Factors Influencing River Bank Stability in the
Tigris and
Shatt Al-Arab Water Ways, Iraq
A.A. Albadran
Environment-Agency.UK
(E-mail:
[email protected])
Abstract
Excessive bank
erosion is occurring at several places along the main
waterways of Iraq, the Shatt al Arab and Tigris, and
their tributaries. The stability of the banks of these
rivers has been investigated, employing the techniques
of soil mechanics and the principles of hydraulics. The
bank recession was monitored using the erosion pin
method and the direct measurement of fallen soil blocks.
In the Mis'hab waterway, a major tributary of Shatt al
Arab river, the bank recession rate was around
0.4m/year. Block failure of soils is the most common
erosion mechanism, whereas thin falls due to alternating
drying and wetting are less common in this area. The
riverbank materials which are generally highly cohesive
clayey soils, showed plastic failure during triaxial
compression tests except for one distinct layer in the
soil profile. The materials of the Tigris riverbank are
silty clay and silty clay with sand. Some showed brittle
failure under triaxial compression tests. The erosion
rate was 0.67m/year and the mean volume of soil removed
per meter of bank was 1.2m3/year. A variety
of mechanisms of bank failure have been identified. The
analysis of flow velocity measurements carried out in
situ from a bout using a propeller type current meter,
indicate that the position of the maximum velocity core
and generation of secondary flow circulations lead to
the exertion of high boundary shear stresses on the base
of the outer banks causing basal scouring of the
meandering Tigris. The flow spirals in the straight
reaches of the Mes'hab river are mainly due to bank
irregularities of flow diversion due to fallen soil
blocks at the bank foot. The river traffic induced waves
are believed to be more highly destructive of riverbanks
than the tidal current action in the restricted
waterways of the Shatt al Arab. The characteristics of
shipping generated waves in a restricted waterway (
Mis'hab) were investigated using the currently most
commonly operated boats. The long boat of shallow draft
( Shachtor) generates the lowest waves at the riverbanks
where it is sailed in the central zone of the waterway
and at a limited speed of not more than 15km/hr.
However, the broad old transport boat of deep draft (Mator)
generates highly erosive waves even at low speed of
operation. If possible the latter should not be used in
the restricted waterways. Slope stability analysis
conducted for different bank profiles at three sites on
the two rivers using commonly applied techniques,
indicates that the stability of low banks is strongly
dependent on the geotechnical properties of the bank
materials such as effective cohesion. High banks show
less response to the variation of cohesion. The geometry
of the bank profile is the principle factor controlling
the stability. The stability of high profile banks is
generally lower than for those of low profile. Factors
such as tension cracks and pore water pressure serve to
reduce stability. The present study is a preliminary
investigation and further work will be required before
adoption of the findings to form the basis of standard
bank protection recommendations throughout the waterways
concerned.
Unpublished Ph.D. Thesis,
University of Dundee, UK, 1987
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