Hydraulic assessment of environmental flow regimes to facilitate fish passage through natural riffles: Shoalhaven river below Tallowa Dam, New South Wales, Australia

Abstract

Proposed environmental flow regime changes downstream of a major water supply dam have been assessed in terms of effects on depth, velocity and fish passage across natural, gravel-bed riffles and rapids. This study focussed on passage requirements for Australian bass, Macquaria novemaculeata (Perciformes, Percichthyidae), a catadromous fish of considerable ecological and recreational fishing importance. Some 23 major riffles and rapids occur between the dam and the tidal limit over a river length of 25 km. Reconnaissance investigations of riffle slope, length, width, depth and morphologic characteristics indicated that wideshallow, steep-turbulent and bifurcating riffle morphologies were most likely to cause problems for upstream bass passage under low flow conditions. Two approaches were used to investigate riffle depths and velocities over a range of flows. A rapid assessment approach directly measured thalweg depths and velocities under two controlled flow release rates in riffles identified as being potentially problematic to upstream bass passage. Detailed topographic surveys and two dimensional hydraulic modelling with River2D was undertaken for two riffles identified as 'worst case' examples of wide-shallow and steep-turbulent morphologies. Results from both approaches were consistent and complementary. Both approaches identified riffles where minimum depths and maximum velocities were likely to be problematic for upstream passage by Australian bass at a flow rate of 130 MLd À1 (the current regulated flow release) but were mitigated at flow rates above 300 MLd À1. Assessment of environmental flow regime transparency and translucency threshold options with regard to a 300 MLd À1 target flow indicated that options where the transparency threshold was set at the 80th flow duration percentile (flows equalled or exceeded for 80% of time), and varied according to the monthly pattern of natural flows, improved hydraulic conditions for upstream bass migration.

Alluvium of antiquity: Polycyclic terraces in a confined bedrock valley

“…This type of combined 1D and 2D hydraulic modelling approach where a 1D model is used to objectively develop downstream boundary conditions for input into River2D has been successfully applied in studies where 2D as opposed to 1D models are needed for depiction of complex hydraulic environments (e.g. Bartsch et al, 1996;Reinfelds et al, 2010).…”

“…Tallowa Dam was completed in 1976 and has a low operational capacity relative to annual inflows (3.3% and 5.5% of mean and median annual inflows, respectively). Water extraction from Tallowa Dam to supplement the water supply of Australia's largest city, Sydney, primarily impacts on small flood events b1.1 years average recurrence interval (ARI) (90% annual exceedance probability), with peak flow rates for larger flood events being largely unaffected (Reinfelds et al, 2010).…”

Modern depositional processes in a confined, flood-prone setting: Benches on the Shoalhaven River, NSW, Australia

“…Owing to its small operational capacity relative to mean and median annual inflows (3.3 and 5.5%, respectively; Reinfelds et al, 2010), Tallowa Dam frequently overtops, creating unmanaged spills of water to the river downstream. Peak flow rates for events equal to and greater than the 90% annual exceedance probability (AEP) flood (instantaneous maximum flow~10,000 ML d −1 ) are largely unaffected, but their receding hydrograph limbs are typically truncated (Reinfelds et al, 2010).…”

Where technology meets ecology: acoustic telemetry in contemporary Australian aquatic research and management

“…However, over the past decade acoustic telemetry has revealed critical aspects of the life history and movement ecology of freshwater et al (2014) species in Australia, and this information is now being broadly incorporated into water resource policy and management (e.g. Reinfelds et al 2010;. Koster and Crook (2017) summarise information from telemetry studies for four Australian freshwater species of management interest to demonstrate how results from telemetry research can be integrated as conceptual models to support the development of environmental flow rules and other conservation measures.…”