372
17 Environmental Flow Definition and Management: A Case Study …
to maintain sediment supply could include stopping gravel extraction activities and
allowing bank erosion to proceed.
The flow and sediment processes in the Jiaojiang River estuary are largely
controlled by marine rather than fluvial inputs. The estuary requires regular dredging
regardless of any periodic scouring that might be provided by high river flows. During
large floods, freshwater extends further into Taizhou Bay, and sediment is transported
offshore. This may create ecological responses in the estuary and offshore. During
low flow periods, the rate of fine sediment deposition on the bed of the estuary tends
to increase. At these times, saltwater migrates further upstream.
Thus, the issues for environmental flows in the estuary are mainly related to
maintaining the mean position of the salt wedge by maintaining river flows. Inland
migration of the salt wedge due to reduced flows will not change the overall character
of the estuary, but it may slightly increase the size of the tidal prism.
17.3.2.3 Environmental Flow Requirements Definition
Based on the identified environmental assets, environmental flow recommendations
were developed to achieve the following broad objectives: (1) maintaining fish diversity and abundance; (2) maintaining subsistence and commercial fisheries; (3) maintaining water quality at a level capable of supporting objectives 1 and 2, together
with existing uses of the river; (4) maintaining riparian vegetation in its current form
in riparian and low-lying floodplain habitats; and (5) maintaining the geomorphic
processes required to support objectives 1–4.
(1) Links between environmental assets and flow components
Specific environmental flow objectives pertaining to geomorphology, vegetation,
and fish were determined independently from one another for each reach. Associations were then made between each flow objective and the components of the flow
regime on which the objective depends. These associations are broadly represented
as the conceptual model of flow-ecology-geomorphology relationships depicted in
Fig. 17.4.
The conceptual model illustrates the role of multiple specific flow components
in achieving the overall flow objectives. The linkages were developed based on a
general scientific understanding of the interrelationships that exist in river systems
of this kind between ecology, flows, and geomorphology. While individual flow
components may relate to several objectives, typically there is one objective that acts
as the key constraint. During the balance of the environmental flows assessment,
these were treated as the “controlling objectives.” In addition to the component of
the flow regime important to the specific environmental objectives, the timing of the
flows is also important.
(2) Hydraulic criteria to define recommended environmental flow release rules
To implement environmental flow recommendations, each component comprising the
regime has to be specified as a flow volume, with associated frequency, timing, and
17 Environmental Flow Definition and Management: A Case Study …
to maintain sediment supply could include stopping gravel extraction activities and
allowing bank erosion to proceed.
The flow and sediment processes in the Jiaojiang River estuary are largely
controlled by marine rather than fluvial inputs. The estuary requires regular dredging
regardless of any periodic scouring that might be provided by high river flows. During
large floods, freshwater extends further into Taizhou Bay, and sediment is transported
offshore. This may create ecological responses in the estuary and offshore. During
low flow periods, the rate of fine sediment deposition on the bed of the estuary tends
to increase. At these times, saltwater migrates further upstream.
Thus, the issues for environmental flows in the estuary are mainly related to
maintaining the mean position of the salt wedge by maintaining river flows. Inland
migration of the salt wedge due to reduced flows will not change the overall character
of the estuary, but it may slightly increase the size of the tidal prism.
17.3.2.3 Environmental Flow Requirements Definition
Based on the identified environmental assets, environmental flow recommendations
were developed to achieve the following broad objectives: (1) maintaining fish diversity and abundance; (2) maintaining subsistence and commercial fisheries; (3) maintaining water quality at a level capable of supporting objectives 1 and 2, together
with existing uses of the river; (4) maintaining riparian vegetation in its current form
in riparian and low-lying floodplain habitats; and (5) maintaining the geomorphic
processes required to support objectives 1–4.
(1) Links between environmental assets and flow components
Specific environmental flow objectives pertaining to geomorphology, vegetation,
and fish were determined independently from one another for each reach. Associations were then made between each flow objective and the components of the flow
regime on which the objective depends. These associations are broadly represented
as the conceptual model of flow-ecology-geomorphology relationships depicted in
Fig. 17.4.
The conceptual model illustrates the role of multiple specific flow components
in achieving the overall flow objectives. The linkages were developed based on a
general scientific understanding of the interrelationships that exist in river systems
of this kind between ecology, flows, and geomorphology. While individual flow
components may relate to several objectives, typically there is one objective that acts
as the key constraint. During the balance of the environmental flows assessment,
these were treated as the “controlling objectives.” In addition to the component of
the flow regime important to the specific environmental objectives, the timing of the
flows is also important.
(2) Hydraulic criteria to define recommended environmental flow release rules
To implement environmental flow recommendations, each component comprising the
regime has to be specified as a flow volume, with associated frequency, timing, and