Hydrology and Flood Evaluation - Enviro Dynamics Namibia

PROPOSED FLOOD MITIGATION MEASURES TO 

BE IMPLEMENTED FOR THE 

OSHAKATI/ONGWEDIVA AREA 

Environmental Impact Assessment (EIA) 

Specialist study: Hydrology and Flood Evaluation 

Prepared by: A C Muir 

August 2012

COPYRIGHT © 

PROJECT NAME 

Environmental Impact Assessment for the Proposed 

Mitigation Measures to be implemented for the 

Oshakati/Ongwediva area 

STAGE OF REPORT 

Final Draft 

CLIENT 

Emviro Dynamics 

LEAD SPECIALIST 

A C Muir 

DATE OF RELEASE August, 2012 

CONTRIBUTORS TO THE 

REPORT 

A C Muir, L C Hattingh 

DECLARATION 

I ,A C Muir hereby declare that I do: 

(a) have knowledge of and experience in conducting assessments (my area of expertise), 

including knowledge of the Act, these regulations and guidelines that have relevance to the 

proposed activity; 

(b) perform the work relating to the application in an objective manner, even if this results in views 

and findings that are not favourable to the applicant; 

(c) comply with the Act, these regulations, guidelines and other applicable laws, as relevant to 

my area of expertise. 

I also declare that there is, to my knowledge, no information in my possession that reasonably has or 

may have the potential of influencing – 

(i) any decision to be taken with respect to the application in terms of the Act and the 

regulations; or 

(ii) the objectivity of this report, plan or document prepared in terms of the Act and these 

regulations. 

Signed: ___________________

ii 

EXECUTIVE SUMMARY 

In recent years, the central northern areas of Namibia have experienced heavy 

flooding. The Town of Oshakati was severely affected by the floods, and the 

Namibian Cabinet accepted a proposal for the implementation of a Concept 

Master Plan for the Town. The main purpose of the Master Plan is to keep the 

floodwaters out of Oshakati by constructing a flood protection dike around the 

town. 

The consultants appointed for the design of the flood protection dike, BAR Namibia, 

developed a hydrology and hydraulic model to gauge the effect that the 

construction of the dike may have on the areas susceptible to inundation through 

flooding. 

The EIA team carried out an evaluation of the work done by BAR and reached the 

following conclusions: 

• Despite the lack of hydrology and flow data in the Cuvelai catchment the 

consultant developed a realistic basis on which to generate rainfall data which is 

acceptable taking into account the conservative approach adopted. 

• The flood flows generated from the hydrological model compare well with the 

flows experienced in 2011 based on the modeled inundation compared to the 

actual inundation in 2011. 

• The calibration of the model using actual inundation maps of 2011 plus measured 

flood levels provide a measure of confidence in the hydraulic model. 

• The design of the flood diversion channel was based on the modeling objectives 

of providing sufficient flow capacity such that the inundation before the channel 

construction and after construction was similar. The design achieved this 

objective. 

• The construction of the flood diversion channel will have no effect downstream of 

Oshakati after the flow has merged with the natural oshanas flow. Taking the 

footprint of Oshakati around which the flow will be diverted and comparing that 

with the ex of the Cuvelai flow area, any local disruptions which may occur near 

Oshakati are insignificant. 

• Construction of the dike and the flood diversion channel will affect certain 

households that fall within the construction boundaries. It will also affect 

traditional access routes into Oshakati. These are reported on in more detail in the 

Social Impact report. 

The following recommendations are therefore made: 

• The lack of rainfall, flow and water level data in the catchment of the Cuvelai is a 

serious shortcoming that should be addressed as a matter of urgency, particularly 

in light of similar developments which are likely to take place within the 

catchment. 

• It is further recommended that funds should be made available on the budget of 

the current project to design a strategy to implement the collection of critical 

Environmental Impact Assessment: Oshakati Flood Mitigation Project 

Specialist Study: Hydrology and Flood Evaluation 

Date July 2012

iii 

data. This data should then be used to monitor future flood events and compare 

actual measurements with the model results. 

• The construction programme and a flood management plan must be designed 

for the construction phase when the town will be most susceptible to flooding. 

• The design of the dike should take into consideration the access of the rural 

communities near Oshakati into the town. 



Date July 2012

iv 

CONTENTS 

EXECUTIVE SUMMARY ........................................................................................................... ii 

CONTENTS ............................................................................................................................ iv 

TABLES AND FIGURES ........................................................................................................... vi 

APPENDICES ........................................................................................................................ vii 

ABBREVIATIONS AND ACRONYMS.................................................................................... viii 

1 INTRODUCTION ............................................................................................................. 1 

1.1 BACKGROUND ........................................................................................................... 1 

1.2 SPECIALIST STUDY LEADER ......................................................................................... 1 

1.3 TERMS OF REFERENCE................................................................................................ 1 

1.4 assumptions and limitations ..................................................................................... 2 

1.5 METHODOLOGY ........................................................................................................ 3 

1.5.1 Hydrological Model ............................................................................................ 3 

1.5.2 Hydraulic Model.................................................................................................. 4 

1.5.3 Design of the Flood Diversion Channel ........................................................... 5 

1.5.4 Evaluation of Consultants Methods and Results ............................................ 6 

2 PROJECT DESCRIPTION ................................................................................................. 7 

3 LEGAL AND REGULATORY REQUIREMENTS ................................................................... 9 

4 THE RECEIVING ENVIRONMENT .................................................................................. 10 

4.1 INTRODUCTION ........................................................................................................ 10 

5 IDENTIFICATION OF KEY IMPACTS .............................................................................. 14 

5.1 KEY POTENTIAL IMPACTS TO BE ASSESSED............................................................. 14 

6 IMPACT ASSESSMENT .................................................................................................. 15 

6.1 METHODOLOGY FOR IMPACT ASSESSMENT ......................................................... 15 

6.1.1 Description of impact ...................................................................................... 15 

6.2 IMPACT ASSESSMENT ............................................................................................... 23 



Date July 2012

v 

7 CONCLUSIONS AND RECOMMENDATIONS ............................................................... 24 

8 REFERENCES ................................................................................................................ 25 



Date July 2012

vi 

TABLES AND FIGURES 

Table 1: Environmental sensitivity and the potential impact ....................................... 13 

Table 2: Flood levels north and to the west of Oshakati ............................................... 17 

Table 3: Flood levels within Oshakati ................................................................................... 17 

Table 4: Inundation of rural households .............................................................................. 18 

Table 5: Inundation of access routes .................................................................................. 18 

Table 6: Changes in flow velocities ..................................................................................... 19 

Table 7 Higher turbidity and salinity ..................................................................................... 19 

Table 8: Flood levels north and to the west of Oshakati ............................................... 20 

Table 9: Flood levels within Oshakati ................................................................................... 20 

Table 10: Inundation of rural households ............................................................................ 21 

Table 11: Inundation of access routes ................................................................................ 21 

Table 12: Changes in flow velocities ................................................................................... 21 

Table 13: Higher turbidity and salinity .................................................................................. 22 

Table 14: Summary of impact assessment .................................................................... 23 

Figure 1: Extent of hydraulic model ....................................................................................... 5 

Figure 2: Overview of the Project Area showing the position of the Dyke ...................... 8 

Figure 3: Flow of Oshanas through Oshakati ..................................................................... 11 

Figure 4: Oshana flow intercepted by Diversion Channel ............................................... 12 

Figure 5: Inundation map for pre and post dyke construction ....................................... 16 



Date July 2012

vii 

APPENDICES 

BAR Namibia, IMDC (2012). Design and Supervision of the Construction of the Dike 

around Oshakati. Report on the Hydrological and Hydraulic Model 



Date July 2012

viii 

ABBREVIATIONS AND ACRONYMS 

1D 

2D 

BAR Namibia 

DTM 

EIA 

PDM model 

PMF 

RMF 

UK 

WCE 

1 - dimensional 

2 - dimensional 

Buro of Architects Namibia 

Digital Terrain Model 

Environmental Impact Assessment 

Probability Distributed Moisture model 

Probable Maximum Flood 

Regional Maximum Flood 

United Kingdom 

Windhoek Consulting Engineers 



Date July 2012

1 

1 INTRODUCTION 

1.1 BACKGROUND 

In recent years, the central northern areas of Namibia have experienced heavy 

flooding. In 2008 the floods were considered the highest in living memory. However, 

these floods were exceeded in 2009, and again in 2011. 

The Town of Oshakati was severely affected by the floods, and the Namibian 

Cabinet accepted a proposal for the implementation of a Concept Master Plan for 

the Town. The main purpose of the Master Plan is to keep the floodwaters out of 

Oshakati by constructing a flood protection dike around the town. 

BAR Namibia has been appointed as the consultants on the project. IMDC was 

appointed by BAR as sub-consultants to perform the hydrological and hydrodynamic 

study. This specialist report forms part of the Environmental Impact Assessment and 

will evaluate the work done on the hydrology and hydraulic modeling and will assess 

the various impacts of the project on the environment. 

1.2 SPECIALIST STUDY LEADER 

Mr Muir trained as a Civil Engineer at the University of Cape Town, South Africa. After 

graduation he joined the Department of Water Affairs, Namibia in 1974. During this 

period he worked as a design engineer and also spent four years on site as a 

construction engineer. He was promoted to head of the design office in 1981 and 

thereafter promoted to Director of Works of the Department. He left the government 

service in 1991 and joined Windhoek Consulting Engineers (WCE) as an Associate. 

During his time with WCE he was involved in numerous flood investigations which 

included both flood hydrology and hydraulic modeling. 

His experience is also related to the design, construction and management of water 

supply projects and township services as well as the investigation into water 

resources. He also has experience in water and waste water treatment systems. 

1.3 TERMS OF REFERENCE 

The purpose of the hydrological and hydrodynamic study is to provide the basis for 

the design of the flood protection measures with all necessary hydraulic information. 

For this, a detailed hydrodynamic model was developed, that allows the impact of 

flooding due to the construction of a dyke around Oshakati to be assessed. To 

deliver input to this hydrodynamic model, a hydrological model was first developed. 

With the model, changes in water levels and flows in and around Oshakati can be 



Date July 2012

2 

calculated. Hence design variables, such as dyke crown height, culvert capacity 

and bridge width, can be determined. This will then divert floodwater around the 

town. The dike will extend from the north of Okatana and will divert the floodwaters 

around Oshakati, to the west. The floodwater will then join with the natural drainage 

system south of Oshakati. 

The EIA component of this study will evaluate the methods used, the model 

calibration techniques applied and the level of confidence that can be attributed to 

the results. Impacts on the environment of the construction of the dike will also be 

assessed. 

1.4 ASSUMPTIONS AND LIMITATIONS 

A major limitation in conducting a study of the hydrology and flow regime of the 

Cuvelai catchment is a serious lack of records. This includes rainfall records, flow 

data and flood levels 

Although the Namibia met office has rainfall records at a few stations in the Cuvelai, 

some of which go back to 1913, many have not been updated for some time and 

there are significant gaps in the records. Very little data is available on surface water 

flows in the catchment. There are 13 gauging stations located south of the border 

which measure water levels, but no flow data are available. These flow gauging 

stations were installed prior to the 2008 floods at Shakambebe and Shanalumono just 

south of the Angola border. No depth-discharge curves exist, which renders it 

impossible to convert the stage records into flow records. 

No data is available for the major part of the catchment which lies north of the 

Namibia/Angola border. 

The lack of data has made the hydrological study a challenging task and the level of 

accuracy of the results should be seen in this context. For this reason a conservative 

approach was adopted in determining the flows in the catchment. 

Flow data are available in the adjoining catchments of the Kunene River at 

Ruacana and the Okavango River at Rundu. However, this information was not used 

in estimating the flows in the Cuvelai catchment. 

The limitations imposed on the methodology and accuracy of the study can thus be 

summarized as follows: 

 

 

No flow records exist for the Cuvelai catchment; 

The catchment of the Cuvelai is extremely flat – an average slope of less than 

0.1%; 



Date July 2012

3 

 

 

The conventional methods for determining flood peaks are not applicable to 

these very flat catchments – this was confirmed by some control calculations 

done by the EIA team using deterministic methods for the Kunene as well as 

Okavango catchments. It was clear from the results of these deterministic 

methods that the attenuation effect of the oshanas cannot be effectively 

modeled by the use of conventional methods as the results tend to give 

extremely unrealistically high flood peaks; 

Previous hydrology calculations for the Cuvelai catchment (KPE: 2009) used 

the results of extremely conservative empirical calculations to determine flood 

peaks for different sub-catchments assuming that the use of the RMF and PMF 

of the Kunene River with probabilities of exceedance of less than 10 -9 is of 

relevance. This is considered to be overly conservative and not realistic; 

1.5 METHODOLOGY 

1.5.1 Hydrological Model 

The following methodology was applied by the consultants, BAR Namibia, in 

generating the flood data for the hydrological model. For detailed information on 

the techniques used to generate the modeling data for the catchment area, refer to 

the BAR report on the hydrology and hydraulic model. 

i. A DTM of the catchment area was compiled from available data. 

ii. 

iii. 

iv. 

Rainfall data was generated for the Cuvelai catchment using available 

rainfall records and satellite imagery. 

Time series evaporation data was derived based on the average potential 

evaporation rate per year (i.e. 2800-3000 mm). 

Additional modeling parameters such as soil storage capacity, reservoir time 

constants and infiltration rate to groundwater were developed. 

v. The above information was used to develop the hydrological model which 

transforms rainfall and potential evaporation data to flow at the catchment 

outlet. 

vi. 

The model that was used for the study is the Probability Distributed Moisture 

model (PDM model) which is a fairly general conceptual rainfall-runoff model 

which was developed and is maintained by the UK Centre for Ecology and 

Hydrology. 



Date July 2012

4 

1.5.2 Hydraulic Model 

The hydrodynamic model was developed using InfoWorks RS v10.5.12 (Innovyze Ltd, 

UK). The program can model open channel flow and overbank flows in any network 

of channels. 

The program can be used to solve systems under both steady and unsteady flow 

conditions, either flows with a pronounced 1D character or flows with no principal 

component, i.e. 2D-flow can be modeled. 

A digital terrain model (DTM), combined with on-site topographical measurements of 

all relevant hydraulic structures, were used to define river cross sections, bridges, 

culverts etc., as well as information on the bed characteristics which was modeled 

using Manning’s coefficient. 

Due to the very flat topography in the region, there is an exchange of flow between 

the interconnected oshanas. The invert level of the oshanas is not constant, neither is 

the water level constant during periods when there is flow in the oshanas, and also, 

significantly, during times of high floods. The direction of the flow between the 

interconnected oshanas depends on the difference in water levels between the 

oshanas. The location of the interconnections were determined from the flood map 

of April 5, 2011, while the flow between the oshanas was determined using the weir 

flow equation. 

After the initial modeling runs, the model was calibrated using the flood inundation 

areas recorded in 2011. More detail of the calibration methods can be found in the 

BAR report on the hydrology and hydraulic modeling. The flood levels obtained from 

the model were also checked against measured flood levels in the area which 

corresponded within acceptable limits taking into account the expected modeling 

tolerances. 

The extent of the model can be seen in Figure 1. 



Date July 2012

5 

Figure 1: Extent of hydraulic model 

1.5.3 Design of the Flood Diversion Channel 

The flood diversion channel has been designed to transfer flood waters arriving in the 

oshanas north of Oshakati, flowing around the western side of Oshakati and joining 

up again with the natural flow of the oshanas south of Oshakati. One of the aims of 

the project is to ensure that the dike has a minimal impact on the inundation of the 

area upstream and to the west of Oshakati. The effect of the inundation will be 

reported on in more detail in the social impact study. 

However, based on the method used to calibrate the hydraulic model, the effect of 

further inundation should be limited. 



Date July 2012

6 

1.5.4 Evaluation of Consultants Methods and Results 

A number of discussions and meetings were held between the EIA team, the 

Department of Water Affairs’ Hydrology Division and the consultants, BAR Namibia, 

on the work undertaken in determining the flow magnitudes and flood inundation 

areas. These discussions were held in Windhoek and Oshakati in January 2012. At 

this stage the first draft of the hydrology and flood modeling report was available. 

The EIA team’s initial comments were then addressed in the final model which was 

presented in Windhoek in March 2012. 

Taking into consideration the lack of rainfall, flow and flood level data in the Cuvelai 

catchment, and considering also that the major part of the catchment falls in 

Angola, where no data is available, the detail of the work done by BAR Namibia as 

well as the techniques used to generate the flood data is noteworthy. It is the 

opinion of the EIA team that the results of the hydrology and hydraulic modeling are 

sufficiently accurate to be used in the design of the flood protection dike around 

Oshakati. 



Date July 2012

7 

2 PROJECT DESCRIPTION 

The Town of Oshakati, the capital of the Oshana Region, has in recent years, been 

severely affected by flooding. To make Oshakati flood free, a dyke will be 

constructed, extending from the north of Okatana around the western edge of 

Oshakati and joining up again with the natural flowing oshanas south of Oshakati. 

This will divert the main flows arriving from the north, around Oshakati. 

The proposed dyke for Oshakati will be approximately 26km long and 44m wide. The 

height of the crest of the dyke will be between 2.2-2.5 m above the existing oshana 

bed level. This level corresponds to the maximum water level of the design flood 

(return period of 100 year). An additional freeboard of 500mm has also been 

provided. The 100 year flood level has been determined using a hydrological and 

hydraulic model which was developed by the4 consultants. 

The dyke has been extended to the south to avoid backwater flowing into the town 

from the south. 

The dyke will have a slope protection such as grouted stone pitching at the water 

side to avoid erosion, the details of which will be designed according to the soil 

characteristics. At the land side a cemented gravel slope protection is foreseen. 

All natural vegetation including large trees will be cleared from the footprint area of 

the dyke to ensure a good foundation. 

On top of the dyke a dual carriage way (60m road reserve) will be constructed 

which will act as a bypass or ring road around Oshakati. 

The ring road will have a limited access only at the intersections with the trunk roads 

entering Oshakati. These intersections will be designed as roundabouts with a large 

radius to avoid the need for and maintenance of robot controlled intersections. The 

ring road is being considered in conjunction with the Roads Authority. 

The dyke around Oshakati will be used to divert flood waters from the north around 

the western side of the town. The diversion channel will use existing oshanas as far as 

possible to convey the water around the town. However, where high ground 

between these oshanas is encountered it will be excavated to form part of the 

channel. 

An overview of the project area is shown in Figure 2 



Date July 2012

8 

Figure 2: Overview of the Project Area showing the position of the Dyke 



Date July 2012

9 

3 LEGAL AND REGULATORY REQUIREMENTS 

The Local Authorities Act requires that no residential development is allowed below 

the 1 in 50 year flood level. 

A long period of relatively dry rain seasons has been experienced in the north of 

Namibia between 1995 and 2008. This period coincided with rapid urbanization and 

development, particularly informal housing, in Oshakati. This also resulted in large 

low lying areas being used for the construction of residential units. 

The project now being proposed will be based on keeping Oshakati flood free for 

floods exceeding a return period on 1 in 100 years. 



Date July 2012

10 

4 THE RECEIVING ENVIRONMENT 

4.1 INTRODUCTION 

The town of Oshakati lies within the area in the north of Namibia which is 

characterized by very flat terrain and an ill-defined drainage network that originates 

in Angola and converges into the Etosha Pan. The town was originally developed on 

higher lying areas between oshanas. With the rapid urbanization that took place 

after 1990 developed flowed over into the lower lying areas of Oshakati. These are 

the areas most affected by flooding. 

The ephemeral channels, or oshanas, in the Cuvelai Delta normally fill up from local 

rains in the rainy season but with little continuous flow. In good rainy seasons, 

floodwaters from the upper catchment in Angola reach Namibia, resulting in floods 

known as the efundja. Under normal circumstances the local population welcomes 

the efundjas because of the opportunity for fishing and because open water is 

available for stock drinking. 

• Floods are mainly the results of local rainfall and rainfall just north of the border in 

the Cuvelai Delta. 

• Efundja floods are mainly the result of a flood in the “main” Cuvelai River 

spreading out over the full width of the Cuvelai Delta north of the border. 

• Floods in the Cuvelai Delta are often made worse by road embankments which 

obstruct the flow of water in the oshanas. 

Flooding in Oshakati is caused by the flow from a few major oshanas that flow from 

the north. Currently this flow goes through the centre of Oshakati and causes serious 

inundation of the low lying areas where a lot of informal settlement has occurred 

over the last number of years. 

Figure 3 provides an overview of the existing oshanas that flow into Oshakati and the 

extent of the inundation that occurred in 2011. 



Date July 2012

11 

Figure 3: Flow of Oshanas through Oshakati 

Flow from the major oshanas north of Oshakati will be combined. Atherse flows will 

be intercepted by the dyke and diverted to the west of the town. 

Figure 4 shows how these flows will be combined and intercepted by the diversion 

channel. 



Date July 2012

12 

Figure 4: Oshana flow intercepted by Diversion Channel 

Land use in the affected area is dominated by communal farming in the rural areas 

and urban development within the Oshakati Townlands. 

In Oshakati Town flooding takes place in the lower lying areas that have developed 

following rapid urbanization over the last 20 years. This has been made worse by the 

recent good rain seasons which caused major flows in the oshanas that flow through 

and around the town. 

The rural areas are less affected by the floods as development has traditionally taken 

place on the higher lying ground next to the Oshanas. However, recent floods have 

caused major problems with access where roads and other traditional access routes 

have bee4n either breached or inundated. 



Date July 2012

13 

Table 1: 

Environmental sensitivity and the potential impact 

ENVIRONMENTAL FEATURE SENSITIVITY POTENTIAL IMPACT 

Low lying areas in Oshakati 

The floods cause major flooding 

in the low lying areas 

increasingly being used as 

informal settlements 

Positive as flood will be 

diverted around the 

town 

Inundation of rural areas 

Changes in the flood regime 

may inundate new areas. 

If larger areas are 

inundated, this will 

affect rural residents. 

Access from rural areas close to Oshakati 

into the town 

Rural residents current5ly use 

numerous alternative, more 

informal routes into Oshakati. 

During times of flood 

access will be 

negatively affected as 

the whole western side 

of Oshakati will only be 

accessed along major 

roads. 



Date July 2012

14 

5 IDENTIFICATION OF KEY IMPACTS 

5.1 KEY POTENTIAL IMPACTS TO BE ASSESSED 

The following are the key impacts related to the hydrology and hydraulic modeling 

study: 

• Flood levels north and to the west of Oshakati 

• Flood levels within Oshakati 

• Inundation of rural households 

• Inundation of access routes. 

• Changes in flow velocities 

• Possible increase in turbidity and saline content of the oshanas water caused by 

excavations required to construct the diversion channel 



Date July 2012

15 

6 IMPACT ASSESSMENT 

6.1 METHODOLOGY FOR IMPACT ASSESSMENT 

The methodology followed in determining the impacts listed above is the same for all 

impacts. All the impacts are affected by the flows in the oshanas and the flood 

levels. The methodology is described in detail in paragraph 1.5: 

6.1.1 Description of impact 

The construction of the dike will have an effect on the flood levels north and to the 

west of Oshakati. Although the diversion channel has been designed to be large 

enough to divert the full incoming flow from the north there are still areas which will 

now be inundated during the 1 in 100 year flood. The inundation of these areas will 

only occur under extreme flood conditions and the impact is thus not considered to 

be severe. Figure 5 below presents the inundated areas pre and post dyke 

construction. 

Flood levels within Oshakati will be reduced to the flooding that will take place from 

local stormwater runoff. Flooding from the oshanas will not occur after construction 

of the dike. 

Inundation that can be expected of the rural households in the vicinity of Oshakati 

should not increase since the hydraulic model was calibrated to ensure that the 

areas inundated by floods are not greater after the construction of the dike. 

Inundation of access routes to Oshakati will be affected during periods of flood and 

the rural population will have to make use of the major roads to gain access to the 

town. This may cause inconvenience since longer distances may have to be 

traversed. 



Date July 2012

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Figure 5: Inundation map for pre and post dyke construction 



Date July 2012

17 

Table 2: 

Flood levels north and to the west of Oshakati 

DESCRIPTION 

EXTENT 

Site specific 

Local 

Regional 

National 

International 

A 

minimal 

A 

minimal 

No impact 

No impact 

No impact 

backwater 

effect 

backwater effect is 

is created to the 

created to the 

north and west of 

north and west of 

Oshakati 

under 

Oshakati 

under 

extreme 

flood 

extreme 

flood 

conditions 

conditions 

DURATION 

Very Short Term 

Short term 

Medium term 

Long term 

Permanent 

≤ 1 week for local 

For duration of 

No impact 

No impact 

No impact 

flooding 

flood event 

INTENSITY/ 

No lasting effect 

Minor effects 

Moderate effects 

Serious effects 

MAGNITUDE 

No environmental 

Effect will depend 

No impact 

No impact 

functions 

and 

on accuracy of 

process 

are 

model calibration 

affected 

Table 3: Flood levels within Oshakati 

DESCRIPTION 

EXTENT 

Site specific 

Local 

Regional 

National 

International 

Improvement 

Improvement 

No impact 

No impact 

No impact 

DURATION 


Short term 

Medium term 

Long term 

Permanent 

No impact 

No impact 

No impact 

No impact 

No impact 

INTENSITY/ 


Minor effects 



MAGNITUDE 

No environmental 

No 

No impact 

No impact 

functions 


environmental 

process 

are 

functions 


affected 

process 

are 

affected 



Date July 2012

18 

Table 4: Inundation of rural households 

DESCRIPTION 

EXTENT 

Site specific 

Local 

Regional 

National 

International 

Similar to previous 

Similar 

to 

No impact 

No impact 

No impact 

natural condition 

previous natural 

condition 

DURATION 


Short term 

Medium term 

Long term 

Permanent 

From local rain 


No impact 

No impact 

No impact 

flood event 

INTENSITY/ 


Minor effects 



MAGNITUDE 

Minor 

Minor 

No impact 

No impact 

Table 5: Inundation of access routes 

DESCRIPTION 

EXTENT 

Site specific 

Local 

Regional 

National 

International 

Inconvenience 

as 

Inconvenience 

No impact 

No impact 

No impact 

alternatives 

are 

as 

alternatives 

available 

are available 

DURATION 


Short term 

Medium term 

Long term 

Permanent 



No impact 

No impact 

No impact 

flooding 

flood event 

INTENSITY/ 


Minor effects 



MAGNITUDE 

Minor 

Minor 

No impact 

No impact 



Date July 2012

19 

Table 6: Changes in flow velocities 

DESCRIPTION 

EXTENT 

Site specific 

Local 

Regional 

National 

International 

Flow velocities are 

Could 

be 

No impact 

No impact 

No impact 

on 

average 

dangerous 

approximately 0.5 

during floods 

m/s and can 

increase up to 0.8 

m/s. This could be 

dangerous 

for 

people wanting to 

cross an oshanas 

during floods 

DURATION 


Short term 

Medium term 

Long term 

Permanent 



No impact 

No impact 

No impact 

flooding 

flood event 

INTENSITY/ 

Will have a lasting 

Will have a 

No impact 

No impact 

MAGNITUDE 

effect 

lasting effect 

Could be major 


Table 7 Higher turbidity and salinity 

DESCRIPTION 

EXTENT 

Site specific 

Local 

Regional 

National 

International 

Not expected to 

Not expected to 

No impact 

No impact 

No impact 

be significant if 

be significant if 

fish not affected 

fish not affected 

DURATION 


Short term 

Medium term 

Long term 

Permanent 



No impact 

No impact 

No impact 

flooding 

flood event 

INTENSITY/ 

Should find a 

Should find a 

No impact 

No impact 

MAGNITUDE 

balance after a 

few flood events 

balance after a 

few flood events 

Could be major in 




Date July 2012

20 

DESCRIPTION 

the short term in the short 

term 

Status and degree of confidence in predictions of the impact are described in the 

table below : 

Table 8: 

Flood levels north and to the west of Oshakati 

NONE LOW MEDIUM HIGH 

IMPACT 

SIGNIFICANCE 

The impact is expected 

to be moderate as 

relatively small areas 

are affected and then 

only under extreme 

flooding conditions 

Table 9: Flood levels within Oshakati 


IMPACT 

SIGNIFICANCE 

The situation of 

flooding within 

Oshakati will improve 

with the construction 

of the dyke 



Date July 2012

21 

Table 10: Inundation of rural households 


IMPACT 

SIGNIFICANCE 

Rural households may 

be affected by 

inundating larger 

areas. The impact can 

be mitigated by 

ensuring adequate 

capacity in the 

diversion channel to 

pass all the flood 

waters. 

Table 11: Inundation of access routes 


IMPACT 

SIGNIFICANCE 

The impact is expected 

to be low as there are 

alternative routes into 

town. The duration of 

the impact is also 

limited to the flood 

events 

Table 12: Changes in flow velocities 


IMPACT 

SIGNIFICANCE 

The impacts could be 

serious if people try 

and cross the diversion 

channel during 

flooding, particularly 

young children. 



Date July 2012

22 

Table 13: Higher turbidity and salinity 


IMPACT 

SIGNIFICANCE 

The impact may cause 

the normal behaviour 

of fish to change. If 

fish stay away from 

the area it could affect 

a source of food for 

the local population. 



Date July 2012

23 

6.2 IMPACT ASSESSMENT 

Table 14: 

Summary of impact assessment 

DURING CONSTRUCTION 

DURING OPERATION 

NATURE OF THE IMPACT Care must be taken 

when construction takes 

place during the rain 

season that temporary 

protection is provided 

against flooding of the 

town 

Maintenance must be 

undertaken of the canal and 

any signs of erosion must be 

immediately attended to 

EXTENT Could be major during 

the rainy season 

 

Very important 

DURATION ± 3 months Permanent 

INTENSITY High Medium 

PROBABILITY High High 

STATUS (+ OR -) - ve - ve 

SIGNIFICANCE (NO 

MITIGATION/ENHANCEMENT) 

Enhancement Enhancement 

MITIGATION Yes Yes 

SIGNIFICANCE (WITH 

MITIGATION/ENHANCEMENT) 

High High 

CONFIDENCE LEVEL High High 



Date July 2012

24 

7 CONCLUSIONS AND RECOMMENDATIONS 

With reference to the evaluation of the hydrology study and the hydraulic modeling 

carried out by BAR Namibia. The following can be concluded: 

• Despite the lack of hydrology and flow data in the Cuvelai catchment the 

consultant developed a realistic basis on which to generate rainfall data which is 

acceptable taking into account the conservative approach adopted. 

• The flood flows generated from the hydrological model compare well with the 

flows experienced in 2011 based on the modeled inundation compared to the 

actual inundation in 2011. 

• The calibration of the model using actual inundation maps of 2011 plus measured 

flood levels provide a measure of confidence in the hydraulic model. 

• The design of the flood diversion channel was based on the modeling objectives 

of providing sufficient flow capacity such that the inundation before the channel 

construction and after construction was similar. The design achieved this 

objective. 

• The construction of the flood diversion channel will have no effect downstream of 

Oshakati after the flow has merged with the natural oshanas flow. Taking the 

footprint of Oshakati around which the flow will be diverted and comparing that 

with the ex of the Cuvelai flow area, any local disruptions which may occur near 

Oshakati are insignificant. 

• Construction of the dike and the flood diversion channel will affect certain 

households that fall within the construction boundaries. It will also affect 

traditional access routes into Oshakati. These are reported on in more detail in the 

Social Impact report. 

The following is recommended: 

• The lack of rainfall, flow and water level data in the catchment of the Cuvelai is a 

serious shortcoming that should be addressed as a matter of urgency, particularly 

in light of similar developments which are likely to take place within the 

catchment. 

• It is further recommended that funds should be made available on the budget of 

the current project to design a strategy to implement the collection of critical 

data. This data should then be used to monitor future flood events and compare 

actual measurements with the model results. 

• The construction programme and a flood management plan must be designed 

for the construction phase when the town will be most susceptible to flooding. 

• The design of the dike should take into consideration the access of the rural 

communities near Oshakati into the town. 



Date July 2012

25 

8 REFERENCES 

Enviro Dynamics (2010). Environmental Impact Assessment and Environmental 

Management Plan of Orange River Life of Mines Extension Project, Sendelingsdrift. 

Oranjemund: NAMDEB. 

Mendelsohn, J., Jarvis, A., Roberts, C., & Robertson, T. (2009). Atlas of Namibia: A 

portrait of the land and its people. Cape Town, South Africa: Sunbird Publishers (PTY) 

LTD. 

BAR Namibia, IMDC (2012). Design and Supervision of the Construction of the Dike 

around Oshakati. Report on the Hydrological and Hydraulic Model 

G van Langenhove (2008). Hydrological Perspective on 2008 Floods in Northern 

Namibia, with Special Reference to Cuvelai Area. Report Department of Water 

Affairs 

L C Hattingh (2012). Hydrology for the Omafo to Outapi Road (DR3608). Hydrology 

report for the design of the upgrading of the road between Omafo and Outapi. 

KPE Joint Venture (2008). Study Report on Flooding of Proclaimed Roads in North 

Central Namibia (RA/CS-NP/04-2008). Report for the Roads Authority 



Date July 2012

Hydrology and Flood Evaluation - Enviro Dynamics Namibia

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