SUMMARY OF HYDROLOGICAL SPECIALIST STUDY

Transcription

1 SUMMARY OF HYDROLOGICAL SPECIALIST STUDY The Government of the Republic of Mozambique (GOM) has received a grant from the Millennium Challenge Corporation (MCC), an innovative United States established foreign assistance program designed to reduce poverty by promoting sustainable economic growth. The MCC operates on the principle that aid is most effective in countries that promote good governance, economic freedom and investments in people. The GOM, through MCA (Millennium Challenge Account-Mozambique, a public institution created by the GOM) and its National Directorate of Water (DNA-GOH) organ, intends to use a portion of the proceeds of this grant, to carry out feasibility studies relating to the rehabilitation and augmentation of the Nacala Dam and Reservoir. The Nacala Dam is located on the Muecula River approximately 30 kilometres south west of Nacala. The dam is the primary water source for the town of Nacala which is situated approximately 200 kilometres north east of Nampula City. This underlies the importance of the dam for the future development of Nacala Town and its port. The Nacala Dam was designed and constructed from 1968 to In 1982 the dam wall was overtopped for 10 consecutive hours due to the non operation of the spillway gates. This resulted in high seepage through the defective concrete on the right hand side of the spillway which flooded the fill downstream of the core and may have been the trigger for the concentrated erosion of the embankment in this area. In 1983 the wash away area was reinstated and a layer of stone pitching was placed on the downstream face of the wall as a protective measure against the possibility of future overtopping. Further repairs were undertaken in 1995 and 2002 resulting in a pre-feasibility study into the possibility of rehabilitating and raising the dam wall which was undertaken by Michael Baker Jr. Inc. (Baker) in In June 2009 Jeffares and Green (Pty) Ltd (J&G) were appointed to undertake the Nacala Dam Feasibility Study, Environmental and Social Impact Assessment, Design and Supervision Project. The Feasibility Study consists of many aspects and in addressing these aspects J&G has developed a suite of reports comprising six volumes, with a volume for each of the key parts of the study. This document represents the Hydrological Report (Report No. FS/2010/HI02) which is Volume 2 of the suite of reports. The requirements of the hydrological investigation were as follows: Review the quality of the existing yield and flood hydrology undertaken by Michael Baker Jnr in 2006 (Baker 2006) during the pre-feasibility stage of this

2 project. Take adequate measures to resolve the possible implications for the use of the data in subsequent hydrological analyses for the design and operational phases of the project. Undertake a yield analysis of the reservoir system currently in place as well investigating the impacts on the yield of raising the Full Supply Level (FSL) by a minimum of two metres. A minimum of a 30 year climate record was required to be used. Assess the Environmental Flow Requirements of the Muecula River and include this in the yield analysis to assess its impact on the yield of the dam. Using this information along with sedimentation information develop operating rules for the dam. Undertake the design flood hydrology to be used in spillway design. Ensure the peak flow figures make allowance for the tropical cyclones and large storms that frequent the northern Mozambique coast line. The return periods for the design floods should be in accordance with local standards but if there is a significant discrepancy between this standard and international best practice it is required to carry out an assessment of both. Undertake an assessment of the historical sedimentation rates from construction of the reservoir to present, including the bathymetrical survey undertaken in Utilise the sedimentation rates to assess the potential impacts on the reservoir volume-area-storage rating curves. As a result of the above mentioned requirements the investigation was broken down into four main sections: Yield hydrology Environmental flow requirements Sediment yield Flood hydrology Yield Hydrology In essence, the purpose of the yield hydrology analysis was to assess the water resource capability (or yield ) of Nacala Dam, both currently and after it has been raised. In addition to this assess the impacts of making releases to support downstream environmental flow requirements and the possible long-term loss of available storage in the dam as a result of sedimentation. The analysis included a number of aspects such as the following: The analysis and processing of hydro-meteorological data (i.e. rainfall,

3 evaporation and streamflow) and the application of these in a rainfall-runoff modelling process The determination of the physical basin characteristics of Nacala Dam, the impacts of sedimentation and the magnitude and temporal distribution of appropriate environmental flow releases. Yield analyses were undertaken for selected scenarios based on the above information. A thorough data collection process was undertaken resulting in a comprehensive patching exercise to develop a catchment representative rainfall record of sufficient length and reliability. This was used in the generation of streamflows in conjunction with the WRSM2000 model with the results of this used as the input time series to the enhanced Water Resources Yield Model (WRYM). A summary of the scenarios used during the yield analysis is provided in the table below. Summary of Yield Analysis Scenarios # Name Nacala Dam characteristics EFR (3) Sediment allocation FSL (1) Live storage Average to live Raised req. (m storage (4) (m) Class AMSL) m 3 % MAR (2) ) m 3 /a) m 3 ) A Present % B +2m % C +2m&EFR_C % C D +2m&EFR_C&Sed % C E +2.25m % Notes: (1) Full supply level, shown in metres above mean sea level. (2) Mean annual runoff, which equals 11.7 million m 3 /a (as discussed in Section 2.6.3). (3) Desktop Class C environmental flow requirement (EFR). (4) Sediment allocation to live storage over a planning period of 50 years. A summary of the results obtained from the yield analysis is provided in the table below. Summary of Yield Analysis Results # Name HFY (1) Yield m 3 /a) at indicated RI (2) (and annual assurance of supply)

4 m 3 /a) 1:20 (95 %) 1:50 (98 %) 1:100 (99 %) 1:200 (99.5 %) Difference at 1:50 A Present B +2m C +2m&EFR_C D +2m&EFR_C&Sed E +2.25m Notes: (1) Historical firm yield, based on an analysis over a 50-years period, from 1956 to 2005 (hydrological years). (2) Recurrence interval of failure, in years, based on a long-term stochastic yield analysis of year generated streamflow sequences. Environmental Flow Requirements Due to the Muecula River having insufficient flow for the initial part of the study, the Rapid Level IV Reserve determination has not yet been undertaken. Hence it was decided that a desktop reserve determination be completed as an interim measure. A conservative approach was adopted (taking into account that there have been no environmental releases from the dam since it was commissioned), thus selecting an environmental flow release equivalent to a Desktop Model Class C for the catchment. The Desktop Model produced the following figure for the EFR: Mm 3 /ann = 21.4 % MAR Sediment Yield Analysis The sedimentation deposition rate was calculated using the results of the bathymetric survey carried out in 2001 combined with the results of the bathymetric survey carried out as part of the current study. This provided 36 years of actual recorded sediment deposition when assessing the change in capacity of the reservoir from the time of construction until now. This figure was then converted into an annual average over the 36 years and this was extrapolated forward to obtain a 50 year value. The 2009 survey showed that 14 % of the sediment fell within the dam s dead storage, hence leaving 86% within the live storage portion of the dam. The sediment figure used in the yield analysis was: Nacala Dam V 50 = 86 % 2.17 Mm 3 Nacala Dam V 50 = 1.86 Mm 3

5 Design Flood Hydrology A review of the design flood hydrology was undertaken in order to determine the peak discharge figures to be used in the design of the new spillway and the freeboard of the raised embankment. Due to a shortage of information empirical methods were largely used as a basis for the calculation of the Safety Evaluation Flood (SEF) and the Recommended Design Flood (RDF). The local Mozambican Guidelines for dam classification were consulted along with the South African National Committee on Large Dams (SANCOLD) Guidelines. The Francou Rodier Method was used for the calculation of the SEF with a K factor of 5.2 being applicable as per the specifications from the SANCOLD Guidelines. Five methods were used to calculate the intermediate return periods (specifically the 1:200 year return period) for the RDF. Some of these methods were empirically based and some deterministic. The RDF figure was then routed through the impoundment using the process of Level Pool Routing. The outcome of the design flood hydrology was the following: RMF + = SEF = m 3 /s RDF = 800 m 3 /s Routed RDF = 734 m 3 /s The results presented in this report were then used in the preliminary design of the spillway and the raised embankment (refer to Volume 4, Preliminary Design Report, FS/2010/PD04).