Flood Routing in the Catchment of Urbanized Lakes

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ScienceDirect Aquatic Procedia 4 (2015) 1173 – 1180

INTERNATIONAL CONFERENCE ON WATER RESOURCES, COASTAL AND OCEAN ENGINEERING (ICWRCOE 2015)

Flood Routing in the Catchment of Urbanized Lakes Sumaiyah Tazyeena, Shivakumar J Nyamathia* a

Department of Civil Engimeering, UVCE, Bangalore University, Bangalore-560056, India

Abstract The number of the lakes in Bengaluru has fallen from 262 in 1960 to 81 in 1985 due to the rapid urbanization. As the catchment and command area becomes more urbanized, the impact of more impervious area, decreased potential for infiltration, and loss of natural depression storage has changed the response to runoff due to rainfall and thus the shape (peak and time base) of the resulting runoff hydrograph. For routing peak runoff through the lake, a flood hydrograph is determined. The study area is Hulimavu lake situated in the south east part of Bengaluru of Pennar River Basin, Bellandur/Varthur Series and Madivala sub Series at an average elevation of 922 m above MSL, spread out in area of about 0.566 km2. The catchment area of 11.11 km2 is located between Latitude 12º50′00′′ N and 12º52′45′′ N and Longitude 77º34′30′′ E and 77º37′00′′ E . The lake area is being encroached for urban activities, decreasing the surcharge storage capacity thereby inundating downstream and upstream areas due to backwater effect. Flood routing studies are carried out for existing storage capacity of the reservoir (0.8Mm3) and for the revised / enhanced storage capacity of the reservoir (1.023Mm3) by de-silting by an amount of 0.2 Mm3. Flood study shows peak discharge of 148.82 m3/s occurs for a return period of 50 years. Flood routing analysis shows peak flood flowing over the surplus weir is 29 m3/s and 17.5 m3/s for existing and enhanced storage respectively. This enhanced storage will overcome reduction in submerged area and the submergence level from 896.3 m to 895.95 m which is less than the Maximum Water Level (896.00 m) for the peak discharge of 148.82 m3/s. © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license © 2015 The Authors. Published by Elsevier B.V. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-reviewunder under responsibility of organizing committee of ICWRCOE 2015. Peer-review responsibility of organizing committee of ICWRCOE 2015 Keywords: Unit hydrograph; Urbanized Lake; Peak discharge ; Flood routing

* Corresponding author. Tel.: +91-994-541-6878; fax: +91-802-296-1930. E-mail address: [email protected], [email protected].

2214-241X © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of organizing committee of ICWRCOE 2015 doi:10.1016/j.aqpro.2015.02.149

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1. Introduction A catchment’s flood response to rainfall may have to be quantified for a variety of reasons. Among the most common are peak flow and flow volume estimation, flood duration, flood warning and the design of hydraulic structures. Flood estimation is inherently more difficult on smaller catchments than larger ones. Catchment characteristics, used in the estimation of flood parameters at ungauged sites, are more difficult to extract from smaller catchments; errors that escape detection will have a proportionally greater effect on the final estimate. Any flood estimation procedure in only as good as the data used in its construction. The relative deficiency of small, lowland, dry, permeable catchments in past analyses has so far meant that accepted procedures are less able to predict flood parameters accurately in such cases (Marshall & Bayliss, 1994). In general, estimation of floods can be done by empirical flood formulae, Envelope Curves, Rational Formula, Unit Hydrograph application and frequency analysis (Reddy, 2004). For catchments with insufficient rainfall or corresponding concurrent runoff data, it is necessary to develop synthetic unit hydrograph. These are unit hydrographs constructed form basin characteristics (NPTEL). The Central Water Commission (CWC, 1986) recommends the use of the Flood Estimation Reports brought out for the various sub–zones in deriving the unit hydrograph for the region. These sub–zones have been demarcated on the basis of similar hydro – meteorological conditions and a list of the basins may be found. The design flood is estimated by application of the design storm rainfall to the synthetic hydrograph developed by the methods outlined in the report. The application of modifiedpuls method of flood routing helps in evaluating the relative accuracy of flood routing methods to a natural river. The modified-puls method has at its core the postulate that storage depends only on outflow rate (Strelkoff, 1980).

1.1. Study Area The study area is Hulimavu lake (Fig 1) situated in the south east part of Bengaluru of Pennar River Basin, Bellandur / Varthur Series and Madivala sub Series at an average elevation of 922 m above MSL with Latitude 12052′5.88′′ N and Longitude 77036′4.56′′ E. It is spread out in area of about 0.566 km2 having a catchment area of 11.11 km2. The lake area is being encroached for urban activities, decreasing the surcharge storage capacity thereby inundating downstream areas. Since the catchment is urbanized, the time to peak decreases, increasing the peak discharge.

Fig 1. Location map of Study Area

In the past, this rain-fed water body was used as a storage pond primarily for agricultural purpose, fishing, drinking water source, etc., Rapid urbanization and change in the land use pattern in the surrounding vicinity has stressed the existing infrastructure facilities which has aggravated due to improper maintenance and lack of awareness. Raw sewage is getting mixed with rainwater and finding its way into the lake, polluting the groundwater. In addition, unhygienic activities are seen at several places on the foreshore of the lake and dumping of solid wastes into the lake. Change in land use pattern in the catchment area, modification in the original alignment and blockages in the inlet channels have reduced the runoff into the lake. This runoff gets diverted resulting in bringing raw sewage from un-sewered area. During peak monsoon, the water surcharges into low-lying residential areas (Lake Development Authority).

Sumaiyah Tazyeen and Shivakumar J. Nyamathi / Aquatic Procedia 4 (2015) 1173 – 1180

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2. Methodology Flowchart gives the methodology for the Derivation of Flood hydrograph for the Study area (Fig 2). Extraction of the catchment from Survey of India Topomap 57H/9 Determination of Physiographic parameters like Area, L, Lc Slope Determination of 1-hr Synthetic Unit hydrograph parameters Drawing of a Synthetic Unit Hydrograph Estimation of Design Storm Duration TD Estimation of Point rainfall and areal rainfall for storm duration TD Distribution of Areal rainfall during the design storm duration TD Estimation of rainfall excess units Estimation of Base flow Computation of design flood peak Computation of Design Flood Hydrograph

Fig. 2. Methodology for development of Flood Hydrograph (Source: CWC, 1986) Table 1. Parameters of 1- hr unit hydrograph for Hulimavu watershed. tp

qp

W50

W75

WR50

WR75

TB

Tm

Qp

(hr)

m3/s/km2

(hr)

(hr)

(hr)

(hr)

(hr)

(hr)

m3/s

1.09

1.90

1.11

0.66

0.38

0.26

5.35

1.59

21.12

2.1. Estimation of storm duration The design storm duration is TD = 1.1 x tp = 1.1 x 1.09 = 1.2 hrs. Adjusting the design storm duration to next one hour, the adopted design storm duration (T D) is 2 hrs. This is because the methodology is designed for storm duration of 2 hrs and above. 2.2. Estimation of point rainfall and areal rainfall The point rainfall estimate for 50-yr return period and for duration of 24-hr is read against 50-yr 24-hr isopluvial map (CWC, 1986). The value of 50-yr 24-hr point rainfall is 16 cms. The design storm duration for the catchment is 2 hrs. The point rainfall estimate for 2 hrs was obtained by multiplying the 50-yr 24-hr point rainfall of 16 cms with the value of 0.53 read and interpolated from Section 4.2 of Flood Estimation report for Kaveri Basin Subzone – 3(i). 50-yr 2 hr point rainfall = 16 x 0.530 = 8.48 cm. The above point rainfall estimate of 8.48 cm was multiplied by areal reduction factor of 0.98 corresponding to a catchment area of 11.11 km2 and for design storm duration of 2 hrs

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as interpolated from Table A-3 (areal to point rainfall ratios (CWC, 1986). Therefore 50-yr 2 hr areal rainfall = 8.48 x 0.98 = 8.31 cm 2.3. Time distribution of areal rainfall The areal rainfall estimate for 50-yr 2 hr areal rainfall of 8.31 cm was distributed to give one hourly gross rainfall units by using the Distribution co-efficients for duration of 2 hrs from Table A-2: Time Distribution Coefficients of Areal Rainfall (CWC, 1986) as shown in Table 2. Table 2. Time distribution of areal rainfall. Duration (hrs)

Distribution Co-efficients

Areal Storm Rainfall (cm)

1-hr Rainfall (cm)

(1)

(2)

(3)

(4)

2 1

1.0 0.8

8.31 6.65

1.66 6.65 (Source: CWC, 1986)

Areal storm rainfall values in column (3) for durations of 2 and 1 hrs in column (1) were obtained by multiplying the 2-hr storm rainfall value of 8.31 cm with the distribution co-efficients in column (2) for respective durations. 1hr rainfall units in column (4) were obtained by subtraction of successive values of storm rainfall from 1-hr onwards in column (3). 2.4. Estimation of base flow The design base flow rate vide section 3.12 (Flood Estimation report for Kaveri Basin Subzone – 3(i)) is 0.05 m3/s/ km2. Therefore, the total base flow for a catchment area of 11.11 km2 = 11.11 x 0.05 = 0.56 m3/s. 2.5. Estimation of design flood (peak only) The maximum discharge ordinate of unit hydrograph is 21.12 m3/s at 2 hrs. The maximum 1-hr rainfall excess unit of 6.15 cm (after deducting the loss rate of 0.5 cm/hr from the 1-hr rainfall) was placed against the maximum discharge ordinate of 21.12 m3/s. Likewise the next lower rainfall excess unit was placed against the next lower unit hydrograph ordinate in the table 3 and so on. Summation of the products of columns (2) and (3) gives the total direct runoff to which base flow is added to get the maximum discharge. Table 3. Direct runoff. Time (hrs)

Unit hydrograph ordinate (m3/s)

1-hr rainfall (cm)

Loss rate (cm/hr)

1-hr rainfall excess (cm)

Direct runoff (m3/s)

(1)

(2)

(3)

(4)

(5)

(6)

1 2

15.84 21.12

6.65 1.66

0.5 0.5

1.16 6.15

18.41 129.85

Total

148.26

Base flow Total Peak Discharge

0.55 148.82

2.6. Computation of design flood hydrograph The 1-hr Rainfall Excess sequence shown in column (3) of the table 3 was reversed to obtain the critical sequence as shown in Table 4.

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Sumaiyah Tazyeen and Shivakumar J. Nyamathi / Aquatic Procedia 4 (2015) 1173 – 1180 Table 4. Critical rainfall excess sequence. Time (hrs)

Critical 1-hr Rainfall Excess (cm) sequence

1

6.15

2

1.16

Table 5. Flood hydrograph ordinates for Hulimavu watershed. Time (hrs)

1-hourly rainfall excess (cm)

UHO (m3/s)

6.15

1.16

Direct Runoff (m3/s)

Total Direct Runoff (m3/s)

Base flow (m3/s)

Total Flood flow (m3/s)

(1)

(2)

(3)

(4)

(5)

(6)

(7)

0

0

0.00

-

0.00

0.56

0.56

1

10.56

64.93

0.00

64.93

0.56

65.48

1.3

15.84

97.39

12.27

109.66

0.56

110.22

2

21.12

129.85

18.41

148.26

0.56

148.82

2.15

15.84

97.39

24.54

121.93

0.56

122.49

2.3

10.56

64.93

18.41

83.33

0.56

83.89

3

2.5

15.37

12.27

27.64

0.56

28.20

4

0.6

3.69

2.91

6.59

0.56

7.15

5

0.1

0.61

0.70

1.31

0.56

1.87

5.35

0

0.00

0.12

0.12

0.56

0.67

Unit hydrograph (fig 3a) is plotted for the Hulimavu watershed. For computation of design flood hydrograph, the unit hydrograph ordinates for 1-hr interval were tabulated against time (hrs) as shown in table 5. The critical rainfall sequence of 1-hr rainfall excess units given in table 4 were entered horizontally. The direct runoff resulting from each of the 1-hr rainfall excess units was obtained by multiplying the 1-hr rainfall excess unit with the unit hydrograph ordinates with a successive lag of 1-hr, since the unit duration of unit hydrograph is 1-hr. The direct runoffs were added horizontally to get the total direct runoff to which total base flow of 0.56 m 3/s was added to give the design flood hydrograph ordinates. The total discharge ordinates in column (7) were plotted against time in column (1) to get the design flood hydrograph as shown in the fig 3b.

TB=5.35 hr

a 25

120 W75 = 0.66 hr WR75= 0.26 hr

100

W50=1.11 WR50= 0.366hr

Trial 1 Trial 2

5

Discharge m3/s

Discharge m3/s

10

148.82 m3/s

140

Tm=1.59hr

20

15

160

b

80 60 40 20

0

0 0

1

2

3 Time (hrs)

4

5

6

0

1

2

3 4 Time (hrs)

5

Fig. 3. (a) Unit hydrograph for Hulimavu watershed; (b) Flood hydrograph for Hulimavu Watershed for 50-yr Return Period.

6

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3. Results and Discussions 3.1. Storage capacity of a lake Storage capacity of a lake depends upon the topography of the site and the height of the Bund. The storage capacity and the water spread area at different elevations can be determined from the contour map. In addition to finding out the capacity of a lake, the contour map of the lake can also be used to determine the land and property which would be submerged when the reservoir is filled up to various elevations. Table 6 provides the submerged area at different elevations along with the capacity at that contour before and after restoration. Fig 4 gives the Elevation-Area-Capacity curve for the Hulimavu Lake based on these areas and capacities. It is also seen that the present capacity of the lake is 0.8 Mm3. After restoration of the lake, de-silting and shoreline developments, the capacity of the lake is increased to 1.023 Mm3. Table 6. Submerged bed levels for Hulimavu lake. Existing Contour (m)

After Restoration

890.5

219.86

Cumulative Volume of water (m3) 11.94

891

4770.15

498.59

4770.15

498.59

891.5

28059.54

5755.27

28059.54

5755.27 30272.91

Cumulative Area (m2)

Cumulative Area (m2) 219.85

Cumulative Volume of water (m3) 11.94

892

113026.7

30272.91

113026.66

892.5

280841.6

92839.45

267430.7

87839.45

893

505271.5

191265.9

480459.68

181265.91

893.5

729701.4

314424.9

761335.09

335400.18

894

1034668

457470.2

1130260.1

508445.47

894.5

1370175

616786.9

1613181.7

737762.15

895

1738998

792119.2

2213396.8

1023094.5

Fig. 4. Elevation-area-capacity curves (existing and after restoration).

3.2. Reservoir planning If I and Q denote the inflow into and outflow from a reservoir, and S the storage in the reservoir, suffixes 1 and 2 can be used to denote a given quantity at the beginning and the end of the time interval and can be expressed as § I1  I 2 · § Q  Q2 · ¨ ¸'t  ¨ 1 ¸'t 2 ¹ © 2 ¹ ©

S1  S 2

(1)

In the above equation, I1 and I2 are known given inflow hydrograph to be routed through the reservoir, Q 1 and S1 are the initial outflow from the reservoir and the initial storage in the reservoir which are either known or assumed and Q2 and S2 are the two unknown quantities which must be determined. Thus to solve for Q 2 and S2 one more relation is needed. It can be assumed that the storage in the reservoir is independent of inflow and the outflow is dependent only on storage (Reddy, 2004). To find the storage-discharge relationship, the discharge from the reservoir for different elevations for existing and after restoration conditions are computed and an elevation-discharge curve (fig 5a) and a curve of discharge versus storage (existing and after restoration) (fig 5b) called as the routing curve are prepared.

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a

b

897

Discharge (m3/s)

896 Elevation (m)

895 894 893 892 891 890 0

5

10

15

Discharge

20

25

20 18 16 14 12 10 8 6 4 2 0

30

Existing After Restoration

0

0.5

(m3/s)

1 Storage

1.5

2

(Mm3)

Fig. 5. (a) Elevation-discharge curve; (b) Routing curves for Hulimavu watershed (existing and after restoration).

The Inflow-Storage-Discharge (ISD) method of Reservoir routing, first developed by L.G.Puls of the U.S. Army Corps of Engineers, uses the equation (1) by re-arranging and writing as I1  I 2 Q Q · · § § 't  ¨ S1  1 't ¸ ¨ S 2  2 't ¸ 2 2 2 ¹ ¹ © ©

(2)

The routing curves of ISD method for the existing and after restoration prepared are shown in the fig 6 (a) & (b). a

b

30

30 25 Discharge (m3/s)

Discharge (m3/s)

25 20 15 10 S-Q/2*∆t S+Q/2*∆t

5

20 15 10 S-Q/2*∆t S+Q/2*∆t

5 0

0 0

0.5

1

1.5

(S-Q/2*∆t) and (S+Q/2*∆t) in Mm3

2

0

0.5

1

1.5

2

2.5

(S-Q/2*∆t) and (S+Q/2*∆t) in Mm3

Fig. 6. Routing curves of ISD method (a) existing; (b) after restoration.

The graph of water surface elevation (existing and after restoration) against time (fig 7a) is obtained from the fig 5a for different values of outflow. The inflow and outflow hydrographs for existing and after restoration conditions are plotted on the same scale. It is observed that the peak-flow of outflow hydrograph is less than the peak-flow of the inflow hydrograph (Fig 7b) i.e., the peak-flow is reduced. Similarly, the time to peak in the outflow hydrograph is more than the time to peak in the inflow hydrograph. These are the effects of reservoir storage on the movement of flood wave through the reservoir. The reduction in peak known as the attenuation is 119.82 m 3/s and 131.32 m3/s for the existing and after restoration conditions respectively. The difference in times to peak known as the reservoir lag is 1 hour for the both the conditions. The attenuation and reservoir lag are very much dependent on the initial outflow from the reservoir which existed at the time of arrival of flood which is assumed to be 0.5 m3/ for both existing and after restoration conditions.

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a

b

896.4

180

Peak elevation = 896.3 m

160

Atte nuat ion = 119. 32 m3/s

140

896.0

Peak elevation = 895.5 m

895.8

Existing

895.6

After restoration

895.4

Discharge (m3/s)

Elevation (m)

896.2

Reservoir lag = 1 hr

120 100 80 60

Atte nuat ion = 131. 32 m3/s

Inflow Hydrograph Outflow hydrograph (Existing) Outflow hydrograph (After restoration)

40

895.2

20

895.0

0 0

5

10 Time in hrs

15

20

0

5

10

15

20

Time in hrs

Fig. 7. (a)Water surface elevation (existing and after restoration) (b) Inflow and routed hydrographs for Hulimavu watershed (existing and after restoration)

4. Conclusions The peak discharge estimated as per the present study using CWC method is 148.82 m3/s for 50 years return period. Even though the CWC method is applicable for a catchment area of more than 25 km2, still an attempt has been made to analyze flood studies of an urban lower stream order (Third order) catchment as there are no other appropriate methods readily available and to understand flood runoff versus time in shorter intervals. This flood runoff versus time will help to derive flood routing studies. Flood routing studies are carried out for existing capacity of the surplus weir of 18.4 m3/s and it is observed that during peak flood, outflow of 29 m3/s occurred which is more than the discharge capacity of the surplus weir. Hence capacity of the weir is insufficient for the safe passage of flood discharge which results in back water effect as the outflow is observed even at 896.3 m which is 30 cm higher than the Maximum water level (896.00 m) of the lake. An attempt has been made for the safe passage of flood discharge by increasing storage capacity from the existing 0.8 Mm3 to 1.0 Mm3. For that increased capacity of the reservoir flood routing studies were carried out which shows an outflow of 17.5 m3/s during peak flood which will be far less than the estimated discharge capacity of 18.4 m3/s. Accordingly, it is also observed that the elevation during the peak flow after restoration will be 895.95 m which is less than the Maximum Water Level (896.00 m) of the lake. References CWC (1986). Flood estimation report for Kaveri basin subzone 3(i). Directorate of Hydrology (small Catchments), Central Water Commission, New Delhi. Jayaram Reddy P (2004). “A text book of Hydrology”, 2nd Edition, Laxmi publication pvt limited, New Delhi, Flood routing, Design flood, 441485. Lake Development Authority, Government of Karnataka, Marshall & Bayliss (1994), “Flood estimation for small catchments”, Natural Environment Research Council, Institute of Hydrology, Oxfordshire, United Kingdom 124:1-2. NPTEL, Indian Institute of Technology, Kharagpur, (2010), Module 2- The science of Surface and Ground water, Lesson 3- Rainfall-Runoff Relationships. Theodor Strelkoff, (1980), “Modified Puls routing in Chuquatonchee Creek” prepared for Hydrologic Engineering Center, US Army Corps of Engineers, California under contract number DACW05-80-P-0324.