GriffithGateway.com - 2012 Floods

Griffith Floods

Hi, thought I'd keep you all posted about what's happening.

Out here in Nericon, 10km north of Griffith, we are in a different valley to the Murrumbidgee (different bioregion, in fact) and because we are on a hill, we are high, dry and safe.

Background

Griffith is classified semi desert and lies directly on the line which describes the 'mainly summer rainfall' of the north from the 'mainly winter rainfall' of the south. So rain can be equally expected at any time of the year, from the remnants of the tropical storms which wander south in summer, or the south westerly fronts which bring cold wet winter weather from off the Southern Ocean.

Griffith lies under Scenic Hill which is at the limit of both the South West Slopes to the east, and the Cobar Peneplain to the north. It looks out across vast flat river valleys which extend almost to Melbourne and Adelaide. These deep soil flats are the result of millions of years of deposited river gravels, sands and silts.

The Murrumbidgee River is 30km to the south. It has never threatened Griffith with flooding. Mirrool Creek begins around Temora and may not run for years at a time. In many places it is difficult to even tell the course of the creek as the typical 'creek line' seems to disappear in paddocks and reappear further on.

Beginning 1911, the system of supply and drainage channels were constructed throughout the Leeton and Griffith area, intertwined like the veins and arteries of our bodies, to create the Murrumbidgee Irrigation Area, a construction which has created vast wealth and security for the nation.

The Main Canal (boring name, eh!) carries water from the Murrumbidgee River near Narrandera, and supplies farms all the way from there to well west of Griffith.

In the Irrigation Area, Mirrool Creek has become a drainage canal for excess water off the irrigated farms and empties into Barren Box Swamp for reuse further out. Surprisingly, although it is in the Murrumbidgee River Valley, Mirrool Creek actually empties into the Lachlan River near Booligal, and this last occurred in 1974.

In its natural state Mirrool Creek would not flood the Griffith Area, but changes to the landscape over the last hundred years have had their effect.

The Rain

At the end of February and the beginning of March 2012 Griffith had two thirds of its annual average rainfall in one week, 250mm, with the final 150mm in one day. The land is flat and the water is slow to move on. There is a heavy reliance on the drainage system to carry water south and west to Mirrool Creek and Barren Box Swamp, but the rainfall far exceeded the capacity of the drains, and there was localised stormwater flooding in many areas - including the farmlands and the lower lying villages around Griffith, especially Yoogali and Hanwood.

While Main Drain J was running well, the backlog of water was vast.

At this time of year, it is normal to have a full river and full canals because farmers are watering before harvest of grapes, cotton, rice, soy beans, sorghum - you know the drill. So we have a full water system and then a significant rain event.

The Later Flooding

Just as Griffith scored record rainfall, so did the entire southern and western half of the state. And just as the local stormwater was abating, it became apparent the Mirrool Creek had awoken from a long slumber and was heading our way. Prior to farming settlement, this would have meant a harmless steady flowing across flat land, and a good drink for the parched flora and fauna. But now the creek hits man made obstructions.

That area under the red circle is where Mirrool Creek meets the Main Canal just east of Yenda, at the Yenda Regulator and is expected to pass underneath - those with a bit of Dutch heritage might better understand how this is possible. But the volume of water is too great and the Main Canal is acting as a dam with the Creek backing up and overflowing into the Main Canal and into the Northern Branch Canal, and threatening to break the banks of both. Realise that in most cases, these canals run higher than ground level, otherwise, how would the water run easily onto the crops?

The Northern is in turn overflowing towards the west, quickly flooding the triangular area marked in blue bounded by the Main Canal, the Northern Canal and the Sydney road and railway line

which includes Yenda village and vineyards, to a depth of about a metre. In this section is the Casella Winery (think Yellowtail) which has had to shut up shop mid vintage.

Too much for the drainage system to cope with, the water is continuing to move further west along the road toward Beelbangera and Griffith, constricted by the Main Canal and the railway, also flooding farmland to the north, and at the same time entering other drains all of which head back south to Main Drain J, which is then a second threat to the low lying villages of Yoogali, Bilbul, Beelbangera and Hanwood which have not yet been cleared of their own local flooding.

At this point the water movement is quite complicated and difficult to predict and hence there are flood alerts which may or may not come to a head. It is a slowing unfolding scenario because the land is so flat. After all, that's why they put the irrigation scheme here in the first place!

The bulk of the residential area of Griffith is on the higher ground around Scenic Hill, on the land that was considered less suitable for irrigation.

The Future

The Mirrool Creek / Main Canal crossroad was the cause of similar flooding in 1956 and so may be a site considered for redeveloping in the future. Or will we never have a similar flood again? Or will it happen again next year?