Anissa Zuhrita, Hyundra Zakiya Putri Wahyu, Nelly Handayani, Helny Yofin Mega Milla, Nabila Zalianti Safitri, Sigit Heru Murti, Sudaryatno

Flood Potential Assessment of the Way Urang Sub-Watershed Based on Peak Discharge Using the Rational Method

Introduction

Flood potential assessment of the way urang sub-watershed based on peak discharge using the rational method. Assess flood potential in Way Urang sub-watershed, Pesawaran, Lampung, using peak discharge, remote sensing, and GIS. Findings show high flood risk due to peak discharge far exceeding channel capacity in downstream areas.

Abstract

Peak discharge is a key indicator for assessing flood potential in a river basin. This study estimates peak discharge in the Way Urang sub-watershed, Pesawaran, Lampung, by integrating remote sensing and Geographic Information Systems (GIS) to derive physical parameters that control surface runoff. The Rational Method was applied, combining the runoff coefficient (C), rainfall intensity (I), and drainage area (A). The runoff coefficient was calculated using the Cook Method, which takes into account soil type, slope gradient, vegetation density, and drainage density. Rainfall intensity was derived from daily records using the Mononobe equation, with time of concentration estimated from the Kirpich formula. Data sources include Sentinel-2 imagery, DEMNAS, rainfall records from 2014 to 2023, and field measurements. The results show a peak discharge of 217.19 m³/s for a basin area of 20.20 km², with a coefficient of variation (C) of 69.20% and an intensity (I) of 55.89 mm/h. High runoff reflects the combined effects of low-infiltration soils, steep slopes, and high annual rainfall. Morphometric measurements yielded a total channel cross-sectional area of 27.91 m² and an estimated bankfull discharge of ~9.53 m³/s, indicating that the channel capacity is far below the peak discharge. This imbalance suggests a high flood potential in downstream areas, particularly in Bunut Village. The findings underscore the importance of integrating spatial data, field surveys, and remote sensing to analyze watershed physical characteristics and to support more effective, spatially informed flood planning and mitigation.  

Review

This study presents a timely and relevant assessment of flood potential within the Way Urang sub-watershed, employing a well-structured methodology that integrates remote sensing and Geographic Information Systems (GIS). The approach of using the Rational Method, enhanced by the Cook Method for runoff coefficient, Mononobe equation for rainfall intensity, and Kirpich formula for time of concentration, demonstrates a comprehensive use of established hydrological techniques. The leveraging of modern data sources such as Sentinel-2 imagery, DEMNAS, and extensive rainfall records from 2014-2023, coupled with field measurements, strengthens the foundation of the analysis by grounding the model parameters in empirical data. The findings reveal a significant flood risk, with a calculated peak discharge of 217.19 m³/s for the 20.20 km² basin, driven by factors like low-infiltration soils, steep slopes, and high annual rainfall. A crucial insight is the stark discrepancy between the estimated peak discharge and the channel's bankfull capacity (~9.53 m³/s), indicating a severe inadequacy of the existing channel to convey flood flows. This imbalance clearly points to a high flood potential, particularly highlighting Bunut Village as a vulnerable downstream area. The study effectively underscores the utility of integrating spatial data and remote sensing for understanding watershed physical characteristics and informing flood planning and mitigation efforts. While the study provides a robust initial assessment, a key area for further clarity and perhaps expansion concerns the return period associated with the estimated peak discharge. The Rational Method is typically applied for specific design storm frequencies, and explicitly stating the return period for the calculated 217.19 m³/s would provide crucial context for flood risk management and allow for more direct comparison with engineering design standards. Additionally, discussing the inherent limitations and assumptions of the Rational Method for a basin of this size, particularly when parameters are derived rather than empirically measured for an ungauged watershed, would add further scientific rigor. Future work could also explore the sensitivity of the peak discharge to variations in input parameters, or integrate the findings into a broader flood risk assessment framework that considers socio-economic vulnerability alongside physical flood potential.

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