Environmental Impact Assessment Project Number: 52111-001 February 2020 Samoa: Alaoa Multi-purpose Dam Project Volume 2: Geomorphic Impact Assessment (Part 8 of 9) Prepared by Hydro-Electric Corporation for the Asian Development Bank. This environmental impact assessment is a document of the borrower. The views expressed herein do not necessarily represent those of ADB's Board of Directors, Management, or staff, and may be preliminary in nature. Your attention is directed to the “terms of use” section on ADB’s website. In preparing any country program or strategy, financing any project, or by making any designation of or reference to a particular territory or geographic area in this document, the Asian Development Bank does not intend to make any judgments as to the legal or other status of any territory or area. ALAOA MULTIPURPOSE DAM Geomorphic Impact Assessment 26 June 2019 Prepared by Hydro -Electric Corporation ABN48 072 377 158 t/a Entura 89 Cambridge Park Drive, Cambridge TAS 7170 Australia Entura inAustralia is certified to the latest version of ISO9001, ISO14001, and OHSAS18001. ©Entura. All rights reserved. Entura has prepared this document for the sole use of the client and for a specific purpose, as expressly stated in the document. Entura undertakes no duty nor accepts any responsibility to any third party not being the intended recipient of this document. The information contained in this document has been carefully compiled based on the client’s requirements and Entura’s experience, having regard to the assumptions that Entura can reasonably be expected to make in accordance with sound professional principles. Entura may also have relied on information provided by the client and/or other parties to prepare this document, some of which may not have been verified. Subject to the above conditions, Entura recommends this document should only be transmitted, reproduced or disseminated in its entirety. - Revision No: 0 26 June 2019 Alaoa Multipurpose Dam Geomorphic Impact Assessment ConsultDM no. 1. Document information Alaoa Multipurpose Dam - Geomorphic Impact Assessment Revision No: 0 ConsultDM no. 26 June 2019 2. Executive summary The Asian Development Bank (ADB) commissioned Entura and Fluvial Systems to carry out a geomorphological characterisation and impact assessment for the proposed Alaoa Multi- Purpose Dam Project, on the Vaisigano River, Samoa. The proposed dam site is located mid- catchment, capturing flow from two eastern branches of the headwaters, which together comprise 47.8 percent of the total catchment area (Figure 1.1). This report is intended to be incorporated into the overall social and environmental impact assessment (SEIA) for the project. The objectives of this report are to describe the physical characteristics of the Vaisigano River catchment and Apia Bay under existing conditions, to assess the impacts of the proposed Alaoa Dam on those physical characteristics, and to recommend appropriate monitoring and mitigation measures. 2.1 Methodology and data This report used standard, up to date and appropriate data and methodologies to undertake the assessment. Topographic data were from a LiDAR (Light Detection and Ranging) survey. Terrain analysis, involving mapping of topography and slope, definition of sub-catchments and drainage lines, and calculation of reservoir capacity as a function of water level, was undertaken using GIS (Geographic Information System). Hydrology data were a modelled 48- year long hourly time-step discharge series at various locations throughout the catchment, and a water level series for the reservoir. The flow regimes of the modelled flow series were characterised using a range of standard hydrological statistics that covered the full range of flow components: minimum flows, low flows, baseflows, high flows and flood flows, and also characterised the seasonal distribution of flow components. A number of approaches were used to estimate suspended sediment load and bedload of the river, and rate of reservoir sedimentation. A model was developed for assessing reservoir shoreline erosion. This report took a standard 3-step approach to impact assessment, first characterising the existing environment, second, assessing the Project’s impact to the environment, and third, recommending mitigation measures and a monitoring program. 2.2 Existing environment The topography, geology, soils, climate and hydrology of the Vaisigano River catchment were described in detail. Hydrological analysis revealed that the current regime is highly altered relative to the natural regime. The load of suspended sediment from the catchment of the proposed Alaoa Dam was estimated using a number of methods. The results covered a wide range, suggesting a high level of uncertainty in the estimate of sediment load. However, the results suggest that the Vaisigano River has a high specific suspended sediment yield by world standards. Bed material particle size was measured in the field, and these data were used in a model to predict the bedload transport rate. This established that bedload transport rates were relatively low. Alaoa Multipurpose Dam - Geomorphic Impact Assessment Revision No: 0 ConsultDM no. 26 June 2019 2.3 Key impacts of the proposed Alaoa Dam The main potential impacts of the proposed dam and its operation on geomorphic processes and forms are: • Trapping of sediment in the dam reservoir, reducing the load of sediment to the river downstream, and to Apia Bay; • Scour of the shoreline of the reservoir rim due to focus of wind-generated waves at the normal full supply level (NFSL); i Alaoa Multipurpose Dam - Geomorphic Impact Assessment Revision No: 0 ConsultDM no. 26 June 2019 • Potential mortality of trees, shrubs and ground cover on hillslopes within the reservoir flood storage area during extended periods of inundation during high inflows, with subsequent risk of hillslope instability; • The rate of reservoir water level drawdown on flood recessions exceeding the rate at which hillslope soils can drain, with subsequent risk of hillslope slumping; and • Altered hydrology reducing the frequency of bed material mobilisation and reducing bed material load. Each of these impacts were assessed using numerical models. 2.4 Mitigation and monitoring The likelihood and consequence (risk) of each of the identified direct impacts of the proposed Alaoa Dam were assessed. The impacts were judged almost certain to occur, but the consequences were subjective and partly of an ecological nature. Reduced suspended sediment delivery to the river system downstream of the dam cannot be mitigated, but the geomorphic consequence of this is insignificant because nearly all of the sediment that would otherwise have passed through the river system would have entered Apia Bay and then flowed out to deeper water. Deposition of a large volume of fine sediment within the reservoir can only be mitigated by occasional flushing of the sediment by opening the low level offtake. This procedure was estimated to be required every 15 – 20 years, but the estimate is uncertain. The consequences of sediment flushing can be mitigated by arranging it to be done at a time when the catchment is experiencing a natural flood event, to assist transport of the turbid water to Apia Bay. Downstream of the Dam, the rate of bedload transport will be reduced due to reduced magnitude of flood peaks. However, it appears that the natural rate of bedload transport is low, so the consequences of a reduction are likely to be minor. With the dam operational, channel forming flows will operate at a lower magnitude, so the river will likely slowly adjust by contracting in width. The potential for mitigating this impact by environmental flows was investigated by Elvey and Gippel (2019). Their conclusion was that the most appropriate course of action was to allow the channel to contract and stabilise at a new dynamic equilibrium. Scour of the shoreline of the reservoir rim due to the action of wind waves is inevitable. Mitigation of this would be impractical, so it is recommended to allow the shoreline to erode to bare rock. 2.4.1 i i Submergence and waterlogging of trees, shrubs and ground cover on hillslopes within the reservoir flood storage area, as well as rapid drawdown, will create the risk of loss of soil and vegetation. It is recommended to seek additional expert advice on this matter. Mitigation could take the form of management of water levels and maintenance of good cover of vegetation that will tolerate the hydrologic and hydraulic conditions within the flood storage area. Monitoring of geomorphic form and process should focus on indicators that effectively characterise the main project impacts identified in this report, to determine if and to what extent the predicted impacts occur, and also the implemented mitigation measures, to determine the effectiveness or otherwise of the measures. The monitoring program would be a component of an adaptive approach to management of the dam and the Vaisigano River system. ii Alaoa Multipurpose Dam - Geomorphic Impact Assessment Revision No: 0 ConsultDM no. 26 June 2019 Monitoring that will assist sediment and erosion management includes: Annual sounding survey of the reservoir to determine sedimentosition dep rate Surveillance of the reservoir flood storage area immediately following drawdown of raised water levels in the wet season. Initial survey could be done by AUV (aerial unmanned vehicle), with any areas of potential vegetation death or soil instability inspected on the ground. iii Alaoa Multipurpose Dam - Geomorphic Impact Assessment Revision No: 0 ConsultDM no. 26 June 2019 3. Contents 1. Introduction 1 2. Methodology and data 3 2.1 Morphology of Vaisigano River catchment and Apia Bay 3 2.2 Geology and soils 3 2.3 Climate 4 2.4 Hydrology 6 2.5 River sediment character, transport and deposition 8 2.5.1 Introduction 8 2.5.2 Suspended sediment load 9 2.5.3 Bedload 14 2.5.4 Hydraulic modelling 15 2.5.5 Bed material particle size distribution measurement 16 2.5.6 Reservoir sediment trap efficiency 17 2.5.7 Reservoir shoreline erosion 19 2.5.8 Stability of hillslopes within the reservoir 22 3.