Floodplains and Levees

Floodplains

A floodplain is a flat area of land bordering a river that is subjected to periodic flooding. It is made up of silts and sands which have been deposited oever many years by the river.

When the river floods over the surrounding land it loses energy and deposits its suspended load. Regular flooding results in the building up of layers of nutrient which forms a flat and fertile floodplains.

Levees

A raised river bank building up by deposition of the river during flooding period.

When the river water burst its bank, the water loses its energy and therefore deposit occurs. The heaviest materials are deposited first as these required most energy to be transported and therefore build up around the sides of the river forming raised banks known as levees.

Done by Frank JH406 (19)

MEANDERS!!!

Alright, let's start off by refreshing your memory of what a meander is. In simple terms, a meander is a feature of a river, at where there is a bend in the river course. Meanders are formed by mainly 2 processes, namely deposition and erosion.

Flow of fastest water, Thalweg, swings from bank to bank, and when it hits the outer bend, erosion occur due to hydraulic action, resulting in the undercutting of the river bank, forming a steep sided river cliff, a concave bank. This can be observed in the cross section diagram below.

Next, due to the helicoidal flow of water in the meander, sediments are brought to the inner bend, and water being slow flowing at the inner bend deposits the sediments, forming a slip off slope, a convex bank.

Overtime, the convex and concave bank would be more obvious, thus showing the bends in a river course - a.k.a MEANDERS!

An example is the stretch of River Conwy in the North Wales, which use to be a river in the past, but now having complex pattern of meandering.

Overtime, meanders actually leads to the formation of an Ox Bow lake too. As we have learnt that erosion takes place on the outer bend, it causes the neck of the meander to become narrower, and eventually joining together. Water will then takes the shortest route, which is the straight river course without passing through the meander bend. after some time, deposition will then seal off the old meander bend, forming a straight river channel. The old meander bend that is left isolated will be called an ox bow lake!

Sean Quek

JH406.21

Effects of Agricultural and Grazing on Water Balance and Hydrograph (Pham Minh Tri)

1. Water balance:
P=E+Q+deltaS
P= Precipitation
E=Evapotranspiration
Q=Run off.
deltaS=Change in storage.

Basically, water balance is just an expression of the idea of conservation of water: If there is water precipitated, that amount of water has to either evaporate back or run off to somewhere else.

2. What we do in agriculture and how that affects water balance:
Assume that the considered land was previously a forest, what we do is basically to chop down all the trees and vegetate the land with crops. To determine how that affects various factors in the water balance, let's make further assumption that the previous concentrated forest is replaced by corn. Based on that assumption, evapotranspiration will obviously drops since the ability to intercept and hold water of the concentrated forest is much better than the corn field. Hence E drops.
Through agricultural activity, the surface soil may be soften and made more spongy. However, as the vegetation layer is much thinner than before, this surface soil is easily washed off. The root of the crop plants is also unable to penetrate deep down into the soil, hence the effect of plants on deeper soil's capacity is eliminated. Hence, deltaS drops.
Since E and deltaS drop, Q increases (Assuming that the amount of precipitation is constant)
3. Changes in Hydrograph.
As surface run off increases, the peak of the hydrograph is expected to be higher (more overlan, the lag time is expected to be shorter and the gradient of the rising limb is steeper.

Erosion and deposition process

EROSION consists of three components, which are Corrasion, Hydraulic Action and Solution.
Any surplus energy is used by a river to carry out erosion. Erosion is the picking up and removal of material. There are three main processes by which a river will erode its channel.

Solution: water contains dissolved carbon dioxide from the air and this may react with limestone and chalk, causing it to dissolve.

Corrasion: this is where particles of rock carried by the river grind away at the bank and bed. It

is the most significant type of erosion in most rivers.

Hydraulic action: this is the sheer power of the water as it crashes onto the bed or against the banks. It is particularly significant at waterfalls and raids or during times of flood. Air bubbles may burst in areas of great turbulence sending our shockwaves which may increase erosion. This is cavitation.

DECOMPOSITION occurs throughout the course of a river wherever the speed of flow drops such that particles can no longer be carried. This could occur on the inside bend of a meander on the bed of the ricer or close to its banks where friction is at its greatest or where the ricer enters the sea or a lake and its flow is checked.

In addition, erosion mainly happened at upper course and middle course of a river, where holds the largest energy, and also the fastest speed. Decomposition mainly happened at the lower course of a river, where has the lowest speed or energy, even though its volume may be the largest.

Done by Fangwei

Waterfalls and plunge pools

A WATERFALL is a vertical step in the river. It is usually found along the upper course of the river where velocity is usually greater. Waterfalls usually form when there is a band of more resistant rock cutting across less resistant rock, as seen in the picture below (:



As the river flows, the more resistant rock gets eroded slower than the less resistant rock. Thus, a step is formed. Over time, as water goes down the vertical step its kinetic energy hits the bottom, causing a deep pool under the waterfall. This is called the PLUNGE POOL.

As the plunge pool gets larger and larger, the overhang of the waterfall (the more resistant rock, in the picture) will eventually drop into the plungepool due to a lack of support. Overtime, the waterfall receeds. Apparently, the niagara falls retreats about three feet a year! :O

The biggest waterfalls in the world attract many tourists. These include niagara falls, victoria falls etc. Here's a pic of the picturesque Niagara Falls (:




Grace.

Urbanisation

As familiar as this question may seem to us, only a few selected ones actually know the true answer to this puzzling question - how does urbanisation affect the hydrograph? First of all, in order to answer this question with precision, this has to be broken down into 2 parts, the water balance equation as well as the hydrograph.

Water Balance Equation: Ppt = Q + E + Change in S
Legend: Ppt = percipitation
E = evapotranspiration
Q = surface runoff
(delta S) = change in soil moisture

During urbanisation, there is a decrease in Evapotranspiration (E) due to the fact that trees are being cut down for the sake of development. As a result, this results in a decrease in soil moisture as well, as when trees are cut down, this reduces interception capabilities through throughfall and stemflow. As such normally, we would expect an increase in surface runoff (Q) by rainfall soil erosion. However, this is not the case as in urbanised areas, there is little soil exposed as soil is mainly covered up by concrete which is impermeable. As such, this reduces the infiltration capacity of soil greatly. As a direct result, as surface runoff = total ppt - rate of infiltration, there will be an increase in Q.



Now that we know that there is an increase in surface runoff, this would also lead us into discovering that there is basically a higher peak now in the hydrograph as well as a steeper rising limb as those 2 are dependant on amount of surface runoff generated as surface runoff is the fastest to enter the river. There will too be a lesser lag time as seen in graph above as basically, in urbanised areas, to protect against flooding as a result of high amounts of surface runoff, there are advanced drainage systems which will too speed up in the generating of surface runoff, decreasing lag time. As such, we now know more about effects of urbanisation on hydrographs. Oh yea. Great..

And ya, kudos to MOE for the graph above.

Joshua
JH40615

The River System! :D

Yesterday, Miss Tang introduced the river system to us!

She started off by showing us a picture of the part of the Nile that ran through Egypt. As we observed the picture, we noticed that the Nile looked GREEN. o.O However, as we were shown the zoomed in picture of the delta, we saw geometrical plots of green, indicating that these were agricultural plots of land, and not forests. Forests won't grow such perfectly straight and rectangular boundaries. Further up the Nile, away from the delta, there were buildings and much infrastructure as well as some agriculture. Upon further analysis of the picture from Google Earth, we noticed that Egypt was mainly established along the River Nile. Then, we were shown a picture indicating the population density of Egypt. Along the Nile, the population density was between 1000 to more than 5000 per sq km! However, further away from the Nile, was the Sahara, which had a population density of 0-10 per sq km. There was also a high population density in the Nile Delta, where there is much agricultural activity.

After we were shown the pictures, Miss Tang distributed clay to us, and we were to use it to make a model of the river system. We were given the various parts of the system, namely the delta, waterfall, meander, floodplains and levees, ox-bow lake and rapids, but were not told what went where. We were also not allowed to refer to our notes to find out what was what and what went where. After making our very professional and artistic models of the river system, we had a class discussion about the location of the various features of a river system. We learnt that the Upper Course Features (near the source) included waterfalls and rapids. Meanders and ox-bow lakes make up the Middle Course Features, and Lower Course Features would be deltas and floodplains and levees. Levees are mounts at the side of the river that normally consist of bigger and heavier sediments that the river had deposited. Floodplains contain smaller sediments deposited by the river.

Miss Tang then discussed some questions with us.

1. Why are waterfalls and rapids near the source?
   Because they need high ground and mountainous areas to exist.
   
   
2. Are meanders really slower (velocity)?
   It looks like it! However, rapids and waterfalls may only look faster because there is more turbulence from the rocks, and so it looks faster and more violent. However, the middle course (i.e. meanders) may have a higher velocity as well because of the bigger volume and larger amount of energy.
   
   
3. What's the difference between meanders/ox-bow lakes and rapids/waterfalls?
   The middle course features are on lower and flatter ground compared to the upper course features.
   
   
4. What do you see at the lower course?
   - Sediment Deposition
   - Sediments too heavy to be carried by river
   - Velocity drops further

For the last part of the lesson, Miss Tang showed us pictures from one of her field trips. I personally thought that the pictures of all the features were really cool and breathtaking! :D

Done By,
Wong Hui Ying. (:

PS: Sorry if the font size is too small, I didn't know it was so puny and now I can't change it back. :P