Paddling flooded rivers

Comprehensive article that shows how flood-prone New Zealand rivers are and suggests ways of keeping safe.

It's rainin' in the mountains and the river's on the rise
- Hoyt Axton

[image:1222 size=original nolink=1]
Gunslinger rapid on the Buller River, in high flow.

Floods are right at the top of the catalogue of natural disasters, creating the maximum of human misery. Bangladesh, China, the USA, Italy, Australia - all have experienced floods in recent times. Millions of people died in China when the Yellow River flooded. Closer to home, the January 1984 Invercargill flood caused over $1,000,000,000 (in today's dollars) worth of damage. To kayakers, however, it means that little creek just out of town will be bank to bank. Phone around your paddling mates and it's all on.

What are the risks of paddling flooded rivers and is it worth it?

It's a cliche, but it all comes down to judgement. The essential ingredient in making good judgement calls is knowledge. Of yourself, of your companions, and the river. Too many tragedies start with a person who failed to say NO. Better to be a live chicken than a dead duck. No one got drowned in a portage, or even driving the shuttle, for that matter.

To raise the odds in your favour, you need to understand what goes on in a river when the flow rises. Lots of experience, physical strength and skill and lucky escapes help a lot, but a little physics goes a long way, so please don't switch off just yet.

(River) Physics 101

Most rivers have been flowing in their beds for millions of years. The Clarence River, for example, was flowing before the Kaikoura mountains rose. The Clarence just kept cutting down as the land kept rising. What it means is that the flow patterns at normal river levels are really well established. As a paddler you will be very familiar with flow patterns that exist in the normal river situation. As the river floods, water flows in places that only rarely see water, where rocks are sharp, not worn smooth, where trees grow, banks don't follow a natural water worn shape, and the power and energy of the water increases faster than most people appreciate. Physics tells us that twice the speed of the current means four times the force it can generate if something gets in its way.

Different channel shapes have totally different characteristics as the flow increases. Pool drop rivers turn into continuous swift water with river wide hydraulics at narrows. Braided rivers spread out bank to bank and the big rivers get deceptively dangerous. Flow patterns which are not apparent at normal flows, start to make their presence felt. All water in flowing channels moves in a vortex like a big double corkscrew running downstream. This is just a swirl on the surface normally, but in a flood water mushrooms up from river banks and folds down on the eddy lines with tremendous power. The nett effect is that if you are in the river you will be unable to reach the safety of the banks. Each time the flow slows down you get pulled under as the river's kinetic energy converts into vertical axis rotational flow. In canoeing English, it means that eddy lines have very powerful whirlies and a lot of flow is going down. At bends and sharp expansions of width, the power increases and the randomness multiplies. Familiar resting places transform into "forever eddies".

[image:1224 size=original nolink=1]
Don't forget your spare paddle and throw rope, my son!

A child can push a swimming kayaker under, but it takes the weight of three adults to sink a kayak.

How a river behaves in a flood is affected by rainfall, the catchment and the channel you are considering. The type of catchment affects the rate at which falling rain ends up in the river. How fast will the river rise and fall, and what peak flow is expected compared with the normal flow? A river catchment with bare hills, hard underlying rock layers and steep creeks will rise suddenly and have a peak flood which is many times higher than the normal flow. A river with native forest, lakes and pumice soil will rise slowly, with only a percentage increase over the normal flow. It will also fall slowly.

Hydrologists use the term Coefficient of Variation or CV to measure this characteristic.

Some examples:

River Catchment Mean flow (cumecs) CV
North Island
Kaituna 100% lake fed 22 0.3
Motu native forest 92 1.4
Manawatu pasture 102 1.33
Tarawera pines/native, pumice, lakes 72 0.4
Whakatane native forest 56 1.3
South Island
Hurunui lake, native forest 51 0.9
Karamea native forest, lakes 116 1.3
Cleddau native forest, high rainfall 29 1.8
Ashley some native forest 13 1.6

What does this tell us? How important is a river's CV?

Compare the Tarawera with its neighbour, the Whakatane. They get similar rainfall and have similar gradient, but the Tarawera gets its water through fractured pumice soils and has much of its direct rain absorbed by pine forest. Rainfall hardly affects this river. The Whakatane flows out of the Urewera National Park and can rise to a flood after heavy rain.

In the South Island, the Ashley rises and falls quickly, whereas its cousin the Hurunui, fed by Lake Sumner and extensive tussock lands, holds its water, you might say. West Coast rivers like the Whataroa have a CV of 1.5, despite being partly snow fed and having dense native forest cover. West Coast rivers have massive rainfall and steep, 'hard' catchments. Some have snowfields and glaciers at their heads. What does that tell us? Huge floods, quick to rise, and freezing cold.

Look at the flow graphs for two rivers as rain arrives in the South Island. The Buller, a very big river, which is fed at this gauge point by Nelson Lakes National Park, rises slowly from 40 cumecs to 90 cumecs, and then slowly drops away until the next front arrives.

[image:1225 size=original nolink=1]

The Aorere, which has a smaller catchment in the western ranges of Nelson, rises from its normal 12+ cumecs to 750 cumecs, which is quite a flood in this river. By the following day the Aorere has dropped back to normal. The 16th of July was a SE storm and the 4th of August was a northerly, which explains why the rivers behaved differently from each other. Knowing the river and keeping your eye on the flow gauge builds up your knowledge base, and knowledge is power.

[image:1226 size=original nolink=1]

What's my advice? Know your river. In the absence of personal references, ask for its CV.

Trees, wires, pipes, car bodies, hand rails, supermarket trollies, fences, all artfully concealed in cold brown water. Often forgotten is that flooded rivers contain quite a lot of stuff that isn't water; mud, stones, boulders, trees and branches.

Flooded rivers are colder, dirtier, faster and are harder to get out of if you're swimming. There are no nice friendly eddies, just swirling evil places with trees, fences and debris in them. If you come out of your boat in a flooded river, you are at extreme risk. Floating objects keep getting pushed to the centre of the flow making it nigh impossible to self-rescue. This is when you need companions who are up to the task of getting to you and getting you back in your boat in the same conditions that caused you to go for a swim.

Whitewater accident statistics from all over the world make sobering reading. The three top killers are - took a long swim in high water; foot entrapment; caught in keeper hydraulic. In the case of the high water deaths, the cause was either hypothermia or injuries received during a long swim. In many cases the swimmer deaths were preventable if there had been capable rescuers available downstream of the capsize.

Urban rivers present a special set of dangers. Nasty bits of steel stick out from bridges, and there are weirs, fences, trees and even park benches that can become deathtraps once you add water. Plus it's a public place, so please remember the copycat factor. Would you like to have some kid on an inner tube think it's OK to do what you're doing? It happens all the time.

[image:1227 size=original nolink=1]
This rise in flow is typical of New Zealand's rain fed rivers. (For USA visitors - 1150 cubic metres per second is over 40,000 cfs)

[image:1228 size=original nolink=1]
The Aorere River at 20 cumecs. Two kayakers drowned at this spot in a flood some years ago.

Even easy rivers demand respect when they are flooded. It is quite common to understimate the dangers because the level of paddling ability required is not high, but there are other dangers not present in normal flows.

When the clouds lie on the mountains
And the rain begins to fly
Keep your eye upon the river
Or you'll kiss your ass goodbye.

Five Point River Checklist

  1. Rescue. Have we all got the skills and the right gear to look after one another?
  2. I. Am I up to this? Who am I paddling with? Have I seen them under stress?
  3. Volume. What's this rain doing to this river? Rising or falling? By how much?
  4. Escape. Where from and where to?
  5. River. Where are the crunch spots going to be?


District councils throughout New Zealand have flow phones where you can call in and get a voice message giving flow rate, rising or falling, mean flow etc. Look in your phone book under the local council. The data is oriented towards fishers, but is one of the most useful pretrip checks you can make. The Canterbury Regional Council and the Tasman District Council have excellent websites with flow graphs and rainfall data.

River conservation and flow data

After a few trips you will be able to relate the scientific data to your experience on the river. Keep a logbook and include the actual flow rate. This data has important conservation value when the NZRCA goes to bat in tribunals and hearings, where we are fighting for our remaining wild rivers. Flow rate versus the canoeing experience is especially relevant in water rights negotiations, eg. Tekapo, Pukaki, Tongariro, Waikato, Rangitaiki, Whanganui, Whakapapa, Mangahao.

A little secret to reward those who have read all this stuff

At 500 to 700 cumecs a surfing wave of stunning perfection, alarming size but friendly disposition develops below Gunslinger rapid on the Lyell Creek section of the Buller River.

A warning

The Kawarau River below Nevis Bluff is a lot more dangerous than it looks when the flow rises above 400 cumecs. Citroen rapid, Retrospect, Natural Bridge (lethal) and even the Roaring Meg section have permanently altered the mental state of a number of top paddlers. There is a fine line between a fun mystery move and a possibly fatal outcome. A wet exit here in high flows is no laughing matter.