Assessment of Flood Vulnerable Areas Downstream of Mangla Dam

A Case Study on Jhelum River

Saqib Ehsan*, Naqi Irtiza Haider, Shoaib Ghani, Tahir Zaman

NFC-Institute of Engineering & Fertilizer Research Faisalabad, Pakistan

*Corresponding Author: hod.civil@iefr.edu.pk

Abstract—The purpose of flood modeling is to understand and manage the mechanisms at work in floodplains. The objective of this study is to assess the flood vulnerable areas along the Jhelum River downstream of Mangla dam in Pakistan. For this purpose, a project reach of about 70km downstream of Mangla dam up to Rasul barrage has been taken into consideration. Due to existence of populated area along the project reach, this particular reach length was considered in order to assess the possible risk of flooding. One dimensional flood modeling has been carried out by the application of Hydrologic Engineering Centre River Analysis System (HEC-RAS) for different flooding scenarios. The recent floods of 2010 and past floods of 1992 and 1997 have been focused in this study. The simulated results were analyzed and the possible extent of flooding at downstream locations was estimated. In extreme scenario of 1992 flood, many surrounding populated areas existing along the project reach from 13050m to 69204m were found vulnerable. This study recommends the rehabilitation and improvement of existing flood protection measures downstream of Mangla dam and also introduction of new useful measures to ensure the flood safety in vulnerable areas. .

Keywords: Flood Vulnerability, Flood Modeling, Rasul barrage, HEC-RAS, Jhelum River, Mangla Dam

I. INTRODUCTION

A flood is an overflow of water that submerges land which is usually dry. Floods are common natural disasters that can affect millions of people around the world. They destroy houses and buildings, and carry soil away from valuable farming land. Floods can also contaminate drinking water and can cause various water-borne diseases. They are often caused by overflowing of river, but overflowing lakes and seas can also cause Flooding. Flooding has always been a part of human history. Many ancient civilizations developed along waterways and rivers because people needed water for their fields. Main types of flood include: cat-III flood, coastal flood, urban flood, river flood and flash flood. The major causes of flooding are: heavy rainfall, snow melting, failure of dam, river overflow and strong winds in coastal areas. Floods can have devastating consequences and can have effects on the economy, environment and people. [1-5]

The flow pattern in a river changes according to the geometry of river valley. In case of high flooding in a river valley, the extent of flooding also changes with respect to the typical

shape of the river valley. For different rivers, the response of the river valley to flood water would certainly be different. There are many parameters of river geometry which are responsible for the conveyance of regular river flows and floods. [6,7]

The flood modeling is one of the engineering techniques which provide precise information of the flood profile. The rainfall, runoff, catchment characteristics and return period are the parameters which govern the flood. Flood inundation models enable us to make hazard predictions for floodplains and to mitigate increasing impact of flooding on people and property. The goal of such tools is to simulate probable inundation damage in a given area depending on many flooding scenarios with different intensity, duration and return period. Model reliability is assessed by comparing the simulation results and real/observed data in a calibration process. [8-12]

There are different models available for carrying out flood inundation modeling. The results of flood routing on a river reach are further analyzed for the assessment of possible risks of flooding to people and property. In this research, one dimensional flood modeling using the HEC-RAS program has been conducted. HEC-RAS is widely used all over the world for modeling 1-D open channel flow conditions in rivers, lakes/reservoirs, irrigation canals and other inland water systems with the provision of different hydraulic structures. The program package is an integrated system of software designed for interactive use in a multi-tasking environment. HEC-RAS can model steady and unsteady flow water surface profile computations with both, SI and US customary units. This useful modeling tool is available free of cost with almost all extensions. [2, 3, 13, 14]

In this study the focus is on the one dimensional flood routing on the project reach length (69204m) starting from downstream of Mangla dam to Rasul Barrage using HEC-RAS in order to assess the possible risk of flooding for nearby populated areas. For this purpose, past flood events of 1997, 1992 and 2010 have been taken into consideration.

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DOI: 10.24081/nijesr.2016.1.0007

II. MATERIALS AND METHODS

A. Project Area

The Jhelum river valley downstream of Mangla dam up to Rasul Barrage has been taken into consideration. The project reach is about 69204 m downstream of Mangla dam up to Rasul Barrage. Figure 1 shows the Jhelum river valley up to the upstream of Trimmu barrage. The contribution of five Tributaries like Suketar Nallah, Bandar Kas, Jabba Kas, Kahan river and Bunha river at different downstream locations within the project reach are also shown in Figure 1. [7,15]

Fig. 1. Jhelum river valley downstream of Mangla dam [7], [15]

B. Data Collection from NESPAK

The input data required for modeling in HEC-RAS was collected from the Water Resources Division of NESPAK, Lahore in soft form. Further, in subsequent meetings useful information about the project area was also gathered through personal communication with relevant engineers. The collected data mainly comprises the following:

- Geometrical data of project reach - Flow data at upstream and downstream of project

reach for past floods of 1992, 1997 and 2010 - Data of tributaries and bridges - Data of existing flood protection measures

(levees/dykes) along the project reach

C. Input of Data in Model

Depending upon the available data, 25 cross sections were inserted in HEC-RAS for the definition of project reach. As there was no exact information available regarding the roughness in cross sections so

an assumed value for Manning’s n was initially considered for each cross-section. [13, 14]

Available outflow hydrographs downstream of Mangla Dam were used as upstream boundary condition for different scenarios.

As there was no reliable data available at downstream reach location so the available option of normal depth (in terms of channel slope) in HEC-RAS was considered. [13]

Rail Bridge in Sarai Alamgir at 34869m downstream of Mangla dam has been defined by using the available bridge data.

Based on the available data from NESPAK, the existing levees/dykes were also defined at respective cross-sections downstream of Mangla dam up to Rasul barrage.

D. Theory of Unsteady Flow Modeling in HEC-RAS

The unsteady flow modeling in HEC-RAS is based on the principle of mass (continuity) and principle of conservation of momentum. In HEC-RAS, the unsteady flow simulations are run through the interaction of flow in main channel and floodplains. When the water level in the river increases then water flows laterally away from the main channel, flooding the floodplains and filling the available storage areas. Due to rising water depth, the floodplains starts conveying water downstream normally along a shorter path than that of the main channel. While in case of decreasing water level, water flows towards the main channel from overbanks. [10, 12, 14]

E. Flood Routing in HEC-RAS

Based on the available data, one dimensional flood routing in HEC-RAS has been carried out for different flooding scenarios with unsteady flow conditions. Several model runs were made to overcome the numerical instability. As per available data, the contribution of tributaries was also incorporated. The outflow hydrograph for 1997 flood (peak Q=12798 m3/sec) was taken from NESPAK to run the unsteady flow simulation. The model was calibrated for 1997 flood at Rasul barrage by comparing the simulated maximum discharge of 14743.60 m3/sec with the observed maximum discharge of 15563 m3/sec at Rasul barrage. Figures 2 & 3 show the values of maximum discharge and maximum water surface elevation along the project reach for 97 flood respectively. The simulated water surface elevation is also close to the observed value as shown in Figure 3.

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Fig. 2. Maximum discharge along project reach for 97 flood

Fig. 3. Maximum water surface elevation along project reach for 97 flood

There is some variation in simulated results with respect to the observed values. This variation could be due to following reasons:

Reliable data of manning’s value n was not available so model was run with an assumed value.

The actual contribution of tributaries for different

flooding scenarios was not known.

There might be some errors in the measured data of

cross-sections.

III. RESULTS AND DISCUSSIONS

A. Results of 1997 Flood

The peak value of discharge decreases along the downstream reach due to retention of outflow hydrograph. The increase in discharge values at specific downstream locations is due to the contribution of tributaries. The maximum water surface elevation also decreases along the project reach because of flood expansion at downstream river locations with respect to the geometry of cross-sections. Based on the expansion of flooding at downstream river locations for 97 flood and available information about the surrounding areas, the vulnerable populated areas existing along the project reach have been identified. The vulnerable areas for 97 flood are:

Rahiyaan, Dhal, Pakhwal and Jandila at 34603m

Kastila and Pira Gaib at 35769m

Narwal and Nougran at 43018m

Khuhar, Dhoki, Nathwala, Chotala, Fatehpur, Khurd,

Puran, Bhambar, Darapur and Rasul village from

56847m to 66992m

B. Flood Routing for 2010 Flood

The outflow hydrograph (peak Q=6431.549 m3/sec) was taken from NESPAK to carry out flood routing of 2010 flood along the project reach. Figures 4 & 5 show the values of maximum discharge and maximum water surface elevation along project reach for 2010 flood respectively.

Fig. 4. Maximum Discharge along project reach for 2010 flood

The observed maximum discharge at Rasul barrage was 6560 m3/sec which is very close to the simulated value of 6523.75 m3/sec. The increase in the value of maximum discharge at some locations is due to the contribution of tributaries. The maximum water surface elevation decreases along the reach.

Fig. 5. Maximum water surface elevation along project reach for 2010 flood

The vulnerable areas for 2010 flood are:

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Dhoki, Nathwala, Chotala, Fatehpur and Khurd from

56847m to 61941m

C. Flood Routing for 1992 Flood

The outflow hydrograph (peak Q=26293 m3/sec) was taken from NESPAK to run the flood routing simulation for 1992 flood which is the highest flood event occurred in the past. In this study, the flood of 1992 has been considered as extreme flooding event. Figures 6 & 7 show the values of maximum discharge and maximum water surface elevation along the project reach for 1992 flood respectively. The observed discharge at Rasul barrage was 26500 m3/sec which is close to the simulated discharge of 25830.18 m3/sec.

Fig. 6. Maximum discharge along project reach for 92 flood

The maximum discharge decreases along the project reach due to retention of outflow hydrograph except the locations of tributaries. Further the maximum water surface elevation also varies in the same pattern along the downstream reach as for the other flooding scenarios.

Fig. 7. Maximum water surface elevation along project reach for 92 flood

The simulated results of maximum discharge and maximum water surface elevation are quite close to the observed results for all considered floods. The same flow pattern clearly justifies the calibration process and reliability of the model. The vulnerable areas for 1992 flood are:

Ring Sharif, Mora surat shah, Dalyal, Mangola,

Barala and Mughalabad at 13050m

Pulanda and Rungpur at 22377m

Rahiyaan, Dhal, Pakhwal and Jandila at 34603m

Kastila and Pira Gaib at 35769m

Pahirwal and Bagga at 38444m

Narwal and Nougran at 43018m

Khuhar, Dhoki, Nathwala, Chotala, Fatehpur, Khurd,

Puran, Bhambar, Darapur, Rasul village, Rasul tehsil

and Dilawar from 56847m to 69204m

The people living in these areas are at risk of flooding. The 1992 flood has been considered as the worst case scenario in this study. The extent of flooding has been found maximum in 1992 flood scenario as compared to other simulated scenarios. Most of the areas adjacent to project reach at different river locations have been found to be exposed to flooding. The depth of floodwater is varying at different locations with respect to the geometry of cross-sections. The vulnerable areas are located on both right and left sides of the reach. The population at risk along the project reach is mostly rural. The results of flood routing simulations show that at respective river locations of vulnerable areas, the flood protection measures (levees/dykes) are either inadequate or not existing at all. This situation clearly justifies the urgent need to enhance the existing flood protection measures along the project reach and also to introduce new measures at respective locations to ensure long-term flood mitigation.

IV. CONCLUSIONS AND RECOMMENDATIONS

Flood modeling is an important task for decision making in the field of flood risk management. Due to existence of populated areas along the reach of Jhelum River, this

particular reach length was considered for the purpose of modeling in order to assess the possible risk of flooding.

Three past flood events of 1997, 2010 & 1992 were simulated in HEC-RAS for unsteady flow conditions to estimate the

possible extent of flooding along the project reach. The past flood of 1992 was considered as the worst case scenario. The expansion of flooding at different downstream locations has

been found the largest for 1992 flood scenario in comparison to other flooding scenarios.

In case of extreme flood event equivalent to 1992 flood, which is the highest past flood in Jhelum River, most of the

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surrounding populated areas along the project reach have been found vulnerable to flooding. The results clearly reflect the inadequacy of existing flood protection measures at respective river locations. The population downstream of Mangla dam along the project reach is mostly rural. The extended presence of floodwater in some vulnerable areas depending on the duration of flood event may turn the fertile agricultural land into barren land by water logging and salinity. The results of this study should be further utilized to estimate the possible loss of life and property damage at downstream locations in case of severe flood like 1992 flood event.

The people of vulnerable areas should be aware of flood severity and possible flood damages. The flood protection

measures existing along the flood vulnerable areas need early rehabilitation and improvement. There should also be improvement in flood warning, emergency preparedness and

evacuation techniques in order to minimize the possible risks of flooding. This study would be useful for flood safety

management in Pakistan as well as in other parts of the world.

ACKNOWLEDGMENT

Authors would like to acknowledge the support of National Engineering Services Pakistan (Pvt.) Limited (NESPAK) for the provision of the required data for this research project.

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