Guidelines and Recommendations for Mitigation of Hydrological and Flow Impacts Kees Sloff and Jenny Pronker

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1 Mekong River Commission Office of the Secretariat in Vientiane 184 Fa Ngoum Road, Ban Sithane Neua, P.O. Box 6101, Vientiane, Lao PDR Tel: (856-21) Fax: (856-21) Office of the Secretariat in Phnom Penh 576 National Road, no. 2, Chok Angre Krom, P.O. Box 623, Phnom Penh, Cambodia Tel: (855-23) Fax: (855-23) ISH Consultancy for the Development of Guidelines for Hydropower Environmental Impact Mitigation and Risk Management in the Lower Mekong Mainstream and Tributaries Guidelines and Recommendations for Mitigation of Hydrological and Flow Impacts Kees Sloff and Jenny Pronker What will you learn? The ISH0306 approach Know your basin Risks, impacts and vulnerabilities of HP development Using the mitigation hierarchy From individual schemes to a joint operation of multiple schemes 2 1

2 About your ISH trainers for these 2 sessions Kees Sloff PhD on reservoir sedimentation in 1997 Deltares since 1995: specialist Assistant Professor at Delft University since 2001 Jenny Pronker MSc Civil Engineering 2017 Thesis on Impacts of Hydropower on the Mekong Delta Starting at CDR International next month Lois Koehnken PhD Sediment transport in the Orinoco River Basin in 1990 Director L Koehnken Pty Ltd, Australia Sediment speciailist in several MRC programs Introduction to Mitigation HPPs have the potential to directly alter flow and sediment movement in river systems Rivers will respond to these changes, and these responses may have negative impacts with respect to: - The physical integrity of the river - The ecology of the river - The social uses of the river Ideally negative impacts are avoided but this is rarely possible When negative impacts cannot be avoided, need to minimise and mitigate 2

3 Consider the life cycle : each dam will influence the river for generations Many dams are built to exist for a century Impacts will last at least 4 generations, and probably even beyond that (even if dam is removed) A HPP scheme is just an element of the system or basin: impacts extend basin wide 3

4 Guidelines and Recommendations for Mitigation of Hydrological and Flow Impacts The ISH0306 approach Hydrology and flows 7 Step 1: know your system Hydrology of the Mekong Monsoon June Nov Dry season Lot of rain in the Eastern Lao catchments Timing of start and end of wet season is very regular (2 weeks variation) 4

5 Step 1: know your system Hydrology of the Mekong Lancang (China) only contributes 16% to average annual flow Effect China more relevant in Vientiane than Kratie, and more relevant in dry season Step 1: know your system: establish baseline Collect your required data - Historic data - Targeted surveys (install your equipment) - Remote sensing data (inundated areas, etc) Complete assessments using models (computational, physical) 5

6 Step 2: risks, impacts, vulnerabilities of HP development Lancang Rainfall to runoff Tributary run off River runoff hydrograph Storage Evaporation/rain Dams Tributary run off Evaporation/rain modified hydrograph cascade Storage Tonle Sap Salinity intrusion Step 2: risks, impacts, vulnerabilities of HP development For impacts on hydrology and flow the reservoir and how it is operated is more relevant than the dam ponce.sdsu.edu 6

7 laotiantimes.com Fishbio.com 15/01/2018 Step 2: risks, impacts, vulnerabilities of HP development: storage versus run-of-the-river Storage reservoirs (often high dams) are used to capture the fluctuations in the inflow discharge: downstream of such dams the flow is more constant (reduced high flow, increased low flow) Run-of-the River reservoirs (often low dams) operate with a more-orless constant water level: all incoming discharge is going through the turbines and spillways. Downstream no impacts. Nuozhadu dam, Lancang: storage Xayaburi dam: run of the river Step 2: risks, impacts, vulnerabilities of HP development: values and risk Understanding values & risks What needs to be protected after HPP development? Hydrology and flows examples: - Physical: Timing of onset of wet season; Tonle Sap reversal; inundation depth and inundated areas of flood plains; - Ecological: Sustain wetlands; trigger for fish migration; deep pools; - Social: availability of clean fresh water; tourism; navigability; 7

8 flow flow 15/01/2018 Importance and value of flow in the Mekong basin What needs to be protected after HPP development? Annual / inter-annual changes to flow Changes in seasonality & Change of timing & duration of continuous uniform release floods and low flows, changes in flows Tonle Sap and changes in salt intrusion in the delta Modification of flood intervals: Reduction in occurrence of minor floods & no change in large events Peaks in flood and low flow change, smoother hydrograph jan jan dec dec Daily / short-time period changes in flow Hydro-peaking Safety and navigation related changes caused by sudden rise or drop of water levels Pulses by operations idem Secondary (indirect) risks through impacts on sediment movement, fish, navigation, water quality 8

9 flow flow 15/01/2018 Annual / inter-annual changes to flow Changes in seasonality & Change of timing & duration of continuous uniform release floods and low flows, changes in flows Tonle Sap and changes in salt intrusion in the delta Modification of flood intervals: Reduction in occurrence of minor floods & no change in large events Peaks in flood and low flow change, smoother hydrograph jan jan dec dec primary risks that have been identified for the mainstream dams Daily / short-time period changes in flow Hydro-peaking Safety and navigation related changes caused by sudden rise or drop of water levels Pulses by operations idem Secondary (indirect) risks through impacts on sediment movement, fish, navigation, water quality Example: annual/seasonal impacts of storage schemes Lower reaches of the Mekong River will: have higher flow in dry season have later start of wet season have lower high flows in start of wet season UMB (main) LMB (main) 3S 9

10 Discharge 15/01/2018 Example: annual/seasonal impacts of storage scheme in the Colorado River USA Glen Canyon dam Lake Powel 1296 MW 1963 Year Example: Cumulative impacts of small hydropower North Vietnam Nam Chien 1 Limited sediment entrapment PS Nam Chien 2 PS Q Large reservoir blocks much sediment and discharge Bypass: river only wet during flood Q low high flow, high low flow Q Pa Chien PS t t 10

11 Example: impacts daily fluctuations of HP operation: Colorado River USA Daily fluctuation related to power peaking: Causes erosion of sand bars (bank erosion) 15 January 2018 =0.6 m Water level Mile January range of water-level variation 46 Step 3: Quantification: Indicators and monitoring Table 2.2. Hydrology and Flows indicators. System components Hydrology And Flow LMB level Flow (sub-daily/ every hour or minute) Water level (sub-daily/ every hour or minute) National/ local level Onset of wet season Duration of wet season Minimum flows average wet season peak daily flow average flow volume entering Tonle Sap monthly average dry season flow (i.e. flow in march) total wet season flow volume 23 11

12 Step 3: Quantification: use of modelling tools 0-D Catchment & basin models Detailed 3D reservoir models (cascade), ½-D hydropower models Downstream impacts: 1D hydraulic models Step3: Testing impacts: compliance with PMFM 1 flow procedures (example) 90% FDC Dry season planning purpose flow criteria (Vientiane) Wet season planning purpose flow criteria (Vientiane). 1 Procedures for the Maintenance of Flows on the Mainstream 12

13 Step 3: Testing impacts: compliance with PMFM flow procedures: result for upper cascade BDP2030 & mainstream dams 90% historical 90% FDC (criteria) Scenario calculations with PMFM flow criteria (dry season at Vientiane) 90% values (monthly values). Step 3: Compliance with PMFM flow procedures: result for upper cascade BDP2030 & mainstream dams 90% historical 13

14 Step 3: Upper cascade: seasonal changes Indicator: Flow volume of the wet season Indicator: daily flow characteristics UPSTREAM LUANG PRABANG Violin plots historic BDP 2030 Sc Wet season onset 14 Jun ±36 days 26 Jun ±39 days 25 Jun ±39 days Wet season duration 152 days ± 36 days 142 days ± 37 days 142 days ± 37 days Conclusions: the wet-season flow volume and the daily characteristics do not differ between the cascade-scenarios; major difference between historic and BDP2030 future (Chinese dams and tributary dams) Step 4: Identify mitigation options appropriate to values and risks Wide range of mitigation options: - Designing HPPs that minimise impacts & maximise mitigation options - Implementing infrastructure to enable mitigation - Developing operating rules to achieve mitigation goals - Coordinating operations with other HPPs minimise impacts and maximise mitigation - Implementing catchment management to reduce overall impacts and maximise benefits Mitigation approaches may change over life-cycle of project - Construction / operations / decommissioning 14

15 Step 4: Avoidance Mitigation - Compensation The risks, impact and vulnerabilities is the basis for the detailed mitigation options Using the full mitigation hierarchy (avoidance, minimization, compensation) should be the New Frontier for the LMB countries 30 Step 4: Identify mitigation options Use of the Tables in Volume 1 and find details on solutions in the Manual Distinguish: planning/design (mostly new dams) Operation (mostly existing dams) Consider the mitigation hierarchy 31 15

16 Risks / Impacts Hydrology and downstream flows 1) Change of timing & duration of floods and low flows 2) Peaks in flood and low flow change, smoother hydrograph 3) Changes in Tonle Sap flows and salt intrusion in the delta Geomorphology and Sediments 1) Water logging & loss of vegetation leading to increased bank erosion Increased erosion due to increased scour 2) Winnowing of smaller sediment leading to bed armouring & reduction in downstream sediment supply 3) Channel narrowing through encroachment of vegetation 4) Decoupling of tributary & mainstream flows. Erosion and / or deposition due to tributary rejuvenation 5) Backwater sedimentation causing flood-level increase upstream Water quality 1) Changes / loss of seasonal temperature patterns downstream 2) Increased water clarity increasing water temperature and risk of algal growth Table 5.1 (I) Annual/Inter Annual Changes to Flow Planning / design / construction MP=Master Plan; F=Feasibility Stage; D=Design; C=Construction (I.1) Avoidance Operation Options Indicators Options Indicators (I.1.1) Dam siting in Master Plans River length affected; to avoid risks and impacts in contribution to LMB flow and themes hotspot areas sediment loads (I.1.2) Selection of sites with less hydrological and sediment impact (I.2.) Mitigation (I.2.1) Development of flow Minimum flow, hydraulic rules (MP and F) parameters, magnitude, (I.2.2) Develop joint operation duration, timing of wet and rules for releases (F) dry season flows (I.2.3) Design multiple large gated spillways/outlets at multiple levels, and low level sediment outlets (D) (I.2.4) Design bypass channels (F and D) (I.3) Compensation Plan for and implement; (I.3.1) Creation of offsets of residual impacted habitats and areas (F and D) (I.3.2) Floodplain and wetland rehabilitation (F and D) (I.4) Adaptation Implementation of operating rules Area of offsets and improved floodplain and wetland habitats (I.2.5) Mimic natural flow regime (artificial releases, environmental flows) (I.2.6) Maintain seasonal patterns through HP operations (I.2.7) Annual sediment sluicing to maintain seasonal pulse (I.2.8) Monitoring of impacts (I.3.3) Monitor offsets and floodplain and wetland rehabilitation Minimum flow; onset of wet season; magnitude, duration of wet/ dry season flows (flow duration curve); changes to fish diversity/ biomass, sediment loads and timing of sediment delivery, extent and timing of salinity intrusion Changes to diversity/ biomass of fish and other aquatic organisms 32 Risks / Impacts Hydrology and downstream flows 1) Short term flow fluctuations 2) Safety and navigability Geomorphology and Sediments 1) Rapid wetting and drying of banks 2) Increase in shear stress on river channel Water quality 1) Fluctuating temperature and water quality 2) Altered concentrations of downstream WQ parameters Fisheries and Aquatic Ecology 1) Degradation of riparian and instream habitats 2) Thermopeaking 3) Increased fish/ macroinvertebrate drift and stranding 4) Offset of migration triggers 5) Loss of food sources Biodiversity, Natural Resources and Ecosystem Services 1) Degradation of function, dynamics and ecosystem services of wetland and Table 5.2 (II) Short-term flow fluctuations / Hydro-peaking Planning / design / construction Operation MP=Master Plan; F=Feasibility Stage; D=Design; C=Construction Options Indicators Options Indicators (II.1) Avoidance (II.1.1) Dam siting in Master Plans to River length affected; avoid risks and impacts in themes quickly dewatered hotspot areas (MP) area (II.1.2) Selection of sites where impacts are reduced by entering tributaries (MP) (II.2.) Mitigation (II.2.1) Development of flow rules (F and D) (II.2.2) Design of re-regulation weir (D) (II.2.3) Coordination of multiple hydropeaking HPP (II.2.4) Design of aeration weir (D) (II.2.5) Avoidance of flow fluctuations during construction (C) (II.2.6) Establish protected areas and evacuation paths for inundation zones (C) (II.2.7) Flexible mooring structures for ports (D and C) (II.2.8) River-bank stabilisation works (C) (II.3) Compensation Plan for and implement; (II.3.1) Habitat improvement (F & D) (II.3.2) Floodplain and wetland rehabilitation (D and C) Ramping frequency, amplitude, ramping rate, minimum flow temperature, dissolved oxygen, downstream damping of water-level fluctuations (II.2.9) Re-regulation warning systems (II.2.10) Operating rules to minimise flow changes, management of reregulation weir to provide appropriate downstream flow (II.2.11) Monitoring of impacts Area of improved (II.3.3) Monitor habitat floodplain and wetland improvement and habitats rehabilitation (II.4) Adaptation Monitoring, adaptive management (based on monitoring data) Catchment management to maximise water quality in and discharged from impoundment Ramping frequency, ramping amplitude, ramping rate, minimum flow, changes to fish diversity/ biomass. Bank / bed erosion rates Downstream temperature, D.O. downstream damping of water-level fluctuations Changes to fish diversity/ biomass 33 16

17 Mitigation measures (examples) Mitigation examples Ramping (limit water level change) Diversion channel Artificial flood Flow regulator 34 Mekong Example: mitigation seasonal impacts Tonle Sap Tonle Sap Impacts to be mitigated: Timing of flow reversal Change in volumes exchanged Blue: increase Green: decrease Mitigation: - Increase profile of Tonle Sap River - Create upstream diversion channel - Generate flood pulse ( Million m 3 ) Not a solution for the delta (Tonle Sap en flood plains act as storage and will absorb the pulse! Flood plains 35 17

18 Example: daily fluctuations (power peaking) Ramping (rate in m/hours at which water levels may change): - Relevant for ecology (fish stranding), navigation, safety, etc. - Consider natural rates of water-level variation (for instance 0.05 m/hour) 36 Example: Regulation weir at Nam Ngiep 1 The re-regulation reservoir is to store water discharged from the main dam for 16-hour peak power generation, re-use it for power generation and release it downstream evenly on a 24-hour basis on weekdays and Saturday Operation level between EL and EL (eff. Storage 4.6Mm 3 ) 37 18

19 Example: backwater upstream of Pak Beng reservoir Pak Beng dam: transboundary issue. Impact: During dry season Keng Pha Day reefs should be emerged. Due to backwater of dam the reefs get submerged. Mitigation: Model results show that water level can be reduced at low discharge by applying a 5 m lower operation level during dry season 38 Step 5: Take home messages Hydrological mitigation is mostly for impacts which are not directly hydrological, but more aimed for indirect effects, for instance for fish and sediments Mitigation for flow covers the full life cycle of the reservoir (many generations) and a full basin (including other developments in the Mekong basin) The large extent of the flow impacts of any scheme means that transboundary issues are relevant The ISH0306 guideline does not cover all aspects (socio-economics, dam-safety, navigation), but that does not mean these have no consequences for mitigation of HP impacts Mitigation of annual/seasonal impacts often involves joint operation of multiple dams 39 19

20 Summary of approach for hydrology and flow mitigation in ISH0306 Review literature, regional experience, international experience - ISH Manual contains many examples of mitigation approaches relevant to the LMB - Aim of mitigation should be to retain values and minimise risks through Avoidance, minimisation, mitigation Maximise operational flexibility Mitigation includes: - Planning siting and design of projects - Infrastructure gates, re-regulation weirs, aeration weirs, by-pass channels - Operating regime - environmental flows (low, med & high flows),ramping rates, lake level constraints, etc Questions? Nam Ou, Muang Ngoi, Laos 20