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.