FLOOD LOSS MITIGATION APPENDIX I INTRODUCTION The

FLOOD LOSS MITIGATION APPENDIX I INTRODUCTION The hazard from flooding of the Heretaunga Plains is described, and the purpose of flood protection measures is discussed. Possible ways of reducing flood losses are described in general terms, with natural overflow paths being noted and secondary defence stopbanking measures outlined. FLOOD HAZARD The stopbanking and associated river works on the Ngaruroro River have been undertaken primarily to reduce the frequency of flooding on the adjacent flood plain. The main economic benefit of these works arises from the more intensive land use that can be undertaken because of the reduced risk of flooding. The protection from flooding allows the more damage-sensiti ve intensive land uses to be carried out economically. This intensification, however, increases the f 1ood hazard, as there is a greater potential damage from flooding. River control schemes only improve the level of protection; they do not eliminate flooding. The standard of flood control depends on the likelihood of occurrence of the flood flow that fills the controlled area to capacity, the strength of the river edge protection works against river attack, and the ability of the stopbanks to retain water over flood durations without failing. Whatever the standard of any scheme, flooding will still occur, albeit less frequently. There will always be over design flood events, with flooding occurring because of stopbank overtopping or breaching. The intensification of land use with improved levels of flood protection will inevitably give rise to a greater flood hazard. The protection works are undertaken to allow a more intensive occupation of the flood plain land, and the economic return results from a combined investment in protection works and intensification of land use. At the same time, the damage caused by flooding when it does occur will be very much greater than previously. Occasional more disastrous losses are an inevitable cost of the greater use of flood prone land. Given this greater flood hazard, the minimising of damage when flooding does take place, and the prompt re-establishment of production, is vital. The long term economics of the whole protection/intensification undertaking may depend upon it. If rare flooding causes production losses over a long period of time, and re-establishment costs are high enough or difficult to finance, the average annual benefit may not be sufficient to warrant the original investment, in spite of the rarity of the overdesign event. Insurance and flood relief ensure that the burden of the flood damage does not fall unduly on the people directly affected, and facilitate reconstruction and the re-establishment of normal production. In fact, many of the decisions and costs of both flood protection and f1,ood damage are shared by the community as a whole. This has arisen because of the community wide impact of flooding, and the disastrous effect on individuals of rare event flooding. However, this only reinforces the need for community preparedness and the importance of communal preventive measures. Individuals often underestimate the risks of flooding and the damage caused by flooding, and increasing the awareness of people about flooding is an essential part of improved community preparedness. The proposed scheme is essentially one of reconstruction to re-establish a given standard of protection. The protection works themselves are being rebuilt to generally accepted standards for major river schemes. The intensification of land use has been taking place for many years, and is continuing, essentially in response to prevailing social and economic circumstances. The present land use is, however, relatively intensive and dependent on a high level of protection for economic viability, while further intensification is taking place in spite of the general economic constraints of the present time. The well developed nature of the flood plain means that there is little opportunity for the setting aside of specified relief flood paths. There are natural flood paths across the plains from the Ngaruroro and Tutaekuri Rivers, but development in these areas is generally similar to the rest of the plains. The most preferred overflow pattern would be a uniformly distributed flow, but even with uniform overtopping of the stopbank the natural topography and features such as drains and roadways will concentrate overflows. Apart from the construction and maintenance of river control works, the main actions the Board can take to minimise flood losses can be categorised under the following headings: (1) Definition of the flood hazard - indicating the damage potential from flooding, which is a combination of land use and susceptibility to flooding. (2) Flood warning - predicting the likely magnitude and duration of flood events from telemetered rainfalls and river level data, and providing forecasts of possible flooding. (3) Determination of flood paths - estimating the likely extent and depth of flooding from failure of the protection works at various places. The Board has prepared a Flood Manual to assist in flood prediction and warning during a flood emergency, and is at present undertaking a flood plain study of the Heretaunga Plains to better define the flood hazard and the range of alternatives for flood loss mitigation. This flood plain study is, however, a major investigation, which will take many years to complete. The area at risk from flooding by the Ngaruroro River, and by the combined effect of the Tutaekuri and Ngaruroro Rivers at their common mouth is shown on plan 1893, sheet 614. The areas where floodwaters are likely to pond for more than 3 days in the event of stopbank overtopping or breaching have been determined approximately, using estimated one metre contours on 1:25000 scale base maps, and these - 184 areas are also shown on the ~lan. Maior trans~ortationfeatures such as the railway, state highwa)s and bridges are'shown as well. FLOOD OVERFLOW PATHS The Heretaunga Plains is essentially a large fan, built up by the aggradation of river channels and the continual breaking out and shifting of the channels. At any point in time there is naturally low lying land, and overflow floodwaters will flow towards and be concentrated along the lower lying depressions and basins. The old channel of the Ngaruroro River from below Roy's Hill, across Flaxmere, and down the Irongate Stream to the Karamu Stream remains as a natural pathway for overflows. On the other hand the land around Omahu Road is relatively high and would act as a divider of overflows. Below Fernhill overflows will naturally concentrate along the path of the Raupare Stream on the right bank and of the Tutaekuri-Waimate Stream on the left bank. There are no natural pathways that are relatively less developed over a sufficient length for them to be usefully delineated as flood overflow paths. Overflows between Fernhill and Chesterhope can, however, be significantly contained by additional stopbanks. In this way large areas of land can be given extra protection, which would be particularly useful in the event of minor breaches of the main scheme stopbanks above Chesterhope. On the left side there is an existing double banked outlet and the old stopbank along the left bank of the Tutaekuri-Wairnate Stream is still intact over substantial lengths. The rebuilding of this stopbank, and its connection to the outlet channel, would result in the full containment of minor breach flows and divert them down the outlet. In the event of major overflows this bank would provide an extra degree of protection to the orchard land along Pakowhai Road, at the expense of deeper ponding on the pasture land on the right bank of the Tutaekuri-Waimate Stream. On the right side an additional stopbank can be constructed to contain overflows and divert them to the Raupare Stream outlet to the Clive River. In this case a more extensive and disruptive stopbank is required, but additional protection could be given to a large area of land, including a semi-urban fringe along the north side of Hastings. The proposed Hastings to Napier motorway is sited between Fernhill and Chesterhope, and crosses the Raupare and Tutaekuri-Waimate streams as well as the Ngaruroro River. The formation of this road could have a pronounced effect on the path taken by overflows. Any embankment across the low lying land around the Raupare Stream would impound overflows and direct the floodwaters towards the urban areas of Hastings. This would not only negate the advantages of the secondary defence stopbank, but would also significantly increase the flood damage potential of overflows. On the Tutaekuri-Waimate side the stream itself is well away from the Ngaruroro River, but there is a drainage way closer to the primary stopbank. If vertical curves are used to form a wide dish in the motorway profile where it crosses the minor waterway.s, and the secondary defenke'banks are joined to the roadway, which would then act as an embankment, the motorway and f ood overflow requirements can be made complementary, rather than being in conflict (see Figure 1-1). Alternatively, additional openings to ake overflow floodwaters can be incorporated in the minor wate way crossings. In this case, though, there would be a definite limit to the flow that can be passed without causing additional ponding and overflow diversion effects. The existing motorway beside Napier would have an effect on the spread of floodwaters, and for overflows from below the motorway bridge over the Tutaekuri River, the existing road formation would tend to deflect floodwaters further into Taradale than would otherwise have occurred. FLOOD PLAIN MANAGEMENT The amount of damage sustained when flooding does occur can be reduced by sound planning and appropriate design with little or no increase in land development and building construction costs. Shelter belts and orchard trellises will quickly trap debris and deflect floodwaters, and appropriately sited subdivision roads and drainage ways can be used as relief paths, carrying floodwaters directed towards them through an area with minimal damage.

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