FLOW AND EROSION PROCESSES AT BENDS AND AROUND RIVER TRAINING WORKS IN A SAND BED BRAIDED RIVER

Transcription

1 FLOW AND EROSION PROCESSES AT BENDS AND AROUND RIVER TRAINING WORKS IN A SAND BED BRAIDED RIVER MOHAMMAD NAZIM UDDIN DOCTOR OF PHILOSOPHY INSTITUTE OF WATER AND FLOOD MANAGEMENT BANGLADESH UNIVERSITY OF ENGINEERING AND TECHNOLOGY DECEMBER

2 ABSTRACT Riverbank erosion is one of the major problems in Bangladesh. Annually thousands of hectares of land are eroded along the major rivers. Millions of people became homeless and landless, and displaced due to erosion. The erosion also affects livelihood, agriculture and environment. To protect banks from erosion about thirty bank protection structures have been constructed along both banks of the Jamuna River. But almost every year some of the bank protection works are damaged. So, it is essential to investigate the insight causes of failure of the bank protection structures. It is basically a field based research. The present study has been conducted (i) to investigate flow and erosion processes at bends, (ii) to investigate flow and erosion processes around bank protection works and failure mechanism of some selected bank protection structures, (iii) to develop of an analytical erosion prediction model and finally, (iv) to integrate indigenous and technical knowledge for sustainable erosion management. In the present study it is found that the secondary current in a bend of the third order channel within a large scale sand bed braided river is similar to that of the single thread meandering channel. But the secondary cell is divided into multi-cell due to flow over the sandbar. It is also found that the bend channel migrate downstream, lateral direction and upstream direction. The maximum rate of erosion is observed at the upstream and downstream sides of the bend. The rate of erosion is higher at upstream and downstream side of the bend due to the shear velocity is six times higher than critical shear velocity and the thalweg is near the bankline. The flow and erosion processes around Sirajgang hardpoint, Betil and Enayetpur spurs and bandal at Randhunibari have been investigated in the present study. The radical morphological change in a sand-bed braided river is a great challenge for the structural stability of the river training works. Because the flow processes near a river training works is guided by the sandbar which is a prominent characteristic of a dynamic braided river. It is found that the strength of vortices, flow intensity and velocity considerably changes due to drastic morphological change around the Sirajgang hardpoint. From the present study it is clarified that the oblique flow a prime factor for the failure of the river 2

3 training works. Other factors also accelerate the failure event of the river training works. From the analysis of flow processes and morphology around the Sirajganj hardpoint it has been investigated that the following are the reasons of the failure of the Sirajganj hardpoint such as: generation of oblique flow by sandbar and attacked the hardpoint, washed away of sandbar adjacent to the hardpoint, thalweg shifting at the vicinity of the hardpoint, movement of five meter high dune through the channel passing adjacent to the hardpoint, riprap failure due to movement of dune, development of scour hole because of riprap failure, flow slides from the hardpoint side. The causes of failure of the Betil and Enayetpur spurs have also been investigated from the present study. The causes are as follows: generation of oblique flow as a result of upstream morphology, strong parallel flow upstream of the spurs and development of deep channel due to flow circulation normal to the spurs. It is also found from field measured data analysis that the bandal-like structure is very effective as flow diversion and sediment deposition measure. But selection of site for implementing of such structures is a major factor in a large scale sand bed braided river. An analytical erosion prediction has been developed for the third order channel of a large scale sand bed braided river. The model is developed using the flow parameters, planform parameters, properties of bank materials and indigenous knowledge. The indigenous knowledge regarding effective period of erosion has been integrated into model. The model basically produces an envelope curve. The envelope curve covers eighty seven percent of the observed data and seventy eight percent of EGIS data. The field engineers could estimate the rate of erosion for the third order channel of a sand bed braided river. It is evident from the present study that the riparian people are enriched with experienced based indigenous knowledge on flow and erosion processes around different bank protection works, flow and erosion processes at bends, erosion processes at the straight bankline, historical morphological change, bend development, the trend of the rate of erosion, effective duration of erosion. It is clarified from the present study that there are strong similarities among experience-based indigenous knowledge, field engineer s knowledge, field measured result and laboratory based scientific knowledge. Interesting outcome of the present study is that the local people can guess the bank 3

4 protection structure is going to be failed observing bustling out of air bubble, floating bamboo and geo-textile and turbid water. Observing such signs, the local people convey message of probable failure of the bank protection works to the BWDB personnel for taking necessary steps to avoid failure event. Therefore, management program during emergency situation should be formulated integrating the indigenous knowledge. 4

5 Assessment of Irrigation Efficiency and Techno-Socio-Economic Opportunities for Its Improvement in Minor Irrigation MD. EFTEKHARUL ALAM DOCTOR OF PHILOSOPHY Institute of Water and Flood Management BANGLADESH UNIVERSITY OF ENGINEERING AND TECHNOLOGY March

6 Abstract In Bangladesh, about 63% of the arable land is now irrigated and there is immense pressure to bring more areas under irrigation. But the expansion of irrigation has been thwarted due to reduced transboundary flow, unplanned and unregulated withdrawal of groundwater and inefficient management. Improvement of irrigation efficiency (IE) and efficient management would have a dominant role in enhancing the development of irrigation. In this context, this study was taken up with the objectives of estimating the present level of IE and water productivity (WP), determining the parameters that affect the IE, and assessing the techno-socio-economic potentials for improving the IE. The study was carried out in 7 districts of 5 agro-ecological zones. In total, 85 irrigation schemes were selected in such a way that they represent all the major variables of irrigation performance namely, type of pumping technology (STW, DTW and LLP), source of water (surface and groundwater), management system (public and private), pricing system (area based, time based and crop share based), water distribution system (buried pipe, lined and earthen canal) and source of energy (diesel and electricity). In addition, field experiments were also carried out to study the impact of improved on-farm water management (alternate wetting and drying: AWD) technology over the traditional practice of continuous flooding. Field data on pump discharge (Q), total hours of pump operation (H), total irrigated area (A), crop yield, and rainfall data were collected, and evapotranspiration (ETo) were estimated during the 2008 and 2009 irrigation seasons. Participatory rural appraisal tools like, FGD and test of farmers' knowledge were conducted in 5 upazilas in order to find out the reasons of using excess irrigation water and their probable solutions, areas of farmers knowledge gap, acceptability of different technologies, etc. From the analysis of field data on water requirement and supply and crop yield, the present average IE and WP of the country were estimated as 34% and kg/ m3(2006 lit/kg), respectively. It was also found that the source of energy and types of irrigation equipment have no significant impact on IE. The opportunity of improving the IE by implementing time based water pricing system over area based system is 3.85%, that of lined canal system over earthen canal is 3.37% and that of AWD technology is about 6.28%. The improvement of IE due to management system i.e. public-private-partnership (PPP) over privately managed scheme is about 2.65%. Buried pipe system and crop 6

7 shared method increased the IE by 6.38% and 1.63%, respectively over earthen-area based system. From mathematical calculation and regression analyses, the combined contribution of water pricing system (time based), water distribution system (buried pipe), management system (PPP) and AWD, in increasing the IE from the base condition of earthen-area based-private system varied from 17.55% to 18.84%. From the farmers survey, it was found that the knowledge gap of farmers about on-farm water management technology is the highest among all the four important agricultural inputs and services. Besides, the four selected indicators are also found to be techno-socio-economically viable and sustainable, if these are implemented in integrated manner. Creation of awareness among different stakeholders through training and demonstration for rapid dissemination of AWD technology and introduction of demand based water marketing system instead of supply based system can be achieved with minimum financial implication. Replacement of earthen canal by construction of lined canal and buried pipe may be done in phases but would require active participation of all the concerned stakeholders and introduction of PPP approach for participatory irrigation water management. Considering the food security, economic development, poverty reduction and protection of natural environment, the government should take the opportunity of the abovementioned variables in an integrated manner for increasing IE from 34% to close to its potential value of 50%. 7

8 POVERTY ALLEVIATION THROUGH IMPROVED WATER MANAGEMENT IN A DELTAIC TIDAL PLAIN Md. Mahbubul Alam DOCTOR OF PHILOSOPHY Institute of Water and Flood Management BANGLADESH UNIVERSITY OF ENGINEERING AND TECHNOLOGY February

9 ABSTRACT The southwest region of Bangladesh that represents about 27 per cent (40,450 sq. km) of the total area of the country is crisscrossed by large tidal rivers. A large number of Tidal Basins, which are flat, low-lying alluvial lands interspersed with tidal streams and waterfilled depressions are found in this region. Though affected by water management problems like flood, drought, drainage congestion and salinity intrusion, the southwest area of Bangladesh is still very much dependent on agricultural crops, particularly rice, as the main source of income and livelihood for its rural population. The region is among the highest poverty-stricken parts of the country. Improved water management (irrigation, flood control, drainage improvement and salinity intrusion control) is the key to agricultural productivity improvement and have a positive impact on the poor. The objectives of this research work were establishment of linkages between water management interventions and poverty, assessment of impact of water management interventions on poverty and selection of suitable water management options for further alleviation of poverty in the tidal basin. Linkage between water management interventions were established through focus group discussions (FGDs) of the stakeholders. Impact of water management interventions on poverty were assessed through household survey. For selection of options for further poverty alleviation, water resources demand and availability were assessed analytically. The feasibility of different water management options for productivity increase was evaluated. Finally, the selected options were prioritized by the stakeholders through focus group discussions. Study results of linkage analysis showed that in most cases water management interventions increase the income from agriculture by increasing crop yield and cropping intensity, by encouraging fish culture, commercial agriculture (orchard, betel leaf plantation etc.) and livestock rearing. They also generate employment opportunities for the resource poor. Combination of necessary agricultural water management interventions reduces poverty at different level. Study results on impacts of different water 9

10 management options showed a reduction in poverty incidence by 10.0, 6.7, 6.7 and 6.7 percent, respectively due to the presence of irrigation, flood control, drainage improvement and salinity intrusion control interventions. The benefits of irrigation, flood control and drainage on poverty alleviation are additive when implementation is feasible. Flood control, drainage and irrigation (FCDI) projects had the highest impact on poverty alleviation compared to any other water management interventions. Besides, these interventions, further poverty alleviation is possible through efficient use of available water resources. Canal rehabilitation for increasing surface water availability with promotion of conjunctive use, use of polyethylene pipe instead of earthen canal, alternate wetting and drying (AWD) instead of keeping continuous standing water and shifting from traditional Aus (pre-monsoon) rice varieties to supplementary irrigated HYV rice varieties are some of the ways for increasing productivity and alleviation of poverty. Conjunctive use after rehabilitation of existing canals could increase the present irrigation coverage by 70% and reduce the present overexploited groundwater use by 57%. Use of plastic pipe and polyethylene pipe could reduce the conveyance loss from 30% to around 5%. Alternate wetting and drying (AWD) irrigation method can save around 25% of irrigation water or fuel consumption cost compared to the conventional practice of continuous standing water. An analysis of prioritization of the water management options by the stakeholders showed that conjunctive use of water after canal rehabilitation was considered as the top priority for productivity improvement of the study area. Moreover, compared to the other water management options, which are farmer-based with limited potential, conjunctive use is system-based and would have impact on poverty alleviation over the whole project. 10