April 2010

SINSKE CONSULT

HYPOTHETICAL

FLOOD RETENTION/DETENTION DAM GROOTKLOOF RIVER

TIERKLOOF FARM 4/789, WORCESTER

Dr B.H. SINSKE, Pr Eng

Specialist Consultant Floodline Investigation and Flood Protection Measures

50 Brandwacht St 7600 Stellenbosch

Tel. 021 886 5140 Fax. 021 886 5164

E-mail: mailbox@sinske.com Website: www.sinske.com

HYPOTHETICAL FLOOD RETENTION/DETENTION DAM

GROOTKLOOF RIVER TIERKLOOF FARM 4/789, WORCESTER

CONTENTS

1 INTRODUCTION 1 1.1 Terms of Reference 1 1.2 History 1 1.3 Related Study 1 1.4 Dam Site 1 2 DAM INFORMATION 2 2.1 Dam Basin 2 2.2 Dam type 2 3 CATCHMENT INFORMATION 2 3.1 Catchment Area 2 3.2 Topography 2 3.3 Land Use 2 3.4 Mean Annual Precipitation (MAP) and Rainfall Intensity 2 4 FLOOD DETERMINATION 3 4.1 Flood Peaks 3 4.2 Flood Hydrographs 3 5 DAM SITE 1 3 5.1 Flood Retention Dam 3 5.2 Flood Detention Dam 4 6 DAM SITE 2 4 6.1 Flood Retention Dam 4 6.2 Flood Detention Dam 5 7 SUMMARY, DISCUSSION and CONCLUSION 6 7.1 Summary of Results 6 7.2 Discussion of Results 6 7.3 Conclusion 6 REFERENCES 7 APPENDIX 7 Figures 1 to 3 Tables 1 to 7

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HYPOTHETICAL FLOOD RETENTION/DETENATION DAM

GROOTKLOOF RIVER TIERKLOOF FARM 4/789, WORCESTER

1 INTRODUCTION 1.1 Terms of reference

It was requested by Mr Nik Wullschleger of BolandEnviro, Environmental & Resource Management Consultants, Worcester, on behalf of the Department of Agriculture, that the benefits of a flood retention dam on the Grootkloof River upstream of Farm Tierkloof (Portion 4 of Farm 789), Worcester, be conducted and reported on.

The purpose of the study is to investigate how the previously determined flood peaks for the design of the rehabilitated watercourse of the Grootkloof River (vid. § 1.3) could be attenuated by such a dam Presently a detailed survey to select a suitable dam site does not exist. Therefore the investigation concentrates on a hypothetical dam.

1.2 History

In a Background Information Document (BolandEnviro, 2010) the events leading to the necessity of the present study are in detail explained.

1.3 Related Study

In March 2010 the author conducted a study entitled "Design flood determination, Grootkloof River, Tierkloof Farm 4/789, Worcester". The purpose of the study was the estimation of flood peaks with various return periods for the design of the rehabilitated waterway of the Grootkloof River. The following design flood peaks, based on the rational method were recommended:

Grootkloof River, Point 1 Q10 = 8 m3/s Q20 = 10 m3/s Q50 = 14 m3/s Q100 = 17 m3/s Grootkloof River, Point 6 Q10 = 9 m3/s Q20 = 11 m3/s Q50 = 15 m3/s Q100 = 19 m3/s The situation of Points 1 and 6 is shown on an aerial photograph,

(Source: Google) Figure 1, APPENDIX

1.4 Dam Site The criteria for the selection of a suitable dam site are: (1) The access to the dam site should be easy. (2) The storage capacity of the reservoir should be as large as possible. (3) The catchment of the dam should be as large as possible. Two sites for a hypothetical dam have been chosen accordingly, on the Grootkloof River ca. 180 m and ca. 300 m upstream of Point 1. The dam sites are also shown on an aerial photograph,

(Source: Google) Figure 1, APPENDIX

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2 DAM INFORMATION 2.1 Dam Basin

The two sites for a hypothetical dam are indicated on the 1:10 000 orthophoto 3319 C 23 (Contour interval 10 m). (Scale 1:3 000) Figure 2, APPENDIX The chosen bottom levels (BL) of the two reservoirs are: DAM SITE 1 BL = 260,00 m DAM SITE 2 BL = 270,00 m Measured surface areas and calculated reservoir capacities are given, (y = height above BL) Table 1, APPENDIX With the data of Table 1, surface area A (m2) and reservoir capacity V (m3) are given by the following power relationships: DAM SITE 1 A = 335· y1,55 and V = 73,60· y2,73

DAM SITE 2 A = 244· y1,38 and V = 45,15· y2,64

2.2 Dam Type

The Department of Agriculture suggested the realization of a retention dam for a 20-year flood in the upper reaches of the Grootkloof RIver. It has been decided, however, to investigate a flood detention dam as well.

Definition:

In retention storage, runoff is not released downstream and usually is removed from storage only by infiltration through a porous bottom or by evaporation. Detention storage involves detaining or slowing runoff and then releasing it. Detention dams (also called retarding basins) are usually designed with an open outlet and a spillway for the evacuation of floods larger than the design flood. (Bedient and Huber, 1988)

In order to use a retention dam for flood control, the dam must be equipped, apart from a spillway, also with a bottom outlet (i.e. a small-diameter pipe with a valve) to drain the reservoir after the flood event and have it ready to receive the next flood.

3 CATCHMENT INFORMATION 3.1 Catchment Area

Catchment of the Grootkloof River upstream of the hypothetical dam: A = 1,00 km2 Relevant Maps: 1: 50 000 Topo Series 3319 CB Worcester

1: 10 000 Orthophoto Series 3319 C 23 The catchment (scale 1:25 000) is shown, Figure 3, APPENDIX

3.2 Topography The topographic characteristics of the catchment are summarised, Table 2, APPENDIX

3.3 Land use A weighted runoff coefficient C = 0,5 has been adopted (vid. March 2010 study, §1.3).

3.4 Mean Annual Precipitation (MAP) and Rainfall intensity Relevant information: (vid. March 2010 study, §1.3)

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4 FLOOD DETERMINATION 4.1 Flood Peaks

The following flood peaks to be expected at the dam site have been calculated, using the rational method with t = tC = 0,4 h (vid. March 2010 study, §1.3):

Q10 = 8,2 m3/s Q20 = 10,1 m3/s Q50 = 13,5 m3/s Q100 = 16,8 m3/s

4.2 Flood Hydrographs

For reservoir routing procedures, inflow hydrographs with variable rainfall duration must be used, as usually not the peaked ("thin") short-duration hydrograph, but rather the flat ("fat") long-duration hydrograph produces the largest outflow peak. Using the extended rational method with rainfall duration t > tC, flood peaks Q and flood volumes V have been calculated for various return periods T, and listed, Table 3, APPENDIX

5 DAM SITE 1 5.1 Flood Retention Dam

Design parameters: Design flood: 20-year flood Levels: Bottom level of reservoir BL = 260,00m (y = 0) Overflow crest level FSL = 269,50 m → y = 9,5 m → VFULL = 35000 m3

Non -overflow crest NOC = 272,00 m → y = 12,0 m Spillway: Broad-crested weir situated west flank of dam wall. Trapezoidal cross-section: b = 10 m, 1: m = 1: 3 Dam inflow: Inflow hydrographs: Table 3, APPENDIX Dam outflow (spill): Initial condition: Reservoir water level WL = 260,25 m (reservoir empty). Inflow hydrographs are routed through the reservoir with Qout = 0 (bottom outlet closed). Maximum reservoir levels and outflow peaks (spill) are given, Table 4, APPENDIX The dam will probably be classified a Category II dam. The recommended design discharge will be a 100-year or a 200-year flood, SANCOLD (1991 and 1990). With the non-overflow crest of the dam at level 272,00 m, there would be a freeboard available of f = 1,30 m, respectively f = 0,95 m. Attenuation: The 20-year flood (and smaller floods) will be totally absorbed by the reservoir. The peaks of the 50- and 100-year flood will be attenuated to a mere 1 or 2 m3/s. Drainage time: The time required to empty the reservoir depends on the size of the bottom outlet, e.g.:

Pipe diameter D = 300 mm: Drainage time t = 21 h Pipe diameter D = 450 mm: Drainage time t = 9 h

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5.2 Flood Detention Dam

Design parameters: Design flood: 20-year flood Levels: Bottom level of reservoir BL = 260,00 m (y = 0) Outlet invert level IL = 263,50 m → y = 3,5 m → VDEAD = 2300 m3

Overflow crest level FSL = 266,50 m → y = 6,5 m → VFULL = 12200 m3

Non -overflow crest NOC = 268,50 m → y = 8,5 m Outlet: Pipe diameter D = 750 mm Spillway: As for retention dam:

Broad-crested weir situated in saddle west of dam wall. Trapezoidal cross-section: b = 10 m, 1: m = 1: 3 Dam inflow: Inflow hydrographs: Table 3, APPENDIX Dam outflow (spill): Initial condition: Reservoir water level WL = 263,50 (invert of outlet pipe) Inflow hydrographs are routed through the reservoir (outlet open). Maximum reservoir levels and outflow peaks (outlet and spill) are given, Table 5, APPENDIX The dam will probably be classified a Category I or II dam. The recommended design discharge will be a 100-year or a 200-year flood, SANCOLD (1991 and 1990). With the non-overflow crest of the dam at level 268,50 m, there would be a freeboard available of f = 1,30 m, respectively f = 0,75 m. Attenuation: The outflow peak of the 10- year flood will be attenuated to 2,2 m3/s The outflow peak of the 20- year flood will be attenuated to 2,5 m3/s The outflow peak of the 50- year flood will be attenuated to 2,9 + 0,7 = 3,6 m3/s The outflow peak of the 100- year flood will be attenuated to 3,1 + 1,8 = 4,9 m3/s

6 DAM SITE 2 6.1 Flood Retention Dam

Design parameters: Design flood: 20-year flood Levels: Bottom level of reservoir BL = 270,00m (y = 0) Overflow crest level FSL = 282,50 m → y = 12,5 m → VFULL = 35500 m3

Non -overflow crest NOC = 285,00 m → y = 15,0 m Spillway: Broad-crested weir situated in saddle west of dam wall. Trapezoidal cross-section: b = 10 m, 1: m = 1: 3 Dam inflow: Inflow hydrographs: Table 3, APPENDIX 2 Dam outflow (spill): Initial condition: Reservoir water level WL = 270.25 m (reservoir empty). Inflow hydrographs are routed through the reservoir with Qout = 0 (bottom outlet closed). Maximum reservoir levels and outflow peaks (spill) are given,

Table 6, APPENDIX

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The dam will probably be classified a Category II dam. The recommended design discharge will be a 100-year or a 200-year flood, SANCOLD (1991 and 1990). With the non-overflow crest of the dam at level 285,00 m, there would be a freeboard available of f = 1,20 m, respectively f = 0,75 m. Attenuation: The 20-year flood (and smaller floods) will be totally absorbed by the reservoir. The peaks of the 50- and 100-year flood will be attenuated to a mere 1 or 2 m3/s. Drainage time: The time required to empty the reservoir depends on the size of the bottom outlet, e.g.:

Pipe diameter D = 300 mm Drainage time t = 19 h Pipe diameter D = 450 mm Drainage time t = 8 h

6.2 Flood Detention Dam Design parameters: Design flood: 20-year flood Levels: Bottom level of reservoir BL = 270,00 m (y = 0) Outlet Invert level IL = 273,50 m → y = 3,5 m → VDEAD = 1200 m3

Overflow crest level FSL = 277,50 m → y = 7,5 m → VFULL = 9200 m3

Non -overflow crest NOC = 280,50 m → y = 10,5 m Outlet: Pipe diameter D = 750 mm Spillway: As for retention dam:

Broad-crested weir situated in saddle west of dam wall. Trapezoidal cross-section: b = 10 m, 1: m = 1: 3 Dam inflow: Inflow hydrographs: Table 3, APPENDIX Dam outflow (spill): Initial condition: Reservoir water level WL = 273,50 (invert of outlet pipe) Inflow hydrographs are routed through the reservoir (outlet open). Maximum reservoir levels and outflow peaks (outlet and spill) are given,

Table 7, APPENDIX The dam will probably be classified a Category I or II dam. The recommended design discharge will be a 100-year or a 200-year flood, SANCOLD (1991 and 1990). With the non-overflow crest of the dam at level 280,50 m, there would be a freeboard available of f = 1,40 m, respectively f = 0,80 m. Flood attenuation: The outflow peak of the 10- year flood will be attenuated to 2,7 m3/s The outflow peak of the 20- year flood will be attenuated to 3,0 m3/s The outflow peak of the 50- year flood will be attenuated to 3,4 + 1,1 = 4,5 m3/s The outflow peak of the 100- year flood will be attenuated to 3,7 + 2,8 = 6,5 m3/s

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7 SUMMARY, DISCUSSION and CONCLUSION 7.1 Summary of Results The attenuated flood peaks are summarized, Table 8. Also shown in Table 8 are size and volume content of a hypothetical earth fill dam. Table 8 Attenuated flood peaks

Dam Site

Dam Type H (m) L (m) V (m3) Attenuated Flood Peak QOUT (m3/s)

Q10 Q20 Q50 Q100 1 Retention

Detention 12,0 8,5

230 160

36900 14300

--- --- 0,9 1,8 2,2 2,5 3,6 4,9

2

Retention Detention

12,5 10,5

110 65

22000 11000

--- --- 1,0 2,0 2,7 3,0 4,5 6,5

Note: H = height of dam, foundation depth not included

L = approximate length of non-overflow crest V = approximate volume content of an earth fill dam 7.2 Discussion of Results Flood attenuation:

The attenuated flood peaks of Table 8 must be compared with the maximum flood peaks resulting from the natural catchment (vid. § 4.1). The degree of flood attenuation achieved is quite substantial: In case of a retention dam: 90% to 100% In case of a detention dam: 70% to 75% (site 1) and 65% to 70% (site 2) Dam site:

An earth fill dam at site 1 would have a larger volume than a dam at site 2: In case of a retention dam: 1,7 times larger In case of a detention dam: 1,3 times larger

Retention dam vs. detention dam:

Flood attenuation of a retention dam is slightly better than that of a detention dam The volume content of a retention dam is 2,5 times (site 1) and 2,0 times (site 2)

larger than that of a detention dam The outlet of the retention dam is a small-diameter pipe equipped with a valve.

The valve must be closed during a flood, but must be opened soon after to release the water stored in the reservoir. The time to drain the reservoir is 10 to 20 hours. The detention dam is equipped with a large-diameter pipe without a valve.

7.3 Conclusion

It is obvious that a detention dam (preferably at site 2) is the better choice, also because human error or malfunction (operation of the valve) is eliminated.

If the channel from Point 1 to Point 6 is to be designed for a 50-year flood, the design flood for the channel could be lowered from 14 m3/s to 4 or 5 m3/s, which justifies a cost analysis using an alternative design.

B.H. SINSKE, Pr.Eng. Stellenbosch, April 2010

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REFERENCES Bedient, P.B., Huber, W.C. (1988). Hydrology and Floodplain Analysis. Addison-Wesley Publishing Company, Reading, Massachusetts. SANCOLD (1990). Interim Guidelines on Freeboard for Dams. Safety Evaluation of Dams, Report No. 3 South African National Committee on Large Dams, Pretoria SANCOLD (1991). Guidelines on Safety in Relation to Floods. Safety Evaluation of Dams, Report No. 4 South African National Committee on Large Dams, Pretoria APPENDIX Figures

Fig. 1 Study area on aerial photograph (© Google) Fig. 2 Dam Site (scale 1:3 000)

Fig. 3 Catchment area (scale 1:25 000) Tables

Table 1 Reservoir data

Level (m)

DAM 1 y (m) A (m2) V (m3)

DAM 2 y (m) A (m2) V (m3)

260,00 270,00 280,00 290,00

0 0 0 10 11945 39815 20 35075 264790

0 0 0 10 5855 19517 20 15235 121299

TABLE 2 Catchment characteristics, Grootkloof River upstream of dam site

Catchment Area Highest point in catchment

A (km2) RL (m)

1,00 780

Height at catchment outlet RL (m) 265 Max. height difference Hmax(m) 515 Longest watercourse L (km) 2,30 (1) (2) Average height difference Havg (m) 452 507 Average slope J (%) 19,7 22,0 Orohydrographic factor K= L/√J 0,52 0,49 Lag time (3) tL (min) 14 14 Time of concentration: Dyck/Peschke (tC = 2tL) Dyck/Peschke (tC = 1,67tL) Bransby Williams US Dept. of Agriculture Kirpich (SCS) formula Kerby Kirpich/Kerby

tC (h) tC (h) tC (h) tC (h) tC (h) tC (h) tC (h)

0,47 0,45 0,39 0,38 0,38 0,37 0,29 0,27 0,24 0,23 0,45 0,44

0,45 Note: (1) Average height difference and average slope calculated with equal area method (2) Average height difference and average slope calculated with 1085 method (3) Lag time calculated with Ramser/Kirpich/Rowe, Ven Te Chow or Thiele/Euler formulas,

depending on K (Sifalda, 1996)

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TABLE 3 Peak and volume of inflow hydrographs

T = 10 yr T = 20 yr T = 50 yr T = 100 yr T = 200 yr t

(h) Q V (m3/s) (m3)

Q V (m3/s) (m3)

Q V (m3/s) (m3)

Q V (m3/s) (m3)

Q V (m3/s) (m3)

0,4 0,6 0,8 1,0 1,5 2,0 3,0 4,0 5,0 6,0

12.0 24,0

8,2 12000 6,5 14000 5,4 15500 4,6 17000 3,6 20000 3,0 22000 2,3 23000 1,9 27000 1,6 28000 1,4 30000 0,8 35000 0,5 40000

10,1 15000 7,9 17000 6,9 19000 5,7 20000 4,5 24000 3,7 27000 2,8 30000 2,3 33000 1,9 35000 1,7 37000 1,0 43000 0,6 49000

13,5 19000 10,6 23000 8,8 25000 7,6 27000 6,0 32000 4,9 36000 3,8 41000 3,1 44000 2,6 47000 2,3 49000 1,3 57000 0,8 66000

16,8 24000 13,2 29000 11,0 32000 9,4 34000 7,4 40000 6,2 44000 4,7 50000 3,8 55000 3,2 58000 2,8 61000 1,6 71000 0,9 82000

20,5 30000 16,1 35000 13,4 39000 11,5 42000 9,1 49000 7,5 54000 5,7 57000 4,6 67000 3,9 71000 3,4 74000 2,0 87000 1,2 100000

Note: The Q and V values for t = 12 h and t = 24 h are approximate only. The rational method stipulates constant rainfall intensity during the rainfall event. For longer rainfall durations, however, this assumption is not acceptable any more. .

TABLE 4 DAM SITE 1 Retention Dam: Max. reservoir levels and outflow peaks (spill)

T = 20 yr T = 50 yr T = 100 yr T = 200 yr t

(h) WL QSPILL (m) (m3/s)

WL QSPILL (m) (m3/s)

WL QSPILL (m) (m3/s)

WL QSPILL (m) (m3/s)

0,4 0,6 0,8 1,0 1,5 2,0 3,0 4,0 5,0 6,0

266,40 --- 266,80 --- 267,20 ---267,30 --- 267,75 --- 268,05 --- 268,45 --- 268,80 --- 268,90 --- 269,10 ---

267,15 --- 267,60 --- 267,90 --- 268,15 --- 268,70 --- 269,00 --- 269,50 --- 269,85 0,2 270,00 0,5 270,20 0,7

267,80 --- 268,30 --- 268,65 --- 268,85 --- 269,45 --- 269,85 0,2 270,30 0,8 270,45 1,2 270,60 1,5 270,65 1,7

268,40 --- 268,95 --- 269,35 --- 269,60 0,1 270,20 0,7 270,50 1,3 270,85 2,1 271,00 2,5 271,05 2,7 271,05 2,7

Table 5 DAM SITE 1 Detention Dam: Maximum reservoir levels and outflow peaks

T = 20 yr T = 50 yr T = 100 yr T = 200 yr t

(h) WL QOUT QSPILL (m) (m3/s) (m3/s)

WL QOUT QSPILL (m) (m3/s) (m3/s)

WL QOUT QSPILL (m) (m3/s) (m3/s)

WL QOUT QSPILL (m) (m3/s) (m3/s)

0,4 0,6 0,8 1,0 1,5 2,0 3,0 4,0 5,0 6,0

265,85 2,2 --- 266,10 2,3 --- 266,35 2,4 --- 266,35 2,4 --- 266,40 2,5 --- 266,30 2,4 --- 266,00 2,3 --- 265,70 2,1 --- 265,20 1,9 --- 265,00 1,7 ---

266.60 2,6 0,1 266,90 2.7 0,3 267,00 2,8 0,4 267,10 2.8 0.5 267,20 2,9 0,7 267,15 2.8 0,6 267,00 2.7 0,4 266,65 2,6 0,1 266,20 2,4 --- 265,85 2,2 ---

267,15 2,8 0,6 267,50 3,0 1,2 267,60 3.0 1,5 267,65 3.1 1,6 267,75 3,1 1,8 267,75 3,1 1,8 267,50 3,0 1,2 267,20 2,9 0,7 266,90 2,7 0,3 266,60 2,6 0.1

267,70 3,1 1,7 268,00 3,2 2,5 268,15 3,2 2,9 268,20 3,3 3,0 268,25 3,3 3,3 268,15 3,2 3,0 267,90 3,1 2,2 267,60 3,0 1,4 267,30 2,9 0,9 267,10 2,8 0,5

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Table 6 DAM SITE 2 Retention Dam: Max. reservoir levels and outflow peaks (spill)

T = 20 yr T = 50 yr T = 100 yr T = 200 yr t

(h) WL QSPILL (m) (m3/s)

WL QSPILL (m) (m3/s)

WL QSPILL (m) (m3/s)

WL QSPILL (m) (m3/s)

0,4 0,6 0,8 1,0 1,5 2,0 3,0 4,0 5,0 6,0

278,10 --- 278,60 --- 279,20 ---279,30 --- 280,00 --- 280,40 --- 280,90 --- 281,40 --- 281,60 --- 281,90 ---

279,10 --- 279,70 --- 280,20 --- 280,50 --- 281,20 --- 281,70 --- 282,40 --- 282,90 0,3 283,10 0,6 283,30 0,9

280,00 --- 280,70 --- 281,20 --- 281,50 --- 282,30 --- 282,80 0,3 283,40 1,1 283,60 1,6 283,80 2,0 283,80 2,0

280,90 --- 281,70 --- 282,20 --- 282,50 --- 283,20 0,9 283,70 1,8 284,10 2,9 284,20 3,2 284,25 3,3 284,15 3,1

Table 7 DAM SITE 2 Detention Dam: Maximum reservoir levels and outflow peaks

T = 20 yr T = 50 yr T = 100 yr T = 200 yr t

(h) WL QOUT QSPILL (m) (m3/s) (m3/s)

WL QOUT QSPILL (m) (m3/s) (m3/s)

WL QOUT QSPILL (m) (m3/s) (m3/s)

WL QOUT QSPILL (m) (m3/s) (m3/s)

0,4 0,6 0,8 1,0 1,5 2,0 3,0 4,0 5,0 6,0

277,05 2,7 --- 277,35 2,9 --- 277,60 3,0 --- 277,50 2,9 --- 277,50 2,9 --- 277,20 2,8 --- 276,50 2,5 --- 275,90 2,2 --- 275,30 1,9 --- 275,00 1,7 ---

278.00 3,2 0,4 278,35 3.3 0,9 278,45 3,4 1,1 278,45 3.4 1.1 278,45 3,4 1,1 278,30 3.3 0,8 277,90 3.2 0,3 277,30 2,9 --- 276,50 2,5 --- 276,00 2,3 ---

278,75 3,5 1,8 279,00 3,6 2,6 279,10 3.7 2,8 279,10 3.6 2,8 279,05 3,5 2,7 278,90 3,4 2,2 278,50 3,3 1,2 278,10 3,2 0,5 277,60 3,1 0,1 276,90 2,8 ---

279,35 3,7 3,7 279,65 3,8 4,7 279,70 3,8 5,0 279,65 3,8 4,9 279,55 3,7 4,5 279,30 3,6 3,5 279,85 3,5 2,1 279,25 3,4 1,2 278,15 3,3 0,6 277,80 3,1 0,2