Introduction:
It is said that “complete immunity from floods is a practical impossibility”. but mitigation measures coupled with modern technology can play a significant role in subsiding the risks from flood events to the people who are located in flood-prone areas. With this principle in mind, soon after the catastrophic floods of September 2014 in J&K, it was decided by the Government that in view of the flood vulnerable topography of J&K, besides the events of flood being stochastic have caused immense damage to the life and property time and again, hence it was felt that there is an urgent need of conducting the comprehensive studies in respect of the Jhelum and Tawi basins for mitigation of floods under World Bank aided $ 250 million Jhelum Tawi Flood Recovery Project (JTFRP) so that a mechanism to predict and reduce the flood risk shall be put in place. With the advent of technology “flood modeling- a flood prediction/information system” was also introduced for the first time in the erst-while State of Jammu and Kashmir by engaging national and international consultancy services being the widely adopted component of any flood mitigation project currently. Flood modeling is used for the assessment of flood risks by developing computer logical programs using real time or forecasted hydro-metrological data as an input to generate the extent of flooding, the depth of flood water and identifying source of potential flooding. The results generated helps in predicting the areas vulnerable to floods and thus helps in proposing mitigation and protection measures well before the event of flooding including alerting the residents to take early action to evacuate quickly to safer places.
How does it work?
A model is a three dimensional replica of any physical entity, but as lot of computations are involved in the flood modeling which cannot be dealt by physical modelling hence use of softwares is must in order to simplify an extremely complex situation that allows a user to simulate a real hydrological phenomenon or process with a set of mathematical formulas.
Summarized below is the functioning of the flood modeling:-
Flood modeling requires use of two hydrological computer modelling softwares namely HEC-HMS (Hydraulic Engineering Center-Hydrologic Modeling System) and HEC-RASH (Hydraulic Engineering Center-River Analysis System). HEC-HMS is used for carrying out the rainfall-runoff modeling for estimating flows for a range of events in the form of runoff hydrograph. The data required for the rainfall-runoff modeling in HEC-HMS may comprise of terrain, soil, land use land cover, hydro-meteorological etc. The hydrograph (plot of Discharge vs Time) obtained is shown as under :
The output (hydrograph) obtained from HEC-HMS becomes input for HEC-RAS (Hydraulic model) meant to aid hydraulic engineers in channel flow analysis and floodplain determination. The response of the river basin to the run-off hydrograph as an input will generate the flood hazard mapping in the form of the extent of flood inundation and the depth of floods associated with specified areas. After running the model we will be able to determine how far the extent of the flood inundation goes, how fast it is going to be, which can help us in preparing evacuation maps and emergency action plans. An example of the flood modeling depicting the extent of flood inundation in the catchment of the river Jhelum is shown as under ;
It is clear that the correctness of results will depend on the nature of input given as well as the perfection of the catchment used for the flood modelling. Therefore, one has to be meticulous while choosing the data set in respect of the inputs and the catchment used for the flood modeling.
Catchment/river basin required for the purpose of the flood modeling is obtained from the remote sensing techniques or using satellite data known as Digital Elevation Model which is the three-dimensional representations of earth’s surface representing the morphological surface and to simulate catchment responses without any objects, like plants and buildings. Digital Elevation Model has a specific resolution and to obtain the error free results viz-a-viz extent of inundation, flow patterns and flood depth, it is necessary to use digital elevation model of very high resolution for getting accurate representation of terrain morphology (e.g., slope, flow direction), and therefore a better prediction of hydrological responses. For example, Digital Elevation Model of resolution say 10 meters means one grid cell represents an area of 10 × 10 m of physical land. So it is clear that the resolution plays an important role in defining the topographical terrain features and hydrological information. Digital Elevation models of coarser resolutions can be downloaded from the websites and online portals like U.S Geological Survey (USGS), ISRO’s Geoportal BHUVAN etc. The Shuttle Radar Topography Mission (SRTM) 30m DEM was widely used before major advancements took place in the remote sensing sector. Today even 1m Digital Elevation Models are possible for studying the river morphology.
2. Integrated Operating Forecasting System (IOFS)—Good news for J&K.
An application of the flood modeling will soon see the light of the day in J&K in the form of a web based application by the name of Integrated Operating Forecasting System (IOFS) wherein information related to floods and other disasters will be made accessible to general public. This will allow the general public to know the real time and forecasted inundation levels at different Gauge and Discharge (G&D) stations like Sangam, Ram-Munshi Bagh, Asham etc besides will simulate submergence condition of different areas on the basis of the forecasted IMD data. As such, the general public as well as authorities will be in a better position to prepare themselves well in advance to combat any eventuality caused by the floods.
Moreover, by virtue of 2D flood modeling with 1 in 100 year flood discharge as reference, it is possible that one of the important and long pending non-structural measure which is yet to be in place i.e flood plain zoning can be implemented wherein the entire area on both sides of the river Jhelum/Tawi will be divided into different zones on the basis of their vulnerability to floods, this will be depicted by a comprehensive map which would be generated after running the model. An example of the same is shown as under:
Keeping in view of the foregoing map, the authorities may pass flood zone laws that restrict land use in certain areas and allow the development of the infrastructure accordingly on the basis of flood vulnerability of that specific area e .g. areas indicated in the map which may get inundated to the depths greater than 3 meters are classified as red zones need to be strictly prohibited for construction of any critical infrastructure but can be used for play fields, plant nurseries etc. This can surely help us in minimizing the loss of life, livestock, agricultural activities, property in case of a flooding event. It may be noted here that the map shown in fig. 5 depicting the extent of inundation and flood depths has been generated on the basis of SRTM-30m DEM, however the same cannot be taken as final flood hazard map till high resolution digital elevation model generating more accurate hazard maps is available with the agencies working on the project. However, once the high resolution Digital Elevation Model of the river catchment becomes available and it is found that the land use pattern even at the potential red or orange zones have already changed at many places by way of massive growth of built up area, one may argue then what is the need for the implementation of flood hazard mapping? It is worth mentioning here that it is very likely that some areas, which may get categorized in red zones, shall comprise of only wetlands/water bodies like Anchar lake, Hokersar Lake, Narkara Wetland etc which still have not become victims of increasing anthropogenic activities and human negligence to a substantial degree, thus still possess enormous potential to mitigate floods. It is only on this consideration, the Government may initiate a process of notifying them as permanent flood detention basins. It may also be the case that some red or orange zones may fall very close to vicinity of the river Jhelum/Tawi with massive built up areas, the land left out even in those areas need to be saved from haphazard construction and may require now very careful planning (specific building standards and construction materials may be required to reduce potential flood damage) for the protection of life and property of the people as waiting any further will give rise to further massive encroachments till these flood plains, water bodies and wetlands will remain only a fraction of their original size or the worst case they may go extinct.
Mohammed Akeeb Dar is an Assistant Engineer (Civil)