ANNEX K: WEIRS Feasibility Study of Kilombero Valley ...

ANNEX K: WEIRS Feasibility Study of Kilombero Valley Irrigation Schemes Technical Assistance to Support the Development of Irrigation and Rural Roads Infrastructure Project (IRRIP2)

May 2016 – Final Version

This document was produced for review by the United States Agency for International Development. It was prepared by CDM Smith for the Technical Assistance to Support the Development of Irrigation and Rural Roads Infrastructure Project, Task Order number AID-621-TO-12-00002, under the USAID Global A&E IQC (Contract No.: EDH-I-00-08-00023-00).

ANNEX K: WEIRS Feasibility Study of Kilombero Valley Irrigation Schemes

Technical Assistance to Support the Development of Irrigation and Rural Roads Infrastructure Project (IRRIP2)

Prepared by: Philip Harvey, Civil Engineer Keith F. Williams, Chief of Party

Organization: CDM International, Inc. (CDM Smith)

Submitted to: Thomas Kaluzny, United States Agency for International Development (USAID) Contracting Officer’s Representative (COR)

USAID Contract No.: EDH-I-00-08-00023-00, Task Order AID-621-TO-12-00002

Report Date: 07 May 2016

DISCLAIMER

The author’s views expressed in this publication do not necessarily reflect the views of the United States Agency for International Development or the United States Government.

ANNEX K: WEIRS FEASIBILITY STUDY OF KILOMBERO VALLEY IRRIGATION SCHEMES i

Using this Report

The feasibility study of the Kilombero Valley irrigation schemes covers the Kisegese, Udagaji-Mgugwe, and Mpanga areas in Kilombero District. The feasibility study is documented in three main reports (one for each study area) with a series of associated annexes covering specific components. This report is one of those annexes. A single executive summary, prepared as a standalone document, covers the three study areas, given the similarities between them. The figure below illustrates the layout of the feasibility study reports for the three study areas.

The feasibility study is intended to be accessed at three levels of detail:

The executive summary provides a summary of the main reports for the three study areas in the Kilombero Valley for those who wish to get a brief overview of the subjects addressed and the main conclusions. Those wishing to understand particular issues in more detail should read the appropriate section in the main report.

The main report for each study area provides a thorough description of the work carried out, including objectives, study area, identification and development of alternatives, evaluation, cost estimates, conclusions, and recommendations.

The annexes (such as this report) to the main report provide supporting details of various components contributing to the feasibility assessment. Several component annexes are common to all study areas (e.g., Sociology, Land Tenure); therefore, a single annex has been prepared and is included with each study area report.

ANNEX K: WEIRS FEASIBILITY STUDY OF KILOMBERO VALLEY IRRIGATION SCHEMES ii

Table of Contents

Using this Report .......................................................................................................... i

Table of Contents ......................................................................................................... ii

List of Tables ............................................................................................................... v

List of Figures.............................................................................................................. vi

List of Acronyms and Abbreviations .......................................................................... viii

INTRODUCTION ................................................................................................ 1-1

SOURCE DATA AND DESIGN CRITERIA ....................................................... 2-1

KISEGESE: CHIWACHIWA OFFTAKE ............................................................ 3-1

ANNEX K: WEIRS FEASIBILITY STUDY OF KILOMBERO VALLEY IRRIGATION SCHEMES iii

KISEGESE – RUIPA OFFTAKE ........................................................................ 4-1

UDAGAJI OFFTAKE ......................................................................................... 5-1

MGUGWE .......................................................................................................... 6-1

ANNEX K: WEIRS FEASIBILITY STUDY OF KILOMBERO VALLEY IRRIGATION SCHEMES iv

MPANGA ........................................................................................................... 7-1

CONSTRUCTION COST ESTIMATES .............................................................. 8-1

SUMMARY AND CONCLUSIONS .................................................................... 9-1

REFERENCES ................................................................................................. 10-1

ANNEX K: WEIRS FEASIBILITY STUDY OF KILOMBERO VALLEY IRRIGATION SCHEMES v

List of Tables

Table 1.1: Catchment Areas at Weir Sites ........................................................ 1-2

Table 2.1: Design Flood Flows (m3/s) ................................................................ 2-1

Table 3.1: Chiwachiwa Irrigated Area Main Canal Parameters ......................... 3-5

Table 4.1: Ruipa River Weir Sites Visited .......................................................... 4-1

Table 4.2: Kisegese Blocks 1 and 2 Irrigated Area Main Canal Parameters ....................................................................................... 4-6

Table 5.1: Udagaji River Weir Sites Visited ....................................................... 5-1

Table 5.2: Udagaji Irrigated Area Main Canal Parameters ................................ 5-9

Table 6.1: Mgugwe Weir Sites Visited ............................................................... 6-1

Table 6.2: Mgugwe Irrigated Area Main Canal Parameters .............................. 6-7

Table 7.1: Mpanga Weir Sites Visited ............................................................... 7-1

Table 7.2: Mpanga Irrigated Area Main Canal Parameters ............................... 7-7

Table 8.1: Estimated Construction Costs for Headworks .................................. 8-1

Table 9.1: Principal Design Parameters for Each Site ...................................... 9-1

ANNEX K: WEIRS FEASIBILITY STUDY OF KILOMBERO VALLEY IRRIGATION SCHEMES vi

List of Figures

Figure 1.1: Irrigation Areas ................................................................................. 1-1

Figure 2.1: Operation of Structure during Flood Flows ....................................... 2-3

Figure 2.2: Head Drop across Structure ............................................................. 2-4

Figure 2.3: Sediment Flushing at Irrigation Intake .............................................. 2-6

Figure 2.4: Example Cutoff Arrangement ........................................................... 2-6

Figure 2.5: Larinier Super-active Fish Pass ........................................................ 2-7

Figure 2.6: Schematic Diversion Layout ............................................................. 2-8

Figure 2.7: Schematic Diversion Layout for Udagaji ........................................... 2-8

Figure 3.1: Proposed Chiwachiwa River Weir Location – Aerial Imagery .......... 3-2

Figure 3.2: Proposed Chiwachiwa River Weir Location – Ground Levels .......... 3-2

Figure 3.3: Final Weir Location Relative to Irrigated Area – Chiwachiwa ........... 3-3

Figure 3.4: Chiwachiwa Final Site –Topography and Headworks ...................... 3-4

Figure 3.5: Chiwachiwa Offtake – Downstream Cross Section .......................... 3-5

Figure 3.6: Chiwachiwa Offtake – Downstream Rating Curve ............................ 3-6

Figure 3.7: Chiwachiwa Offtake – Weir Rating Curve ........................................ 3-7

Figure 3.8: 3D Model of Proposed Chiwachiwa Offtake Structure ..................... 3-8

Figure 4.1: Ruipa Offtake: Weir Locations Visited – Aerial Imagery ................... 4-1

Figure 4.2: Ruipa River Proposed Weir Location ............................................... 4-3

Figure 4.3: Proposed Location for Weir Structure – Ruipa River ........................ 4-3

Figure 4.4: Final Weir Location Relative to Irrigated Area – Kisegese Blocks 1 and 2 .................................................................................. 4-4

Figure 4.5: Ruipa Offtake Site Topography and Headworks .............................. 4-5

Figure 4.6: Proposed Ruipa Offtake Location – Ground Levels ......................... 4-6

Figure 4.7: Ruipa Offtake – 5 m Downstream Cross Section ............................. 4-8

Figure 4.8: Ruipa Offtake – Downstream Rating Curve (In-Bank Flow) ............. 4-8

Figure 4.9: Ruipa Offtake – Downstream Rating Curve (Out-Of-Bank Flow) ..... 4-9

Figure 4.10: Ruipa Offtake: Weir Rating Curve .................................................. 4-10

Figure 4.11: 3D Model of Proposed Ruipa Offtake Structure ............................. 4-11

Figure 5.1: Udagaji Offtake: Weir Locations Visited – Aerial Imagery ................ 5-2

Figure 5.2: Existing Upstream Udagaji Weir at Weir 1 ....................................... 5-3

Figure 5.3: Final Weir Locations Relative to Irrigated Area – Udagaji ................ 5-4

Figure 5.4: Udagaji Offtake Locations – Ground Levels ..................................... 5-5

Figure 5.5: Udagaji Weir Site 1 Topography with Headworks ............................ 5-6

Figure 5.6: Udagaji Weir Site 2 Topography ....................................................... 5-7

Figure 5.7: Udagaji Weir Site 2 Looking Upstream ............................................. 5-8

Figure 5.8: Exposed Gneiss Rock at Udagaji Weir Site 2 .................................. 5-8

ANNEX K: WEIRS FEASIBILITY STUDY OF KILOMBERO VALLEY IRRIGATION SCHEMES vii

Figure 5.9: Udagaji Weir Site 1 – 5 m Downstream Cross Section .................... 5-9

Figure 5.10: Udagaji Weir Site 1 – Downstream Rating Curve ........................... 5-10

Figure 5.11: Udagaji Weir Site 2 – 5 m Downstream Cross Section .................. 5-10

Figure 5.12: Udagaji Weir Site 2 – Downstream Rating Curve ........................... 5-11

Figure 5.13: Udagaji Offtake – Weir Rating Curve ............................................. 5-12

Figure 5.14: Udagaji Weir Site 2 – Weir Rating Curve ....................................... 5-13

Figure 5.15: 3D Model of Proposed Offtake Structure – Udagaji River .............. 5-14

Figure 6.1: Mgugwe Offtake: Weir Locations Visited .......................................... 6-2

Figure 6.2: Mgugwe Offtake: Proposed Alternative Weir Location ..................... 6-3

Figure 6.3: Final Location of Weir Relative to Irrigated Area – Mgugwe ............ 6-4

Figure 6.4: Mgugwe Offtake Locations – Ground Levels .................................... 6-5

Figure 6.5: Mgugwe Site Topography and Headworks ....................................... 6-6

Figure 6.6: Mgugwe Offtake – 5 m Downstream Cross Section ......................... 6-7

Figure 6.7: Mgugwe Offtake – Downstream Rating Curve ................................. 6-8

Figure 6.8: Mgugwe Offtake – Weir Rating Curve .............................................. 6-9

Figure 6.9: Mgugwe Offtake – 3D Model of Proposed Structure ...................... 6-10

Figure 7.1: Mpanga Offtake – Proposed Weir Sites ........................................... 7-1

Figure 7.2: Final Weir Location Relative to Irrigated Area – Mpanga ................. 7-3

Figure 7.3: Mpanga Offtake Locations – Ground Levels .................................... 7-4

Figure 7.4: Mpanga Weir Site 2 Topography and Headworks ............................ 7-5

Figure 7.5: Mpanga Offtake – River Bed Condition (Weir Site 2) ....................... 7-6

Figure 7.6: Mpanga Offtake – River Bed Condition 30 m Downstream of Proposed Site (Weir Site 2) .............................................................. 7-6

Figure 7.7: Mpanga Offtake – Downstream Cross Section ................................ 7-7

Figure 7.8: Mpanga Offtake – Downstream Rating Curve .................................. 7-8

Figure 7.9: Mpanga Offtake – Weir Rating Curve ............................................... 7-9

Figure 7.10: Mpanga Offtake – 3D Model of Proposed Structure ....................... 7-10

ANNEX K: WEIRS FEASIBILITY STUDY OF KILOMBERO VALLEY IRRIGATION SCHEMES viii

List of Acronyms and Abbreviations

AADF Annual Average Daily Flow FS Feasibility Study/studies FSL Full Supply Level IRRIP2 Technical Assistance to Support the Development of Irrigation and

Rural Roads Infrastructure Project LiDAR Light detection and ranging mAD Meters above datum (Survey of Tanzania) USBR United States Bureau of Reclamation USAID United States Agency for International Development

ANNEX K: WEIRS FEASIBILITY STUDY OF KILOMBERO VALLEY IRRIGATION SCHEMES 1-1

INTRODUCTION

Under the “Technical Assistance to Support the Development of Irrigation and Rural Roads Infrastructure Project (IRRIP2)”, funded by the United States Agency for International Development (USAID), feasibility studies (FS) are required to assess the potential for developing irrigated agriculture in four areas: Kisegese (comprised of the Chiwachiwa Block and Kisegese 1 and 2 Blocks), Udagaji, Mgugwe and Mpanga. These areas are shown on Figure 1.1.

Figure 1.1: Irrigation Areas


Different options for the river offtake structures have been considered for the IRRIP2 irrigation schemes. These structures generally comprise a controlling weir with a diverting offtake to the irrigation areas. Table 1.1 provides catchment areas for the five weir structure locations where water would be extracted. These catchment areas have been digitized from Survey of Tanzania 1:50,000 mapping.

Table 1.1: Catchment Areas at Weir Sites

Weir Location Catchment Area at Weir Site(1)

(km2)

Ruipa 761

Chiwachiwa 439

Udagaji 24

Mgugwe 226

Mpanga 2,579 Notes: (1) Catchment areas digitized from Survey of Tanzania 1:50,000 mapping.

Soil studies have been carried out to identify the gross land area that is suitable for irrigation (this is summarized in FS Annex A: Soils and Land Suitability). Hydrometric data has been used to determine the flows at the weir sites that would be available for irrigation (the assessments are described in FS Annex C: Hydrology). These studies have been used to investigate the suitable flows to be diverted at each of the five weir structures and the irrigable area that could therefore be supported. The flood flows that the weirs need to be capable of passing have also been considered in FS Annex C: Hydrology.

Outline designs of the weir structures have been based on the required offtake flows and flood flows to be safely passed over the spillways. This Report outlines the design process for each proposed weir structure including details on:

1. Design data sources;

2. Overall design criteria;

3. Site-specific design philosophy; and

4. Proposed general arrangements.


SOURCE DATA AND DESIGN CRITERIA

Source Data

The principal data sources that have been used in the weir design development are:

1. Topographic data;

2. Hydrological data;

3. Geotechnical data;

4. Irrigated scheme design data; and

5. Site visits.

The accuracy, uncertainties and risks associated with the above data sources are described in the following sections.

2.1.1 Topographic

The principal source of topographic data used in the feasibility-level design development of the weirs was the 10-cm resolution Light Detection and Ranging (LiDAR) data. The LiDAR and aerial imagery (flown by Southern Mapping Inc. of South Africa in 2013 as part of IRRIP2) covered all the proposed irrigation areas (including the proposed areas for the weir structures). This data represented the most up-to date and reliable topographic data source to use for the weir structure feasibility designs.

Data collection by LiDAR has inherent inaccuracies due to the nature of the collection method. However, the level of accuracy provided by the LiDAR was found acceptable for the purpose of feasibility-level design. As the project progresses, there will be a need for site-specific topographic surveys to carry out detailed design work.

2.1.2 Hydrological

Appropriate flood flows to be passed over the weir structures needed to be identified and used as an input into the feasibility designs. For the weir structures, a 1-in-100 year return period has been adopted. This is in line with standard Tanzanian design guidance as outline within ‘Irrigation Design Manual’, Ministry of Agriculture and Cooperatives, Government of Tanzania. The design floods for each of the structures are presented in Table 2.1. The method for calculating flood flows is described in FS Annex C: Hydrology.

Table 2.1: Design Flood Flows (m3/s)

Return Period Mpanga Ruipa Chiwachiwa Udagaji Mgugwe

100 Year 458 227 165 31 113

Although under normal circumstances the structure would operate at lower flow rates, the design flood is required during the design process to determine the spillway needed to safely pass this flood and to ensure the structures’ stability under these flood conditions.

2.1.3 Geotechnical

Site visits were undertaken to better understand the potential geotechnical conditions at the proposed weir locations. The site visits and desk-based geotechnical


assessments are discussed further in the appropriate sections below and in FS Annex G: Geotechnical.

The geotechnical investigations that have been carried out are non-intrusive surveys and desk studies. The primary source of data was the 1:125,000 scale geological maps of Tanzania with useful background information provided from Dypvik & Nilsen (2001). Consequently, only the type of foundation has been assessed for each site and, as no formal sub-surface site investigations have been undertaken, this has been based on a ‘worst credible case’ for the ground that could be encountered. This implies that the proposed designs might be considered conservative in nature.

2.1.4 Irrigable Area Design Criteria

The following parameters were determined for the canals that would be supplied with water diverted at the weir structures. These needed to be considered in the design of the weir structures themselves.

1. Offtake design flow

2. Minimum water level required to command the irrigated areas

3. Canal bed width

4. Design depth of flow

5. Canal side slopes

6. Initial weir location

Following initial design work for the weir structures, it was important to feed elevation and structure arrangement information back into the canal and irrigation layout design work.

2.1.5 Site Visits

Field visits to the project areas were conducted from 4 to 7 June 2014 by the Geotechnical Engineer. The Hydraulic Structures Engineer subsequently visited the sites from 21 to 23 August 2014.

These visits aimed to identify potential locations for weir structures that would command the identified irrigable areas. During each visit, a number of alternative sites were assessed against the following criteria:

Assessment of foundations for the weir and offtake structure

Location and orientation of the weir to be able to feed a main canal

Elevation of the weir with respect to the elevation of the main canal at the start of the irrigable areas

Discharge capacity of the downstream river section, i.e., potential backwater against the structure

Findings from the field visits were subsequently used to inform the feasibility weir designs.

Headworks Design Criteria

The principal criteria adopted for the weir design is described in the following sections.

2.2.1 Structure Location

For each weir structure, several potential locations were considered during field visits by the Hydraulic Structures Engineer in August 2014. The sites that were visited and


the subsequent recommendations on the preferred locations are summarized below. The final weir sites were selected based on the following criteria:

1. Hydraulic conditions at the site

2. River profile and floodplain alignment

3. Foundation conditions

4. Location relative to proposed head of canal

2.2.2 Water Levels and Design Floods

It was necessary to define a range of allowable water levels for the structures during operation.

Under normal operating conditions, no storage of water above the existing floodplain level was considered, i.e., the water level control is proposed to be retained within the existing river channel.

Under flood conditions, flows upstream of the structure are likely to be out of bank. However, the control structure would return the flood to the river channel as shown in Figure 2.1. Under such conditions the drop in hydraulic head across the structures will be low, see Figure 2.2, the proposed structures are unlikely to constitute a significant risk if they were to breach, and no dam break analysis is warranted.

Figure 2.1: Operation of Structure during Flood Flows

River alignment

Flow direction

Flood bund

Regulating structure

Auxiliary spillway

High ground

High ground


Figure 2.2: Head Drop across Structure

The above assessment indicates that the design flood for all the weir structures should be set as the 1-in-100 year event. It was therefore necessary to consider the weir lengths required to allow the 1-in-100 year floods to pass over the structures. The flood rise above sill level at each structure was calculated in order to determine the required height of abutments and wingwalls.

2.2.3 Downstream Conditions

The downstream conditions for each weir structure have been assumed to be normal depth of flow based on the natural cross section. This assumes no downstream influence on water levels in the river reaches downstream of the weirs.

A typical representative cross section of the river channel was extracted from the LiDAR data at a location immediately downstream of the proposed structure. It is noted that the LiDAR data will have only provided the water surface elevation on the day of flight. Should the schemes proceed beyond feasibility stage, topographic and bathymetric surveys would be required at the weir locations to inform the detailed design of the weirs.

The channel long section was also extracted from the LiDAR data, with a representative channel gradient calculated based on the prevailing topographic conditions. A standard Manning’s free flow calculation was carried out using a conservative manning coefficient of 0.04 to determine the normal depth of flow within this channel.

While the methodology outlined above is likely to give reasonable approximations to the downstream water surface profile in most of the proposed sites, downstream infrastructure might influence the profile at others. For instance, should the flood passing the designed structure exceed the capacity of the river channel further downstream, the backwater effect might raise the surface water profile up to the control structure. In these instances, further discussion is given for the particular structure location.

Finally, it should be noted that the derived rating curves do not make reference to the recently completed floodplain mapping, as this was not available at the time of the design work being carried out. As such, for the purposes of this report, the simplified exercise was undertaken to identify the likely downstream water levels. Any further design should make reference to the likely flood conditions as described in FS Annex I: Flood Mapping.

2.2.4 Foundation Conditions

The geotechnical understanding for each site has been based on visual inspection only. The likelihood of bedrock at the weir structure foundation level was considered

Head drop

D/S Flood Water Level

U/S Flood Water Level


for each site. Based on these site observations the structures have been designated as either ‘rock foundations’ or ‘permeable foundations’.

2.2.5 Structure Levels

The sill levels of the structures have been set so that they are high enough to provide sufficient command of the irrigable area while minimizing canal excavation requirements. However, setting the sill levels too high would have resulted in difficulties in providing sufficient head rise and freeboard to the top of the structures while passing the design flood. To limit flood rise during the design flood, an ogee weir has been used for all the sites. Furthermore, submergence has been allowed for where appropriate.

Where the 1-in-100 year design flood could not be contained within the channel, the design provides for safe passage of the flood through the headworks structure as well as over a length of lowered crest of an auxiliary embankment with appropriate protection designed to withstand this portion of the flood flow.

It should be noted that at the sites where such an arrangement has been designed, the natural flow regime will include out-of-bank flow during floods. Thus, the downstream conditions when floods pass over the lowered auxiliary embankment will be safely returning into prevailing out-of-bank flows.

Where an over-toppable embankment has been used, a weir discharge coefficient of 1.5 m1/2/s has been assumed. For the main structure, the weir discharge coefficient has been developed in line with recommendations in ‘Design of Small Dams’ (United States Bureau of Reclamation (USBR) 1987).

2.2.6 Flood Protection

The proposed irrigated areas and main canals are situated within existing floodplains. Therefore, flood embankments will be required at several of the weir sites, such as shown in Figure 2.1, and will need to connect into flood embankments that are to be provided around the irrigated areas (discussed in more detail in FS Annex I: Flood Mapping and FS Annex J: Engineering).

2.2.7 Sediment Control

The build-up of sediment behind the weir and in front of the offtake canal was identified as a potential issue. Therefore, sluice gates have been provided on each weir, which will allow accumulated sediment to be periodically flushed out of the head ponds. The pier along the side of the sluice gate has been extended further upstream than other piers. The aim is to provide a ‘pocket’ of low velocity flow which would encourage sediment to fall out of the water column in a location where it can then be flushed through the adjacent sluice gate, see Figure 2.3.

In the case of Udagaji, owing to the steep nature of the river, an offline sedimentation basin and flushing channel has been provided to allow for effective sediment management. For Ruipa and Chiwachiwa rivers, concerns regarding potential sediment inflows were such that one additional sediment basin at each site has also been provided.


Figure 2.3: Sediment Flushing at Irrigation Intake

2.2.8 Cutoff Design

Water retaining or control structures that are not founded on rock usually incorporate upstream and downstream cutoffs, as shown in Figure 2.4 to protect against the effects of erosion at the downstream toe; reduce exit gradients; increase the seepage path; and reduce the uplift force. A cutoff is often constructed using sheet piles, or, where depths are sufficiently shallow, a sub-surface concrete wall can also be acceptable.

In this IRRIP2 study, where the weir structures are to be founded on ‘permeable foundations’, it has been assumed that cutoffs are required. The cutoff is designed on a structure-by-structure basis with the design criteria recommended in Hydraulic Structures (4th Edition) (Novak 2007):

Upstream cutoff to limit the hydraulic exit gradient at the toe to 0.16 m/m

Downstream cutoff to protect against anticipated scour depths based on Lacey regime theory

Figure 2.4: Example Cutoff Arrangement

Simplified empirical equations were used to investigate whether piling would be required for seepage control. Finite element calculations have not been carried out at this stage of the design. However, it is recommended that these should be incorporated into a future detailed design phase.

Flushing flows

Canal intake

Area of potential sediment build-up

U/S Cutoff D/S Cutoff

Head difference across structure

Seepage flows


2.2.9 Fish Passes

It has been determined that fish passes will be required for all of the weir structures apart from Udagaji. The appropriate type of fish pass is dependent on the species of fish that are expected to migrate across the structure. Fish species (and therefore fish pass requirements) have not yet been identified at each site. Therefore, for the purposes of this feasibility study, it has been assumed that Larinier super-active fish passes (shown in Figure 2.5) will be appropriate for all weir structures. These Denil type passes have generally been shown to be one of the more flexible options with respect to fish species and is therefore an appropriate assumption given the current level of design.

Figure 2.5: Larinier Super-active Fish Pass

In fish pass design, it is necessary to understand the specific periods of fish migration and to evaluate the likely range of flows and water levels during periods when the fish pass would be required. The Q95 water levels have been taken as the lower limit of operation. An upper flow rate and associated percentile above which the fish pass may not operate were also assessed. Q5 and Q95 flow rates were based on the Flow-Duration curves developed during the hydrological studies and presented in FS Annex C: Hydrology.

Although general guidance is to accommodate the Q5 river flow water levels, in most cases that was not possible. Efforts have been made to accommodate the maximum range of conditions. In most cases, this equated to the Q10 condition.

The discharge that will pass through the fish pass is an important aspect of the design. A reasonable proportion of flow must pass through the fish pass in order to attract the fish towards the entrances. This is generally set as a percentage of Annual Average Daily Flow (AADF), as developed during the hydrological studies and described in FS Annex C: Hydrology, and flow rates in excess of 10% are usually recommended. However, where there are high flows in the river this percentage can lead to very large fish passes and it has been suggested that the percentage could then be reduced to 5%. For the Mpanga weir (which has large flows), 5% of the AADF has been assumed. For all other structures, 10% of the AADF has been adopted.

Larinier super-active fish passes have a maximum recommended flight length of 10 m and a maximum longitudinal gradient of 15%. The relatively low heads across all but one of the structures mean that they only require one flight each. The higher


head drop across the Mpanga weir structure means that three flights would be required.

For all but one of the fish passes a baffle height of 150 mm has been assumed. The larger flows at the Mpanga weir structure means that 200-mm baffles are required. The width of each unit is taken as 6 x baffle height, therefore 0.9 m in all cases except Mpanga, which is 1.2 m.

Further species-specific data collection must be carried out to determine whether the fish pass designs described above are appropriate. Any additional design work must incorporate a full assessment of the fish requirements at each site in order to make appropriate adjustments.

2.2.10 Weir Constructability

Where the existing channel is relatively flat or shallow, the proposed weir will be constructed by creating a temporary diversion channel on the same bank as the offtake structure. The temporary channel will either be excavated or formed of a cofferdam, made from sheet piling, and is to be backfilled once construction is complete. The temporary channel should be designed to pass a 1-in-10 year flood event, see Figure 2.6. This methodology is likely to be suitable at all sites except Udagaji, which is shown in Figure 2.7.

Figure 2.6: Schematic Diversion Layout

Figure 2.7: Schematic Diversion Layout for Udagaji

Flow direction

Temporary cofferdam

Diverted river channel

Phase 1 Phase 2

Diverted flow Diverted flow

Temporary cofferdam

Temporary cofferdam

River channel

Constructed sluice


The temporary diversion structure required at Udagaji will be different from the other four structures as the channel is much steeper. The expected flows at Udagaji, however, are relatively low. This should allow the sluice to be used for the temporary diversion. Therefore, the following sequence could be considered:

construction of the sluice and offtake

divert river through the sluice

construct the main weir

This assumes that it would be possible to pass the 1-in-10 year flow rate through the sluice. This sequencing would need further attention at later stages of the design.


KISEGESE: CHIWACHIWA OFFTAKE

Initial proposals for the Kisegese scheme indicated there would be a single irrigated area supplied by a weir site along the Ruipa River. Therefore, initial site visits were based on the assumption of one structure in an upstream location. However, further design reviews in October 2014 led to the development of three irrigated areas fed from two different weirs: one on the Chiwachiwa River (feeding the Chiwachiwa Block) and one on the Ruipa River (feeding Kisegese Blocks 1 and 2). Therefore, feasibility designs were developed for these two weir locations without the benefit of site visits. The design work has been based on information obtained from desk-based studies.

Site Visit

The site for the Chiwachiwa River weir structure was selected after the site visits were undertaken. Therefore, no site-specific information was available for this study. The findings of this study must be considered in light of the uncertain site conditions.

Weir Location

The initial location for the weir structure was at the most preferential site with respect to the main canal layout. After further review, the weir site was moved upstream by 90 m in order to:

1. Locate the weir on a straight reach of river.

2. Provide a more defined river channel with improvements in flood flow control, along with better proximity to high ground.

The initial and proposed locations are shown overlaid on the aerial imagery and the LiDAR data in Figure 3.1 and Figure 3.2, respectively. Figure 3.3 shows the final weir location (highlighted in a red box) with respect to the irrigated area and other irrigation infrastructure.


Figure 3.1: Proposed Chiwachiwa River Weir Location – Aerial Imagery

Source: LiDAR and aerial imagery, Southern Mapping Inc., 2013

Figure 3.2: Proposed Chiwachiwa River Weir Location – Ground Levels


Initial location

Proposed location

Flow direction

Initial location

Proposed location


Figure 3.3: Final Weir Location Relative to Irrigated Area – Chiwachiwa

Topography

As can be seen from Figure 3.1 and Figure 3.2, the site topography is dominated by a wide, flat floodplain on the left bank meeting higher ground 300 m from the proposed site, along with ground which rises more steeply on the right bank. The river channel at the site is approximately 15 m wide with limited meandering. A well-defined river channel with limited river bends provides reasonable approach


conditions. There is space on the right bank that could be used to provide an auxiliary spillway.

Figure 3.4 shows the local site topography with the main river channel well defined and the structure location within a suitably straight reach. On the right bank, the ground begins to rise within reasonable proximity to the proposed site.

Figure 3.4: Chiwachiwa Final Site –Topography and Headworks


Geology

No visits were made to the proposed weir site; therefore, the geological conditions are based on preliminary desk studies of the site. These are documented in FS Annex G: Geotechnical.

The site is underlain by “Migmatitic biotite gneiss, locally with hornblende and garnet” and “Garnetiferous biotite gneiss”, both of the Usagara series. These have generally been found to be very strong and durable where encountered in the area. Weathering is possible, but has generally found to be thin. On the flanks of the river course, these Pre-cambrian rocks are overlain by the Neogene stratum, “lateritic earths (loamy sands, brown, pink, red to yellow)”. No alluvium is shown at this location on the geology map, but it is possible that the Pre-Cambrian rocks are overlain by a thin layer of more recent alluvial soils, most probably coarse, rounded cobbles and boulders of similar material.

Based on the available information, it is most likely that the sub-soil under the weir is rock. However, it may be firm ground of less strength. Additional geotechnical studies would be required to confirm the presence/absence of rock at reasonable

Flow direction

To canal

Flood bund


depth. To remain consistent with the adopted ‘worst credible case’ condition, Chiwachiwa weir has been assumed to be founded on ‘permeable foundations’.

Irrigation Design Criteria

The design criteria for the Chiwachiwa weir are provided in Table 3.1.

Table 3.1: Chiwachiwa Irrigated Area Main Canal Parameters

Location Bed level Bed

width Full supply level (FSL)

Flow depth Flow

mAD m mAD m m3/s

Kisegese (Chiwachiwa)

282.1 1.25 283.7 1.1 1.54

General Arrangement Design

The following sections provide further details on the proposed general arrangement of the Chiwachiwa weir structure.

3.6.1 Downstream Condition and Channel Conveyance

The downstream cross section has been extracted from the LiDAR data immediately downstream of the structure. The section is shown in Figure 3.5. The associated rating curve was developed using Manning’s normal depth calculations as described in Section 2.2.3 and is shown in Figure 3.6.

Figure 3.5: Chiwachiwa Offtake – Downstream Cross Section


280.0

281.0

282.0

283.0

284.0

285.0

286.0

287.0

0 100 200 300 400

Ele

vati

on

(m

AD

)

Station (m)

LB RBFlood plain


Figure 3.6: Chiwachiwa Offtake – Downstream Rating Curve

Figure 3.6 shows that channel conveyance at this location is approximately 40 m3/s (less than the 1-in-100-year Design Flood of 165 m3/s). Therefore, it would be impractical to try to retain flood flows up to the 1-in-100 year return period within the river channel through the regulating structure. To lower upstream water levels during flood events, an auxiliary spillway has been incorporated into the design.

3.6.2 Hydraulic Configuration

Initial weir sizing was based on the natural river channel width at the site. The bankfull level is approximately at elevation 283.5 mAD1. The design of the weir crest level considered the need to command the irrigated area against the need to minimize the increase in flood risk upstream of the structure.

The auxiliary spillway was located on the right bank to avoid any conflict with the irrigation infrastructure proposed on the left bank. Based on the natural topography of the site, an auxiliary weir of approximately 35 m in length was seen as most practical.

The hydraulic assessments resulted in an auxiliary weir length of approximately 35 m at an elevation of 284 mAD with a 15-m wide main spillway crest at an elevation of 283 mAD. The evaluation of the auxiliary spillway was completed by calculating the total flow over the embankment using the standard weir equation (ICE 2015). The flow over the main structure was calculated as described in Section 3.6.3. With

1 mAD refers to meters above datum, referenced from the Survey of Tanzania

280.0

281.0

282.0

283.0

284.0

285.0

286.0

287.0

0 50 100 150 200 250 300 350

Wat

er L

evel

(m

AD

)

Discharge (m3/s)

Limit of in‐bank flow


these dimensions and elevations, the peak upstream water level during the 1-in-100 year flood event was found to be 285.1 mAD.

The level of the flood protection bund on the left bank was then designed to retain the 1-in-100 year flood event with 0.7 m of freeboard.

3.6.3 Weir Hydraulic Performance and Flow Gauging

Flow measurement can be incorporated by using an ogee crested weir. The rating curve for the weir is presented in Figure 3.7. This has been developed based on the known performance of Ogee weirs as described in ‘Design of Small Dams’ (USBR 1987).

Figure 3.7: Chiwachiwa Offtake – Weir Rating Curve

During higher flow flood events, significant submergence is expected resulting in a reduction in the efficiency of the weir. Under the current design scenario the flow rate above which submergence starts to impact flow is approximately 35.1 m3/s at an upstream water level of 284.1 mAD. The hydraulic performance of an ogee weir under submerged conditions would not preclude the measurements of flows during extreme events. However, with upstream water levels above 284.1 mAD, additional flows over the auxiliary spillway would not be included in the flow rating shown in Figure 3.7 and should be accounted for separately.

The rating curve presented in Figure 3.7 is for the fixed crest spillway only. The rating curve allows for the expected submerged conditions based on the downstream rating curve as presented in Figure 3.6. However, further investigations should be undertaken to evaluate the expected conditions in the lower reaches of the river to improve the accuracy of any flow gauging at this site.

The performance of the main spillway weir under submerged conditions results in a lower weir coefficient at the design head than would otherwise be expected under modulus conditions. The reduction in efficiency was evaluated using ‘Design of Small Dams’ (USBR 1987) as described above.


3.6.4 Energy Dissipation

A stilling basin has been included in the feasibility design to protect against erosion of the channel downstream of the weir. Given the hydraulics expected in the 1-in-100 year event, a USBR “Low Froude Number Stilling Basin” has been selected.

3.6.5 Cutoff Design

In order to reduce the impact of seepage, 4.5-m deep piles measured from the underside of the base slab on both the upstream and downstream end are required. This depth of downstream cutoff should also be sufficient to protect against downstream scour.

Concrete cutoffs would be more practical and less expensive than sheet piling as they would not require a sheet piling rig to be mobilized to site. The use of concrete cutoffs has therefore been considered more appropriate for the feasibility designs. Alternative construction methods could be considered during later design phases.

3.6.6 Final Arrangement

Figure 3.8 shows a 3D representation of the proposed weir structure on the Chiwachiwa River.

Figure 3.8: 3D Model of Proposed Chiwachiwa Offtake Structure

15 m

35 m

Irrigation intake

Stilling basin

Flushing sluice

Fixed crest spillway

Auxiliary spillway

Flow to irrigation canal


KISEGESE – RUIPA OFFTAKE

Site Visit

Table 4.1 and Figure 4.1 show the locations of the weir sites visited (Weir Site 1 and Weir Site 2). In this stretch of the Ruipa River, there are meanders with historical river channels apparent from the LiDAR mapping. Colluvium is likely throughout this area; however, suitable foundations for a concrete structure may potentially be found at depth. A summary of the visual assessment of each site is given in the following sub-sections.

Table 4.1: Ruipa River Weir Sites Visited

Site No.

Easting Southing GPS

Waypoint No. Deg Mins Secs Deg Mins Secs

Weir Site 1 36 19 14 8 7 24 51

Weir Site 2 36 19 5.2 8 7 7.7 88

Figure 4.1: Ruipa Offtake: Weir Locations Visited – Aerial Imagery


Weir Site 1

Weir Site 2

Flow direction


4.1.1 Weir Site 1

The following was observed from the left bank:

the river is approximately 20 m wide with 2-m high banks

no rock was visible at the site

the floodplains were found to be farmed

4.1.2 Weir Site 2

Weir Site 2 was identified on site through a change in the vegetation on the left bank. The following was observed from the left bank:


immediately upstream there is a meander in the river toward the left bank where the river is narrower with higher banks

the floodplains were found to be farmed

4.1.3 Downstream Sites

Some effort was made to reach potential offtake sites further downstream. However, this was not possible due to high water levels at the sites and an impassable access road.

Weir Location

The site that was adopted for the feasibility design of the Ruipa River weir structure was selected after site visits had been undertaken to the two sites further upstream. Therefore, no site-specific information was available for the feasibility design. However, information obtained from visits to the two upstream sites gave an indication of the ground conditions that might be expected.

Weir Site 3 has been taken forward for further design development. The proximity of Weir Site 3 to the visited sites is shown on Figure 4.2. The site was selected to ensure supply to the irrigation head canal as well as to allow for a structure alignment corresponding to both in-channel flows as well as out-of-bank flows. The direction of these types of flows is indicated in Figure 4.3.

Figure 4.2 shows that the Ruipa River meanders significantly. This characteristic, when considered alongside the LiDAR survey data in the area, suggests that the channel is in flux and there is a propensity for the river to change course. There is the potential for movement of the channel to such an extent that it would bypass the weir structure. This would result in a need to re-align the channel so that the weir and canal would still be supplied with water.

Figure 4.4 shows the final location of the weir (highlighted in a red box) with respect to the irrigated areas and other irrigation infrastructure.


Figure 4.2: Ruipa River Proposed Weir Location


Figure 4.3: Proposed Location for Weir Structure – Ruipa River


Weir Site 2

Weir Site 1

Flow direction

Weir Site 3

Weir Site 3

River flow direction

Floodplain flow direction


Figure 4.4: Final Weir Location Relative to Irrigated Area – Kisegese Blocks 1 and 2

Topography

The site topography is dominated by low-lying floodplains on either side of the river with a river channel exhibiting high sinuosity. The low conveyance of the river channel at this location has meant that out-of-channel flow has needed to be incorporated into the design.


This out-of-channel flow is protected by flood embankments provided around the irrigation infrastructure allowing the flow during flood seasons to be passed around it as shown in FS Annex I: Flood Mapping.

Figure 4.5 shows the site topography with the river channel and the low-lying banks on either side. The structure will create a head pond that should help mitigate the poor approach conditions at this site during low flow. At high flow, it is expected that the approach conditions would improve significantly in line with the structure alignment.

Figure 4.5: Ruipa Offtake Site Topography and Headworks


In-channel flow direction

Out-of-bank flow direction

To canal

Flood bund


Figure 4.6: Proposed Ruipa Offtake Location – Ground Levels


Geology

No site visit has been made to Weir Site 3 (the chosen location). Therefore, the following assessment has primarily been based on a desk study, supplemented with information collected from site visits made further upstream. The study is described in more detail in FS Annex G: Geotechnical.

The site is underlain by Neogene alluvium, comprising sands, silts and clays. Slightly to the south, there is an area of Mbuga earths, also in the Neogene series, which are believed to be waterlogged sandy clays and organic soils. On the left bank, this area is underlain by Neogene stratum. However, there is a suggestion that the main embankment on the right bank extends over an area underlain entirely by alluvium.

It has therefore been assumed that the Ruipa weir structure is to be founded on ‘permeable foundations’. For later design phases, site-specific geotechnical information would need to be obtained and analyzed.


The design criteria for the Ruipa weir are provided in FS Annex J: Engineering and are shown in Table 4.2.

Table 4.2: Kisegese Blocks 1 and 2 Irrigated Area Main Canal Parameters


width Full supply level (FSL)

Flow depth Flow

mAD m mAD m m3/s

Kisegese Blocks 1 and 2 (Ruipa offtake)

271.0 2.0 273.2 2.2 6.3

Weir Site 3



The following sections provide further details on the proposed general arrangement of the Ruipa weir structure.


The development of a downstream rating curve to be used for the design of the structure is complicated by the fact that the flow in the river becomes out-of-bank during the 2-year return period flood as described in FS Annex I: Flood Mapping. The results of the flood modeling with LiDAR water surface elevations were not available at the time of developing the rating curve.

The following two assumptions were made for the development of the downstream rating curve:

There is no downstream influence to the river flow at the weir site.

The river flows perpendicularly to the alignment of the weir structure across the river. Given the variable flow conditions between low-flow and flood conditions, this may not be the case resulting in differing water levels.

The assumptions should be re-evaluated at later stages of design development.

The cross section shown in Figure 4.7 demonstrates the difficulties of siting a weir at this location. A clear river channel is well-defined with banks as shown in Figure 4.7. However, the floodplain is only slightly above the elevation of the riverbed itself. Therefore, once the channel capacity is exceeded the entire valley begins to act as the de-facto riverbed. This is then likely to lead to saturation of the soils once flows have subsided and the flow has returned to the river channel.

Similarly, the right bank shows no significant raised ground within several kilometers of the river. Figure 4.7 extends only for the high ground located near the riverbed on the right hand side to maintain clarity in the figure. The proposed command area on the right bank is located within the low-lying area that continues from chainage 1,500 m for several kilometers.

Because of this relatively unusual topography, the downstream rating curve at this location has two modes: when flows are in-bank and when flows are out-of-bank. This reflects the relatively complex flow conditions under in-bank and out-of-bank flows with likely variations of downstream conditions between the two scenarios.

Therefore, the rating curve shown in Figure 4.8, developed using Manning’s normal depth calculations as described in Section 2.2.3, is for in-bank flows only and was used to determine the low-flow structure capacity.

The rating curve shown in Figure 4.9 is the preliminary downstream rating curve developed for out-of-bank flow, and was used primarily to determine the required embankment and wing wall levels. The hatched area shown on the rating curve below is the level below which the rating curve is not applicable due to the unusual topography of the site. Consequently, consideration of flow rates below the bankfull capacity using Figure 4.9 should be avoided. Both rating curves were developed based on the methods outlined in Section 2.2.3 using the cross sections as shown in Figure 4.7.


Figure 4.7: Ruipa Offtake – 5 m Downstream Cross Section


Figure 4.8: Ruipa Offtake – Downstream Rating Curve (In-Bank Flow)

270

272

274

276

278

280

282

284

286

288

290

0 500 1000 1500 2000

Elevation (mAD)

Chainage (m)

River thalweg

272.0

272.5

273.0

273.5

274.0

274.5

275.0

0 20 40 60 80 100

Wat

er L

evel

(m

AD

)

Discharge (m3/s)

LB RB


Figure 4.9: Ruipa Offtake – Downstream Rating Curve (Out-Of-Bank Flow)

From Figure 4.7, it can be seen that flows greater than elevation 274.7 mAD, with a corresponding flow of approximately 90 m3/s from Figure 4.8, exceed the channel capacity. Given the assumptions above, with the design flood at this location (taken as 214 m3/s), retaining flood flows within the river channel at this location up to the 1-in-100 year return period is impractical. Therefore, a form of auxiliary spillway to pass the additional flows is necessary.


The hydraulic design of the Ruipa weir was challenging due to the limited hydraulic head available at the site to pass the 1-in-100 year flood event without significantly raising water levels. In particular, the use of a formal auxiliary spillway in the form of an over-toppable embankment with controlled flows proved to be impractical in this instance due to the local topography and the need to minimize increases in water levels, since they would negatively affect adjacent farmlands.

Therefore, the most cost-effective hydraulic design for the Ruipa weir would use the existing floodplain of the right bank as an effective auxiliary spillway with sufficient protection on the right flank of the structure to prevent damage to it during high-flow events. This flooding should be no worse than current flooding which inundates the entire floodplain during high flows. The design is such that minimal increases in water level upstream of the structure would be expected limiting the impact on upstream stakeholders.

Based on these design assumptions, the proposed structure would then preserve the bankfull capacity upstream of the offtake site as close to the existing channel as possible through providing minimal head losses through the structure during the spillway design flow, taken to be that of bankfull capacity (90 m3/s).

272.0

272.5

273.0

273.5

274.0

274.5

275.0

275.5

- 500 1,000 1,500 2,000

Wat

er L

evel

(m

AD

)

Discharge (m3/s)


A significant risk associated the arrangement is the possibility of the river migrating from its current position. However, the risk of such an event could be reduced through minor re-profiling of the local topography on the right bank to develop a preferential flow path towards the proposed structure. In a situation where the channel does move away, it would be relatively easy to carry out excavation works to re-direct the river back towards the weir and offtake structure.



Figure 4.10: Ruipa Offtake: Weir Rating Curve

During higher flow flood events, significant submergence is expected with a corresponding reduction in weir efficiency. Under the current design scenario the flow rate above which submergence starts to impact flow is approximately 28.5 m3/s at an upstream water level of 274.3 mAD. For upstream water levels above 274.7 mAD, flows are expected to bypass the weir structure.

The rating curve presented in Figure 4.10 accounts for the expected submergence and reductions in flow rate resulting from the downstream effects using the rating curve as presented in Figure 4.8. The reduction in efficiency was evaluated using ‘Design of Small Dams’ (USBR 1987) as described above. Additional investigations should be undertaken to evaluate the expected conditions in the lower reaches of the river to increase accuracies of any flow gauging at this site.


A stilling basin has been included in the feasibility design to protect against erosion of the channel downstream of the weir. Given the hydraulics expected in the 1-in-100 year event, a USBR “Low Froude Number Stilling Basin” has been selected.


4.6.5 Cutoff Design


Concrete cutoffs would be more practical and less expensive than sheet piling as they would not require a sheet piling rig to be mobilized to site. The use of concrete cutoffs has therefore been considered more appropriate for the feasibility designs. Alternatives construction methods could be considered during later design phases.


Figure 4.11 shows a 3D rendering of the proposed weir structure on the Ruipa River. Details are presented on drawing KI 240-241 in FS Annex L: Drawing Album.

Figure 4.11: 3D Model of Proposed Ruipa Offtake Structure

20 m

Irrigation intake

Stilling basin

Flushing sluice




UDAGAJI OFFTAKE

Site Visit

Three alternative irrigation systems have been considered for the Udagaji command area: drip irrigation, sprinkler irrigation and basin irrigation. For drip and sprinkler irrigation, additional head would be required to drive supply; therefore, a higher weir site would be required.

Initially, two sites were investigated on the Udagaji River for weir structures (Table 5.1): a downstream location (Weir Site 1) that was intended to supply the basin system, and an upstream location (Weir Site 2) that was originally envisaged to supply either a drip or sprinkler system.

At a later stage of the feasibility study, it was recognized that an additional 17 m of head would be necessary for a sprinkler system over and above that for a drip system. Therefore, a potential site was located based on the LiDAR topography that would be suitable to give the required head for the sprinkler system (Weir Site 3). Although this site was not visited, the riverbed and riverbanks would be similar to the other two sites being upstream of them and having a similar river grade.

During the initial site visits to Weir Site 1 and Weir Site 2, frequently exposed rock and rapids were observed. This was expected, based on the relatively steep gradient of the river. It was also noted that a partially constructed weir exists close to Weir Site 1 (Figure 5.1 and Figure 5.2). A summary of the visual assessment of each site is given in the following sub-sections.

Table 5.1: Udagaji River Weir Sites Visited

Site No.



Weir Site 1 35 52 29 8 36 31.1 56

Weir Site 2 35 52 25.7 8 36 20.5 58


Figure 5.1: Udagaji Offtake: Weir Locations Visited – Aerial Imagery


5.1.1 Weir Site 1

At Weir Site 1 and at the partially constructed weir it was noted that:

The existing weir appears to be located at sufficient elevation to serve the proposed Udagaji basin irrigation area.

The existing weir is founded on solid rock and its abutments would be likely to accommodate a high weir.

Local areas of flatter ground in close proximity are available for associated infrastructure.


Figure 5.2: Existing Upstream Udagaji Weir at Weir 1

Photo: Site visit, 23 August 2014

5.1.2 Weir Site 2

At Weir Site 2 it was noted that:

The route for a delivery canal would pass through difficult ground with highly variable local topography dominated by a steep-sided valley.

The wider valley is likely to lead to larger construction costs when compared to Weir Site 1.

Being located at a greater elevation than Weir Site 1, it provides the potential to supply drip irrigation systems.

Weir Location

For the basin irrigation option, the preferred location for the Udagaji offtake structure is at Weir Site 1, just downstream of an existing partially-constructed weir located approximately 3 km east-northeast of Kihansi village and less than a kilometer upslope (north) of part of Udagaji village.

For the development of a drip option, the preferred location is Weir Site 2. To develop sufficient head for a sprinkler alternative, Weir Site 3, further upstream from that of Weir Site 2 was identified.

Weir Sites 1 and 2 show similar characteristics and exhibit reasonable rock foundations, and it is assumed that similar conditions prevail at Weir Site 3. It should be noted that a structure at each of the three sites will have to allow for large-scale sediment transport up to boulder size.

Figure 5.3 shows the final weir locations (highlighted in a red box) with respect to the irrigated areas and other irrigation infrastructure.


Figure 5.3: Final Weir Locations Relative to Irrigated Area – Udagaji


Topography

All three identified weir sites are located within a steep sided valley with a steep, fast flowing river, although the presence of a tributary at Weir Site 2 increases the valley width.

Figure 5.4 shows that the sites exhibit similar characteristics. However, Weir Site 1 is located at the point where the valley opens out into wider flood plains making access easier, while Weir Site 2 and Weir Site 3 are located within the steep-sided valley.

Figure 5.4: Udagaji Offtake Locations – Ground Levels


5.3.1 Weir Site 1

The right bank rises steeply with a rise up to the floodplain of 10 m above stream level in only 10 m horizontal distance from the stream edge. However, the left bank differs significantly with a small rise of only 2-3 m over approximately the same distance (Figure 5.5) Beyond the bank edge there is an 8-m wide flat area before the bank rises steeply again at approximately 30°. It would be preferable if the weir structure could be retained within the river channel so that the adjacent flat area can be used to house any additional irrigation infrastructure.

Weir Site 2

Weir Site 1

Tributary

Weir Site 3


Figure 5.5: Udagaji Weir Site 1 Topography with Headworks


5.3.2 Weir Site 2

The site is dominated by a relatively wide channel between a very steep-sided valley. This is indicative of the confluence between the main Udagaji River and a small tributary joining further downstream from the east. The location of the axis is upstream of the confluence and is located within the main river channel. On the left bank of the main river, the structure is able to tie into the bank without the need to control the tributary. On the right bank, the ground rises steeply providing a reasonable location for the structure abutment.

Flow direction

To pipeline

Sedimentation basin

Washout to river


Figure 5.6: Udagaji Weir Site 2 Topography


Geology

The 1:125,000 scale geological maps of Tanzania indicated that the location of the proposed weir sites lies on the boundary of fine grained quartzo-feldspathic Gneiss with thick layers of quartzitic gneisses, occasionally with microcline phenocysts and garnet (right bank) and partly migmatized, fine to medium grained quartzo-feldspathic gneiss with hornblende and layers and lenses of amphibolitic gneisses. No alluvium is shown at this location on the map, but the base of the stream course is strewn with cobbles and boulders washed down from upstream and it is suggested that rockhead is at relatively shallow depths. The assessment is described in more detail in FS Annex G: Geotechnical.

The assessment was corroborated during the site visit with large rock exposures and large boulders present in the streambed, as shown in Figure 5.7. At the existing weir, one of these exposures is a strong to very strong, black cream and orangey beige, striated, fine to medium grained banded gneiss with occasional patches of biotite, slightly moderately weathered on surface only, as shown in Figure 5.8.

It is possible that Weir Site 1 could be located on talus; however, this was difficult to confirm at the time of the site visit with little exposure near the lower weir site. At this stage of the design process, this uncertainty will have no significant impact on the proposed structure design at this site. Therefore, for preliminary design the proposed founding conditions for both Udagaji weir sites are assumed to be rock.

Structure axis

Flow direction

Tributary

Main river


Figure 5.7: Udagaji Weir Site 2 Looking Upstream

Photo: Site visit, 6 June 2014

Figure 5.8: Exposed Gneiss Rock at Udagaji Weir Site 2

Photo: Site visit, 6 June 2014



The design criteria for the Udagaji weir sites are provided in Table 5.2.

Table 5.2: Udagaji Irrigated Area Main Canal Parameters

Location Bed level

Bed width/pipe diameter

Full supply level (FSL)

Flow depth Flow

mAD m mAD m m3/s

Udagaji (Basin) 293.5 0.75 294.1 0.54 0.4

Udagaji (Drip) 307 0.6 307.0 N/A 0.2

Udagaji (Sprinkler) 324 0.6 324.0 N/A 0.2


The following sections outline the proposed general arrangement at each of the Udagaji weir structures, with the Weir Site 3 arrangement the same as that of Weir Site 2 but at an elevation 17 m higher.


The downstream cross sections have been extracted from the LiDAR data downstream of the structure. The sections are shown in Figure 5.9 and Figure 5.11 and the associated rating curves (developed using the methodologies as described in Section 2.2.3) are shown in Figure 5.10 and Figure 5.12.

Figure 5.9: Udagaji Weir Site 1 – 5 m Downstream Cross Section


290

292

294

296

298

300

302

304

306

308

310

0 20 40 60 80 100

Elevation (mAD)

Chainage (m)

LB RB


Figure 5.10: Udagaji Weir Site 1 – Downstream Rating Curve

Figure 5.11: Udagaji Weir Site 2 – 5 m Downstream Cross Section


291.5

292.0

292.5

293.0

293.5

294.0

294.5

0 50 100 150 200 250 300

Wat

er L

evel

(m

AD

)

Discharge (m3/s)

305

310

315

320

325

330

0 20 40 60 80 100 120

Level (mAD)

Chainage (m)

LB RB

Tributary Main river


Figure 5.12: Udagaji Weir Site 2 – Downstream Rating Curve

The design flood for the 1-in-100 year return period at both sites is taken as 31 m3/s compared to a channel conveyance of more than 200 m3/s. The Udagaji River therefore has a significantly greater conveyance capacity than the 1-in-100 year flood event. The main driver for both structures will therefore be to provide sufficient water level control to the irrigation canal.

The presence of the tributary joining the Udagaji River (mentioned previously in Section 5.3) can be seen on the left bank at Weir Site 2 where there is a localized dip in level.


The hydraulic conditions at the three sites are expected to be the same and therefore the proposed arrangements are similar.

The existing weir (shown in Figure 5.2) is only partially complete, but could potentially be incorporated into any new weir at the site. However, since the hydraulics of the existing weir are unknown, a new weir downstream of the current one is proposed.

The weir configuration is designed to fit within the natural river channel at the structure site. Bankfull level is approximately 298 mAD at Weir Site 1 and 314.5 mAD at Weir Site 2. However, given the relatively low 1-in-100-year design flood in comparison to bankfull capacity, water levels would be retained within the channel.

307.50

308.00

308.50

309.00

309.50

310.00

310.50

311.00

311.50

312.00

312.50

313.00

0 500 1000 1500 2000

Wat

er L

evel

(m

AD

)

Discharge (m3/s)


Weir Site 3 will be at an elevation 17 m higher where the water levels would also be retained within the channel.

Taking account of the local bank levels and site topography, a peak design water level of 295.5 mAD was considered with a main spillway crest level of 294.2 mAD for Weir Site 1. For Weir Site 2 the peak design water level was 310.0 mAD and the crest level was 308.6 mAD. For Weir Site 3 the peak design water level was 327.0 mAD and the crest level was 325.6 mAD. A spillway width of 10 m was considered for each site.

A low-level sluice has been provided at both sites in order to remove smaller sediment deposits from in front of the irrigation offtake at both sites. It should be noted that at this weir site larger sized boulders could be deposited which may require mechanical clearing.


Flow measurement can be incorporated by using an ogee crested weir. The rating curve for the weirs are presented in Figure 5.13 and Figure 5.14. This has been developed based on the known performance of Ogee weirs as described in ‘Design of Small Dams’ (USBR 1987).

Figure 5.13: Udagaji Offtake – Weir Rating Curve

294

294.2

294.4

294.6

294.8

295

295.2

295.4

295.6

0 5 10 15 20 25 30 35

Lev

el (

mA

D)

Discharge (m3/s)

y = 294.2+0.150x0.625


Figure 5.14: Udagaji Weir Site 2 – Weir Rating Curve

The flow is expected to remain modulus, with a maximum flow rate of approximately 32.7 m3/s at an upstream water level of 295.5 mAD for Weir Site 1. The water level is 210.0 mAD for Weir Site 2.


The Udagaji weir (at the three locations) is assumed to be founded on rock; therefore, no downstream stilling basin is required at either site.

5.6.5 Cutoff Design

The Udagaji weir (at the three locations) is assumed to be founded on rock, therefore no cutoff has been provided at either site.

5.6.6 Sediment Control

Due to the high velocity flow and associated sediment transport capacity expected at each of the Udagaji weir sites, a sedimentation basin is required. The sediment basin has been sized based on simple sediment settling calculations assuming 90% efficiency, removing a sediment size of 10 mm.

A trapezoidal invert profile and low-level flushing sluice discharging into the river channel have been provided for sediment flushing and will be required at each site.

The sediment basin is based on preliminary calculations using broad assumptions of trap efficiency and particle sizing. The proposed sizing should be verified during subsequent design phases.


Figure 5.15 shows a 3D rendering of the weir structure at Weir Site 1 on the Udagaji River. The 3D rendering of the weir structure at Weir Sites 2 and 3 will be similar to that of Weir Site 1.

308.6

308.8

309

309.2

309.4

309.6

309.8

310

0 5 10 15 20 25 30 35

Lev

el (

mA

D)

Discharge (m3/s)

y = 308.7+0.150x0.625


Figure 5.15: 3D Model of Proposed Offtake Structure – Udagaji River

10 m

Irrigation intake

Flushing sluice

Fixed crest spillway Flow to basin

and pipeline

Sedimentation basin


MGUGWE

Site Visit

Initial site visits to the proposed locations for the Mgugwe River weir structure identified that they are within a floodplain area (Table 6.1 and Figure 6.1). Being in a floodplain means there are a number of inherent risks including:

Uncertainties of foundations for concrete structures within a colluvium zone

A high proportion of costs associated with a relatively large spillway required to pass the design flood flow

A main road crossing the floodplain and river was also identified during the site visits. The weir structure was initially positioned upstream of the existing road bridge (Weir Site 1), thus avoiding the potential increase in flood risk to the existing road structure that could be caused by the new weir structure. However, this location would necessitate a form of road crossing for any associated canal infrastructure transferring water to the irrigation area.

In order to avoid the need for a road crossing, the possibility of locating the weir structure downstream of the bridge was investigated (Weir Site 2). This assessment was initially undertaken as purely a desk-based study with no specific site investigations or survey, and prior to the outcome of the flow and drainage study of the commanded areas downstream of the proposed location.

Table 6.1: Mgugwe Weir Sites Visited

Site No.

Easting Southing GPS Waypoint

No. Deg Mins Secs Deg Mins Secs

Weir Site 1 35 50 33 8 39 48 60

Weir Site 2 Downstream of road bridge (not visited)


Figure 6.1: Mgugwe Offtake: Weir Locations Visited


6.1.1 Weir Site 1

At this location, the Mgugwe River meanders across to the left bank.

The left bank of the river channel rises abruptly before rising another 70 m to a steeply sloping hill, thus restricting access on this bank, as well as developing a high point that a delivery canal from any site further upstream would have to pass. The right bank extends over some 450 m of flat floodplain that is farmed.

6.1.2 Weir Site 2

This proposed location was not visited. However, an offtake location was identified based on local topography that enabled command of the irrigable area while resulting in minimal impact on the road upstream of the proposed location.

Weir Location

During subsequent design development of the irrigation network, an alternative location was identified downstream of Weir Sites 1 and 2, as shown in Figure 6.2. This is referred to as Weir Site 3 and is the location that was taken forward for design development.


Figure 6.2: Mgugwe Offtake: Proposed Alternative Weir Location


Weir Site 3 was identified and then verified by considering the channel capacity and the local topographic features, including meanders and bank levels. Further discussion of the hydraulic characteristics is given in Section 6.6. Figure 6.3 shows the final weir location (highlighted in a red box) with respect to the irrigated area and other irrigation infrastructure.

Weir Site 2

Weir Site 1 Weir Site 3

Flow direction

Existing main road

Mgugwe River

Road bridge


Figure 6.3: Final Location of Weir Relative to Irrigated Area – Mgugwe


Topography

The main features that define the site topography at Weir Site 3 (the proposed location) are: a well-incised river channel of approximately 20 m width; and a wide, well-developed floodplain extending 340 m and 670 m on the right and left bank, respectively. This is shown in Figure 6.4 and Figure 6.5.

The riverbed exhibits a slight degree of meandering but with reaches of reasonably straight channel that should be suitable for the weir structure.

Figure 6.4: Mgugwe Offtake Locations – Ground Levels


Weir Site 1

Weir Site 3

Existing road

Flow direction

Weir Site 2


Figure 6.5: Mgugwe Site Topography and Headworks


Geology

Site visits to the chosen weir site (Weir Site 3) did not take place. The main findings of the site visit to the upstream site (Weir Site 1) are presented below. In absence of more site-specific data at the preferred location, observations at Weir Site 1 have been used to inform the preliminary design while also considering the information available within the 1:125,000 scale geological maps of Tanzania.

There were no exposures of soil or rock identified during the site visit to Weir Site 1. However, based on surface soil strata visible at the site, it is likely that the superficial soils will extend to considerable depth in the center of the river valley. On the flanks of the valley, the 1:125,000 scale geological maps of Tanzania suggest that Neogene red earth strata may be present over weathered Pre-Cambrian basement rock. This would require geotechnical confirmation for later stages of design.

FS Annex G: Geotechnical provides more information and notes that the solid geology beneath the proposed weir site is likely to be fine to medium grained quartzo-feldspathic Gneiss with biotite. However, the map shows that this bedrock is overlain by Quaternary/Neogene River deposits (silt, sand and gravel).

Based on the above assessment, it is unlikely that suitable rock could be found to form the structure foundation. Therefore, the foundation type for the feasibility level design has been designated as ‘permeable’.


The design criteria for the Mgugwe weir are provided in Table 6.2.

Flow direction

To sedimentation basin and canal


Table 6.2: Mgugwe Irrigated Area Main Canal Parameters


width Full supply level (FSL) Flow depth Flow

mAD m mAD m m3/s

Mgugwe 294.7 0.75 295.2 1.0 1.3


The following sections provide further details on the proposed general arrangement of the Mgugwe weir structure.


Figure 6.6 and the associated rating curve, developed using Manning’s normal depth calculations as described in Section 2.2.3, shown in Figure 6.7, demonstrates that channel conveyance at this location is approximately 120 m3/s and therefore the 1-in-100-year design flood (113 m3/s) would be retained within the river channel. This indicates that an auxiliary spillway would not be required. Hydraulic calculations (as discussed in Section 6.6.2) are required to confirm this.

The minimum level of the existing road taken from the LiDAR is 298.8 mAD. This is considered in the calculation of the proposed weir level and maximum flood water level. This level should be confirmed at later stages if additional topographic survey data is procured.

Figure 6.6: Mgugwe Offtake – 5 m Downstream Cross Section


293.0

293.5

294.0

294.5

295.0

295.5

296.0

296.5

297.0

297.5

0.0 20.0 40.0 60.0 80.0

Ele

vati

on

(m

AD

)

Chainage (m)

LB RB


Figure 6.7: Mgugwe Offtake – Downstream Rating Curve


Initial weir sizing was based on the natural river channel width at the site. As noted above, the flood water levels are unlikely to exceed bankfull level (296.8 mAD) during the 1-in-100 year event. However, at the design flood water level a reduction in flow rate of 10% to allow for submergence has been considered.

Taking account of the various site constraints and setting the peak water level during the 1-in-100 year flood event to 297.2 mAD it was possible to develop a weir configuration with a main spillway crest level of 295.2 mAD and a spillway width of 20 m. In this way, the maximum upstream water level would remain within the channel, and below the level of the road on the bridge upstream. This would indicate that this weir structure would not increase the risk of flooding of the upstream road crossing.



293.0

293.5

294.0

294.5

295.0

295.5

296.0

296.5

297.0

297.5

0 50 100 150 200

Wat

er L

evel

(m

AD

)

Discharge (m3/s)



Figure 6.8: Mgugwe Offtake – Weir Rating Curve

During higher flow flood events, significant submergence is expected with a reduction in flow rate of 10% at the design water level. Under the current design proposals, the flow rate above which submergence starts to impact flow is approximately 30.3 m3/s at an upstream water level of 296.0 mAD. With no auxiliary spillway at this site, the total flow rate during extreme events can be effectively monitored, given that the hydraulic performance of an ogee weir under submerged conditions is well understood. The reduction in efficiency was evaluated using ‘Design of Small Dams’ (USBR 1987) as described above.

The rating curve presented in Figure 6.8 allows for the submergence of the weir, developed using the downstream rating curve as presented in Figure 6.7. Additional investigations should be undertaken to evaluate the expected conditions in the lower reaches of the river to increase accuracies of any flow gauging at this site.


A stilling basin has been included in the feasibility design to protect against erosion of the downstream toe of the structure. Hydraulic calculations to identify where flow would progress from modulus to submerged flow over the weir have been carried out. The calculations indicate that a Type I stilling basin would be acceptable and that a 6-m long basin would be required to control the hydraulic jump and protect the structure from downstream erosion.

6.6.5 Cutoff Design


Concrete cutoffs would be more practical and less expensive than sheet piling as they would not require a sheet piling rig to be mobilized to site. The use of concrete cutoffs has therefore been considered more appropriate for the feasibility designs. Alternative construction methods could be considered during later design phases.

y = 303+0.10x0.615



Figure 6.9 shows a 3D rendering of the proposed weir structure for the Mgugwe River.

Figure 6.9: Mgugwe Offtake – 3D Model of Proposed Structure

20 m

Irrigation intake

Stilling basin

Flushing sluice




MPANGA

Site Visit

The Mpanga River defines the west side of the proposed Mpanga irrigable area. In order to provide irrigation to this area, two potential weir locations were considered. Weir Site 1 is located at the furthest possible point downstream that would still command the whole of the identified irrigable area. Weir Site 2 is upstream where site conditions are more favorable – reduced lengths of embankments are required, and suitable rock foundations were observed in the riverbed. Table 7.1 and Figure 7.1 detail the proposed locations.

Table 7.1: Mpanga Weir Sites Visited

Site No.



Weir Site 1 35 47 36.4 8 55 39.3 69

Weir Site 2 35 47 10.1 8 55 49.3 70

Figure 7.1: Mpanga Offtake – Proposed Weir Sites


Weir Site 1

Weir Site 2

Flow direction


7.1.1 Weir Site 1

At this location:

the river meanders toward the left bank


the floodplain extends out on the right bank

no rock is visible, meaning typical colluvium foundation uncertainties exist

7.1.2 Weir Site 2

At this location (which is about 1 km upstream of Weir Site 1), an exposed rock bar is visible across the river.

Weir Location

Weir Site 2 has been taken forward for further design development. This decision was based on site visit observations that are documented in FS Annex G: Geotechnical. The proximity of high ground and a favorable river channel profile were important factors resulting in Weir Site 2 being the preferred option.

Weir Site 1 exhibited lower hydraulic capacity in the channel, which would have been likely to result in a need for a taller structure. Furthermore, for a structure at Weir Site 1, the presence of a wide flood channel on the right bank would have resulted in significantly greater construction costs. In addition, the foundation materials at Weir Site 2 were considered more certain at this stage of design development. Figure 7.2 shows the location of the final location of the weir (highlighted in a red box) with respect to the irrigated areas and other irrigation infrastructure.


Figure 7.2: Final Weir Location Relative to Irrigated Area – Mpanga


Topography

The topography at Weir Site 2 is characterized by a 65-m wide river channel with a reasonably steep-sided bank on the left side as shown in Figure 7.3. This contrasts with the right bank where a shelf of lower gradient land is present which continues for 35 m to 40 m before rising more steeply. Therefore, the main weir structure and offtake structure would be provided on the left bank with the potential for providing an auxiliary spillway on the right bank.

Figure 7.3: Mpanga Offtake Locations – Ground Levels


Weir Site 1

Weir Site 2


Figure 7.4: Mpanga Weir Site 2 Topography and Headworks


Geology

Some rock exposure was observed at Weir Site 2 during the field visit, although the rapids in the water (Figure 7.5) indicated that the riverbed is probably rocky.

Approximately 30 m downstream of the actual weir site there is a broad section of river with numerous boulders and exposed bedrock in the banks, as shown in Figure 7.6. At this location, the rock was comprised of a very strong, light grey, homogeneous, granitic textured rock with little evidence of banding or foliation: meta-diorite or homogeneous Gneiss. The material making up the left bank appeared to be compact sand, which may be the Neogene lateritic earths.

This visual assessment appears to be consistent with the 1:125,000 scale geological maps of Tanzania, which indicate that the solid geology underlying the proposed site consists of fine to medium grained quartzo-feldspathic Gneiss with biotite, occasionally with garnet, streaky, sometimes homogeneous. Remnants of the Neogene stratum are shown to overlie these Pre-cambrian rocks on the left bank, but may be present under the right bank too. The geological map shows river deposits to be present in the river channel, but this is thought to be unlikely based on the findings of the site visit.

Based on the assessment above, it has been assumed that the Mpanga weir could be founded on rock.

Flow direction

To sedimentation basin and canal

Auxiliary spillway


Figure 7.5: Mpanga Offtake – River Bed Condition (Weir Site 2)


Figure 7.6: Mpanga Offtake – River Bed Condition 30 m Downstream of Proposed Site (Weir Site 2)




The design criteria for the Mpanga weir are provided in Table 7.2.

Table 7.2: Mpanga Irrigated Area Main Canal Parameters

Location Bed level Bed width Full supply level (FSL)

Flow depth Flow

mAD m mAD m m3/s

Mpanga 302.0 4.0 303.0 2.2 21.2


The following sections provide further details on the proposed general arrangement of the Mpanga weir structure.


The downstream cross section has been extracted from the LiDAR data immediately downstream of the structure. The section is shown in Figure 7.7 and the associated rating curve was developed using Manning’s normal depth calculations as described in Section 2.2.3 and is shown in Figure 7.8.

Figure 7.7: Mpanga Offtake – Downstream Cross Section


295.0

300.0

305.0

310.0

315.0

320.0

325.0

0 50 100 150 200 250 300

Elevation (mAD)

Chainage (m)

LB RB


Figure 7.8: Mpanga Offtake – Downstream Rating Curve

The hydraulic conditions at the weir site are predominantly characterized by a channel of relatively low hydraulic conveyance capacity when compared to the 1-in-100 year event, with floodplains located on both banks but primarily on the right bank.

Figure 7.7 and Figure 7.8 show that channel conveyance at this location is approximately 356 m3/s (less that the 1-in-100 year design flood of 458 m3/s). Therefore, flood flows cannot be retained within the river channel at this location. To lower upstream water levels during flood events, an auxiliary spillway has been incorporated into the design. The main spillway is likely to encounter a degree of submergence during passing of the higher floods.


Initial weir sizing was based on the natural river channel width at the site. The bankfull level is approximately at elevation 302 mAD. The design of the weir crest level considered the need to command the irrigated area against the need to minimize the increase in flood risk upstream of the structure.

The auxiliary spillway was located on the right bank to avoid any conflict with the irrigation infrastructure proposed on the left bank. As described above, the topography lends itself well to such an arrangement.

The hydraulic assessments resulted in an auxiliary weir length of approximately 35 m at an elevation of 304 mAD with a 65-m main spillway crest at an elevation of 303 mAD. With these dimensions and elevations, the peak upstream water level during the 1-in-100 year flood event was found to be 304.9 mAD. Clearly, this assumes flooding upstream on both the left and right banks. However, based on the rating curve as shown in Figure 7.8 and as described in Section 7.6.1, the 1-in-100 year flood conditions would result in flooding of the banks even without the structure

297.0

298.0

299.0

300.0

301.0

302.0

303.0

304.0

305.0

306.0

307.0

0 500 1000 1500 2000

Wat

er L

evel

(m

AD

)

Discharge (m3/s)



in place – the actual increase in flood extent due to the introduction of the structure would be relatively limited.



Figure 7.9: Mpanga Offtake – Weir Rating Curve

The flow conditions will remain modulus, with a maximum flow rate of approximately 379.8 m3/s at an upstream water level of 304.9 mAD.


The Mpanga offtake has been assumed to be founded on rock; therefore, no stilling basin has been provided.

7.6.5 Cutoff Design

The Mpanga offtake has been assumed to be founded on rock; therefore, no cut-off has been provided.


Figure 7.10 shows a 3D rendering of the proposed weir structure for the Mpanga River.


Figure 7.10: Mpanga Offtake – 3D Model of Proposed Structure

35 m

65 m Irrigation

intake

Flushing sluice


Auxiliary spillway



CONSTRUCTION COST ESTIMATES

Introduction

The estimated construction costs include the total expenditure required to construct the headworks and excludes any lifetime operation or maintenance costs. Unit rates have been derived and applied to the quantities calculated from the feasibility designs. Unit rates were determined by considering the following:

The construction methodology and specification of works (e.g. concrete mix proportions or level of compaction for earthworks)

Quantity of the materials used

Labor, plant and equipment used

Transportation of materials including the distance from the source of the material to the site

The cost rate of all of these individual elements

Contractor’s overhead

Contractor’s profit

The methodology and assumptions used are presented in detail in FS Annex J: Engineering.

Quantities

The quantities associated with headworks structures (river weir, intake, sedimentation basin) have been calculated through design spreadsheets. The dimensions of standard structures have been provided in the typical structure drawings in FS Annex L: Drawing Album.

Table 8.1 presents costs for the principal cost categories for each site. The amounts show the figures rounded to the nearest 1,000 USD and do not include for 20% contingencies.

Table 8.1: Estimated Construction Costs for Headworks

Headworks

Earthworks

Concrete, including

ReinforcementStone

Pitching/Rock

Steel Works and

Miscellaneous

Total (excluding 20% Contingencies)

Chiwachiwa 12.3 386.0 22.7 5.4 426.4

Ruipa 114.8 433.7 38.1 6.4 593.0

Mpanga 127.5 1,543.4 0 5.4 1,676.3

Mgugwe 75.4 341.7 1.5 5.4 424.0

Udagaji (Site 3) 22.5 120.0 0 0 142.5 Source: IRRIP2 cost estimate spreadsheets. Costs in ‘000 USD.


SUMMARY AND CONCLUSIONS

Table 9.1 summarizes the principal design parameters for each site.

Table 9.1: Principal Design Parameters for Each Site

Headworks Offtake

Flow River Bed

Full Supply Water Level

Intake Invert

Canal Depth

Spill- way

Width

Auxiliary Spill- way

Length

Design Flood Flow

Design Flood Level

Top of

Bank

Existing Flood Level

m3/s mAD mAD mAD m m m m3/s mAD mAD mAD

Chiwachiwa 1.54 281.2 283.2 281.8 1.25 15 35 165 285.1 283.5 284.4

Ruipa 6.30 271.1 273.5 271.1 2.4 20 - 227 275 274.6 275.0

Mpanga 21.2 299 303 301.4 4.0 65 35 458 304.9 302.0 303.0

Mgugwe 1.3 293.8 294.7 294.4 0.75 20 - 113 297.2 296.8 296.7

Udagaji (Site 1)

0.2 298.7 294.1 293.4 0.75 10 - 31 295.5 298.3 293.9

The conditions existing at each of the proposed sites vary significantly. Some of the river channels (for example at the Udagaji River weir structure) have high conveyance and limited channel width, allowing for a relatively efficient weir design. Other river channels (for example at the Ruipa River weir structure) have limited channel conveyance and problems associated with out-of-channel flooding.

The conditions at each of the five weir sites were rationalized in order to provide a consistent approach to design. Hydraulic, geotechnical, and topographic constraints were considered during the design to determine the preferable arrangement at each site.

The drawings of each of the structures are provided in FS Annex L: Drawing Album. The costs of each structure have been estimated and details are provided in the Bill of Quantities.


REFERENCES

Reference Title

Beck 1964 The Kilombero Valley of South-Central Tanganyika, E. African Geographical Rev, No, 2, April 1964 pp 37-43.

CDM Smith 2013a

Field Reconnaissance Report. Kilombero Valley Irrigation Schemes: 10-14 December 2012. Irrigation and Rural Roads Infrastructure Project. CDM Smith for USAID/Tanzania. January 2013.

CDM Smith 2013b

Review of Rapid Appraisal Reports. Kilombero Valley Irrigation Schemes. CDM Smith for USAID/Tanzania. February 2013.

CDM Smith 2014 Preliminary Hydrology, Kilombero Valley Irrigation Schemes, Irrigation and Rural Roads Infrastructure Project. CDM Smith for USAID/Tanzania. 25 March 2014.

CDM Smith 2016 Annex G: Geotechnical. Feasibility Study of Kilombero Valley Irrigation Schemes. Irrigation and Rural Roads Infrastructure Project. CDM Smith for USAID/Tanzania. 01 April 2016.

Dypvik & Nilsen 2001

Contributions to Geology and Paleontology of Gondwana, Geological Institute, University of Cologne.

ICE 2015 Floods and Reservoir Safety, 4th Edition, Institute of Civil Engineers, 2015

Novak 2007 Hydraulic Structures (4th Edition), P. Novak, 2007.

RISC 2013 Independent Technical Specialist’s Report on The Kito Prospect, Kilosa-Kilombero License, Tanzania, 12 December 2014.

USBR 1987 Design of Small Dams. A Water Resources Technical Publication. United States Department of the Interior Bureau of Reclamation. Third Edition, 1987.

WREM 2012 Rufiji Integrated Water Resources Management and Development Plan: Interim Report. Volume I: Socio-economic, and Management Profile. Volume II: Water Resources Availability Assessment. Volume III: Current Water Use and Infrastructure Assessment. Volume IV: Rufiji Decision Support System. United Republic of Tanzania Ministry of Water. WREM International Inc. January 2012.

Error! Reference source not found. - Error! Reference source not found. 2

Technical Assistance to Support the Development of Irrigation and Rural Roads Infrastructure Project (IRRIP2)

U.S. Agency for International Development

686 Old Bagamoyo Road, Msasani P.O. Box 9130 Dar es Salaam

Tanzania

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ANNEX K: WEIRS Feasibility Study of Kilombero Valley ...

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