Hydrology 48

Aside from the geological history, landscapes continue to evolve geomorphologically through predominantly fluvial processes, with water being the key element, particularly on the Central Plateau. All of the above mentioned soil profiles are impacted by the harsh freeze-thaw pattern of the Plateau, in particular the process of needle ice formation, also referred to as frost heave (Storey and Comfort, 2007). The ice forms beneath the soil surface from frozen water lifting the crust and material frozen to it as the soil surface freezes. Needle ice is capable of lifting a 5 kg rock 1 cm off the ground surface (Storey and Comfort, 2007). While this may not sound significant, this can have severe implications for rehabilitation efforts, such as sites around the Lake Augusta lunettes.

Although a fine scale impact, processes of frost heave can impact areas on a catchment scale due, especially given the altitude of the Plateau and the extent of bare ground. However, rivers too exert a catchment influence, particularly relative to their drainage patterns and headwater storage capacity. Lakes or peat land in particular, can act as sponges soaking up flows, storing the water for slower and longer discharging periods (Jerie et al. 2003). This will have an impact on the erosive potential and bedload carrying capacity of a given river. Gordon et al., (2004) identify catchment area as one of the more important basin descriptors as it influences the number and size of each stream along with the potential water yield. In order to expand on the catchment hydrology relative to the study area, a watershed model was created for three sites that define the sub-catchment area (the whole study site occurs within the upper Ouse

Catchment) for the current study (Figure 3-5). These are the James River catchment, the Ouse River reference catchment and the Ouse River study catchment. The watershed delineation in Table 3-1 shows the sub-catchment area and water body extent. The sub-catchments indicate that they represent good paired study catchments due to their similarities in catchment size (Table 3-1).

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Figure 3-5 Water-body extent and drainage network for each of the sub-catchments

50 | P a g e

Table 3-1 Sub-catchment and water body size within the study area

Sub-catchment Catchment area (km2_{) Water body Area (km}2₎

James River 81.518 8.688

Ouse River Reference 79.649 4.408

Ouse River Regulated 104.488 12.351

As can be seen from Table 3-1 there are some similarities between the sub-catchments. The James River sub-catchment at 81.5 km2 _{is almost exactly the same size as the Ouse River}

reference sub-catchment size of 79.75 km2_{. The Ouse River regulated sub-catchment is slightly}

larger with nearly 105 km2 of surface drainage. Interestingly, the water body area of the two reference sub-catchments added together is almost the same as the study sub-catchment area, largely due to Lake Augusta. It is also note-worthy to highlight the water body difference in Table 3-1 between the reference sites with the James River catchment having almost double the storage capacity in tarns and larger water bodies to that of the Ouse reference sub-catchment. This aspect further strengthens the evidence of the glacial limits that extended over the James River catchment but failed to reach the lower parts of the Ouse River catchment.

Flows downstream of Lake Augusta are controlled by Augusta Dam. The dam was constructed between January 1951 and May 1953, the latter including the period of initial filling. The dam is a 13 m rock lined dam with a sloping clay core and a concrete gravity wall crest that discharges water downstream via twin 1.52 m square reinforced concrete conduits. Water is controlled by upstream still-water valves and control valves on the downstream side (Hydro Tasmania, 2005b). The full supply level (FSL) of the reservoir is 150.62 m ASL and the nominal minimum operating level (NMOL) is at 1141.63 m ASL. The storage operating rules recommend the lake be maintained at no less than 1146.63 m ASL (Hydro Tasmania, 2005c). Lake Augusta holds 35 discharge days of water, meaning that with no inflows, water can continuously be discharged for downstream uses for 35 days (Greg Carson, pers. comm.., 2010). The crest of the dam is at 1152.98 m ASL, however a separate concrete gravity wall spillway exists 500 m-1 km further to the west. The spillway crest level is FSL (1150.62 m ASL) and beyond this level flows spill, flowing downstream over the access road and overland down to the Ouse River – regulated reaches (Hydro Tasmania, 2005b). There is no defined channel natural or otherwise, the water flowing over the spillway simply takes the most direct route down to the Ouse River. The upper section of the spillway has a semi-defined channel, while closer to the Ouse River it is difficult to distinguish where flows enter as the water appears to spread and have several flow paths joining the Ouse River.

51 | P a g e The Ouse River below Augusta Dam forms part of a myriad of diversion channels, canals, catchment transfers and inter-basin transfers (Figure 3-6). Water is primarily used for the generation of hydro-electricity; however, water is also allocated for other uses each year including irrigation (9,718 ML), stock and domestic (64 ML), water supply (69 ML) and other uses (107 ML) within the catchment (DPIW, 2006). The primary off-take in both distance and water volume is Liawenee Canal, situated ~9 km downstream of Augusta Dam. The canal transfers water between Lake Augusta and Great Lake, primarily for hydro-electricity

generation. There is no consideration given in the Storage Operating Rules (Hydro Tasmania, 2005c) as to how water is transferred between these storages (e.g. discharge volume per day or flow velocities), the river is essentially utilised as a conduit for water based on lake level (Mundoview, 2007).

Figure 3-6 Ouse River and Neighbouring Catchments showing the network of Water Transfers and Storages (Cox and Graham, (2006)