All about floods

• What is a flood?
• How flooding happens
• Why flooding happens
• Why do some places flood more than others?
• Working out flood probabilities
• Will flooding happen more often in future?
• Economic effects of flooding
• Social effects of flooding
• Environmental impacts of flooding
• Why things go wrong

What is a flood?

A flood is water that flows or pools on land where it is not usually found. This water can be fast flowing and deep, it can be swift and cause property and people to be washed away. It can cause water to pond on land, sitting for hours or even days causing the crops and land beneath to be damaged. Floods can pick up and carry objects (debris) and then drop these objects further downstream, causing damage as they pass by and leaving a big mess to be cleaned up when the flood passes. Flooding is a fact of life. Floods created the fertile flats which we have taken advantage of to settle on and to develop agricultural and horticultural industries.

How flooding happens

Most commonly flooding is caused when more rainfall falls than the land and waterways can contain, or the failure of an embankment holding large amounts of water behind it. This can cause water to puddle and pool on the lands surface land. Streams and rivers will swell as more water flows into them, if there is enough water they can spill out of their usual channels. Lakes will rise, and flooding can occur around their edges.

Why flooding happens

All soils have tiny spaces (pores) between the soil particles, these are usually filled with air, however when it rains these spaces get filled with water.
All soils have different amounts of water they can hold in their pore space, once these pore spaces are filled with water any more water falling onto the soil will either flow over the land and into a stream or waterway, or it will pond.
Hard surfaces like concrete, steel and asphalt have no pore spaces, this means that none of the water that falls on these surfaces can be absorbed, and we must have drainage systems to take the water away safely.      

Why do some places flood more than others?

There are a number of things that can make a place flood more often, these include;

• The ability of the soil or drainage system to hold the water, how much can it hold before the water pools on the surface?
• Does the water have somewhere to go once the soil cannot contain any more? Does it flow way into a waterway or does it create a pool of water where it is?
• Is it a place where the water from a lake or waterway will overflow onto when it the amount of water increases? Some areas are designed to give rivers and streams space to spread out when they have extra water, these can be man-made (flood ways, ponding basins), or natural, (wetlands, swamps, floodplains) these locations can flood regularly.
• Is it a place that has a high amount of rainfall each year? Are storms common? 

Working out flood probabilities

Have you heard people talk about a 100yr flood, or a 50yr flood? Ever wondered what that means? And how do they work it out?
Say you have a bag of 50 balls, 49 of them are blue and one is yellow. If you pick one out you have a 1 in 50 chance of taking the yellow ball. It is the same with flood probabilities. However if you get a 50yr flood this year it does not change the probability of getting a 50yr flood next year, to represent this with the balls you need to put the ball back that was taken out and only then take another, again you have a 1 in 50 chance of taking the blue ball out.
A 100yr flood has a one in one hundred (1%) chance of happening in any year and a 20yr flood has a 1 in 20 (5%) chance of happening in any year.
Regional councils collect data from the rivers and streams that are significant to their area. The data is most commonly  recorded in the form of a river level, this is then converted to river flow. [Include a diagrams of info on how this is done].The data is stored to create a record of river level and flow.
This data is then analyzed and used to set river flow probabilities. This is used to help predict how large a floods of certain size is, e.g. the 100yr flood.
There are several methods we can use to predict this information if the stream or river has no or not enough reliable recorded data.

The following examples may help put risks of flooding of the Whakatane Waimana floodplains into context:

Will flooding happen more often in future?

The Bay of Plenty region has experienced significant flooding in the past and will continue to experience flooding into the future, with the risk predicted to slowly increase over time due to climate change factors. Climate change researchers are predicting a possible increase in the frequency and intensity of extreme weather events, with the biggest threat coming from heavy rainfall and the resulting floods. This means the amount and magnitude of protection required to protect the public from large flood events will increase, as will the effects of a failure of protection works. People of the Bay of Plenty need to be aware that flooding is an issue that can and will affect us, and there is a risk that protection works may fail, there are things we can all do to minimise the effects of future flood events and become more resilient communities.  

  Factors that influence the frequency of flooding in the Bay of Plenty

• El Nino Southern Oscillation (ENSO)
• Inter-decadal Pacific Oscillation (IPO)
• Climate Change

 Click to link for more info on Climate change

Economic effects of flooding

Depending on the location and severity of a flood there can be a wide range of economic impacts.
The cost of repairing our communities following a flood event can be millions of dollars. The cost includes
• repair or replacement of damaged property and goods
• the loss of income during the event and time taken to clean up
• restoration and repair to flood protection schemes and town infrastructure
• providing emergency and temporary shelter and care for evacuees

Rural

When flooding occurs in a rural area there can be costly damages to agriculture and livestock including:

• Loss of crops
• Loss of livestock
• Damage to grazing land
• Damage to fences and gates
• Damage to houses, barns, and other rural structures

Urban

When Flooding occurs in an urban environment it can be very damaging to residences and commercial buildings. Potential impacts include:

• Damage/destruction of homes and buildings
• Damage to landscape features (gardens, lawns, etc)
• Damage to civil infrastructure (water, sewage and gas pipelines, stormwater systems, roads, power lines, etc)
• Disruption and damage to utilities and public services such as:
  o Transportation (roads and public transport services)
  o Potable drinking water
  o Wastewater treatment facilities
  o Natural gas
  o Power generation and delivery     
  o Emergency services

In most cases, regardless of where a flood occurs, the clean-up costs people money. Often areas that take a long time to recover from the effect of flooding will also experience a decrease in tourism and spending on leisure activities during this recovery time in the area.

Social effects of flooding

There are many impacts of flooding that cannot be measure with money. Flood waters can cause damage to water and sewage supplies to be cut off, potentially causing health issues to many people. People can suffer loss and grief through loosing irreplaceable possessions such as heirlooms, photos and pets. Some may experience stress and trauma through having to evacuate their homes due to rising flood waters.
The social impacts of flooding include:

• Injuries
• Fatalities (most commonly drowning)
• Stress both mental and physical, resulting from damaged property, disrupted plans and lives, and loss of family or friends 
• Displacement of people resulting from water damage to homes and infrustructure areas
• Increased levels of disease and gastrointestinal illness due to contaminated water supplies and over loading of storm and wastewater systems.

Environmental impacts of flooding

There are many positive and negative environmental effects of flooding. However in areas that are densely populated by humans the natural processes that flooding play a key role, in can be compromised, prevented, or undervalued.

Positive Effects

Floodwaters will usually carry with them a lot of sediment that is picked up from the river bed or washed in with runoff. As the water rises and waterways overflow their banks onto surrounding flood plains and flat land, the water velocity decreases and the sediment is deposited onto the ground. This sediment is often full of good nutrients that can help to boost the health of the land beneath it.
So while the flood may damage some vegetation or crops that are growing when the flood happens, the newly deposited sediment is critical for the ongoing health and productivity of low lying soils.  This occasional deposition of soils helps to raise the level of the land and balance out the effects of land compaction, shrinkage and subsidence. Over time as land is compacted and groundwater is extracted for various uses, land can begin to slowly sink. The regular deposition of sediment held in flood waters helps to keep floodplains at their natural level.

Negative Effects

Flood waters will often be very damaging to wetlands and vegetation in areas that are inundated either as a result of high water velocities or lack of oxygen for the plants due to prolonged submersion. The deposition of thick layers of sediment can also kill plants. High water velocities and high sediment loads can also be detrimental to aquatic ecosystems. Prolonged heavy rain can also cause or contribute to slips and the degradation of hill slopes and flood waters within a waterway can eat into the neighbouring land turning paddocks into riverbeds as rivers and stream find new paths. 

Why things go wrong

Here are some reasons, natural events and human errors.

Human errors include;

• Unlawful human activities (lowering stopbank heights, taking flood gates off culverts)
• Failure to maintain flood protection measures to a safe standard
• Pump levels are set wrong, or power is lost
• Infrastructure (houses and businesses) built in high risk zones

Natural events include;

• Floods that are larger than systems were designed to cope with
• The collapse of banks or land causing waterways to divert from their existing course
• Flood waters rise faster than systems can cope with 

Flood protection measures are planned and built with the best information, knowledge and materials available at the time, within the budget. As time passes, we learn more about how our local waterways behave and how effective different flood protection measures are, and new products become available. Installed flood protection assets will deteriorate or settle over time.