By Pat Pepper, NCWQ Environment Adviser
Update on impact of lack of reliable water on regional communities and industries and the environment.
The Problem: In my NCWQ Environment Adviser’s Report, November 2019, I drew attention to the fact that many billions of megalitres of water can flow out to sea while other parts of the country suffer extreme drought. The freshwater flood plumes can cause environmental damage to the reefs in the Great Barrier Reef Marine Park. Drought significantly impacts the environment, industries and communities including Murray Darling Basin.
Regional towns were running out of water with dam levels dangerously low. While some rain has fallen, emergency water restrictions remain in place in some areas. e.g. While the level and volume for Stanthorpe increased from 9.5% and 196 megalitres (ML) at 3rd February 2020 to 17.5% and 362 ML. at 17th February, water carting is continuing to Stanthorpe. Water remains in Storm King Dam as a contingency for emergency events and to provide a habitat for aquatic life. https://www.sdrc.qld.gov.au/living-here/water-wastewater/water-update
Rain has fallen across Murray Darling Basin recently but not enough to end the drought. The flows in many rivers will be boosted and dam storage levels lifted but threats to water quality persist, including contamination from bushfire debris.
Rainfall Projections: Rainfall in the near future (2030) and late in the century (2090) has been projected by CSIRO and the Bureau of Meteorology for clusters of Natural Resource Management Regions including the following:-
- The Wet Tropics cluster which contains the Wet Tropics and Great Barrier Reef World Heritage Areas, as well as a high proportion of the Great Barrier Reef catchment,
- The Monsoonal North East with the Mitchell, Gilbert, Norman and Staaten River catchments, all of which flow into the Gulf of Carpentaria and the Burdekin region,
- The Central Slopes cluster comprising NRM regions to the west of the Great Dividing Range from the Darling Downs in Queensland to the central west of New South Wales with a number of important headwater catchments for the Murray Darling Basin and
- The Murray Basin cluster comprises NRM regions across New South Wales, Victoria and South Australia. The cluster extends from the flatlands of inland New South Wales to the Great Dividing Range.
In the near future, natural variability is projected to predominate over trends due to greenhouse gas emissions for all of the above clusters. However for the Wet Tropics and Monsoonal North East Clusters it is noted that because global climate models offer diverse results, and models have shortcomings in resolving some tropical processes it is difficult to provide confident rainfall projections.
By late in the century,
- for the Wet Tropics, projections generally have low confidence,
- for the Monsoonal North East, projections generally have low confidence,
- for the Central Slopes, climate models indicate decreasing winter rainfall with high confidence. There is a good understanding of the physical mechanisms driving this change (southward shift of winter storm systems together with rising mean pressure over the region). Decreases are also projected in spring, with medium confidence. The direction of change in summer and autumn cannot be confidently projected due to the complexity of rain producing systems in this region, the large spread of model results, and some inconsistent results from finer scale modelling.
- For the Murray Basin cool season (April to October) less rainfall is projected with high confidence. In the warm season (November to March), there is medium confidence that rainfall will remain unchanged.
Increased intensity of extreme daily rainfall events is projected with high confidence for Wet Tropics, Monsoonal North East and Central Slopes clusters. Even though mean annual rainfall is projected to decline, heavy rainfall intensity is projected to increase, with high confidence.
For the Murray Basin and Central Slopes clusters time spent in drought is projected, with medium confidence, to increase over the course of the century. For Wet Tropics and Monsoonal North East clusters, drought will continue to be a feature of the regional climate variability, but projected changes are uncertain.
WWW.CLIMATECHANGEINAUSTRALIA.GOV.AU CSIRO and Bureau of Meteorology Brochures (WET_TROPICS; MONSOONAL_NORH; CENTRAL_SLOPES; MURRAY_BASIN)
Various Proposals to address the problem:
In my NCWQ Environment Adviser’s Report, November2019, the following proposals were discussed:-
- Bradfield Scheme 1938
- Revised Bradfield’s Scheme 1981
- Moore-Hielscher Updated Bradfield Scheme: 2019
- NSW Proposals (diverting flows from the Manning, Macleay and Hunter rivers inland)
- Hell’s Gate Dam in North Queensland :
- Northern Australia Water Resource Assessment program (Fitzroy, Darwin, Mitchell catchments)
- National Water Grid
Environmental impacts of dams: Dams can supply significant sources of hydropower, water supply or flood protection but a dam must have a sustainable purpose and operating mission to provide a greater benefit than the environmental impacts of its existence or the risks associated with its ageing structure. https://www.waterpowermagazine.com/features/featurelarge-or-small-4978245/
Environmental issues with dams to be addressed include:-
- riparian habitat loss,: upstream of a dam, impounded water can drown riparian communities; downstream the shore line changes and with it the riparian communities,
- sedimentation: Dams can trap sediments normally deposited downstream. The storage capacity of the dam can be reduced with high sedimentation. Seasonal flooding which would fertilise and water flood plains can be interrupted and debris in river channels not cleared or redistributed downstream,
- erosion can reshape river channels below the dam, once sediment deposition ceases,
- water quality may deteriorate in reservoirs (e.g., thermal stress, low dissolved oxygen, acidification), especially close to the bottom. It can decline as a result of drainage water returning from irrigation projects . If the reservoir becomes shallower through sedimentation, in arid regions evaporation could increase leaving behind salts and decreasing the water quality.
- groundwater: With seepage into bedrock, river water could enter groundwater and water tables rise around a reservoir,
- fish migration and reproduction could be disrupted
POSITIVE AND NEGATIVE IMPACTS OF DAMS ON THE ENVIRONMENT
M. Sait TAHMİSCİOĞLU, Nermin ANUL, Fatih EKMEKÇİ and Nurcan DURMUS INTERNATIONAL CONGRESS ON RIVER BASIN MANAGEMENT 2011 P759=69
Dams: Ecological Impacts and Management Stefan Schmutz and Otto Moog (http://creativecommons.org/licenses/by/4.0/),
The location of the dam, size of reservoir (height of dam, volume of reservoir), and water residence time affect the impact. The dam operation mode can determine the seasonal variation of stored water, water level fluctuations, sediment capture and release, as well as daily and seasonal downstream flow patterns.
Sediment Management options depend on storage capacity, mean annual runoff, and
mean annual sediment load and include sediment sluicing, sediment flushing, sediment bypass, and sediment augmentation downstream of reservoirs .
Habitat Improvements in Reservoirs. Mitigation measures can comprise instream structures such as gravel bars, islands, etc., lateral widenings of the cross profiles in riverine sections of impoundments, creating artificial habitats in lacustrine section, and bypass systems within the alluvial floodplains.
Riverine Zone Lacustrine Zone
Dams: Ecological Impacts and Management Stefan Schmutz and Otto Moog (http://creativecommons.org/licenses/by/4.0/),
Large and small dams can provide water storage but the size and type of dam needs to suit both the site conditions and satisfy the objectives for its construction. The impact of a dam varies with the river, the dam’s design, and the projected use. Small dams can be best suited for small hydroelectric developments utilising low diversion and storage, for smaller scale irrigation projects, flood control on smaller tributaries, ground water recharge basins, and off-site storage of recycled water or desalinated water.
Off-stream reservoirs constructed on smaller streams which store water pumped from a nearby river or adjacent basin typically have less environmental impact. A small dam can have less impact on the environment if designed to be more effective in safely passing fish species both upstream and downstream, and to bypass sediment https://www.waterpowermagazine.com/features/featurelarge-or-small-4978245/
While fluvial characteristics are maintained to some extent in small reservoirs, e.g., run-of-the-river
hydropower plants, lentic conditions prevail in large storage reservoirs.
Dams: Ecological Impacts and Management Stefan Schmutz and Otto Moog (http://creativecommons.org/licenses/by/4.0/),
Water security throughout the country and in all sectors is vitally important. Surely an extensive feasibility study with bipartisan support, both Federal and State, is needed to address this perennial problem of lack of reliable water for regional communities and industries and the environment. Surface and groundwater capture-and-storage options, land suitability, the commercial viability of primary production should be considered with potential environmental, social, indigenous and economic impacts and risks.
