Lower Tapi Basin Research Articles

A Comparative Study for Provision of Environmental Flows in the Tapi River

Environmental flows are defined as the flow required into a stream to maintain the river’s ecosystem. The notion of Environmental Flow Allocation (EFA) ensures that a sufficient amount of water is delivered to the stream to maintain ecological integrity. The objective of this study is to examine environmental flows and determine the best acceptable strategy for providing flows into the river in the Lower Tapi Basin. To achieve this objective, daily discharge data from three sites, Ukai (period 1975–2020), Motinaroli (period 1990–2021), and Ghala (period 1995–2005) were collected and analyzed using the Tennant, Tessman, variable monthly flow (VMF), and Smakhtin methodologies. A comparative analysis was carried out on all three sites using the four methodologies. The Tessman and VMF approaches have a strong connection with the computed environmental flow requirements (EFR), according to the results. The calculated EFR was found to be in the range of 30–35% of mean annual flows (MAF). The maximum EFR found at station Ghala is about 54.5% of MAF according to the Tessman method. Such research will help to prevent future degradation of the river by supplying flow in accordance with the EFR, and it will also be used by stakeholders and policymakers to allocate water to preserve the ecosystem.

Dam site suitability analysis using geo-spatial technique and AHP: a case of flood mitigation measures at Lower Tapi Basin

Dam site suitability analysis using geo-spatial technique and AHP: a case of flood mitigation measures at Lower Tapi Basin

Spatio-Temporal Variability of Rainfall Indices and their Teleconnections with El Niño-Southern Oscillation for Tapi Basin, India

The long-term spatio-temporal variability in twelve rainfall indices across Tapi basin are investigated for period 1944–2013. The temporal trends in aforesaid indices, representing magnitude, intensity, frequency and duration of rainfall, are analysed using non-parametric tests. The total annual rainfall exhibit uniformly increasing trends over Lower Tapi basin (LTB), while heterogeneous trends are observed across Upper (UTB) and Middle (MTB) Tapi basins. The rainy days portrays decreasing trends at 75% stations in the basin, while rainfall intensity displays increasing trends across LTB and UTB. On other hand, LTB and Purna sub-catchment (in UTB) show coherent increasing trends in extreme rainfall, while decreasing trends are observed over Burhanpur sub-catchment (in UTB) and MTB. A considerable increase in duration of dry spells is reported at 92.5% stations in the basin, indicating a marked shift towards drier climatic regime. Further, Principal Component Analysis (PCA) is carried out to address the space-time variability of rainfall indices. The results of PCA were adopted in cluster analysis to identify the stations exhibiting similar characteristic variations in rainfall trends. The hydroclimatic teleconnections between rainfall indices and large-scale oceanic-atmospheric circulations of the tropical Pacific Ocean (viz., El Nino-Southern Oscillation) revealed the prevalence of wet conditions in the basin during moderate to strong La-Nina, while dry conditions were encountered during moderate to strong El-Nino and weak La-Nina events. The spatial estimates of rainfall indices are derived corresponding to 2-, 10- and 50-year return periods which would be helpful in hydrologic design and risk assessment of rainfall-induced disasters in near future.

Assessing the Impact Index of Urbanization Index on Urban Flood Risk

The process of urbanization has changed overall Land use/Land cover patterns which are being attributed to flooding and resulting in the economic damages from flooding events. This current study aims to evaluate the implication of spatiotemporal changes of LULC Pattern on the flood risk of Surat city (Gujarat, India), Lower Tapi Basin. The Topographical maps and satellite imagery of Resources-1 of the year 1968 and the year 2006 respectively are used for analyses the urbanization index. As the flood risk is a combination of flood hazard, and vulnerability of the urbanized area, flood losses are expected to rise due to change in each of these aspects. The remote sensing and spatial analysis tools of Geographic Information System (GIS) have been used to statistically examine the flood risk index along with their different land-use scenarios. It has been observed that other than natural processes, rapid urbanization obstructions are being considered as one of the main drivers of flood risk aggravation, and if so, it has made essential for the implementation of flood management approach at the top priority for reducing the risk of flood.

Assessment of flood inundation mapping of Surat city by coupled 1D/2D hydrodynamic modeling: a case application of the new HEC-RAS 5

Surat city of India, situated 100 km downstream of Ukai Dam and 19.4 km upstream from the mouth of River Tapi, has experienced the largest flood in 2006. The peak discharge of about 25,770 m3 s−1 released from the Ukai Dam was responsible for a disaster. To assess the flood and find inundation in low-lying areas, simulation work is carried out under the 1D/2D couple hydrodynamic modeling. Two hundred ninety-nine cross sections, two hydraulic structures and five major bridges across the river are considered for 1D modeling, whereas a topographic map at 0.5 m contour interval was used to produce a 5 m grid, and SRTM (30 and 90 m) grid has been considered for Surat and the Lower Tapi Basin. The tidal level at the river mouth and the release from the Ukai Dam during 2006 flood are considered as the downstream and upstream boundaries, respectively. The model is simulated under the unsteady flow condition and validated for the year 2006. The simulated result shows that 9th August was the worst day in terms of flooding for Surat city and a maximum 75–77% area was under inundation. Out of seven zones, the west zone had the deepest flood and inundated under 4–5 m. Furthermore, inundation is simulated under the bank protection work (i.e., levees, retaining wall) constructed after the 2006 flood. The simulated results show that the major zones are safe against the inundation under 14,430 m3 s−1 water releases from Ukai Dam except for the west zone. The study shows the 2D capability of new HEC-RAS 5 for flood inundation mapping and management studies.

Snyder Unit Hydrograph and GIS for Estimation of Flood for Un-Gauged Catchments in Lower Tapi Basin,India

In the flood prone catchments, it is needful to estimate the discharge, standard lag time, time of peak, and flood response of each watershed in the basin. The SUH method offers considerable advantage over others, and thus, has been chosen for estimation of flood response, contribution of flooding potential, percentage of flood volume for 25 sub-watersheds. The discretion of sub-watershed for estimation of peak discharge, time of peak, alternate lag time, and width of SUH at 50% and 75% of peak found to offer advantages over other methods. This paper considers Snyder Unit Hydrograph (SUH) with GIS based spatial database for calculating discharge at Lower Tapi Basin (LTB). The hydrological parameters of each sub-watershed such as river length, length of centroid, spatial area, land use, lateral slope, and terrain and soil factors have been extracted from GIS database. The geo-data has been combined with topographical maps to produce a digital elevation model (DEM) of 50 m cell size. The analysis for all 25 subwatersheds exhibit that 35.07 m3/s and 4.55 m3/s and 13.23 hours and 4.33 hours have been highest and lowest peak flow and time of peak respectively. The SUH model has been validated for peak discharge at a gauge site Amli (E73023' N21023') where discharge data were collected during 2010 and 2011 monsoon. A comparison between measured and SUH modelled discharge shows good fit within a mean variability range of 5-7%. The SUH methods ability to estimate hydrological parameters including peak flow discharge shows wider replication for un-gauged catchments.

Assessment of the Flood Potential on a Lower Tapi Basin Tributary using SCS-CN Method integrated with Remote Sensing & GIS data

The purpose of this research paper is to identify watersheds with high flood potential based on watershed characteristics for formation of surface runoff. The SCS-CN method relies on remote sensing and GIS data for obtaining watershed characteristics. A 30 m raster grid size digital elevation model (DEM) has been generated from field survey using Global Positioning System (GPS) of 3 m accuracy integrating with Survey of India topographical maps of 1: 50,000 scale having 10 m contour interval. The undisturbed soil samples from field have been collected and laboratory analysis was carried out using modified proctor compaction test as per ASTM D1557 and sieve analysis as per ASTM C136. This has helped in establishing hydrological soil map while land use map has been prepared using Landsat 7ETM+ image band 2, 3, 4 [30 m] merged with PAN band 8 [15 m] for classification. The supervised classification approach using maximum likelihood classifier has been employed for preparation of land use map for Varekhadi catchment having 442 km2 of geographical coverage. The major land use categories classified on 10 Nov 2001 Landsat 7ETM+ image have been agriculture (32%), forest (29%), wasteland (20%), fallow land (14%), built-up (4%) and water bodies (2%). The hydrological soil groups generated in GIS environment have identified two soil groups viz. group B and group C that exist under study area. The Varekhadi catchment has been delineated into five watersheds viz. Amli, Zankhwaw, Visdaliya, Godsambha and Wareli delineated using DEM and stream network. The SCS-CN model was applied for estimating of daily run-off for each sub-watershed. The results obtained on the flood potential analysis shows that Wareli watershed has highest flood potential while the Amli watershed lowest. It should be noted that highest value of flood potential belongs to lowest part of watershed, where high population density can be found. This analysis reflects an increased vulnerability and risks to floods and inundations for Wareli watershed. Stream gauge data has been used for result validation with a common event of 2010 and it shows good fit with the model. The flood potential analysis within the lower Tapi basin tributary suggests that the SCS-CN method with hydrological parameters derived using remote sensing and GIS data can be applied to predict run-off in poorly gauged watersheds

Water Harvesting Structure Positioning by Using Geo-Visualization Concept and Prioritization of Mini-Watersheds Through Morphometric Analysis in the Lower Tapi Basin

Geo-visualization concept has been used for positioning water harvesting structures in Varekhadi watershed consisting of 26 mini watersheds, falling in Lower Tapi Basin (LTB), Surat district, Gujarat state. For prioritization of the mini watersheds, morphometric analysis was utilized by using the linear parameters such as bifurcation ratio (Rb), drainage density (Dd), stream frequency (Fu), texture ratio (T), length of overland flow (Lo) and the shape parameter such as form factor (Rf), shape factor (Bs), elongation ratio (Re), compactness constant (Cc) and circularity ratio (Rc). The different prioritization ranks were assigned after evaluation of the compound factor. 3 Dimensional (3D) Elevation Model (DEM) from Shuttle Radar Topography Mission (SRTM) and DEM from topo contour were analyzed in ArcScene 9.1 and the fly tool was utilized for the Geo-visualization of Varekhadi mini watersheds as per the priority ranks. Combining this with soil map and slope map, the best feasibility of positioning check dams in mini-watershed no. 1, 5 and 24 has been proposed, after validation of the sites.

Temporal variations in the specific stream power and total energy expenditure of a monsoonal river: The Tapi River, India

In this study, an attempt has been made to evaluate the temporal variations in specific stream power and the total energy available for geomorphic work during the monsoon season for the Tapi River, in central India. Continuous daily discharge data (1978–1990), hydraulic geometry equations and the relationship between discharge and water surface slope were used to compute the daily specific stream power ( ω) for the Savkheda gauging site in the lower Tapi Basin. The total amount of energy generated by all the monsoon flows was estimated by integrating the area under the ω-graph derived for the monsoon season. The analyses of the 13-year daily discharge data reveal that the average and maximum ω values range from 4–20 W m − 2, and 22–964 W m − 2 respectively. Specific stream power duration curve derived for the site shows that for 25% of the time the power per unit area is > 10 W m − 2. Furthermore, unit stream power was found to be above the Williams' [Williams, G.P., 1983. Paleohydrological methods and some examples from Swedish fluvial environments. I. Cobble and boulder deposits. Geografiska Annaler 65A, 227–243.] threshold of pebble-movement (1.5 W m − 2), cobble-movement (16 W m − 2) and boulder-movement (90 W m − 2) for 71%, 15% and 2% of the time, respectively. Computations further indicate that the total amount of energy generated by the flows during the monsoon season is in the range of 37 MJ (deficit monsoon years) to 256 MJ (excess monsoon and/or flood years). Large floods have one-third share in the total monsoon energy expenditure. In the absence of appropriate data on the yearwise geomorphic effects, the geomorphic work was evaluated in terms of the total suspended sediment load transported. The total monsoon sediment load is strongly related to the total monsoon energy. The results of the study indicate that the average flow competence and capacity are remarkably higher during wetter monsoon seasons and flood years than during the shorter and drier monsoon seasons. The present analyses demonstrate that the flows are geomorphically effective for a greater part of the monsoon season, except during the deficient monsoon years, and there is little doubt that large-magnitude floods are effective agents of geomorphic change in monsoonal rivers.