Port Hills Erosion

What would happen if Christchurch experienced a rain event such as that which Auckland or Gisbourne recently endured?  And what lessons are there to be learnt about Port Hills erosion?

It’s late February 2040.  Residents of Christchurch have endured a miserable beginning to the year – it has rained so often that the hills are oozing water like a full sponge and the plains are pooled with stormwater. And then, the forecast brings more bad news.  An atmospheric river of rain is deflected by a blocking area of high air pressure and makes landfall over Christchurch. It drops five hours of rain over the already sodden Port Hills at a rate of over 50mm per hour.

The consequences are not pretty. On the upper slopes of the Port Hills, the highly erodible loess soils, already sodden, turn to porridge and begin to slide down the hill; slips merge and large parts of the slopes flow, overwhelming and blocking stormwater facilities. Stormwater, unable to be contained by its normal drains and pipes, breaks out new paths, flowing swiftly, worsening the erosion and finding the most direct path down the hill.  That direct path is often through properties and even through houses themselves.  House foundations not pinned to underlying rock are undermined and the massive force of debris-reinforced mud pushes some of these houses down the hill onto adjacent and newer homes.  Cars are swept into the melee to add to the weight and pressure of moving material.

At points on the lower slopes of the hills, the accumulation of stormwater, mud, building materials, fences, bins and cars descending the steeper slopes causes a complete nightmare of piled-up and churned mud, wood, plant matter, metal, rubbish and random plastic objects. 

The retention ponds in the headwaters of the Ōpāwaho Heathcote River, already half-full from recent rain, exceed their capacity within the first hour, the overflow causing the river to burst its banks. Before long, houses nearer the river are flooded, with rescues and retreat soon happening on a wide scale.   

Before high tide slows the escape of waters into the estuary to back up the flood waters, let’s turn off the imagination: the picture is pretty clear.

This is an all-too-realistic scenario.  But can the results of such a deluge be avoided?  That will depend on what we do in the next fifteen years, always hoping that we have that long before we experience this.  If we do nothing to lessen erosion risk on the hill, this is what will happen – we have already seen it happen in parts of the North Island.

While such a rainfall event will always have poor outcomes for many, at least some of the consequences could be mitigated by actions that are within our capacity if we act now.

• Through appropriate revegetation of the hills, we can reduce exposure of loess soils to overland water flows
• We can investigate, identify and armour likely overland flow routes, making sure that such flows will avoid buildings and critical infrastructure
• We can change land use to lessen exposure of loess soils
• We can make changes to and enforce higher building standards to reduce the impact of stormwater from residential homes and sub-divisions on the hill
• We can improve the provision and capacity of stormwater infrastructure, and maintain it to higher standards so that is better able to cope with anticipated high rainfall events
• We can improve general awareness amongst residents of the hill suburbs so that each can contribute appropriately to the on-going maintenance and readiness of stormwater facilities, many of them privately owned and maintained
• We can help the City Council identify weaknesses in the current public stormwater system.

Everyone has a role to play but there needs to be motivation for action; recent rainfall events in the North Island have given us a poke and there are conversations now taking place.  Just last week, sixty people representing a wide range of interested parties – iwi, Council, ECAN, Water Zone Committees, landowners, community groups and the Ōpāwaho Heathcote River Network – workshopped possible next steps to tackle the issue of erosion on the Port Hills. 

One of the most important next steps that came from that workshop is for the City Council to take the lead in developing a Port Hills Plan – a plan that urgently draws together the combined knowledge of those who are already making a difference on the Port Hills. A Port Hills Plan will need to bring together the threads of revegetation/regeneration, erosion control, three waters, roading, biodiversity, pest management, forestry, farming and recreation.  The plan will need adequate funding and urgency to be collaboratively written and then adequate funding and urgency for its implementation.

If the high rainfall scenario above eventuates, then having a fancy stadium and a rate rise below that of inflation will be something of a sick joke as we gaze on the visible remains of a silt-filled city wondering if it is possible to move it out of the swamp.