Landslide and flooding risk analysis - Cox's Bazar Site Planning and Site Management Sector

The flood layer prepared by IOM and UNHCR, combined a mix method:

1. High quality drone imagery from December 2017, was analysed to verify non-usable[1] locations and to determine the extent of their boundaries. A buffer of approximately 2-3 m was added to the outlines of these areas to accommodate the differing conditions during the rainy season. These outlines were then validated onsite and where needed, adjusted to reflect a more realistic flooding scenario, taking inputs from local residents as needed. The onsite verification revealed that in most instances, the assumptions were either accurate or slightly underestimated thus requiring only minor expansion of the boundaries in some cases, but no reductions were made.

2. The main river flood levels, with an enormous and complicated catchment area, was calculated empirically, based upon field measurements taken of high water levels as indicated from people who have lived in the area for over 20 years. The river was surveyed, both in section and longitudinally, and the volume of water was back calculated based on the river arrangement where the high water level was known. For small tributaries, with well-defined and small catchment areas, rainfall intensity data (from International Journal of Scientific & Engineering Research, Volume 6, Issue 5, May 2015) for Chittagong, with a 10 year return period was used to calculate the volume of water. The height of the flood waters for the smaller tributaries was based on the channel sections, the slope and volume of water calculated from the rainfall intensity.

Landslide Layer prepared ADPC, UNHCR and IOM:

The impact area is calculated based on the fact that slopes of more than 35 degrees have a risk of failure. The slope was calculated based on the DEM dataset at 0.5 meter spatial resolution, gathered by IOM drone imagery. The DEM was adjusted match a geographical point of reference, and trees and buildings removed. The additional area of susceptibility of landslide was extended by manually drawing polygons by UNHCR and ADPC, with the support of DEM topography and contour lines. These polygons are extensions of 40 degrees and above until reaching the base of the respective slope. Spatial analysis was carried out in order to provide the statistical results of the population at risk.

Risk Management Criteria and assumptions made:

• The crucial landslide trigger factor is pore pressure • The land slide failure would be sudden • When it does fail, it will have an aspect ratio of 1 to 1 • 35 degrees slope and above, a risk of failure • 40 degree slope and above has a 50% chance of failure • 45 degree slope and above 85% chance of failure