This is to prove that the thesis work titled “Study on Morphological Changes and Navigation Problems of Pussur River in Bangladesh” was carried out by Motiur Rahman in Department of Civil Engineering, Khulna University of Engineering &. Mongla Port is located at the confluence of the Pussur River and the Mongla Nulla, approximately 71 nautical miles upstream from the Fairway Buoy (approaching the Pussur River) of the Bay of Bengal.
INTRODUCTION
- General
- Objectives
- Scope of Work
- Organization of Thesis
The power plant is located about 13 km upstream from Mongla Port on the left bank of the Pussur River. The overall aim of the study is to investigate the temporal and spatial change of the morphological behavior of the Pussur River and propose some technical interventions for sustainable navigation in the Pussur Channel.
LITERATURE REVIEW
General
Previous Studies on Hydrodynamic & Morphological Modelling
IWM (2013a): “Environmental Impact Assessment (ELA) of the proposed dredging project in the outer bar area of Pussur Channel”. The drainage system on both sides of the river has deteriorated due to massive siltation in the river.
Summary
Capital dredging was determined taking into account a 3-hour tidal window, and the design of the approach channel was carried out in accordance with PIANC guidelines. Current and wave conditions are favorable for vessel operation, but the approach channel is in an accretion zone and requires regular maintenance dredging.
APPROACH AND METHODOLOGY
General
Study Area
The average annual rainfall of the study area (Mongla) is approximately 158.9 mm, with approximately 88% of the average annual rainfall occurring during the period from May to October, and 36% during June - July (Table 3.2). The predominant wind directions in the study area are from the south during March to October with a minimum magnitude of 0.94 m/s in October and maximum of 1.91 m/s in May is shown in Table 3.3.
MongIa
- Data Collection
- Assessment of Effect of Dredging and Siltation Rate
- Proposal for Interventions
- Overall Methodology
MPA bathymetric maps surveyed for different years from Chalna to Fairway buoy have been collected to analyze the past changes in Pussur river morphology. The water level at a location within the survey range has been collected by installing pressure cell as well as by installing staff gauges for cross-checking. The time delay of the reflected sound is related to the depth of the river.
The river water sample (1 liter) was collected in each vertical at 0.2, 0.6 and 0.8 of the total depth using a point-integrated pump bottle system. Based on the availability of hydrographic maps, erosion deposition scenarios at different annual intervals were assessed. The longitudinal profile of the selected reach and the cross-section in both the dredging area and outside the dredging area are prepared to assess the effect of dredging.
PUSSUR RIVER AND ITS NAVIGABILITY
General
River Systems in the Southwest Region of Bangladesh
The Gorai-Nabaganga is the other drainage route for the Gorai water, which now mainly reaches the bay via the Passur and Sibsa rivers. In the downstream of Bardia, Kumar river (which is a branch of Anal kha river) is connected with Modhumoti river.
Legend
- River System near Mongla Port
- Topographical Features of Pussur River
- Hydrological Characteristics of Pussur River .1 Water Level
- Sediment Transport in the Pussur River
- Navigation Route and Its Navigability
- Traffic Movement
- Summary
Jhapjhapia river rises from the upstream of Pussur river and meets the Sibsa river at its upstream point. The ebb flow from Mongla Nullah flows directly into the west bank of the Pussur River and causes minor erosion (IWM, 2004). Between Mongla harbor and the sea, the Pussur river channel is generally straight, with slight meanders.
DHI (1993) has collected a large amount of data on this river from which the controlling physical processes and the nature of sediment transport processes in the Pussur River can be understood. Silt is generally absent in the channel along the main course of the Pussur River, indicating that suspended silt contributes significantly to erosion/deposition processes along the river. The main finding about the navigability of the river Pussur is that the river has been experiencing navigability problems for the last two decades especially from Chalna to Chilla Bazar.
GEOMORPHOLOGY OF PUSSUR RIVER
General
Chalna to Digraj (Chart-lO)
The transverse profile of water depth for different years is prepared in selected sections and presented in Figures 5.5 to 5.9. After one year of this deepening, i.e. in 2015, the depth at the front of the jetty decreases to almost -7.5 m CD, and in the middle of the fairway it increases by about 0.25 m. The Pauline Wreck is located at the cross section of the meander, where the navigable channel changes its course from a clockwise bend on the east bank to a counter-clockwise bend on the west bank.
Within the fairway along the wrecked pavlina, the bed is silted to approx. 0.25 m on the west side and the shed approx. 0.75 m on the east side within one year of dredging (figure 5.9). It has been observed that the channel until 2013 is of a degrading type, with the rate of scour varying from 0.12 to 0.32 m per year. From figure 5.4 it can be seen that the channel is on the concave side of the river.
Mongla Nulla to Base Creek (Inner Bar) [Chart-81
The cross section of the Danger Khal area shows that there is a deep pocket of 18-22 m depth at the Danger Khal confluence (Figure 5.10). The profile of the Sultan Khal area (Figure 5.12) shows that the river bed of the channel in the year 2005 was more than -5.0 m CD, which means that for this section with a ship's draft at 7.00 does not need to be dredged for hours. The riverbed of the channel at the food silo area shows a mild silt pattern (Figure 5.13).
The channel is observed to be of a mildly degrading type until 2011, with the scour rate varying from 0.03 to 0.1 inches per year. A typical cross section near Joymonirgol is shown in Figure 5.14 and Harbaria Khal is shown in Figure 5.15. The long profile, as shown in Figure 4.9, indicates that the channel of this reach has sufficient draft for movement and anchoring of ships with a draft of up to 10.5 meters.
Tinkona Dwip to Kagaboga Khal IChart-31
Hiron Point to Tinkona Dwip [Chart-21
Fairway Buoy to Hiron Point IChart-il
The coastal topography seems to be connected with the presence of a very deep trough in the middle of the seabed, running in a NE direction, the so-called Swatch of noground which enters its northern end in the immediate vicinity of the Pussur entrance. The northern end of the Swatch of no ground is directly opposite the Pussur outlet. The proximity of this deep trough causes the seaward slopes of the central part of the bar to be extremely steep.
The main physical process controlling the morphology of the outer bar is the decrease in tidal velocity seaward from Hiron Point, where the tidal prism is no longer confined to a distinct channel. Towards the mouth of the Pussur-Sibsa inlet, where the banks widen, there is a considerable difference between the flood and ebb channels. The approach channel near Hiron Point was found to be more markedly bifurcated after the 1987 and 1988 floods, also after the 1998 flood, this may be related to the massive amount of sediment carried by the flow from upstream and deposited in the wider part of the entrance to the Pussur River .
Summary
INTERVENTIONS FOR NAVIGATION ENHENCEMENT
- Introduction
- Potential Causes of Navigation Problem in Pussur River
- MPA Interventions to Enhance Navigability .1 Maintenance Dredging
- Back-filling Rate and Effectiveness of Dredging
- Proposals for Enhancement of Navigability
- Chapter 7
For example, one of the wrecks in the outer bar area (wreck ocean wave), as shown in Figure 5.24, causes silt formation downstream. In the meantime, it was seen that silting has started in the port area (about 13 km downstream of the port jetty). The most recent investment project entitled "Harbour Area Dredging" was completed in the year 2014.
TSHD will be deployed during the dredging work and transport dredged material to facilitate the dumping of the materials at the designated location. For this purpose, the capital dredging works in the year 2014 in various segments of the Pussur navigation route are taken into account. Before dredging of the west side channel, the depth in this area was reduced to -4.5 in CD.
CONCLUSION AND RECOMMENDATION
General
Then about 16 km river reach called outer bar at the entrance of Pussur river does not have enough draft to 7.5 in draft. The ship's berth area has scoured about 0.5 in instead of the predicted silting of 0.22 rn/yr and the channel along the wreck pavlina has scoured about 0.25 in the west side and scoured about 0.75 in the east side. The navigation channel in this range has sufficient draft for movement and anchoring up to 10.5 in draft.
On the western side of the broken ocean wave, the channel has been fairly stable for the past ten years. MPA is now planning to dredge the outer bar up to -8.50 in CD and if that project can be implemented then vessels having more than 10.0 in draft will be able to enter the berthing area of Mongla Port. So far, ships having 7-7.5 in draft are easily reaching the harbor berth without any maintenance cleaning in the channel.
Conclusion
Out of 37 years of record, about 3.0 Mm3 dredging was carried out in the jetty to maintain the jetty pocket along the jetty. The dredging operations at the jetty front area were primarily necessary to clean up very shallow spots right along the berths. In order to improve the landing pocket depth in front of the jetty, in addition to maintenance dredging, piling could be proposed as a solution.
The rapid siltation in front of the jetty is caused by the sliding of the sediment from the underlying jetty. For the canalization effect near the jetty area, extension dikes/groins/guide wells opposite the jetty may also be necessary as a structural intervention. The deflected flow as a result of the structural intervention on the eastern bank can cause bank erosion upstream of the jetty area; can propose a bank revetment to prevent this.
Recommendations for Further Study
If the flow velocity can be increased in vulnerable parts like MPA Jetty area and danger khal area, then the bed can be scoured naturally. For the channelization of the Mooring Buoy area, structural measures can be proposed to close the eastern channel along the inner bar. For a more effective result, a bench lining may be required at the west bank, opposite the inner beam.
