Spatiotemporal variations of Manning’s roughness coefficient in the estuary of Langsa River based on field measurements and hydraulic modeling

Study region

Langsa City is one of Indonesia's coastal areas prone to flooding due to tidal flows and seasonal rainfall.

Study focus

This study aims to determine the Manning roughness coefficient (n) dynamically using the Manning distribution permutation method for tidal flow and upstream discharge. A HEC-RAS 1D hydraulic model is used to predict flood levels by combining water level data in the dry season and rainy season in tidal rivers. The model is calibrated statistically at four locations based on the width-to-depth (W/D) ratio to ensure accuracy. River segments are classified as wide-SHALLOW if W/D > 10 and narrow-DEEP if W/D < 10.

New hydrological insights for the region

The study reveals that the Manning coefficient needs to be adjusted according to the ratio of river segments, especially those with a width-to-depth ratio of less than 10. Dynamic changes in the Manning coefficient are observed in tidal rivers, especially during upstream flood periods; the n value increases to 0.090 during low flow conditions, transitioning to high tide, and decreases to 0.055 during the flood period in the upper reaches. Additionally, a 29.35 % increase in upstream discharge during the rainy season highlights the sensitivity of local hydrographs to environmental changes. These findings support the development of early warning systems for tidal rivers in coastal areas, thereby strengthening flood management and mitigation strategies.