Appendix A-7
DSF Demonstration Scenario No.7:
Floodplain Embankments
Mekong River Commission - Water Utilisation Project Component A:
Development of Basin Modelling Package and Knowledge Base (WUP-A)
Technical Reference Report:: DSF 650 DSF Testing and Evaluation
Contents
7
Scenario 7: Impact of Floodplain Embankments
7.1 Scenario Objective
7.2 Specification of Scenarios in the DSF
7.2.1 Climate Component
7.2.2 System Demand Component
7.2.3 Assumed Interventions Component
7.3 Model Setup
7.3.1 SWAT Models
7.3.2 IQQM Model
7.3.3 iSIS Model
7.4 Demonstration Scenario 3: Impact of Catchment Cover Change – Test
Results: Primary Level
7.5 Scenario 7: Embankment Scenario – Test Results: Secondary Level
7.5.1 Test 1: Acceptable Minimum Monthly Dry Season
Flow
7.5.2 Test 2: Acceptable Reverse flow of Tonle Sap in Wet
Season
7.5.3 Test 3: Average Daily Peak Main Stream Flow in the
Flood Season
7.5.4 Test 4: Extent and Duration of Flooding
7.5.5 Test 5: Extent and Duration of Saline Intrusion
7.5.6 Test 6: Irrigated agriculture performance
7.5.7 Test 7: Degree of Connection for Fisheries Purposes
7.5.8 Impacts on flows in the tidal areas
7.6 Conclusions and Recommendations
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A.7-ii
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Figures
Figure A 7-1:
Location of the 150 km long embankment intervention (pink line)
................................................................................................................... 2
Figure A 7-2
Maps of inundated area downstream Kratie for peak flood
conditions in year 2000 (Baseline Conditions and Embankment
scenario) ................................................................................................... 8
Figure A 7-3
Map showing the difference in inundated area downstream Kratie
for peak flood conditions in year 2000 (Baseline Conditions verses
Embankment scenario).......................................................................... 9
Figure A 7-4
Inundation-depth maps for >0.5 m depth during peak flood
conditions in year 2000 (Baseline Conditions and Embankment
scenario) .................................................................................................10
Figure A 7-5
Map showing the difference in the depth-duration >0.5 m for
year 2000 (Baseline Conditions verses Embankment scenario)....11
Figure A 7-6
Map showing the extent of Embankment scenario saline intrusion
for maximum dry-season salinity intrusion in year 2000................13
Figure A 7-7
Map showing the difference in saline intrusion for maximum dryseason salinity intrusion in year 2000 (Baseline Conditions verses
Floodplain Embankments) .................................................................13
Figure A 2-12
Comparison of flows at Chau Doc for year 2000 (Baseline
Conditions versus LMB dams scenario) ...........................................14
Figure A 2-13
Comparison of flows at Tan Chau for year 2000 (Baseline
Conditions versus floodplain embankments scenario)...................15
Figure A 2-14
Comparison of flows at Phnom Penh (Mekong) for year 2000
(Baseline Conditions versus floodplain embankments scenario)..15
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A.7-iii
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Tables
Table A 7-1: Maximum inundation areas by depth class in the year 2000 ............11
Table A 7-2: Inundation duration > 0.5 m depth for the year 2000 ......................12
Table A 7-3: Maximum dry-season salinity intrusion areas .....................................14
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A.7-iv
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7
Scenario 7: Impact of Floodplain
Embankments
7.1
Scenario Objective
The objective is to assess the effect of restricting inundation of the floodplain
downstream of Kratie. It is analysed in terms of impacts in both the wet and dry
seasons with respect to flood levels and depth-durations and salinity penetration.
There is ample evidence to demonstrate that there is a slow but accelerating pace of
floodplain fragmentation and alienation from flooding. In the Cambodian portion of
the floodplain, this is occurring principally through such developments as upgrading of
existing roads, construction of new roads and new irrigation channels, rather than
deliberate flood control works. In Vietnam on the other hand, it is the construction of
irrigation channels, levees and regulators that cause restrictions to flooding and indeed
to salinity penetration in the delta.
Of course, these changes have both positive and negative aspects, such as for
agriculture, aquaculture and capture fisheries. It is up to the users of the DSF to
determine what their planning objectives are and to correspond these with specific
assessment criteria. This then needs to lead on to the identification of appropriate
indicators in each case that can be simulated by the DSF models, together with the
thematic layers stored in its Knowledge Base.
Of all the possible interventions that could and should be modelled, this demonstration
scenario is but one. Its sole purpose is to demonstrate how it can be done and the scale
of impact if an (unlikely) project was undertaken was made to protect the entire left
bank floodplain in Cambodia from Kampong Cham to downstream of Phnom Penh
where Cambodian National Road No.1 crosses the Mekong River.
7.2
Specification of Scenarios in the DSF
7.2.1
Climate Component
There are no differences compared with the Baseline Scenario.
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A.7- 1
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7.2.2
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System Demand Component
There are no differences in system demands compared to the Baseline Scenario. Any
changes that would in fact take place by virtue of making an large area of the floodplain
flood free, have not been taken into account in this scenario.
7.2.3
Assumed Interventions Component
The map in Figure A 7-1 shows the 150 km long embankment in Cambodia. It extends
from upstream of Kampong Cham to Cambodian National Road No.1 where it crosses
the Mekong River. Its height was set to a very high level such that it would never be
overtopped during floods. The majority of the area that is leveed off by the
embankment is agricultural land, although there are some areas of reed and grassland
wetlands and forest. Towards its downstream end there is one long permanent water
body.
Two areas of flow constriction would be created, one adjacent to Kampong Cham and
one adjacent to Phnom Penh. It is the latter constriction that is likely to raise water
levels in Tonle Sap Lake and cause additional flooding in and around Phnom Penh.
Figure A 7-1:
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Location of the 150 km long embankment intervention (pink line)
A.7- 2
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7.3
Model Setup
7.3.1
SWAT Models
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Model Configuration
No changes were made, however, it is worth noting that the SWAT sub-basins that abut
the iSIS hydrodynamic model extend only to the wet-season flood limit, i.e the area
covered by the iSIS schematisation. Any rain falling on the floodplain, between the
river and the SWAT sub-basins when not flooded, is ignored in iSIS as though it all
soaks into the ground; the ground being very flat and dry after the dry season. Thus
for any particular floodplain area, rain is only added to the water balance when river
flood waters inundate that location.
Thus when the embankment is introduced, a large area of the floodplain is made
permanently free of river flooding, but rain falling on it is not taken into account.
Whilst this may have been a reasonable assumption under Baseline Conditions, as the
period of the year when rain falls on a dry floodplain prior to river flooding is relatively
short at the beginning of the wet season, it may not be appropriate under such extreme
flood mitigation scenarios as this one, where rain falling on the protected part of the
floodplain is unaccounted for over the entire wet season.
Model Input Variables
No changes required.
7.3.2
IQQM Model
Model Configuration
Whilst no changes were made in this demonstration scenario, in practice many changes
could be anticipated that should reasonably be modelled as an integral part of such a
development. This might include such things as increased dry-season irrigation in the
protected area, increased wet season irrigation demands, inclusion of the rain falling on
the protected floodplain area in the wet season, even though not flooded (see s.2.3.1
above) and changes in socio-economic conditions, including population growth and
distributions.
Model Input Variables
No changes required.
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7.3.3
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iSIS Model
Model Configuration
The variables remain the same as for the Baseline Conditions scenario, with the sole
differences being the raised elevation of the new embankment. The embankment was
located such that it used existing features included in the model, such as the natural
river levee, roads and other structures. Hence, no changes to the cell or node structure
of the model were required.
Model Input Variables
Other than changing the height values (‘z’ values) of the above mentioned features that
were used to form the raised embankment, no other changes were required.
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7.4
Demonstration Scenario 3: Impact of Catchment Cover Change – Test Results:
Primary Level
Tests 1 to 3 : MRC Agreement, Article 6 requirements
Tests against Article 6
Scenario
conforms?
Most Critical
Location?
By how much?
Test 1:
Acceptable Minimum Monthly Dry
Season Flow
Yes
N/A
No changes
Test 2:
Acceptable Reverse flow of Tonle
Sap in Wet Season
Yes
Tonle Sap
Lake levels are raised by up to 0.5 m over
most of the lake, a result of increased
flow reversal
Test 3:
Average Daily Peak Main Stream
Flow in the Flood Season
No
Kampong
Cham
Peak wet season river levels are raised
by up over 1.0 m near Kampong Cham
Test 4: Extent and Duration of Flooding (km2) – downstream of Kratie. Awaiting iSIS results
Flooded Area >0.5 m by Duration (km2)
Year 2000 (Wet Year)
Scenario
Baseline Conditions
> 1 day
59,300
> 1 mth
55,444
> 2 mths
51,687
> 4 mths
43,679
> 6 mths
29,650
Embankment Scenario
Difference
Area (km2)
Difference
Percent (%)
Test 5: Extent and Duration of Saline Intrusion (km2) – in the delta. Awaiting iSIS results
Maximum Saline Intrusion by Salinity Class (km 2)
Jan 2000 – June 2000
Scenario
> 1 g/l
Baseline Conditions
21,603
> 4 g/l
19,287
> 8 g/l
17,279
> 15 g/l
14,195
Embankment Scenario
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Difference
Area (km2)
Difference
Percent (%)
A.7- 5
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Test 6: Irrigated agriculture performance
Total Deficit Upstream of Kratie
(million hectare days)
Baseline Conditions
42.77
Scenario
Illustrations of approach in Scenarios (1) – (3)
Difference (million hectare days)
-
Difference (%)
-
Test 7: Degree of Connection for Fisheries Purposes
Maximum Connected River Length (km)
Baseline Conditions
45,571
Scenario 7: Embankment scenario
Difference (Km or
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Maximum Flooded Area (km2)
43,328
45,571
Km2)
0
Difference (%)
0
A.7- 6
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7.5
Scenario 7: Embankment Scenario – Test Results: Secondary Level
7.5.1
Test 1: Acceptable Minimum Monthly Dry Season Flow
This and subsequent time-series tests below, were not conducted as the results are the
same as the Baseline Conditions for all mainstream assessment sites from Kratie
upstream. Downstream of Kratie, iSIS has only been run for the year 2000 at the time
of the demonstration testing, hence it is not possible to provide flow regime-based
comparisons for sites downstream of Kratie as the IQQM river simulation model ends
at the upstream end of the iSIS hydrodynamic model.
7.5.2
Test 2: Acceptable Reverse flow of Tonle Sap in Wet Season
As above, the results of the Embankment scenario will be the same as for Baseline
Conditions for peak wet season flows at Kratie (the basis for determining if an
acceptable level of flooding is achieved in Tonle Sap).
Maps and tabulated data of flood depth, depth-durations and changes to these
indicators between the Embankment scenario and Baseline Conditions, are provided in
s.2.5.4 below.
7.5.3
Test 3: Average Daily Peak Main Stream Flow in the Flood Season
As above, the results of the Embankment scenario will be the same as for Baseline
Conditions for peak wet season flows from Kratie upstream. Changes downstream of
Kratie have been explored through the various spatial analyses presented in the sections
below.
7.5.4
Test 4: Extent and Duration of Flooding
The extent of flooding is shown in Figure A 7-2 for both Baseline Conditions and the
Embankment scenario. The difference in water levels is shown in Figure A 7-3. The
triangular yellow area (i.e. agriculture) below Kampong Cham in the Baseline map is an
artefact of the DeltaMapper process. Most of that area would in fact be flooded. With
more time, users can avoid such problems when creating the TIN used by DeltaMapper
to map the point data simulated by iSIS. However, the same aberration is present in all
the maps generated in the demonstration testing process (although not evident in the
Embankments scenario due to the absence of flooding in that area) and accordingly,
between scenario comparisons of tabulated areas calculated from the maps are true
measures of relative changes (but not absolute areas).
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Baseline
Conditions
Embankment
scenario
Figure A 7-2
106750408
Maps of inundated area downstream Kratie for peak flood conditions in year 2000
(Baseline Conditions and Embankment scenario)
A.7- 8
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Figure A 7-3
Mekong River Commission - Water Utilisation Project Component A:
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Technical Reference Report:: DSF 650 DSF Testing and Evaluation
Map showing the difference in inundated area downstream Kratie for peak flood
conditions in year 2000 (Baseline Conditions verses Embankment scenario)
The changes in flooding are clearly apparent in the difference map (Figure A 7-3)
above. Levels are as much as 2 m lower over large areas of Cambodia and Vietnam
protected by the embankment and in some areas more than 4 m lower. These
reductions are offset by increased water levels in the remaining inundated areas from
Kratie downstream, being most severe (>1 m) in the vicinity of Kampong Cham where
it can be seen the flow is highly constricted between the new embankment and the high
ground to the north of the river. Height increases in Tonle Sap Lake are less, but
substantial, at 0.5 – 0.55 m higher over the whole lake.
Figure A 7-5 below shows the difference in duration of inundation for areas receiving at
least 0.5 m depth of flooding. Durations are typically reduced by two months
throughout the area protected by the embankment and by up to four months in some
areas that flooded for long periods under Baseline Conditions.
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Within Tonle Sap Lake the changes are the reverse of those above, the increases in
flood duration being indicated by blue colours. Increases in durations of 14 to 30 days
occur in a narrow band around the periphery of the lake, whilst changes in other areas
are generally less than 14 days, whilst the center of lake remains unchanged.
Baseline
Conditions
Embankment
scenario
Figure A 7-4
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Inundation-depth maps for >0.5 m depth during peak flood conditions in year 2000
(Baseline Conditions and Embankment scenario)
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Figure A 7-5
Map showing the difference in the depth-duration >0.5 m for year 2000 (Baseline
Conditions verses Embankment scenario)
The area of each depth class and differences tabulated in Table A 7-1. The values
substantiate the differences observed in the above maps. The Embankment scenario
decreases the total area flood by 3.4%, although in terms of depth classes, the greatest
decrease of 4.3% is seen for the areas having depths greater than 0.5 m.
Table A 7-1:
Maximum inundation areas by depth class in the year 2000
Flooded Area by depth class (km 2)
Year 2000 (Wet Year)
Scenario
Baseline Conditions
>0m
Embankment scenario
106750408
(km2)
Difference
Area
Difference
Percent (%)
>0.5 m
> 1.0 m
> 2.0 m
> 4.0 m
43,328
42,003
38,612
29,664
16,409
41,847
-1,481
40,180
-1,823
37,501
-1,111
29,350
-314
16,728
318
-3.4
-4.3
-2.9
-1.1
1.9
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As observed in Table A 7-2 the reduction in flood duration at locations having at least
0.5 m flooded depth is more uniform across the depth-duration classes but is never
greater than 5.2%.
Table A 7-2:
Inundation duration > 0.5 m depth for the year 2000
Flooded Area >0.5 m by Duration (km2)
Year 2000 (Wet Year)
Scenario
> 1 day
> 1 mth
> 2 mths
Baseline Conditions
59,300
55,444
51,687
43,679
29,650
Embankment scenario
56,225
52,668
49,461
42,014
28,112
-3,075
-2,776
-2,226
-1,664
-1,538
-5.2
-5.0
-4.3
-3.8
-5.2
7.5.5
Difference
Area (km2)
Difference
Percent (%)
> 4 mths
> 6 mths
Test 5: Extent and Duration of Saline Intrusion
Figure A 7-6 below shows the maximum saline intrusion area under the Embankment
scenario in the year 2000. The differences are shown as zones of coloured bands in the
difference map Figure A 7-7, from which areas of change can be seen on the west and
east sides of the delta. The total area of each salinity threshold class is provided in
Table A 7-3 which also presents the differences to the Baseline Scenario.
Whilst there is no more than a 1% decrease in any salinity class area, interestingly all
changes are towards fresher conditions, except for a very small area in the west of the
delta. Intuition might have lead to the reverse case being assumed prior to the analysis
because of the large area of reduced flooding in the eastern portion of the Cambodian
floodplain. However, what is driving the trend to fresher conditions in the delta is the
0.5 m peak flood levels in Tonle Sap. This provides a correspondingly higher volume
that takes longer to discharge in the dry season. Thus flows are slightly higher
downstream of Phnom Penh (not analysed in this demonstration scenario report, but
the data are in the DSF for interest parties to analyse them for themselves), even into
the driest months.
106750408
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Figure A 7-6
Map showing the extent of Embankment scenario saline intrusion for maximum dryseason salinity intrusion in year 2000
Figure A 7-7
Map showing the difference in saline intrusion for maximum dry-season salinity
intrusion in year 2000 (Baseline Conditions verses Floodplain Embankments)
106750408
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Table A 7-3:
Maximum dry-season salinity intrusion areas
Maximum Saline Intrusion by Salinity Class (km 2)
Jan 2000 – June 2000
Scenario
> 1 g/l
Baseline Conditions
Embankment Scenario
7.5.6
Difference
Area
Difference
Percent (%)
> 4 g/l
> 8 g/l
> 15 g/l
21,603
19,287
17,279
14,195
21,477
19,155
17,154
14,083
-125
-131
-125
-112
-0.6
-0.7
-0.7
-0.8
(km2)
Test 6: Irrigated agriculture performance
See Scenarios (1) – (3) for demonstration of this test.
7.5.7
Test 7: Degree of Connection for Fisheries Purposes
There is no change in the maximum longitudinal fish migration network extent as there
were no physical barriers introduced that would reduce it.
Changes in lateral connectivity to the floodplain can be observed through the
inundation and inundation-duration maps and tabulated data presented above.
7.5.8
Impacts on flows in the tidal areas
Impacts of LMB dams scenario on flows below Kratie are illustrated by reference to
representative flows in the year 2000 at Chau Doc, Tan Chau and Phnom Penh
(Mekong) in the figures and table below.
Figure A 7-8 Comparison of flows at Chau Doc for year 2000 (Baseline Conditions versus LMB
dams scenario)
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Time Series Analysis Tool
10,000
9,000
8,000
Flow(cumecs)
7,000
6,000
5,000
4,000
3,000
2,000
1,000
15/01/2000
15/03/2000
14/05/2000
13/07/2000
[S1] Chau Doc: Flow
11/09/2000
10/11/2000
[S7] Chau Doc: Flow
01/Jan/2000 - 30/De c/2000
Figure A 7-9 Comparison of flows at Tan Chau for year 2000 (Baseline Conditions versus floodplain
embankments scenario)
Time Series Analysis Tool
28,000
26,000
24,000
22,000
Flow(cumecs)
20,000
18,000
16,000
14,000
12,000
10,000
8,000
6,000
4,000
2,000
15/01/2000
15/03/2000
14/05/2000
13/07/2000
[S1] Tan Chau: Flow
11/09/2000
10/11/2000
[S7] Tan Chau: Flow
01/Jan/2000 - 30/De c/2000
Figure A 7-10 Comparison of flows at Phnom Penh (Mekong) for year 2000 (Baseline Conditions
versus floodplain embankments scenario)
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Time Series Analysis Tool
42,000
40,000
38,000
Flow(cumecs)
36,000
34,000
32,000
30,000
28,000
26,000
24,000
22,000
20,000
18,000
16,000
14,000
12,000
10,000
8,000
6,000
4,000
2,000
15/01/2000
15/03/2000
14/05/2000
13/07/2000
[S1] Phnom Penh (Mekong): Flow
11/09/2000
10/11/2000
[S7] Phnom Penh (Mekong): Flow
01/Jan/2000 - 30/De c/2000
Table A 7-4: Impacts of floodplain embankments scenario on flows and water levels at Chau Doc,
Tan Chau and Phnom Penh (Mekong) for year 2000
Flow (m3/s)
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Stage (mAD)
Max
Value
Max
Date
Min
Value
Min
Max
Date
Min
Value
Min
Date
Max
Value
Phnom Penh Mekong upstream
– Floodplain Embankments
43,323
20/07
1,633
03/04
10.37
15/09
0.79
02/04
Phnom Penh Mekong upstream Baseline
40,815
18/07
1,632
03/04
9.98
16/09
0.79
02/04
Phnom Penh Mekong upstream Difference
2,508
2 days
1
0 days
0.39
-1 day
0.00
0 days
Tan Chau – Floodplain
Embankments
27,999
16/9
1,811
03/04
5.60
18/09
0.37
01/04
Tan Chau - Baseline
28,063
20/9
1,826
03/04
5.61
19/09
0.37
01/04
Tan Chau - Difference
-64
-4 days
-15
0 days
-0.01
-1 day
0.00
0 days
Chau Doc - Floodplain
Embankments
10,527
21/09
71
06/04
5.12
20/09
0.18
29/04
Chau Doc - Baseline
9,813
23/09
73
06/04
4.92
22/09
0.18
29/04
Chau Doc - Difference
714
-2 days
-2
0 days
0.20
-2 days
0.00
0 days
Date
A.7- 16
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7.6
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Conclusions and Recommendations
Substantial changes in flood patterns are observed for this scenario, as would be
expected from such a dramatic intervention. The most significant changes are the
major reductions in flood depths and durations in the area protected by the floodplain
embankment, i.e. that area to the south of Kampong Cham and to the east of Phnom
Penh.
Balancing these decreases are the increases in Mekong River and Tonle Sap Lake levels.
An unforeseen impact is the (slight) decrease in saline intrusion in the delta due to the
higher volume stored in Tonle Sap. This additional volume is discharged over the dry
season months, including the driest month corresponding to the time of maximum
saline intrusion. It is the constricted flow in the river reach between Kampong Cham
and Phnom Penh that leads to the higher levels in the lake. These increases would not
be without consequence, as can be seen from the difference map (Figure A 7-3) that
many roads would be inundated for longer periods and to deeper levels, as would
villages around the lake. A more comprehensive analysis and presentation could
enumerate the number of villages and people in them, as well as impacts on flooded rice
and other socio-economic activities.
106750408
A.7- 17