Introduction
Since the last century, due to increased urbanization of our modern society and various alterations in landform use related to such large scale urban development, flooding had been affected in numerous ways. Urbanization has been achieved by construction of drainage systems, grading of the surface of land and removal of soil and vegetation. This has increased runoff from snowmelt and rainfall due to decreased infiltration to the streams.
Runoff volumes have increased due to the addition of numerous constructed areas, like parking lots and driveways, which are impermeable and thus, do not allow the water to enter into the soil. Also, this runoff water, due to reduction in its travel time, moves faster and such a situation is a direct result of concrete channels, gutters, curbs and storm sewers. All of these lead to an increase in the frequency, peak discharge and volume of the floods.
Stream channels are also sometimes changed, which highly limits their capability of conveying floodwaters into nearby streams. Buildings and roads that are built on flood prone zones increase the risk of erosion, thus completely altering the land’s capacity to drain water. Urban development has turned low risk flood zones into high risk ones by completely altering the topography. (Collier, 2007)
Hydrologic Effects
Huge volumes of runoff water which rapidly flow into rivers and streams as surface or overland flow causes floods. Hydrological setting previous to snowmelts and gale, their extent and strength, geology and landscape of river basins and foliage massively influence the highest point discharge of floods. Urban development and high land use by humans have affected this peak discharge of floods by altering the manner by which snowmelt and rainfall is stored on land surfaces and runoff into streams.
Urban areas, unlike grasslands and forests, have very little capacity for storing snowmelt and rainwater since they are covered with buildings and roads and thus, does not have any vegetation in the surface depressions or soil columns to collect them. (Monaghan, 2006) Since construction of buildings and roads requires us to remove depressions, vegetation and soil from the surface, permeable soil gets substituted with impermeable surfaces lessening permeation of water underground.
This speeds up runoff into streams and ditches. Also, these dense systems of culverts and ditches lessen the length runoff needs to pass both underground and overland for reaching the rivers and streams. The urban basins have very little capacity for storing water and due to rapid runoff during high peak discharges and storms they raise quickly causing floods. Such hydrologic effects due to urban development are even more apparent in smaller stream basins.
Before undergoing urbanization such areas due to the presence of a lot of precipitation could develop into underground flow thus, discharging and recharging aquifers into the network of streams. Urban development totally converts landscapes in smaller stream basins whereas the natural soil and vegetation of larger river basins are somewhat retained. (Alila, 2007)
Hydraulic Effects
Urban construction along floodplains and stream channels immensely change channel capacity in conveying water and also increases the altitude of the water, called stage, with respect to a discharge. Constructions like bridges, which infringe floodplains also amplify upstream flooding because due to them the breadth of the conduits get lessened and their resistance to flooding increases. Due to this, the water flows past any barrier and rises to a higher stage, thus forming a backwater further flooding a bigger area, upstream. Debris and sediments that are contained in the floodwaters additionally narrow the channel further increasing flooding.
Such a hazard is more likely to occur in upstream areas having bridges and culverts where the debris gets collected. Either the deposits and residues block up the less important watercourse channels or they are congested by the wastes due to the underdeveloped culverts thus, generating a closed basin which has no passage to discharge the overspill. Although the debris and floodwater can be transmitted rapidly downstream through the construction of channels, its short term benefits should be balanced next to the possibility of an increase in flooding downstream. Another most important effect of urban expansion which causes flooding is attrition of topography in the inner-city streams, since due to recurrent flooding in these streams, bank and conduit wearing away increases considerably.
In those areas where the channels are rolled out and straightened or the vegetation from the channel banks are removed, the velocity of the streamflow increases, enabling the stream to transfer sediments, more than usual. Urban development frequently leads to stream bank erosion which poses a major threat to the various structures, like bridges and roads, as they are difficult to even after inuring the stream banks. (Miller, 2003)
Effects on Flood Frequency and Discharge
Increase in the frequency and discharge of floods is a general consequence of urban development. Normally, urban development causes the annual maximum discharge of runoff water into a stream to increase, but such an increase is, at times, covered by the significant variations in the storms which take place from year to year. When we face moderate storms that are followed by dry periods, the consequences of urban development become even more evident. Larger storms that occur during wet periods saturate the soil in undeveloped basins and the additional snowmelt and rainfall runoff just like they do in the developed urban basins.
This is where the difference lies. Thus, we see that an increase in peak discharge will be more for smaller frequently occurring floods than larger infrequently occurring floods. (Ashley, 2008) Therefore, urban development substantially increases the frequency and size of flooding exposing people to increased flood hazards. For example, researchers have found that when flood frequency occurs every 2, 10 or 100 years the peak discharge of flood increases due to urban development by almost 600%, 300% and 250%, respectively, when it should have increased by only 50%, 10% and 1% respectively, had there been no urban development. (Douben, 2006)
Conclusion
Researchers have found that urbanization of land increases runoff by almost 6 times than what would have naturally occurred on that particular terrain. Commercial and urban developments within flood plains and sometimes near the oceans lead to increased flooding. Urban development in mountain regions causes a rapid rise in the river waters after a spell of heavy rainfall due to the drainage system of the area.
Changes in land use will cause erosion and runoff since there is no vegetation to stop the water. If small streams are converted into culverts, then the area becomes impervious and the runoff will join the storm sewers and other smaller streams causing floods. Urban development is a necessity and thus, it should be properly planned, since in the absence of planning we will have to face extremely damaging floods which will destroy all of our valued structures.
References:
Alila, Y. (2007). Implications of heterogeneous flood-frequency distributions on traditional stream-discharge prediction techniques. Hydrological Processes, 16(5), 1065-1084.
Ashley, ST. (2008). The storm morphology of deadly flooding events in the United States. International Journal of Climatology, 28(4), 493-503.
Collier, C.G. (2007). Flash flood forecasting: What are the limits of predictability? Quarterly Journal of the Royal Meteorological Society, 133(622), 3-23.
Douben, K. (2006). Characteristics of river floods and flooding: a global overview, 1985-2003. Irrigation and Drainage, 55(S1), S9-S21.
Miller, A.J. (2003) Flood hydrology and geomorphic effectiveness in the central Appalachians. Earth Surface Processes and Landforms, 15(2), 119-134.
Monaghan, MT. (2006). Stream ecosystem response to multiple experimental floods from a reservoir. River Research and Applications, 20(4), 359-377.