Study on Morphological Change and Navigation Problems of Pussur River in Bangladesh

Abstract

Pussur River is situated in the south western hydrological zone of Bangladesh. Mongla Port has established at the confluence of Pussur River and Mongla Nulla, approximately 71 nautical miles upstream from the Fairway buoy (approaches to the Pussur River) of the Bay of Bengal. Mongla Port, the second gateway of Bangladesh was designed for an average 8.5m draft ship. But after the construction of Jetties at Mongla Port, the depths in several areas of Pussur Channel reduced significantly and regular maintenance dredging is required to provide adequate depth alongside the berths, in the approaches to the berths and in the Southern Anchorage areas. The objective of the study is to investigate the temporal and spatial change of morphological behavior of Pussur River and to suggest some engineering interventions for sustainable navigation in the Pussur channel. In this study, bathymetric charts of Pussur River surveyed for different years (2005 to 2016) from Chalna to Fairway buoy have been collected from Mongla Port Authority (MPA) and the siltation/scouring rate and pattern of morphological change has identified through the analysis. Some field studies are performed to collect some baseline primary data such as tidal variation of water level, suspended sediment concentration and bed material characteristics. In the navigation route of Pussur river, the areal distance (straight) of Hiron Point to Chalna is found as 55 km where the channel length is 91 km. The ratio of straight length to channel length is found as 1:1.65, which implies that the channel is mildly meandering. Based on MPA Hydrographic charts, it is observed that the width of the Pussur River varies at different sections between 700 m to 3000 m and approach to Pussur is about 6000 m. The minimum depth is found to be varied from 1.4 m to 11.7 m below CD. It can be noted that the lowest depth is found at upstream (Chalna) that gradually increases towards downstream with the highest minimum depth at Mazhar Point to D’Suza Point and again decreases at further downstream near Fairway Buoy. The maximum depth is found to be varied from 7.5m to 29.6 m below CD from upstream to downstream, respectively. The suspended sediment concentration in river water is measured for a sampling time of 13 hrs. At 0.8d depth, the concentration is found to be varied from 360 mg/l to 2096 mg/l in flood tide which changes inversely with water level and velocity. At 0.2d depth, the concentration varies from 154 mg/l to 1478 mg/l. Sieve analysis of bed material shows that the Fineness Modulus (FM) of the collected 04 samples are 0.30, 0.60, 0.48 and 0.49, i.e. the bed material is mostly sandy. From the average grain size distribution of bed material, d50 is found as 0.052 mm. It is observed that the MPA jetty area and its approaches are mildly scoured where the upstream i.e Chalna to Digraj is being silted up. This upper segment is very shallow having depth about 1 to 5 m CD and have a siltation rate of 0.22 to 0.95 m/year. From 2008 to 2013, there was a natural deepening near jetty front and approaches (scour rate 0.20 m/yr) due to increasing of ship movement and increase of fresh water flow from upstream after Gorai river dredging. In 2014, jetty front of MPA was dredged upto 8.0 m CD which has silted up by 1.0 m within one year. But at upstream of jetty no.9 i.e at front of kleenheat LPG jetty, the channel scoured naturally about 1.5 to 2.0 m between 2008 to 2015. MPA vessel berth and wreck pavlina area hasn’t changed significantly after one year of dredging. The vessel berth area has found to be scoured about 0.5 m and the channel along wreck pavlina has silted about 0.25 m in west side and scoured about 0.75 m in east side in one year. The backfilling rate in mooring buoy dredging area has found v very high. Near the danger khal the dredged area has filled up 1.5 m within one year. The main reason of siltation here is creating a deep pocket where the surrounding areas are shallower. The base creek area was dredged upto -5.2 m CD in the year 2014. After one year of dredging the main navigation channel hasn’t changed significantly. But the East side (Convex side) has deepened about 0.8 m naturally. The bed at Karamjol F.O area is found to be naturally scoured more than 01 m in last 05 years. Similarly the sultan khal area scoured more than 0.5 m in last 10 years. The food silo area has a mild siltation pattern with a rate of 0.2 m/year. The river reach from Joymonirgoal to Harbaria is quite stable in last few decades. The navigation channel of this reach has sufficient draft for movement and anchoring upto 10.5 m draft vessel. This reach is the anchorage point of bulk cargo vessels. After Joymonirgol the river has found very much stable and have sufficient draft for 10.50 m draft vessel upto Hiron Point. After that, about 16 km river reach called outer bar between Hiron point to Fairway Buoy is very critical and don’t have enough draft for more than 8.5 m draft vessel. After the cyclone in 1988, the outer bar has been raised up to 0 to 2 m CD and widening in the east-west direction and the trend is continuing until the recent year. At fairway buoy channel has sufficient depth for mother vessels (20-25 m draft). From fairway buoy to outer bar, this 16 km is quite stable in last few decades. At outer bar area presently shipping channel is on the eastern side of wreck ocean wave. But this channel is being silted up and the shallow portion is extending towards east side day by day. In the downstream of buoy 14, the sedimentation rate at two points is found to be about 0.35 m/year. As a result the width of channel is also decreasing. However, in the west side of wreck ocean wave, the channel is quite stable is last ten years and in degrading type. From this study, it is found that capital dredging is required to get required draft for navigation from Sabour Beacon to Rampal Power Plant as well as for outer bar area to remove the sediment in the existing route or fix up a new route in the nearby area. The total volume dredged in Pussur River over the period 1979 to 2016 is about 12.872 Mm3, which is equivalent to an average of 3,47,000 m3 per year. Out of 37 years record, around 3.0 Mm3 dredging was done in the Jetty Front to maintain the berthing pocket alongside the jetty. But after every dredging the re-siltation rate becoming very high and it’s becoming a very big challenge for the existence of Port. Therefore, only dredging could not sustain the channel effectively; Permanent measures such as structural interventions are essential for long term improvement of navigability. To improve the berthing pocket depth in front of the jetty, in addition to maintenance dredging, sheet piling could be proposed as a solution. The rapid siltation rate in front of the jetty is due to the sliding of the sediment from the jetty underneath. The main reason of siltation is low velocity of flood and ebb flow. If the velocity of flow could be increased in vulnerable sections such as MPA Jetty area and danger khal area, then the bed could be scoured naturally. Velocity could be increased by contracting the channel using structural interventions. For the canalization effect near the jetty area, spur dykes/groins/ guide bund opposite to jetty may propose as a structural intervention. The deflected flow due to the structural intervention in east bank may cause bank erosion at the upstream of the jetty area, a bank revetment may propose to prevent this. For the canalization of the Mooring Buoy area, structural measures can be proposed to close the western channel along the inner bar. For more effective result, a bank revetment at the eastern bank, opposite to inner bar may be required. However, these structural measures require in depth study to establish the effectiveness.