Extent of Urbanization Affecting River Flows of a Drainage Basin
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Extent of Urbanization Affecting River Flows of a Drainage Basin

Factors affecting River flows of a drainage basin which would cause different speed, duration and amount of flows to and within a river channel.

Urbanisation of an area includes clearing the land of natural vegetation and landforms to make way for housing development and industries, creating many man-made structures that can affect the flow of water. Hence, when urbanisation occurs within a drainage basin, it can greatly affect the speed, duration and amount of flows to and within a river channel.

Urbanisation also involves the laying of tarmac and pavements, and also the construction of buildings, all made of concrete, glass and other impermeable surfaces. The creation of such impermeable surfaces affects flows and transfers throughout the river basin system. Under normal circumstances, when precipitation occurs, rainwater is partly intercepted by vegetation, and will move to the river channel either by surface runoff or infiltration into the soil followed by base flow through the soil and to the river. Urbanization is not the only factor that affects the speed of water flow to and within the river channel. As such, I only agree to a small extent that that it is a more determining factor that affects the speed to and within the river channel.

Firstly, when a drainage Basin becomes urbanized and the ground around the river channel is replaced with tiles and concrete.  The presence of such impermeable surface would cause the amount of infiltration into the ground to be greatly reduced and so, there would be lesser through flow and base flow. Hence the water that are unable to infiltrate into the ground would have to flow to the river as overland flow, thus increasing the rate of flow to the river channel. Hence in storms the peak flow would take a shorter time to come as water would enter the river channel at a greater speed.

When the river channel itself gets more urbanized, meaning having the channel sides lined with concrete, this makes the wetted perimeter of the river channel smoother. As the velocity in a river channel is affected by its shape and the channel roughness, having a channel side lined with concrete would make it smoother and hence this would reduce the friction the water has to overcome in order to flow. Also, humans will modify the river channel if meanders are present, so as to make it as symmetrical as possible, hence increasing the velocity of the river. Another modification to the river to prevent possible floods is to deepen and widen the river channel which increases the bank-full capacity. This increases the hydraulic radius of the river, increasing the energy and speed of the river, hence affecting the flow.

Of course, there are other reasons such as physical factors that do affect the flows, such as presence of vegetation and the type of drainage basin.

Vegetation is also a factor that affects the flows to and within a river channel. During rainfall, water would have land on the leaves of the plants, reaching the ground in 2 possible flows, stem flow and through flow. The rain drops would be intercepted by the leaves of the plants and hence slowing down the rate of flow onto the ground. As such, there would be more time for water to infiltrate into the ground and percolate into subsoil and bedrock, hence there would be a delayed and lesser overland flow as water are flowing to the river channel through through-flow and base flow as well. It is only when the soil is saturated that overland flow would occur. Therefore it can be seen vegetation can affect the flow to the river channel.

Also, vegetation helps to hold the soil together and prevents it from being loose. Vegetation helps to hold the soil around the river channel tightly, preventing it from falling into the river channel itself easily. As such, vegetation around the river channel would prevent soil from falling into the river easily and also, it would prevent the soil from being eroded away into the river easily. As more load in the river would increase the channel roughness and decrease its velocity, having vegetation would decrease the amount of load the river would have to carry with it. Thus vegetation can also affect the flows within the channel.

The type of drainage basin would affect the rate of flow to and within the river channel as well. Having a larger drainage basin would mean that the volume of water would be greater, as there is a larger catchment area to collect the rainfall during storms. On a hydrograph, the lag time to peak flow would be longer as the rainfall collected would have to travel a longer distance before reaching the river channel. As for an elongated drainage basin, there would be a longer lag time and a lower peak flow when compared to a rounder drainage basin. This is due to the fact that the distance water has to travel is higher.

Overall, there are many factors that affect the flows and this includes human and physical factors as well. Urbanisation affects the rate of flow by increasing the speed to and within the river channel by the use of concrete. But without enough rainfall, it is difficult to see the differences urbanisation has made to the flow. Factors such as vegetation play a significant role in affecting the rate of flow to the river as the amount of vegetation can affect the types of flows and the rate of flows depending on the intensity of the rainfall. Lastly, the drainage basin itself has a large impact on the overall rate of flow to the river channel.

In conclusion, vegetation affects the types of flows towards the river channel and also the speed of which water flows to the river channel greatly. As compared to urbanisation, it only affects by increasing the speed of the flow to and within. Although it is undeniable that urbanisation does affect the flow as well, I still feel that Vegetation is a more determining factor that affects the type and speed of flows to and within a channel. Thus I agree to the statement to a small extent.


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Comments (1)

Very good points.