Floods are one of the most common, frequent and serious natural disasters throughout the entire world. Flood is a sudden excess level in a river normally at which river overflows both its banks and causes inundation of the nearby area. Urban flood is a flood that is mainly caused due to: 19050790870Heavy Rainfall accompanied by poor drainage. The land converted from agricultural fields or woodlands to roads and parking etc loses its ability to absorb rainfall. Hence due to urbanized cities much water due to heavy rainfall is not infiltrated into soil and they get flooded.
Rise of River level above bank level due to flow from upstream due to backwater effect caused by dangerous tides at the mouth of river.High Tides or Storm upsurges can also lead to excessive flooding in cities accompanied by health problems.Urban floods have many devastating impacts. Floods break the backbone of the transportation system of the city.
Flood to some point destroy sewage disposal facilities and seriously effect drinking water facility in the city.
Many people are displaced from their homes and have to struggle for a times meal in those testing times. Communication devices such as Mobile Phones, Landlines all get suspended during these times. Also due to submergence of power stations power supply also gets suspended. Long term effects include loss in income of the state due to decreased Tourism, Closure of small business, Respiratory problems in the inhabitants. Hence effects of flood extend over a wide range from physical to economic, social and environmental.
Flood risks cannot be entirely avoided, thus they have to be managed. Consequently, flood management does not strive to eliminate flood hazard but to mitigate them. So as to curb flood risk, it is necessary to evaluate the performance applied measures and to revaluate left risks. Risk assessment involves identifying all the possible water related hazards. Hence quantifying of risks has to start with the hydraulic modelling of floods. A number of various scenarios should be modelled in order to overcome the consequences of likely future changes on urban floods. The results of such models provide information about the expected flood frequencies and magnitudes such as depth and duration of floods. To model the floods we have to do their routing and mapping. Flood maps are of great utility and help visualising outcomes from flood evaluations. Routing also can be used to predict the hydrograph shape (and thus lowland flooding potential) subsequent to multiple rainfall events in different sub-catchments of the watershed. Then we can move forward in direction to mitigate them.
This is a model used to obtain depths and velocities of the water along its flow path. This uses Mass and Momentum equations to solve the required variables. This represents flood plain in form of grid cells predicting [h-height, u-horizontal velocity, v-vertical velocity] at each grid. This uses Alternate Direction Implicit Scheme where equations in each direction are solved are solved once per time step. We need Hydro-Meteorological Data, Topographical details, Roughness, Floodplain and channel locations, XML control file for obtaining the desired results. In this method Shallow water equations are solved so as to model the urban floods. In this method aerial photos and maps are needed to provide background to this Model. Input Data types in this model are: Flow vs Time, Side Inflow, Area Inflow, Water Level vs Time, Water Level vs Discharge, Direct Rainfall, Flow vs h, Z vs TimeOutput Data Types are as follows: Water Levels (h) Velocities (V) Depths (d) Unit Flow (q) Several Hazard Categories (Zx) Energy ( ) E ) Froude No. (F) Flow Regime (R) Mass Error (MB1) and more.Equations used in this Model are: TELEMAC Model TELEMAC model solves shallow water equations using finite element method. It chooses a triangular mesh for its computations.
At each point in the mesh TELEMAC calculates depth(h), velocity components in two directions(u,v). Flood mapping plays a significant role in decision-making, planning process and implementation of urban flood management options. The overall goal of flood maps is to provide reliable information on the past and the likely or potential extent of floods and their impacts (sometimes in combination with other related information), which help in making decisions on various aspects of integrated management of floods. Development of Flood maps requires a systematic process including specification of data sets and methodology. Process is explained in detail below:Selection of objective: Firstly it is imperative to decide the Purpose of Mapping, Target Audience (one who will use the map) and areas to be covered in the Mapping process.Selection of Type of Map: The choice of mapped parameters and type will depend upon the objectives of the project, the resources available and the potential benefit achievable. The following maps are relevant: Hazard Maps: Event Maps
Flood hazard maps are alone serving many goals. They can be directly implemented (e.g. in land use planning processes), but are also the vital inputs for vulnerability and risk maps. Therefore, flood hazard maps are fundamental to the entire mapping process, particularly in the beginning. The choice of approach is mostly governed by the mapping stage, the purpose, the availability of data and to a small extent the availability of experts of this field in the country. The historic and geomorphologic approaches are used in the initial stage, whereas the modelling approach is mainly used in the later stages. Above are needed in sufficient quantity and quality for successful completion of the project.A comprehensive capacity development exercise should be planned that would include technical training in all topics where there is dearth of required expertise, such as analysis of hydrological data, data management using GIS, developing hydraulic models, analysis of geomorphologic field etc.
For mitigating floods main requirement is to reduce the surface Runoff since due to urbanization the natural infiltration reduces. So we have basically two options either to store the rainwater temporarily or to use devices that would enable to infiltrate into the soil. When heavy rainfall occurs these ponds receive water and release at a predetermined slower rate. This helps to reduce the peak runoff to the storm sewers and decrease the chance of water inundation.Rain Water Harvesting: These systems can be installed in homes in urban areas and they store rainwater during rainfall which reduces the runoff. Also stored water may be used for various domestic purposes.Green Roofs: On roofs of houses good amount of vegetation is planted. These absorb water during floods and in a way reduce runoff to a good extent if they are used in most of the buildings in flood prone areas. Also thy keep the building cool.
Infiltration Trenches are shallow excavations with substances like rubber or stone which create temporary subsurface storage of surface runoff and in this way enhance natural capacity of ground to store water thereby reducing the surface runoff.Pervious Pavements: Porous pavements provide the structural support just like a conventional pavement, but allow stormwater to drain directly through the pavement surface into an underlying stone bed and the soil below, thereby reducing surface stormwater runoff. Hence they are a great asset in controlling urban floods.Infiltration Basins: This is a shallow artificial pond designed to infiltrate water slowly through permeable soils and reduce surface Runoff so as to mitigate urban floods.
Traffic engineering Department in state can collaborate with Disaster mitigation department to prevent chance of floods in urban areas. What I want to say is laying of sidewalks, Traffic islands, avoiding under passes, providing porous pavements at proper places can reduce runoff to a great extent. However doing this would require simultaneous effort of Traffic and Hydrology engineers. Also water absorbing fertilizers like Cow dung can be used at the required places in cities so as to facilitate the decrease of stormwater runoff. Nature is our dear most friend but may behave as a enemy if not properly handled. Present day pollution, deforestation is compelling nature to show its precarious form in form of disasters like urban floods. Recent Kerela Floods showed how big damage can floods do. I read in a newspaper that it would take atleast 25 years for Kerela to be the same again. Hence in addition to calculations, techniques, mitigation measure described above one initiative we can take is to make sure that we take every possible step To Be Eco-Friendly in every way possible. The more God’s gift Nature will be happy, the safer we will feel in its Lap. References:Class Slides and provided Documents for projectsWikipedia ( Source of image of equations) to NRSC/ISRO for the Flood Map image.