INTEGRATED URBAN FLOOD MANAGEMENT IN INDIA
Controlled Watershed in IISc Campus, Bangalore
IISc being the lead institute for this project has a controlled watershed within which the development, calibration and validation of a hydrological model is carried out for half of its total area (192 acres). The terrain data of fine resolution (0.2 m x 0.2 m) is acquired by LiDAR technology and the calibration of the models is achieved by the sensors installed at the outlet of the drainage network. The precipitation data and the real time runoff data from Centre for Atmospheric and Oceanic Sciences (CAOS) and the installed sensor respectively are collected and analyzed for rainfall-runoff modelling.
Figure 1: IISc Watershed
2D Overland flow model
Urban Flooding is a result of several factors e.g., the formation of Urban Heat Islands (UHIs), formation of local climate zones (LCZs) which combine in a rather complex form, thereby leading to these flooding situations. As a result of climate change, there has been a considerable increase in the frequency of occurrence of short duration - high intensity precipitation events. In particular, convective rainstorm events often induce extreme floods, known as ash floods that are one of the most destructive natural hazards in the country today. So the use of modeling approaches seems to be necessary for both predicting the flood-prone areas and, consequently, planning the damage minimization policies. The change in the Intensity-Duration-Frequency relationship owing to the climate change scenario is a major reason for the occurrence of such events.
Figure 2: IDF Cures corresponding to Annual Maximum Rainfall Series
The hydraulic features of overland flow are significantly related to the characteristics and spatial variability of rainfall. In particular, convective rainstorm events often induce extreme floods, known as flash floods that are one of the most destructive natural hazards. The use of numerical modeling seems to be necessary for both predicting the flood-prone areas and, consequently, planning the damage minimization policies. The use of an accurate model is very important to manage the risk associated with potential extreme meteorological events at basin scale. For simulating the flow of water over the ground, the depth-integrated form of three-dimensional Navier-Stokes equations is used since the horizontal length scale is much larger than the vertical length scale. The resulting 2D equations, called the Saint-Venant equations have been widely used to model the overland routing. The full Saint-Venant equations are, however, sometimes avoided since the simplified forms in most cases, do not cause much difference in the accuracy of results. Thus there are two simplifications which are generally used, namely Diffusive and Kinematic shallow water equations. Diffusive form is generally adopted for overland flow modeling as it has some advantages as compared to the Kinematic form which develops issues with stability and accuracy in some cases. To give a wholistic approach to the problem, details like backwater effects, feedback system between surface and sewer systems and effect of land use land cover of watershed etc. need to be addressed.
Diffusion flow models essentially neglect the inertia terms in the St. Venant equations. The diffusion flow equation can be written in terms of discharge, for hydrologic applications, and in terms of flow depth for hydraulic applications.
One Dimensional Channel Flow Routing
Flood routing is a procedure to compute output hydrograph when input hydrograph and physical dimensions of the storage are known. It is broadly classified into two major categories- namely, reservoir routing and channel routing. Flood routing methods can be classified as hydraulic - in which both continuity and dynamic equations are used –or hydrologic, which generally uses the continuity equation alone.
Three conservation laws mass, momentum and energy are used to describe open channel flows. Two flow variables, such as the flow depth and velocity or the flow depth and the rate of discharge, are sufficient to define the flow conditions at a channel cross section. Therefore, two governing equations may be used to analyse a typical flow situation along a channel, which are popularly known as St.Venants Continuity equations and St.Venants Momentum equation.
Kinematic wave form of the St. Venant's equation, obtained by neglecting the inertial and pressure terms from the continuity and momentum equations.
Illustration of 1D Kinematic Wave Model for Channel Routing
Figure 3: Output hydrographs of Channel Routing (hypothetical case)
Length of Channel, L = 15000ft
Manning's Coefficient, n = 0.035s/ft^0.3
Width of Channel, B = 200 ft
Distance increment, dx = 3000 ft
Time increment, dt = 180s
Overland Flood Routing
The hydrodynamics of overland flows are modelled as 2D shallow flows using the diffusive wave form of St. Venants equations. The irregular shape of the overland flow surface is represented by an approximate shape that resembles the actual shape of the catchment, square grids of uniform size. This often results in discontinuities in spatial lattice along the principal directions. Numerical solutions for the finite volume formulation of partial differential equations are obtained through finite difference method. Since the governing equation is based on conservation principles, a finite volume type formulation is used.
Illustration of 2D Diffusive Wave Model for Overland Routing
Approach- Using MATLAB to solve two dimensional form of Shallow Water Equations. The numerical scheme being used is Alternate Direction Explicit scheme. A hypothetical plane is considered and a spatially uniform rainfall of constant intensity of 180 mm/hr is taken initially as shown in Figure 3.
The finite volume approach is used to simulate the grid as cell storage model.
Figure 4: Solution of 2D Diffusive Wave Model
EFFECT OF LAND USE LAND COVER PATTERN IN OVERLAND ROUTING
- Catchments will have spatial and temporal changes in land use land cover pattern
- Different land use features can be modeled by means of Manning’s roughness coefficient
- Different categories of land use land cover features used in study are: highly pervious, highly impervious areas
Figure 5: Schematic representation of the basin
Figure 6: Water depth along a section at mid-length of the basin (at various time steps)
Case Study: IISc Campus
Figure 7a, 7b, 7c: 3D model of IISc watershed, bird's eye view of IISc Campus and 3D model of IISc campus with water depth shown at time (t) = 1620s into simulation, respectively (top to bottom)