IKMP Discharge and Sediment Monitoring Program Review, Recommendations and Data Analysis

Transcription

1 IKMP Discharge and Sediment Monitoring Program Review, Recommendations and Data Analysis Part 2: Data analysis of preliminary results L. Koehnken Technical Advice on Water May 212

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3 Executive summary The Information and Knowledge Management Programme (IKMP) of the Mekong River Commission (MRC) is reviewing the 211 sediment monitoring component of the Discharge and Sediment Monitoring Project (DSMP). The review includes an assessment of the sampling locations, procedures and protocols employed by the DSMP, and data analysis and assessment. The results of the review will be used to refine the present monitoring program and inform the MRC and others about sediment transport and sediment characteristics in the catchment. The 211 monitoring program collected suspended sediment samples from 15 sites and bedload sediment from 2 sites over high and low flow conditions. Grain size analyses were completed at several locations and provided an indication of the material moving through the catchment. Although the monitoring did not begin at all sites until June 211, the sampling captured the wet season and the vast majority of sediment transport for the year. Overall, the sediment monitoring program has been very successful at monitoring a very large and highly variable river at appropriate temporal and spatial scales. Recommendations for refining the program include: reviewing sampling locations and the depth integrated sediment sampling method to reduce sampling bias associated with cross-channel sediment gradients, extending the collection of depth integrated sediment sampling into major tributaries, standardising grain-size analysis size classes and analytical techniques, and the development of an integrated sediment, water quality and hydrologic database. Preliminary analysis of the monitoring results show 211 was a wet year, with TSS and sediment transport typically following flow patterns. Sediment transport (and flow) at the three upstream sites (Chiang Sean, Luang Prabang, Chiang Khan) was relatively uniform (13-26 Mt/yr) and showed low variability compared to the downstream sites, and compared to historical results (6 12 Mt/yr). Much higher sediment fluxes were documented at Mukdahan (17 Mt/yr) and Khon Chiam (166 Mt/yr), but these loads were not recorded at Pakse (73 Mt/yr) raising questions about the upstream results. Loads increased at Stung Treng (16 Mt/yr) and Kratie (116 Mt/yr) consistent with the inflow of sediment bearing rivers. The calculated sediment transport at Kratie in 211 was considerably lower than the average estimate of 166 Mt/yr, even though it was a wet year. A large decrease in sediment transport was recorded at Chroy Chang Var (82 Mt/yr) relative to Kratie. This flux cannot account for the combined sediment transport at the downstream sites of Koh Norea (87 Mt/yr), OSP MRC (12.2 Mt/yr) and the inflow to the Tonle Sap (6.4 Mt/yr) suggesting either a large additional sediment source is present in the region, and / or the actual sediment transport at Chroy Chang Var was higher than recorded. The outflowing Tonle Sap transported ~1.5 Mt/yr of sediment consistent with previous estimates and the lake being a net sediment trap. Rough nutrient flux estimates for the Tonle Sap show that in 211 ~92, t of TN and 14, t of TP entered the Tonle Sap, with 21, t/yr TN and 3,3 t/yr TP returning to the Mekong during outflow.

4 The suspended solids in the Mekong were dominated by sands and silts at monitoring sites upstream of Kratie, and silts at the downstream sites. The absence of suspendedsand during peak flow-periods at Kratie and downstream is surprising and warrants additional investigation. Bedload at Kratie was dominated by sand sized material, with transport rates generally 1% - 3% of the total suspended solid transport rate. Bedload transport became active at flow rates >3, m 3 /s, and the total June to December bedload flux at Kratie is estimated at Mt/yr, with Mt/yr estimated to be sand. Bed materials were collected from ~6 sites in the Mekong and reflected the conditions at the time of sampling, with fine and very fine sand present at sites monitored during the dry season, and sands and gravels present at sites between Nong Khai and the delta during higher flows. Fine sand and silt increased in the delta. Recommended additional data analysis includes: extending the total suspended sediment transport analysis back through 29 where data is available, examining cross-sections and ADCP profiles to identify channel changes, determining sediment transport rates through the delta, integrating water quality results with flow and sediment transport results, re-examining sediment transport in the Chroy Chang Var Tonle Sap area to account for missing sediment flux, and interpretation of bed materials with respect to channel and flow characteristics, and any available historic results.

5 1 Contents Executive summary Contents Introduction Summary of monitoring and analytical methodology Overview of 211 hydrology Flow balances Sediment results Suspended sediment concentrations Sediment fluxes Annual / wet season sediment fluxes Sediment transport in the Tonle Sap River at Prek Kdam Tonle Sap nutrient fluxes Grain size characteristics of suspended sediments Bedload transport at Kratie nad Nong Khai Bed Materials Additional data analysis References Appendix Bed material grain-size results... 47

6 Table of Figures Figure 2.1. Summary of samples collected in 211. Number indicates the number of results reported during the 211 monitoring year. The number in parentheses indicates the total number of samples prescribed in the Terms of Reference for the year. No entry if parameter not specific in the TOR for the site Figure 2.2. MRC monitoring locations in the Lower Mekong River catchment in Figure 3.1. Average daily flow at sediment monitoring sites in Mekong River catchment. Flows calculated from average daily river level and rating curve based on results Figure 3.2. Flow results for sediment monitoring sites at Kratie and downstream. Average daily flow based on flow measurements obtained on sediment monitoring dates. Flow at Prek Kdam is into the Tonle Sap between June and end of September, and out of the Tonle Sap from October to December Figure 3.3. Water balance for monitoring sites during period of inflow to Tonle Sap River. Flow at Chroy Chang Var accounts for all flow at downstream sites and at Prek Kdam Figure 3.4. Water balance during period of outflow from Tonle Sap River. Combined flow at Koh Norea and OSP MRC accounts for inflows at Chroy Chang Var and Prek Kdam Figure 4.1. Depth integrated total suspended sediment concentrations and average daily flow at monitoring sites Chiang Sean to Kratie for Figure 4.2. Comparison of flow and TSS at Luang Prabang and Pakse in 1961 (Walling, 25) and Figure 4.3. Comparison of TSS concentrations at Chiang Khan to Kratie. Results from depth-integrated samples Figure 4.4. Box and whisker plot of TSS at monitoring sites. The box encompasses the 25th to 75th percentile TSS concentrations with the labelled line indicating the median value. The minimum and maximum values are shown by the whiskers extending from the box, with the maximum value labelled Figure 4.5. Total suspended solid concentrations and where available flow results from sediment monitoring sites downstream of Kratie. Flow measurements based on ADCP results on sediment monitoring days Figure 4.6. Box and whisker plot of TSS concentrations from Kratie to OSP MRC. The box encompasses the 25th to 75th percentile TSS concentrations with the labelled line indicating the median value. The minimum and maximum values are shown by the whiskers extending from the box, with the maximum value labelled Figure 4.7. Box and whisker plot of TSS concentrations in the upper and lower delta in the Mekong and Bassac Rivers. The box encompasses the 25th to 75th percentile TSS concentrations with the labelled line indicating the median value. The minimum and maximum values are shown by the whiskers extending from the box, with the maximum value labelled Figure 4.8. Suspended sediment fluxes on monitoring days at sediment monitoring sites upstream of Phnom Penh Figure 4.9. Suspended sediment fluxes on monitoring days at sediment monitoring sites upstream of Phnom Penh, excluding Khong Chiam Figure 4.1. Suspended sediment fluxes for monitoring days at sites downstream of Phnom Penh

7 Figure Sediment balance for monitoring sites near the confluence of the Tonle Sap for the period of inflow to the Tonle Sap Figure Sediment balance for monitoring sites near the confluence of the Tonle Sap for the period of outflow from the Tonle Sap Figure Google Earth image of the confluence of the Tonle Sap and Mekong River in August 21 showing large differences in suspended sediments concentrations in the waterways and at Koh Norea. PPP=Phnom Penh Port, PK=Prek Kdam, CCV=Chroy Chang Var, KN=Koh Norea Figure Sediment rating curves for sediment monitoring sites. Results from Figure Suspended sediment flux estimates for June Dec 211 using interpolation method, and for Jan-Dec 211 using sediment rating curves Figure Annual sediment flux at Chiang Sean Figure Box and whisker plot of daily TSS values at Chiang Sean in 1998 compared to the TSS concentrations recorded at Chiang Sean in June through December Figure Water level at Prek Kdam and average flow velocity from ADCP transect in Figure TSS at Prek Kdam compared to average water velocity at Prek Kdam, Figure 4.2. Comparison of measured TSS at Prek Kdam and at Chroy Chang Var, and river level at Prek Kdam in Figure Comparison of average flow velocity from ADCP transects at Chroy Chang Var and Prek Kdam Figure Time-series of TSS, Total Phosphorous, Nitrate + Nitrite (NO3+2) and Ammonium at Prek Kdam in Figure Total suspended solids (mg/l) compared to total phosphorous (mg/l) at Prek Kdam in 211. Values from MRC water quality data base Figure Total suspended solids (mg/l) compared to Nitrate + Nitrite (NO3+2) (mg/l) at Prek Kdam in 211. Values from MRC water quality data base Figure Total suspended solids (mg/l) compared to Total Nitrogen (mg/l) at Prek Kdam in 211. Values from MRC water quality data base Figure Grain size distribution by percentage mass for TSS at Luang Prabang. 32 Figure Grain size distribution by mass for TSS at Luang Prabang. There is some confusion over the mass of sediment used in the analysis and the masses indicated may not be correct Figure Grain size distribution by percentage mass for TSS at Pakse Figure Grain size distribution by mass for TSS at Pakse. There is some confusion over the mass of sediment used in the analysis and the masses indicated may not be correct Figure 4.3. Grain-size distribution of suspended load at Kratie, Figure Grain-size distribution by mass for TSS at Kratie, Figure Grain size distribution of suspended load at Prek Kdam, Figure Grain-size distribution by mass for TSS at Prek Kdam Figure Grain size distribution of suspended load at Tan Chau, 211. The clay fraction includes medium and fine sand for the 15 May sample due to insufficient sediment to complete hydrometer analysis Figure Grain-size distribution by mass for TSS at Tan Chau. The clay fraction includes medium and fine sand for the 15 May sample due to insufficient sediment to complete hydrometer analysis

8 Figure Grain size distribution of suspended load at Chau Doc, 211. The clay fraction includes medium and fine sand for 15 May, 15 June, 1 July and 15 November samples due to insufficient sediment to complete hydrometer analysis Figure Grain-size distribution by mass for TSS at Chau Doc. The clay fraction includes medium and fine sand for 15 May, 15 June, 1 July and 15 November samples due to insufficient sediment to complete hydrometer analysis Figure Average flow velocities from ADCP profiles at Pakse, Kratie and Prek Kdam, Figure Bedload (tonnes/day) at Nong Khai and Kratie in Figure 4.4. Suspended and bedload sediment fluxes at Kratie in Figure Sediment rating curve for Kratie based on bedload monitoring results. 35 Figure Bedload rating curve for Kratie based on sand fraction of bedload monitoring results Figure Bedload transport of gravel (>2 mm), sand (.63-2 mm) and silt (<.63 mm) at Kratie for monitoring periods in Figure Bedload transport of gravel (>2 mm), sand (.63-2 mm) and silt (<.63 mm) at Nong Khai for monitoring periods in Figure Detailed grain-size distribution at Nong Khai, Figure Detailed grain-size distribution at Kratie, Figure Map of bed material samples collected from the Mekong in 211. Location labels denote sample ID of grain-size distribution results in Appendix 1. Delta samples shown in Figure Map provided by MRC Figure Location of bed materials collected from delta in 211. Grain size distribution results shown in Appendix 1. MK denotes Mekong, BS denotes Bassac. Map provided by Vietnam National Mekong Committee Figure Grain size distribution results for bed material collected in 211. Results for each transect ar shown, with the site average indicated by thicker orange line Figure 4.5. Grain size distribution at sediment monitoring sites. No analyses are available for Chiang Sean, Nakhon Phanom or Kong Chiam

9 1 Introduction The Information and Knowledge Management Programme (IKMP) of the Mekong River Commission (MRC) is reviewing the 211 sediment monitoring component of the Discharge and Sediment Monitoring Project (DSMP). The review includes an assessment of the sampling locations, procedures and protocols employed by the DSMP, and data analysis and assessment. This report is presented in two parts. The first part summarises the recommendations arising from the review of the logistical, sampling and analytical aspects of the field monitoring component of the DSMP. Part 2 (this report) of the report summarises the preliminary findings from a review of the 211 data set. Interpreting the results collected by the DSMP is a useful way to evaluate the temporally and spatially consistency of the data, as well as providing an overview of sediment characteristics and transport within the catchment. This review is not intended to be an exhausted interrogation of the dataset, and in Section 5, recommendations for future data interpretation are discussed. The interpretation of the results includes an overview of the hydrologic conditions during 211, suspended sediment characteristics, concentrations and fluxes, bedload concentrations and fluxes, bed material characteristics and comparison of 211 sediment fluxes with historic results. 2 Summary of monitoring and analytical methodology The MRC sediment monitoring program in the Lower Mekong Basin (LMB) in 211 consisted of the following components: Discharge measurements and depth integrated suspended sediment sampling at 15 monitoring locations, including 12 in the Mekong, 2 in the Bassac and 1 in the Tonle Sap as shown in Figure 2.2; Grain-size analysis of the Total Suspended Sediments at 6-sites (Luang Prabang, Pakse, Kratie, Prek Kdam, Tan Chau, Chau Doc); The collection of bedload samples at three sites (Chiang Sean, Nong Khai and Kratie) although results from only 2 sites were received by the MRC; The collection of bed materials at approximately 6 sites throughout the catchment The frequency of monitoring varied through the wet and dry season, with 4 samples per month collected during the wet (July through October for most sites, July through November at the delta sites), and 2 samples per month during the dry season resulting in 32 to 34 samples per monitoring site. Because of funding and equipment constraints, monitoring did not commence at several of the sites until June 211. Because the vast majority of sediment (8 95%) is transported during the period June through December, this is not considered a constraint to the interpretation of the

10 results. Part 1 of this report discusses field and laboratory techniques and should be consulted for details of the field and laboratory methodology. A summary of monitoring techniques is contained in Table 1. Figure 2.1. Summary of samples collected in 211. Number indicates the number of results reported during the 211 monitoring year. The number in parentheses indicates the total number of samples prescribed in the Terms of Reference for the year. No entry if parameter not specific in the TOR for the site. Site Flow Results TSS SGSA Bedload Chiang Sean 27 (32) 27 (32) (17) (18) Luang Prabang 23 (32) 23 (32) 23 (17) Chiang Khan 35 (32) 32 (32) Nong Khai 28 (32) 28 (32) (17) 4 (18) Nakhon Phanom 36 (32) (32) Mukdahan 26 (32) 26 (32) Khong Chiam 36 (32) 36 (32) Pakse 2 (32) 2 (32) 19 (17) Stung Treng 21 (32) 2 (32) Kratie 21 (32) 2 (32) 8 (17) 13 (18) Chrouy Changvar 22 (32) 21 (32) Prek Kdam 22 (32) 21 (32) 11 (17) Koh Norea 22 (32) 21 (32) OSP MRC 21 (32) 21 (32) Tan Chau Not 26 (34) 15 (17) specified in TOR Chau Doc Not specified in TOR 26 (34) 15 (17) Table 1. Summary of sampling and analytical methods used in 211 in IKMP monitoring program. Sample Type Collection Method Comment Discharge ADCP & Current meters Equal discharge method prescribed in TOR, some sites used Equal width method TSS sample collection D-96 Flow proportional depth integrated suspended sediment sampler ( TSS determination Filtration & weight analysis Bedload BL-84 bed sampler Bedload analysis Sieves Grain size analysis Sieves & hydrometer Techniques varied between countries Bed materials BM-54 bed load sampler Monitoring results were reported to the IKMP on standardised forms on a quarterly basis.

11 Figure 2.2. MRC monitoring locations in the Lower Mekong River catchment in 211.

12 3 Overview of 211 hydrology This review did not audit the methods and processes used to collect river level information at gauging sites. Average daily river discharge was calculated at the sediment monitoring sites from Chiang Sean to Kratie, and at Prek Kdam using average daily river level and a rating curve based on measured discharge results from At the downstream sites of site Chroy Chang Var (1981), OSP MRC (No site number), Koh Norea (No site number), and Prek Kdam (212) only discharge results from the monitoring days are available for analysis. At the delta sites of Tan Chau and Chau Doc, discharge results are not available for the monitoring days, so no flow information is presented in this report. Flow results (Figure 3.1 and Figure 3.2) show pronounced dry and wet seasons, with two distinct flow peaks occurring in August and September 211. A comparison of annual flood season flows (June to November) and peak flow with annual recurrence intervals (MRC, 25) for 211 at Vientiane (using flow results from Nong Khai) and Kratie are presented in Table 2. At Nong Khai, the year was wetter than average and in the range of a 1:2 to 1:5 year flood season flow event. Peak flows were also higher than average, with the 211 peak about the same as a 1:2 year peak event. At Kratie, the flow year was relatively wetter, with total flow in the range of a 1:1 to 1:2 year event, and peak flows between a 1:2 and 1:5 year event. The high flood volume flows at both sites are attributable to an extended high flow period, with moderate peak flows, rather than one large flood event. The larger statistical flow event at Kratie, compared to Vientiane may reflect flow regulation in the upper catchment, and / or variable distribution of rainfall within the catchment. Table 2. Comparison of 211 annual flood season flow (June-November) with Annual Recurrence Intervals (ARI) at Vientiane and Kratie from Mekong River Commission (25). Site Flood Season Flow Peak Flow Vientiane 211: 13 km 3 211: 15,7 m 3 /s (Nong Khai) 1:2 ARI: 12 km 3 1:2 ARI: 16, m 3 /s 1:5 ARI: 135 km 3 1:5 ARI: 18,5 m 3 /s Kratie 211: 468 km 3 211: 52,2 m 3 /s 1:1 ARI: 455 km 3 1:2 ARI: 51, m 3 /s 1:2 ARI: 475 km 3 1:5 ARI: 59,5 m 3 /s : The flow results show a very large increase in flow volume between Chiang Khan and Mukdahan, reflecting the inflow of the tributaries entering predominantly from the northern and eastern side (left bank) of the catchment, including the Nam Ngum, Nam Theun and Nam Hinboun. Peak wet season flows are proportionately lower at the three upstream sites as compared to the lower sites, as shown by a comparison of 5 th and 95 th percentile flows at the sites (Table 3). The entrance of the left tributaries results in a more variable flow regime with peak flows approximately 15-fold greater than dry season base flow. Flow regulation in the upper Mekong catchment may also be affecting the flow variability at the upstream sites.

13 Flow results from sites downstream of Kratie (Figure 3.2) show that substantial volumes of water are lost to the flood plain between Kratie and Chrouy Chang Var during the wet season, resulting in higher flows at Chrouy Chang Var later in the year as compared to the upstream sites, as this water returns to the main channel. The flows into and out of Tonle Sap occur in distinct periods, with inflow occurring between June and September, and outflow from September through December. 6, 5, Average Daily Discharge (m 3 /s) 4, 3, 2, 1, Chiang Sean Luang Prabang Chiang Khan Nong Khai Mukdahan Khong Chiam Pakse Stung Treng Kratie Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Figure 3.1. Average daily flow at sediment monitoring sites in Mekong River catchment. Flows calculated from average daily river level and rating curve based on results. 6, 5, Discharge (m 3 /s) 4, 3, 2, Kratie Chrouy Changvar Koh Norea Prek Kdam OSP MRC 1, Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Figure 3.2. Flow results for sediment monitoring sites at Kratie and downstream. Average daily flow based on flow measurements obtained on sediment monitoring dates. Flow at Prek Kdam is into the Tonle Sap between June and end of September, and out of the Tonle Sap from October to December. Table 3. Ratio of 95th percentile flow and 5th percentile flow for sediment monitoring sites in Mekong catchment. Average daily flow used as basis for comparison. Site Ratio Q 95th /Q 5th Site Ratio Q 95th /Q 5th Chiang Sean 4.2 Khong Chiam 15.3 Luang Prabang 8.2 Pakse 15. Chiang Khan 7.3 Stung Treng 17.2 Mukdahan 14.6 Kratie 16.5

14 3.1 Flow balances Flow near the Mekong and Tonle Sap confluence is complex owing to the reversal of flow in the Tonle Sap River, and the increased contribution of water from the Tonle Sap catchment late in the year. A flow balance during the period of inflow to the Tonle Sap (Figure 3.3) shows that the flow at Chroy Chang Var can account for the flow present in the lower Mekong (Koh Norea), the Bassac (OSP MRC) and into the Tonle Sap River (Prek Kdam). During August and September there is a small increase of flow relative to Chroy Chang Var at the other sites presumably related to unchannelised overland flow. Following the reversal of flow in the Tonle Sap, the combined flow of Chroy Chang Var and the Tonle Sap can account for the flows measured at Koh Norea and OSP MRC (Figure 3.4). The discharge of the Tonle Sap increases in volume, and accounts for a larger proportion of the flow present at the downstream sites as the wet season subsides. 45, Inflowing Tonle Sap 4, 35, Discharge (m 3 /s) 3, 25, 2, 15, OSP MRC Koh Norea Prek Kdam Chrouy Changvar 1, 5, Figure 3.3. Water balance for monitoring sites during period of inflow to Tonle Sap River. Flow at Chroy Chang Var accounts for all flow at downstream sites and at Prek Kdam. 45, Outflowing Tonle Sap 4, 35, 3, Discharge (m 3 /s) 25, 2, 15, Prek Kdam Chrouy Changvar Koh Norea+OSP MRC 1, 5, 1-Oct-11 8-Oct Oct Oct Nov Nov-11 8-Dec-11 Figure 3.4. Water balance during period of outflow from Tonle Sap River. Combined flow at Koh Norea and OSP MRC accounts for inflows at Chroy Chang Var and Prek Kdam.

15 4 Sediment results 4.1 Suspended sediment concentrations Depth integrated total suspended sediment (TSS) concentrations are compared to average daily flow in Figure 4.1. In these graphs the TSS results are presented against the same scale for each site, but the flow scale increases at successive downstream sites. TSS concentrations show similarities to the flow patterns, with higher concentrations associated with higher flows, and lower variability at the three upstream sites. Maximum TSS values tend to be associated with the first or second flood peak of the wet season and decrease through the wet season. TSS concentrations change markedly between Chiang Khan and Kratie (Figure 4.3), but do not show a consistent decrease in a downstream direction between Chiang Sean and Mukdahan as reported by Xue et al., (21). The large increase in flow between Chiang Khan and Mukdahan is accompanied by a large increase in TSS, and this trend of increasing TSS persists through Khong Chiam where maximum values of ~1 g/l were reported. A similar increase in TSS values between these sites has not been observed in past data sets (Walling, 28, Figure 4.2), and is inconsistent with the present understanding of sediment sources in the Mekong (Kondolf et al., 211). The very high concentrations at Kong Chiam are not present at any of the other sites, which may suggest that these values are attributable to sampling bias and localised inflows. TSS at Pakse is much lower than at Kong Chiam, which is surprising given the proximity of the sites. This reduction may reflect conveyance losses as identified by Walling (28) of suspended sediments derived from the left bank tributaries, but the magnitude of loss is very high and has not been previously documented. From Pakse to Kratie, TSS concentrations increase, presumably reflecting tributary inflows. The distribution of TSS concentrations at these sites is summarised in Figure 4.3 which shows the much greater range of concentrations at Mukdahan and Khong Chiam. The TSS results from sites downstream of Kratie show generally similar trends to the upstream sites with peak suspended sediment concentrations coinciding with early flood peaks (Error! Reference source not found.). At Prek Kdam, on the Tonle Sap, TSS results are higher when the flow is derived from the Mekong and entering the lake, as compared to the outgoing flow which is characterised by low and uniform sediment concentrations, consistent with the lake being a net sediment sink. The range of TSS concentrations and median values in the Mekong and Bassac are similar to the upstream sites of Kratie and Stung Treng, with median values of ~2-22 mg/l. The sites in the lower delta, Tan Chau and Chau Doc have higher TSS concentrations compared to the upstream sites of Koh Norea (Mekong) and OSP MRC (Bassac), with maximum values of ~1,5 mg/l which may reflect the presence of marine sands (Figure 4.7).

16 Total Suspended Solids (mg/l) Total Suspended Solids (mg/l) Total Suspended Solids (mg/l) Total Suspended Solids (mg/l) Total Suspended Solids (mg/l) TSS Flow Chian Sean211 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec TSS Flow Chiang Khan 211 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec TSS Flow TSS Flow Mukdahan 211 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec TSS Flow Pakse 211 Kratie 211 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Average Daily Discharge (m3/s) Discharge (m 3 /s) Discharge (m 3 /s) Discharge (m 3 /s) Discharge (m 3 /s) Total Suspended Solids (mg/l) Total Suspended Solids (mg/l) Total Suspended Solids (mg/l) Total Suspended Solids (mg/l) Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Figure 4.1. Depth integrated total suspended sediment concentrations and average daily flow at monitoring sites Chiang Sean to Kratie for TSS Flow TSS Flow Luang Prabang 211 Nong Khai 211 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec TSS Flow Khong Chiam 211 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec TSS Flow Stung Treng 211 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Average Daily Discharge (m3/s) Discharge (m 3 /s) Discharge (m 3 /s) Discharge (m 3 /s)

17 Luang Prabang Total Suspended Solids (mg/l) TSS Flow Average Daily Discharge (m3/s) TSS Flow Figure 4.2. Comparison of flow and TSS at Luang Prabang and Pakse in 1961 (Walling, 25) and 211. Total Suspended Solids (mg/l) Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Pakse Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Discharge (m 3 /s) 12 Comparison of TSS Chiang Khan to Kratie Chiang Khan Nong Khai Mukdahan Khong Chiam Pakse Stung Treng Kratie TSS (mg/l) Figure 4.3. Comparison of TSS concentrations at Chiang Khan to Kratie. Results from depthintegrated samples Jun Jul-11 9-Sep Oct Dec-11 Figure 4.4. Box and whisker plot of TSS at monitoring sites. The box encompasses the 25th to 75th percentile TSS concentrations with the labelled line indicating the median value. The minimum and maximum values are shown by the whiskers extending from the box, with the maximum value labelled.

18 Total Suspended Solids (mg/l) Chrouy Chang Var 211 TSS Flow Discharge (m 3 /s) Total Suspended Solids (mg/l) TSS Flow In Flow Out Prek Kdam Discharge (m 3 /s) Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Total Suspended Solids (mg/l) TSS Flow Koh Norea Discharge (m 3 /s) Total Suspended Solids (mg/l) TSS Flow OSP MRC Discharge (m 3 /s) Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Total Suspended Solids (mg/l) TSS Tan Chau Total Suspended Solids (mg/l) TSS Chau Doc Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Figure 4.5. Total suspended solid concentrations and where available flow results from sediment monitoring sites downstream of Kratie. Flow measurements based on ADCP results on sediment monitoring days TSS (mg/l) Figure 4.6. Box and whisker plot of TSS concentrations from Kratie to OSP MRC. The box encompasses the 25th to 75th percentile TSS concentrations with the labelled line indicating the median value. The minimum and maximum values are shown by the whiskers extending from the box, with the maximum value labelled.

19 Figure 4.7. Box and whisker plot of TSS concentrations in the upper and lower delta in the Mekong and Bassac Rivers. The box encompasses the 25th to 75th percentile TSS concentrations with the labelled line indicating the median value. The minimum and maximum values are shown by the whiskers extending from the box, with the maximum value labelled. 4.2 Sediment fluxes Average daily sediment fluxes for each monitoring day were calculated using the ADCP or current meter flow values and TSS results. Results for sites upstream of Phnom Penh are shown in Figure 4.8 and Figure 4.9, with sites downstream of Phnom Penh summarised in Figure 4.1. With the exception of Mukdahan and Khong Chiam, the sites upstream of Phnom Penh show similar and consistent characteristics, with two major sediment peaks and increasing sediment loads in a downstream direction. Khong Chiam and Mukdahan have higher suspended sediment fluxes owing to higher TSS values compared to the other site. At Mukdahan, the main flood peak is lacking and examination of the data shows that this is due to a low average TSS result relative to the previous and post sampling runs. The TSS values from the individual verticals collected on August 16 show a strong cross-channel gradient, with values declining from 411 mg/l on one side of the river, to 169 mg/l on the opposite bank. The relatively low sediment flux value may be attributable to using an equal width sampling approach with an insufficient number of sampling verticals in conditions of strong sediment gradients. The sediment fluxes at Mukdahan and Khong Chiam during the wet season are much higher than at Chiang Khan, and Khong Chiam is also higher than the downstream site of Pakse. If these flux estimates are reliable, then there is a very large sediment input between Mukdahan and Khong Chiam which is not being transported 5 km downstream to Pakse. This is unlikely and inconsistent with the present understanding of sediment sources in the Mekong (Kondolf, et al., 211), and probably indicates sampling bias or error at Khong Chiam, and / or the downstream sites. Between Pakse and Stung Treng, sediment fluxes increase reflecting tributary inflows, but show little change at Kratie. Sediment fluxes are relatively lower at Chroy Chang Var in September compared to the other sites. If the results are accurate, than about 25% of the sediment load is being lost from suspension between the two sites. Average flow velocities at Chroy Chang Var were above 1.5 m/s through September so the loss is unlikely due to reduced sediment carrying capacity of the river, and is

20 more likely attributable to flood plain losses, and / or sampling errors, however, a more detailed examination of the flow regime between the two sites is warranted as in channel storage may also be occurring. 4,, Suspended Sediment Fluxes 211 Suspended Sediment (Tonnes/day) 3,5, 3,, 2,5, 2,, 1,5, 1,, 5, Chiang Sean Luang Prabang Chiang Khan Nong Khai Mukdahan Khong Chiam Pakse Stung Treng Kratie Chroy Chang Var Figure 4.8. Suspended sediment fluxes on monitoring days at sediment monitoring sites upstream of Phnom Penh. 1,8, - 1-May-11 2-Jun-11 9-Aug Sep Nov-11 Suspended Sediment Fluxes 211 Suspended Sediment (Tonnes/day) 1,6, 1,4, 1,2, 1,, 8, 6, 4, 2, Chiang Sean Luang Prabang Chiang Khan Nong Khai Mukdahan Pakse Stung Treng Kratie Chroy Chang Var Figure 4.9. Suspended sediment fluxes on monitoring days at sediment monitoring sites upstream of Phnom Penh, excluding Khong Chiam. - 1-May-11 2-Jun-11 9-Aug Sep Nov-11 Sediment fluxes at Koh Norea are lower than at the upstream site of Chroy Chang Var during the first half of the wet season, consistent with the loss of sediment to the floodplain and Tonle Sap River. Sediment fluxes at Koh Norea are higher than Chroy Chang Var later in the year, possibly reflecting the inflow of sediment from the Tonle Sap River (This is discussed in the next section in more detail). Maximum sediment transport occurs at the OSP MRC site on the Bassac during August, with a rapid decrease in sediment fluxes once the wet season peak passes.

21 1,8, Suspended Sediment Fluxes 211 1,6, Sediment Flux (Tonnes/day) 1,4, 1,2, 1,, 8, 6, 4, Chroy Chang Var Prek Kdam Koh Norea OSP MRC Figure 4.1. Suspended sediment fluxes for monitoring days at sites downstream of Phnom Penh. 2, - 17-Apr-11 6-Jun Jul Sep-11 3-Nov Dec-11 A sediment balance based on the daily sediment fluxes during the period of inflow to the Tonle Sap (Jun September, Figure 4.11) shows excellent agreement until mid- September between the total sediment load at Chroy Chang Var, and the combined sediment loads at the downstream sites of Koh Norea and OSP MRC, and in the Tonle Sap at Prek Kdam. The balance shows that only a relatively small percentage of the sediment load at Chroy Chang Var enters the Tonle Sap, with the values ranging from 1% - 2% of the Chroy Chang Var value. In late September the sediment flux at Chroy Chang Var is too low to account for the sediments present downstream. The sediment flux at Kratie for this monitoring period, 1.1 x 1 6 tonnes/day, is in good agreement with the total of the downstream sites, and may indicate that the Chroy Chang Var sediment flux is inaccurate. 1,6, Sediment Balance Inflowing Tonle Sap 1,4, Sediment Flux (Tonnes/day) 1,2, 1,, 8, 6, 4, OSP MRC Koh Norea Prek Kdam Chroy Chang Var Figure Sediment balance for monitoring sites near the confluence of the Tonle Sap for the period of inflow to the Tonle Sap. 2, - In late September the Tonle Sap reversed flow direction back into the Mekong, and the sediment input from Chroy Chang Var and Prek Kdam is compared to the downstream fluxes at Koh Norea and OSP MRC (Figure 4.12). The sediment balance is poor, even though the flow balance is good at these sites (Figure 3.4). The discrepancy may be due to additional sediment pickup in the Tonle Sap downstream of Prek Kdam, an additional sediment input to the Mekong in the area or sampling

22 errors. The evidence for each of these possibilities is discussed in the following dot points: Increase in TSS in Tonle Sap downstream of Prek Kdam: A comparison of TSS values from Prek Kdam and Phnom Penh Port (Table 4) shows TSS increases between the two sites. The TSS value at OSP MRC is likely to reflect Tonle Sap water due to the configuration of the bifurcation, but in October 211 the difference in TSS between the sites was over 1 mg/l, suggesting that either the Phnom Penh Port value was erroneously low (it was a grab sample) or there is an additional sediment input between the two sites. However, even if TSS at Prek Kdam increased by 1-fold, it could not account for the large missing sediment flux because the Tonle Sap only contributes ~ 15% of the downstream flow; Erosion of both banks and bed in the bifurcation area has been observed, and investigated by DHI Haecon (21). Sand mining, estimated in 1991 by DHI to be equivalent to approximately 35% of the bedload sand transport in the region, is predicted to lower the river bed level by approximately 1 m per year, with the change propagating downstream at a rate of about 1 km/yr (DHI Haecon, 21). Input from this bed and bank adjustment would contribute to the sediment load of the region, but is unlikely to be equivalent to the missing flux of up to 2, tonnes/day (~1, m 3 /day). An additional sediment source in the region could also be the return of water to the main channel from the flood plains, but the good flow balance between the sites suggests this is not a major contributor. Sampling errors could be associated with the strong sediment gradients which exist in the Mekong downstream of the Tonle Sap during outflow due to the differences in TSS concentrations between the two source waters. An example of this is evident in October 21 (Figure 4.13), and shows elevated TSS levels in the Tonle Sap relative to the mainstem Mekong. It is possible that monitoring at Koh Norea in October and November 211 over sampled the Tonle Sap contribution relative to the Mekong contribution, resulting in high suspended sediment flux values. TSS concentrations at individual transects show strong cross-channel sediment gradients at both Chroy Chang Var (which is surprising) and Koh Norea during October and November 211, making sampling bias a possibility. The other possibility is that TSS concentrations at Chroy Chang Var are erroneously low. The sediment fluxes at Kratie in October decreased from ~1.2 x 1 6 tonnes/day to.5 x 1 6 tonnes per day. If the Kratie and Chroy Cang Var sediment fluxes are accurate, than over half of the sediment being transported at Kratie is being stored in the river and floodplains during the falling stage of the river upstream of Chroy Chang Var. In summary, there is a large sediment balance discrepancy at the Tonle Sap confluence which warrants additional investigation.

23 6, Sediment Balance Outflowing Tonle Sap 5, Sediment Discharge (tonnes/day) 4, 3, 2, Prek Kdam Chroy Chang Var Koh Norea+OSP MRC Figure Sediment balance for monitoring sites near the confluence of the Tonle Sap for the period of outflow from the Tonle Sap. 1, 1-Oct-11 8-Oct Oct Oct Nov Nov-11 8-Dec-11 PK PPP OSP MRC CCV KN Figure Google Earth image of the confluence of the Tonle Sap and Mekong River in August 21 showing large differences in suspended sediments concentrations in the waterways and at Koh Norea. PPP=Phnom Penh Port, PK=Prek Kdam, CCV=Chroy Chang Var, KN=Koh Norea Table 4. A comparison of TSS values at the sediment monitoring sites at Phenom Penh Port on October, 211. Phnom Penh Port is a water quality grab sample and is likely to underestimate the actual TSS value. Site Average TSS Prek Kdam 13 mg/l Phnom Penh Port 37 mg/l (from water quality database) Chroy Chang Var 92 mg/l Koh Norea 142 mg/l OSP MRC 148 mg/l Annual / wet season sediment fluxes Annual sediment fluxes were calculated using two methods. The first used linear interpolation of sediment fluxes on successive monitoring dates: Sediment Flux (tonnes/day)=average (Sed Flux date 1, Sed Flux date 2 ) x (Date 2 Date 1 ). This approach could only be applied to periods for which monitoring results were available, generally June through December at most sites.

24 Where flow rating curves and river level data for 211 were available the following approach was also used to estimate annual sediment fluxes: A hydrograph of average daily flows at the site for 211 (Figure 3.1) was constructed using average daily water levels and a discharge rating curve based on measured flow results from ; The TSS and measured flow results from 211 were used to construct a sediment rating curve for each site. The best fit relationship was used to derive an average sediment concentration for each day in 211 (Figure 4.14); The modelled sediment concentrations were used with the hydrograph to calculate average daily sediment fluxes; The average daily sediment fluxes were integrated to arrive at an annual sediment flux value for the site. The sediment rating curves (Figure 4.14) are based on power relationships. The R 2 values for the sites were variable, with Pakse having the poorest result (R 2 =.35). The upstream sites of Chiang Sean and Luang Prabang had R 2 values of respectively, with the remaining sites returning values of between.77 and.81. High variability in these types of rating curves is not surprising given that sediment flow relationships vary over time naturally, with rising limbs generally transporting higher concentrations of suspended sediment as compared to falling limbs. Flow regulation or sediment capture can also affect sediment flow relationships. The R 2 values are similar to the range obtained by Walling (28). Future analysis could examine these relationships in more detail, and attempt to refine the flow TSS relationships used to calculate sediment fluxes. Of the two methods, interpolation and rating curve, the interpolation method is probably more reliable for the 211 year because the monitoring frequency during high flows was high (weekly). The sediment rating curves tend to underestimate the sediment fluxes associated with the first flood peak at each of the sites, which results in lower overall sediment transport estimates. Chiang Sean Luang Prabang TSS (mg/l) y =.19x R² =.6686 TSS (mg/l) y = x.4929 R² = Discharge (m 3 /s) Discharge (m 3 /s) Chiang Khan Nong Khai TSS (mg/l) y =.313x.9245 R² =.787 TSS (mg/l) y =.148x R² = Discharge (m 3 /s) Discharge (m 3 /s)

25 TSS (mg/l) y =.915x.8229 R² =.791 Mukdahan Discharge (m 3 /s) TSS (mg/l) y =.33x R² =.887 Khong Chiam Discharge (m 3 /s) Pakse Stung Treng TSS (mg/l) y = x.3273 R² =.3542 TSS (mg/l) y =.142x.7328 R² = Discharge (m 3 /s) Discharge (m3/s) TSS (mg/l) y =.1332x.798 R² =.7654 Kratie Discharge (m 3 /s) Figure Sediment rating curves for sediment monitoring sites. Results from 211. The sediment flux results for the two methods are summarised in Table 5 and Figure The two approaches are in general agreement, with the greatest differences occurring at Kratie and Stung Treng. These sites are missing TSS results for late June 211 and at Kratie for late October 211 as well. These periods were characterised by rapidly changing flow and TSS values, and the higher flux estimates obtained using the interpolation method is probably attributable to linearly calculating sediment fluxes over the extended periods of missing data. Comparing the estimates for the June to December period with the January to December period highlights how little sediment is transported through the system during the dry season, with the 7-month wet period contributing up to 98% of the total sediment load. The dry season sediment input is highest at the upstream sites, where the flow regime is also more uniform as compared to the downstream sites. Spatially, the 211 sediment flux estimates show an increase between Chiang Sean and Luang Prabang, with a decrease between Luang Prabang and Chiang Khan. Between Chiang Khan and Nong Khai there is an increase in sediment flux, but a much larger increase occurs between Nong Khai and Mukdahan. This is consistent with the inflow of the left bank tributaries in this reach. Sediment estimates at Khong Chiam are the highest of all the sites, but a large decrease occurs at Pakse. As previously discussed, the proximity of the two sites (~5 km) combined with the large discrepancy in sediment loads at the two sites raises questions about the accuracy of the data.

26 Between Pakse and Stung Treng, fluxes increase, with the Stung Treng estimate similar to the value calculated for Mukdahan, again raising questions about the Khong Chiam value, although temporary storage and episodic transport of material in the river could also account for the values. At Kratie, sediment fluxes increase, reflecting the input of additional tributaries. The flux at Chroy Chan Var is substantially lower, presumably reflecting flood plain losses between the sites, although the higher value recorded at Koh Norea may also indicate that the Chroy Chang Var value is low. Relatively low sediment fluxes enter the Tonle Sap during the inflowing period, with ~75% of this material trapped in the lake. Table 5 also contains a summary of previous sediment flux estimates for the Mekong. For the upstream sites of Chiang Sean, Luang Prabang and Chiang Khan, the 211 results are considerably lower than historic values. This is a continuation of the trend depicted by historic data held in the MRC data base, which shows decreasing sediment fluxes at Chiang Sean based on daily TSS results for the period 2-28, and weekly to fortnightly measurements in 29 to 211 (Table 6, Figure 4.16). Compared to daily TSS results collected in 1998 the 211 values are lower, and more consistent through the wet season (Figure 4.17). At Mukdahan the estimated 211 load is considerably lower than the estimate for 199, but similar in magnitude to the estimates for At Khong Chiam the 211 estimate is much higher than the 1998 result, and combined with the Mukdahan values, show there is a very large increase in sediment being transported in this region of the river. What proportion of this sediment is derived from tributary input as compared to the reworking of floodplain sediments is not possible to establish without sediment fluxes from tributaries.

27 Table 5. Summary of sediment flux estimates for Jun-Dec 211, and Jan-Dec 211 where possible. Jan-Dec Jun-Dec Jan-Dec Sediment Rating Site Interpolation Interpolation x1 6 Curve (Jun-Dec) Tonnes x1 6 x1 6 Tonnes Tonnes Walling (25) Chiang Sean (9.7) (1961) 57 (1994) 68 (1997) 96 (1998) 12.8 (1999) 81.1 (22) Luang Prabang (24.9) 13 (1961) 66 (1968) 78 (1987) (1997) Sarkkula et al (21) Wang et al (211) annual mean Thai Data (*=Kummu et al 28) 91 ~ (1998) Chiang Khan (17.7) (1998) Nong Khai (1973) 67 (1978) ~ (199) 78.1 (1998) Mukdahan (11.) (1962) 99 (1973) 98.4 (1998) 158 (1999) ~ (199) 15 (1998) Khong Chiam (157.1) ~ (1998) Pakse (Thru Nov 11 only) (64.4) 159. (1961) 188 (1962) 168 (1997) Stung Treng (81.5) Kratie (1.6) 166 (199-2) Chroy Chang Var 81.6 Koh Norea 86.9 OSP MRC (Bassac) 12.2 Prek Kdam: Inflow (Tonle Sap) Outflow In: 5.1/Out: 1.4 ( ) ~92 In: (avg=5.1) Out: 1.6 *

28 Suspended Sediment Flux (Mt/yr) Jun-Dec Interval method Jan-Dec sediment rating curve Figure Suspended sediment flux estimates for June Dec 211 using interpolation method, and for Jan-Dec 211 using sediment rating curves. Table 6. Annual sediment flux at Chiang Sean based on daily TSS measurements for 2 28, 38 measurements per year in 29-21, and 27 measurements in 211. Values in millions of tonnes per year. Year Sediment Flux X1 6 tonnes/yr Year Sediment Flux X1 6 tonnes/yr Annual Sediment Flux (MT/yr) Chiang Sean Figure Annual sediment flux at Chiang Sean Figure Box and whisker plot of daily TSS values at Chiang Sean in 1998 compared to the TSS concentrations recorded at Chiang Sean in June through December Sediment transport in the Tonle Sap River at Prek Kdam The Chaktomuk confluence is of specific interest because of the importance of the Tonle Sap flow regime to fishery and ecological processes. TSS at Prek Kdam shows a complex relationship to flow, with TSS increasing as the inflow to the lake

29 increases, and then showing a marked and rapid reduction during peak flow periods (Error! Reference source not found.). Sediment concentrations decrease markedly prior to the reversal of flow, and remain relatively uniform and low throughout the outflow period. These sediment characteristics are related to the flow regime of the Tonle Sap as shown in Figure 4.18 where river level at Prek Kdam is compared to the average water velocity as measured by ADCP. During the period of inflowing water, flow velocity decreases rapidly after the water level reaches approximately 8 m. The prolonged period at which the river remains at approximately 8 m suggests that this is the level where extensive floodplain inundation occurs, which is consistent with the rapid decrease in flow velocities. Flow velocities continue to decrease, even as water levels increase, and remain low through the reversal of flow (on September 29 in 211). As water levels decrease and flow becomes contained within the channel, flow velocities increase. The sediment behaviour and flow regime observed in 211 is consistent with that described by DHI and HAECON HV (21). TSS concentrations (Figure 4.19) increase during the onset of the incoming flow, but rapidly decrease as flow velocities decrease. Following the reversal of flow in late September, TSS levels remain low, even though flow velocities increase to similar and even higher velocities, as compared to the onset of the inflow. This suggests that the sediment load transported during the initial inflow phase is not remobilised during the outflow, and is trapped in the lake. The concentration of TSS at Prek Kdam during the initial inflow is similar to the concentrations present at Chroy Chang Var during the same period (Figure 4.2), but is much lower compared to Chroy Chang Var once water velocities decrease. Sediment concentrations (Figure 4.2) and the average water velocity at Chroy Chang Var (Figure 4.21) decrease in August at about the same time as at Prek Kdam consistent with the floodplains being inundated at this time. River Level (m) Figure Water level at Prek Kdam and average flow velocity from ADCP transect in 211. River Level (m) Jan Mar May Jul River Level Avg Vel River Level Prek Kdam TSS Chroy Chang Var Jan Mar May Jul Sep Nov Sep Nov Figure 4.2. Comparison of measured TSS at Prek Kdam and at Chroy Chang Var, and river level at Prek Kdam in Water Velocity (m/s) TSS (mg/l) Avg Velocity (m/s) Average Velocity Prek Kdam TSS Jan Mar May Jul Sep Nov Figure TSS at Prek Kdam compared to average water velocity at Prek Kdam, 211. Average Velocity (m/s) Chroy Chang Var Prek Kdam Figure Comparison of average flow velocity from ADCP transects at Chroy Chang Var and Prek Kdam. 5 TSS (mg/l). Apr-11 Jun-11 Jul-11 Sep-11 Nov-11 Dec-11 Feb-12

30 As discussed in the previous section, the sediment flux into the Tonle Sap in 211 is estimated at 6.4 million tonnes per year, with 1.5 million tonnes being returned to the Mekong during the period of outflow. These values are consistent with those estimated by Kummu et al. (28) Tonle Sap nutrient fluxes Water quality samples are collected at Prek Kdam on a bi-monthly basis. Total phosphorous, nitrogen, nitrate, and ammonium results are shown in Figure Results are variable, but show similarity to TSS values over much of the year, and reasonable to good linear correlations exist between TSS and Total Phosphorous (Figure 4.22), nitrate + nitrite (Figure 4.24), and Total Nitrogen (Figure 4.25). Nutrient fluxes into and out of the Tonle Sap were derived for TP, NO (3+2), and TN using the TSS nutrient relationships. Ammonium fluxes were estimated using the average ratio of ammonium to TSS values for the incoming and outflowing periods, as there was a very poor correlation between ammonium and TSS overall. These estimates should be considered as indicative only, as the relationships are based on grab samples which are likely to underestimate coarse particulate material in the water column. The estimates suggest that by mass, approximately 35% of the TN can be accounted for by nitrite, nitrate and ammonium. The TN:TP molar ratio during the outflow period was much higher in 21 and 211 (33 37) as compared to the inflow periods (TN:TP = 4 7). The higher values are in the range of fertile soils, and the lower values of typical river water (Downing and McCauly, 1992). TSS TP TN NO3+2 NH4 TSS (mg/l) Dec-1 Jan-11 Feb-11 Mar-11 Apr-11 May-11 Jun-11 Jul-11 Aug-11 Sep-11 Oct-11 Nov-11 Dec TP, NO3+2, NH4 (mg/l) Figure Time-series of TSS, Total Phosphorous, Nitrate + Nitrite (NO3+2) and Ammonium at Prek Kdam in 211. TP (mg/l) TSS and TP at Prek Kdam y =.17x R² = TSS (mg/l) Figure Total suspended solids (mg/l) compared to total phosphorous (mg/l) at Prek Kdam in 211. Values from MRC water quality data base.

31 NO 3+2 (mg/l) TSS and NO 3+2 at Prek Kdam y =.43x R² = TSS (mg/l) Figure Total suspended solids (mg/l) compared to Nitrate + Nitrite (NO3+2) (mg/l) at Prek Kdam in 211. Values from MRC water quality data base. TN (mg/l) TSS and TN at Prek Kdam y =.143x R² = TSS (mg/l) Figure Total suspended solids (mg/l) compared to Total Nitrogen (mg/l) at Prek Kdam in 211. Values from MRC water quality data base. Table 7. Estimated nutrient fluxes into and out of the Tonle Sap in 211 based on correlations between TSS and nutrient concentrations in water quality data base, and annual sediment flux estimates from depth integrated sediment sampling. Parameter Estimated inflow (tonnes/yr) TP 14,8 3,3 TN 91,52 21,45 NO (3+2) 27,52 6,45 NH 4 5,88 3,935 Estimated Outflow (tonnes/yr) Collecting water samples at the same time as depth integrated sediment samples are collected would allow the understanding of nutrient transport and processes at a finer scale, however field sampling techniques would have to be altered to include the collection of water samples suitable for nutrient analyses. 4.3 Grain size characteristics of suspended sediments Suspended sediment grain-size distribution results are available for Luang Prabang, Pakse, Kratie, Prek Kdam, Tan Chau and Chau Doc for 211 (Figure 4.26 through Figure 4.37). The results were derived from a variety of analytical techniques. At Luang Prabang and Pakse, grain size analysis was completed using sieves only. At Kratie and Prek Kdam, the samples were passed through a 63µm sieve prior to hydrometer analysis, but no >63µm material was captured in the sieve. At the delta sites, a combination of sieves and hydrometer were used. The grain-size distribution results show that sand predominates at Luang Prabang during the onset of the wet season, with the contribution of silt increasing during periods of highest flow. In the dry season the final four results show a return to sand, and then a shift to medium silt, but only low quantities of TSS were analysed during these periods which reduces the reliability of the results. At Pakse, the proportion of sand increases during the onset of the wet season, and decreases through the year, as the proportion of finer material increases. These results

32 are consistent with the flow regime which has peak flow velocities in August and September. The estimated masses of sand, silt and clay transported at Luang Prebang and Pakse is summarised in Table 8. Table 8. Estimated masses (Tonnes/day) of sand (.63 mm), Coarse silt ( mm) and Medium silt ( mm) transported at Luang Prababng and Pakse during June December 211. Site Sand Coarse Silt Medium Silt Total (>.63 mm) ( (.2.45 (Tonnes/yr) (Tonnes/yr) mm) (Tonnes/yr) mm) (Tonnes/yr) Luang Prabang Pakse The grain-size distribution results show a substantial change in suspended material downstream of Pakse at Kratie. No sand was reported in the suspended load at Kratie and the grain-size distribution was uniform throughout the wet and dry seasons. Average flow velocities at Kratie exceed those at Pakse (Figure 4.38) during much of the wet season, suggesting that either the sand is not reaching the downstream site, or sampling and / or analytical issues at Kratie are responsible for the absence of sand. Table 9. Estimated masses (Tonnes/day) of Coarse silt ( mm) and Fine silt ( mm) and clay (<.2 mm) transported at Kratie. Site Coarse Silt Fine Silt (.2 Clay Total ( mm) (<.2 mm) (Tonnes/yr) mm) (Tonnes/yr) (Tonnes/yr) (Tonnes/yr) Kratie Gain size characteristics at Prek Kdam are very consistent and similar to Kratie, even though there is a large range in flow velocities at the site. There is no change in sediment textures between the inflow and outflow periods which is also surprising. At the delta sites of Tan Chau and Chau Doc, the suspended load is predominantly comprised of clay sized material with silt and small volumes of sand present on most sampling dates. No flow velocity data is available for these sites so the grain-size distributions cannot be interpreted with respect to river flow or tidal cycle. Percentage Sand Coarse Silt Med Silt TSS (g) Sand Coarse Silt Med Silt Figure Grain size distribution by percentage mass for TSS at Luang Prabang. Figure Grain size distribution by mass for TSS at Luang Prabang. There is some confusion over the mass of sediment used in the analysis and the masses indicated may not be correct.

33 Percentage Sand Coarse Silt Med Silt 17-Jun 8-Jul 18-Jul 22-Jul 26-Jul 8-Aug 15-Aug 22-Aug 3-Aug 9-Sep 19-Sep 3-Sep 5-Oct 1-Oct 19-Oct 25-Oct 18-Nov 9-Nov 16-Dec TSS (g) Sand Coarse Silt Med Silt 17-Jun 8-Jul 18-Jul 22-Jul 26-Jul 8-Aug 15-Aug 22-Aug 3-Aug 9-Sep 19-Sep 3-Sep 5-Oct 1-Oct 19-Oct 25-Oct 18-Nov 9-Nov 16-Dec Figure Grain size distribution by percentage mass for TSS at Pakse. Percentage Figure 4.3. Grain-size distribution of suspended load at Kratie, 211. Percentage Figure Grain size distribution of suspended load at Prek Kdam, 211 Percentage Coarse Silt Fine & Med Silt Clay 7 Jul 22 Jul 26 Jul 8 Aug 22 Aug 18 Sep 5 Oct Coarse Silt Fine & med silt Clay 7 Jul 15 Jul 21 Jul 28 Jul 5 Aug 14 Aug 2 Aug 27 Aug 9 Sep Coarse Sand Med Sand Fine Sand VF Sand Coarse Silt Med Silt Clay 15-May 15-Jun 1-Jul 8-Jul 15-Jul 23-Jul 1-Aug 15 Nov Figure Grain size distribution of suspended load at Tan Chau, 211. The clay fraction includes medium and fine sand for the 15 May sample due to insufficient sediment to complete hydrometer analysis. 8-Aug 15-Aug 23-Aug 1-Sep 15-Sep 15-Oct 25 Sep 15-Nov 15-Dec 23 Nov Figure Grain size distribution by mass for TSS at Pakse. There is some confusion over the mass of sediment used in the analysis and the masses indicated may not be correct. TS S (g) Coarse Silt Fine & Med Silt Clay 7 Jul 22 Jul 26 Jul 8 Aug 22 Aug 18 Sep 5 Oct 15 Nov Figure Grain-size distribution by mass for TSS at Kratie, 211. TSS (g) Coarse Silt Fine & med silt Clay 7 Jul 15 Jul 21 Jul 28 Jul 5 Aug 14 Aug 2 Aug 27 Aug 9 Sep 25 Sep 23 Nov Figure Grain-size distribution by mass for TSS at Prek Kdam. TSS (g) Coarse Sand Med Sand Fine Sand VF Sand Coarse Silt Med Silt Clay 15-May 15-Jun 1-Jul 8-Jul 15-Jul 23-Jul 1-Aug Figure Grain-size distribution by mass for TSS at Tan Chau. The clay fraction includes medium and fine sand for the 15 May sample due to insufficient sediment to complete hydrometer analysis. 8-Aug 15-Aug 23-Aug 1-Sep 15-Sep 15-Oct 15-Nov 15-Dec

34 Percentage Figure Grain size distribution of suspended load at Chau Doc, 211. The clay fraction includes medium and fine sand for 15 May, 15 June, 1 July and 15 November samples due to insufficient sediment to complete hydrometer analysis. Average Velocity (m/s) Coarse Sand Med Sand Fine Sand VF Sand Coarse Silt Med Silt Clay 15-May 15-Jun 1-Jul 8-Jul 15-Jul 23-Jul 1-Aug 8-Aug 15-Aug 23-Aug 1-Sep 15-Sep Pakse Kratie Prek Kdam 15-Oct 15-Nov 15-Dec. 17-Apr 6-Jun 26-Jul 14-Sep 3-Nov 23-Dec 11-Feb TSS (g) Coarse Sand Med Sand Fine Sand VF Sand Coarse Silt Med Silt Clay 15-May 15-Jun 1-Jul 8-Jul 15-Jul 23-Jul 1-Aug Figure Grain-size distribution by mass for TSS at Chau Doc. The clay fraction includes medium and fine sand for 15 May, 15 June, 1 July and 15 November samples due to insufficient sediment to complete hydrometer analysis. Figure Average flow velocities from ADCP profiles at Pakse, Kratie and Prek Kdam, Aug 15-Aug 23-Aug 1-Sep 15-Sep 15-Oct 15-Nov 15-Dec 4.4 Bedload transport at Kratie nad Nong Khai In situ bedload monitoring was completed at Kratie on 14 occasions and at Nong Khai on 4 occasions during the 211 monitoring year. Bedload was collected using a BL- 54 sampler and the collected mass was weighed and analysed for grain-size distribution. The mass of bedload collected on each monitoring day at Kratie and Nong Khai is presented in Figure Due to the limited number of results from Nong Khai, interpretation is limited however the results suggest that during the early part of the wet season, bedload transport increases as flow increases. Bedload fluxes at Kratie continue to increase through early September whereas the one data point for Nong Khai shows a bedload rate similar to earlier in the year. Compared to suspended sediment transport, bedload is a small component of the total sediment load at Kratie, contributing 1 3% for all monitoring periods except November 211 when bedload contributed 9% of the total sediment flux. A similar analysis cannot be completed for Nong Khai as there are no total suspended sediment results available for the period. The Kratie results were used to develop a sediment rating curve for all size fractions (Figure 4.41), and for sand sized material only (Figure 4.42). Using the same hydrograph as used to determine total suspended solid transport, total annual bedload transport at Kratie is estimated at 1.6 Mt/yr, with 1.5 Mt/yr attributable to sand. Using an average interval approach yielded an annual estimate of 1.8 Mt/yr for total sediment transport with 1.7 Mt/yr attributable to sand

35 Sediment Flux (Tonnes/day) 18, 16, 14, 12, 1, 8, 6, 4, 2, - Bedload Transport Nong Khai Kratie Figure Bedload (tonnes/day) at Nong Khai and Kratie in ,, 1, 1, 1, 1 Sediment Flux (Tonnes/day)1,, Suspended Bedload Figure 4.4. Suspended and bedload sediment fluxes at Kratie in 211. Bedload Sediment Flux (tonnes/day) 18, 16, 14, 12, 1, 8, 6, 4, 2, Kratie Bedload All sizes y =.221x R² =.529 Figure Sediment rating curve for Kratie based on bedload monitoring results Discharge (m 3 /s) Figure Bedload rating curve for Kratie based on sand fraction of bedload monitoring results. Dividing the bedload into gravel, sand and silt at Kratie (Figure 4.43) and Nong Khai (Figure 4.44) show that the greatest variability in transport occurs in the sand sized sediment fraction, although there is limited data at Nong Khai.

36 Kratie Bedload Bedload (Tonnes/day) 18, 16, 14, 12, 1, 8, 6, 4, 2, Sand = mm 7-Jun Jun Jul Jul Aug Aug-11 5-Sep-11 5-Sep Sep-11 5-Oct Oct Nov Nov Dec-11 Figure Bedload transport of gravel (>2 mm), sand (.63-2 mm) and silt (<.63 mm) at Kratie for monitoring periods in Grav t/d Sand t/day Silt t/day Nong Khai Sand = mm 5 Bedload Flux (Tonnes/day) Jun-11 8-Jul Jul-11 2-Sep-11 Figure Bedload transport of gravel (>2 mm), sand (.63-2 mm) and silt (<.63 mm) at Nong Khai for monitoring periods in Grav t/d Sand t/day Silt t/day More detailed grain-size distribution results from Nong Khai show that bedload is dominated by very coarse sand at the site, whereas at Kratie, there is a higher proportion of medium sand. The September bedload sample collected at Nong Khai contains a very different range of grain-sizes which may indicate that the sampler dredged the bottom rather than collecting material being transported bedload. 4 Nong Khai 35 Bedload Sediment Flux (Tonnes/day) Figure Detailed grainsize distribution at Nong Khai, Jun-11 Jul-11 Jul-11 Sep-11 F Silt <.2 Med Silt.2-.45mm C silt mm F+VF Sand mm Med Sand mm VC+C Sand.43-2mm Gravel >2mm

37 18 16 Kratie Bedload 5 45 Bedload Sediment Flux (Tonnes/day) Discharge (m3/s) F Silt <.2 Med Silt.2-.45mm C silt mm F+VF Sand mm Med Sand mm VC+C Sand.43-2mm Gravel >2mm Discharge (m3/s) Figure Detailed grainsize distribution at Kratie, 211. Jun-11 Jun-11 Jul-11 Jul-11 Aug-11 Aug-11 Sep-11 Sep-11 Oct-11 Oct-11 Nov-11 Nov-11 Dec Bed Materials Bed materials were collected from throughout the Mekong catchment at the sites indicated on Figure 4.47 and Figure The bed samples were collected at different times of the year (Table 1), reflecting different flow regimes. The grain-size results for all monitoring sites are contained in Appendix 1, with the results for the sediment monitoring sites presented in Figure 4.49 and compared in Figure 4.5. Table 1. Bed material monitoring dates for sediment monitoring sites. Site Sampling date River stage Luang Prabang 9 November 211 End of falling stage Nong Khai 23 August 211 Peak flow Mukdahan 14 September 211 Peak flow Pakse 28 November 211 End of falling stage Stung Treng 8 June 211 Rising stage Kratie 7 June 211 Rising stage Chroy Chang Var 5 June 211 Rising stage Koh Norea 4 June 211 Rising stage Chau Doc 16 July 211 Rising stage Tan Chau 16 July 211 Rising stage The samples collected in Laos (L1 L11), were collected at the end of the wet season, show low variability and are dominated by very fine and fine sand, consistent with the deposition of fine material under low flow conditions. The timing of sample collection probably accounts for the similarity between Luang Prabang and Pakse in Figure 4.5. The grain-size distribution results for Luang Prabang (Figure 4.27) show that the suspended sediment load at the beginning of the wet season is composed of sands, reflecting the remobilisation of this deposited material. The samples collected upstream of the delta during the rising and peak flow periods contained higher percentages of medium and coarse sands and gravels, and show greater variability across the transect, consistent with higher flow velocities. Differences in sediment textures between Pakse and Stung Treng probably reflect temporal differences in the river, owing to the different sampling times, rather than spatial changes downstream.

38 The proportion of medium and fine sands, and silt increases downstream of Kratie, and at Koh Norea contribute ~88% of the sample. In the delta, the fining of the bed material continues, with very fine sand and silt comprising the majority of the bed material. This is consistent with the reduction of flow velocities occurring within the delta region. The individual transect results from the delta sites show a bimodal grain-size distribution of medium sands and silt. There is very little clay present in the bed at any of the bed material monitoring sites.

39 L1 L2 L3 L4 L5 L6 L7 L1 L9 T3 L8 T4 T5 T8 1 T7 T6 T1 T11 T12 T14 T15 T16 T17 T18 T19 T2 L11 K1 K3 K5 K4 K6 K7 K9 K8 K2 Figure Map of bed material samples collected from the Mekong in 211. Location labels denote sample ID of grain-size distribution results in Appendix 1. Delta samples shown in Figure Map provided by MRC.

40 Figure Location of bed materials collected from delta in 211. Grain size distribution results shown in Appendix 1. MK denotes Mekong, BS denotes Bassac. Map provided by Vietnam National Mekong Committee. Cumulative Percentage T15-Mukdahan Average Size Fractions (mm)

41 Figure Grain size distribution results for bed material collected in 211. Results for each transect ar shown, with the site average indicated by thicker orange line.

42 Luang Prabang Gravel >2mm Coarse &VC Sand.5-2mm Med Sand.25-.5mm Fine &VFSand mm Silt Nong Khai Pakse Mukdahan Stung Treng Kratie CCV Koh Norea Tan Chau Chau Doc Figure 4.5. Grain size distribution at sediment monitoring sites. No analyses are available for Chiang Sean, Nakhon Phanom or Kong Chiam.