17.3 Environmental Flow Assessment and Water Resources…
385
High Flow
A wet season (April–September) baseflow of 46 m
3 /s is recommended to maintain
the flow of low-salinity water on the ebb tide in the upper part of the reach, which
should help to maintain the position of the fresh/brackish/saline interface.
Bankfull Flow
The upper limit of Reach 4 defines the approximate upstream extent of the significant
tidal influence in Ling River, while the downstream limit, just below Linhai City,
defines the upstream extent of saline water intrusion. As a result of the tidal influence, hydraulic conditions within the river are controlled more by tidal processes than
inflows from upper reaches. The exception is under high flow conditions. As water
levels are controlled by river discharge only during high flow events, recommendations for bankfull flows have been made primarily to maintain periodic inundation
of an important riparian wetland area.
The estimated flows required to achieve bankfull and inundate the fringing wetland
areas range between 3,110 and 3,980 m
3 /s (instantaneous). This uncertainty is due to
the lack of a detailed survey of the bank height adjacent to the wetland to accurately
determine the sill level. Monitoring of the flows required to inundate the wetland
could quickly address this uncertainty. The frequency of these events ranges between
0.45 and 0.59 times per year (i.e., about once every two years).
Pulses
A convincing ecological basis for pulses could not be established in this reach, and
pulses have therefore not been recommended. Pulses will not influence water level
but may have a minor and temporary influence on the downstream salinity gradient.
Overbank Flow
Overbank flows were not recommended for this reach as the bankfull component
achieves inundation of the important environmental asset (the wetland). A separate
overbank flow component was not specified due to the widespread floodplain levees
in this reach.
(5) Reaches 5 and 6 Recommendations
While the Jiaojiang River estuary is dominated by marine influences, freshwater flows
are also ecologically important. During high river flow events, the saline estuarine
water is pushed out to sea, which causes a major disruption to the normal ebb and
flow cycle. The biota has evolved to either take advantage of this sudden change in
the hydraulics and salinity regime or to minimize negative impacts.
Due to a lack of resources, it was not possible to derive specific flow rules for
Reaches 5 and 6 using the methodology used in the other reaches. The hydrodynamics
of the estuary are complex, and derivation of flows based on an understanding of the
hydrodynamics would require a separate investigation.
For Reaches 5 and 6, a risk assessment was undertaken to explore the potential
impacts associated with the key hydrological change indices, namely, reduction in
385
High Flow
A wet season (April–September) baseflow of 46 m
3 /s is recommended to maintain
the flow of low-salinity water on the ebb tide in the upper part of the reach, which
should help to maintain the position of the fresh/brackish/saline interface.
Bankfull Flow
The upper limit of Reach 4 defines the approximate upstream extent of the significant
tidal influence in Ling River, while the downstream limit, just below Linhai City,
defines the upstream extent of saline water intrusion. As a result of the tidal influence, hydraulic conditions within the river are controlled more by tidal processes than
inflows from upper reaches. The exception is under high flow conditions. As water
levels are controlled by river discharge only during high flow events, recommendations for bankfull flows have been made primarily to maintain periodic inundation
of an important riparian wetland area.
The estimated flows required to achieve bankfull and inundate the fringing wetland
areas range between 3,110 and 3,980 m
3 /s (instantaneous). This uncertainty is due to
the lack of a detailed survey of the bank height adjacent to the wetland to accurately
determine the sill level. Monitoring of the flows required to inundate the wetland
could quickly address this uncertainty. The frequency of these events ranges between
0.45 and 0.59 times per year (i.e., about once every two years).
Pulses
A convincing ecological basis for pulses could not be established in this reach, and
pulses have therefore not been recommended. Pulses will not influence water level
but may have a minor and temporary influence on the downstream salinity gradient.
Overbank Flow
Overbank flows were not recommended for this reach as the bankfull component
achieves inundation of the important environmental asset (the wetland). A separate
overbank flow component was not specified due to the widespread floodplain levees
in this reach.
(5) Reaches 5 and 6 Recommendations
While the Jiaojiang River estuary is dominated by marine influences, freshwater flows
are also ecologically important. During high river flow events, the saline estuarine
water is pushed out to sea, which causes a major disruption to the normal ebb and
flow cycle. The biota has evolved to either take advantage of this sudden change in
the hydraulics and salinity regime or to minimize negative impacts.
Due to a lack of resources, it was not possible to derive specific flow rules for
Reaches 5 and 6 using the methodology used in the other reaches. The hydrodynamics
of the estuary are complex, and derivation of flows based on an understanding of the
hydrodynamics would require a separate investigation.
For Reaches 5 and 6, a risk assessment was undertaken to explore the potential
impacts associated with the key hydrological change indices, namely, reduction in