REDESIGNING OF STORMWATER DRAINS LEADING

TO KUKKARAHALLI LAKE AND SUGGESTING

MITIGATIVE MEASURES - A CASE STUDY

Nisha.S, Mamatha A.S. and Pavithra.M.Department of Environmental Engineering,Sri Jayachamarajendra College of Engineering, Mysore-570 006, Karnataka, IndiaPhone: 0821-410693E-mail: twitterin@yahoo.co.uk

Stormwater drains are designed mainly to control the quantity,

quality of runoff, resulting from storm events. Stormwater drains are also

designed for erosion control, boosting of natural ground water table,

prevention of eutrophcation and safeguarding other aquatic bodies's life,

enrichment of fresh water resources, maintenance of hygiene and aesthetic

appeal of the lake.

In order to achieve the above goals ,a detailed study of the existing

stormwater drains leading to kukkkarahalli lake was carried out and it was

found that 3 out of the 4 drains required redesigning. The catchment area

was divided into 3 subcatchements, district A, district B and district C

having an area of 37.5 ha, 75 ha and 75 ha respectively. The peak

discharge was found to be 8.43 cum,16.87cum and 16.87cum respectively.

The depth of flow was calculated and was found that all the drains A, B and

C had sufficient depth to carry the discharge, whereas drain D was short by

0.82 m. Hence redesigning is done to increase the total depth of the drain.

Next, it was observed that the actual shape of the drain was not maintained

due to erosion. It was found that drain A was compeletely distorted for about

a length of 315 m, and drain B had lost its shape at different chainages.

Hence by retaining the bedwidth and side slope the same as that existing,

redesigning with respect to earth filling and earth cutting was done as per the

redesigned data. As the result of redesigning, an additional flow of 15 cum

can be allowed into the lake along with the present inflow ,which helps to

increase the fresh water inflow to the lake.

INTRODUCTION

“The natural or man made channels that carry stormwater are referred

to as stormwater drain”[1].

Stormwater drains are designed to control the quality, quantity, timing and

distribution of runoff resulting from storm events. If these drains are

designed properly and modified suitably for the local hydrological scenario,

maximum benefits could be obtained like enrichment of fresh water

resource, improvement of ground water table, improvement of aesthetics of

town or city ,facilitating the safety of adjoining structure of stormwater

drains in the catchment area.

Keeping the above points in mind, a case study of stormwater drains

connecting to Kukkarahalli Lake in Mysore city has been undertaken.

For this purpose, the data including rainfall and topographical data

were collected . Other field data was collected by site surveying.

Objectives of the Study

1. To study the fate of existing stormwater drains leading to

Kukkarahalli lake.

2. To redesign stormwater drains for channelising the storm runoff

into the lake.

3. To estimate the probable financial requirement for rehabilitation of

the drains.

4. To suggest mitigative measures for the management of stormwater

drains.

DESCRIPTION OF THE STUDY AREA AND METHODOLOGY.

The garden city, Mysore is one of the historical places in Karnataka. It

is situated at a latitude of 120 30' N and a longitude of 760 37' 5" E. Tourism

is the major source of income to the city. Some of the major attractions

contributing to its charm are the scenic lakes like Kukkarahalli, Karanji,

Lingambudi, Dalvoi and Devanoor lake, Palaces and Chamundi Hills.

Kukkarahalli lake is situated in a peaceful and scenic atmosphere close to

the Mysore University. It was built by Dewan Poornaiah in 1867.

Unfortunately the beautiful water body has slowly deteriorated. Excessive

inflow of sewage and sludge has resulted in the over nourishment or

eutrophication of this historic and beautiful lake. The exact location along

with the catchment is shown in Figure 1. For the purpose of stormwater

drainage studies, the catchment area of Kukkarahalli lake is divided into

three subcatchments namely District A containing 1 drain, District B

containing 2 drains and District C containing 1 drain. Here drains of width

greater than 1m only are considered.

FIGURE 1 Location And Catchment of Kukkarahalli Lake.

Discharge Estimation

Rational method, is the most widely used peak runoff method in urban

hydrology. A large number of municipalities and local utilities use the

rational formula to design stormwater networks. The rational formula relates

the peak discharge to the rainfall and topographic characteristics of the

drainage area. The primary attraction of the rational formula is its simplicity.

However its drawbacks relate primarily to the fact that the intensity-

duration-frequency relationships on which it is based were originally

intended for the calculation of a peak-flow rate and not for the calculation

of either the runoff hydrographs or flow volumes [3]. The peak flow at any

given point in drainage system is estimated using following formula.

Q=CAR/360 …………… Equation 1

Where Q = Peak, runoff in m3/s

C = Runoff coefficient

A = Catchment area in hectares

R = Rainfall intensity in mm/hour

Use of Rational method is not recommended in any catchment where

ponding of stormwater might affect the peak discharge. The runoff

coefficients used for different types of drainage area were taken from [2].

Manning’s equation was used to calculate the velocity and water depths in

the drains as described in [4].

Table 1

Existing Conditions of Drain - ALocation – Gangothri Campus

Drain

Type

Total

Lengt

h (m)

Chainage Bed

Width

(m)

Total

Depth

(m)

Longitudi

nal

Slope

Side

Slop

e

Peak

Flow

(m3/s)

From

(m)

To

(m)

Trapezoi

dal

400

0 200 6.5 1.63 0.050 1:1.

5

16.87

200 400 18 It is a

flat

terrain

0.034 - 16.87

TABLE .2

EXISTING CONDITIONS OF DRAIN - B

Location – Gangothri Layout

Drain

Type

Total

Lengt

h (m)

Chainage Bed

Width

(m)

Total

Depth

(m)

Longitudi

nal

Slope

Side

Slop

e

Peak

Flow

(m3/s)

From

(m)

To

(m)

Trapezoi

dal

960

0 200 20.1 3.8 0.034 1:1.

5

16.87

200 400 8.2 3.8 0.038 1:1.

5

16.87

400 600 7.5 3.8 0.033 1:1.

5

16.87

600 800 7.5 3.8 0.033 1:1.

5

16.87

800 100

0

30 3.8 0.031 1:1.

5

16.87

TABLE 3

EXISTING CONDITIONS OF DRAIN - C

Location – Open University Campus Gangothri

Drain

Type

Total

Lengt

h (m)

Chainage Bed

Width

(m)

Total

Depth

(m)

Longitudi

nal

Slope

Side

Slop

e

Peak

Flow

(m3/s)

From

(m)

To

(m)

Trapezoi

dal

592

0 200 6.4 3.4 0.014 1:1.

5

8.43

200 400 6.4 3.4 0.0138 1:1.

5

8.43

400 600 6.4 3.4 0.0133 1:1.

5

8.43

TABLE 4

EXISTING CONDITIONS OF DRAIN – DLocation - Paduvarahalli

Drain

Type

Total

Lengt

h (m)

Chainage Bed

Width

(m)

Total

Depth

(m)

Longitudi

nal

Slope

Side

Slop

e

Peak

Flow

(m3/s)

From

(m)

To

(m)

Rectangu

lar

576

0 200 1.5 0.7 0.0065 0 16.87

200 400 2 0.7 0.0071 0 16.87

400 600 2 0.7 0.0076 0 16.87

Existing Drainage Conditions

Detailed study of drains at different chainages is shown in Tables 2.1

to 2.4. Survey carried out revealed that two of the drains are heavily silted

and weeded. This has reduced the discharge capacity of the drains

considerably. The drains were also found to be carrying sewage and sludge

flow thereby increasing the quantity of flow in the drain. The major

bottlenecks observed which obstructed the smooth functioning of the

stormwater drains were :

Silting and weeding of drains

Partial and haphazard lining of drains.

Flow of sewage and disposal of solid waste into the drains.

Silting and blockage of tertiary drains.

Encroachment of flow channels.

Lack of adequate maintenance of the channels and associated structures

like culverts.

Dumping of construction spills and other materials into the drains.

Blockage of the drains due to electric poles and stones being used to

cross the drains.

RESULTS AND REDESIGNING OF STORMWATER DRAINS

After the field analysis of present stormwater drain conditions, it was

observed that the drain C is properly designed for peak discharge but drain

A, drain B and drain D required redesigning. Table 5 shows the redesigned

details.

TABLE 5

SUMMARY OF REDESIGNED DETAILS OF DRAINS

Drain

Type

Total

Length

(m)

Peak

Discharge

(m3/s)

Bed

Width

(m)

Total

Depth

(m)

Side

Slope

Longitudina

l Slope

Drain

A

400 21 6.5 0.86 1:1.5 0.042

Drain B 960 21 20.1 0.68 1:1.5 0.034

Drain D576 21 2 1.52 1:1.5 0.007

Cost Estimation

The major factor, which is to be considered in the redesigning aspect

is to estimate the cost for rehabilitating the entire drain area. Table 6 gives

the unit cost and the total cost for the reconstruction of the drains. Unit

rates have been collected from M/s Dalal Consultancy, Mysore[5].

TABLE 6

COST ESTIMATION FOR RECONSTRUCTION OF

STORMWATER DRAINS

Sl

N

o

Item Unit Quantity Rate/Unit

(Rs)

Cost

(Rs)

Remarks

1 Desilting cum 300.66 46 13830.36 For 509 m length

2 Deweeding sq.m 3538.5 2 7077.00 For 824 m length

3 Lining cum 49.2 1800 808560 For 290 m length

4 Revetment cum 107.49 260 27947.4 For 1275 m length

5 Earth Cutting cum 14167.5 72 1020060 For 800 m length

6 Filling and

Compacting

cum 11855.35 34 403081.9 For 475 m length

Total 22,66,726.3

CONCLUSIONS AND RECOMMENDATIONS

Conclusions

The present condition of the stormwater drains leading to

Kukkarahalli lake were found to be silted, weeded, blocked

with construction debris and polluted by the entry of sewage at

few places.

The reasons for the above condition of these drains were

mismanagement and improper usage of stormwater drains.

.In order to facilitate the easy flow through the drain, redesigning

was done to improve the Lake water quality.

The total cost of remodelling for all the drains worked out to be

around Rs. 23 lakh.

After remodelling there is an additional inflow of 15 m3/sec

along with present discharge thereby increasing the fresh water

quantity in the lake.

Recommendations

Necessity of Cleaning

The system should be periodically inspected and cleaned. Better

maintenance includes removing blockages, cleaning catch basins and

repairing where necessary. Pre-monsoon cleaning up of the storm water

system is very important in order to ensure efficient functioning of the

system during rainy days.

Role of Authorities Concerned

In order to achieve better maintenance of the storm water drainage

system, the following actions have to be taken, so that the drainage system

meets the desired goals.

Prevent flow of sewage and disposal of solid wastes into drains.

Prevent silting, weeding and blockage of tertiary drains.

Prevent encroachments of flow channels and tanks.

Proper maintenance of channel and associated structures (eg.

culverts).

Prevent dumping of construction spills (debris) and other materials into

drains.

REFERENCES

1. David A. Chin, 2000, " Water Resources Engineering". Prentice

Hall.

2. Warren Viessman, Mark J. Hammer, 1992, " Water Supply and

Pollution Control", 5th Edition, Harper Collins College

Publishers.

3. Chance Constrained Model For Stormwater System Design and

Rehabilitation “Journal of Water Resources Planning and

Management”,1997,ASCE..

4. Ven Te Chow ,”OpenChannel Hydraulics”,International

Students,Edition,Mc Graw Hll Publications.

5. Master Plan of Remodelling of Stormwater Drains ,2000 - A

report, Dalal Consultancy.