Dealing with sediment24 September 2003
Flushing sediment at Lake Roxburgh in New Zealand has helped mitigate flooding effects in nearby towns, as Ken Roberts and Peter Foster explain
IN 1996, the Electricity Corporation of New Zealand transferred its hydro generation and storage dams on the Clutha river to Contact Energy. Contact Energy has been required to apply under New Zealand’s Resource Management Act for resource consents to operate its dams and reservoirs for the next 35 years. Dealing with the historic and potential future effects of sedimentation in the Clutha river has been an important issue in this process.
In 1956, the Roxburgh dam was constructed on the Clutha river in New Zealand to provide hydroelectricity. Lake Roxburgh, the reservoir created by Roxburgh dam, has an operating range of 1.85m. It is a long narrow lake with an approximate length of 30km and a width varying from typically 150m to 300m. In the late 1950’s a dam was also built upstream of Lake Roxburgh at the outlet of the natural Lake Hawea to raise the lake and provide water storage for use in the winter when natural river flows are generally lowest and the demand for electricity is highest. The present normal operating range of Lake Hawea is 8m. Lake Dunstan, which is formed by the Clyde dam and is situated between Lakes Roxburgh and Hawea, commenced filling in 1992. The operating range of Lake Dunstan is 1m. Figure 1 shows the Clutha catchment and the hydro dam locations.
In the period from 1956 to 1992, Lake Roxburgh was progressively filling with sediment, sourced mainly from the Shotover river. After 1992 this sediment began to be trapped in Lake Dunstan.
The survey says...
Lake bed surveys commenced in Lake Roxburgh in 1961. Repeat surveys have been carried out at various intervals through to 2000. The data indicates that prior to the filling of Lake Dunstan, some 51Mm3 of sediment accumulated in Lake Roxburgh, at an average rate of 1.42Mm3 per year. This represented a 44% loss of reservoir volume. Figure 2 shows the thalweg level of the lake bed in 1961 and 1993. The sediment does not have any effect on power generation, but it does affect flood water levels at Alexandra township, which is located on the upstream shores of Lake Roxburgh. Under normal flows and operating conditions the water level at Roxburgh dam is up to elevation 132m, and water levels at Alexandra are typically about elevation 132.5m. Under flood conditions the upper reach of Lake Roxburgh develops a more riverine characteristic due to the narrowness of the reservoir and the higher flow velocities. This causes water levels to rise at Alexandra.
The mean flow into Lake Roxburgh is 510m3/sec. In 1957, shortly after Lake Roxburgh was filled a flood of 2570m3/sec produced a water level of 136.68m at Alexandra, and in 1994 a lesser flood of 2340m3/sec produced a higher water level of 140.03m. The effect of the sediment accumulation had produced a water level rise under flood conditions of about 3.9m from 1957 to 1994 for an equivalent flow of 2570m3/sec.
In 1994 a programme commenced of partially drawing down Lake Roxburgh, by up to 6m, in advance of floods. The purpose of this is to re-distribute the sediment within Lake Roxburgh and this redistribution of sediment, combined with the effects of drawing the lake down, reduces the water levels at Alexandra. In 1995 and 1999 major floods occurred with the hourly peak flows into Lake Roxburgh of 3212m3/sec and 3620m3/sec respectively. Figure 3 shows Lake Roxburgh dam partially drawn down during flood conditions and with the spillway gates fully open.
The 1995 and 1999 floods were the two largest floods in the river since 1878. The 1999 flood level reached elevation 142.25m at Alexandra and caused the flooding of some properties within the town. If the partial drawdown procedures had not started in 1994 the flood levels would have been 1.5m higher than they were.
The thalweg bed profile in Lake Roxburgh subsequent to the 1999 flood is indicated in Figure 4. Relative to 1994, some 11.5Mm3 of sediment has been moved, with 6.3Mm3 being re-distributed downstream within the reservoir and 5.2Mm3 being flushed out of the reservoir. The net effect of partial reservoir drawdown and sediment re-distribution has been a reduction in flood levels at Alexandra by 1.7m compared to the 1994 situation.
Other actions taken to mitigate the effects on flood levels due to the sediment in Lake Roxburgh include shutting down outflows from Lake Hawea to the minimum allowable discharge prior to peak flows reaching Alexandra. Lake Hawea is the only controlled lake upstream of Alexandra that has any significant flood storage capacity. This action reduces flood peaks by about 200m3/sec. Furthermore, in 2000 the Government and Contact Energy funded a flood mitigation package for Alexandra that included construction of flood barriers to a level of 143.25m, which is 1m higher than water levels in the 1999 flood. The mitigation package also allowed for the purchase of some properties and the purchase of flood easements over other properties.
Following implementation of the mitigation package, it is now possible – with partial drawdown of Lake Roxburgh and reducing outflows from Lake Hawea during flood peaks – to provide a very similar level of flow capacity protection at Alexandra to that which existed prior to the construction of Roxburgh dam in 1956. Sediment transport and hydraulic modelling indicate that
continuation of partial drawdown during floods will move more sediment and can reduce flood levels by a further metre in about 10 years, depending on flood sizes. Such a situation can be
maintained while Lake Dunstan is accumulating sediment upstream of Lake Roxburgh.
Lake bed surveys in Lake Dunstan indicate that from
1994 through to December 1999 some 5.7Mm3 of sediment had accumulated in the arm of the lake being fed with sediment from
the Shotover river. The 1999 flood alone contributed 2.8Mm3
of sediment, which compares to an expected long-term rate of 1.3Mm3 per year.
The volume of Lake Dunstan is such that there is in excess of 100 years sediment storage available in the lake. Land was purchased around the lake shore at the time of construction of Clyde dam to mitigate against the effects of flood level rises as the sediment
tipping face advances down the lake. Lake Dunstan has a normal operating range of 1m and therefore the sediment accumulation is not a restriction on the run-of-river hydro power generation. It is possible that once the sediment storage volume remaining in the lake is significantly reduced, a sediment flushing regime similar
to that at Roxburgh may be considered. If undertaken during
floods and in tandem with Lake Roxburgh partial drawdown,
the finer sediment flushed out of Lake Dunstan should also pass through Lake Roxburgh and then down the remainder of the Clutha river to the sea.
In 2002/2003 Contact Energy applied for resource consents to operate its hydro stations and Lakes Roxburgh, Dunstan and Hawea over the next 35 years. Sediment management was an important issue in this process. Some communities and river users downstream of Roxburgh dam have become accustomed to a river that carries significantly less sediment than it did prior to 1956. They would like to see these conditions maintained. Others would like to see a return of the pre-dam situation. Some communities upstream of Roxburgh dam and recreational users of Lakes Roxburgh and Dunstan would like to see less sediment being captured by the lakes to improve the recreational amenities and enhanced protection against floodwaters. The Commissioner’s decision determining the balance between the various competing considerations was awaited at the date of preparing this article.