SAMOA FLOOD MANAGEMENT ACTION PLAN 2007-2012 - Pacific ...

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constructed.o Make a preliminary assessment of the environmental impacts including the visualimpacts and health and safety of constructing this option.Figure 3-7: Possible route for a flood diversion channelTop width of channel varies dependingon the flow to be carried221130 mFigure 3-8: Cross-section of a possible flood diversion channel for the lower VaisiganoRiverFlood management Action Plan <strong>2007</strong> 26
Table 3-7:Returnperiod(years)Preliminary dimensions of a flood diversion channel for the LowerVaisigano 6Capacity of thechannel(m 3 /s)Depth of channel(m)Top width of channel(m)1 in 2 74 0.9 33.61 in 5 183 2.0 38.01 in 20 318 2.1 38.41 in 50 370 2.4 39.61 in 100 450 2.7 40.83.2.4. Increasing channel conveyanceThe capacity of a river channel to discharge floodwater can be increased bywidening, deepening or realigning the channel, and by clearing the channel banksand bed of obstructions to flow. Such “improvements” can increase the velocity andpossibly the depth of flow. A particular example of a method of increasing theconveyance capacity providing enhancement opportunities is the creation of a “twostage” channel. In this case the existing river channel is retained for low and mediumflows, and floodplains are created parallel to the river channel to accommodate floodflows.Increasing the channel conveyance capacity can, like diversion channels, reduceflood attenuation and increase the flood risk downstream. Other disadvantagesinclude the cost of maintenance of oversized channels, where sediment depositionmay occur.A preliminary investigation into increasing of the channel capacity has indicated thatthe Vaisigano River would have to be widened by some 70 m at the downstream endwith the bed level dredged to -1 m below mean sea level. At the upstream end nearLeone Bridge the channel needs to be widened by 50 m to prevent flooding duringthe 1 in 100 year flood. The widening and dredging of the channel upstream ofLeone Bridge has not been considered because this part of the Vaisigano River isconstrained by bed rock meaning that the costs of channel widening and dredgingare likely to be prohibitive.The key disadvantages of the increase in channel conveyance for the lowerVaisigano are as follows:o The issue of land tenure and the socio-economic cost and impacts of relocatingcommunities that live adjacent to the Vaisigano between Leone and VaisiganoBridges.o The cost of maintenance of the oversized channels, where sediment deposition islikely to occur.o The destruction of riparian habitat and the visual impact of replacing naturallyvarying channel sections with a section of more uniform geometry.6 Note: The channel is assumed to be trapezoidal with 1 in 2 side slopes and a bottom width of 30 mNo allowance for freeboard has been made when estimating the depth of the channelFlood management Action Plan <strong>2007</strong> 27
Page 1 and 2: SAMOAFLOOD MANAGEMENTACTION PLAN200
Page 3 and 4: AcknowledgementThe development of t
Page 5 and 6: Table of Contents1. Introduction ..
Page 7 and 8: Vaisigano River ...................
Page 9 and 10: 1. IntroductionThis Flood Managemen
Page 11 and 12: 1.2. PurposeThe purpose of the Plan
Page 13 and 14: 1.4. Frameworks that Encompass the
Page 15 and 16: 1.4.4. Existing Policies and Legisl
Page 17 and 18: 1.5. Implementation StrategyThe Pla
Page 19 and 20: 2. Background to flood management i
Page 21 and 22: 2.4. Previous flood risk management
Page 23 and 24: Figure 2-2: Flood wall and bank pro
Page 25 and 26: een set up using the HEC-HMS and HE
Page 27 and 28: Location of floodembankmentsFigure
Page 29 and 30: Table 3-4 provides an estimate of t
Page 31 and 32: Table 3-6:Return period(years)Numbe
Page 33: o Estimate the volume of the fill r
Page 37 and 38: Recommended actions:o A detailed as
Page 39 and 40: Figure 3-9: Flood depth map for the
Page 41 and 42: The degree of flood hazard to peopl
Page 43 and 44: Figure 3-12: Flood hazard map for t
Page 45 and 46: Recommended actions:The following a
Page 47 and 48: process guidance through which the
Page 49 and 50: Table 3-10: Actions required for no
Page 51 and 52: OBJECTIVE KEY ACTIONS EXPECTED RESU
Page 59 and 60: GOAL 5 : SUSTAINABLE WATERSHED MANA
Page 63 and 64: 6. AnnexesAnnex 1: The Process carr
Page 65 and 66: sustainable development.DISASTER WA
Page 67 and 68: Annex 3: Stakeholder Consultation W
Page 69 and 70: SOPACDr. Netatua PelesikotiSustaina
Page 71 and 72: Flood management Action Plan 2007 6

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