Video 2 C2: What is a weir? Flashcards
(26 cards)
historically, mills relied on ____ power
water
what are some examples as to why we would want to raise water levels? (3)
- generate power
- allows for boats and ships to navigate areas that would otherwise be inaccessible
- mitigates the damaging impact of flooding
what is a weir?
a small dam built across a river to control the upstream water level
what is the top of the weird also known as?
the crest
the depth of water is controlled by…
the downstream conditions
ex. adding a weir at the end will increase the water level upstream
adding a weir at the end of a river will _______ the water level upstream
increase
but by how much the depth increases (from adding a weir) depends on…
the flow
the flow in a weir is dependent on what 3 factors?
- the length of the weir
- the height over the crest
- the discharge coefficient (changes based on weir geometry)
the graph of a hydraulic structure’s flow versus water level is called
the rating curve
what does the typical rating curve look like for a weir?
like the water surface profile graph I did in A8
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in many cases, a weir is a passive structure, meaning…
once it’s installed there’s no way to change this rating curve
why would not being able to change the rating curve for a weir not be ideal?
- because streams and rivers are subject to tremendous variability in flow rate (flooding vs not flooding):
- since you have to manage variability, you would normally have to build weirs at their maximum height to handle the water level that occurs during extreme cases- this could be very costly
what would the ideal rating curve for a hydraulic structure look like?
a flatter curve
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why is a flat rating curve more ideal?
because it means over a wide range of flows, you only get small changes in water level
how can we flatten the rating curve?
- could improve weir geometry (max C value of 4 tho)
- length of the weir (but normally fixed or costly) BUT can make the length “folded” = non-linear wear
what is the most efficient shape for a weir?
to match the curve that the water would take off a sharp crest- OGEE SHAPE
what is the part of flow when off a sharp crest called?
the weirs nappe
What is a crest gate?
acts like a weir that can be raised or lowered
what is a benefit of a crest gate?
the structure’s capacity can be increased while flows are high by opening gates, and decreased when flow returns to normal
controlled structures provide more ______ in how water gets released or held back
flexibility
what is a disadvantage of controlled structures?
increased complexity; requires someone to be available 24/5 to operate the gates based on inflows
what is a non-linear weir?
where the length of the weir is non-linear (folded, squiggles, curves, etc)
in what situations is a non-linear weir used?
where you want greater discharge within a greater footprint but without the need to actively controlled outlets
whats another word for non-linear weirs with folded cycles (triangle folds)?
labyrinth
