Iraj Gholami
Engineer,
Water Works Standard Div., Ministry of Power, Tehran, Iran
Alireza Daemi
Manager,
Water Research Centre, Tehran, Iran
and
Ali A. Salehi
Neyshabouri
Assistant
Professor, Dept. of Civil Engineering, Tarbiat Modarres Univ., P.O.Box
14155-4838, Tehran, Iran, tel: +98-21-8005040, E-mail: salehi@modares.ac.ir
Abstract:
In design of diversion dams; it is usually necessary to include a sluiceway to
remove the deposited sediment in front of the inlet structures. One of the main
points regarding the removal of deposited sediment upstream of weir and mainly
in the vicinity of the inlet structures is the extent of the eroded area. It has
been observed that sluiceway operation result in the eroded bed to extend the
upstream of the weir and have the shape of a circular arc in plan. Since the
cleaning of the sediment in front of the intake structures are more important
than the upstream side of the weir, it is usual to use dividing wall in front of
the inlet structures to concentrate the flow strength close to this area. The
extent of this dividing wall is one of the design parameters affecting the
efficiency of the sluiceway. In this paper the experimental investigation on the
effects of the dividing wall on the sediment removal by a sluiceway is reported.
It is shown that introducing the dividing wall causes the efficiency of
sluiceway to clean the area in the vicinity of intake to be improved. However,
it is not correct to increase the length of dividing wall more than a certain
limit, because in such a case, even the volume of the eroded material is
increasing, but it does not reach equilibrium stage quickly. From 4 different
cases considered, the length of dividing wall equal to about 27% of the channel
width could be selected as the optimum length. It is also concluded that
increasing sluice opening results in increase of sediment removal.
Keywords:
sluiceway, dividing wall, sluice, weir, sediment
Experiences
on the operation of diversion dams have shown the importance of the measures to
exclude the deposited sediment upstream of the weir. Some problems associated
with the excess sediment deposition include the closure of intakes and
decreasing the discharge coefficient of the spillway. Also sediment discharge in
channel network causes many problems in the channels and irrigated areas.
Sluiceways are used mainly to discharge sediment deposited in front of inlet
structures and also to help the fast discharge of water in emergency cases. The
discharge capacities of sluiceways are mainly dependent on sediment inflow of
the river. Based on the previous investigations, following guidelines are
available for designing different parameters of sluiceways [1]:
Discharge
capacity of sluiceways should be about 2 times of diversion discharge.
Sluiceways
could be able to pass 10% to 15% of maximum flood.
One of the main points regarding the removal of deposited sediment
upstream of weir and mainly in the vicinity of the inlet structures is the
extent of the eroded area. It has been observed that sluiceway operation result
in the eroded bed to extend towards the upstream side of the weir and have the
shape of a circular arc in plan [2]. Since the cleaning of the sediment in front
of the intake structures are more important than the upstream side of the weir,
it is usual to use dividing wall in front of the inlet structures to concentrate
the flow strength close to this area. The extent of this dividing wall is one of
the design parameters affecting the efficiency of the sluiceways. In this paper
the experimental investigation on the effects of the dividing wall length on the
sediment removal by a sluiceway is reported.
Experimental setup consists of a 12 m. flume that has 75 cm width and 80
cm depth (Fig. 1). The channel bed is horizontal and a diversion weir is built 1
m upstream of the channel end. Sluiceway of 20 cm. width is constructed on the
right hand side of weir. The sidemen materiel with d50=1 mm and
standard deviation of 1.18 (rather uniform) was covered the upstream part of the
weir with a thickness of 15 cm. Four different cases of the dividing-wall length
namely, L = 0, 10, 20 and 30 cm was considered. Also for each case three
different sluice openings are considered while the flow velocity at the sluice
gate opening was kept constant at 4 m/s by using three different discharges.
Each experiment was run until the equilibrium condition was reached (except for
the case of L = 30 cm), after which no sediment removal is observed. After each
experiment the bed profile and the amount of removed sediment was measured.
Based on the
experimental results it was observed that the sediment particles are removed
from the bed by flow and transported through the gate opening. The sediment
concentration in the flow at sluiceway is initially increases reaching a maximum
after which it starts to decrease to reach the equilibrium stage at which there
is no sediment movement. The extent of the erosion is dependent on the flow
strength and the length of dividing wall. In the case of no dividing wall,
usually a quarter of a circle is shaped in plan at the equilibrium stage.
However by installing the dividing wall in front of the intake structures, the
geometry of eroded bed is affected and it is focused mainly close to the intake.
For example the eroded bed for 3 different sluice opening are shown in Figures
2-4. In all of these cases the flow velocity under the sluice gate is kept
constant at 4 m/s and in each Figure 4 different topography in equilibrium stage
(except for L = 30 cm) corresponding with 4 dividing-wall length are presented.
As can be seen, in all of these Figures, introducing the dividing wall causes
the efficiency of sluiceway to clean the area in the vicinity of intake to be
improved. However, for the case of L=30 cm it was observed that the equilibrium
stage occurs at a very long time and the eroded area extends upstream far from
the intake structure as can be seen from Figures 2-4. From Fig. 5 it is possible
to compare the volume of eroded material at the equilibrium time for the first
three cases (L=0,10 and 20 cm) at different sluice openings. Since the case of
L=30 cm is not 989888appropriate, between the other cases, L=20 cm could be
selected as the optimum length for the dividing wall. It can also be concluded
from Fig. 5 that increasing the sluice opening results in increase of sediment
removal. It is noticeable that the slope of the curve between gate opening of
1.5 to 2.3 cm is less than the one for 2.3 to 3.5 cm interval for the cases of L
= 0 and 10 cm. These two slopes are rather equal for L=20 cm and their behavior
is changing for the case of L=30 cm where the slope of the second part (2.3 to
3.5 cm) is less than the first part (1.5 to 2.3 cm).
From the
experimental results on the effects of the dividing-wall length on the sluiceway
efficiency it can be concluded that:
Introducing
the dividing wall causes the efficiency of sluiceway to clean the area in the
vicinity of the intake to be improved. However, it is not correct to increase
the length of dividing wall more than a certain limit, because in such a case
even the volume of the eroded material is increasing, but it does not reach
equilibrium stage quickly. From 4 different cases considered, the length of
dividing wall equal to about 27% of the channel width could be selected as the
optimum length. It can also be concluded that increasing the sluice opening
results in increase of sediment removal.
References
[1]
Ernest Razvan, 1989, River Intakes and Diversion Dams.
[2]
Iraj Gholami Alam, 1998, Study on effective parameters on design of sluiceway
for diversion dams, Msc. Thesis, Dept. of Civil Engineering, Tarbiat Modarres
University, Tehran, Iran.
Fig.
1 Experimental setup
Fig.
2 Effects of dividing-wall length on the sediment removal
(Gate
opening = 1.5 cm)
Fig.
3 Effects of dividing-wall length on the sediment removal
(Gate
opening = 2.3 cm)
Fig.
4 Effects of dividing-wall length on the sediment removal
(Gate
opening = 3.5 cm)
Fig. 5 Sediment removal for different dividing-wall lengths and sluice openings