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Hydro power, desilting chamber model studies
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VITAL ISSUES DURING PHYSICAL MODELING FOR
FURTHER OPTIMIZATION OF HYDRAULICALLY
DESIGNED DE-SILTING CHAMBERS FOR VPHEP
THDC INDIA LIMITED (A Joint venture of GOI & Government of UP)
Type of dam :Concrete gravity dam
Height of dam above deepest :65 m
Foundation level
Top of dam :EL 1270 m
Length of dam :89.55 M (NOF 28.65 m, OF 60.9 m)
Catchment Area at Dam Site :4672 km2
Design Flood :SPF 6700m3/sec (For Design)
:PMF 10840m3/sec (For Checking)
SPILLING ARRANGEMENT:
Sluices : 4 Nos.
Div. cum Spillway Tunnel :1 no., 10 m dia, Circular
Spill Tunnel :1 no., 12m dia, Circular
Ogee spillway :1no., 7 m (H) x 6 m (W)
POWER INTAKE :Right bank, 3 nos. 6 m modified Horse
shoe type
HEAD RACE TUNNEL :Length: 1 no., 13.4 km long, 8.8 m dia
Salient Features of VPHEP
General Layout of Project in Dam Area
3 No. DESILTING CHAMBERS
(390.0 M LONG)
Salient Features De-silting Chamber
De-silting Chamber
• Numbers :3
• Size :390 m (L) x 16m (W) x 20.6 m (H)
• Particle size :0.2 mm & above to be removed
• Gates :3 nos. 5.24 m x 6 m (H),
(Vertical lift fixed wheel)
• Gate chamber :6 m (W) x 9 m (H) x 155 m (L)
• Operation level :EL 1270 m
Silt Flushing Tunnel
• Size :3.6 m x 4.0 m (D shaped)
• Flushing discharge :45.8 m3/sec
• Length :680 m
• Gates :3 nos. 2.5 m x 2.85 m,
(Vertical lift slide Gate)
• Gate chamber :4.80m x 4.80m x 118 m
• Operation level :EL 1233.5 m
Salient Features De-silting Chamber
PLAN OF DE-SILTING CHAMBER
DESILTING CHAMBER - I (390000 x 16000 x 20600mm)
DESILTING CHAMBER - II (390000 x 16000 x 20600mm)
DESILTING CHAMBER - III (390000 x 16000 x 20600mm)
TO GATE OPERATION CHAMBER
Section of Sedimentation Chamber
OBJECTIVE OF STUDY
Physical model study was carried out for the validation
of hydraulic parameters of mathematically modeled
De-Silting Chambers and further to optimize the
shape and size for the removal of 0.20mm and above
particle size of the sediments up to a sediment
concentration of 5000 ppm.
Scope of Studies The studies were carried out for sediment handling
optimization of de-silting chambers in the following
frame work :
• Length of upstream transition to obtain equal velocity
distribution at the entrance chambers
• Dimensions of the de-silting chamber
• Shape, size and number of hoppers to be provided
• Shape and size of flushing ducts along with its carrying
capacity
• Silt deposition in upstream and downstream transitions
as well as in the de-silting chambers
• The proposal of conventional isolated hoppers and
single de-silting trough with perforated slab shall be
tested on the model.
Description of Model • Model studies shall be carried out on a 1:15 scale geometrical
similar model. All the three compartments of de-silting chamber
are similar, therefore only one is represented on the model
• About 50 m length of 6 m dia. intake tunnel followed by 48.7 m
long upstream transition, de-silting chamber, full length of
downstream transition, the collector troughs and sediment
flushing ducts were constructed in the model
• The ppm and gradation of the sediment was same as adopted
by D.H.I. in their mathematical modeling
• The discharge fed into the model was measured over a sharp
crested weir provided in the upstream of the model
Sediment data • Petrography
The overall similarity of the catchment of Vishnuprayag
HEP (situated at Lambagarh 16 kms upstream of
VPHEP) and VPHEP suggests that the Vishnuprayag
petrographic data may be assumed to representative for
the Pipalkoti site. The sediment at Vishnuprayag project,
as reported , are very hard, angular and injurious to
turbines. So the sediment will cause heavy erosion of
several tonnes of steel structure in the waterway of the
power channel, if not removed. The desilting chambers
downstream of the power intake are designed to remove
90% sediments of size 0.2 mm and above.
Sediment data
• The sediment gradation at the barrage intake of
Vishnuprayag HEP during monsoon at different
sediment load was collected.
• The average of the gradation was plotted with the
sediment gradation curve used for model studies of de-
silting chambers of Vishnuprayag HEP. It is observed
that both these gradation are almost similar.
• So, the sediment curve of Vishnuprayag HEP was
adopted for the present studies.
Methodology The de-silting chambers are dimensioned on a simple
principle of fall velocity of sediment particles. The fall
velocity is dependent on many factors but mainly on size
and shape of settling particles. The feeding mixture of
sediment is prepared through the following steps.
1. The fall velocity of particular size particle is known from
literature, (Hunter Rouse/Ruby’s curve).
2. The fall velocity is reduced in the model velocity as per
scale ratio given below:
Geometry : 1/scale of the model.
Discharge : 1/(scale)5/2
Time : 1/(scale)1/2
Velocity : 1/(scale)1/2
Sediment : Reduced in terms of PPM depending
upon the amount of the water quality flowing in the model
Methodology (cont.) 3. The particle size corresponding to the reduced velocity is
known from the same curve.
4. The procedure is repeated to all graded particles provided by
the site and according to scaled particle size, the feeding
sediment mixture is prepared such that its projection to proto
matches with the gradation curve of the site.
On running the model, when flow conditions were
established the prepared mixture was fed into the model for a
definite period of time at a constant rate to maintain the ppm
as desired. During the run of the model the water samples
from silt feeding point, flushing duct and downstream of the de-
silting chamber were collected and analyzed for the sediment
concentration passing through the flushing conduit and power
channel. The silt trapping efficiency of the de-silting chamber
in respect to overall sediment concentration was computed
using the standard formulae.
View of de-silting chamber
OUTER VIEW OF THE MODEL OF DE SILTING
CHAMBER
Inside view of the Desilting
Chamber
Perforated Slab Inside the Desilting Chamber
Desilting Chamber Outlet
Model Investigations Proposal 1:
• The proposal consists of 81 orifices provided only in the
centre of the flushing duct. The size and distances of the
orifices were as follows: Orifice no. Size in (m.) Orifice no. Size in (m.)
1 .38 x .52 53 to 66 .165 Ф
2 to 10 .210 Ф 67 to 79 .150 Ф
11 to 21 .195 Ф 80 .20 x.20
22 to 52 .180 Ф 81 .30 x .20
Orifice no. Distance (m)
1 0.30 m from the start
2 to 15 @ 3.0 m c/c
16 to 32 @ 4.0 m c/c
33 to 50 @ 5.0 m c/c
51 to 78 @ 6.0 m c/c
79 to 81 @ 7.0 m c/c
Model Investigations
• The model was run at a flow of 91.55 cumec (total design
discharge for all the three chambers is 274.65 cumec) out of
which 15.26 cumec passed through the flushing duct and 76.29
cumec passed through the power channel. The performance of
the proposal was tested for different sediment concentration
varying from 2000ppm to 5000ppm.
• During each test run, the sediment with required concentration
was fed in the flow for 1 hour model with constant rate. After
about 30 minutes from the start of the sediment feeding, water
samples were collected from the out flows of the HRT and
flushing conduit.
Model Investigations
It was observed that upto 3500 ppm, desilting chamber had
deposits throughout the length. The silt was deposited in a
length of 15 m in upstream transition. The thickness of deposits
varied from 0.40 m to 1.20 m at different locations on the both
sides of transition.
About 40% of the orifices were closed and silt was
deposited over the perforated slab between the orifices. The
analysis of water samples collected during the test run
indicated that silt trapping efficiency of the system for +0.2 mm
size in proto is of the order of 92% and silt flushing efficiency of
the system is of the order of 63% of the trapped sediment.
Model Investigations
When the system was tested for 5000ppm sediment
concentration, it was observed that discharge in the
flushing ducts decreased slowly from 15.26 cumec to
9.20 cumec in one hour (corresponding to 3.87 hours
proto).
The silt deposition of the order of 2.0 m thickness
was observed at different locations on both sides of
upstream transition. 75% of the orifices were found to be
choked and silt was deposited over the perforated slab
between the orifices.
Model Investigations
Proposal-2:
• After inspection of the model, additional orifices (making
the total number of orifices 149 ) were provided in sub
flushing ducts (on both sides of the central flushing duct)
of the de-silting chamber to facilitate flushing of the
trapped sediment.
Model Investigations
The model was run at sediment concentration of 3000 ppm,
with similar operating conditions as in the proposal-1.
After running the model for one hour, it was observed that
the silt deposition in the chamber and on perforated slab was
almost of the same order as observed with earlier proposal. In
upstream transition, the silt deposition of the order of 1.0 m
thickness was observed.
Silt was deposited over the perforated slab in between the
orifices in the full length of the chamber. A few of the first
hoppers were found open after the run of the model. In all about
50% orifices were found open in the chamber.
Model Investigations
When the system was tested for 5000 ppm under the
same conditions, it was observed that the silt deposition in the
upstream transition was of the order of 1.50m thickness.
No orifice was open in the first 1/3 length of the chamber
while orifices in last 1/3 length were mostly open. However
there was no silt deposition on the downstream transition.
The analysis of water samples collected during the test
run indicated that silt trapping efficiency of the system for +
0.20 mm size particles in proto was about 92% and silt
flushing efficiency of the system was about 63% of the
trapped sediment.
Deposition in upstream transition
Deposition at upstream transition and
perforated slab of Desilting Chamber
Deposition at upstream transition and
perforated slab of Desilting Chamber
Deposition at upstream transition and
perforated slab of Desilting Chamber
(Choking of orifices)
Results
1. Since the overall silt trapping efficiency of the system was
found of the order of 92% with one hour test run, the size
of de-silting chamber is in order to settle the sediment.
2. The silt flushing efficiency of the system was however
very poor. About 50% orifices were found choked and silt
was deposited over the perforated slab in the de-silting
chamber.
3. The proposal needs to be modified with respect to number
and size of orifices and size of the flushing duct.
In order to increase the flushing duct efficiency of the duct
a proposal on mathematical modeling was framed. Optimized
proposal was tested at the existing physical model to ensure its
hydraulic performance.
The proposal consists of only one flushing duct of size
0.70m x 0.71m at the start to 1.80m x 2.12m at the end. The
section of the flushing duct assures more than 3.0m/s flushing
velocity at every point of the duct.
The total length of the upstream transition is 48.70 m, out
of which 42.384m length of upstream transition was constructed
at mild slope and the rest 6.316m with a steep slope, so as to
prevent the settling of sediment at the end of upstream
transition.
Modified Proposal with original section
(20m wide) of de-silting chamber
MODEL INVESTIGATIONS • The performance of the proposal was tested for
different sediment concentration varying from 2000 ppm
to 5000 ppm.
• During each test run, the sediment with required
concentration was fed in the flow for 1 hour (model) with
constant rate. The discharge passing through the
flushing duct was continuously observed to assure that it
did not decrease.
• After about 30 minutes from the start of sediment
feeding, water samples were collected from the out flows
of the HRT and flushing conduit. At the end of the test
run, the chamber was inspected, by opening the panel.
MODEL INVESTIGATIONS • It was observed that upto 2000 ppm, the thickness of silt
deposited in upstream transition varied from 0.075m to
0.10m at different locations on both sides of transition
• About 10 % of the orifices were closed and the thickness
of silt deposited over the perforated slab between the
orifices was of the order of 0.15 m in the upstream reach
and .04m in the downstream reach in terms of proto type
dimensions.
• The analysis of water samples collected during the test
run indicated that silt trapping efficiency of the system for
+0.2 mm size in proto is of the order of 93% (Table-1)
and silt flushing efficiency of the system is of the order of
90 % of the trapped sediment.
Table-1
OBSERVED SILT REMOVAL EFFICIENCY OF DE-SILTING
BASIN OF VPHEP
In Power
Channel
IN
Flushing
Duct
In Power
Channel
In
Flushing
Duct
Intake
Discharge
(cumec)
Pow er
Discharge
(cumec)
Flushing
Discharge
(cumec)
Res.
Level
1I.R.I.
Proposal 1000 315 3815 73.81 30 1064 93.00 87.15 90.50 75.24 15.26 FRL
2 -do- 2000 635 7580 73.60 60 2100 92.10 86.83 90.50 75.24 15.26 FRL
3 -do- 3000 1000 11000 72.29 90 3036 92.00 85.53 90.50 75.24 15.26 FRL
4 -do- 4000 1375 14560 71.42 125 4000 91.40 85.94 90.50 75.24 15.26 FRL
5 -do- 5000 1760 17140 70.74 160 4680 91.00 81.72 90.50 75.24 15.26 FRL
Silt
Flushing
Efficiency
in %
Operating Conditions
Sl. No.Details Of
proposal
Silt
Concentrat
ion fed at
Intake
(ppm)
Observed Silt
Concentration (ppm)
Overall Silt
Trapping
Efficiency
in %
Concentration of
+0.2mm size particle
Silt
Trapping
Efficiency
in % for
+0.2mm
MODEL INVESTIGATIONS The system was tested for higher sediment
concentration upto 5000 ppm also.
At 5000 ppm, the maximum deposition of silt between
the orifices over the slab is of the order of 0.50m only. 62
of the total 77 orifices remain open after the model run.
There is practically no deposition over the down stream
transition
It was observed that over all efficiency of the system for
trapping of the silt load is more than 70% and the
efficiency for +0.2 mm size particle is more than 91%.
The silt flushing efficiency of the system is 82.50%.
Post Run Conditions in the Basin (20.0 m wide) after 2000 ppm
Deposition at upstream transition of Desilting Chamber (20.0m wide)
after 2000 ppm
Post Run Conditions in the Basin (20.0 m wide) after 5000 ppm
Deposition at downstream portion of desilting chamber (20.0 m wide)
after 5000 ppm
Modified Proposal with revised section
• The proposal was tested with revised section (width reduced to
16m from 20m) of the de-silting chamber.
• This proposal was also tested under the same test conditions.
The test results are compiled in Table-2.
• It may be seen that over all efficiency of the system for
trapping of the silt load is more than 70% and the efficiency for
+0.2 mm size particle is around 92%. The silt flushing efficiency
of the system is more or less the same as with earlier proposal.
• No remarkable deposition of silt is noticed at any sediment load
upto 5000 ppm. The model tests indicate that silt deposition of
the order of 0.30m to 0.45m is observed on the cover slab in all
the experiments. Most of the orifices remain open after the
model run. Practically, no deposition over the down stream
transition is observed.
Table-2
OBSERVED SILT REMOVAL EFFICIENCY OF
DESILTING BASIN OF VPHEP
In Power
Channel
IN
Flushing
Duct
In Power
Channel
In
Flushing
Duct
Intake
Discharge
(cumec)
Pow er
Discharge
(cumec)
Flushing
Discharge
(cumec)
Res.
Level
1I.R.I.
Proposal 1000 325 3767 72.98 27 1076 92.50 87.04 90.50 75.24 15.26 FRL
2 -do- 2000 670 7450 72.15 57 2058 92.00 87.06 90.50 75.24 15.26 FRL
3 -do- 3000 1050 10840 70.90 86 2990 91.60 85.93 90.50 75.24 15.26 FRL
4 -do- 4000 1400 13992 70.90 134 3836 91.00 83.19 90.50 75.24 15.26 FRL
5 -do- 5000 1800 16997 70.07 172 4700 90.80 81.80 90.50 75.24 15.26 FRL
Silt
Flushing
Efficiency
in %
Operating Conditions
Sl. No.Details Of
proposal
Silt
Concentrat
ion fed at
Intake
(ppm)
Observed Silt
Concentration (ppm)
Overall Silt
Trapping
Efficiency
in %
Concentration of
+0.2mm size particle
Silt
Trapping
Efficiency
in % for
+0.2mm
Layout plan of Intake and de-silting chamber
CHAMBER-I
CHAMBER-II
CHAMBER-III
Section of de-silting chamber
Post Run Conditions in the Basin (16.0 m wide) After 2000 ppm
Deposition at upstream transition of Desilting Chamber (16.0m wide)
after 2000 ppm
Post Run Conditions in the Basin (16.0 m wide) After 5000 ppm
Deposition at upstream transition of Desilting Chamber (16.0m wide)
after 5000 ppm
Conclusion The aspects which resulted in the optimized
performance of de-silting chamber of VPHEP are as
follows:-
Increase in the slope of the last 6.316m length of the
upstream transition resulted in the rolling over action of
the sediments which considerably reduced the sediment
deposition in the upstream transition.
Change in the section of the flushing duct assured more
than 3.0m/s flushing velocity at every point of the duct,
hence the required flushing efficiency was attained.
Change in the size, shape and distribution of orifices
resulted in the desired passing of sediments to the
flushing duct thereby increasing the silt trapping
efficiency of the de-silting chamber.
Conclusion The over all silt trapping efficiency of the proposal is
more than 70%, while silt trapping efficiency of +0.2mm
size particle is above 91% even at the sediment
concentration of 5000 ppm. Silt flushing efficiency of the
system is 90 % at sediment load of 2000 ppm and
82.50% at 5000 ppm.
Recommended