## 3 System Development: Methodologies and Analysis
### Flood Frequency Analysis
#### 3.2.1 Rainfall Analysis  
Five (5) and ten (10) minute historical rainfall data held within the Samoa Tideda hydrological and Climate Database for the Environment (CliDE) archives were used to assess and/or validate previously established rainfall return period thresholds (Table 1) [22]. Both daily and sub-daily records for Afiamalu, Nafanua, Alaoa, Le Pue and Tiavi sites were particularly crucial as they represented the spatial variability of available data within the catchment (Figure 1), with maximum daily rainfall extremes of over 400 mm recorded at Afiamalu, Nafanua, and Alaoa. The most significant large rainfall event since 2010 was associated with tropical cyclone Evans in 2012. Overall rainfall records suggest that the event was between a 20- to >100-year return period event (5% to <1% annual exceedance probability), and was compounded by antecedent rainfall of 174 mm at Afiamalu and 120.5 mm at Alaoa during the preceding 24-h period. This caused multiple landslides within the catchment [25]. On average it was estimated that up to 206 mm of rainfall fell within \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Parameter**} & **Monitoring** & \multirow{2}{*}{**Station**} & \multirow{2}{*}{**2009**} & \multirow{2}{*}{**2010**} & \multirow{2}{*}{**2011**} & \multirow{2}{*}{**2012**} & \multirow{2}{*}{**2013**} & \multirow{2}{*}{**2014**} & \multirow{2}{*}{**2015**} & \multirow{2}{*}{**2016**} & \multirow{2}{*}{**2017**} & \multirow{2}{*}{**2018**} & \multirow{2}{*}{**2019**} & \multirow{2}{*}{**2020**} \\  & & & & & & & & & & & & \\ \hline \multirow{8}{*}{Rainfall} & Lake & & ✓ & ✓ & ✓ & ✓ & ✓ & ✓ & ✓ & ✓ & ✓ & ✓ \\  & Lantoo & & & & & & & & & & & \\  & Mt. Le Pue & & & & & ✓ & ✓ & ✓ & ✓ & ✓ & ✓ & \\  & Tiavi & & & & & ✓ & ✓ & ✓ & ✓ & ✓ & ✓ & \\  & Alaoa West & & & & ✓ & ✓ & ✓ & ✓ & ✓ & ✓ & ✓ \\  & Afiamalu & ✓ & ✓ & ✓ & ✓ & ✓ & ✓ & ✓ & ✓ & ✓ & ✓ \\  & AWS & & & & & & & & & & & \\  & Nafanua & & & & & & & & & & \\  & AWS & & & & & & & & & & & \\ \hline River & Lelata & & & & & & & & & & ✓ & ✓ \\ Level & Bridge & & & & & & & & & & & ✓ & ✓ \\  & Alaoa West & & & & & & & & & & ✓ & ✓ \\  & Alaoa East & ✓ & ✓ & ✓ & ✓ & ✓ & ✓ & ✓ & ✓ & ✓ & ✓ & ✓ \\ \hline \hline \end{tabular}  
* Tropical cyclone Evans impacted Samoa between 11–15 December 2012.
* Tropical cyclone Gita impacted Samoa on 8–9 February 2018.  
\end{table} 
Table 1: Historic rainfall and flow timeseries for the Vaisigano catchment used in this study. a 2-h period [31]. The 24-h rainfall recorded at Afiamalu was 413.8 and 467 mm at the Alaoa gauge. Other gauges within the catchment recorded less (e.g., 144 mm at Apia and 207 mm at Nafanua), illustrating the high spatial variability of rainfall even for large events. Maximum hourly rainfall for the on 13 December 2012 event varied from 56 mm/h between 12 and 1 pm to 85 mm/h from 2 to 3 pm at Afiamalu, and from 12.2 to 156.8 mm/h at the Alaoa gauge [31]. Again, this illustrates the local spatial variability of rainfall in the catchment. Data from seven operational rain gauges located within or near the catchment (i.e., Afiamalu, Nafanua, Alaoa, Mt Le Pue, Tavi, Maagiagi and Togitogigga) were used to produce area-weighted catchment averaged rainfall and rainfall-surface runoff relationships (detailed in Section 3.2.2), to enable reliable estimation of real-time nowcast and forecast peak flow and level for river gauges at Alaoa West, Alaoa East and Lelata Bridge (Figure 1). Thiessen polygons were drawn around each of the above rainfall gauges with only the gauges at Nafanua, Afiamalu, Maagiagi and Le Pue intersecting with the polygons (Figure 3). This enabled calculation of the area associated with each rainfall polygon (Equation (1)) to underpin the estimation of area-weighted average rainfall for each sub-catchment (Equations (2)-(4)):  
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