An integrated GIS and Geophysical-based approach for geohazards risk
assessment in coastal region: a Case study in Suez city, Egypt
Abstract
In coastal regions, more precaution for urban expansion should be
considered regarding hidden geological hazard from the existence of
expansive clay soil and shallow groundwater level. These conditions
cause huge damage to engineering constructions that leads to losses of
human lives and financial property. However, very little studies were
focused on hazard assessment of swelling clay potentiality using GIS
approach, because its natures as occur slowly by time. The purpose of
the present work is the analysis of geological–geotechnical factors to
present a geo-hazard risk assessment model for the identification of
high risk-prone regions in the coastal zone using an integrated GIS
statistical system and Electrical Resistivity Tomography (ERT) mapping.
Suez city (the case study area) is located in the northwest of the Gulf
of Suez that has a vital and attractive global location for trading and
investment. The geo-hazard model was created in a GIS environment by
integrating thematic maps of possible controlling factors from ALOS-DEM
image and available geological and geotechnical boreholes. These factors
showed a varied ranges: slope (0-35°), elevation (0-50 m), superficial
soil (Conglomerate to silty clay), clay thickness (0-25 m), depth to
groundwater (0-20 m), TDS (1000-35000 ppm), Cl-1 (500-17000 ppm), So4
(250-9000 ppm) and pH (6.8-8.7) and in conjunction with swelling
potentiality (Liquid Limit (LL) (35-88 %), Plastic Index (PI) (12-62
%) and free Swelling (Sf) (35-150 %)). Assessment these factors causes
kind of uncertainty during the evaluation procedure, thus all thematic
maps were scored and weighted according to specified criteria to produce
severity distributions maps. The crucial relationships between surface
and subsurface factors were identified by employing geospatial
qualitative and Pearson’ correlation analysis. In the considered case,
the clay soil is an inorganic cohesive type of high to a very high
swelling degree. Among tested factors, the existence of clay soil and
its thickness seem to play an important role to increase groundwater
level that approach surface in several sites of Suez city, hence rising
the risks of Total Dissolved Salts (TDS), Chloride (Cl-1), and
augmenting swelling potential. Accordingly, the designed suitable
relative weights were determined and all thematic maps were combined to
produce a geo-hazard map using Simple Weighting Overlay (SWO) method.
This approach illustrates the significant factors and the arithmetic
weights assigned to each level of severity, and therefore they should be
considered in similar studies. Geo-hazard map was produced with four
degrees of severity for urban development: low, marginal, severe and
very severe comprising 36.38, 40.28, 22.13 and 1.2 %, respectively, of
the study area. The very severe area was observed mainly around El
Kapanon district. An extensive field survey was performed, noted
significantly high saline groundwater move into the sewage system. Seven
2D- ERT pro