Encroachment on Floodplains
Most major cities and towns are located in river floodplains, which causes additional
stress to river basins. During the past decades more developmental activities have
taken place in urban and agricultural lands, leading to additional stress in these
floodplains and resulting in loss of flood spread areas and increase in flood risk in
downstream portions of the basin. For example, most major European cities in
England and Germany are located in floodplains that have induced burdens in the
rivers, and preventive measures have been made to recreate floodplain storage
(Wheater 2006). Indian cities such as Delhi, Vijaywada, Hoshangabad, Surat, and
others located along the banks of rivers often experience floods for this reason,
aggravated by several other factors.
Land Use Change
In natural catchments, peak flow is usually prolonged by the pervious nature of the
basin. The direct and indirect LULC changes that replace green cover in urban
regions have effects on surface runoff. The LULC changes, such as occur in the
construction of built-up areas increases imperviousness in the basin, causing rapid
overland flow and reducing the time to peak reduction, which increases flood peaks.
However, urban catchments are not affected to the same extent by antecedent
conditions and respond more rapidly to rainfall. Hence, intense rainfall in any season
may become a major cause of flooding, leading to a change in flood seasonality
(Aliani et al. 2019).
Urban Flooding
Increased urbanization alters the hydrologic cycle in the urban basin, affecting the
rainfall–runoff relationship. During urbanization, small streams joining the main
streams are affected and subsequently affect the natural drainage of rainwater that
flows in the basin. Thus, proper drainage structures should be designed to overcome
these overflow processes. The development of storm water drainage systems
involves two steps: estimation of runoff hydrography and routing the runoff
hydrograph through the drainage system to obtain the outflow hydrograph. The
calculation of surface water runoff is quite a challenging task. So, initially the rainfall
intensity has to be designed, being known as storm design, an appropriate method
adopted for obtaining the hydrograph storm over the catchment and recommending
suitable remedial methods, as discussed next. However, the methods adopted for
estimating storm runoff in the catchment and the superiority of flood modeling over
these methods is explained in this chapter in which an overall view of flood
modeling parameters and the techniques used for generating flood modeling is
3 Flood Risk Assessment for a Medium Size City Using Geospatial Techniques with. . .
41
Most major cities and towns are located in river floodplains, which causes additional
stress to river basins. During the past decades more developmental activities have
taken place in urban and agricultural lands, leading to additional stress in these
floodplains and resulting in loss of flood spread areas and increase in flood risk in
downstream portions of the basin. For example, most major European cities in
England and Germany are located in floodplains that have induced burdens in the
rivers, and preventive measures have been made to recreate floodplain storage
(Wheater 2006). Indian cities such as Delhi, Vijaywada, Hoshangabad, Surat, and
others located along the banks of rivers often experience floods for this reason,
aggravated by several other factors.
Land Use Change
In natural catchments, peak flow is usually prolonged by the pervious nature of the
basin. The direct and indirect LULC changes that replace green cover in urban
regions have effects on surface runoff. The LULC changes, such as occur in the
construction of built-up areas increases imperviousness in the basin, causing rapid
overland flow and reducing the time to peak reduction, which increases flood peaks.
However, urban catchments are not affected to the same extent by antecedent
conditions and respond more rapidly to rainfall. Hence, intense rainfall in any season
may become a major cause of flooding, leading to a change in flood seasonality
(Aliani et al. 2019).
Urban Flooding
Increased urbanization alters the hydrologic cycle in the urban basin, affecting the
rainfall–runoff relationship. During urbanization, small streams joining the main
streams are affected and subsequently affect the natural drainage of rainwater that
flows in the basin. Thus, proper drainage structures should be designed to overcome
these overflow processes. The development of storm water drainage systems
involves two steps: estimation of runoff hydrography and routing the runoff
hydrograph through the drainage system to obtain the outflow hydrograph. The
calculation of surface water runoff is quite a challenging task. So, initially the rainfall
intensity has to be designed, being known as storm design, an appropriate method
adopted for obtaining the hydrograph storm over the catchment and recommending
suitable remedial methods, as discussed next. However, the methods adopted for
estimating storm runoff in the catchment and the superiority of flood modeling over
these methods is explained in this chapter in which an overall view of flood
modeling parameters and the techniques used for generating flood modeling is
3 Flood Risk Assessment for a Medium Size City Using Geospatial Techniques with. . .
41