The NT2 impacts on the Tonle Sap system were by flu11339

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									4.5 Scenario Simulations
4.5.1 NT2 Impact Simulations
The NT2 impacts on the Tonle Sap system were simulated by substituting the Kratie input
hydrograph from baseline to the post-NT2 hydrograph for Kratie. Figure 14 shows the Kratie
hydrographs for the baseline situation and for the post-NT2 situation. The latter is the result
of simulations with MikeBasin, as accounted for in Section 3.1.2. Only a small time window is
shown in order to make the difference more visible, since the difference is very small in
proportion to the actual discharge.
Figure 15 shows the simulated post-NT2 water levels in Tonle Sap. For clarity, only a three-
year time window is shown. Figure 16 below shows the difference between the post-NT2
water levels and the baseline situation, i.e. the net impact due to NT2. It is seen that in
September 1969, when the water level peaks at around 7.5 masl, there is a water level
reduction of 8 cm due to NT2. The following two years, the reduction of the maximum water
level are smaller, only 5 and 3 respectively. Figure 35 shows the difference for the entire 50
years simulation period. It is seen that there is a very large variation of the impact from year
to year.
Figure 17 shows a summary plot of the NT2 impact on the lake level. The figure shows, that
in September, the mean reduction of the lake level will be 3-4 cm. In 10 % of the years
however, the reduction may exceed 6 cm.



                                         EcoLao
Asian Development Bank                                                                    Government of Lao PDR
Cumulative Impact Analysis                                                                  Annex 2: Hydrology                                                  Page 17

                                           Kratie baseline [m^3/s]
                                           Kratie post NT2 [m^3/s]
                                                                                       Discharges at Kratie (input to hydraulic simulations)
                                35000

                                30000

                                25000
Discharge m3/s




                                20000

                                15000

                                10000

                                 5000


                                              Feb      Mar        Apr    May     Jun     Jul   Aug    Sep     Oct    Nov    Dec     Jan   Feb    Mar     Apr    May     Jun    Jul
                                              1953     1953      1953    1953   1953    1953   1953   1953   1953    1953   1953   1954   1954   1954   1954    1954   1954   1954


Figure 14                                               Input to hydraulic simulations, discharge at Kratie. Baseline and post-NT2


                                       Lake level post NT2 [m]
                                                                                       NT2 Impact: Post-NT2 Tonle Sap Water Levels
                               9.0

                               8.0

                               7.0

                               6.0
level [masl]




                               5.0

                               4.0

                               3.0

                               2.0

                               1.0
                                                                 1969                                        1970                                        1971



Figure 15                                               Daily Simulated Tonle Sap water level: Post-NT2.
.
                                        Change [m]
                                                                                       NT2 Impact: Change of Tonle Sap Water Levels
                               0.06

                               0.04
meters change of water level




                               0.02

                               0.00

                               -0.02

                               -0.04

                               -0.06

                               -0.08
                                                                  1969                                        1970                                       1971



Figure 16       NT2 impact on Tonle Sap lake levels: Simulated change of lake level (post-NT2
minus baseline). Negative figures means reduced level. Should be related to Figure 15.




                                                                                                EcoLao
Asian Development Bank                                                     Government of Lao PDR
Cumulative Impact Analysis                                                   Annex 2: Hydrology                                      Page 18



                                                             NT2: Change of Tonle Sap Water Levels,
                                                                        The Great Lake
                      0.06


                      0.04
  Changed level (m)




                      0.02


                      0.00


                      -0.02                                    90%
                                                               Mean
                      -0.04                                    10%


                      -0.06
                                   Jan          Feb      Mar         Apr      May    Jun     Jul     Aug      Sep      Oct     Nov     Dec


Figure 17    Summary of simulations 1950-2000 of NT2 impacts on Tonle Sap lake levels. Mean,
10% and 90% percentiles of 50 years of simulations.



                                                                Annual Maximum Water Levels,
                                                                       Tonle Sap Lake
                      9.0
                                                                                                               Baseline
                      8.5                                                                                      post-NT2


                      8.0
  m above sea level




                      7.5


                      7.0


                      6.5


                      6.0
                            1950         1955         1960      1965         1970     1975    1980     1985         1990     1995     2000


Figure 18                            Annual Maximum Water Level, Tonle Sap. Baseline and post-NT2.




                                                                               EcoLao
Asian Development Bank                                                  Government of Lao PDR
Cumulative Impact Analysis                                                Annex 2: Hydrology                                                   Page 19



4.5.2 Cumulative Impacts 2010 and 2025
Input to the hydraulic simulations were the results from the MikeBasin water balance
simulations in terms of the discharge at Kratie, which is shown as Figure 19. The hydraulic
model simulated the response of the Tonle Sap/Mekong system due to the changed
upstream conditions. The resulting simulated water levels of Tonle Sap are shown as Figure
20 for a three-years time window together with the baseline simulation.
As a summary of the simulation results, Figure 21 shows the impact variation over the
average year of the 50 years of simulations. The annual maximum lake level (October-
September) is reduced by around 20-25 cm in 2010 and by 55-60 cm in 2025. It should be
noted that the impacts are largest in September, but the lake level is highest more often in
October than in September.



Table 4 gives a summary of the most essential impacts as results of NT2, Cumulative
Impacts 2010 and 2025, respectively.


                         Baseline                           [m^3/s]
                         Simulation Result: Cumulative 2010 [m^3/s]
                         Simulation Result: Cumulative 2025 [m^3/s]
                                                                                    Mekong Discharge at Kratie
              35000

              30000

              25000

              20000

              15000

              10000

               5000

                     0
                          Feb      Mar         Apr    May      Jun     Jul   Aug     Sep     Oct     Nov    Dec     Jan   Feb    Mar     Apr   May     Jun    Jul
                          1953     1953       1953    1953    1953    1953   1953    1953   1953     1953   1953   1954   1954   1954   1954   1954   1954   1954

Figure 19      Input to hydraulic simulations: Discharge at Kratie as a result of cumulative impacts
2010 and 2025.


                    Lake level Baseline [m]
                    Lake level 2010     [m]
                    Lake level 2025     [m]
                                                                       Cumulative Impacts: Tonle Sap Water Levels

              8.0


              6.0
water level




              4.0


              2.0


              0.0
                                               1969                                           1970                                         1971

Figure 20                            Simulated Tonle Sap water levels: Cumulative Impacts 2010 and 2025.




                                                                              EcoLao
Asian Development Bank                                           Government of Lao PDR
Cumulative Impact Analysis                                         Annex 2: Hydrology                                              Page 20



                                              Cumulative Impacts Change of Tonle Sap Water Levels,
                                                                 The Great Lake
                     0.6
                     0.5
                     0.4
                     0.3
                                                                                                     2010
 Changed level (m)




                     0.2                                                                             2025
                     0.1
                     0.0
                     -0.1
                     -0.2
                     -0.3
                     -0.4
                     -0.5
                     -0.6
                                 Jan     Feb       Mar   Apr     May     Jun          Jul   Aug     Sep      Oct    Nov      Dec


Figure 21    Change of Tonle Sap water level. Average of entire 50 years simulation period, 2010
and 2025 Cumulative Impacts.



                                                         Annual Maximum Water Levels,
                                                                Tonle Sap Lake
                     10

                      9

                      8

                      7
 m above sea level




                      6

                      5

                      4
                                                                                                  Baseline
                      3
                                                                                                  2010
                      2                                                                           2025

                      1

                      0
                          1950         1955       1960    1965    1970         1975       1980      1985     1990     1995
                                                                                        month

Figure 22    Annual Maximum Water Level in Tonle Sap (September or October). Baseline, 2010
and 2025 Cumulative Impacts.




                                                                       EcoLao
Asian Development Bank                Government of Lao PDR
Cumulative Impact Analysis              Annex 2: Hydrology                                Page 21




Table 4         Summary of key simulation results for impacts on Tonle Sap Lake: Wet Season
                 Average                   Average                     Average
                 Annual                    Annual                      Annual
                 Maximum Q   Discharge     Maximum       Level         Maximum       Area
                 at Kratie   Change        Lake Level    Change        Lake Area     Change

                 [m3/s]      [%]           [masl]        [cm]          [km2]         [km2]

Baseline         35,250                    7.64                        9895

                                                                                               2
Post-NT2         35,050      -0.6%          7.61          -3 cm        9855           -40 km

                                                                                               2
CIA 2010         33,565       -5%          7.42          -22 cm        9540          -355 km

                                                                                               2
CIA 2025         31,020      -17%          7.10           -54 cm       9030          -865 km




Table 5         Summary of key simulation results for impacts on Tonle Sap Lake: Dry Season.
                 Average                   Average                     Average
                 Annual                    Annual                      Annual
                 Minimum Q   Discharge     Minimum       Level         Minimum       Area
                 at Kratie   Change        Lake Level    Change        Lake Area     Change

                 [m3/s]      [%]           [masl]        [cm]          [km2]         [km2]

Baseline         2000                      0.89                        2410

                                                                                               2
Post-NT2         2040           +2%         0.91          +2 cm        2420           +10 km

                                                                                               2
CIA 2010         3050         +53%          1.14         +25 cm        2555          +145 km

                                                                                               2
CIA 2025         4470        +124%          1.52         +63 cm        2785          +375 km




                                           EcoLao
Asian Development Bank                         Government of Lao PDR
Cumulative Impact Analysis                       Annex 2: Hydrology                        Page 22



4.5.3 Impacts in the Delta
In the present baseline situation, saltwater intrusion is an important problem in the Delta. The
mechanism that is causing saltwater intrusion is the combination of an extremely low
gradient through the Delta, i.e. for around 300 km from the Sea, and the rather high tidal
amplitudes, namely around 3m (+/- 1.5m). When discharges are small in the dry season, the
high tides cause an inflow of seawater into the rivers. The tidal oscillations are measured as
far upstream as Phnom Penh, although the saltwater itself does not propagate so far
upstream. Flow reversal, i.e. flow going upstream, occurs as far upstream as Tan Chau and
Chau Doc close to the Vietnamese/Cambodian border. Figure 23 below shows observed
discharges at Tan Chau and downstream and illustrates the frequent occurrence of negative
flows at My Thuan.

                                 Comparison of Hourly Discharges on Mekong
                                          (June 10 - July 15, 1992)
                    16,000                                                          My Thuan
                                                                                    Tan Chau


                    12,000



                     8,000
 Discharge (m3/s)




                     4,000



                        0



                    -4,000



                    -8,000
                             0         240                    480      720                     960
                                             Time in hour (36 days)

Figure 23     Observed Mekong discharges under tidal influence. Stations Tan Chau at
Cambodian/Vietnamse border and my Thuan 90km further downstream. Source:WUP-JICA 2003.


Due to the regulation by hydropower, river discharges through the Delta are increased in the
dry season. The increased dry season discharge reduces saltwater intrusion. The results
from the hydraulic modelling are used to quantify the reduction of saltwater intrusion. It
should be noted that the model is somewhat simplified in the Delta, by reducing the large
number of parallel river branches to five model river branches. The results should therefore
be considered as indicative. The main focus is however the difference between baseline and
scenario simulation, and it is believed that the inaccuracy in the difference, i.e. net impact, is
less significant.
Figure 24 below shows the tidal discharge variations in an example time interval in the dry
season, when seawater intrusion is significant (large negative discharges). The location is at
the beginning of the Delta. The figure shows simulated river discharges at 6 hour intervals,
thus illustrating the in- and outflow of seawater in terms of negative, resp. positive
discharges. In the shown example, discharges are above 4500 m3/s in 3 out 4 timesteps (18



                                                    EcoLao
Asian Development Bank                Government of Lao PDR
Cumulative Impact Analysis              Annex 2: Hydrology                                Page 23


of 24 hours) every day, but in one time step (6 hours interval) the discharges are negative
and between 1000 and 2000 m3/s most of the time.
Concerning the impact of NT2, the figure shows that the generally increased dry-season flow
which is a result of the regulation, generally “lifts” the NT2 curve. This implies that the
negative discharges become “less negative”. In order words the seawater intrusion is
reduced. In the shown example, the inflow is reduced by approximately 300 m3/s. The
change of the negative flows is outlined by the distance between the two envelope curves
through the lowest marker points for the two simulations (baseline and post-NT2, resp.).
The above example was presented as an illustration. At other times of the year, the impact
may be different. Therefore a more representative way to assess the impact, is by looking at
duration curves, i.e. the relation between a discharge value and the proportion of time that
flows are smaller than that value. By inspecting the value zero, the duration curve yields the
proportion of time with negative flows, i.e. seawater intrusion. The impact on saltwater
intrusion by scenarios can be expressed as a changed duration of negative flows.
Figure 25 shows the Flow Duration Curve computed for Tan Chau at the beginning of the
Delta at the Cambodian/Vietnamese boundary. The input data to each curve are the
discharge time series simulated for a 50 years period (1950-2000) with 6 hours time step.
The figure however only shows part of the curve for discharges below 6,000 m3/s, i.e. dry
season flows. It is clearly visible that more drastic scenarios have more impact in terms of
increasing the discharges (in the dry season) and correspondingly, reducing the duration of
time with negative discharges.
For a closer look at the impact on seawater intrusion, Figure 26 provides a zoom to the
smallest and negative discharges. The Baseline duration curve (bold) crosses the Q=0 m3/s
horizontal line at 9.5%, i.e. in the baseline situation, the flow direction is upstream in 9.5% of
time. The Post-NT2 curve (located closely to the left of the Baseline curve) intersects the
zero line at 9.3%. This means that negative discharges occur 2% less time. This is a
beneficial impact, although marginal.
For the two Cumulative Scenarios, the impacts are seen more easily. As shown by Figure
26, the duration curve for the 2010 scenario intersects the Q=0 m3/s at 8.4% and for the
2025 scenario at 6.1%. This corresponds reductions of the time with negative discharges by
12% and 36%, respectively. This means substantial beneficial impacts with respect to
saltwater intrusion.




                                          EcoLao
Asian Development Bank                                                           Government of Lao PDR
Cumulative Impact Analysis                                                         Annex 2: Hydrology                                                           Page 24


                                 8000


                                 7000


                                 6000


                                 5000
       Total Discharge (m3/s)




                                 4000


                                 3000


                                 2000                                                                                                  Baseline
                                                                                                                                       NT2

                                 1000


                                       0


                                -1000


                                -2000
                                       01/01/50                              11/01/50                                 21/01/50                                   31/01/50



Figure 24       Simulated tidal oscillations of discharge in the Mekong Delta. Two lines outline the
“envelope” of negative flows for the two scenarios: pre- and post-NT2.



                                                                             FLOW DURATION CURVES:
                                                                      Discharge at Tan Chau (Vietnam/Cambodia border)
                                6000
                                              baseline
                                5000          NT2
                                              2010
                                              2025
                                4000


                                3000


                                2000                                                                                                      Freshwater to
                                                                                                                                             the Sea
  Discharge




                                1000


                                  0


                        -1000

                                                                                                                        Seawater
                        -2000
                                                                                                                        intrusion

                                                         Zoom frame
                        -3000
                                                         for next figure

                        -4000
                                       0%          5%    10%               15%          20%        25%          30%              35%      40%             45%          50%
                                                                                              Time proportion



Figure 25       Flow Duration Curve (Discharge interval <6000 m3/s, i.e. dry season). Illustrates
scenario impacts on flow duration: Duration of negative discharges is reduced due to regulation. The
bold frame indicates the area that is zoomed to on Figure 26.



                                                                                        EcoLao
Asian Development Bank                                 Government of Lao PDR
Cumulative Impact Analysis                               Annex 2: Hydrology                                       Page 25




                                                        FLOW DURATION CURVES:
                                                Discharge at Tan Chau (Vietnam/Cambodia border)
              2000



              1500

                                2025 Scenario
              1000



               500

                                                                                                          Baseline
  Discharge




                 0



               -500                                                                                           baseline
                                                                                                              NT2
                                                                                                              2010
              -1000
                                                                                                              2025


              -1500
                                                       Post NT2


              -2000
                      5%   6%                   7%                8%                     9%       10%   11%              12%
                                                                       Time proportion



Figure 26       Flow Duration Curve (Discharge interval <2000 m3/s). Illustrates scenario impacts on
flow duration: Duration of negative discharges is reduced due to regulation..




                                                             EcoLao

								
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