research article improving carrying capacity of … ii/ijaet vol ii issue ii... · research article...

International Journal of Advanced Engineering Technology E-ISSN 0976-3945

IJAET/Vol.II/ Issue II/April-June, 2011/231-238

Research Article

IMPROVING CARRYING CAPACITY OF RIVER TAPI (SURAT,

INDIA) BY CHANNEL MODIFICATION Agnihotri P.G

a and Patel J.N.

b

Address for Correspondence

a. Associate Professor b. Professor

Civil Engineering Department, S.V. National Institute of Technology, Ichchhanath, Surat – 395 007 (INDIA).

ABSTRACT

Surat city is situated at the bank of river Tapi (India) near its delta region. The flow of water and water level in the river Tapi is

controlled by Ukai dam which is 100 kms away from Surat city. The city has faced many floods since long. The aspects of

channel modification of river Tapi using geospatial technologies are proposed in this research paper. This is helpful in the

preparation of Flood Mitigation Plan for Surat city as a curative measure for the control of flood in the river Tapi. For channel

modification, software HEC-RAS is used. The flood inundation map of Surat city is prepared in Arc GIS using software HEC-

Geo RAS.

INTRODUCTION

River Tapi is originating from a Multai Hills

(Gavilgadh hill ranges of Satpura) and flowing

through three states Maharastra, Madya Pradesh and

Gujarat having length of 725 Kms. The flow of water

and water level in the river Tapi is controlled at Ukai

dam which is 100 kms away from Surat city. The

foundation of dam is resting on Dolerite dykes

(Basalt). It is constructed for irrigation purpose

mainly and also served the purpose of flood control,

generation of hydropower and supply of industrial

and drinking water. The average rainfall in the

catchment area is about 785 mm and average yearly

run off is 17,226 MCM. The area of Surat city

situated at delta stage of the river is 326.51 sq.km.

and population is about 40 lacs. The city is having

60,000 Shops & Establishment in trading activity.

The city is also famous for diamond industry. The

Major industries like Essar Steel, Reliance, ONGC, L

& T, Gail, Kribhco, Shell, NTPC, GSPC, Torrent

Power etc. are situated in the city. The study area is

shown in Fig. No.1. Floods are occurring in river

Tapi time to time, due to which major portion of the

city is submerged creating lot of damage in

residential as well as industrial areas. There is a need

of reducing the effect of flood. In this paper the

aspects of river channel modification are considered

for enhancing the carrying capacity and reducing the

effect of flood in the city.

NECESSITY OF CHANNEL MODIFICATION

Flood occurs at Surat city frequently due to sudden

release of water from Ukai dam in river Tapi. At the

time of floods in river Tapi, Surat city and

surrounding regions are most affected. The city has

faced many floods since long back. There was a flood

in the Surat in 1959, 1968, 1998 and 2006. The

summary of the flood is given in the Table No. 1. The

Surat city and surrounding villages are part of flood

drainage of Tapi River. The carrying capacity of river

was about 6 Lacs cusecs. Since 1883 floods are

recorded in the month of August and September.

Major flood event took place in the year 1883, 1944,

1959, 1968, 1998 and 2006. The effective waterway

of river Tapi is reducing day by day with respect to

width and depth due to silting, which affect the

carrying capacity of the river. The dredging of river

in certain reaches can be carried out by conducting its

feasibility project. The computation for channel

modification of river Tapi has been carried out and

the enhanced cross section of river Tapi is suggested.

The channel modification is suggested in the reach of

river Tapi between Kathor to Magdalla. The

modification of river channel is done to increase the

carrying capacity of river Tapi and thus reducing the

effect of flood in Surat city and surrounding region.

CHANNEL MODIFICATION METHODS

Most widely used methods for channel modification

are discused below.

Levees

Levees are seldom equated with a channel

modification because, in most cases, they are

constructed well away from the channel. However, a

levee on one or both sides of a stream represents a

new and higher channel bankline for flood flows.

Levees confine the flood to a smaller cross section of

the floodplain and thus serve to channel flood flows

downstream. The benefits of levees are that they have

the least impact on the stream environment of any of

the structural flood reduction alternatives, and they

are nearly always the most effective and least

expensive method of reducing flooding to the

protected area.



Fig. 1 River Basin Map of India

Table 1: Flood History at Surat

Sr. No. Flood Event Discharge

(Lac Cusecs)

Water Level at

Hop Bridge(m) Period

1 1883 10.05 11.05 July

2 1884 8.46 10.05 September

3 1894 8.01 10.33 July

4 1942 8.60 10.56 August

5 1944 11.84 11.32 August

6 1945 10.24 11.09 August

7 1949 8.42 10.49 September

8 1959 12.94 11.55 September

9 1968 15.5 12.08 August

10 1994 5.25 10.10 Aug.-Sep.

11 1998 7.00 11.40 September

12 2006 9.09 12.40 August



High-Flow Diversion Channel and Weir

A high-flow diversion channel and weir refers to a

formal control structure that diverts some amount of

flow (can vary from a portion to most of the higher

flows) out of the river and into a separate channel,

usually moving the diverted flow along a different

flow path or to a different watershed. The diversion

of flow results in increased flood protection for land

and communities downstream of the flow split. The

diverted flow often rejoins the existing stream farther

downstream. The benefits of using this method are

that no modifications are made to the river channel,

and the diversion often takes place well upstream

from the protected area, improving visual aesthetics

(as opposed to when a levee or channel modification

is constructed to protect the area).

High-Flow Cutoff/Diversion Channel

A high-flow cutoff/diversion channel differs from a

high-flow diversion channel/weir in that a gated weir

structure is not needed at the diversion channel

entrance; the diversion channel is smaller, and the

diversion flow path is shorter-often just the distance

across the neck of a meander loop. During higher

flows, the diversion channel allows a portion of the

flow to follow a shorter, more efficient flow path.

The advantages are that the existing channel remains

unaffected and additional capacity is provided during

higher flows. As with the previous channel

modification types, the disadvantages are the land

costs required for the diversion channel, the loss of

this land for other purposes, and the need for erosion

control structures at the upstream and downstream

diversion boundary

Clearing and Snagging

Clearing and snagging involve removing vegetation

from the channel sides and along the bankline

(clearing) and removing trees, debris, and stumps

from the channel (snagging). The channel geometry

and alignment usually remain unchanged with this

solution, with the modification simply resulting in a

lower Manning's n value. Clearing and snagging is

therefore modeled in HEC-RAS by reducing the

channel n value. However, significant environmental

effects may result from this solution. Fish habitat and

cover are removed, the shade given by vegetation is

lost, and bottom sediments are resuspended by the

snagging.

Clearing and Enlarging One Side of the Channel

This technique combines clearing and snagging with

cutting, but only on one side of the existing channel.

Additional capacity is gained from the enlargement

and clearing of part of the channel. The same

environmental negatives and potential problems exist

for this solution as for the clearing and snagging and

compound channel options, but only for one side of

the channel. This solution is modeled in HEC-RAS

by adjusting both the geometry and the n value.

Widening the Upper Channel and Using the

Original Channel for Low Flow

Widening the upper channel and using the original

channel for low flow involves clearing and enlarging

both sides of the existing channel. The lowest

(deepest) portion of the existing channel is left as

undisturbed as possible to act as a low flow channel

during regular small events. Sedimentation in the cut

areas along with vegetative growth will make

maintaining the new capacity of the modification

difficult. This can be modeled in HEC-RAS similarly

to clearing and enlarging one side of the channel.

Realigning the Channel

When a channel is realigned, portions of the original

channel may be abandoned, as the modified portions

follow a new flow path for at least part of the reach.

If the total modified channel length is shorter than the

original channel length, a steeper invert slope will

occur. This will ultimately result in faster velocities,

and may significantly increase scour and deposition

problems along the realigned reach. One or more

erosion control structures are usually necessary for

this method. Realigned channels can be modeled in

HEC-RAS by locating the centerline station for the

new channel and adjusting the channel and overbank

reach lengths. The realignment may also include

increased cross-sectional area and a reduced n value.

HYDRAULIC DESIGN OF RIVER CHANNEL

The channel modification of River Tapi is done from

Kathor to Magdalla using concept of Most Efficient

sections. The channel is designed for carrying

different discharge of water using theory of practical

lined channel sections.

Design Procedure

The data given:

Discharge Q, Bed Slope of Channel S, Rugosity

Coefficient n, Maximum permissible velocity V and Side

slope of Channel m (m Horizontal to 1 Vertical)

To be evaluated:

Hydraulic Mean radius R, cross section area A,

wetted perimeter P, Bed width b and Depth of

water h

Equations used:

1. Continuity Equation Q = A*V



2. Maning’s Equation 2/13/21

SRn

V =

Design Steps

1. Area of cross section

( )θθ cot2 ++= hbhA

2. Perimeter of cross section

( )θθ cot2 ++= hbhP

3. Hydraulic mean radius of cross section

( )( )θθ

θθcot2

cot2

++++

=hbh

hbhR

Where, b = Bed width of Canal (m)

h = Depth of Water in Canal (m)

θ = Inclination of Side of Canal with

Horizontal at Top

4. P = A/R

From step 2 and 3

5. ( ) RAhbh /cot2 =++ θθ

( )θθ cot2/ +−= hRAb

From step 1 and 5

6. ( )( )θθ cot2/ ++−= hhRAA

( )θθ cot2 ++ h

7. ( ) ( )θθ cot/ 2 +−= hRAhA

( ) ( ) 0/cot2 =+−+ AhRAh θθ

02 =+− APhKh

Where K = θ + Cot(θ), P = A/R

8. In the above equation, value of A, R and P is

evaluated as follow:

A = Q/V

2/3

×=

s

vnR

P = A/R

9. The Equation shown in step-7 is Quadratic

Equation. Solving the equation and taking the

relevant root (the other root gives a negative

value); the value of h can be evaluated.

Sample Calculation

Data given

Discharge Q (Cumecs.) = 14158.42 (5,00,000

Cusecs)

Rugosity Coefficient n = 0.0225

Side Slope 1in m, m = 3

Bed Slope of Channel S = 0.00025

Velocity of Flow V (m/Sec.) = 2.35

1. Here ( ) 3cot =θ and θ = 0.321751 radian

2. K = θ + Cot(θ)

K = 0.321751 + 3 = 3.321751

3. Area of Cross section

A = 850/2.35 = 6024.86 m2

4. Hydraulic Mean Radius

2/3

000225.0

35.20225.

×=R = 6.11 m

5. Wetted Perimeter

P = A/R

P = 6155.83/5.92 = 985.20 m

6. Put the values of K,A and P in Quadratic

Equation shown in step-7,

3.321751h2 – 985.20h + 6024.86 = 0

Solving the quadratic equation,

h = 6.24 m (The other root h = 290.34 m

will give negative value of b)

7. The bed width of canal

3.321751* 6.24*26.11/ 6024.86 −=b

b = 943.70 m Say b = 944 m

In the Triangle ABC (Fig. 2)

Angle C = 90°, Angle A = θ and

Angle B = 90° - θ

8. b1 = h * Sin(θ)

b1 = 6.24 * Sin (0.321751) = 1.97 m

9. b2 = m * 6.24 * Cos (θ)

b2 = 3 * 5.413025 * Cos (0.321751)

= 17.78 m

10. y = h – h * Cos (θ)

y = 6.24 - 6.24 * Cos (0.321751)

= 0.32 m

11. The top width of canal T

T = b+2b1+2b2

T = 943.70 +(2 * 1.97) + (2 * 17.78)

T = 983.2 m

In case a larger or smaller depth is needed, some

adjustment can be made with the slope since the

given value is only the average slope of the terrain

and minor changes are always possible.

For Discharge 500000 Cusecs (14158.42 Cumecs)

Data Given

Rugosity Coefficient n = 0.0225

Side Slope 1in m = 3

Bed Slope of Channel S = 0.00025

Velocity of Flow V (m/Sec.) = 2.3

Calculated Quantities

Area of Cross Section A = 6024.86 m2

Hydraulic Mean Radius R = 6.11 m

Wetted Perimeter P = 985.2 m

Cot (θ) = 3

Value of θ in radian = 0.321751



Factor K = 3.321751

Depth of Water h1 = 290.34 m (Not considered)

Depth of Water h2 = 6.24 m

Bottom Width = 944 m

b1 = 1.97 m

b2 = 17.78 m

y = 0.32 m

Top Width = 983.2m

Design of River Channel for different discharge is

shown in Table No. 2.

MODIFICATION OF RIVER CHANNEL

The channel design/modification tools in HEC-RAS

allow the user to perform a series of trapezoidal cuts

into the existing channel geometry or to create new

channel geometry. The current version of HEC-RAS

has two tools for performing channel modifications.

These tools are available from the Tools menu of the

Geometric Data editor and are labeled Channel

Design/Modification and Channel Modification

(original). The tool labeled Channel

Design/Modification is a new tool for HEC-RAS

version 4.0. The tool labeled Channel Modification

(original) is the original channel modification tool

developed for HEC- RAS. The original channel

modification tool has been left in HEC-RAS for those

user’s who may prefer this tool to the new one. In

general, these tools are used for planning studies, but

it can also be used for hydraulic design of flood

control channels. In the present study, three different

modifications of river channel for three value of

discharge is discussed. The design parameters for

three different alternatives are mentioned in Table

No. 2. The modification of existing River section is

carried out using software HEC RAS 4.0. The sample

details of modification of river cross-section are

shown in Table 3. A typical modified river cross-

section is shown in Fig. 3. The flood inundation map

is prepared in ArcGIS 9.2 along with HES-Geo RAS

for all the three alternatives of channel modifications.

Fig. 2 Designed Practical Section of Trapezoidal

Channel

Figure 3 Existing and Modified Channel Section of River Tapi for flow of 500000 cusecs

Modified

Cross-Section

Original

Cross-Section

Distance (m)

Elevation (m)



Table 2 Channel Design for Different Alternatives

Sr.

No.

Discharge

(Cusecs)

Bottom

Width (m)

Depth of

Water (m)

Top Width

(m) b1 (m) b2 (m) y (m)

1 500000 944 6.24 983.2 1.97 17.78 0.32

2 450000 846 6.26 884.7 1.98 17.82 0.32

3 400000 746 6.28 786.2 1.99 17.88 0.32

Table 3 Channel Modification Details

FLOOD SUBMERGENCE

Flood submergence map of the city is prepared using

software HEC-RAS, HECGeo-RAS and ArcGIS 9.2.

Pre-processing of the modeling including preparation

of geometric data is done in ArcGIS 9.2. Further

editing of Geometric data and assigning of Flow data

is accomplished in software HEC-RAS. The model is

also run in HEC-RAS. The flood inundation map is

prepared in ArcGIS 9.2. The software HECGeo-RAS

is working as a link between the ArcGIS 9.2 and

HEC-RAS. It displays in the window of ArcGIS 9.2

as a special toolbar. It is used for the creation of

different themes (Layers) for modeling as part of

Pre-processing. The Post-processing of model for the

preparation of flood inundation map is also done

using HECGeo-RAS. The model comprises of three

stages

1. Pre-processing

2. Running the model

3. Post-processing

Flood inundation map for flood event of

2006 (9.1 Lac Cusecs) is shown in the Fig. 4. For the

same area flood inundation map is prepared after

modifying the section of river Tapi for different

alternatives for flow of 9.1 Lac Cusecs and shown in

Fig. 5 to Fig. 7. The reduction in flood submergence

due to proposed modification is shown in Table 4.



Table 4 Comparison of Inundated Area after Channel Modification for Flow of 9.1 Lac Cusecs

Modified

Carrying

Capacity

Total Area

of

Inundation

(% )

Area of

Inundation

< 2 m (% )

Area of

Inundation

2 - 5 m (% )

Area of

Inundation

5 - 8 m (% )

Area of

Inundation

8 - 11 m (% )

Area of

Inundation

> 11 m (% )

Existing 100 19.54 58.09 12.58 4.71 5.08

400000 Cusecs 87.51 35.27 36.12 7.64 4.65 3.82

450000 Cusecs 81.03 32.64 33.35 6.83 4.66 3.56

500000 Cusecs 78.30 31.77 32.34 6.22 4.61 3.35

Fig. No. 4 Flood Depth Map of Surat City for 9.1 Lac Cusecs Flow before modification of River Channel

Fig. No. 5 Flood Depth Map of Surat City for 9.1 Lac Cusecs Flow after modification of River Channel

(Modified Carrying Capacity 400000 Cusecs)







CONCLUSION

The study area (Surat City, INDIA) is highly affected

by the flood and it is necessary to develop flood

reduction plan for the study area which helps to

control big disaster in future. As apart of flood

reduction plan, modification in the cross section of

river Tapi is proposed in this paper. Different

alternatives of river channel modification are

considered. The hydraulic design of the river section

is carried out and reduction in area of submergence

for modified river section is calculated using

geospatial technologies. The total inundation area for

the flood of 9.1 lacs cusecs reduced to 87.51 %,

81.03 % and 78.30 % for modified carrying capacity

of river 400000 cusecs, 450000 lacs cusecs and

500000 cusecs respectively.

REFERENCES

1. AjayKumar (1995), “Flood Control and

Settlement Planning” Mohit Publication, New

Delhi.

2. Akash Acharya, Biswaroop Das, Kiran Pandya et

al (2006), “Surat 2006 Floods: A Citizens’

Report“Centre for Social Studies (CSS) Surat

and Department of Human Resource

Development (DHRD), Veer Narmad South

Gujarat University, Surat.

3. Agnihotri P.G. and Patel J.N (2008), “Preparation

of Flood Reduction Plan for Surat City and

Surrounding Region (India)” International

Journal on Transactions on Fluid

Mechanics ; Issue 2 Volume 3 ; 116-125 ;

WSEAS

4. Agnihotri P.G. and Patel J.N (2008). “ Study of

Flood at Surat City and Its Remedial Measures”

The 3rd IASME / WSEAS International

Conference on Water Resources, Hydraulics &

Hydrology , (WHH'08) University of Cambridge,

Cambridge, UK, during February 23-25, 2008

5. H.M. Raghunath (2005), “Hydrology: Principles,

Analysis, Design”, New age International

Publishers, New Delhi

6. Handerson F.M. (1966), ” Open Channel Flow”

The Mac Millan Company, New York

7. Patel J.N. “Flood management over region of

river Tapi using GPS” Journal of Geospatial

Today, Vol-I, Issue – 4, October-November,

2002.

8. Subramanya K. (1991), “Flow in Open

Channels”, Tata McGraw-Hill, New Delhi.

9. S.N. Ghosh (1986), “Flood Control and Drainage

Engineering”, Oxford and IBH Publishing Co.

Pvt. Ltd., New Delhi

research article improving carrying capacity of … ii/ijaet vol ii issue ii... · research article...

Documents