DESIGN FLOOD ESTIMATION OF PAZHASSI BARRAGE FOR EFFECTIVE MANAGEMENT OF FLOODS

Abstract

In the changing climate scenario, many river basins are susceptible to floods by natural or anthropogenic reasons. The frequency and severity of floods have been increasing in recent decades due to a combination of factors including climate change, rapid urbanization, deforestation and inadequate water management practices. As a result, accurate estimation of flood magnitudes has become increasingly crucial in mitigating the impacts of floods and designing resilient water resource management systems. In 2012 August, Pazhassi barrage in Kannur District had an overflow of flood waters for 20 hours with the water level rising above the free board level to 28.35 m. It has been reported during the inspection by the dam safety review panel in Kerala in 2022 that the design capacity of the barrage has to be re-estimated in the scenario of changing

rainfall patterns and climate change (DSRP, 2022). Hence the study was proposed to re- estimate the design flood value of Pazhassi barrage.

Design flood is defined as the flood which a project (involving a hydraulic structure) can sustain without any substantial damage, either to the objects which it protects or to its own structures. In order to re-estimate the design flood value of Pazhassi barrage, hydrometeorological approach was followed. It includes the design storm estimation by rainfall frequency analysis using HEC-SSP and Probable Maximum Precipitation (PMP) and Standard Project storm (SPS) obtained with help of PMP atlas(CWC, 2015). Further step was the derivation of catchment response function by Arc-GIS using the Digital Elevation Model (DEM). The flood estimation reports of CWC provided the criteria to develop the Synthetic Unit Hydrograph (SUH) of the basin(CWC, 2001). Then distribution of design storm of 24 hours into two 12 hours bells, application of loss rate ,time adjustment and critical sequencing were done to obtain the effective rainfall hyetograph. Convolution of effective rainfall on SUH was done to obtain the Direct Runoff Hydrograph (DRH). Adding the baseflow to DRH, the flood hydrograph was generated (CWC, 1992). The same procedure was repeated for all the design storms and the peak values of flood hydrographs were obtained as the design flood values. The criteria for design flood value of the structure based on (Indian Standards) IS specifications has been assessed (IS: 11223-1985). Critical design flood value for the structure was identified. The estimated values of flood for different recurrence interval storms, Standard Project Flood (SPF) and Probable Maximum Flood (PMF) values were compared with the existing design flood value to check adequacy of

capacity. Pazhassi barrage doesn’t have a rule curve at present and operations are based on an accepted policy. The present operation policy was analysed for safety considerations and adequacy of storage for meeting Crop Water Requirement (CWR) of the command area. CROPWAT 8.0 software was used to estimate the CWR of Pazhassi command area crops. Calculated value of volume based total irrigation water requirement of the command area was compared with the storage capacity of barrage available for irrigation (KERI, 2011). SPS, PMP, design storm values of 100 years and 200 years return periods (T) for Pazhassi barrage were 36.4, 40.11, 49.36 and 56.66 cm respectively. SPF, PMF, 100 years T and 200 years T floods were 7099, 7756, 9841 and 11351 m3

/sec respectively. Based on IS specification (11223-1985), structure qualifies for SPF. The estimated flood value SPF was 7099 m3

/sec and designed flood value of Pazhassi barrage was 3500

m3 /sec with a difference of 3599 m3/sec. Gross irrigation requirement of command area for 75% overall efficiency was 88.43 m3

/sec. Available storage for irrigation purpose was only 25.80 Mm3 which is not enough to cater the demand. Additional spillways need to be provided for the structure to contain the flood and to ensure safety of the barrage. A rule curve has to be designed for the barrage in order to meet multi-purpose objectives of irrigation, drinking water supply and power generation with additional spillways to carry the flood flows.