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In-stream and off-stream dams by catchment and bioregion. This indicator is also related to 'river structures'. Dam development (in- and off-stream) is listed as one of the 'drivers' of the environmental state of aquatic systems in Tasmania (Davies 2001). In-stream dams impact on aquatic health through disrupting the exchange of water and materials (e.g. energy flow, nutrient cycling, fish migrations). However, off-stream dams also have cumulative consequences for water quantity, aquatic health and estuarine health. A negative impact of farm dams is the reduction of water yields and run-off reaching rivers and streams and eventually the sea. This can impact negatively on river mouths by building up sand bars and blocking flow. This is occurring in the north-east and north-west of the State (DPIWE 2001).
Tasmania has a large amount of storage capacity in reservoirs. The storage capacity of non-hydro dams which includes mining, industrial and farm dams, is shown in the following table. Storage capacity of non-Hydro dams Size Number Total capacity > 5,000 ML 8 dams 108,255 1,000-5,000 ML 27 dams 57,680 <1,000 5,685 dams 128,505 Source: WIMS database, DPIWE The first SoE Report (SDAC 1997) noted that apart from two strong periods of growth-one in the mid-1960s, and the other in the early 1980s-there has been a general upwards trend in dam approvals since records have been kept. Since the last SoE report (from January 1998), some 907 dams have been approved, although the majority of these are listed as having a dam status as 'proposed' rather than 'existing'. As at September 2002, the WIMS database registered 5,516 existing dams (both in-stream and off-stream) for irrigation, stock, and stock and domestic uses combined. These dams have a total capacity of 159,586 ML representing about 55% of the registered existing non-hydro dam capacity in the State of 294,440 ML. The map provides an indication of non-Hydro dam density across Tasmania showing concentration in a band extending across the north as well as the east and south-east. The majority of dams registered are for irrigation and stock. The greatest dam densities occur in catchments of the north and north-west. The Inglis and Blythe catchments have farm dam densities of nearly one dam for every square kilometre.
Concentration of non-Hydro dams in planning and management catchments Catchment name Catchment area (km
2
) Total dams In-stream dams Off-stream dams Dam concentration (no. per 100 km
2
) Inglis 615.7 509 500 9 82.7 Blythe 377.2 304 302 2 80.6 Cam 288.6 191 185 6 66.2 Leven 727.4 435 419 16 59.8 Rubicon 717.5 391 355 36 54.5 Black-Detention 646.2 279 279 0 43.2 Emu 254.6 109 109 0 42.8 Duck 552.4 236 228 8 42.7 Forth-Wilmot 1,179.6 387 382 5 32.8 Great Forester-Brid 783.0 256 247 9 32.7 Mersey 1,908.9 593 554 39 31.1 Meander 1,568.6 301 191 110 19.2 Pitt Water-Coal 919.8 159 137 22 17.3 Derwent Estuary-Bruny 1,091.5 140 119 21 12.8 Ringarooma 982.8 118 113 5 12.0 Tamar Estuary 1,074.4 123 86 37 11.4 Jordan 1,253.2 136 110 26 10.9 Boobyalla-Tomahawk 652.2 63 61 2 9.7 Tasman 927.1 75 58 17 8.1 Huon 3,807.9 253 197 56 6.6 Pipers 753.7 47 43 4 6.2 Lower Derwent 1,603.7 97 52 45 6.0 Montagu 476.1 24 20 4 5.0 Little Swanport 878.9 36 33 3 4.1 Little Forester 353.6 12 12 0 3.4 Brumbys-Lake 1,508.5 50 44 6 3.3 North Esk 1,065.5 35 30 5 3.3 Macquarie 2,732.4 83 72 11 3.0 Prosser 1,148.5 33 29 4 2.9 Swan-Aspley 1,360.3 34 26 8 2.5 South Esk 3,349.5 67 57 10 2.0 Welcome 674.8 13 8 5 1.9 Scamander-Douglas 686.6 13 7 6 1.9 King Island 1,094.0 20 18 2 1.8 Clyde 1,117.5 19 16 3 1.7 Ouse 1,482.4 17 12 5 1.1 George 615.0 7 3 4 1.1 Musselroe-Ansons 972.1 8 7 1 0.8 Arthur 2,505.4 15 15 0 0.6 Great Lake 396.4 2 2 0 0.5 Nelson Bay 867.6 4 4 0 0.5 King-Henty 1,792.7 8 8 0 0.4 Furneaux 1,887.9 7 4 3 0.4 Upper Derwent 3,541.3 7 7 0 0.2 Pieman 4,148.9 4 4 0 0.1 Gordon-Franklin 5,893.6 0 0 0 0.0 Port Davey 2,843.2 0 0 0 0.0 Wanderer-Giblin 1,756.4 0 0 0 0.0 The same data analysed for bioregional boundaries shows the higher concentration of farm dams in the Northern Slopes, South-East and Ben Lomond bioregions. Northern Slopes has the greatest concentration of farm dams of 42 dams/100 km2. Bioregion
Bioregion
Total farm dams In-stream dams Off-stream dams Total farm dam concentration
West 15,507 10 10 0 0 Central Highlands 7,528 32 29 3 0 Southern Ranges 13,737 240 184 56 2 Northern Midlands 4,876 359 249 110 7 Flinders 4,151 321 283 38 8 King 7,673 703 680 23 9 Ben Lomond 4,261 523 492 31 12 South-East 6,231 800 640 160 13 Northern Slopes 6,570 2,733 2,599 134 42 Source: DPIWE
Referable dams are dams which because of their height and/or storage volume pose a potential threat if dambreak occurs. The owners of referable dams have the responsibility for regularly reviewing the safety of their dams. The growth in referable dams is shown in the following figure. The Hydro designed, constructed and now owns and operates 54 large and referable dams as well as a number of smaller dams within Tasmania. Its reservoirs account for the majority of the State's storage capacity at 26,149 GL (NLWRA 2001). Hydro storage development grew rapidly after the Second World War, with an average of one reservoir built (or expanded) each year from the late 1940s to the mid-1980s. This rate slowed into the 1990s, with the last hydro-dam completed in 1993. Additional hydro-electric dams are unlikely to be built in the foreseeable future, although mini-hydro schemes have been suggested including on the proposed Meander Dam. Mining dams had a boom period during the 1970s which was an intensive period of investment in minerals. Although some mining dams are used to supply water for mine workings and drinking, the majority are for the retention of tailings (73% of the dams and 88% of the storage capacity). A tailings dam with a capacity of 11,700 ML was built in 1999 on Princess Creek near Mt Lyell. The WIMS database records 720 existing (built) non-Hydro dams, with a combined capacity of 780,545ML, that are within the specifications of large dams as defined in the ANCOLD Guidelines on Dam Safety Management, namely:
These referable dams are shown on the following map.
Since the last SoE Report (SDAC 1997), some 38 large dams have been built with a combined capacity of 21,023 ML. Thirty six dams were built on-stream, one was described as off-stream and two were described as 'catchment'. The Water Development Plan is currently reviewing opportunities for irrigation dam development in the State (DPIWE 2001). The specific opportunities identified by the Water Development Plan are:
The proposed Meander Dam is at the southern end of the Meander Gorge, approximately 450 m downstream of the confluence of the Meander River and Warners Creek. The 50 m dam would capture 43,000 ML to provide 24,000 ML for irrigation annually (DPIWE 2002). The dams currently being assessed are shown in the following table: Dam name River Capacity (ML) Meander Dam Meander River 43,000 Chimney Hill Elizabeth River 14,000 Benham St Pauls River 21,000 Maloneys Hill Macquarie River 15,000 Waterhouse Boobyalla River 32,000 and 20,000 (Tomahawk) Edith Creek Junction of Duck River 5,000 Christian Marsh Shannon River 18,000 State of the Environment Tasmania Indicator |
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