The placement of a weir depends on Many things:
As it turns out, the most effective
placement of a fish weir can be determined using the principles
of Classical Physics! In the Photo above, the fish weir on the
Buskin River in Kodiak Alaska (located inside the red circle) was
placed for its ideal conditions of water quality, erosion,
attractiveness to fish, and the abundance of river sediments conducive
to weir placement. The considered elements are: Water
velocity and
Potential for Flooding,
Erosion of the river banks at the site, Fish's
desire to move upstream of the weir
Water Velocity and Potential for Flooding:
Since water is like a gas, and moves from areas of higher pressure to areas of lower pressure, it will move from the confines of a Lake, depicted in the left side of the picture, towards the ocean, via the river, shown as the dark blue meandering stream shown to the right of the red circle. This move from higher pressure to lower pressure areas that the water takes part in causes an increase in water velocity at the immediate outlet of the lake, where the weir is placed. The increased water velocity is necessary to the function of the weir as a fish-counting mechanism, as it is the essential indicator to fish and how fish know to go through the weir.
Although the outlet of the lake provides a good location with significantly higher water movement than that of a pool, or eddy down river, it also provides an optimum location for the aversion of flash flooding. In the event of great rainfall, the mountains surrounding the lake become saturated with water. When the soil is saturated, additional water to the soil is no longer absorbed by vegetation or drawn into groundwater. Rather, it becomes runoff, which is water that rolls downhill due to gravity in a reduced friction area (such as on the surface of a water saturated soil). Since water has a shear stress of nearly zero, run-off moves quickly and willingly downhill into the lake, where it accumulates until the holding capacity of the lake is exceeded and the water moves down river in a sudden manner.
Because the outlet of the lake is a considerably wider area in the river, with no curves that would cause water to move quickly in circular motion from centripetal force, the placement of a weir allows the fish to be seen more easily as it is a laminar, not a turbulent flow of water. The ability for water to move quickly and efficiently from the lake to a further downstream position with very little resistance from the weir, or winding in the river, the outlet of the lake is an area less susceptible to the ravages of a flash flood.
Erosion at the Weir Site
Another key component of choosing a weir site is finding a site where you can be assured that the banks of the river won't simply wash away in the event of a flash flood. As greater amounts of rainfall, more water will come out of the lake in the same amount of time, so essentially you have increased water acceleration as it moves from the lake down stream because of the pressure of the water in the lake increasing the force on the water in the lake, it will move with greater speeds (that is it will accelerate more because F=ma as it leaves the river. The increase in the force of the water allows it greater destructive properties, likewise greater ability to move rocks, silt, and other debris when collisions with the water and the soil occur on the river banks. Therefore, it is ideal to place your weir in an area where the water will not move at higher velocities, thus it will have less force and be less destructive in erosional processes. Further, it is helpful to place your weir in an area where the banks are vegetated so that the vegetation will hold soil in place, as opposed to a sandy bank.
Fish Moving Upstream
The final indicator of a good weir location is that you must find a location where the fish WANT to move upstream of the weir. It is virtually useless to build a weir to count fish through if the fish will not move up stream. To get the fish to move upstream, the fish-counter opens a gate on the weir, and the fish move from the more quickly moving waters of downstream to the more placid waters upstream.
Water Velocity and Potential for Flooding:
Since water is like a gas, and moves from areas of higher pressure to areas of lower pressure, it will move from the confines of a Lake, depicted in the left side of the picture, towards the ocean, via the river, shown as the dark blue meandering stream shown to the right of the red circle. This move from higher pressure to lower pressure areas that the water takes part in causes an increase in water velocity at the immediate outlet of the lake, where the weir is placed. The increased water velocity is necessary to the function of the weir as a fish-counting mechanism, as it is the essential indicator to fish and how fish know to go through the weir.
Although the outlet of the lake provides a good location with significantly higher water movement than that of a pool, or eddy down river, it also provides an optimum location for the aversion of flash flooding. In the event of great rainfall, the mountains surrounding the lake become saturated with water. When the soil is saturated, additional water to the soil is no longer absorbed by vegetation or drawn into groundwater. Rather, it becomes runoff, which is water that rolls downhill due to gravity in a reduced friction area (such as on the surface of a water saturated soil). Since water has a shear stress of nearly zero, run-off moves quickly and willingly downhill into the lake, where it accumulates until the holding capacity of the lake is exceeded and the water moves down river in a sudden manner.
Because the outlet of the lake is a considerably wider area in the river, with no curves that would cause water to move quickly in circular motion from centripetal force, the placement of a weir allows the fish to be seen more easily as it is a laminar, not a turbulent flow of water. The ability for water to move quickly and efficiently from the lake to a further downstream position with very little resistance from the weir, or winding in the river, the outlet of the lake is an area less susceptible to the ravages of a flash flood.
Erosion at the Weir Site
Another key component of choosing a weir site is finding a site where you can be assured that the banks of the river won't simply wash away in the event of a flash flood. As greater amounts of rainfall, more water will come out of the lake in the same amount of time, so essentially you have increased water acceleration as it moves from the lake down stream because of the pressure of the water in the lake increasing the force on the water in the lake, it will move with greater speeds (that is it will accelerate more because F=ma as it leaves the river. The increase in the force of the water allows it greater destructive properties, likewise greater ability to move rocks, silt, and other debris when collisions with the water and the soil occur on the river banks. Therefore, it is ideal to place your weir in an area where the water will not move at higher velocities, thus it will have less force and be less destructive in erosional processes. Further, it is helpful to place your weir in an area where the banks are vegetated so that the vegetation will hold soil in place, as opposed to a sandy bank.
Fish Moving Upstream
The final indicator of a good weir location is that you must find a location where the fish WANT to move upstream of the weir. It is virtually useless to build a weir to count fish through if the fish will not move up stream. To get the fish to move upstream, the fish-counter opens a gate on the weir, and the fish move from the more quickly moving waters of downstream to the more placid waters upstream.