Appendix A3.2

Additional Hydrology Issues

 

Long Valley – Dry Season Monitoring programme

 

1.1              In order to maximise discovery of the transient tidal effects on the groundwater within the main alluvial aquifer in Long Valley, a 26 hr programme of Neap and Spring tide cycle observations was arranged amongst the 13 standpipe piezometers and 8 piezometers. Neap Tide observations were ordered on 26/27th November 2001 and Spring tide on 3/4th December:

 

·        The HKO predicted tide cycles are shown in Figs 1 &2.

·        The actual plotted tidal and peizometric observations in Figures 3 & 4.

·        The preliminary analysis of the measured variation in dry season groundwater levels in the main alluvial aquifer for the combined Neap and Spring tides is given on Figure 5. The groundwater regime across a 200m wide band centred on the railway is characterised.

 

1.2              Each period of peizometer reading was coupled with general observations of other non-tidal stimuli in order to build up a picture of the groundwater regime for normal agricultural operations within Long Valley. These outside influences include:

 

·        Temporary deflation of AFCD fabridam and alternative bypass pumping, (daytime only);

·        Pumped irrigation supply from the main River and the new River Beas wetland;

·        Temporary pumping from individual private wells;

·        Temporary daytime pumping from the irrigation channel and cascade flooding of fields/ponds; and

·        Consequences of major coffer dam blocking 80% of the main channel at the confluence with the River Indus (Spring Tide).

 

1.3              Tables A3.2.1 and A3.2.2 present the accompanying field notes for parts of the monitoring period to demonstrate some of the varied influencing factors.

 

Table A3.2.1

Long Valley – Neap Tide – 26th /27th November

Field Notes – Inspection at 16.00 to 18.00hrs

 

AFCD Fabridam deflated and a single 8inch pump in action feeding the irrigation system in its stead;

Visit to WSD fabridam/entrance of River Indus revealed a temporary channel works situation with a major Coffer dam/bund. 80% of River Beas / Sutlej blocked off but tidal regime still in place.

Floor of channel exposed above Ho Sheung Heung footbridge. Perforated channel floor bleeding rust coloured groundwater upon retreat of the tide;

Herons, Great Egrets feeding in the mud deposited on the combined channel floor. More than a dozen birds standing within the mud bottom at 100 to 150 m of the main construction activity. East Rail passenger trains to Lo Wu run alongside the eastern bund and delivery of goods train, laden with pigs to the Sheung Shui Abattoir, also in evidence. Disembarking pigs present the most disturbing and noisy aspect of the whole scenario. Construction activity on both banks with movements of mixer trucks and dump trucks and backhoe excavation and loading activity within the coffer dam. Large rip-rap also being tipped and placed within the bunded area of combined channel floor.

Also, Little Egrets feeding on Beas Channel floor within 50 m of filling and spreading activity at the top of the channel bund.

Farmers using several submersible pumps to lift irrigation water from the central channel to supply water spinach beds above new tunnel alignment. Groundwater table lower at the eastern side of Long Valley with substantial shrinkage cracks in the fields (several metres long by 20 – 40mm wide and up to 200mm deep.) 

 

 

Table A3.2.2

Long Valley – Spring Tide – 3rd/4th December

Field Notes - Inspection at 16.30 to 18.00hrs

 

AFCD Fabridam deflated, but a single 8inch pump delivering to the irrigation system;

Ying Kong covered channel bifurcation. Flow 90% directed to Long Valley; The other 10% escaping through perforated weir towards one of the main channel wetlands.

Natural watercourse leading to Long Valley has been newly cleared of vegetation and channel dug out in places to promote flow;

Adjacent to Beas River- Farmers observed using two submersible pumps in new wetland as their source;

Pumps in Long Valley watercourse present but not in service. Marsh area remains flooded and the well drowned. Lots of evidence of irrigation water having been used earlier in the day with many of the fields still waterlogged and normally dry channels showing signs of usage;

Almost all fields on the rail tunnel alignment have gone into service. Fallow and totally overgrown plots are now all cleared and newly planted;

Major channel construction at north end immediately upstream of WSD fabridam. Channel floor completion works entail coffer dam/bund across 80% of the width of the channel and some 150m length. Tidal flow quite restricted. Coffer dam purposely breached at 17.00 hrs by backhoe to flood the new floor and presumably feed flows back up the River Indus towards Ping Che; Ebb tide from Beas and Sutlej wholly diverted up the River Indus.

Inspection at 23.00 to 00.30hrs

AFCD Beas Fabridam deflated and pump switched off; No flow in irrigation channel across Long Valley, residual flow draining back to wetland;

WSD Fabridam area large proportion of the incoming tide feeding towards River Indus, but Sutlej and Beas also rising. Heavily restricted flow and head loss in coffer dam channel;

High tide occurred at approx 23.15 and reached top of Ho Sheung Heung bridge flap valve plus 50mm. Tideflood and ebb very sharp. Coffer dam restricted flow;

Met piezo dipping teams and took a preliminary view of results. Response to overnight high tide will be the key; No noticeable response to small tidal peak during the day;

Dogs barking but not a hazard to piezo ops;

Large numbers of herons and great egrets flying as a consequence of disturbance by piezo team and the undersigned; Flying typically in groups of 8 to 12 and sighted mainly over the two drainage channels;

Long Valley extremely well lit by reflected street lighting and abattoir flood lighting. Hand torches only necessary to read piezometer cable marking; Farmers also cropping at midnight under flood lights.

 

 

1.4              In the centre of Long Valley the influence of the tide is not as great as that from the irrigation channel crossing the Valley from Beas to Sutlej. The measured groundwater levels vary by as much as 1.5 m over a 10 days period in the centre of the valley. In the east and west extremities the groundwater level variation is less pronounced being typically 0.7 and 0.8m. The combined plot in Figure 5 gives a reasonable low water Neaps (Dry Season) boundary/envelope for the main aquifer.

 

1.5              The bottom of the dry season groundwater envelope is the key controller necessary to safeguard the wetland ecology and the agricultural practices. This minimum groundwater envelope must not be compromised by the tunneling works. The production piezometer monitoring programme needed to uphold this criteria is set out in Table A3.2.3.

 

Table A3.2.3

Long Valley Tunnel Works

 

Piezometer Monitoring

Groundwater levels in Long Valley are to be monitored across the four sections defined in the EIA/EM&A. Each section or gate comprises 3 boreholes and 5 piezometers (2 discrete piezometers and 3 standpipes);

 

Environmental controls on groundwater during the tunnelling contract require the following periods of monitoring:

 

Baseline

 

·         Baseline controls each year, or as directed by the Engineer; – a full suite of hourly monitoring across all 20 instruments for a 26 hour period, wet and dry season, spring and neap tides. Concurrently, tidal reference shall also be taken hourly from the Beas River footbridge opposite Ho Sheung Heung for each of the four monitoring periods;

·         On the basis of the four sets of results the Engineer will define an acceptable baseline groundwater envelope for each of the four sections, for the two seasons and two tides.

 

Production

 

·         Production Monitoring – As each tunnel drive approaches one of the four piezometer gates, daily monitoring of all 5 piezometers and the tide will commence;

·         Daily piezometer monitoring, at a time of day to be agreed with the Engineer, will be required throughout the whole period when any part of the TBM lies within +/- 50m of the gate in plan; Part of the TBM shall be defined as the length lying between the cutter face and the first whole fully completed segment of permanent tunnel lining;

·         The frequency of monitoring may be increased, as directed by the Engineer, should the monitored groundwater levels step outside the defined acceptable groundwater envelope for the particular gate, season and tide;

·         Additional ad hoc piezometer monitoring may be ordered by the Engineer, if any form of recharge is required to restore the local groundwater to the acceptable envelope or that any part of the Action Plan (as defined in the EIA) has to be invoked as a consequence of the passage of the tunnel drive.

 

Post Production

 

·         After full passage of the TBM through the piezo-gate, Post production groundwater monitoring may be required to be undertaken weekly, for a period of 6 weeks, or as directed by the Engineer.

 

 

1.6              The upper limit line of the dry season groundwater envelope has not been truly defined because the cofferdam facilitating the reconnection of the River Indus has curtailed the maximum water level in the Beas and Sutlej during the two reading periods. The Spring tide top water level was about 0.35m below expectations.

 

Provisional Dry Season Groundwater Level Action and Limit levels

 

1.7              The provisional dry season absolute lower limit line for arresting any adverse effects of groundwater lowering in the main alluvial aquifer should be set at the bottom of the groundwater envelope shown in Figure 5. In order to adequately control the works:

 

·        The action line for commencement of groundwater augmentation in consultation with the Long Valley farmers should be set at 0.3m above this limit line.

·        Notification for preparation to augment water supply and take groundwater re-charge actions as described in the Action Plan should be set at 0.5m above the absolute limit line.

 

1.8              Prior to commencement of tunneling operations a field sensitivity check should be run of the consequences for the groundwater regime of pre-emptive changes to the irrigation regime. Typically, an understanding should be achieved of the likely groundwater response over a three day period of the following:

 

·        Doubling of the flow in the irrigation channel by increasing the impounding level of the AFCD dam and adjustment of flows in the channel by changing the sluice valve settings;

·        Pumping from the main irrigation channel and maintaining standing water levels in various combinations of the fallow fields;

·        Direct pumped re-charge to existing wells close to the tunnel alignments

 

1.9              A re-run of the piezometer monitoring programme when the rivers are fully operational at the end of the dry season is required to confirm these provisional groundwater Action and Limit levels. A suite of wet season Spring and Neap readings will need to be run in July 2002.

 

 

2          ADDITIONAL GEOLOGICAL INFORMATION

 

2.1              In October / November 2001, 14 drillholes, DTP/20 to 25 and LDD200/DH/117 to 124, were sunk at location in close proximity to the 4 representative sections across the tunnel alignment within Long Valley. Locations of these drillholes are shown in Figure M1 of appendix A3.2b.

 

2.2              Drillholes DTP/20 to 25 are located along the center line of the tunnel alignment. The drillholes were sunk to 5m below Grade III or better rock. U100 or Mazier soil samples were collected at 2m interval. Permeability tests were carried out in drillholes DTP/(P)/20 to 22 and 25 at the alluvial and CDV layers. Standpipe piezometers were installed in these drillholes at the alluvial layer. Drillholes DTP/(P)/20 to 24 revealed sandy/silty/gravelly alluvial materials overlain by Fill. Drillhole DTP/P/25 revealed interbedded layers of sandy/clayey and silty/clayey alluvial materials overlain by Fill. Underneath the alluvial materials is completely decomposed volcanic tuff.

 

2.3              Drillholes LDD200/DH/117 to 124 are located at 60m, with 4 drillholes on each side of the future tunnel alignment. The drillholes were sunk to 1m below Grade III or better rock. Mazier soil samples were collected at 2m interval. Permeability tests were carried out, in all the drillholes, at the Alluvial and CDV layer. Standpipes and piezometers were installed in these drillholes for ground water monitoring. Drillholes LDD200/DH/121 revealed interbedded layers of sandy/clayey and silty/clayey alluvial materials. Drillholes LDD200/DH/117, 118 & 122 revealed sandy/gravelly alluvial materials overlain by Fill and Estuarine deposits. LDD200/DH/121 revealed sandy/gravelly alluvial materials overlain by residual soil. LDD200/DH/124 revealed fill overlying silty and sandy alluvial materials. Underneath the alluvial materials is completely decomposed volcanic tuff.

 

2.4              The borehole logs are attached in this Appendix A 3.2a. The 4 sections across the tunnel alignment have been updated with the new geological information and are presented in Appendix 3.2b. The geological material revealed from these boreholes are fully consistent with that previously obtained.

 

2.5              Permeability tests were conducted at selected drillholes and the findings are summarized below:-

 

Borehole No.

Response zone

(m,BGL)

Materials with response zone

Permeability

(m/s)

LDD200/DH/117 (I)

4.50 – 5.50

Alluvium, A3/A4

6.82E-05

LDD200/DH/118 (I)

5.50 – 6.50

Alluvium, A3/A4

4.56E-05

LDD200/DH/118 (II)

12.50 – 13.50

CDV

3.94E-07

LDD200/DH/119 (I)

3.50 – 4.50

Alluvium, A4

1.48E-04

LDD200/DH/119 (II)

11.00 – 12.00

CDV

4.81E-07

LDD200/DH/122 (I)

2.50 – 3.50

Alluvium, A3

3.01E-07

LDD200/DH/122 (II)

10.50 – 11.50

CDV

6.02E-07

LDD200/DH/123 (I)

3.50 – 4.50

Alluvium, A3

2.34E-05

LDD200/DH/123 (II)

11.00 – 12.00

CDV

2.38E-06

LDD200/DH/124 (I)

5.00 – 6.00

Alluvium, A3

8.28E-07

LDD200/DH/124 (II)

14.00 – 15.00

CDV

7.65E-06

DTP/20 (I)

4.50 – 5.50

Alluvium, A3

6.62E-05

DTP/20 (II)

12.00 – 13.00

CDV

1.26E-06

DTP/21 (I)

4.50 – 5.50

Alluvium, A3

1.88E-06

DTP/21 (II)

11.00 – 12.00

CDV

2.26E-06

DTP/22 (I)

5.50 – 6.50

Alluvium, A3

8.89E-07

DTP/22 (II)

12.00 – 13.00

CDV

2.15E-07

DTP/25 (I)

4.00 – 5.00

Alluvium, A3

1.94E-05

DTP/25 (II)

12.00 – 13.00

CDV

5.64E-06

 

 

2.6              The permeability test results obtained are mostly within the range of values of the corresponding soil types previously reported.

 

 

3          SENSITIVITY ANALYSIS

 

3.1              The variation in groundwater level under steady state condition after installation of the twin tunnel has been presented in Appendix A3.1. In this appendix, two sensitivity analyses had been carried out to supplement the results previous reported.

 

3.2              In the first analysis, the upper bound value (ie. most permeable) of the falling head tests results at CDV has been used to estimate the groundwater table variation under steady state condition after construction of the twin tunnel.

 

3.3              In the second analysis, the change in ground groundwater table with time after tunnel construction has been estimated.

 

3.4              Same assumptions, as stated in paragraph 10.2 of Appendix A3.1, except the permeability data input data has been adopted in the sensitivity analyses. The permeability data adopted in these sensitivity analyses are as follows:-

 

Soil

Permeability

Fill

7.67 x 10–6 m/s

Alluvial Clay/ Silt (A1/A2)

5.32 x 10–6 m/s

Alluvial Sand/Gravel (A3/A4)

4.36 x 10–5 m/s

Completely Decomposed Tuff (V)

 6.50 x 10–6 m/s

 

 

3.5              The results of the analyses are presented in Appendix A3.2 c and are summarized as follows:-

 

SECTION A-A

 

 

Distance From Centre of Twin Tunnels

60mS

-

60mN

120mN

Total Head (mPD)

Initial Condition

3.84

3.59

3.32

2.97

With Twin Tunnels

3.84

3.58

3.31

2.96

Difference

 -

 10mm Drop

 10mm Drop

 10mm Drop

 

 

SECTION B-B

 

 

Distance From Centre of Twin Tunnels

60mS

-

60mN

Total Head (mPD)

Initial Condition

3.80

3.56

3.38

With Twin Tunnels

3.80

3.55

3.37

Difference

 -

 10mm Drop

 10mm Drop

 

 

SECTION C-C

 

 

Distance From Centre of Twin Tunnels

60mS

-

60mN

Total Head (mPD)

Initial Condition

3.85

3.66

3.45

With Twin Tunnels

3.86

3.66

3.41

Difference

 10mm Rise

 -

 40mm Drop

 

 

SECTION D-D

 

 

Distance From Centre of Twin Tunnels

60mS

30mS

20mS

10mS

70mN

90mN

Total Head (mPD)

 

Initial Condition

3.62

3.37

3.30

3.24

2.48

2.24

With Twin Tunnels

3.72

3.54

3.48

3.45

2.26

2.07

Difference

100mm Rise

170mm Rise

180mm Rise

210mm Rise

220mm Drop

170mm Drop

With Twin Tunnels

3.62

3.37

3.30

3.24

2.47

2.12

Difference

-

-

-

-

10mm Drop

120mm Drop

 

 

3.6              The sensitivity analyses indicate that only minor changes in groundwater table would be resulted under i) a more permeable CDV layer and ii) the transient period. Therefore, groundwater drawdown due to the presence of the twin tunnel so as to adversly impact either the local ecology or the agriculture would be insignificant, as compared to other factors, such as seasonal and tidal (Neap/Spring) variations or local drawdown due to pumping water from wells for irrigation purposes.

 

 

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