west gode irrigation project report on flood protection

Somali Regional State

Irrigation and Basin Development Bureau

West Gode Irrigation Project

Report on

Flood Protection, Rehabilitation and Maintenance Work

November 2019, Jijiga

Project, Flood Protection, Rehabilitation & Maintenance Works



Contents

1 Introduction .......................................................................................................... 3

2. Background and Present Situation ...................................................................... 5

2.1 Background .................................................................................................. 5

2.2 Present Situation ........................................................................................... 6

2.2.1 Head Works ............................................................................................ 6

2.2.2 Main Canal and Main Canal Structures .................................................. 6

2.2.3 Farm Works ............................................................................................ 7

2.3 Major Constraints Observed.......................................................................... 7

2.3.1 General ................................................................................................... 7

2.3.2 Engineering (Technical) Constraints ....................................................... 8

3 Flood Protection and Maintenance Works at the Headwork .............................. 10

3.1 General ....................................................................................................... 10

3.2 Anticipated Flood at the Headwork ............................................................. 10

3.3 Flood From the Western Escarpment ......................................................... 10

3.4 Flood From Wabi Shebelle .......................................................................... 12

3.4.1 General ................................................................................................. 12

3.4.2 Flood Mitigation Measures at The Headwork ....................................... 13

3.5 Proposed Remedial Measures for Head Works and Main Canal ................ 15

4 Flood Protection and Maintenance Works at Farm Work .................................. 19

4.1 General ....................................................................................................... 19

4.2 Anticipated Flood at the Farm Work ............................................................ 19

4.3 Present Situation ......................................................................................... 20

4.3.1 Farm Works .......................................................................................... 20

4.4 Proposed Remedial Measures for Farm Works .......................................... 21




List of Figures

Figure 3-1, Flood Generated From Western Escarpment 1 ................................................. 11

Figure 3-2, Flood Scenario After Construction of Gode-West/Gode Road 1 ........................ 12

Figure 3-3, Anticipated Flood at Headwork & Proposed Mitigation Measures 1................... 14

Figure 3-4, Flood Mitigation Measures Af2013 Flood 1 ....................................................... 15

Figure 3-5, Profile & Cross Section of W/Dyke 1 ................................................................. 17

Figure 3-6, New Dyke from IP-A up to 2km Downstream1 .................................................. 18

Figure 4-1, Flood Generating Wadis 1 ................................................................................ 20

Figure 4-2, Location and Layout of Floodway 1 ................................................................... 22

Figure 4-3, Typical Cross Section of the Floodway 1........................................................... 23




1 Introduction

This is a field mission Report for flood protection, rehabilitation and maintenance works to

be done at West-Gode Irrigation Project. The report presents the findings of the field

assessment team and elaborates measures to be taken with full justification and

validation based on the knowhow, experience and empirical knowledge of the team.

To this end, a team comprising a Senior Hydraulic/Structural Eng., Elector-Mechanical

Eng. and a Senior Eng. from Irrigation Bureau has visited the project site from 30/10/2019

to 2/11/2019. Apart from this the project personnel at the site have given valuable input

for the contents of this report.

To keep the consistency of the present study and maintenance work with earlier works in

Gode-West area, former design reports and design criterion as prepared by

NEDECO/WRDA were closely referred and used. Moreover, drawings as well as

hydrological studies prepared by NEDECO/WRDA, SWWDSE/YESHI-BER Consult were

of immense assistance in the preparation this report.

It is believed that a small briefing secession in combination with this report will give the

Irrigation Bureau in particular and the policy makers of the region in general a good

insight on the envisaged flood protection, rehabilitation and maintenance work proposed

for Gode-West Irrigation Project.




2. Background and Present Situation

2.1 Background

In the early 1980’s the Ethiopian Government has intended to start intensive agricultural

development program on a total of 27,600 ha land at Gode West and Gode South. The

prevailing physical conditions of the area, climate, soils and water, allows year round

irrigated agricultural practices on large number of crop commodities in West and South

Gode (7,600ha in West Gode and 20,000 ha land in South Gode).

Water for irrigation will be won from the prominent Wabi Shebelle River by means of the

already constructed diversion structure. The entire irrigation water both for Gode West

and Gode South will be diverted at West Gode and will be conveyed via a main canal

that runs along the left bank of the river (Gode West Side).

To this end the Ethiopian Government and SRS in association with international

consultants and local experts has completed the detailed study and design of;

a. A Diversion Weir which can abstract water for the overall irrigation of West and South

Gode Area.

b. A Main Canal that conveys the diverted water to West Gode and up to the river

crossing site to South Gode (the total length of the main canal up to the river crossing

site is 27km)

c. A River crossing structure (Siphon) to covey water beneath Wabi Shebele river to

South Gode

d. Flood protection works

e. Detailed on farm work design on 7,600 ha land in West Gode

Ever since 1987, the development of the project was transformed to the construction

stage and to date the construction of the following works is substantially completed.

Head works, including diversion weir, scouring sluice, desilting basin, sediment

ejector and protection works

Main canal, 13.75 km main canal and associated main canal structures

Farm work development on 3,200 ha




2.2 Present Situation

2.2.1 Head Works

At present the project is not functioning at its full capacity. This is mainly due to a number

of obstacles. At the head works, the major bottle neck observed is the siltation of the

main canal and the desilting basin as well as the non-functionality of the electro-

mechanical parts of the gates and hoisting devices.

Currently the sluice gates are staked and are not functioning. Although the two intake

gates are open their desirable maneuverability is not guaranteed, while the remaining

two intake gates are stacked and nonfunctional. Moreover, large amount of debris are

accumulated at the intake gates. Except for one major divide wall all civil works at the

intake are found to be in good shape.

Since the two desilting basins are totally silted up, the sediment ejectors and the vortex

tube are not functional and the gates that control the flow through them are stacked.

Another major problem at the head works is the damage inflicted by external flooding.

During the site visit it was observed that the flood coming from the Northern escarpment

has breached the main canal and has caused considerable damage.

2.2.2 Main Canal and Main Canal Structures

The total length of the main canal that is constructed has a total length of 13.75 km and

have a capacity of 46.6m3/s. After Km 13+750 (immediately downstream of CR-2) the

main canal ends at a dead end. The flow quantity (46.6m3/s) is the total discharge

required to irrigate both Gode West and Gode South. The constructed main canal

structures include a bridge at the head end of the canal, four main off-takes, two head

regulators and two rejection spillways.

At present due to various reasons the main canal is silted up and its conveyance

capacity is very much reduced. In some places the sedimentation is critical that siltation

has taken place from bed level up to the bank of the canal. From the visual inspection

made, the inlet, transition and out let of the four main off-takes was found intact contrary

to this all main off-take gates and their hoisting devices are staked and lifting of the gates

is impossible.

As for Head Regulator-1, except for minor maintenance, involving plastering and

pointing further maintenance is not required. However, due to over loading and heavy




traffic Head Regulator-2 requires heavy maintenance. On both head regulators the stop

logs are stacked in their grooves and have started to rot in place.

2.2.3 Farm Works

The command area at Gode West is about 7,560 ha. The irrigable area is divided into six

primary command units. Based on geographic positioning the first primary unit is situated

on the Northern side while the other five primary command units are on the Southern

side.

The first primary unit comprises of 1,920 ha land while primary unit two and three

combined have 1,740 ha and the area under primary unit four is 2,070 ha and the

combined command area of primary unit five and six is 1,880 ha.

During the site visit the general condition and status of the constructed canals, drains

and irrigation infrastructures was assessed. It was observed that most of the canals and

drains suffer siltation due to lack of maintenance. Further, some part of the embankment

of Primary Canal-1,2 and 3 is damaged as a result of inferior fill material, poor

workmanship and lack of preventive and regular maintenance.

With respect to external floods, as in any low land irrigation project Gode Irrigation

project should be protected from external floods. The flood generated from the Norther

wadis (Wady Bu-Y) and the floods from the Eastern boundary of the farm are major

concerns. To safe guard the farm area from the recent flood, mainly from wadi Bu-Y, an

interception drain is being excavated on the northern boundary of the farm that run

parallel with embankment of primary canal GW1-00. However, it was observed that the

excavation of the intercepting drain has major technical issues and should be rectified

immediately.

2.3 Major Constraints Observed

2.3.1 General

During the site visit a number of constraints and problems were observed on the

sectoral, technical and managerial aspects of the project. These observed constraints

have imposed a barrier for the full functioning of the system to the extent that water

diversion to the farms in West Gode is impossible. Unless proper corrective measures

are taken now, it is believed that the far reaching negative effects of the constraints

could hamper the smooth operation of the project in the future.

For proper functioning of the project the existing constraints on the project should be

addressed and alleviated sustainably. As the whole work on the project i.e. head works,

main canal and farm works are connected and interrelated, the problems of the project




should be approached from a system point of view. To address the problems of the

project in its entirety the observed constraints are classified in to the following two

categories;

Engineering (Technical) Constraints

Sectoral Constraints

2.3.2 Engineering (Technical) Constraints

Currently, most of the technical constraints of the project are being addressed. However,

it is worth mentioning the crucial constraints observed;

All gates and hoisting devises on the head work are not functional

Heavy sedimentation is observed at the intake and the under sluice as well as on the

sediment basin

The entire reach of the main canal is silted up (on some places from bed to the bank

top level)

Although constructed, the western flood protection dyke for the head works was not

constructed as per the working drawing prepared

At the head works and at the beginning of the main canal, the flood coming from the

western escarpments is developing its own course and is endangering the sediment

basin and the main canal.

All main canal off-take gates and head regulator stop-logs are silted up and are

stacked inside the sliding grooves

The second head regulator on the main canal is dilapidated and need major

maintenance

Most of the primary, secondary and tertiary canals are silted up and most all of the

off-take gates and hoisting devices are stalked.

The flood protection dyke along west Gode command area and the flood drain are

not constructed and Primary GW1-00 could easily be breached and cause severe

damage on the farm land.

The following major aspects are believed to contribute for the problems observed;

Lack of operation and maintenance of the head works including the electro-

mechanical parts and in particular the operation of the under sluice gate was not

done as per the rules and regulations

The sedimentation basin was not operated as per the standard norm and rules plus

the vortex tube downstream of the sediment basin was not functional for a prolonged

time




The sedimentation of the main canal and the malfunctioning of main canal structures

is attributable to quite a number of factors such as hydrologic, hydraulics, operation

and maintenance

Hydrologic: from hydrologic point of view the main canal should be protected from

external floods. In fact the left bank of the main canal acts as a dyke to protect itself

from external flood. Due to unaccounted external flood the main canal is breached in

some places and sedimatation is observed at these locations

Hydraulics: From hydraulic point of view although the main canal is constructed to

convey water for both Gode West and Gode South (46.6 m3/s) it has never been

operated to carry this design flow. Rather, the estimated flow in the main canal has

never been more than 3 m3/s, this situation has led to low velocity in the main canal

than the design non silting/non scouring velocity of flow, which highly encourages

sediment deposition along the canal reach.

Operation: The operation of the under sluice, intakes, the main canal and the

sedimentation basin has never been according to the standard rules and regulations

Maintenance: The main canal and the desilting basin should have been maintained

as per the rules and regulations after every irrigation season. However, neither

preventive nor regular maintenance has been done so far.

Regarding the farm works most of the problems are due to non-functionality of the

system for a prolonged time as well as lack of preventive and regular maintenances.

In addition part of the problem is due to lack of construction quality control. Other

major attribute for the problem on the farm works is the damage caused by external

floods and wind erosion and deposition. Parallel with this, the internal drainage of the

system is not well constructed/maintained and the main drain that evacuates excess

runoff from the system is not built.




3 Flood Protection and Maintenance Works at the Headwork

3.1 General

In this section the flood protection works and other maintenance works needed at the

headwork is elaborated.

Being no exception the design of the head-works for Gode Irrigation Project was based

on probabilistic values of certain return periods. The return period for the design of any

hydraulic structure depends on the degree of risk (mainly safety and economy) that the

designer runs in designing a structure during its anticipated service life.

Accordingly the following return periods were adopted for the design of the head-works

of Gode Irrigation Project;

The diversion weir was designed for a 1:100 yrs. return period flood volume

(discharge).

The Western Dyke, abutment and key walls of the weir were designed for a 1:50 yrs.

return period flood height (stage).

Thus;

The diversion weir has a total crest length of 100m, which can safely accommodate a

flood magnitude of 1:100 yrs.

The crest levels of the Western Dyke, abutment and key walls of the weir were fixed

at 328.50 m (a.m.s.l)

3.2 Anticipated Flood at the Headwork

During the study and design of the headwork, two flood cases were considered and flood

protection works were proposed for the same. Then it was noted that the headwork

could be vulnerable from floods that are generated from;

Flood from the Western Escarpment

Flood from Wabi Shebelle River

3.3 Flood From the Western Escarpment

Figure-3.1 shows the general flood scenario that could be generated from the Western

Escarpment. The Western catchment area which brings flood to the head works (at

beginning of the main canal) generates its flow from the local escarpments. The total

catchment area of the North-Western escarpment is estimated to be 5-10 km2. The

Figure shows the general condition at the headwork prior to the construction of the old




Gode-West/Gode road. As shown in Figure-3.1, the flood that could be generated from

the western escarpments is localized and of small magnitude. The flood from this

escarpment will spread on the land between the escarpment and the main canal before

reaching the headwork. Moreover, inspecting the local topography, the excess runoff

from this escarpment will be encouraged to flow along the gently sloping terrain into the

large flood plain between the headwork

Figure 3-1, Flood Generated From Western Escarpment 1

and the irrigation area of West Gode. Hence, except for a spoil bank, which is 1m high,

that runs parallel to the main canal, as such no major flood protection was provided for

the flood from the Western Escarpment.

After the construction of the old Gode-West/Gode road, the original flood scenario as

given in Figure-3.1 is totally changed. And the flood that is generated from the western

escarpment has become one major concern for the safety of the headwork and the main

canal. Figure-3.2 shows the flood generated from the western escarpment after the

construction of old Gode/Gode-West road. Now the runoff from the escarpment crates a

concentrated flow that is channeled between the Western Dyke and the old Gode/Gode-

West road. This is mainly because the embankment of the road has blocked the

Wes

tern

Escar

pmen

t

Wes

tern

Dyke

N

Head Works

Wabi Shebelle

Wabi Shebelle

Main Canal

Figure-3.1

Wabi Shebelle

Wes

tern

Escar

pmen

t

1m High Spoil

Bank

Flood Plain

324

323

322 321 320

325

326

327

328

329

Flood Generated From Western Escarpment




transition of the runoff from the escarpment into the flood plain. Further, the excess

runoff that is coming along the embankment of the old Gode/Gode-West road has

started to create its own channel adding more for further breaching and sedimentation of

the main canal.

Figure 3-2, Flood Scenario After Construction of Gode-West/Gode Road 1

3.4 Flood From Wabi Shebelle

3.4.1 General

As floods generated from the highlands of Bale are regulated by the Melka Wakena

Dam, some 600 km upstream of Imi, the high flood volume that is generated within the

upper valley is significantly attenuated and their influence on the high stage of Wabi

Shebelle is minimal. As a result of this the flow of Wabi Shebelle in the lower valley

(where Gode West Irrigation Project is located) is influenced by the runoff generated in

the middle valley. Within the middle valley quite a number of tributaries join the Wabi

Shebelle both on its right and left banks. Notable tributaries worth mentioning are Ramis,

Golocha, Darole which join Wabi Shebelle on the right bank and Fafen, Ereer and

Daketa which join Wabi Shebelle on its left bank.

Wes

tern

Escar

pmen

t

Wes

tern

Dyke

N

Head Works

Wabi Shebelle

Wabi Shebelle

Main Canal

Figure-3.2

Wabi Shebelle

Wes

tern

Escar

pmen

t

1m High Spoil

Bank

Flood Plain

32

4

323

322 321 320

325

326

327

328

329

Flood Generated From Western Escarpment After The Construction of Gode West/Gode Road

Road From Gode

To Camp

Con

cent

rate

d Flo

w

Con

cent

rate

d R

unof

f

Flood F

rom

North

Eas

tern E

scar

pmen

t




The flow through most of these tributaries is seasonal flush flood with vehement

characteristic. Along the reach where these tributaries join Wabi Shebelle, up to about

30 km upstream of Imi, Wabi Shebelle River flows in a very deep gorge steeply sloping

down to the first five kilometers (25 m/km) after which the gradient gradually decreases.

As a result the river flow is well contained within its banks and flood hazard is

insignificant.

Between Imi and Gode, Madiso River (with a catchment area of about 12,295 km2) joins

Wabi Shebelle on the left bank. From Imi to the frontier, the Wabi Shebelle flows in a

vast alluvial plain with very gentle slope of 0.25 to 0.35 m/km. These small gradients

entail low velocity thereby retarding the incision power of the river. As a result of which

the natural free board of the river (the difference between the river bank and the water

surface) is minimal to accommodate any flow larger than the base flow. Thus, whenever

the river stage is high the entire stretch of land starting from Imi up to the frontiers is

always at high flooding risk.

As West Gode Irrigation Project is situated within the high risk flood zone it has been

affected quit a number of times where the recent major damage observed was by the

floods of 2014.

3.4.2 Flood Mitigation Measures at The Headwork

During the design of the headwork for West Gode Irrigation Project, special attention was

given to safe guard the headwork from the flood that will be generated from the high

stage of Wabi Shebelle. To alleviate the flood hazard due to overflow of Wabi Shebelle

at the headwork, a flood protection dyke with a 300 m long emergency spillway was

taken as the best solution course. Based on its geographic orientation with respect Gode

the flood protection dyke was designated as Western Dyke.

Figure-3.3 shows the mitigation measure for flood that could be generated from the Wabi

Shebelle. The Figure shows the general condition at the headwork prior to the

construction of the old Gode/Gode-West road. As illustrated in Figure-3.3, since the flood

from the over flow of Wabi Shebelle is of considerable magnitude, should the design

flood stage (i.e. 328.50) be exceeded a 300 m long emergency spillway was provided to

evacuate the flow before the Western Dyke is overtopped.




Figure 3-3, Anticipated Flood at Headwork & Proposed Mitigation Measures 1

The expected flood phenomenon given in Figure-3.3 was made in 1987 well before

the old Gode/Gode-West road was constructed and the effect of the road was not

taken into account. Due to this fact, after the construction of the old Gode/West-

Gode road, the original flood scenario is totally changed as given in Figure-3.4.

Now the flood that is generated from the western escarpment and the flood that

would be spilled over the Western Dyke has become one major concern for the

safety of the headwork and the main canal.

Evidently, the embankment of the road, see Figure 3-2, has partially blocked the

flood plain which was supposed to be a safe outlet for excess flood both from the

over flow of Wabi Shebelle and the western escarpment.

Recently (in 2013) the flood generated from the western escarpment was channeled

by the road embankment towards the main canal and the spoil bank was breached

at two locations, and serious damage was inflicted on both banks of the main canal.

To Wabi Shebelle

Flushing Channel

Sediment Ejector

Wabi Shebelle Diversion Channel

Spoil Bank (OGL+1m)

Spoil Bank (OGL+1m)

Wes

tern

Dyk

e (C

L=

328.

50)

Left Compartment Desilting Basin

Right Compartment Desilting Basin

Wei

r Axi

s

Intake

Main Canal

Under

Sluice

Escar

pmen

t

Geb

er-J

ebiy

e Se

tllm

ent

Wabi Shebelle

Wabi Shebelle

Wab

i She

belle

Wabi Shebelle

Clo

sure

Dam

Pro

pose

d 30

0m L

ong

Em

erge

ncy

Spi

llw

ay

Vehicle Bridge

Spoil Bank (OGL+1m)

Flood Plain

Figure-3.3Anticipated Flood Phenomenon at Headwork & Proposed Mitigation Measures

324

325

327

328

326




To alleviate this problem, in 2013 five ford crossings along the old Gode-W/Gode

road were proposed and a meter ditch provided to encourage the flow of the excess

runoff from the escarpment via the ford crossings into the flood plain. Further the 1m

spoil bank along the main canal was proposed to be replaced by a 2m high and 2Km

long compacted earth fill starting from IP-A.

In Figure-3-4 the flood mitigation measures proposed after the 2013 flood are given.

Figure 3-4, Flood Mitigation Measures After 2013 Flood 1

3.5 Proposed Remedial Measures for Head Works and Main Canal

Based on existing situation and the analysis given thereof and experience from

earlier remedial works the following flood protection and maintenance works are

forwarded for the head works and the main canal;

Newly Constructed 2,000m Long Compacted Fill Dyke

Spoil Bank (OGL+1m)

To Wabi Shebelle

Flushing Channel

Wabi Shebelle D

iversion Channel

Spoil Bank (OGL+1m)

Wes

tern

Dyk

e (C

L=

328.5

0)

Left Compartment Desilting Basin

Right Compartment Desilting Basin

Wei

r Axi

s

Intake

Main Canal

Under

Sluice

Geb

er-J

ebiy

e Se

tllm

ent

Wabi Shebelle

Wabi Shebelle

Wab

i Sh

ebel

le

Wabi Shebelle

Clo

sure

Dam

Vehicle Bridge

324

325

327

328

326

Escar

pmen

t

Figure-3.5 Flood Mitigation Measures Taken After The 2013 Flood

Flood Plain

Sediment Ejector

Gode-W

/Gode R

oad

Gode-W/Gode Road

RL

RL

FLO

W

FLO

W

RL

RL

FL

OW

FL

OW

Fo

rd C

/L

RL

FL

OW

FL

OW

Fo

rd C

/L

5 No. Modified

Ford Crossings




a. The operation and maintenance of the head work and the main canal is the

mandate of the Federal Government. With this regard the SRS should request

the concerned federal institution to expedite its responsibility.

b. Maintain all gates and electro-mechanical works at the intake, under sluice,

sediment ejector and vortex tube and make all gates and their hoisting

devices functional. It is recommended to give priority for the under sluice

gates so that they should be operational before the coming flood.

c. Remove all debris and obstructions from the intake and the under sluice and

re-fix of the trash racks.

d. Remove silt along the entire reach of the main canal from cahinage Km 0+000

up to Km 13+750 and maintain the cross section of the main canal as per the

levels and dimensions of the working drawing.

e. The twin segment basins should be reconstructed to the lines and dimensions

of their working drawing.

f. At present the main canal has a dead end at Km 13+600 (downstream of

cross regulator-2). This dead end combined with the hydraulic effects of the

cross regulator has greatly contributed for the sedimentation of the main canal

upstream. Further, the capacity of the two rejection spillways up stream of

Head Regulator-1 and Head Regulator-2 cannot accommodate the design

flow and will further endangers the stability of the head regulators and also

could lead the breaching of the main canal. Therefore, the construction of the

main canal from Km 13+750 up to the head land of South Gode should

commence as soon as possible. In due course, the two rejection spillways up

stream of Head Regulator-1 and Head Regulator-2 will be demolished and

their function will be overtaken by a new rejection spillway to be constructed

up stream of the head land of Gode South.

g. The Western Dyke should be fully maintained as per Figure 4-5.

h. The 2km long spoil bank that runs parallel to the main canal, starting IP-A up

to 2km should be replaced by a compacted fill embankment as shown in

Figure 3-6.




Figure 3-5, Profile & Cross Section of W/Dyke 1

i. On the left side of the main canal from km 3+500 up to Km 9+500 the entire

flat land is supposed to be a flood storage area for the excess flood that would

be generated from Wadi Bu-y. Therefore, during maintaining the left bank of

the main canal along this stretch special attention should be given (attention

both in fill material quality and workmanship quality).

j. Along the main canal starting from Km 0+750 up to km 9+400 there is no

crossing structure over the main canal. As there is no other access to the

river, this situation has forced the local community and their cattle to cross the

main canal at any place thereby adding much to the damage of the canal

banks and deposition of sediments. Therefore, to alleviate this problem

appropriate crossing should be designed and constructed between Km 0+750

until km 9+400.

k. The stop logs at Cross Regulator-1 and 2 should be replaced by a standard

gate with hoisting devices

l. All main off-take gates and their hoisting devise should be maintained.

Cross Section at Cha 0+000 Cross Section at Cha 2+000 Cross Section at Cha 3+000

328.50328.50 328.50

326.50 327.25 327.50 328.50

326.00

326.50

327.00

327.50

328.00

328.50

200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000 3200 3400 3600 3800 4000

Cross Section at Cha 4+000

328.50

OGL

Western Dyke Top Level

WesternEscarpment

Rt. Abutment of

Diversion Weir

Figure-3.7Longitudinal Profile of Western Dyke and Cross Sections

Chainage in m

Ele

vat

ion m

Proposed

Spillway




Figure 3-6, New Dyke from IP-A up to 2km Downstream 1

(OGL+2)

OGL 1

2

1

2

~75m

Dike

C/L Main Canal

C/L

2000

10000

Main

Canal

C/LOGLOGL

0 100 200 300 400 500 2000 2100 2200 2300 2400 2500 2600 2700

ENDIP-A

OGL

(OGL+2)

Ele

vati

on

(m

)

Distance (m)

Longitudinal Profile (Not to Scale)

Plan (Scale 1:100) Section A-A (Scale 1:100)

A A




4 Flood Protection and Maintenance Works at Farm Work

4.1 General

In this section the flood protection works and other maintenance works needed at

West Gode farm area is elaborated.

As in any low land irrigation project West-Gode Irrigation scheme is susceptible for

external floods and should be protected from the adverse effect of the same.

In earlier designs, adequate measures were taken to safeguard the farm area from

external floods. Then it was proposed to construct a dike and a flood way to transit

external floods before reaching the farm work. As a result the left bank of primary

canal GW1-00 (which takes off from the main canal and traverses along the eastern

and northern boundaries of Gode-West) was supposed to act as a dyke to protect

the farm area from floods that are coming from the Western and Northern areas. The

height of the left bank of primary canal GW1-00 was 3m above the natural ground

level.

As the flood from the Northern catchment is huge (about of 580 m3/s) a flood way

was proposed to evacuate the flood generated from this area before it reaches the

farm.

4.2 Anticipated Flood at the Farm Work

Figure 4-1 shows the depicted catchment areas that culminate flood to the farm

area. The two important flood generating areas are;

The Northern catchment area (wadi Bu-y) that is responsible for the flood that

endangers the irrigation system on the Northern boundary. The total catchment

area of Wadi Bu-Y is estimated to be 10,500 km2 with a total flood magnitude of

about 580 m3/s (NEDECO/WRDA 1986).

The North-Eastern catchment area was estimated to be around 500 km2 with a

flood magnitude of 260 m3/s (NEDECO/WRDA 1986).

On the Easter side the flood coming from the Northern wadis are combined with the

flood generated from Eastern wadis that could cause huge danger to the entire farm.




Unless the flood drain and the embankment of primary canal GW1-00 are

constructed (for layout and arrangement of the flood way see Section 4.3) the

occurrence of the maximum probable flood of Wady Bu-y could even negatively

affect the main canal starting from Km 3+500 up to Km 9+500.

Figure 4-1, Flood Generating Wadis 1

4.3 Present Situation

4.3.1 Farm Works

During the site visit the general condition and status of the constructed canals, drains

and irrigation infrastructures was assessed. It was observed that most of the canals

and drains suffer siltation due to lack of maintenance. All of the primary canals are

silted up and primary off-take gates need maintenance.

In the course of the site visit the team has observed the ongoing excavation of an

interceptor drain on the Northern boundary of the farm that runs parallel to primary

canal GW1-00. This is mainly to transit the flood coming from Wadi Bu-y into Wabi

Shebelle. It was noted that the excavation is a kind of makeshift excavation to block

the current flood from entering the farm area. The interceptor drain under excavation

has no defined cross section and other geometric parameters such as depth, bed

slope, side slope etc.

By and large, most of the problems at the farm work are due to lack of preventive

and regular maintenances. In addition part of the problem is due to lack of

Main Canal

Wabi Shebelle

Main

Dra

inN

/E F

lood D

rain

Main CanalLegend

Main Drain

DykeFlood Drain

Esc

arpm

ent

Wes

tern

Dyke

Road From Gode

N

Head Works

To Camp

Wabi Shebelle

Wabi Shebelle

Breached

Main Canal

Main Canal

To Gode

Depicted Catchment Areas at Gode West

Main Canal

To South Gode

Esc

arpm

ent

Escarpment

Esc

arp

ment

Figure 5-4

Northern CatchmentArea (Wadi Bu-y)

North-E

astern

Catchm

ent Area

Nor

th-W

este

rn

Cat

chm

ent A

rea




construction quality control. As stated above, the other major attribute for the

problem on the farm works is the damage caused by external floods. Parallel with

this the internal drainage of the system is dilapidated and the main drain that

evacuates excess runoff from the system is totally silted up.

4.4 Proposed Remedial Measures for Farm Works

Based on existing situation and the analysis given thereof and experience from

earlier remedial works the following flood protection and maintenance works are

forwarded for the farm work;

a. On the outset, the Irrigation Bureau should reestablish a scheme

administration unit that will look after the operation, maintenance and water

management matters. The scheme administration unit should be well

organized and equipped; both in manpower and equipment and its duties and

responsibilities shall be defined.

b. Once the scheme administration unit is reestablished, Water Users

Associations shall be established at Tertiary, and Primary level. The Water

User Associations shall be established based on appropriate bylaw.

c. All primary canals should be desilted and their side and long slope shall be

reconstructed to the levels and dimensions of their working drawings.

d. The left side embankment of primary Canal GW1-00 shall be inspected and

shall be compacted to the levels and dimensions as per the working drawing.

e. Invariably all existing irrigation canals should be desilted and drainage canals

should be deweeded and desilted. Parallel with primary off-take gates and

hoisting devises should be inspected and maintained and/or replaced. As

most of the farm plots are infested by weeds and wild bushes they should be

devoid of any obstacle by clearing and grabbing.

f. Once the left side embankment of of primary canal GW1-00 is built to its

levels and dimensions a flood way that could intercept the flood from Wadi

Bu-y and the flood from the Eastern Wadis should be excavated. The location,

layout and cross sections of the flood way is given in Figure 4-2 and Figure 4-

3.




Figure 4-2, Location and Layout of Floodway 1D

W2-00

31,0

00.0

0

32,0

00.0

0

33,0

00.0

0

34,0

00.0

0

35,0

00.0

0

36,0

00.0

0

37,0

00.0

0

48,000.00

47,000.00

46,000.00

45,000.00

DW2-00

DW2-00

38,0

00.0

0

39,0

00.0

0

40,0

00.0

0

41,0

00.0

0

42,0

00.0

0

44,000.00

43,000.00

Legend

Main Drain

Flood Drain

30,0

00.0

0

29,0

00.0

0

28,0

00.0

0

27,0

00.0

0

49,000.00

50,000.00

51,000.00

52,000.00

53,000.00

54,000.00

48,000.00

47,000.00

46,000.00

45,000.00

49,000.00

50,000.00

51,000.00

52,000.00

53,000.00

GW

1-0

0

GW

1-00-04

DW1-01

DW1-00

DW1-00

GW1-00

GW

1-0

0

DW

1-00

DW

-00

DW-00

Secondary Drain

Tertiary Drain

FW

-1

FW-1

FW-1CHA 0+000 (FW-1)




Figure 4-3, Typical Cross Section of the Floodway 1

21

GW1-00

C/L

(OGL+3)

FW-1

C/L

Typical Section Km 0+000 - End of GW1-00

21

30m Min.

GW1-00-04

C/L

Typical Section End of GW1-00 - End ofGW1-00-04

(OGL+3)

Typical Section End of GW1-00-04 - 11+700

1:21:2

45m

OGLBL

FW-1

C/L

30m Min.

1:21:2

45m

OGLBL

FW-1

C/L

1:21:2

45m

OGLBL

Spoil Tip To Dump Excavated

Material Spoil Tip To Dump Excavated

Material

Spoil Tip To Dump Excavated

Material

10m Min. 10m Min.

10m Min.

10m Min. 10m Min.

OGL




5. Tentative Cost Estimate

West-Gode Irrigation Project Cost Estimate For

Flood Protection, Rehabilitation & Maintenance Work

1 Flood Protection at Headwork Unit Qty. Rate Amount

1.1 Rehabilitation and maintenance of Western Dyke m3 15,000

1.2 Compacted fill for new dyke from IP-A up to 2km d/s m3 24,000

2 Headwork

2.1 Electro-Mechanical Work

2.1.1 Clearing debris from the under sluice and intake gates LS 15,000

2.1.2 Supply and install intake gates and hoisting devices No. 4

2.1.3 Supply and install under sluice gates and hoisting devices No. 2

2.1.4 Supply and install sediment ejector gates and hoisting devices No. 4

2.1.5 Supply and install vortex tube gates and hoisting devices No. 2

2.1.6 Supply and install trash racks Set 1

2.2 Civil Work

2.2.1 Divide wall from intakes to sediment basin transition m3

2.2.2 Channel Excavation for Desilting Basin m3 16,800

2.2.3 Channel Excavation for Desilting Basin m3 5,800

2.2.4 Excavation in Open Cut m3 6,400

2.2.5 Non Woven Fabric m2 13,000

2.2.6 Dry Stone Pitching m3 5,800

3 Main Canal

3.1 Electro-Mechanical Work

3.1.1 Supply and install Main off take gates and hoisting devices No. 4

3.1.2 Supply and install gates and hoisting devices at cross regulator 1 & 2 No. 8

3.2 Civil Work (Main Canal & Main Road)

3.2.1 Excavation m3

3.2.2 Channel Excavation m3 320,000

3.2.3 Compacted Fill m3 150,000

3.2.4 Supply & Compact Gravel Material for Road m3 50,000

3.2.5 Maintenance work on cross regulator-1 and 2 LS 75,000

3.2.6 Maintenance work for emergency spillway-1 LS 25,000

3.2.7 Maintenance work for emergency spillway-2 LS 80,000

4 Farm Work

4.1 Excavation of Floodway m3 2,199,600

4.2 Compacted fill for the left embankment of primary canal GW1-00 m3 388,200

4.2 Clearing silt and forming primary canals PQ 85,000

4.3 Maintaining, forming and gravel surfacing of all connecting roads m3 25,000

4.3 Maintenance work on all primary canal off take gates No. 8

4.4 Clearing silt and forming tertiary canals PQ 15,000

4.5 Clearing silt and forming tertiary drains PQ 15,000

4.6 Clearing silt and forming secondary drains PQ 10,000

west gode irrigation project report on flood protection

Documents