## 3 System Development: Methodologies and Analysis
### Flood Frequency Analysis
#### 3.2.2 Flood Stage and Flow Thresholds  
Stage and flow data from the operational gauges at Alaoa West, Alaoa East and Lelata were used to identify/monitor flow thresholds that are expected to be exceeded based on real-time observations/forecasts of rainfall. Long-term records at these sites along with a rating curve developed in [22] for flows up to 15 m\({}^{3}\)/s at the Alaoa East gauging station were used. The associated rating equations for different stage-flow ranges at Alaoa East up to a stage of 3.2 m and discharge of 75 m\({}^{3}\)/s are shown in and are summarized in Table 3. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Duration**} & \multicolumn{4}{c}{**Rainfall Intensity (mm/h) for Different Return Period Events 1**} \\ \cline{2-6}  & **1 in 5 Year** & **1 in 10 Year** & **1 in 25 Year** & **1 in 50 Year** & **1 in 100 Year** \\ \hline
5-min & 313 & 365 & 412 & 450 & 493 \\
10-min & 242 & 283 & 320 & 350 & 382 \\
15-min & 204 & 240 & 270 & 295 & 323 \\
30-min & 142 & 166 & 187 & 204 & 223 \\
1-h & 90 & 105 & 119 & 129 & 142 \\
2-h & 57 & 66 & 72 & 80 & 87 \\
3-h & 40 & 47 & 50 & 57 & 62 \\ \hline \hline \end{tabular}  
* \({}^{\dagger}\) Thresholds for additional design event durations (up to 60-day duration and 1000-year return period) were adapted from American Samoa based on similar catchment characteristics identified in [33].  
\end{table} 
Table 2: Intensity-Duration-Frequency (IDF) threshold values used in this study. Figure 3: Thiessen polygons for automatic rain gauges (ARG) and automatic weather stations (AWS) in or near the Vaisigano catchment. These stage-discharge data, along with preliminary analysis by [23], provided a basis to derive flood frequency distributions from Alaoa East (Table 4). For design events up to the 10-year return period (10% annual exceedance probability), the flow magnitudes were within 10% of previous flow estimates as detailed in [33]. For higher magnitude design events, flows have increased from those previously available (e.g., the 100-year return period event increased from 331 to 542 m\({}^{3}\)/s) (Table 5) [23]. Modelled flow levels at Lelata for the 5-, 20-, 100- and 1000-year return period events were derived using the area weighted sub-catchment areas and rescaling of peak flows to Alaoa East and Alaoa West (Table 5). Compared to the values shown in Table 5, the rescaled flows for Alaoa East are higher for the 5- and 20-year return period conditions but are lower for the 100- and 1000-year return periods. Similar trends are reflected at the Lelata gauge site [23]. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & \multicolumn{2}{c}{**Design Event Flows at Alaoa East Gauge**} & \multicolumn{2}{c}{**Design Flows at Electric**} \\  & \multicolumn{2}{c}{**(Eastern Catchment)**} & \multicolumn{2}{c}{**Power Corporation Weir**} \\ \cline{2-3}
**Return Period** & \multirow{2}{*}{**Flow (m\({}^{3}\)/s)**} & **Level (m Gauge** & **(Eastern, Central and Western Catchments)** \\  & & & **(m\({}^{3}\)/s)** \\ \hline
1 & 17 & 1.7 & 29 \\
2 & 41 & 2.6 & 69 \\
5 & 82 & 3.2 & 139 \\
10 & 131 & 3.6 & 222 \\
20 & 204 & 4.0 & 346 \\
50 & 358 & 4.8 & 608 \\
100 & 542 & 5.4 & 921 \\
200 & 814 & 6.4 & 1384 \\
500 & 1382 & 7.8 & 2350 \\
1000 & 2052 & 9.3 & 3489 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Design event flows for Alaoa East station adapted from [23].