approach since it calculates the flood depth by using surface runoff and excess water from pipe network at the same time

Analysis of Meandering River Morphodynamics Using Satellite Remote Sensing Data—An Application in the Lower Deduru Oya (River), Sri Lanka

River meandering and anabranching have become major problems in many large rivers that carry significant amounts of sediment worldwide. The morphodynamics of these rivers are complex due to the temporal variation of flows. However, the availability of remote sensing data and geographic information systems (GISs) provides the opportunity to analyze the morphological changes in river systems both quantitatively and qualitatively. The present study investigated the temporal changes in the river morphology of the Deduru Oya (river) in Sri Lanka, which is a meandering river. The study covered a period of 32 years (1989 to 2021), using Landsat satellite data and the QGIS platform. Cloud-free Landsat 5 and Landsat 8 satellite images were extracted and processed to extract the river mask. The centerline of the river was generated using the extracted river mask, with the support of semi-automated digitizing software (WebPlotDigitizer). Freely available QGIS was used to investigate the temporal variation of river migration. The results of the study demonstrated that, over the past three decades, both the bend curvatures and the river migration rates of the meandering bends have generally increased with time. In addition, it was found that a higher number of meandering bends could be observed in the lower (most downstream) and the middle parts of the selected river segment. The current analysis indicates that the Deduru Oya has undergone considerable changes in its curvature and migration rates.publishedVersio

Sensitivity Analysis of Catchment Characteristics in Urban Stormwater Management Modeling

Numerical stormwater management modeling helps to understand the urban hydrological catchment behavior. Urban hydrological modeling is a complex process, when considering all the urban hydrological catchment characteristics. Selecting and/or prioritizing catchment characteristics according to their sensitivity on modeling results helps to generate simplified and accurate catchment models. Sensitivities of selected catchment characteristics depend on land use change have been analyzed by hydrological modeling of an urban catchment. Changes to surface roughness coefficients increased the modeled post development peak flow up to 10.3% and 15.5% in 1 year and 100 year ARI events. Changing of infiltration parameters caused to 8.8% and 0.3% changes in peak flows under two scenarios. These results show that the sensitivity of parameters associated with the portion of impervious land use is more significant than the characteristics associated with the pervious land use portion. Results have suggested that the overall land use change is more sensitive to the modeling results of minor rainfall events than the results of major rainfall events. Results will help to decide most sensitive catchment characteristics and applicable range of their values, when modeling similar urban catchments

Effectiveness of Stormwater Best Management Practices in Urban Land Developments

Australian local governments urge to practice the water sensitive urban designs and best management practices in urban land development processes to protect the pre-development hydrology in the post- development stage. This usually comes with the best management practices guided by the local governments and environmental protection authorities. Under the best management practices, one criterion is retaining runoff from post development 1 year 1 hour Average Recurrent Interval (ARI) event within the development site for runoff quality control by treating the initial runoff as close as to the source. In this study, soakage wells, bio retention swales and rainwater tanks were modeled to cater 1 year ARI runoff within the development. Their impact on reduction of required 5 year and 100 year ARI event runoff storage was studied. One of the Western Australia’s residential development consist of about 118 ha was selected as the case study. The XPSWMM hydrodynamic computer model has been used to do the numerical modeling. It has shown 70% of 5 year runoff volume and 34% 100 of year required storage capacities has been reduced by applying above best management practices. Cost effectiveness of the methods used to cater 1 year ARI event was discussed. The results will be used to develop the urban water management plan for the development site

Numerical Modeling of Flood Vulnerability in Urban Catchments for Flood Forecasting

This study uses two of numerical modeling approaches to represent the urban catchment, and assess the flood risk of an urban catchment; hydrological surface routing approach and hydraulic 2D surface routing approach. Both approaches were combined with 1D hydraulic stormwater drainage network. XPSWMM was used as the modeling tool. This comparison leads to select most suitable approach to assess the possible flood inundations in the study area. Comparison of results of two approaches shows that the both approaches are suitable to represent urban catchment’s hydrological behavior, but the results of hydrological surface routing are more close to observation data. Model was calibrated to one sub catchment and used to generate flood vulnerability maps for whole catchment. These maps will be used as water sensitive urban design guide lines in the future developments of the area, while structural and non structural measures are to be implemented