Urbanization
Urbanization replaces permeable land surface with impermeable one, which is covered by streets, parking lots, buildings etc. Water will run off the surface rather than being absorbed by soil. Base flow will decrease while overland flow will increase. With more overland flow, river will respond more quickly to rainfall, resulting in a shorter lag time. The construction of sewers during urbanization effectively increases the drainage density, resulting in a higher peak discharge.
Deforestation
Trees lengthen the lag time by intercepting the water. They also increase water infiltration by taking up water by their roots. Deforestation will reduce the interception and water-taking up; there will be more overland flow. Rivers in areas that have been cleared by deforestation will also respond quicker to rainfall, resulting in a higher peak discharge and shorter lag time.
Building of dams
The presence of a dam, will allow flow to be controlled, reducing the risk of flooding and allowing rivers to gradually respond to heavy rainfall in a controlled way; the river has a longer lag time.
Done by Nguyen Trung Hieu (23)
REFERENCE:
Channel Geometry and Flow Characteristics. The Physical Environment. Retrieved on 26 April 2011.URL: http://www.uwsp.edu/geo/faculty/ritter/geog101/textbook/fluvial_systems/channel_geometry_and_flow.html
As you mentioned about the influences of urbanization on hydroghraph, in theory, it will result in shorter lag time and high peak discharge. Howerver, urbanization not only brings free space such as parking lots, it also has small lane and street with different height, which will block the flow of water inside a wall of impermaeable land, resulting in the longer lag time. It can be observed during the rainning season in Hanoi, the different in height of different streets and the weak canal system around the city create a flood in the city in about a day after a rain in the morning.
ReplyDeleteIt is not always true that urbanization means shorter lag time, it also depends on the canal system and the gradient of the flow.
Vu Tuan Duc (24)
While deforestation is a human influence, so is afforestation, the planting of vegetation. Afforestation can help to protect the river banks because the tree roots help to hold on to the soil. Therefore, lesser erosion will occur, and there is a reduction in the amount of sediments deposited into the river. As such, the river channel will not become shallower, resulting in lesser floods. Thus, overland flow decreases. As Hieu has already mentioned, trees help to lengthen the lag time by intercepting the water.
ReplyDeleteHence, the hydrograph should have a longer lag time and lesser overland flow.
Ng Jane Wen, Chelsia
JH402 (8)
Rachel Tan: Although urbanization will replace permeability of land, have you considered about the drainage systems in place, which may in term increase the lag time instead?
ReplyDeleteYes, flood do indeed occur in heavily urbanised Singapore, but the cause is not due to the lack in permeability of the land but actually due to the blockage in the drains. Also, as can be seen from other areas in Singapore, floods do not occur. Hence, it can be seen that an effective and useful drainage system is in place. Hence, I beg to differ that urbanization causes a decrease in lag time
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