Padma Multipurpose Bridge Project

off-take for a distance of 12 km. Topographic cross sections lines were surveyed in 2009 over theadjacent floodplains. Figure 3-18 shows a three dimensional rendering of the Padma channel basedon the 2008-2009 bathymetry. The Padma River has a deep meandering thalweg that shifts from theleft (south) bank upstream from the Arial Khan off-take, to the opposite right (north) bank downstreamfrom it. Although the channel width at bankfull level is about 5 km wide, it is constricted in most partsby large bars, especially at the bridge section. Contraction scour caused by these bars during lowflows, plus meandering flow impinging against the inerodible north bank, might help explain the deepchannel located near the north bank at the proposed bridge crossing. The lowest bed elevationsreached values of El. -28.7 m PWD.Figure 3-18: Three-dimensional rendering of the Padma River and adjacent south floodplain near theproposed bridge location39. Bathymetry of the Padma along the bridge site is shown in Figure 3-19 for the years 2008and 2009. The bathymetry of River is changing due to the erosion and accretion process of the River.Two deep channels are noticed near the left Bank and a shallow channel is located along the rightbank in 2009. The depth of deep channel near the left bank increased from -12m to -24m. A plot ofbed elevation difference between 2008 and 2009 near Mawa (Figure 3-19) shows that substantialmorphological changes can occur over a single flood season. The 2008 thalweg was partially filled in2009 and migrated north in response to erosion. At the same time, large shallow bars present in 2008were significantly eroded.3-22