Pasakha Flood Mitigation Proposal

Transcription

1 Pasakha Flood Mitigation Proposal Technical Report NAPA-2 Project Preparatory Grant Flood Engineering Management Division Ministry of Works and Human Settlement 1

2 Contents 1. Introduction Objective Temporary Outputs Potential long term outputs Previous Studies Description of the Study Area Climatic Features of the Study Area Hydrology of the Study Area Rainfall Runoff Analysis Availability of Rainfall Data Maximum Daily One-Day Rainfall Delineation of Catchment Area Discharge Calculations The peak Discharge for the recorded storm period is 255 m 3 /sec River System Analysis Set-Up Simulation Results Sediment Transport and Slope Stability Desk Study Field Investigation and Review of Implemented Mitigation Measures Conclusion Annexure

3 Pasakha Flood Mitigation Project Project Preparatory Grant 1. Introduction All the southern foothills bordering India and Bhutan are highly prone to flash floods and other forms of disasters which cause loss of human lives, properties and damage to natural environment. The areas in and around Pasakha (Chhukha Dzongkhag) industrial estate located on the same belt of foothills are no different. The last decade has witnessed increasing incidences of flooding, which can be attributed to increased rainfall intensity and untimely arrival of monsoons. The area experiences as high as mm of monthly rainfall (Gedu, August, 2000). Tala and Gedu region is the catchment area for the streams that run through Pasakha industrial area. The highest amount of rainfall recorded in a day at Tala was 400 mm in the month of August This high amount of rainfall experienced by this area is the main factor for high incidences of flash floods and erosion. Entire communities are at risk from flashfloods in Pasakha area under Chukha Dzongkhag, owing to the increased frequency and intensity of rainfalls and high rates of siltation. Barsa River that flow in the area swells to unbelievable magnitude that the river training works are submerged during monsoon and pose serious risks of flood and landslides. In view of the increase in such climate hazards in Pasakha and its surrounding areas, it is necessary to safeguard human lives (industrial workers and community in the vicinity of the industrial estate) and properties (industrial infrastructure, roads, water supply, drains etc.). 1.1 Objective This project aims to address the vulnerabilities to flooding and landslides in the Pasakha industrial and residential area through a study on the Barsa River watershed conditions for drawing up an adaptation plan and through river and bank protection works. To effectively intervene in major flood affected areas at Pasakha Industrial Areas before the area poses threats to humans, their livelihood and investments by enabling local stakeholders to undertake effective adaptive measures. The specific objectives of the river training works are (i) To guide the axis of flow at ordinary and low stages and safe passage of floods without overtopping the banks, (ii) To protect the banks from erosion and generally improve their alignment by stabilizing the river channel, (iii) To train the river flow along a safe course, thereby avoiding damage by flooding or erosion of valuable lands, habitations and factories, etc., (iv) To prevent outflanking of the Padzekha bridge by directing the flow in a defined stretch of the river. 3

4 1.2 Temporary Outputs Improved safety to human lives and industrial properties from the impact of floods through timely mitigation activities; Stabilize fallow unstable government land through appropriate measures and development of green areas to avert and mitigate the risks of flood to human lives and livelihoods as well as reduce potential economic losses resulting from damage to industrial properties and disruption of industrial operations. 1.3 Potential long term outputs A safer and stable Barsa watershed leading to a safer area for industries and the downstream communities and infrastructures. After more study on the morphology of the Barsa River and the upstream conditions, flood predictions in the future can be done and other river training works designed. 1.4 Previous Studies In December 2000, a team led by Dr. Daisuke Higaki and Bhutanese counterparts from Geological Survey of Bhutan has conducted a preliminary study on Disaster Mitigation Works around BCCL and BFAL factories along Barsa River basin. The study provides counter measures against erosion, sediment flow and flooding due to high water level. In 2002, a separate study on stability of the Barsa River Catchment Area has been conducted by a team from Department of Geology and Mine, led by Mr. Indra Kumar Chettri. This study has focused on mapping both active and inactive landslides in Barsa River Catchment, encompassing an area of square kilometers. A hazard map using the parameters of slope angle, topography, climate and the mass wasting have been produced Annexed. The study has suggested a combination of measures to alleviate the degree of landslide in the area. 2. Description of the Study Area Pasakha, under the Phuentsholing Thromde (Municipality) in southern Bhutan, is one of the first planned industrial areas of the country. It is located on the confluence of two rivers, Barsa and Singye. Pasakha is connected to proper Phuentsholing Town by a 15-km asphalt road. Pasakha is a growing industrial hub situated just along the international border with India. Geographically, the southern belt of Bhutan is the foothills of the Eastern Himalayas which fall roughly within the latitude corridor of 26º50' to 27º N; and climatically, the foothills are of subtropical nature with broad-leaf rainforests. Except for large north-south rivers that originate from 4

5 temperate to glaciated regions in the north, most foothill drainages (catchments < 75 km 2 ) exhibit torrential discharges under the influence of monsoon precipitation Geologically, the southern foothills of Bhutan fall under a tectonic category known as the Siwalik Tectonic Formation. The Siwalik rocks were laid down and elevated and folded against the older rocks of the Himalaya according to intercontinental geo-tectonic theory. Since very early times, extensive erosion has taken place and the present topography consists of strike valleys, dip slopes and escarpments as well as many areas of landslide instability. The Siwalik zone is predominantly composed of sandstone, siltstone, clay-shale and boulder beds. CHUKHA Pasakha 5

6 Figure 1-1 Overview of Study Area 2.1 Climatic Features of the Study Area In general, the climate in Bhutan varies according to latitude and altitude, the latter being a predominant governing factor. The climate and soils that determine the natural vegetation are generally classified as follows: the alpine tundra (above 3800 m), the cold temperate forest ( m), the warm temperate forest ( m), the semi-humid subtropical forest ( m) and the humid subtropical forest ( m) to the south where the effect of summer monsoon is most pronounced. The Pasakha Industrial Area falls under the sub-tropical forest zone with pleasant but hot summer and chilly cold winters. The rainy seam begins as early as April and it lasts as late as October. However, the upper catchment of the Barsa River is in semi-humid subtropical forest belt. 6

7 Temperature in ºC 2.2 Hydrology of the Study Area Pasakha Industrial Area is at the junction for of two rivers, Singye and Barsa. The past flood events in the area that caused the flooding of the industrial materials, the residential areas and the washing away of the old Barsa Bridge were caused by the monsoon Barsa River. Singye River is relatively mild while Barsa River has been causing problems almost every monsoon. The rivers receive water primarily from monsoon rainfall. Secondary sources such as snowmelt are non-existent though the river does not fall dry in dry season due to ground water. There are no discharge recording stations established in any of these rivers. However, a nearby meteorological station (Phuentsholing and Tala) has daily records of precipitation. Due to the significantly higher risk of events occurring in the Basra river catchment areas and the higher risk level these pose to the Pasakha Industrial Area, the study has concentrated on the catchment and Basin of Barsa River. At its headwaters, Barsa River flows from a very steep terrain at Jumbja. It is then joined by a tributary flowing from Ganglakha Area. The highest altitude of the catchment area is as high as 2482 meters above mean sea level. The general gradient of the area is high with slope angles more than 45º, with very few places with slope less than 20º. With a catchment area of about 61 square kilometers, the Barsa River flows through Pasakha, where the channel is dominantly sand and gravel bed. Some climatic qualities recorded at the two stations of Phuentsholing and Tala for period and respectively are shown in Figure 2.2, 2.3 and 2.4. (courtesy: DHMS, MoEA). Figure Monthly Mean Temperature JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC Mean Maximum Phuentsholing Mean Minimum Phuentsholing Mean Maximum Tala Mean Minimum Tala

8 Axis Title Rainfall in MM Figure 2.3 Average monthly Rainfall ( ) JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC Phuentsholing Tala Figure 2.4 Total Annual Rainfall Phuentsholing Tala 8

9 3. Rainfall Runoff Analysis Hydrologic studies to determine runoff and peak discharge should ideally be based on long-term stationary stream flow records for the area. However, Barsa River does not have a discharge gauging station. It therefore is necessary to estimate peak discharges with hydrologic models based on measurable watershed characteristics. In order to determine the peak discharge of the river, a rainfall-runoff model is necessary. The rainfall-runoff model will predict the runoff for a given period of rainfall. A rainfall runoff model has been set up for the catchment area of Barsa River. A digital elevation model (DEM) at 30 meter resolution was obtained from Watershed Management Division of Ministry of Agriculture and Forestry. Using Geo-HMS, another product of USACE, as an extension of ARCGIS software, the DEM was processed as a basin model file for analysis in HEC-HMS. 3.1 Availability of Rainfall Data Rainfall data (total daily series) was made available by the Meteorology Division, Department of HydroMet Services, Ministry of Economic Affairs, Thimphu. A Class-A met-station has been established in Phuentsholing and Tala since Class A met-stations provide data such as temperature, relative humidity, rainfall (daily), wind speed, sunshine hours and cloud cover. Year JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC Total Table 3-1 Total Monthly and Annual Rainfall at Phuentsholing Station 9

10 Year JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC Total Table 3-2 Total Monthly and Annual Rainfall at Tala Station The 16 years ( ) daily rainfall data series is checked for consistency. It must be confessed that is very difficult to quantitatively assess the quality of data. Although the rainfall recordings for most years seem to be good, some doubts can be cast as observations and recordings are manually done. There are no automatic gauging stations and a trial gauge with automated hourly and even sub-hourly capabilities is reported with technical problems. For the purpose of preliminary analysis, the available data is used. 3.2 Maximum Daily One-Day Rainfall It is very desirable to have hourly date rainfall data or even sub-hourly data in order to adjudge a particular storm and storm period. However, due to lack of the hourly data, the study has considered daily data for the purpose of analysis. The highest One-Day rainfall for Phuentsholing was recorded on 2nd August 2000, where measured up to mm. For Tala Station, the highest One-Day rainfall was recorded on 15 th June 1998, measuring up to 400 mm of rainfall. In absences of sub-daily rainfall data, this study has drawn the data from Phuentsholing Station which shows higher magnitude of storm. The One-Day rainfall from 1 st August 2000 till 3 rd August 2000, are used for the analysis. 10

11 The recorded highest One Day rainfall (Annual maximum) is tabulated in Table 4.3 Year Phuentsholing Tala Table 3-3 Highest One-Day Rainfall for Phuentsholing and Tala Weather Stations 3.3 Delineation of Catchment Area Based on the 30-meter resolution raster image of Bhutan, using ARCGIS software on HEC- GeoHMS extension, the catchment for Barsa River is delineated. Figure 4.1 illustrates the delineated catchment area of Barsa River. The total area of the upper catchment used for the analysis is square kilometers. 11

12 W110 W180 W130 W150 W160 0 Outlet Figure 3-1 Catchment of Barsa River 3.4 Discharge Calculations The Hydrologic Modeling System (HEC-HMS) software is designed to simulate the precipitation-runoff processes of dendritic watershed systems. It is designed to be applicable in a wide range of geographic areas for solving the widest possible range of problems. The input requirements for this SCS Peak method are as follows: Particulars Catchment Area = km 2 Longest Flow Path, in kilometers L W110 = 1.89 L W130 = 3.54 L W150 = 3.57 L W160 = 1.34 L W180 = 9.93 Average Watershed Slope, Calculations Lag Time, T lag by T lagw110 = 0.42 T lagw130 = 0.76 T lagw150 = 0.75 T lagw160 =

13 S W110 = 1.89 S W130 = 3.54 S W150 = 3.57 S W160 = 1.34 S W180 = 9.93 T lagw180 = 2.14 Upper Watershed area km 2 Soil Group = B, shallow loess, sandy loam Woods (good cover) % of Drainage Area Forested, with slope > 6 Curve number CN = 55 Maximum Potential Retention, S 200mm Initial Abstraction, Ia, 40 mm Rainfall Hyetograph Remarks: Record Storm Event dating to 2-4 August Aug 2000, 00:00 03Aug 2000, 00:00 04Aug 2000, 00: mm mm mm Based on the above input data, the flow hydrograph at the Outlet, which is above the BFAL/BCCL Area, is calculated by HEC-HMS software. The Flow Hydrograph for the storm period is illustrated in Figure 4.2.during the peak discharge is as tabulated in Table The peak Discharge for the recorded storm period is 255 m 3 /sec Figure 3-2 Flow hydrograph 13

14 Date Time Outflow m 3 /s Date Time Outflow m 3 /s 1-Aug-00 0: Aug-00 11: Aug-00 1: Aug-00 12: Aug-00 2: Aug-00 13: Aug-00 3: Aug-00 14: Aug-00 4: Aug-00 15: Aug-00 5: Aug-00 16: Aug-00 6: Aug-00 17: Aug-00 7: Aug-00 18: Aug-00 8: Aug-00 19: Aug-00 9: Aug-00 20: Aug-00 10: Aug-00 21: Aug-00 11: Aug-00 22: Aug-00 12: Aug-00 23: Aug-00 13: Aug-00 0: Aug-00 14: Aug-00 1: Aug-00 15: Aug-00 2: Aug-00 16: Aug-00 3: Aug-00 17: Aug-00 4: Aug-00 18: Aug-00 5: Aug-00 19: Aug-00 6: Aug-00 20: Aug-00 7: Aug-00 21: Aug-00 8: Aug-00 22: Aug-00 9: Aug-00 23: Aug-00 10: Aug-00 0: Aug-00 11: Aug-00 1: Aug-00 12: Aug-00 2: Aug-00 13: Aug-00 3: Aug-00 14: Aug-00 4: Aug-00 15: Aug-00 5: Aug-00 16: Aug-00 6: Aug-00 17: Aug-00 7: Aug-00 18: Aug-00 8: Aug-00 19: Aug-00 9: Aug-00 20: Aug-00 10: Aug-00 21: Table 3-4 Time Series Result of Flow for the storm period 14

15 4. River System Analysis 4.1 Set-Up HEC-RAS is a software developed USACE for river modeling. This software has been deployed for modeling of Barsa River. Based on the GIS and Topographic Data from Phuentsholing Thromde, the river profile and river cross-section profiles were created. The schematic representations of the river cross-section and river profile from the critical areas are illustrated in Figure 5.1 Figure 4-1 Schematic Representation of river cross-sections 15

16 The time series flow data generated from the rainfall runoff analysis for the storm period between 1 st August 2004 and 3 th August 2004 has been used as the boundary condition at cross section Simulation Results The river system was simulated for the period of storm event, beginning from 1st August 2000 to 3rd August The some selected results of the simulation are illustrated in Figure 5.2 and Table 5.1 River Station Profile Stage Max WS Max WS Max WS Max WS Max WS Total Discharge Minimum Channel Elevation Water Surface Elevation Critical Water Surface Velocity Channel Flow Area Top Width Looking at the five cross-sections attached below, the bank overflow during the peak discharge will occur at River Station 0. In case of extreme weather event, whereby a storm event leads to a precipitation higher than millimeters in 24 hours, the expected discharge would be even higher than our calculate value. Moreover, in absence of hourly rainfall data, the 24-hourly data has been used. While replacing the hourly rainfall intensity with daily rainfall data, the probability of having an error in discharge data is relatively high. 16

17 Figure 4-2 River Cross-section during maximum Water Surface Elevation 17

18 5. Sediment Transport and Slope Stability 5.1 Desk Study Since year 2000, Department of Geology and Mines (DGM) has undertaken few studies to understand and propose measures to mitigate landslide risk in Barsa Watershed Area. A preliminary study on Disaster Mitigation Works around BCCL and BFAL factories along Barsa River basin has been conducted in December 2000, by a team led by Dr. Daisuke Higaki and Bhutanese counterparts from Geological Survey of Bhutan has conducted. The focus of the study had been to provide counter measures to prevent toe erosion and possible flooding of BFAL and BCCL areas. They have suggest In 2002, a separate study on stability of the Barsa River Catchment Area has been conducted by a team from Department of Geology and Mine, led by Mr. Indra Kumar Chettri. This study has focused on mapping both active and inactive landslides in Barsa River Catchment, encompassing an area of square kilometers. A hazard map using the parameters of slope angle, topography, climate and the mass wasting have been produced (Annexure I). The study has suggested a combination of measures to alleviate the degree of landslide in the area. Some of these suggested measures had been implemented and the present condition of these measures was assessed by a team from the Flood Engineering Management Division from 13 th -16 th February, The following parameters have not been assessed in the previous studies: 1. The peak monsoon river velocity 2. The flood discharge 3. The scour depth 4. The depth of the bed rock 5. The stabilization of landslide slope 5.2 Field Investigation and Review of Implemented Mitigation Measures A team comprising of engineers from Flood Engineering Management Division of Department of Engineering Services, Mr. G.C Birkha of Association of Bhutanese Industries and two staff members of BFAL visited the flood prone areas of Pasakha. The observation made by the team has been detailed below. The previous mitigation works along the BFAL and BCCI areas have failed during the past few monsoon seasons. The existing river training dykes (N26º E 89 º ) have been submerged by the deposits washed down from the active landslide (N26 º E 89 º 18

19 ) above. The training dykes having 3m foundation depth have failed and been submerged completely. Illustrated in Figure 6.1 The gabion walls below the training dykes are still standing though the apron has been scoured by about 150mm. The height of the existing gabion wall measures about 5.7 m with apron width of 5.6m and 2m depth. The flood height reached last year is 3.3m above the river bed level. However, the river bed level is not constant since it changes every season. It increases with the heavy sedimentation caused by the landslides and then decreases when the high rainfall intensity washes away the deposits. Illustrated in Figure 6.2 Figure 5-1 Submerged River training Dykes 19

20 Figure 5-2 Gabion Walls below BFAL Sl. Proposed Mitigation Measures Implemented Mitigation Measures Present status of the implemented mitigation measures 1 Two spurs below the main landslide area. Spur 1: L=10m and Spur 2: L=20m Both the spurs were constructed with RCC structures Spur 1 failed when the monsoon rain scoured the bed of the RCC structures. Spur 2 was constructed with one edge on a rock foundation and the other on the sediment bed. So during the monsoon, the sediment bed was washed leading to the collapse of the RCC structure in half. 2 RRM River Training Dykes along the river below the Spurs. River training dykes were constructed with Plum concrete structures The river training dykes have been submerged by the sediments deposits washed from the main landslide during the monsoon seasons. 3 4 Gabion walls below the RRM Training Dykes Training Dykes to protect the colony area The gabion wall of 6m height below the training dykes was implemented Gabion walls were constructed to protect the area The gabion walls below the training dykes are still standing though the apron have been scoured by about 150mm The gabions walls protecting the colony areas have been completely washed away by the monsoon floods. Table 5-1 Review of Measures Implemented to Alleviate Flooding in Pasakha 20

21 The visiting team has made an observation on a landslide that has been contributing towards fluvial deposits along the Basin of Barsa River around Pasakha industrial Area. The Google Earth Image of the landslide area is illustrated in Figure 6.3 BFAL AREA Figure 5-3 New Landslide Development An extract from the tour report of the field investigation team is given below. The landslide (N26º , E 89⁰ º ) is the main cause of the sediment deposits in the Barsa River. This landslide is encircled by two streams (Stream S1 and S2) that are fed by the seasonal rainfall. The streams are washing away the light sediments leaving behind the boulders. 21

22 Figure 5-4 Landslide Development in Barsa Catchment 22

23 The landslide slope during the dry season is prone to overgrazing by the cows that reside in the upper watershed areas. Overgrazing weakens the stability of the slope thus increasing its proneness to landslide during the monsoon seasons. Along with the identification of the problematic landslide ((N26º , E 89⁰ º ), the investigation team has also proposed some counter measures that can be implemented to alleviate the situation. 6. Conclusion The initial study carried out for Barsa River has confirmed the need for protecting Pasakha Industrial Area by training Barsa River and protecting the watershed area. The most important activity for Flood Risk Mitigation in Pasakha is to conduct a detailed study to prevent landslides in the catchment area. It is equally important to undertake routine maintenance of floodway to put a limitation on changing of the river course. Prior to construction of embankment to protect that Pasakha Industrial Area, hydraulic design should be initiated again in order to incorporate morphological change that would occur in the upcoming monsoon season. 7. Annexure 1. A preliminary study on Disaster Mitigation Works around BCCL and BFAL factories, by Dr. Daisuke Higaki, 2000 (Available at DGM Library) 2. Stability Study of the Barsa River Catchment Area by Department of Geology and Mines,2002 (Available at DGM Library) 23