07/04/09 Daryl Lee
What a rainy day... Perfect for GEOGRAPHY =D. Today, Mr Faizal showed and explained to us what was happening at the 'garden' below our classroom.
So.. from what we have learnt from Mr Heah,
Firstly, there is interception, which is the trees, the grass and whatever that is obstructing adrop of water from falling down to earth to get absorbed by the soil.
SOMETHING LIKE...
YES! That is interception.
Then, there is infiltration, when water infiltrates in the soil.
When the soil reaches infiltration capacity, overland flow might result like we saw from the top of our classroom. Overland flow sometimes results in floods when it gets too serious. That is why vegetation plays an important role in preventing the soil from clumping together to increase infiltration capacity.
<---- OVERLAND FLOW There is also throughflow, which is water moving through the soil.It is not as fast as overland flow, travelling at between 0.005 and 0.3m per hour.
After through flow, there is an even slower kind of flow, which is groundwater flow.
This happens when the underlying bedrock is permeable. Water will then slowly soak up the soil below and it will then flow through the small pores of rocks. This process is extremely slow. (takes a few THOUSAND YEARS)
Also, Mr Faizul also stated that some parts of the soil had clumped together as there is no vegetation on it and water is unable to infiltrate there. Therefore, we could see more overland flow at those parts of the 'garden' then at the parts with vegetation on it.
Human activity has also affected the hydrological cycle as water is unable to infiltrate through concrete and hence we could see (from the side of our classroom) lots of overland flow.
That's not all, we also read on the river channel processes which i will type about now =D.
Firstly, there are three river processes:
1) Transportation
2) Deposition
3) Erosion
1) Transportation
before we can talk about this, we have to know the kinds of load that a river carries
i) Dissolved load- Transport of chemicals in water (Solution)
ii) Suspended load- Sediments that is being carried by the flow of water (Suspension)
iii) Bedload- Material that is too heavey to be suspended in the water.
--> These are transported by rolling (traction) or bouncing (saltation)
so much sediments!
2) Deposition
Happens when velocity begins to fall, it has less energy and can no longer hold the load it is carrying.
Occurs when,
1) The load suddenly increases
2) Shallow waters
3) Low precipitation
4) When a river enters a sea or a lake (less velocity)
5) River overflow its bank hence the velocity outside the channel is reduced
3) Erosion
It is the picking up and removal of material.
3 types:
i) Corrasion- particles picked up by the river is rubbed along the river beds, wearing them away
ii) Hydraulic Action- occurs mostly at waterfalls and rapids. Air bubbles may burst in these areas sending out shockwaves which may increase erosion, this is called cavitation
iii) Solution- Dissolved CO2 from the air which is dissolved in the water may react with limestone and chalk, causing them to dissolve.
Lastly, there is attrition which is the particles that are being transported by a down-river crashing against one another causing them to become increasingly rounder and smaller in size.
ALRIGHT DONE! =D
GET WELL SOON MR HEAH =D
posted by 08ip01 at
5:32 PM
14 Comments:
Haha. Just a comment inspired by that picture of the sediment loaded overland flow, when a river floods, it can also get extremely contaminated as more soil is eroded as the rain/water flows over it, and plants are uprooted as a result. The water can also flow into unwanted areas like garbage dumps, 'transporting' the waste along with it as it flows about. The 'overland flow' really picks up about everything it can over land and 'flows' it about,resulting in water contamination and pollution...
Urbanisation is also one of the main cause of overlandflow (I think). Since the soil is replace by streets. Infiltration rate decreases --> flood
(Vu Ngoc Quang)
Minh:
For the hydraulic action, in waterfall the air bubbles actually IMPLODE. It's an interesting Physics phenomenon that you may want to
I wonder if deltas can be composed of human garbage that the river has picked up along the way, especially in urbanised areas. If such really does exist, i suppose it will have an effect on the soil composition, especially unbiodegradable plastic which will be detriment to the living beings on the delta.
Indeed dumping rubbish into the rivers and seas seem an easy way of clearing our waste but it doesnt really gets washed away. At the end of the day, it might end up polluting areas populated by humans which means we ultimately end up suffering the consequences of our irresponsibility.
Actually I think that urbanization does not really cause flood, if the city plan is good enough in managing the channels, sewers and canals. An example is Singapore. In the past, Singapore used to have flood very frequently. Nowadays, Singapore is indeed free from flood and is even able to fairly alter the water cycle to sustain her own water need. The reason of flooding is rather improper deforestation in hilly areas, since it reduces interception and soil infiltration capacity (without vegetation, the spongy and soft soil is easily washed away which makes it less likely to absorb that much water).
I think that was a really awesome post, but just to add, I think it is also important to know so what if the sediments can deposit and erode? Let me just link this back to something that alot of us are familliar with, deltas and is cross section. We should know by now that there are some features such as the small river cliffs at the bank of the river which is formed due to erosion of the sediments as the flow of the river would be extremely fast at that area of the turning point. We also have the slope in the river itself which is caused by the slower flowing waters depositiong the sediments and leaving them to pile up, forming slope.
From this, what we can see is that not only does the type of the sediments matter in the deposition or erosion but the velocity of the movement of water is also another important variable.
Speaking of interception, perhaps its also a good idea to talk about throughfall and stemflow, which is water falling off leaves or running down plant stems and tree trunks. It shows that interception does not completely bar the water in its way from reaching the ground.
The infiltration capacity of the soil decreases because of the urbanization. In urbanization, the soil is cemented, so no more water is soaked by the ground and hence leading to the floods. This is the reason why we see some of the cities in developing countries like Mumbai in India having floods so often. Mumbai had its worst floods on 26 June, 2006. Go google and type Mumbai floods and look at the images.
Sorry guys, the date i gave was wrong
It was actually on 26th July, 2005
About the effects of urbanisation, I think it then depends on what exactly is being built, and on the existing level of development. Generally, urbanisation increases the rate of transfer by a fair bit because as said, the soil is cemented to create pathways, buildings, etc and there are pipes (drainpipes), roof gutters and so on. I guess the point would be it increases flood risks, which can only be lowered with proper management through alteration of the several factors of a river's flow. It's not a direct and definite relation though, that urbanisation leads to flooding. I guess this is why it is crucial to understand the measures that may be taken to reduce flood risks.
But it goes the other way too, that if there is minimal human activity and no urbanisation, there will be no floods. Each river has its own individual characteristics that have to be explored first. Other factors such as the amount of vegetation, gradient and so on come in.
Also, just to add on, the type of load that is being transported does not occur in equivalence for all rivers. It is dependent on the geology as well. Apart from velocity, the type and size of sediments, whether the bed is made of clay, sandy soil, etc, other factors could include the presence of faults that may allow more water to seep in to increase the amount of chemicals dissolved (solution), etc.
Sorry that was me...
Just in case some of you are wondering why lower infiltration rate will cause floods, it is because when there is less infiltration, there will be less water passed off as throughflow or groundwater flow. Instead, most of it is passed off as overland flow, and since overland flow is faster, the hydrograph will have a higher peak, so thats why there will be floods.
As for floods, normally flash floods occur when infiltration rate is moderately low but the field capacity is extremely low. That is when the floods that normally cause catastrophic damage occur. This is because if infiltration rate is low, there will be more surface runoff which may lead to floods which is what weng kin mentioned. But if the field capacity is extremely low, meaning the amount of water it can hold is low, chances are water is going to overflow very quickly, and during a heavy downpour, we would be swimming instead of walking :O
I believe urbanisation does indeed increase flood risk. Urbanisation and deforestation often go hand in hand, and deforestation is indisputably a cause for increased flood risk due to lesser infiltration. Although cities also provide interception, cement has rather low infiltration capacity, resulting in more overland flow. Also, the drains in fact have an effect similar to increased drainage density, which, as we all know, leads to a more peaked hydrograph, and increased flood risk.
Perhaps the reason we see less floods in Singapore is because the drainage system and management has improved. Also, most drains carry the water, after it is purified and whatnot, to the ocean, which has a much bigger capacity to hold water than most lakes.
And I really doubt Singapore can manipulate the water cycle (in response to Minh's comment) Technologies like NEWater alllow us to circumvent or go around the water cycle, not alter it. We can't command rain at will, unless perhaps by cloud seeding, which I don't think Singapore does.
Post a Comment
Subscribe to Post Comments [Atom]
<< Home