This document summarizes an experimental study on the effect of the gap beneath an impact wall on the performance of stilling basins. Models of stilling basins were tested with an impact wall located at 4 diameters downstream of the pipe outlet and with the gap below the wall varied from 0.5 to 1.5 diameters. Performance was evaluated based on a performance index calculated from flow properties and scour depth and location. The model with a 1 diameter gap performed best with higher performance index values, indicating better energy dissipation and reduced downstream scouring. The study concluded that an appropriate gap size improves the efficiency of the stilling basin design.

1. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308
(Print), ISSN 0976 – 6316(Online) Volume 4, Issue 4, July-August (2013), © IAEME
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ANALYSIS OF BAFFLE WALL GAP IN THE DESIGN OF STILLING BASIN
MODEL
H. L. Tiwari
M A N I T Bhopal, India
ABSTRACT
This paper investigates the gap of baffle wall in stilling basin design for the enhancement of
energy dissipation in order to protect the downstream structures from excessive scouring.
Experiments have been carried out by keeping the baffle wall at same location for Froude numbers
3.85, 2.85 and 1.85 to evolve the new stilling basin model by changing the gap beneath the wall
above the basin. Performance of outlet basins having baffle wall of at same location with varying the
gap were evaluated based on dimensionless number called as performance index (PI), which is the
ratio of grains Froude number to scour index. Higher values of PI indicate better performance of the
basin. It is concluded that by placing the appropriate baffle wall size at suitable location with proper
gap, the efficiency of basin increases.
Key words: Baffle wall, Performance, Scouring, Stilling - basin,
Introduction
Baffle wall also called as impact wall plays an important role in the design of stilling basin
for dissipating the excessive energy downstream of hydraulic structure like over flow spillway,
sluices, pipe outlets etc. The effect of impact wall on the flow or scour characteristics depends upon
the geometry of impact wall, its location and the gap underneath the wall with basin floor. Various
types of recommended stilling basins for pipe outlets using impact wall are by Bradley and Peterka
[1], Vollmer and Khader [2], Garde et al. [3], Verma and Goel [4 &5], Goel [6], Tiwari et al. [7],
and Tiwari [8 &9], etc. The present research paper describes about the effect of the gap beneath the
wall over basin floor for a stilling basin performance. The effects of design of stilling basin affects
on scour characteristics downstream of the basin.
The jet of water strikes against the baffle wall, which equalizes the distribution of flow of
water over the full channel width. The flow gets turned toward the upper portion of the baffle wall
and comes down on the floor of stilling basin after striking the hood portion of impact wall. The
dimensions and location of an impact wall is quite important for improving the dissipation of energy.
The size of hood should be such that there is no possibility of splashing of water. The gap at the
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bottom of the baffle wall may give rise to additional horizontal shear as small portion of water would
pass through this gap dissipating more energy thereby performance of the stilling basin improved.
The opening should be such that no material is entrapped inside the openings. In present work,
performance of stilling basin models were evaluated with varying gap from 0.5d to 1.5d of impact
wall over stilling basin . All models are shown in Figs.1 and 2
MATERIALS AND METHODS
Experimental Set-up
The experiments were conducted in a recirculating laboratory flume of 0.95 m wide 1 m deep
and 25 m long. A rectangular pipe having 10.8 cm x 6.3 cm as cross sectional area was used to
represent the outlet flow. This pipe was connected with delivery pipe of diameter of 10.26 cm in
centrifugal pump. The exit of pipe was kept above stilling basin by one equivalent diameter (1d). A
wooden floor was provided downstream of the outlet for fixing the appurtenances in the basin. To
observe the scour after the end sill of stilling basin an erodible bed was made of coarse sand.
First of all, the erodible material was filled up to the height of end sill and gets leveled then
normal depth was maintained over the sand bed by allowing the water from the overhead tank inside
the flume by operating the tail gate. Later on a centrifugal pump of capacity 20 HP was switched on
while keeping the control valve closed which was fitted into the feeding pipe. The flow into the
flume was increased gradually so as to achieve required Froude number with a minimum possible
disturbance to the erodible sand bed. The discharge was measured by a calibrated venturimeter
installed in the feeding pipe. With the operation of tail gate, the desired steady flow condition with
normal depth of flow was maintained, which was computed by Manning’s formula corresponding to
the inflow Froude number Fr = V/(gd)0.5
,where V is the average velocity in the pipe, g is the
acceleration due to gravity and d is the equivalent diameter of the pipe. As soon as the the required
amount of water flowing from pipe outlet, reached the erodible bed material the movement of bed
materials started and the geometry of scour hole started changing with time. After one hour test run,
the motor was switched off. The value of maximum depth of scour (dm) and its location from the end
sill (ds) were noted. All the models were tested for constant run time of one hour and with the same
erodible material for all Froude numbers.
The time for each test run was kept as one hour. The value of maximum depth of scour (dm)
and its location from the end sill (ds) were noted as shown in Table 4. Impact wall of size 1.5d × 3d
was tested in the stilling basin model (MSM-42) for different flow conditions for all three Froude
numbers at 4d location along with the end sill. Further the gap beneath the wall with basin floor was
varied to 0.5d, 0.75d and 1.5d for same impact wall. Arrangements of models are shown in Table1
and Figs.1&2. Scour parameters along with PI values for all models are given in Table 2.
Table 1 Testing of Models to Study the Effect of Gap of wall above Basin Floor
S.
No.
Model
Name
Impact Wall with hood End Sill
Gap beneath the wall
above basin floor
Location
from
outlet exit
Shape Height Width
Location
from
outlet exit
1 MSM-42 1d 4d Sloping 1d 1d 7d
2 MSM-43 0.75d 4d Sloping 1d 1d 7d
3 MSM-44 0.5d 4d Sloping 1d 1d 7d
4 MSM-45 1.5d 4d Sloping 1d 1d 7d

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Performance Evaluation of Models
A stilling basin model that produces smaller depth of scour at a longer distance is considered
to have a better performance as compared to another stilling basin which results in a larger depth of
scour at a shorter distance when tested under similar flow condition [5]. The performance of a
stilling basin models were evaluated for different Froude number (Fr) which is a function of channel
velocity (v), the maximum depth of scour (dm) and its location from end sill (ds). A non dimensional
number, called as performance index (PI) has been used for evaluating the performance of stilling
basin model [7 &8]. This is given as below:
ܲ‫ܫ‬ ൌ
୚୶ୢ౩
ଶୢౣට୥
ρ౩షρ౭
ρ౭
ௗఱబ
(1)
Where, V - the mean velocity of channel, ds - distance of maximum depth of scour from end sill, dm-
depth of maximum scour, g – gravitation acceleration, ρs- density of sand, ρw density of water, d50-
the particle size such that 50% of the sand particle is finer than this size, A higher value of
performance index indicates a better performance of the stilling basin model. This performance index
takes into consideration of flow, scour parameters (dm & ds) and properties of material used
downstream of the stilling basin.
Model MSM-42 with gap 1d
Model MSM-43 with gap 0.5d
Figure 1 Stilling Basin model with impact wall having gap 1d and 0.75d

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Model MSM-44 with gap 0.5d
Model MSM-45 with gap 1.5d
Figure 2 Stilling Basin model with impact wall having gap 0.5d and 1.5d
Table 2 Performance Index for Models Tested at varying Gap (dm and ds are in cm)
S.
No.
Model
name
Fr = 1.85 F r = 2.85 F r = 3.85
dm ds PI dm ds PI dm ds PI
1 MSM-42 4.2 25.2 4.88 4.2 35.4 7.82 5.2 40.2 7.86
2 MSM-43 2.8 16 4.65 4.8 32.6 6.30 7.8 41.5 5.41
3 MSM-44 2.9 15.7 4.40 4.7 28 5.52 8.1 35.5 4.45
4 MSM-45 5.1 17.9 2.85 7.4 31.5 3.95 7.6 35.8 4.79

5. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976
(Print), ISSN 0976 – 6316(Online) Volume 4, Issue 4, July
RESULTS AND DISCUSSIONS
In order to study the effect of gap between basin floor and impact wall, the impact wall of
size 1.5d × 3d was tested at different gap with the basin floor along with triangular end sill
secured at 7d. Models MSM-42, MSM
0.75d, 0.5d and 1.5d respectively, having the impact wall
sill (1V:1H) at 7d. By keeping the impact wall at 4d mod
of stilling basin model (MSM-42) was
which came as 4.88, 7.82 and 7.86 for Froude numbers 1.85,2.85 and 3.85 respectively.
model keeping the gap as 1d, the impact wall gap was changed to 0.75d and in similar flow condition
model was tested and renamed as MSM
5.41 for Fr = 1.85, 2.85 and 3.85 respectively.
conditions by varying the gap as 0.5d and
and performance indices were computed as shown in Table 2. On analyzing the performances of all
models it reveals that the model (MSM
tested models. Table 6 and Fig. 3 also
as its performance indices (4.88, 7.82 & 7.86
compared to other models, namely MSM
1d, values of performance indices increases, but when gap reached to 1.5d, these values decreases. It
shows that gap 1d is appropriate, which is also established by Bradl
Table 3 Variation of PI with
Gap
Fr MSM-44
0.5d
1.85 4.40
2.85 5.52
3.85 4.45
Fig. 3 Variation of Performance Index with Ratio of Gap to Equivalent
ivil Engineering and Technology (IJCIET), ISSN 0976
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of gap between basin floor and impact wall, the impact wall of
3d was tested at different gap with the basin floor along with triangular end sill
, MSM-43, MSM-44 and MSM-45 were tested at the gap of. 1d,
and 1.5d respectively, having the impact wall located at 4d along with the triangular end
By keeping the impact wall at 4d model was tested with gap 1d and performance
42) was evaluated by computing the values of performance index PI),
which came as 4.88, 7.82 and 7.86 for Froude numbers 1.85,2.85 and 3.85 respectively.
gap as 1d, the impact wall gap was changed to 0.75d and in similar flow condition
model was tested and renamed as MSM-43, and its performance was evaluated as PI =
5.41 for Fr = 1.85, 2.85 and 3.85 respectively. Similarly other models were tested at similar flow
0.5d and 1.5d and renamed as MSM-44 and MSM
and performance indices were computed as shown in Table 2. On analyzing the performances of all
models it reveals that the model (MSM-42) having gap as 1d performs better as compared to other
also explain that model MSM-42, having gap 1d performs better
s its performance indices (4.88, 7.82 & 7.86) for all three Froude numbers are higher side as
compared to other models, namely MSM-44, MSM-43 and MSM-45. As gap increases from 0.5
increases, but when gap reached to 1.5d, these values decreases. It
which is also established by Bradley & Paterka [1].
Variation of PI with Baffle Wall Gap above Basin Floor
Gap beneath Baffle wall with Basin floor
MSM-43 MSM-42 MSM-45
0.75d 1d 1.5d
4.65 4.88 2.85
6.30 7.82 3.95
5.41 7.86 4.79
Variation of Performance Index with Ratio of Gap to Equivalent dia.
ivil Engineering and Technology (IJCIET), ISSN 0976 – 6308
August (2013), © IAEME
of gap between basin floor and impact wall, the impact wall of
3d was tested at different gap with the basin floor along with triangular end sill (1V:1H)
45 were tested at the gap of. 1d,
at 4d along with the triangular end
ted with gap 1d and performance
evaluated by computing the values of performance index PI),
which came as 4.88, 7.82 and 7.86 for Froude numbers 1.85,2.85 and 3.85 respectively. testing the
gap as 1d, the impact wall gap was changed to 0.75d and in similar flow condition
PI = 4.65, 6.30 and
tested at similar flow
44 and MSM-45 respectively
and performance indices were computed as shown in Table 2. On analyzing the performances of all
42) having gap as 1d performs better as compared to other
42, having gap 1d performs better
) for all three Froude numbers are higher side as
45. As gap increases from 0.5d to
increases, but when gap reached to 1.5d, these values decreases. It
[1].
45
. of Pipe.

6. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308
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CONCLUSIONS
An experimental study was conducted in the laboratory to study the role of gap of impact
wall over basin in the design of stilling basin for varying gap (0.5d, 0.75d, 1d and 1.5d) for
rectangular shaped pipe outlet with 12 test runs for Froude numbers 3.85, 2.85 and 1.85. Based on
the experimental results, it is concluded that gap beneath the wall from basin floor affect the
performance of stilling basin significantly due to change in the flow patterns. During the study it
was found that the gap of impact wall affects the flow conditions and hence scours pattern
downstream of the stilling basin.
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