Dealing with Increasing Intensity of Winter Storms.
Climate change predictions indicate that while we are experiencing a period of decreasing annual rainfall, individual rain storms are likely to become more intense. When rain falls at intensities greater than the rate at which the water can infiltrate into the soil, runoff results. Bare or poorly vegetated areas will generally have lower infiltration rates than well vegetated areas, partly because of the pummelling and compacting action of rain drops hitting the soil surface. One millimetre of rain falling on one hectare (100m x 100m) would weigh 10 tonnes. So a rain storm of 20mm represents 200 tonnes of water per hectare hitting the ground at around 35 km per hour (9.81 metres per second) – quite an impact. No wonder bare soil becomes compacted and loses its infiltration capacity.
Added to these natural effects, the impervious areas that shed water (such as roofs and paved areas) are increasing in the Shire.
Risk management strategies need to incorporate plans for dealing with the estimated climate changes. For example:
an increase in extreme daily rainfall intensity needs appropriate systems and structures to accommodate the runoff,
a decrease in mean annual rainfall will lead to the need to develop alternative supplies of potable water,
a combination of increased temperature and decreased rainfall will affect available habitat - and lead to higher dam evaporation rates.
But wait a minute! We are receiving water at some times of the year in such abundance that it becomes a problem, and at other times of the year we are short of water, and all the while the cost of scheme water is rising. It’s time we had a change in mindset and started to look upon runoff water as a resource rather than a problem.
An increase in household rain water tanks would be an obvious first step.
Our hilly terrain introduces special challenges for handling runoff without it causing erosion. Runoff velocity affects its ability to cause erosion and carve out drainage channels. As a rule (affected to an extent by soil types) runoff velocity should be kept below 1 metre per second, and runoff structures and channels should be designed to reflect this. Deeper runoff has more momentum and velocity than shallow water. So, shallower, wider channels may be required to maintain an acceptable runoff velocity.
From the point of view of erosion control, bare soil areas should be minimised. Managing pastures to maintain around 30% soil cover at the break of the season will assist in reducing runoff and the loss of soil and plant nutrients. Cultivating on the contour (on suitable slopes up to around 12% slope) will help to increase infiltration and decrease runoff.
Directing runoff into dams turns the runoff from being a problem into a resource. On farms this is achieved by installing grade banks that end at a dam or in a waterway that leads to a dam. In specific instances there may be opportunities to turn roadside runoff into dams, helping to replenish them at the beginning of the winter rainfall season.
Runoff from intense storms flowing into dams that are already full creates a new set of problems. Runoff flowing into the back of a dam will tend to surge towards the dam wall. Without sufficient bank height between the water surface when the dam is full and the top of the dam wall (referred to as ‘freeboard’), a surge of runoff water could overtop the dam wall, start to erode that front wall and ultimately cause dam wall failure, and severe damage to anything in the path of water from a bursting dam. As another general rule, the freeboard on dams should be around 1 metre. The amount of freeboard is set by the level of the spillway at which excess dam water starts to drain out of the dam. The difference in elevation between the spillway and the lowest part of the dam wall should be around one metre. Livestock watering on a dam can compact and reduce the height of the dam wall and puddle and partially block the spillway. Good managers will inspect and clean out spillways if necessary at the beginning of the winter rain season.
The Difference Between Climate and Weather.
From the Shire’s Sustainability Advisory Committee.
This has been one of the wettest years in recent times. In contrast, 2010 was one of the driest years on record, if not the driest. This demonstrates the vagaries of weather (variable atmospheric conditions of temperature, rainfall, humidity and wind). So, how should we predict what next year is most likely to produce? For this we need to put together all the records from all the years of records to find the average (the arithmetic mean of all the recordings over the years) or, often more useful, the modal value (the conditions we receive most often). We call this long term view ‘climate’. Each new year of records is added to all others to improve our knowledge of the mean and modal values of our climate. The longer the string of annual records, the more confidence we can have in our picture of climate. Here’s a graph of the Bridgetown annual rainfall between 1888 and 2006. It is quite difficult to discern any pattern from that. But if we ‘smooth’ the data by using a technique called the ‘moving ten year average’ a clearer picture starts to emerge. This graph is made by finding the average of years 1 to 10, and then the average of years 2 to 11 and so on.
What people noticed was that the average annual rainfall had started to decrease from the 1950s. Effectively, the long-term climate was starting to change. Similar effects were noticed in other parts of South west WA. In Perth a step down in the level of Mundaring Weir was noticed from 1975 on. A comparison of the long-term modal rainfall for Bridgetown (1888-2006) with the modal rainfall we have received since 1975 (1975-2006) is shown in Figure 3. Remember that modal rainfall is the rainfall we receive most often. The first [blue] column for each month is the long-term modal value; the second [red] columns are the modal rainfall since 1975. The biggest single difference is the modal rainfall for May, which has more than halved. October is drier; November is wetter, and there is slightly more rain in January and March. The change in May has implications for water storage for watering livestock, irrigating gardens and fruit trees and for storage for domestic uses.
There is now a need to store more water for longer - until the more reliable June rainfall arrives.
In conclusion, the weather we receive from day to day and year to year is quite variable, but from the weather records we can discern patterns that we might call ‘climate’. However, those records also show us that climate is changing and getting drier. This will have an effect on water supplies. It’s not time to start to be worried, but it is time to start to be concerned.