International Journal of Advanced Engineering Technology E-ISSN 0976-3945 Research Paper DYNAMIC FLOOD ROUTING AND UNSTEADY FLOW MODELLING: A CASE STUDY OF UPPER KRISHNA RIVER 1Doiphode Sanjay L* 2Oak Ravindra A. Address for Correspondence 1M tech Student, Bharati Vidyapeeth Deemed University, College of Engineering Pune 411043 2Former chief Research Officer, Central Water and Power Research Station, Pune 411024 ABSTRACT The movement of a flood wave in a river channel is a highly complicated phenomenon of unsteady and non uniform flow. Flood routing is a mathematical method (Model) for predicting the changing magnitude, shape and celerity of a flood wave which propagates through a river. Every year Sangli City, in Maharashtra, faces problems of floods and damages during monsoon. The problems are caused by the release of water from the reservoirs located in the Upper reaches of Krishna & Koyna Rivers. During these periods, many bridges over Krishna River get submerged, resulting cutoff in communication, inundation of the city and surrounding area. The HEC-RAS is capable of performing one-dimensional water surface profile calculations for unsteady flow for a full network of channels, a branching system or a single river reach. Unsteady flood model in HEC-RAS was set up using available survey data of Krishna and Koyna River. The Krishna River reach of length about 233 Km from the downstream of Dhom Dam to Sangli city was considered for the flood routing studies. For Koyna River the reach from downstream of Koyna dam to Krishna –Koyna confluence at Karad was also incorporated. In the year 2005, maximum flood occurred in the Sangli. For upstream boundary of the model the available flood release hydrographs of the worst year 2005 from various reservoirs in upper reaches of Sangli was used. Contribution of discharge from the tributaries and the local catchments was also incorporated. Calibration of model was done using the gauge discharge data available at Karad and Sangli. The results of model run showed that the stage and discharge worked out from the model had a good agreement with observed stage and discharge. Hence, it was concluded that the model set up can be reliably used to get the flood flow profiles at Sangli. Studies were also conducted to estimate the changes in the hydrographs under the estimated worst scenarios. The analysis of available flood data was done to identify the flood sensitivity of Sangli, due to Koyna and Dhom dam flood water release. It was found that a flood situation at Sangli mainly depends on the water release from koyna dam. Maximum limit for the flood release from Koyna dam could be 1690 cumecs, so that the flood level would reach the danger level of 540.77m at Irwin Bridge Sangli. KEYWORDS Flood routing, upper Krishna River, Koyna River, Sangli, HEC-RAS 1. INTRODUCTION Dhom dam was 634.92 cumecs which is @ 35.70% Within a hydrologic context, routing is a process of of spillway capacity of Dhom dam. (1778.19 cumecs) determining the spatial and temporal variations in Considering this fact, in future there may be flow rate along a watercourse. Flood routing may be probability of releasing more discharges from these defined as the process of computing the progressive Dams, which may create severe flood situation at time and shape of a flood wave at successive Sangli. locations along a river. The movement of a flood At present in Sangli, flood forecasting is based on wave in a river channel is a highly complicated statistical correlations between upstream and phenomenon of unsteady and non uniform flow. Not downstream gauge stations, measured rainfall in the only does the flow vary with time as the wave catchment area and gauge/discharge data of progressed downstream, but the channel properties tributaries etc. Flood routing aspects are not covered and amounts of lateral inflow/outflow can also vary. properly in these estimations. Thus the analytical solution to the problem becomes The one dimensional numerical analysis software quite complicated. HEC-RAS [Hydraulic Engineering Centre- River Every year Sangli City, in Maharashtra, faces Analysis System] is used in this study to calculate the problems of floods and damages during monsoon. water surface profiles. The problems are caused by the release of water from Main purpose of flood routing studies is to calibrate the reservoirs located in the Upper reaches of Krishna the model for water level and discharge hydrographs & Koyna Rivers. During flood periods, many bridges for specific reach / reaches based on the past data; over Krishna River get submerged, resulting cutoff in and to estimate the changes in the hydrographs under communication, inundation of the city and the estimated worst scenarios. An unsteady Hydraulic surrounding area. In 2005 from 23 rd July to 2 nd flood model of Upper Krishna River from Dhom August there was consecutive heavy daily rainfall in Dam to Sangli City has been prepared for this the catchment ranging from 84 mm to maximum of purpose. Fig: 1 shows the index map of the Krishna 462 mm, resulting in major water release from River and its various tributaries. Koyna and Dhom Dam, which resulted in to the maximum worst recorded flood at Sangli. At Irwin Bridge, Sangli the danger river level is 540.77 m. However, on 04/08/2005, water level at Irwin Bridge, Sangli was 543.38 m. for about 181 Hours. During the analysis of data for the study, it was found that the maximum water released was 2817.40 Cumecs which is @ 73% of the maximum flood release capacity of Koyna Dam at FRL. (3822.70 Cumecs) Similarly maximum water release from Fig: 1 Index Map showing study area. IJAET/Vol.III/ Issue III/July-Sept, 2012/55-59 International Journal of Advanced Engineering Technology E-ISSN 0976-3945 Studies were also conducted to identify the sensitivity Prism storage and transient wedge storage. This of floods in terms of both water levels and discharges method does not account for the time changes in at Irwin Bridge Sangli, due to releases from various water flow that are transmitted as waves, dams on the upstream. instantaneously from one end of the reach to other. In 2. METHODOLOGY AND MODEL SETUP cases where there are significant back water effects 2.1 Methodology and where the channel is either very steep or very The one dimensional equations of unsteady flow flat, then dynamic effects may be significant and in derived by Saint-Venant (1871) is the theoretical such cases hydraulic routing is preferred over foundation for flood routing. The St.Venant hydrologic routing. equations consist of a conservation of mass equation 2.1.2 Hydraulic flood routing models ∂(Av ) ∂ A The hydraulic flood routing models are again + = 0.................(1) ∂x ∂ t classified into A conservation of momentum equation 1. Simplified hydraulic models 2. Complete hydraulic models ∂v ∂ v ∂ h +V + g += S f 0.....(2) 1. Simplified Hydraulic model ∂t ∂ x ∂ x Simplified hydraulic models are based on the In which t is a time, x is distance along the conservation of mass equation and simplified form of longitudinal axis of the water way, A is the cross conservation of momentum equation. sectional area, v is velocity, g is the gravity a) Kinematic Wave Models acceleration constant, h is the water surface elevation In some cases, inertial and pressure forces are much above a datum, and Sf is the friction slope which may smaller than friction and gravity forces. For such be evaluated using a steady flow empirical formula cases, Simplified form of momentum equation is So such as Chezy or Manning’s equation. = Sf, which states that the momentum of the unsteady Due to complexities of the St. Venant equations their flow is assumed to be the same as that of steady solution is not feasible, and various simplified uniform flow as described by the Chezy or Manning approximations of flood wave propagation continued equation. The kinematic wave model is limited to to be developed. Two possible approaches to the applications where single valued stage-discharge flood routing problems are available to engineers, rating exists, and where back water effects are • Hydrologic routing also known as lumped insignificant. The kinematic wave models are very routing is based on the conservation of mass popular in applications to overland flow routing of and an approximation of the relationship precipitation runoff. between flow and storage. The hydrologic b) Diffusion model routing, directly allows computation of the A diffusion model uses the continuity equation and outflow hydrograph at the downstream end of simplified form of momentum equation as the reach. ∂ h S f − = 0 the inclusion of water surface slope • Hydraulic routing also known as distributed ∂ x routing is based on the conservation of mass term allows the diffusion models to describe the and a simplified form of St. Venant attenuation (Diffusion effect) of the flood wave, and conservation of momentum equation. The accounts for backwater effects. As this models hydraulic routing requires computation of the doesn’t consider Inertial terms of momentum discharges along the river in an important equation and therefore, is limited to slow to number of intermediate points, for every time moderately rising flood waves in channels of uniform step, beside the cross sections in these geometry. intermediate points, the hydraulic parameters 2. Complete Hydraulic model like roughness coefficient are needed. In both If the complete St.Venant equations are used, the cases, the inflow hydrograph at the upstream model is known as a complete hydraulic or dynamic end of the observed reach is required. wave model. With the advent of computers, 2.1.1 Hydrologic flood routing models nowadays it is possible to solve complete St.Venant Hydrologic flood routing models are conceptual equations for flood routing.