Home  5  Books  5  GBEzine  5  News  5  HelpDesk  5  Register  5  GreenBuilding.co.uk
Not signed in (Sign In)

Categories



Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

PLEASE NOTE: A download link for Volume 1 will be sent to you by email and Volume 2 will be sent to you by post as a book.

Buy individually or both books together. Delivery is free!


powered by Surfing Waves




Vanilla 1.0.3 is a product of Lussumo. More Information: Documentation, Community Support.

Welcome to new Forum Visitors
Join the forum now and benefit from discussions with thousands of other green building fans and discounts on Green Building Press publications: Apply now.




    • CommentAuthortony
    • CommentTimeNov 20th 2020
     
    I have done this one before - now thinking three gorges

    Is it possible to change the flow of a river?

    You can slow it by filling a dam or reservoir once then the flow will remain the same as it would have been without the dam.

    Once the dam or reservoir is full then the flow has to go down the river, ie if there is flooding it was going to flood anyway, with or without the dam

    Discuss 🙂
  1.  
    You've done this one several times now. The answer is still the same as last times:
    http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16672
    http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16476 in members area

    But I will post this link again which explains how a flood storage dam works
    https://www.theflowpartnership.org/belford
    • CommentAuthorSteveZ
    • CommentTimeNov 20th 2020
     
    You are looking at different cases here. If the solid conventional dam is full then any excess water will carry on as before via the overflow safety process. When the catchment area has the features needed to slow the flow and temporarily store the excess, and release the water slowly, then the downstream area (village etc) is protected from the sudden appearance of extra surface drainage water.
    See the example of the village of Ladock in Cornwall - Hooray for the beavers!
    •  
      CommentAuthorfostertom
    • CommentTimeNov 20th 2020
     
    The point is, a water-supply or hydroelectric dam is kept full all the time, bar drought, so tony is then correct.
    But a 'leaky' flood mitigation dam stays near-empty most of the time, so has lots of capacity to fill up when it pours for days, absorbing peak flow to be released slowly in the following days.
    • CommentAuthortony
    • CommentTimeNov 20th 2020
     
    Once all the storage capacity is full then all the new rain has to go down the river - normally floods happen when it has been raining a lot and storage capacity is full.
    •  
      CommentAuthorfostertom
    • CommentTimeNov 20th 2020
     
    How long, since the beginning of heavy rain, depends on how big the dam's storage capacity is.
    True, once it's full, you're right. But if, during a period of heavy rain, the first x% of the excess is stored by filling up the dam, then that x% is held back from flooding downstream. In many cases that initial relief will allow the downstream land's absorbent capacity to handle the reduced total peak, when it comes.
    • CommentAuthortony
    • CommentTimeNov 20th 2020 edited
     
    But not in China
    • CommentAuthorCliff Pope
    • CommentTimeNov 21st 2020
     
    <blockquote><cite>Posted By: fostertom</cite>
    But a 'leaky' flood mitigation dam stays near-empty most of the time, so has lots of capacity to fill up when it pours for days, absorbing peak flow to be released slowly in the following days.</blockquote>


    That must depend on the soil/rock structure underneath the dam? The pressure at the bottom of any body of water will obviously be greater the fuller it is, so it will leak faster as it fills. I've observed this effect in a garden pond. We have several quite large ponds fed by a stream. When trying to identify and plug a leak it's tempting to imagine that when the level falls but then stabilises at some point that must be the level that has the hole.

    But it doesn't work like that - there are an infinite number of stabilisation levels, each one determined by the rate of ingress of water. Only when the stream is at full flood is the pond "full". As the water flow diminishes after the spring rains, it reaches the point where all the water is absorbed by the ground, and the water level is in equilibrium at any level between full and empty, as inflow always balances leakage/seepage into the ground.
    • CommentAuthortony
    • CommentTimeNov 21st 2020
     
    So in your last sentence there is no change in the overall flow
    • CommentAuthorCliff Pope
    • CommentTimeNov 21st 2020
     
    <blockquote><cite>Posted By: tony</cite>So in your last sentence there is no change in the overall flow</blockquote>

    No change from what?
    There is a flow into the pond, varying over the course of the year, a variable amount of that flow gets absorbed by the ground under the pond, and to the extent that water is absorbed by the ground, the flow out will be less than the flow in.
    But the amount that gets absorbed surely is greater if the pond is full, because the pressure on the ground will be higher? A bit like filtering a liquid through a funnel - the flow is faster when the funnel is full.

    So in general it would follow that any holding back of a river in a pond or reservoir slows the flow simply by reason of the depth of water, regardless of the obvious temporary slowing of the flow while the reservoir is filling.
    • CommentAuthortony
    • CommentTimeNov 21st 2020
     
    No, matter can neither be created or destroyed, river flow can’t either what goes in must come out
    •  
      CommentAuthordjh
    • CommentTimeNov 21st 2020
     
    Posted By: tonyNo, matter can neither be created or destroyed, river flow can’t either what goes in must come out

    Well yes
    (a) but the flow out doesn't have to be via the river, it can soak into the ground instead
    (b) and what matters isn't the total flow over all time, it's the peak flow at any particular time and a flood pond can alter that
  2.  
    Cliff, Tom, DJH, Steve, you're quite right, though I don't think Tony is planning to change his mind!

    For anyone else: a relly who works in flood defence told me this about the Three Gorges dam (I paraphrase):

    'Three Gorges mainly works as a hydro power dam but is also used for flood storage during rainy season. On days with heavy rain, less water is allowed out of the dam than flows in. This stores the flood water temporarily, so the water level rises from 145m up to 175m, or 180m in emergency. They can deal with for about a week of heavy rain. On subsequent dry days when the river is low, extra water is allowed out, to reduce the level down to 145m again, ready for the next flood. Operators use sophisticated weather forecasts and river measurements to work out the best flow for each day, to get the level ready for whatever weather is coming up next. But they must balance this with generating renewable power at the times it is needed, and keeping river level high enough for ships, so it's a compromise.

    "The low land is fed by 5 rivers, of which the dam is located on one, the Yangtze. The dam is successful to reduce flooding coming down the Yangtze, but obviously not for flooding coming down the other 4 rivers, so there are still some floods, there has been weeks of extreme rainfall this year. This caused media reports that 'the dam doesn't work' which is perhaps unfair, they couldn't really have built it much bigger."
    • CommentAuthorbhommels
    • CommentTimeNov 22nd 2020
     
    Austria has some impressive hydropower arrangements. There are installations where reservoirs in neighbouring valleys are linked through tunnels underneath the mountain range separating the two. Although the sum of the flow of the rivers discharging in both reservoirs is not changed, the ratio of flow between the rivers can be altered, albeit probably temporarily, and within limits.
  3.  
    That's interesting. The Scottish hydroelectric schemes have tunnels between valleys, eg water can be diverted from the upper catchment of the River Spey into the Tay, to the Lyon and back again, and to the River Earn. Google suggests the hydropower dams can be manipulated to reduce floodwater in the Tay at Perth city by 10-20%.

    Tangentially, planning consent has just been granted for the long-discussed pumped storage dam at Coire Glas, which will double the UK's pumped storage capacity if it is ever built.
    •  
      CommentAuthordjh
    • CommentTimeNov 23rd 2020
     
    Posted By: WillInAberdeenTangentially, planning consent has just been granted for the long-discussed pumped storage dam at Coire Glas, which will double the UK's pumped storage capacity if it is ever built.

    That sounds like good news. Sadly, I believe you should add 'and if it is still in the UK' to the end of the sentence :devil:
    • CommentAuthorCliff Pope
    • CommentTimeNov 23rd 2020
     
    <blockquote><cite>Posted By: tony</cite>No, matter can neither be created or destroyed, river flow can’t either what goes in must come out</blockquote>

    I've just thought of another means of loss of water, apart from outflow and seepage into the ground - evaporation.
    Perhaps not relevant in a UK winter, but there are hotter parts of the world where the entire river flow evaporates and the river comes to a dead end in a salt pan.
    • CommentAuthortony
    • CommentTimeNov 23rd 2020
     
    I agree that evaporation does take place, in the case of very high flows and flooding it does not seem to work sufficiently quickly.
Add your comments

    Username Password
  • Format comments as
 
   
The Ecobuilding Buzz
Site Map    |   Home    |   View Cart    |   Pressroom   |   Business   |   Links   
Logout    

© Green Building Press