Development of Flood Monitoring Index for daily flood risk evaluation: case studies in Fiji

Abstract

Both fluvial and pluvial floods are a common occurrence in Fiji with fluvial floods causing significant economic consequences for island nations. To investigate flood risk and provide a mitigation tool on daily basis, the Flood Index ($I_{F}$) is developed based on the rationale that the onset and severity of an event is based on current and antecedent day’s precipitation. This mathematical methodology considers the notion that the impact of daily cumulative precipitation on a particular flood event arising from a previous day’s precipitation, decreasing gradually over time due to the interaction of hydrological factors (e.g., evaporation, percolation, seepage, surface run-off, drainage, etc.,). These are accounted for, mathematically, by a time-reduction weighted precipitation influencing the magnitude of $I_{F }$. Considering the duration, severity and intensity of all identified events, the applicability of $I_{F}$ is tested at 9 study sites in Fiji using 30-year precipitation datasets (1990–2019) obtained from Fiji Meteorological Services. Newly developed $I_{F}$ is adopted at flood prone sites, with results demonstrating that flood events were common throughout the country, mostly notable between November to April (or the wet season). Upon examining the variations in daily $I_{F}$, the flood properties were determined, showing that the most severe events generally started in January. Flood events with the highest severity were recorded in Lautoka [$I_{F}^{acc}$ (flood severity) $\approx 149.14$, $I_{F}^{max}$ (peak danger) $\approx 3.39$, $D_{F}$ (duration of flood) $\approx 151\;{\text{days}}$, $t_{onset}$ (onset date) $= 23{\text{rd}}\;{\text{January}}\;2012$], followed by Savusavu $(I_{F}^{acc} \approx 141.65, I_{F}^{max} \approx 1.75, D_{F} \approx 195\; {\text{days}}, t_{onset} = 27{\text{th}}\;{\text{November}}\;1999)$ and Ba $(I_{F}^{acc} \approx 131.57, I_{F}^{max} \approx 3.13, D_{F} \approx 113 \;{\text{days}}, t_{onset} = 9{\text{th}}\;{\text{January}}\;2009)$. The results clearly illustrate the practicality of daily $I_{F}$ in determining the duration, severity, and intensity of flood situation, as well as its potential application to small island nations. The use of daily $I_{F}$ to quantify flood events can therefore enable a cost-effective and innovative solution to study historical floods in both developing and first world countries. Our methodology is particularly useful to governments, private organizations, non-governmental organizations and communities to help develop community-amicable policy and strategic plans to prepare for flood impacts and undertake the necessary risk mitigation measures.