The red river of Town Bush Valley
One morning, some years ago, residents living close to Town Bush Stream, awoke to a roaring sound coming from nearby. On investigating the sound they were shocked to find a river of tumbling, swirling red water flooding over the banks of the normally docile little stream that flowed past the verges of their gardens. It had rained heavily the previous day and right through the night and now the Town Bush River was in full flood. The rich, red torrent of water raged down the watercourse, overflowing and ripping at its banks. Where the stream bed became constricted, and particularly at bends in the course of the stream, the flood surged across its normally dry verges. Vegetation was uprooted, garden walls were flattened, and some houses were flooded. Later, when the flood subsided, home owners took stock of the situation and many questions were asked.
In the first place, why are these damaging floods experienced every few years? That question is fairly easy to answer. The Town Bush River catchment lies in a high rainfall area and is very steep. When heavy storms occur, run off water concentrates very rapidly in the narrow valley through which the river runs. As a result the time of concentration of runoff water is very high, and the flood water of the many minor catchments meet up in the valley bottom within a short space of time. The N3 highway makes a major contribution by carrying water down the hill from the Hilton overpass and releasing it at certain points in the catchment.
Adding to the problem the riverine vegetation which once stabilized the river banks has been destroyed. Now the river bed and its banks are infested with alien weeds which do a poor job of holding the soil and restrict the flow of water causing flood water to overflow the river banks. The fact that many houses have been built within the high flow area of floods results in damage to houses and gardens. Today the Municipality will not sanction the building of houses on ground that falls within these potential flood areas.
The second question which has frequently been asked by residents is the cause of the heavy red load of sediment that the river carries. This is often observed in the artificial river by-pass at Cascades Shopping Centre. When this first came to the attention of the Ferncliffe Catchment Conservancy committee, the chairman at the time, Dr Derek Scotney, undertook to investigate the situation. Several causes became apparent.
One of the causes is the sediment load caused by stream bank erosion during floods. Another problem comes from N3 highway where runoff is concentrated and then released on very steep slopes of highly erodible soil. At one site in particular the flood water has eroded a gully which is 2 m deep in places. The erosion from this gully must deposit eroded soil into the river. Yet another source of erosion is from recently felled plantations, and particularly where heavy machinery has been operating. Mountain bike riders have found that the wash of eroded soil across plantation roads has made them impassable at times. Normally the tree canopy and trash lying below the trees allows very little erosion to take place.
In recent times another source of erosion has occurred in the plantations. Motor bikes and quad bike riders have discovered the thrill of riding straight down hill between the rows of trees. This frequent use has cleared the trash and eaten into the soil causing furrows which carry runoff water and eroded soil down to the river.
A major source of red sediment was discovered during Dr Scotney's investigation which explained why the river would suddenly run red without storms being experienced in the lower catchment. Well up in the catchment, and close to the N3, the river has eaten out a deep valley over many, many years. In this area on the N3 side of the river, there is a bare, vertical bank which is estimated to be 15 metres in height. Parallel to the lip of this face is a crack into which water must run during storms. This saturates the soil of the vertical face, and often with the weight of gum trees growing on the edge of the bank, large chunks of soil peel off the face and crash into the river. The result is the turgid red sediment that we frequently see flowing down the river. In another article the effects that this heavy sediment load has on the aquatic fauna and flora of the river will be discussed.
The manager of the land in which this "Slip" is situated is doing everything in his power to overcome the problem, initially by felling all the gum trees growing between the slip and the N3 highway. Of course the N3 authorities should be concerned about this as it is not inconceivable that the bank will one day erode back to the highway. Most people are well aware of the sink holes and erosion slips that have occurred along N3 in the Town Hill area in the past.
While floods will always be a feature of the catchment and will even be exacerbated by the increased runoff from new developments and tarred roads, many strategies can be followed to improve the threats of flood damage. This will take careful planning by the authorities and land owners in the catchment.