09 December 2020

PGF ProgrammeFlaxmill Inundation Remediation Project

Agenda

• Porongahau Road Raising

• Future Activities Discussion• Ugly Hill Road culverts

• Silt removal

• Debris attenuator

Overview

Online Road Raising – North Section

1 in 10 year LOS with 200mm of freeboard

Proposed Works

Online Offline

(Works within the road corridor) (Works within the Stream / channel)

• Road embankment raising Silt removal

• Ugly Hill Road Culvert Upgrade Debris Arrestor

•

Online Ugly Hill Culvert Upgrades including minimal Road Raise (South section)

• Lift road over slight dip above Ugly Hill culverts

• Replace existing 1,5m pipes with 3Nr Box culverts: W4m x H2m

Background - Flood Extents (100yr ARI)

Offline Debris Arrestor

• Soldier piles (e.g. railway irons) to trap debris before bridge

• Low initial cost but regular maintenance

• Safe access (stairs, working platform, truck?)

• Any opportunities to reduce debris load from catchment?

Example

Debris Arrestor cont.

Proposed Works Summary

Online

• Wanstead Bridge to Flaxmill Bridge (North Section) road embankment raise $6.9M

• Flaxmill Bridge Increase Capacity $0.4M

• Ugly Hill Corner capacity upgrade $0.9M

• Replace existing culverts with large box culvert

• Raise road over 150m length

Offline

• Widen stream upstream and downstream of Ugly Hill Culverts $0.5M

• Taurekaitai Stream - channel widening/silt removal $1.6M

• Total $8.2M

Question from Wallingford Meeting:

Q) Why can’t the removed silt be used for a stockbank? A) The material is not ideal for use as a stockbank as it holds too much moisture and there would also be a high risk that it would get washed back into the river, especially if there was any erosion.

Q) They current culverts are quite high, will they be lowered? A) They will be much larger and reach to a lower point that the current culverts.

Q) There are a lot of trees coming down that river at the moment causing damage, blocking the river channel and potentially increasing flooding risk, will anything be done to prune / remove these? A) The CHBDC will liaise with the Hawke’s Bay Regional Council to see if they may be able to help with this issue. (The CHBDC’s funding only relates to improving the road).

Q) The fenceposts in the paddock with silt build up are almost entirely covered with silt – the current proposed silt removal wont nearly be enough to cover the amount built up there – can more be taken? A) The current silt removal amount in combination with the other design factors will make the project successful, but Stantec & CHBDC to complete a walkthrough of this area with landowners to discuss design and make sure the silt removal numbers are where they should be.

Q) Would the debris arrestor be better further upstream? A) Stantec still to design this element – will investigate placement.

Q) Will the fences be raised too? A) Only the north side fenced will be raised at this stage.