پديد آورندگان :
همتي، محمد نويسنده دانشگاه شهيد چمران اهواز Hemmati, M. , شفاعي بجستان، محمود نويسنده دانشگاه شهيد چمران اهواز Shafai-Bajestan2, M
كليدواژه :
تلاقي رودخانه ها , نسبت دبي , حداكثر عمق آبشستگي , River confluence , غير همكف , Discharge ratio , Maximum scour depth , حداكثر ارتفاع رسوب گذاري , Unequal bed level , Height of
چكيده لاتين :
Introduction
Due to the three dimensional flow structure at river
channel confluences, especially at the downstream flow
mixing zone a deep scour hole is formed. At the
separation zone downstream of the junction comer a
point bar is also developed. The seour hole and the
point bar can cause changes in river morphology and
can be the main reason for bank instability or failure of
the structures near the confluence area.
In nver confluences SIX different regions are
distinguished, as shown in Fig (I). Locations and
dimensions of regions 1 and 3 and flow characteristics
in regions 2 and 4 depend on many factors. As region 3
becomes larger due to sedimentation, the flow velocity
in region 4 will be increased and the scour dimensions
become larger. This will cause bank failure and create a
meander at this location.
The flow vortex developed in this region can also cause
many difficulties for navigation.
According to these phenomena, the study of flow
pattern and scour hole in river confluences, has
attracted the attention of many researchers. Most of
these studies have however been focused on river
confluence of equal bed level. Although an unequal
river bed level at the river junctions are common in
nature, the scour and sedimentation studies at these
junctions have not received the attention of researchers
due to the complexity of the experimental setup. It has been assumed that the bed level has no significant
effect on the scour and the sedimentation patterns.
However it has been reported by a few investigators
that the flow patterns and the zone of maximum shear
stress in junctions of unequal bed level is different from
that of equal bed level cases, (Best, 1987; Ghobadian
and Shafai Bejestan, 2007). Objective
The main objective of this study is to investigate the
dimensions of the separation zone at river confluences
of unequal bed levels and to develop relations for
prediction of these dimensions compared to the case of
equal bed level Melhodology
An extensive experimental study was conducted in the
hydraulic laboratory of the Shahid Chamrao Uoiversity.
By applying the dimensional analysis theory, a
dimensionless relation is developed as shown in Eq.(I):
~=f(&,-.!...-)
d,o QI B]
where cD is a symbol for maximum scour depth (H,)
and height of sediment deposition downstream of scour
hole (Hb) , d,o is the mediao sediment size, Q, aod Q,
are respectively the discharges in the main and lateral
channel, z is the difference of the bed levels at the river
confluence, and 8] is the river bed level downstream of
the confluence. Results and Discussion
Effect of Ihe discharge ratio:
The variation of (H,/d,o) and (Hi/d,,) was plotted
against the discharge ratio Q, for the experimental data.
The results indieated that both these dimensionless
parameters are directly proportional to the diseharge
ratio. Effect of Ihe river bed level:
Variation of (Hjdso) and (Hi/d,o) against the ratio of
ZIB, showed that the bed level can generally decrease
both the scour depth and the height ofsediment.