river flow 2

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River Flow 2Lab experiment

This experiment was carried out on the 25th of October in the Fluids Lab. We

simulated a river flowing and studied how it performs with the addition of

bridge abutments and water features, the results of which are featured in this

report.

2012

Joe McCormack

Engineering Geology, Civil 4

3rd

Nov 2011



Joe McCormack River Flow 2 Engineering Geology

2

River Flow 2

This was our second river flow experiment on the Hydrology bench. This time we

were investigating the behaviour of our river with respect to.....

• Bridge abutments

• “Rip Raps”

• Bank erosion

• “Pools & Riffles”

• Bars

• Sediment transportation

The set up for this experiment was the same as River Flow 1. We turned theapparatus on an hour previous to get the river flowing at full capacity. When we

arrived to the lab, the first thing we did was record the flow, which was 2.5

l/min at the inlet and 3.0 l/min at the outlet. Again, similar to the previous

experiment, the difference in flow had to do with the build up of groundwater

which filters through the system, flows down the bench (2% gradient toward

outlet) and exits through the outlet. This process tends to give a small increase

in flow which we read at the outlet.

We then had a look at the Piezometer’s attached to the bench, we recorded the

readings and this is what it looked like....

Piezometer Trend




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One thing which was visually evident was the increase in sediment transport

compared with the last experiment. This has probably got to do with the

increase in velocity due to flow rate which was not as forceful as before. We

found ourselves cleaning out sand from the trap beneath the outlet more often.

It was easy enough to spot the sediment transport due to the movement of

black particles in the sand, probably mica which were about 1mm in size.

Another outcome of increased velocity was the greater width in the channel.

This was particularly evident at both the inlet and the outlet, the velocity due

to the flow of the river was obviously causing havoc to the banks of the river

through erosion.

One thing we were going to concentrate more on this time was the developmentof “bars” (an elevated region of sediment) as a result of changing circumstances

further up in the river. However there was a bar formed at the start of the

experiment, with no real evident cause for a bar to form. On consulting our

lecturer and through looking closely at the set up, we discovered a slight curve

in the bank, further up, on the opposite which was causing the sediment to be

guided in the direction of the bar (illustrated below). This is commonly known as

sinuosity in a river, where the watercourse naturally meanders, eroding banks

which can turn up again downstream. The level of this erosion can cause real lifeengineering problems and effect people who occupy houses, business or land

adjoining river banks.




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As seen above we tried to barricade the bank by adding an imitation wall, which

would shield the bank and protect it from further erosion. This did not have the

desired effect as there were obvious gaps between the stones which led to the

flowing water passing through and eroding the bank behind the stones. If this

were a full scale project, the erosion taking place behind the rocks could have

been avoided by grouting the joints. The wall did however deflect the flow to

the other side of the bank which brought new issues to the fore. Thinking we

were going to save the bank on the left, we caused the bank on the right suffer

additional erosion. A significant increase in sediment transportation was

recorded as a result of the wall either side.

We then removed the wall and put

the stones across the river creating

a wall and a “rip rap”. The “rip rap”

consists of stones placed flat on the

river bed. Combined with the wall in

front, the system resulted in a

decrease in the transportation of

sediment. When the flowing water

passed over the stones it meant the

velocity of the flow was disturbed,

it also meant the river wasn’t as deep in that section. The two factors combined

led to a decrease in the velocity.




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We then added the first bridge

abutment to the sequence. We left the

“rip rap” in place to see the effect it

would have on the process. We noticed

a pattern develop almost immediately,

where a bar formed either end of the

abutment and a channel grove became

deeper and wider at the sides of the

abutment. It was clear when looking at

the outlet that a change had occurred,

the width of the channel was becoming wider. The banks were being eroded at

considerable pace and the bar which formed at outlet left, had been washedaway.

We then removed the “rip rap” and waited to see what would happen next. Thebars at either end became significantly bigger and new bars were developing.

The increase in velocity, due to the removal of the “rip rap”, lead to bank

erosion becoming a real problem on the right bank. It is wise to observe the

consequences of small change on a river system, if this experiment is anything

to go by.

Next we were to witness another fault arise from the addition of the second

bridge abutment. This abutment was more angular than the previous one and so

we examined it in order to see what would happen in the river system.




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We left the first

abutment in place and it

wasn’t long before we seen

some results. A “pool and

riffle” formation was

developing. The force of

the flowing water

undermines the sediment

at the near face of the

abutment and then dumps most of it relatively close-by. The distances are

generally equal, where the length of the pool is the same as the length of the

riffled out material. You may notice from the diagram that the bars havedisappeared from the original abutment, another observation which should be

noted!

We then added a third

and final abutment to

the trial. Interestingly

we discovered

something similar to

the last step where

previously the bars

disappeared, this time the “pool & riffle” disappeared from the second

abutment and the flow arrangement ran uniformly for a couple of minutes.

However before long, a “pool & riffle” pattern developed at the front face of

the new abutment, which we suspected might happen. Also, the channel width at

the outlet began to increase again and bars were beginning to develop again in

the system as seen below.




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I think one of the most important observations is the varying width of the

channel at the outlet. This is a good representation of the potential

vulnerability associated with river systems, where reasonably small

modifications can have detrimental effects further downstream.

The increasing channel

width became our

priority. In an attempt

to remedy the situation,

we installed a “rip rap”

at the first abutment.

This worked fine for a

while and it seemed as

though the river had

stabilised, with no

expansion of the

channel at the outlet.

After a few minutes, the river began a downhill trend yet again. The rivers

banks began to depreciate and new bar formations were evident. The “pool &

riffle” which was positioned at the first abutment remained there. As shownabove, the “rip rap” enabled the water flow over the top of the stones but after

some time the watercourse began to develop a new path at the bank, left and

right of the “rip rap”. The creation of this new watercourse meant that the

banks were going to erode at a swift pace and so we removed the “rip rap” so as

not to create any more problems than we already had.

For the next 10 minutes, we experimented with different configurations of the

system, trying to find the best means of leaving all of the abutments in placewhilst altering everything else around. This proved to be very difficult as it

seemed that as soon as we repaired one aspect of fault, another would appear

somewhere else. By carrying out this experiment, one could sum up the

proceedings with a quote:

‘Rivers are changed by the landscape through which they flow, and in turn

change the landscape.’ 1

1‘River Systems’ http://www.traleetimes.com/files/Geography/River_Systems_-_NOTES.pdf [accessed on 3

November 2011] p. 1.

http://www.traleetimes.com/files/Geography/River_Systems_-_NOTES.pdf




river flow 2

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