Department of Civil Engineering

This paper presents observed failure modes for a geobag structure from a series of physical model tests. Six hundred bags were employed to observe the failure in terms of friction force, water depth, flow rate and bag aging. Three different failures are reported. An analysis of the effect of bag aging shows that 52% of saturated bags gain weight in the range 0 to 5% and about 70% of initially dry bags gain weight in the range 10 to 20%. The outcome indicates the importance of bag aging in the 'incipient failure'. These observations will eventually be used to validate an existing numerical model.

Sand-filled geotextile bags (geobags) are commonly used for riverbank protection, however, despite their deployment in a significant number of locations, the failure modes of such structures are not well understood. This paper describes a series of 1:10 physical model tests designed to improve our understanding of the basic failure modes of geobag revetments used as river bank protection. In particular, the aim of the experimental programme is to quantify the relationship between hydraulic shear stress on the revetment surface and geobag displacement. The aspect of the work reported is the use of two analytical techniques, the Conveyance Estimation System (CES) and the membrane analogy method developed by Chow to predict hydraulic shear stress on the geobag surface. It is concluded that both methods produce acceptable shear stress predictions for the initially washed away bags, although the CES is recommended for future use because of its better prediction and ease of application.

Floods are triggered by water overflow into drylands from several sources, including rivers, lakes, oceans, or heavy rainfall. Near real-time (NRT) flood mapping plays an important role in taking strategic measures to reduce flood damage after a flood event. There are many satellite imagery based remote sensing techniques that are widely used to generate flood maps. Synthetic aperture radar (SAR) images have proven to be more effective in flood mapping due to its high spatial resolution and cloud penetration capacity. This case study is focused on the super cyclone, commonly known as Amphan, stemming from the west Bengal-Bangladesh coast across the Sundarbans on 20 May 2020, with a wind speed between 155 -165  gusting up to 185 . The flooding extent is determined by analyzing the pre and post-event synthetic aperture radar images, using the change detection and thresholding (CDAT) method. The results showed an inundated landmass of 2146 on 22 May 2020, excluding Sundarban. However, ...

Rapid urbanization poses challenges to meet the increased water supply demands. Apart from the quantity, the distributed water quality often fails to meet the permissible level. This study aimed to conduct a citywide spatio-temporal variation of water quality parameters. Water sampling points were selected by applying the Analytical Hierarchical Process (AHP) technique using ArcGIS considering pipe leakage, source water quality, pipe age, and pipe materials. The Chattogram city comprises 397 km of pipes; pipe material distribution shows 71.28% PVC, 20.94% asbestos, 5.16% mild steel, 2.17% ductile, and 0.45% cast iron. The citywide pipe network was established in 1963; 20.87% of pipes aged over 30 years, 15.07% 20–30 years, 26.38% 10–20 years, and 37.68% pipelines are relatively new, i.e., within 0–10 years. Eight water quality parameters, i.e., pH, temperature, turbidity, biochemical oxygen demand after five days (BOD5), total coliform, fecal coliform, chloride, and residual chlorin...

In recent years, sand filled geotextile bags (geobags) have been used as a means of long term riverbank protection. However, despite their deployment in a significant number of locations the failure modes of such structures are not well understood. The aim of this research is to influence future design guidelines for geobag riverbank protection work. To achieve this it is necessary to enhance the fundamental knowledge of the performance of geobags in a revetment subject to flowing water. To achieve this quasi-physical model studies have been carried out considering two cases: a fixed bed and mobile sediment bed. To further help understand the forces acting on geobags the quasi–physical model observations have been used to validate numerical model simulations using a commercial 3D Discrete Element Model (DEM) software EDEM ® coupled with a 3D water velocity field. From the quasi–physical modeling it was known that there are types of geobag revetment failure common to both fixed bed a...

For more than two decades, sand filled geotextile bags (geobags) have been used as a means of longterm riverbank protection. However, despite their deployment in a significant number of locations, design guidelines for such structures are not well established. To date, there is no guidance at all for river bank applications and only one guideline, from Australia, concerning coastal geobag revetments. To influence future design guidelines for geobag riverbank protection work, fundamental knowledge is required on the performance of discrete geobags in a revetment during revetment construction, as well as post construction. This paper outlines the current state of the art in terms of guidelines on geobag launching/placement and geobag revetment performance evaluation. This paper attempts to quantify both pre and post geobag revetment performance using 3D Discrete Element Model (DEM) software. It is envisaged that, using routine field monitoring data, the validated DEM model could provi...

Identifying urban flood-prone areas with their relative hazard scale is the fundamental work of urban flood management. This may introduce using a probabilistic methodology by flood hazard zoning of potential urban flood hazard area. Based on statistical analysis of US EPA Storm Water Management Model (SWMM) with a threshold value of maximum depth of each node of link network number of overflow events in a specified time period can be obtained. Further using Kernel hazard density the spatial analysis in ArcGIS can be used to obtain a GIS compatible maps for the hazard zoning of the potentially flood prone areas. In this study Chittagong city, the second largest city of Bangladesh has been taken as a case study and the wettest year 2014 was selected for model simulation. The validated model outcome reasonably identified the flood prone vulnerable zones which is comparable to the outcome of recent field studies. Thus, it is expected that the acquired flood hazard mapping will play maj...

This paper presents observed failure modes for a geobag structure from a series of physical model tests. Six hundred bags were employed to observe the failure in terms of friction force, water depth, flow rate and bag aging. Three different failures are reported. An analysis of the effect of bag aging shows that 52% of saturated bags gain weight in the range 0 to 5% and about 70% of initially dry bags gain weight in the range 10 to 20%. The outcome indicates the importance of bag aging in the ‘incipient failure’. These observations will eventually be used to validate an existing numerical model. Key-Words: Geobag, failure modes, bag aging, failure mechanisms, Jamuna River.

Sand filled geotextile bag (“geobag”) revetments are commonly used for riverbank protection schemes in Bangladesh and other countries around the world. The method is effective however the failure modes are not well understood. To address this, the hydrodynamic forces associated with bag failure need to be better understood. The research reported here addresses this through: (a) Physical modelling using 1:10 scale model geobag revetment, and (b) Conveyance Estimation System (CES) modelling, to estimate hydraulic forces on a geobag revetment. In the physical modelling, the velocities associated with the failure of geobag revetments have been measured. The comparison between the experimental data and the CES data indicates that the CES model can predict model velocities with reasonably accuracy. This validated CES model can then be used to estimate shear stress. In the next stage not reported here, the CES predicted velocities will be used to prepare the mapped velocity field for the D...

This paper presents observed failure modes for a geobag structure from a series of physical model tests. Six hundred bags were employed to observe the failure in terms of friction force, water depth, flow rate and bag aging. Three different failures are reported. An analysis of the effect of bag aging shows that 52% of saturated bags gain weight in the range 0 to 5% and about 70% of initially dry bags gain weight in the range 10 to 20%. The outcome indicates the importance of bag aging in the ‘incipient failure’. These observations will eventually be used to validate an existing numerical model. Key-Words: Geobag, failure modes, bag aging, failure mechanisms, Jamuna River.

Low flow or Environmental Flow (EF) assessment is vital to ensure the river and ecosystem remain healthy. Both natural and human interventions might alter a river. Therefore, this study presents EF requirements of the famous Hilsa breeding center in the Padma River, Bangladesh, by applying the Hydrologic Engineering Centers River Analysis System (HEC-RAS) for discharge and water surface levels simulations at different stations. The frequency analysis of 20 years of historical data, spanning 2000-2019, used the Log-Pearson Type III (LP-III) distribution method, while the one-dimensional unsteady flow simulation was performed for the last 10 years (i.e., 2012-2019). Subsequently, the HEC-RAS simulated water level values reasonably correlated with the field observations at four stations, namely Baruria Transit, Mawa, Tarpasha, Sureswar, with Coefficient of determination R 2 =0.86, 0.83, 0.92, and 0.74, alongside simulated minimum water surface levels of 1.57 m, 0.37 m, 0.30 m, and 0.27 m, respectively. Also, the Baruria Transit and Mawa had simulated flows that reasonably correlated with the field observations at R 2 =0.70 and 0.61, with a simulated minimum flow of 3849.51 m 3 /s and 3789.14 m 3 /s, respectively. The minimum flow according to the frequency analysis was 4017 m 3 /s, 3685 m 3 /s, 3449 m 3 /s, 3229 m 3 /s, and 3113 m 3 /s at Baruria Transit and 3304 m 3 /s, 2781 m 3 /s, 2438 m 3 /s, 2141 m 3 /s, 1992 m 3 /s at Mawa station in 5, 10, 20, 50 and 100 years return periods, respectively. This study overlooked to report the ongoing investigations into the water quality issues. Thus, this study is expected to guide the required EF quantity towards a healthy Hilsha fish habitat and surface water source for drinking purposes in this studied river. The stated method is also applicable to other similar rivers around the world.

Assessing fish habitat suitability in a data-scarce tidal river is often challenging due to the absence of continuous water quantity and quality records. This study is comprised of an intensive field study on a 42 km reach which recorded bathymetry and physical water quality parameters (pH, electroconductivity, dissolved oxygen, and total dissolved solids) testing and corresponding water levels and velocity. Frequent water sampling was carried out on 17 out of 90 locations for laboratory water quality tests. Based on this, an interpolation technique, i.e., Inverse Distance Weighted (IDW), generates a map in a Geographic Information System (GIS) environment using ArcGIS software to determine the river water quality parameters. Additionally, a hydrodynamic model study was conducted to simulate hydraulic parameters using Delft3D software followed by a water quality distribution. During validation, the Delft3D-simulated water quality could reasonably mimic most field data, and GIS featu...

Bangladesh is extremely vulnerable to seismic activity. While the earthquake resistant building design and construction code has been developed in sophisticated national/city level, the implementation at the local level has been more of an exception than the rule. People used to make their houses by local materials and local construction technique without any engineering philosophy especially in rural areas. Therefore, there is an imminent need to know the actual physical condition of the rural houses and to estimate the risk associated with them, to bring into force simple, user friendly, non-engineered construction practices for use by the community and the construction artisans. A physical survey has been done on the existing condition of the rural housing. From this information the problems are identified with respect to earthquake resistant non-engineered housing technique. Techniques of strengthening of existing rural houses to make them earthquake resistant are elaborated and...

Seismic structural damage assessment is a part of post earthquake disaster management program. During the Rangamati Earthquake of Chittagong Hill Tracts in July 27, 2003, the only Public Library located at Chittagong was severely affected and undergone various degrees of damages. This paper deals with the physical damages as well as structural strength assessment of the damaged Public Library building. A detailed overview of several seismic damage evaluation documents framed by FEMA 310 has been elaborated and incorporated for seismic structural strength assessment process. Finite element frame analysis is used incorporating Bangladesh National Building Code (BNBC) seismic provisions. The difficulties existing in the structural system planning of the damaged Chittagong Public Library building and design discrepancies are documented and probable suggestive measure are highlighted in this paper.

This study examines the effects of geocell geometry on the settlement behaviour of sandy soils reinforced with HDPE (High-Density Polyethylene) geocells. Rectangular, diamond, and hexagonal geocell shapes were simulated in Abaqus and applied to three soil density types: medium dense, loose, and very loose soils. To maintain consistency in the analysis three type of geocell were designed with height ratio(H/B) as 1, depth ratio (u/B) as 0.1 and pocket ratio as 0.8. The factors for improvement based on settlement measurements at 10% and 15% (s/B%) indicated that hexagonal geocells are the strongest in improving the stability of the soil, closely followed by diamond-shaped geocells. This indicates that hexagonal and diamond arrangements of cellular structures, under uniaxial and biaxial compressive loads, facilitated a prompt transfer and distribution of loads within the structure as a whole and restricted settlement more effectively than the rectangular shape. Higher number of load distribution points on hexagonal and diamond geocells were attributed to their observed superiority by allowing for more confinement, more effective stabilization of soil than being larger in size and spreading the applied stresses evenly. On the other hand, rectangular geocells with fewer intersection points have a relatively low confinement effect which reduces the aiding of load bearing capacity. Again, the results also show that medium dense soils exhibit the highest improvement factor for optimal balance between soil particle interlocking and geocell reinforcement. In looser soils, however, a steeper increase in the improvement factor was noted, reflecting a greater relative benefit from geocell stabilization as it counters the inherent instability of low-density soil structures. This study underscores the potential of HDPE geocells, particularly in hexagonal and diamond configurations, for enhancing soil reinforcement. Such advancements hold promise for sustainable infrastructure development, offering a reliable approach to improving ground stability in various geotechnical applications.