Padma Multipurpose Bridge Project

2.3.2.3 General Alignment of RTW19. Six cross section types are used and embody the following principal components: Upper slope wave protection mainly above low water level, at a slope angle of 1V:3.5 H,starting from the inner edge of a berm at El. -1 m PWD (which forms part of the waveprotection). In general the wave protection extends up to the floodplain level only, but it israised to 3 m above 100-year flood level for certain lengths of revetment, for the guide bund,and for hard points, to provide freeboard during floods and to allow for wave runup. Concreteblocks underlain by geotextile filter material are used to provide the wave protection. Lower (underwater) slope erosion protection placed on a dredged 1V:6H slope with the toe ofthe slope set at various elevations depending on the location and cross section type. Rockriprap placed on a geotextile/bamboo fascine mattress is used to provide the erosionprotection.A falling apron placed along the toe of the underwater slope protection, to provide temporaryprotection from future undermining by launching down the scoured slope. Given the practicaldepth limitation for dredging, all river bank protection depends on falling aprons for scourprotection. Rock riprap is used for the falling apron.20. Table 2-4 summarizes key details for Types 1 through 6 cross sections. The design scour levelis generally El. -47 m PWD, except that a level of El. -55 m PWD is used where flow can impingedirectly against the bank, such as at the head of a guide bund or hard point (Type 1). Where aparticularly robust cross section type is required, the falling apron is dredged to El. -25 m PWD (Types1, 2 and 3), which places the apron as close to the design scour level as is practically possible givendredging limitations. Less deep, but more economical, dredge depths (El. -15 m PWD) are usedwhere a higher level of risk is acceptable, at locations away from the main bridge and end-facilities.21. On the south bank, a raised embankment is required near the bridge to return overbank flowback to the main channel at some distance from the bridge (Types 1 and 2), and at the hard points toprevent overtopping flow (Types 1 and 4). Type 5 is for the north bank around the bridge abutment.Type 6 consists of a geobag revetment placed along the existing north bank upstream of the bridge(Figure 2-6); the existing underwater bank slope is covered with four layers of geobags.Location (Northbank NB, Southbank SB)Design crest level[m.PWD]Design scour level[m.PWD]Dredge[m.PWD]levelUnder waterdredging [m 3 /m]Table 2-4: Summary of bank type details (except Type 7)Type 1 Type 2 Type 3 Type 4 Type 5 Type 6SB SB SB SB SB SB+10.7 +10.7 +5.5 +10.7 +5.5 +5.5-55.0 -47.0 -47.0 -47.0 -47.0 -47.0-25.0 -25.0 -25.0 -15.0 -15.0 adaptive4094 3812 3812 1649 1649 adaptiveEarth-work [m 3 /m] 250.0 250.0 110.0 250.0 110.0 110.0Wave protectionand embankment[m 2 /m]Under water slopeprotection [m 2 /m]Falling apron baselength [m 2 /m]Source: RTW Scheme design, 200986.5 86.5 38.7 86.5 38.7 38.7150.9 150.9 150.9 90.1 90.1 adaptive45.0 33.0 33.0 48.0 48.0 adaptive2-11