1. nacala dam hydrology executive summary

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SUMMARY OF HYDROLOGICAL SPECIALIST STUDY

The Government of the Republic of Mozambique (GOM) has received a grant from theMillennium 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 1975. 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 2006.

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 inaddressing 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

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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

2001. 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,

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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)

Sedimentallocation

to live

storage

(4)

(millionm

3)

Raised(m)

FSL(1)

(mAMSL)

Live storage

Class

Averagereq.

(millionm

3/a)

(millionm

3)

% MAR(2)

A Present - 76.50 3.99 34 % - - -

B +2m 2.00 78.50 6.62 57 % - - -

C +2m&EFR_C 2.00 78.50 6.62 57 % C 2.78 -

D +2m&EFR_C&Sed 2.00 78.50 4.76 41 % C 2.78 1.86

E +2.25m 2.25 78.75 7.01 60 % - - -

Notes: (1) Full supply level, shown in metres above mean sea level.

(2) Mean annual runoff, which equals 11.7 million m3/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)

(million

Yield (million m3/a)

at indicated RI(2)

(and annual assurance of supply)

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m /a) 1:20(95 %)

1:50(98 %)

1:100(99 %)

1:200(99.5 %)

Differenceat 1:50

A Present 4.7 5.7 5.1 4.8 4.6 -

B +2m 6.3 6.5 6.0 5.6 5.3 0.9

C +2m&EFR_C 4.4 4.8 4.3 4.1 3.8 -1.6

D +2m&EFR_C&Sed 3.9 4.2 3.7 3.5 3.3 -0.6

E +2.25m 6.4 6.7 6.1 5.7 5.4 0.1

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 201 50-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:2.497 Mm3/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 the36 years and this was extrapolated forward to obtain a 50 year value. The 2009 survey

showed that 14 % of the sediment fell within the dams dead storage, hence leaving 86%

within the live storage portion of the dam. The sediment figure used in the yield analysis

was:

Nacala Dam V50 = 86 % 2.17 Mm3

Nacala Dam V50 = 1.86 Mm3

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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 = 1 530 m3/s

RDF = 800 m3

/sRouted RDF = 734 m3/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).