PROPOSED FLOOD MITIGATION MEASURES TO BE IMPLEMENTED …€¦ · Mitigation Measures to be implemented for the Oshakati/Ongwediva area STAGE OF REPORT Final Draft CLIENT Emviro Dynamics

Specialist study: Hydrology and Flood Evaluation

Prepared by: A C Muir

August 2012

PROPOSED FLOOD MITIGATION MEASURES TO

BE IMPLEMENTED FOR THE

OSHAKATI/ONGWEDIVA AREA

Environmental Impact Assessment (EIA)

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

Environmental Impact Assessment: Oshakati Flood Mitigation Project Specialist Study: Hydrology and Flood Evaluation Date July 2012

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

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.

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

v


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

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

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

vii


APPENDICES

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

around Oshakati. Report on the Hydrological and Hydraulic Model

viii


ABBREVIATIONS AND ACRONYMS

1D 1 - dimensional

2D 2 - dimensional

BAR Namibia Buro of Architects Namibia

DTM Digital Terrain Model

EIA Environmental Impact Assessment

PDM model Probability Distributed Moisture model

PMF Probable Maximum Flood

RMF Regional Maximum Flood

UK United Kingdom

WCE Windhoek Consulting Engineers

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

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%;

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. Rainfall data was generated for the Cuvelai catchment using available

rainfall records and satellite imagery.

iii. Time series evaporation data was derived based on the average potential

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

iv. 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.

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.

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.

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.

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

8


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

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.

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.

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.

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.

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.

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

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.

16


Figure 5: Inundation map for pre and post dyke construction

17


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

DESCRIPTION

EXTENT Site specific

A minimal

backwater effect

is created to the

north and west of

Oshakati under

extreme flood

conditions

Local

A minimal

backwater effect is

created to the

north and west of

Oshakati under

extreme flood

conditions

Regional

No impact

National

No impact

International

No impact

DURATION Very Short Term

≤ 1 week for local

flooding

Short term

For duration of

flood event

Medium term

No impact

Long term

No impact

Permanent

No impact

INTENSITY/

MAGNITUDE

No lasting effect

No environmental

functions and

process are

affected

Minor effects

Effect will depend

on accuracy of

model calibration

Moderate effects

No impact

Serious effects

No impact

Table 3: Flood levels within Oshakati

DESCRIPTION


Improvement

Local

Improvement

Regional

No impact

National

No impact

International

No impact


No impact

Short term

No impact

Medium term

No impact

Long term

No impact

Permanent

No impact

INTENSITY/

MAGNITUDE

No lasting effect

No environmental

functions and

process are

affected

Minor effects

No

environmental

functions and

process are

affected

Moderate effects

No impact

Serious effects

No impact

18


Table 4: Inundation of rural households

DESCRIPTION


Similar to previous

natural condition

Local

Similar to

previous natural

condition

Regional

No impact

National

No impact

International

No impact


From local rain

Short term

For duration of

flood event

Medium term

No impact

Long term

No impact

Permanent

No impact

INTENSITY/

MAGNITUDE

No lasting effect

Minor

Minor effects

Minor

Moderate effects

No impact

Serious effects

No impact

Table 5: Inundation of access routes

DESCRIPTION


Inconvenience as

alternatives are

available

Local

Inconvenience

as alternatives

are available

Regional

No impact

National

No impact

International

No impact



flooding

Short term

For duration of

flood event

Medium term

No impact

Long term

No impact

Permanent

No impact

INTENSITY/

MAGNITUDE

No lasting effect

Minor

Minor effects

Minor

Moderate effects

No impact

Serious effects

No impact

19


Table 6: Changes in flow velocities

DESCRIPTION


Flow velocities are

on average

approximately 0.5

m/s and can

increase up to 0.8

m/s. This could be

dangerous for

people wanting to

cross an oshanas

during floods

Local

Could be

dangerous

during floods

Regional

No impact

National

No impact

International

No impact



flooding

Short term

For duration of

flood event

Medium term

No impact

Long term

No impact

Permanent

No impact

INTENSITY/

MAGNITUDE

Will have a lasting

effect

Could be major

Will have a

lasting effect

Could be major

No impact No impact

Table 7 Higher turbidity and salinity

DESCRIPTION


Not expected to

be significant if

fish not affected

Local

Not expected to

be significant if

fish not affected

Regional

No impact

National

No impact

International

No impact



flooding

Short term

For duration of

flood event

Medium term

No impact

Long term

No impact

Permanent

No impact

INTENSITY/

MAGNITUDE

Should find a

balance after a

few flood events

Could be major in

Should find a

balance after a

few flood events

Could be major

No impact No impact

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

21


Table 10: Inundation of rural households NONE LOW MEDIUM HIGH

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.

22


Table 13: Higher turbidity and salinity NONE LOW MEDIUM HIGH

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.

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

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.

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

Documents

PROPOSED FLOOD MITIGATION MEASURES TO BE IMPLEMENTED …€¦ · Mitigation Measures to be implemented for the Oshakati/Ongwediva area STAGE OF REPORT Final Draft CLIENT Emviro Dynamics