Potential Stop bank breach upstream of Foxton Beach
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This brief report accompanies a contoured aerial photograph, and explains some of its implications. It describes the flood hazard presented by the potential stopbank breach 800 metres upstream of the Foxton Beach Boat-ramp.
If the currently occurring erosion continues, the remains of the original stopbank, and then the new temporary stopbank are likely to be undermined and destroyed, with the following consequences:
- Flooding of up to approximately 150 hectares of low lying rural land would occur. The land would partially drain with the falling tide, and flood again with the rising tide. At present inundation would be by silty river water. If river flows were to decrease too much before repairs were completed, inundation could occur with salty water.
- A tide high enough to flood the lowest point on the main road into Foxton occurs on average once every 4 months. The flooding from tide alone would not be deeper than about 200 mm, and the duration would be less than 2 hours. In the absence of storm surges or very large river floods, the road would not be floodable for at least the next month.
- The likelihood of houses being flooded in the near future is extremely remote, given the location of the potential stopbank breach. If the February flood were to occur at the same time as the highest tide in the past 8 years, that would be an unlikely combination of two severe events. It would place the lowest houses at the eastern end of Foxton Beach at marginal risk for being flooded.
The tide cycles are favourable for the next week or so. The tide cycles are currently diminishing, and neap tides occur on Friday 22 Oct. The neap tide range is much smaller than the spring tide range.
Further explanation of these conclusions follows below.
Ground Levels at Foxton Beach
The contours on the aerial photograph show ground levels relative to Mean Sea Level. They show that water must reach a level of about 2.7 metres before it enters the Foxton Beach township.
It also has to reach 2.17 metres before it starts to flow across the low point of the main road, about 400 metres east of the Seabury Ave / Ferry St intersection. Note that the plan shows the 1 metre contour crossing the road in one location. This is an error resulting from a break-line being inadvertently omitted. The 2.17 m level for the low point is confirmed.
River level recorder at Foxton Beach Boat-ramp
Horizons Regional Council has operated a river level recorder at the Foxton Beach boat-ramp since 1996. In that time the highest recorded river level was 2.13 metres. This was on 17 April 1999, when a storm surge coincided with a spring tide. The river level at the potential breach site could be a little higher than at the recorder, but is unlikely to be more than about 200 mm higher.
If the April 1999 circumstances occurred after the stopbank breached, the main road into Foxton Beach would be inundated to a depth of about 160mm at high tide. No houses would be flooded.
Interaction of tides and river floods at Foxton Beach
The river level is mainly determined by the tide. High river flows play a lesser role, and then the effect is more pronounced on low tides than it is on high tides. The minor flood during the past weekend had no discernible impact on river level at high tide. It may have raised the river level by 200 to 300 mm at low tide.
The February flood raised the high tide level at the recorder about 600 mm above the expected high tide level. Low tides were not apparent, the river remained at high tide level for three tide cycles. Fortunately this flood coincided with neap tides.
Current tide cycles
The last spring tide was the midday tide on Saturday 16th October. The tide cycles will progressively diminish until the neap tides of Friday 22nd October, after which they will increase again.
The river level at the most recent high tide was 1.25 metres MSL at the recorder, and probably about 1.4 metres at the potential breach site. This level will be progressively lower at each high tide until Friday 22 Oct, when the river level at the breach will be about 0.7 metres. This is approximately ground level at the site.
Scenarios whereby the Manawatu River could flood Foxton Beach township
A stopbank breach would not present a severe threat to Foxton Beach unless it occurred much further upstream. The river level would be higher further upstream, and if water escaped through a breached stopbank it could therefore pond to a greater depth in the flooded area.
At-risk lengths of stopbank further upstream have been identified, some upgrading work was completed last construction season, and the remainder will continue as soon as weather and ground conditions permit.
Graham Doull
SENIOR DESIGN ENGINEER
18 October 2004
Source: Horizons Regional Council.
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