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Holding back the water

by Kerry Wood
Wellington is facing serious flooding risks. Flood-planning is becoming necessary.

This is shown by an article in the December edition of Engineering NZ’s magazine ‘EG,’ titled ‘Giving up the coast?’ The introduction states:

Human activity is changing our climate in unprecedented and irreversible ways, and extreme weather events are appearing with ever-more frequency and intensity. Against these, seawalls and other grey infrastructure may no longer be sustainable. Ultimately, for some settlements, a managed retreat — the planned withdrawal of communities and infrastructure away from coastal areas or other risks — may be the only option…

The main author of the article is Rob Bell, of Bell Adapt Ltd in Hamilton.

If we consider the options for managed retreat, there is a radical and hopefully useful alternative: avoid managed retreat entirely. At this stage many assumptions are needed, and some will inevitably be wrong. (I must admit here to being a retired engineer.)

Progress

The Regional Council has already commissioned sea level rise studies. Local sea level is today rising at about 3mm/yr, corrected for historic land-movement. The anticipated local sea level rise is some 0.3m to 1.3m by the end of the century. However, uncertain factors such as unstable Antarctic ice mean that a rise exceeding 2.4m by 2100 is physically possible.

This uncertainty should improve soon, because quicker and much more accurate measurements are now practical, as recently reported in the DomPost.

Using existing Regional Council data, sea level rise can be taken as perhaps up to 2.0m by 2100, plus 0.5m for storm-surge. If this proves correct the main areas affected will be:

— Lower Hutt, inland to the Ewen Bridge and Woburn Station, as well as at Seaview and lesser flooding down to Eastbourne and beyond.
— Central Wellington, from Kaiwharawhara to Oriental Bay, and inland to Thorndon Quay, Lambton Quay and Courtenay Place.
— Evans Bay, to Evans Bay Parade, and Kilbirnie from Cobham Drive to Coutts St and Kilbirnie Crescent
— Miramar, from 100m north of Miramar Av to Devonshire Rd.

Miramar has 2.5sq km of at-risk urban land, but Lower Hutt’s risks cover a far greater area.

This extent of flooding is massive-yet-potential, which justifies studies. More options will doubtless appear as comprehensive studies begin.

Problem

Sea-level rise in Wellington is already a risk, and the sooner the implications are identified, the better the response can be. There are clear indications that it could become very serious within a decade or two.

An obvious approach would be to use sea-walls to keep the water out, but sea-walls also keep water in. Using sea-walls requires pumping to dispose of rainwater, but sea-walls to keep out a metre or more of water — in all affected areas — are going to need very large-scale pumping.

Where are the optimum sites for pumping?

In the Netherlands, floating houses are already common. This could be very useful in Lambton Harbour, where floating walkways could provide access to the central city.

However, central Wellington is itself one of the most worrying areas, flood-prone as far inland as Thorndon Quay and the golden mile. Worse, heavy rain creates large-scale stormwater flows down Kent and Cambridge Terraces, which might bring too much water into the central city. If this were to happen, LGWM’s light rail plans might well be in serious trouble, which they will doubtless wish to understand.

Raising roads above high tide would help, but what will be the cost of raising all roads in central Wellington, by a metre or more? Adjacent buildings would need revised ground-floors, or even first-floor footpaths and entrances.

Floating houses are also possible in areas needing managed isolation, but again, how to manage roads in low-lying areas? Residents might have to plan their days around tide-tables as well as weather forecasts.

In principle, non-return valves can allow stormwater to escape into the harbour. Experimenting seems worthwhile, but my own experience is that retreating waves draw non-return valves open, letting seawater in. This brings sand and rocks into the valve, jamming it open.

Solution?

Necessary conventional measures would be extensive sea-walls, almost right around the harbour, and potentially large-scale alteration of buildings, roads, railway tracks and so on. Obviously this will be very costly.

The proposed alternative might be cheaper, and probably more effective: build a 2km sea-wall across the harbour, from Breaker Bay (to protect Seatoun) to the road running to Pencarrow Head. It will need these features, potentially high-risk:

— A large canal-lock to allow shipping into the harbour.
— Substantial pumping arrangements, to maintain existing harbour water-levels.

A secondary 1km sea-wall, in shallower water, will be needed in Lyall Bay. It too will need pumping, on a much smaller scale.

Obviously, both sea-walls would need careful design, to be confident that they could withstand earthquakes, tsunamis or both. It may be that lock-gates will be unable to withstand a tsunami. If so, a possible alternative would be closing the harbour entirely.

Pumps in the main sea-wall could be in two seperate chambers, to protect against chamber-flooding. Each chamber would need sufficient pumping capacity to lower the harbour water-level by the maximum stormwater volume that had entered in the previous tide. That way, there would be backup capacity, with either set of pumps capable of completing the job within 12 hours. Sea-wall pumps would also need to pump out sea-water entering the harbour through the lock gates, either directly or indirectly.

If the whole harbour can be kept within the present-day tidal range, by pumping at Seatoun, the only remaining problems will be pumping out the rainwater (including the Hutt River!) and bringing ships into port. A single civil engineering project — even on this scale — might be worth considering.

If this option proves practical it would make the whole harbour available for stormwater storage at high tide. This would eliminate the need for all managed retreat except on Wellington’s south coast.

Even if large-scale tsunami-proof canal-locks prove impractical, this two-seawall, multiple-pump option might still be worthwhile:

— KiwiRail could build a new ferry terminal in the Breaker Bay Area, with extra wharves for Bluebridge and a new freight line to the existing rail yards. KiwiRail might well prefer this to the present site.
— The commercial harbour could be closed, with timber exports shipped by rail to Napier or New Plymouth, and containers shipped to Northport (Marsden Point) by rail, or coastal shipping from South Island ports.
— The commercial harbour land could then be used for inner-city housing.

If pumping the Hutt River is practical, together with all other stormwater, Wellington has a solution.

If not — which seems much more likely — extensive planning will be needed, as well as considering sea-level rise continuing beyond 2100.

7 comments:

  1. RH, 30. December 2021, 8:20

    And we’re building at Shelly Bay. Hmm.

     
  2. Ray Chung, 30. December 2021, 21:18

    Yes, RH, I agree! The developer should be responsible for ensuring that there’s adequate provision for any potential sea level rise including the road through to the Miramar junction.

     
  3. Max Drake, 31. December 2021, 20:27

    I also wondered about the issue. A lock at the harbour entrance could also act as a bridge to open up land over in Wainuiomata and all the undeveloped area east of the harbour entrance.
    I played with some maps.
    There is the issue of the Rongotai area that would need to be protected. Risks and opportunities. A good article, thank you for sharing.

     
  4. Henry Filth, 31. December 2021, 22:13

    Interesting scenario. Perhaps practice by raising or re-routing the sea-level part of the main trunk railway between Porirua & Plimmerton.

     
  5. Kerry, 1. January 2022, 15:40

    Henry. I hadn’t thought of that one. Yes, it will need doing, unless it is already high enough for the next few years and can wait until levels are better known.

     
  6. NigelH, 2. January 2022, 9:00

    “… unless it is already high enough for the next few years and can wait until levels are better known.” I’d suggest waiting 20-25 years to see what the rise actually is. I understand 3mm/year is what has occurred over at least the last 50 years if not more, so no need to become alarmed now. At 3mm/year by 2100 is the rise is just under 240mm. No need to come up with henny penny sky’s falling in of 2.0m rise by 2100, now.
    Some low level preliminary planning is required in the next year or two for 25 years at 3mm/year but nothing more.
    I’m not a Wellingtonian but lived in Lower Hutt for a few years but don’t recall having been there when my route to the Esplanade from Belmont was blocked by the river flooding.

     
  7. Kerry, 2. January 2022, 21:18

    Nigel. Sure it is easy to come up with henny penny and no need for action. When that approach is taken, the next stage is very likely to be real panic. Is there really no point in planning for drastic change by 2050?

     

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