CAMBODIA LAO PDR THAILAND VIETNAM REVIEW ON HYDROLOGY
CAMBODIA LAO PDR THAILAND VIETNAM REVIEW ON HYDROLOGY AND HYDRAULICS, AND SEDIMENT AND RIVER MORPHOLOGY FIRST DRAFT OF THE TECHNICAL REVIEW REPORT FOR SANAKHAM HYDROPOWER PROJECT’S PRIOR CONSULTATION PROCESS THE 10 TH MRC REGIONAL STAKEHOLDER FORUM – REGIONAL CONSULTATION ON THE PRIOR CONSULTATION PROCESS FOR SANAKHAM HYDROPOWER PROJECT 24 NOVEMBER 2020, PAKSE, LAO PDR, VIA VIDEO CONFERENCE Prepared by MRC Secretariat Presented by Dr. Ly Sarann, Water and Climate Monitoring Specialist, Technical Support Division
Hydrology and Hydraulics Hydrology and hydraulics determines how infrastructure is designed and operated for hydropower production, navigation, sediment routing and environmental and social considerations. Hence it is relevant for all topics and addressed up front. • • Data used Derived discharge Tailwater calculations Design floods Operations Hydrological monitoring Modelling tools Impacts related to hydraulics of the structure MEETING THE NEEDS KEEPING THE BALANCE
Background: concerns and risks • Sanakham HPP is sited directly upstream of border with Thailand: how does operation of SNHPP contribute to changes in seasonal flows and daily fluctuations downstream? Laos SNHPP Thailand • Did the developer use proper methods to quantify the physical behavior of the river, Laos and its variability (wet/dry years, upstream dams, detailed flows near the infrastructure, etc. )? • How about the role of SNHPP in the Lao dam cascade, and its contribution to the impacts of the cascade? MEETING THE NEEDS KEEPING THE BALANCE
Pre-project monitoring & analysis • Only Hydrological data of distant stations Luang Prabang and Vientiane are used, while passing over nearby Chiang Khan station discharge • Data only used until 2012 to exclude impacts of Lancang Cascade and upstream tributary flows: these data do not represent current hydrology • Trendlines with poor correlation between Vientiane and Luang Prabang data are used to extend Luang Prabang series for period 1923 -1961 • Insufficient information on the QA/QC procedures and technical details on used equipment for hydraulic data • Only rainfall and evaporation from Vientiane and Xayaburi were collected, mentioned, and not used for further assessment, modelling or validation • Derived daily discharges with measured daily values was not compared. • Area-scaling approach is not accurate for this part of the LMB. • Rainfall not included when deriving daily discharges and flood peak data. MEETING THE NEEDS KEEPING THE BALANCE
Pre-project monitoring & analysis M ONTHLY R UNOFF AT D AM • S ITE Different monthly average discharges used in different submitted documents • Inconsistencies between documents submitted, which has made this review challenging. I MPACT OF UPSTREAMDAMS • Results do not accommodate substantial impacts of upstream tributary storage dams • Hydropeaking operations of Pak Lay HPP will make rapid changes in flow which must be accommodated in SNHPP operating rules Relative contribution of discharge from catchments and variation in the contribution between wet and dry seasons, for station Vientiane, from MRC (2009) % of flow affected by Lancang Cascade • Draw down sediment sluicing in SNHPP has not been tailored to sluicing operations of Pak Lay HPP (joint operation) I MPACTS OF CLIMATE CHANGE: Developer has not included potential impacts of climate change on flow regimes. MEETING THE NEEDS KEEPING THE BALANCE % of flow affected by tributary dams
Tailwater calculations Unknown how flood data point for 2008 has been obtained as measurements did not start before 2009. Data points for 2015 (red triangles) do not cover full range of observed discharge measurements in that year. Add recent data and compare to Chiang Khan data Riverbed downstream will scour, which will lower the tailwater. This will in turn affect the design of the navigation lock and fish pass entrances. MEETING THE NEEDS Rating curve used as tailwater level for SNHPP KEEPING THE BALANCE
Flood frequency and design flood analyses • Final design values for dam are not determined by presented extreme-value Comparison of changes to the magnitude of extreme events for the analyses. Values from the CNR same return period over an estimated project life of 100 years optimization study of 2009 have been (source: ICEM, 2010) copied. • No determination of PMF (Possible Maximum Flood) from hydrological calculation for extreme tropical storms • No account for possible flood releases from upstream dams during high floods • Similarly, potential effects of climate change on peak flows were not considered • Probability of flood volume and flood discharge should be presented jointly MEETING THE NEEDS KEEPING THE BALANCE Past 1/10, 000 yr In 2030 1/1, 000 yr
Impact assessment and risk mitigation Project monitoring and adaptive management H YDROLOGICALMONITORINGPROPOSEDBY THE DEVELOPER • Alternative site for upstream discharge monitoring should be considered, as Luang Prabang site is now in the Xayaburi impoundment • Project can also benefit from connecting to hydrological monitoring systems installed by upstream dam operators M ODELLINGTOOLS FOR IMPACTASSESSMENT • Insufficient detailed information to judge accuracy of calibration of 1 D and 2 D modelling tools. • Hydrological SWAT model has not been applied correctly for representing dam operation during Tb. ESIA&CIA, and it has not been further used to support feasibility study R ESERVOIROPERATIONRULE • Run-of-river operation does not exclude rapid fluctuations in releases, but their downstream impacts and mitigation have not been considered. • Decreased velocities in impoundment will impact on sediment deposition, water quality and fish (larval drift) • Power peaking of SNHPP has not been ruled out: this requires a clear decision and actions MEETING THE NEEDS KEEPING THE BALANCE
Impact assessment and risk mitigation Project monitoring and adaptive management I MPACTS RELATEDTO HYDRAULICSOF THE STRUCTURE • Sanakham dam is sited at a sharp river-bend, which will affect the hydraulics close to the dam structure and outlet structures • Energy dissipation structures (stilling basins) downstream of sluices have been designed such that they fail long before design flood of the dam (see dam safety theme) • Information on calculations for dimensions of the stilling basins and rip rap have not been shared • Coefficients and equations for sluice capacity need clarification, and validation in physical model test MEETING THE NEEDS KEEPING THE BALANCE
Compliance with PDG 2009/Good Practices by draft PDG 2020 • PDG 2009 contains no clauses for hydrology and hydraulics, but EFA is needed to determine how outflows should be managed to maintain downstream ecosystems (important because of transboundary impacts!) • Good Practices by draft PDG 2020: Most clauses have been addressed, but many of them have issues Paragraphs Remarks (selection of issues summarized from Annex B) Risks/Impacts Note comprehensively assessed (downstream impacts missed); impacts of upstream dams inadequately included; impacts on floods and droughts not well assessed Pre-project analyses Historic time series has issues; model and data not well combined; hydraulic models do not extend far enough downstream Guidance for monitoring Some more details required on monitoring framework; consider interaction with other data sources (other dams) and joint monitoring; locations of stations downstream; method of rating curve; use of WMO and HYCOS guidelines not clear Guidance for analysis Effect upstream dams not modelled; Climate change not modelled; Models for downstream reach missing; Downstream impacts ignored; Operational guidance Cascade joint operations not considered; no responses to downstream environmental conditions; ramping rates not addressed; indicators for adaptive management not defined. MEETING THE NEEDS KEEPING THE BALANCE
Overall finding for Review on hydrology and hydraulics • Not full use of the wealth of data and tested methodologies available from the MRC • Information that has later been superseded by newer studies • Considerable inconsistencies and contradictions in the information provided MEETING THE NEEDS KEEPING THE BALANCE
Sediment and River Morphology The downstream transport of both fine- and coarse-grained sediment in the Mekong River is important to maintain the structure and functioning of downstream ecosystems, and to carry nutrients onto the floodplain areas. MEETING THE NEEDS • • • Pre-project monitoring and analysis Post-project sediment transport dynamics Geomorphology Information Design and operation of the sediment facilities Sediment transport modelling Project monitoring and adaptive management KEEPING THE BALANCE
Pre-project monitoring & analysis ► ► ► Old data, predating completion of XHPP, and in many instances, HPP on Lancang Cascade. This must be updated with current monitoring data. Developer’s total sediment loads are about 3 x higher than those determined by MRC (Developer 69. 0 Mt/year vs MRC 22. 0 Mt/year) Difference has profound implications for assessing the likely changes that may occur due to SNHPP Pre-dam sediment loads of ~21 Mt/yr at Sanakham dam site under the ‘no dam’ scenario. Under pre-cascade conditions, most sediment load passing through Sanakham dam site is composed of fine sand. Post completion of cascade, virtually all of sand gravel is captured by HPP impoundments, with only silt and clay and very minor quantities of fine-sand discharged downstream, with a total about 5 Mt/year These changes are not addressed in the FS Will alter downstream habitats MEETING THE NEEDS KEEPING THE BALANCE
Post-project sediment transport dynamics ► ► Models are based on inaccurate input data Not representative of existing or postproject sediment dynamics. Project site is only 1. 5 to 2 km upstream of Lao-Thai border ► ► Lao PDR Alluvial channel Near dam impacts will be transboundary No detailed description of potential impacts ► Sediment loads & grain-size ► Morphology of channel, banks MEETING THE NEEDS Dam site Thailand River downstream of Sanakham is alluvial and highly susceptible to change Area of high biodiversity KEEPING THE BALANCE
Geomorphology Information – Lacking in detail & mitigation MRC modelling shows erosional wave progressing downstream after construction of full cascade Sediment load to decrease from ~20 Mt/yr to ~5 Mt/yr ► ► ► No consideration of potential geomorphic impacts associated with fluctuating water levels Changes to river thalweg, tributary confluences, and potential for mid-bar formation in inundated area have not been considered. No provision of any mitigation approaches, such as offsets, to address permanent loss of habitats. MEETING THE NEEDS ► ► No discussion how erosion could affect ecological processes or socioeconomic aspects, such as riverbank gardens and sand mining. Any riverbank protection works undertaken in Thailand must be undertaken under guidance and with permission from relevant Thai authorities. KEEPING THE BALANCE
Design & operation of the sediment facilities • Right gates have the lowest sill level and are aligned with the thalweg, but they are not aligned with the main flow paths through dam infrastructure • Deep sediment deposits required to accumulate in front of dam before bedload can be passed through gates • Combined area of 4 lower-level gates (800 m 2) is larger than those proposed at the upstream Pak Lay project, and similar to those at Xayaburi. However, sediment management needs to be considered at the cascade level, • Proposal to open all gates and lower water levels when the Pak Lay HPP flushes sediments requires monitoring and modelling to guide operations. • Developers need to describe how hydropeaking at any of the upstream HPPs Low flow velocity in would affect the operations at Sanakham. front of right gates MEETING THE NEEDS KEEPING THE BALANCE
Sediment transport modelling (1 -D) Current modelling studies do not provide a reliable assessment of potential transboundary impacts. No model results for downstream changes 1 -D model does not capture erosion / deposition at sufficient detail Monthly flows used do not reflect present increased higher dry season flows since construction of Lancang and tributary dams. Extreme events, such as droughts are not considered Input sediment load is two to three times higher than recent monitoring results suggest. MRC & SNHPP modelling shows large reduction in flow velocity through impoundment MEETING THE NEEDS KEEPING THE BALANCE
Sediment transport modelling (2 -D) Lack of information about 2 D model Benefits of right bank lower-level gates not demonstrated Results show relatively low water velocities upstream of the right bank sluice gates during flood flows when all gates are opened. High sedimentation behind right gates No results showing opening of right gates only Model shows high sedimentation upstream of the low-level gates consistent with the low velocities, Only removes sediment near dam wall upstream. MEETING THE NEEDS Very limited information about changes to grainsize distribution of discharge over time is provided. No modelling results related to the effects of sediment flushing on downstream river or how effective it will be for minimising sediment deposition in impoundment. No modelling of transboundary impacts Changes to grain-size will have impact on downstream transboundary environment KEEPING THE BALANCE
Project Monitoring & Adaptive Management Proposed monitoring provided at a high level – Requires further development and refinement Use of Luang Prabang as the inflow site for the Sanakham project is inappropriate: • Location of gauge is now in backwater of impoundment • Xayaburi operations will alter the quantity, grain-size distribution and timing of sediment discharged downstream. Sediment inflows need to be monitored at the head of Sanakham impoundment. Potential for cooperative monitoring with Pak Lay Sampling methodology: Proposed point measurements need to be calibrated using depthintegrated, cross-sectional measurements that capture sediment transport through cross-section. Bathymetric monitoring needs to include collection of samples for determination of grain-size. This information is important sediment flushing modelling and management Description of downstream riverbank monitoring needs to include changes or triggers that will be used to determine whether mitigation measures are required. Needs to be implemented cooperatively with Thailand MEETING THE NEEDS KEEPING THE BALANCE
Compliance with PDG 2009/Good Practices by draft PDG 2020 • PDG 2009 requires the use of site-specific data for developing an understanding of sediment transport at site and developing appropriate sediment management strategies. Sediment monitoring should be done weekly (flood season) to monthly (dry season) for a full year prior to submission of HPP for prior consultation. Project does not align with these requirements. • Sediment facilities at SNHPP are only partially aligned with PDG 2009. • Project does not align with requirement to establish target sediment concentrations and flushing strategies. • Potential changes to thalweg and other geomorphic characteristics are not described and included in monitoring strategy. Project does not align with these requirements. • Project does not adequately consider transboundary impacts MEETING THE NEEDS KEEPING THE BALANCE
Recommendations Undertake sediment monitoring over a full year or more to be more accurate Update models based on recent information & provide more details on the 1 -D and 2 -D models, Provide model results based on implementation of full Northern Lao Cascade & for Xayaburi only. Consider redesign of low-level gates with lower base level & better alignment with major flow path Clarify sediment operating rules to address apparent difference between documents submitted. Provide more information on time-step Run-of-River inflow = outflow condition Provide estimated amount of sediment expected to be passed through the sluice gates Identify how much of shorelines upstream & downstream of the SNHPP may be prone to slumping. Work in Thailand must be done with the consent and participation of the Thai Authorities. Identify mitigation options / offsets for the habitats lost by the impounding of water. Update the descriptions of the proposed monitoring programme MEETING THE NEEDS KEEPING THE BALANCE
THANK YOU One Mekong. One Spirit.
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