378
17 Environmental Flow Definition and Management: A Case Study …
Low Flow Pulse
Periodic flow pulses of 20 m
3 /s have been recommended to flush fine sediments
and algae from cobbles and to maintain water quality by “turning over” pools and
re-oxygenating the hyporheic zone. These pulses also provide opportunities for fish
to move between pools that might otherwise be isolated by shallow water over riffles
during low flows and move terrestrial organic material, thereby providing a potentially important source of carbon for aquatic food webs. The flushing of fine sediments, which would only partially be achieved, and the scouring of vegetation from
bars are the controlling objectives for this flow. Two events, each lasting a minimum
of one day, are recommended to reflect the natural frequency of these events.
High Flow
A wet season (Apr–Sep) baseflow of 1.3 m
3 /s is recommended to maintain sufficient
water depths for fish movement into and out of spawning areas; ensure that average
flow velocities in the channel exceed 0.25 m/s, which is necessary to maintain the
buoyant and drifting eggs of grass carp, silver carp, and black carp in the water
column; and ensure that sufficient backwater habitats exist for juvenile and young
fish.
High Flow Pulse
Periodic flow pulses of 20 m
3 /s have also been recommended during high flow
periods. These pulses serve the same objectives as the low flow pulses but at different
times of the year. An event has been specifically recommended during April to coincide with the onset of spawning by several carp species, both to potentially stimulate
spawning and to provide opportunities for upstream migration. An additional four
events are necessary in the period May–September to scour detritus and algae from
the streambed and vegetation from bars. Releases can coincide with natural increases
in discharge downstream to reduce the volume of water released from the dam. Each
high flow pulse should endure for a minimum of two days.
Bankfull Flow Riffles
Zhu Creek is a typical gravel-bed river in which many of the natural channel features
(pools, riffles, and bars) are maintained by bankfull flows mobilizing bed material
and scouring encroaching terrestrial vegetation. These geomorphic features create
unique habitats that must be maintained to meet the fish and vegetation objectives.
The peak discharge required to reach the top of the bank at the surveyed cross section
was estimated to be 524 m
3 /s, but this need only be reached instantaneously. The
associated mean daily flow for input to the IQQM model is 177 m
3 /s. These flows
should occur once every two years on average.
Overbank Flow
Overbank flows have been recommended to maintain the natural disturbance regime
in riparian areas where periodic floods can play a key role in vegetation structure
(including wetland communities). Flows necessary to inundate riparian areas in close
17 Environmental Flow Definition and Management: A Case Study …
Low Flow Pulse
Periodic flow pulses of 20 m
3 /s have been recommended to flush fine sediments
and algae from cobbles and to maintain water quality by “turning over” pools and
re-oxygenating the hyporheic zone. These pulses also provide opportunities for fish
to move between pools that might otherwise be isolated by shallow water over riffles
during low flows and move terrestrial organic material, thereby providing a potentially important source of carbon for aquatic food webs. The flushing of fine sediments, which would only partially be achieved, and the scouring of vegetation from
bars are the controlling objectives for this flow. Two events, each lasting a minimum
of one day, are recommended to reflect the natural frequency of these events.
High Flow
A wet season (Apr–Sep) baseflow of 1.3 m
3 /s is recommended to maintain sufficient
water depths for fish movement into and out of spawning areas; ensure that average
flow velocities in the channel exceed 0.25 m/s, which is necessary to maintain the
buoyant and drifting eggs of grass carp, silver carp, and black carp in the water
column; and ensure that sufficient backwater habitats exist for juvenile and young
fish.
High Flow Pulse
Periodic flow pulses of 20 m
3 /s have also been recommended during high flow
periods. These pulses serve the same objectives as the low flow pulses but at different
times of the year. An event has been specifically recommended during April to coincide with the onset of spawning by several carp species, both to potentially stimulate
spawning and to provide opportunities for upstream migration. An additional four
events are necessary in the period May–September to scour detritus and algae from
the streambed and vegetation from bars. Releases can coincide with natural increases
in discharge downstream to reduce the volume of water released from the dam. Each
high flow pulse should endure for a minimum of two days.
Bankfull Flow Riffles
Zhu Creek is a typical gravel-bed river in which many of the natural channel features
(pools, riffles, and bars) are maintained by bankfull flows mobilizing bed material
and scouring encroaching terrestrial vegetation. These geomorphic features create
unique habitats that must be maintained to meet the fish and vegetation objectives.
The peak discharge required to reach the top of the bank at the surveyed cross section
was estimated to be 524 m
3 /s, but this need only be reached instantaneously. The
associated mean daily flow for input to the IQQM model is 177 m
3 /s. These flows
should occur once every two years on average.
Overbank Flow
Overbank flows have been recommended to maintain the natural disturbance regime
in riparian areas where periodic floods can play a key role in vegetation structure
(including wetland communities). Flows necessary to inundate riparian areas in close