Urban Flood Hazard (both Reverine and Urban)

+Urban Flood hazard maps of flood affected wards of the cities/town in 1:10,000 scale with classification scheme of low hazard, moderate hazard, high hazard and very high hazard areas comprising of major and minor infrastructures overlaid
+Vulnerability and Risk Assessment for urban flood hazard in Guwahati



Background

Urban flood is one of the most recurrent hydro-meteorological disasters presently in the globe.  A sizeable portion of the world population suffers from flood every year in major twon and cities. Conveyance of floodwaters in wash corridors and arroyo floodplains that were once predominantly overland sheet flow or flow in rills and gullies, are now redirected by houses, fences, small buildings, streets, and other obstructionsto flow. In many of these cases, flood containment within the historic mapped floodplain is questionable due to loss of storage and flow path obstruction, and there is a need to quantify the discharge that has been diverted from the floodplain. Traditional one-dimensional backwater models are inadequate to predict the unconfined flow behaviour in such urban environments and if used, require too many assumptions regarding flow diversions and potential confinement. Two-dimensional flood routing programs can efficiently simulate these complex urban flood environments with accuracy and detail.

Flood hazard mapping and flood inundation modelling are the vital components in flood mitigation measures and land use planning, and are prerequisites for the flood insurance schemes.  As flood itself cannot be prevented but the damages due to flood can be mitigated with proper planning and preparedness in community level. For that it is very important to identify the degree of hazard associated with various portions of a flood plain. Hence flood hazard zonation is an important component of overall flood management strategy.

Database

Hydro-Meteorological Data :
+Rainfall data (hourly, daily) for last 25 years
+Daily discharge and water level data of rivers and major sewer-storm / drainagechannels flowing in the study AOIs
+Sewer and drainage layout map and plans of Guwahati
+Cross-section and Longitudinal profiles of rivers and major drainage / sewer channels

Spatial And Ancillary Data:
+Municipal Sewer-Storm / Drainage system map from respectivedepartments for Guwahati
+Flood Inundation reports/maps
+Municipal Sewer-Storm / Drainage system map from respectivedepartments for Guwahati
+Municipal Wards and Town Layout.

Hybrid Digital Elevation Model

The DEM was a fundamental dataset used for development of the urban catchment hydrological model component in HEC-HMS platform and the hydraulic flood simulation model in the HEC-RAS/SWMMplatform. This dataset was a vital input in the hydrological-hydraulic model build-up, and flood hazard layer generation. Thereby, a Hybrid DEM of 1 m spatial resolution was developed using Z-flood points from RTK/ETS survey and CARTOSAT-1 stereo dataset of cell size 2.5 m spatial resolution.

Urban floodplain areas have significant effects on inundation flows. Shallow-water models account for the reduction in storage and in the exchange sections due to presence of buildings and other structures on the floodplains, and are most appropriate to analyse such problems

Hydrologic-Hydraulic Analysis & Modelling

The rainfall data analysis showed that there are storms and surges in the annual intensity-duration-frequency (IDF) trend in the last 10 years. Monthly total rainfall peaks ranged between  200 mm and 400 mm in the last 10 years. From an assay of daily rainfall data, storm peaks were recorded in around 5th June, 26th June, 30th July and 11th Sept last year in 2012 at Silchar (source: AWS_ISRO1107 Christian Basti & AWS_ISRO1101 Khanapara) as shown in Fig.2.5. These rainfall storm events have induced flood inundation and waterlogging in parcels of Guwahati metropolitan. The analysis of the monthly rainfall data showed that the average annual rainfall is about 2355mmand out of this about 22% of the annual precipitation occurs in April & May and about 65% of theprecipitation occurs in the period of June, July, August and September. However, it can be introspected that the rainfall intensity has a bearing with the induced surface overland flow and drainage discharge capacities (over that it was already conveying prior to the storm event). These extreme storm events were used  to generate 1 to 3 hourly flood hydrographs using a quasi-distributed hydrological model.