THESIS ABSTRACT

Abstract

Rapid urbanization in Pokhara has converted large open and agricultural land into impervious surfaces, reducing natural infiltration capacity and increasing the frequency and severity of pluvial flooding during the monsoon. The existing drainage network, designed decades ago on outdated rainfall estimates, is increasingly unable to accommodate current runoff volumes. This study evaluates the potential of Low Impact Development (LID) practices to restore natural hydrological cycle and improve pluvial flood resilience in a 1.24 km² area using the EPA Storm Water Management Model (SWMM) 5.2. A continuous-simulation hydrological model was developed for three representative sub-catchments (SC1, SC2, SC3) within Pokhara Valley, using continuous simulation, Dynamic Wave flow routing, and Horton's infiltration. Rainfall characteristics were analyzed through annual and monthly trends, Gumbel extreme value analysis, and Intensity-Duration-Frequency (IDF) curves, confirming a highly variable, monsoon-dominated climate. Six LID practices were individually modelled and compared. Under continuous simulation, bio-retention cells were the most effective, reducing runoff by 56–73% at 10–20% coverage. Rain gardens track the same pattern 53-72% at 10-20% coverage. Rooftop disconnection outperformed rain barrels (13.78–26.39% versus 0.91–1.58%) due to continuous soil infiltration rather than finite storage. Permeable Pavements aided 7.06%, 10.64%, 15.76% reduction at 10,20 and 50% coverage of the road area. Under discrete 2, 10, and 25 year design storms, rain gardens and bio-retention cells at 20% coverage cut total node flood volume by 49–53%, though no scenario eliminated flooding at the coverage levels, at any of the three chronically flooded junctions, indicating the practiced LID reduced flood severity rather than eliminating it. A one-at-a- time sensitivity analysis on bio-retention soil hydraulic conductivity was conducted; runoff reduction responded consistently but moderately (sensitivity index 0.3–0.5) across all return periods and coverage levels, indicating the study's core conclusions are reasonably robust to parameter uncertainty. 10 The study concludes that infiltration-based LID practices, particularly bio-retention cells and rain gardens, can meaningfully reduce urban runoff and flood volume even at modest coverage levels, and recommends their systematic integration into urban planning and drainage design in rapidly urbanizing cities likes Pokhara,.

Keywords

Low Impact Development, SWMM, Urban flooding, Bio-retention, Node flooding, Pokhara.