Past Esperance floods highlight the effects of erosion and sedimentation occurring in south coast waterways.
In the three days from Tuesday 5th to Thursday 7th of January, 1999, the town of Esperance received 213 millimetres of rainfall compared with the average January rainfall of 17 mm. and the average annual rainfall of 670 mm. The rainfall was widespread around Esperance Shire, but extreme only in the immediate vicinity of the town (50 kilometre radius). Preliminary analysis by the Department of Water suggested that the average expected period of recurrence of this event was approximately 300 years.
Local flooding occurred in Esperance town and many roads in the shire suffered varying degrees of damage, with the most significant being the bridge abutment failure of the South Coast Highway on both branches of the Dalyup River. The Gibson Road crossing at the Coramup Creek was also extensively damaged.
The Department of Water operates several stream gauging stations in the Esperance region on rivers in the catchments of the Stokes Inlet and Lake Warden Wetland system. The Coramup, Bandy, Neridup and Melijinup Creeks are gauged, although they have only been functioning since 1998. This information was used to analyse the features of the flood event.
The impact of the flooding on the local catchments has provided a benchmark for assessing the robustness of south coast waterways protection and enhancement methods. The outcomes in the small catchments affected in the Esperance area are relevant to catchments across the region as far as the Frankland-Gordon Rivers.
The overall impression of the impact of the flooding on the channel environment, given the extreme nature of the event, is that the existing channels adequately confined the bulk of the flow. The drainage efficiency of the channels is therefore quite high and the promotion of revegetation as a tool for floodplain and channel management is on track and unlikely to reduce general flood conveyance as some fear. The flood flow was focused through the middle reaches of the rivers and in these situations vegetation along the slope of the banks was uprooted or severely damaged, and banks looked as though a gigantic mechanical 'slasher' had moved down stream. It appeared that the bulk of the stream power was confined to the main channels, which is evidence that the defined channels are well incised. Broader flood plain areas appear prone to sediment deposition and channel braiding, yet the pre-existing channels managed to persist in many cases.
Many landowners lost fences and stream crossings. Several commented that the road and 'on farm' crossings that fared the best were those with the lowest profiles, ie. those most closely following the cross sectional perimeter of the channel. It was also observed that crossings on the 'straight' reaches of channels were more resilient than those placed on or near bends or in areas where the channel was braided. Many of the latter were completely obliterated.
Significant sediment mobilisation is a major threat to river pools and terminal wetlands or estuaries. The relative importance of the various possible sources of this sediment is uncertain and studies would need to be conducted to determine the contributions from sheet erosion off valley sides (apparently small amounts from large areas), bank collapse and channel scouring (large amounts from smaller erosion hotspots). Some studies in the United States suggest bank and bed erosion as major contributors, and this appeared to be the case for this flood.
The capacity of the river channels appears sufficient to adequately contain significant storm flows, at least up to a 1 in 200 years flood event. They also contain most of the power of the flood flows. During long periods of low flows, bankfull discharge maintains a smaller channel within the larger, but also allows vegetation to freely colonise the main incised channel thus reducing its carrying capacity. When the next extreme event occurs, the flow spills around this vegetation and where there is no resistance the channel widens rapidly. This process had also occurred on bends in some cases, resulting in severe scouring on the inside of the bend, an area usually subject to deposition. This phenomenon was observed in narrow valleys as well as the broader, level flood plains. Stabilisation of the toe, slope and top of banks is essential.
The central stream channels should be maintained at a sufficient degree of openness to reduce the risk of excess water banking up and spilling turbulently around the edges, scouring the banks and possibly creating a new main channel. Alternatively river bank revegetation should be more robust, however in this case flood flows will be pushed out onto the flood plain and the conveyance efficiency reduced. The situation is applicable to stream channels large and small. The concern that revegetation on the banks and floodplain impedes flood conveyance is probably unwarranted.
Fences suffered varying degrees of damage. The placement, orientation and type of fencing appropriate for different parts of the floodplain also needs to be more systematically investigated.
Secondary floodplain channels, even though largely dry, should be treated as rivers in their own right with protection works designed specifically to reduce the risk of excessive scouring without impeding floodplain flow.
Stream bank revegetation is a priority. Dense ground covers such as sedges and rushes are required to complement the tree species such as melaleucas.
Sedges and rushes appear to be very resilient to flood flows even when plants are isolated or occur in clumps separated by bare ground. These would be suitable for protecting the bed and lower parts of the channel bank.
Although some tree plantings suffered, others appeared robust enough to handle the flows. Isolated trees, particularly on the main channel banks, may be undermined by flow around their trunks. Tree roots are not sufficiently dense to hold the soil between them in the case of concentrated flow against the base. The surrounding soil surface should also be armoured. Tree roots have been shown to significantly reinforce the soil around them and this can serve to reduce the risk of slumping as flood flows ebb.
Since the main channel appears to take the brunt of the flow and therefore the stream power, the riparian vegetation away from the main channel serves to diffuse the over bank flow. The distribution and density of revegetation works should be such as to avoid focusing the flow. In the Dalyup and Coramup systems the flood conveyance would probably not have been dramatically reduced by more flood plain vegetation.