18.104.22.168 Urban drainage
Urban precipitation, surfaces and catchment characteristics; impacts on drainage basin storage areas; urban water cycle: water movement through urban catchments as measured by hydrographs.
We have already seen that urban areas can drastically alter the climates of urban areas, but what of the drainage systems that exist in urban locations?
Precipitation is higher in urban areas due to increased presence of hygroscopic nuclei, and urban areas also experience more thunderstorms. These alter the INPUTS to the urban drainage system, but other changes also take place. When looking at the changes in the urban drainage system, it is useful to consider the Urban water cycle and balance;
A natural water balance displays the RELATIONSHIP between inputs (precipitation), stores (soils, rocks, surface, plants) and outputs (evaporation, transpiration, runoff) of a drainage basin system. This is significantly disrupted by the construction of urban areas
The urban water balance also displays the RELATIONSHIP between inputs (precipitation), stores (soils, rocks, surface, plants) and outputs (evaporation, transpiration, runoff) of a drainage basin system BUT tends to have less infiltration and evapotranspiration, greater runoff than natural systems and also has imported water and waste water losses not present in natural systems. These changes are shown in the diagrams below;
The main changes include:
These changes are more pronounced the more urban an environment, so as we progress from a rural urban fringe area into suburbs and on into the CBD the impacts on urban water budgets become more severe.
The Urban Hydrograph
A Hydrograph is a graph which shows the discharge of a river, related to rainfall, over a period of time. As a result of the changes to urban drainage basins, urban hydrographs tend to be flashy, where rivers are more likely to flood due to lower interception and infiltration rates. Although the base flow of rivers is lower in urban rivers that in rural rivers as less if fed into the system via soil and rock stores, these rivers fill quickly during rain events and are more likely to flood. The lag time of an urban river is shorter due to drainage systems which speed the flow of water through urban areas and a higher peak discharge is evident in urban rivers. These rivers also have steep rising and falling limbs as water reaches urban rivers quicker via drainage networks. The contrasts between a rural and urban hydrograph can be seen below.
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Posted by Rob Gamesby April 2020