Northern Victorian Flood Review Part II
This post examines the hydrological influences on past and present flooding leading to the January 2011 floods in northern Victoria. The floods themselves are discussed n Part I here, and affects on the family farm in Kerang are shown here. Part III on climatic influences is here.
Local data used are daily flows at Laanecoorie Reservoir on the mid Loddon River, and daily rainfall from the Newstead and Cairn Curran rain gauges upstream. Hydrological issues concern long-term catchment change and flow data. These affect the assessment of historical floods on the Loddon River.
If there is no real ability to model the likely affect of floodwaters on a floodplain, planning will focus on past floods. This is problematic because catchments have been modified over time for flood mitigation, irrigation management, drainage and the protection of property. The following affects the Loddon River in particular but will be relevant to all the rivers affected by flooding last summer.
The flood of record on the Loddon River is August 1909. A flash flood led to the dam at Laanecoorie giving way, washing out the gauge, so the level and peak flow was estimated. The peak downstream at Kerang was higher than the second largest flood on record in spring 1975 – reflecting the amount of water coming from the dam failure. The flood hydrograph is dominated by this peak but the preceding rainfall is unexceptional. Twenty years ago I made a submission to the Kerang flood study suggesting that the runoff co-efficient assumed for the 1909 flood was unrealistic, its peak was influenced not by rainfall but by dam failure and that 1975 should be used as the flood of record as is the case for most other rivers in the region. This advice was ignored.
The catchment has also changed over time. In the past, the catchment was flashier than it is today. That is, rainfall very quickly became runoff and the resulting floodwater travelled very quickly down the river in high peaks that passed very quickly. This is due to two factors: gold mining, and water storages and river management. Gold mining led to many of the upper catchments in central Victoria being cleared for firewood to run stamping mills to crush the gold ore. Firewood collection for towns and bakeries especially was also a factor. These led to many of the box-ironbark and wetter forests in the upper catchments being cleared. Subsequent recovery after gold mining declined and electricity replaced wood fuel led to the forest recovery. Catchment management to prevent erosion via the Soil Conservation Authority also slowed runoff from cleared slopes in the mid catchment.
Water storages have allowed flow regulation; full-scale flooding does not occur until the storages are full and subsequent runoff has to be released. Even then, storages do slow down floodwaters. There are two reservoirs of significance on the Loddon: Cairn Curran and Laanecoorie. Farm dams on small tributaries in the mid catchments will also slow down runoff, preventing levels of low flow that would otherwise occur.
These changes over time can be illustrated by the August 1909, September–October 1916, December 1933 and October 1975 floods. Shown are daily flows at Laanecoorie and rainfall at Newstead. Using a single station provides a guide rather than full analysis which would require total rainfall on the upper catchment. The 1909 floods clearly stand out as having a peak on the two vertical scales: flow on the left and rainfall on the right. 1916 has a greater rainfall event with lower flows. 1933 was a big flood occurring on a wet catchment. The 1975 event is the largest modern flood and was the best guide to what could happen in terms of flood behaviour. Later, lesser floods such as those in 1981 and 1993 also provided experience for locals and professional managers. These graphs also show that rainfall and flow occurred on the same day in the earlier floods but by 1975 there was a day’s delay between rain events and peak flow. This is evidence of a healthier catchment but greater regulation will also be having an effect.
Floods further downstream have become even less peaked than at Laanecoorie in the mid catchment. Big floods are now large slow-moving bodies of water that move down the main trunk stream. The catchments have different flood behaviour in their upper-mid and lower reaches. Under natural conditions when water reached the floodplains it would move into prior (ancient) streamlines and anabranches (stream that flow out and back into a river) across the plains. The whole mid Murray River system in northern Victoria and southern NSW behaves like this. In Victoria this behaviour can be seen in a whole system of surface- and groundwater-controlled stream and lakes. In NSW, large wetlands have been formed along major rivers such as the Lachlan and Murrumbidgee.
In the modern systems, most of the floodplain away from the main streams is now farmland protected from flooding. In NSW, many of the elongated wetlands are farmed and cropped. Much of the recent expansion in these wetlands has taken place since the 1970s using low security irrigation water and is one reason why the Murray Darling Basin is over-allocated and unsustainable. This expansion is still taking place further northon the Darling River.
This pattern of understanding then suggests we have a typology of floods along the river systems of the Murray Basin:
- Small floods within single streamlines that are largely beneficial.
- Moderate floods in single streamlines that require some level of protection for adjoining property (if that is current policy).
- Large floods that affect multiple rivers and want to join across the floodplain utilising prior streams, anabranches and lakes.
The floods in January 2011 were clearly of type 3. Type 3 floods though, are largely being managed within single river systems and are informed by past records of flood heights. These, as I have already pointed out, are suspect for the first half of the 20th century. We can learn a lot from rainfall events during those periods, but less from the flooding behaviour. While the response to the recent floods was massive they could have been better managed. Understanding the climatic influences on major floods is a key part of this better management.
This has turned into at least a three-part posting. In Part III, I will cover the climatic influences that lead to the third type of flooding and link it to the recent floods.