TERMS OF REFERENCE FOR GEOTECHNICAL AND GEOPHYSICAL SURVEY FOR SAND AND SUBSURFACE DAMS IN SOMALILAND AND PUNTLAND

Drought Resilience and Sustainable Livelihoods Program (DRSLP II)

Annex 1

TERMS OF REFERENCE FOR GEOTECHNICAL AND GEOPHYSICAL SURVEY

FOR SAND AND SUBSURFACE DAMS IN SOMALILAND AND PUNTLAND

BACKGROUND

The Drought Resilience and Sustainable Livelihoods Program (DRSLP) is a program financed by the African Development Bank (AfDB), which is being implemented to address the negative impact of recurrent drought in countries of the Horn of Africa region including Somalia within the framework of the IGAD Drought Disaster and Sustainability Initiative (IDDRSI), under the general leadership and coordination of the Intergovernmental Authority on Development (IGAD).

The Drought Resilience and Sustainable Livelihoods Program– Project II (DRSLP II) in the Horn of Africa is part of a multinational program that covers Eritrea, Ethiopia, Somalia and Sudan. The project is implemented by Save the Children International (SCI) in Puntland, Somaliland and South Central regions of Somalia where over 1,505,959 people and their 4 million animals will be benefiting. The overall goal is to contribute to poverty reduction and accelerated economic growth on a sustainable basis in Somalia. This will be achieved by investing in natural resources (water, pasture) management, integrated land management and ecosystem restoration and protection as well as rebuilding of agricultural and livestock infrastructures, to build resilience and sustainable livelihoods for pastoral and agro-pastoral communities in drought-prone areas of Somalia..

OVERALL OBJECTIVES OF THE ASSIGNMENT

To improve water availability and accessibility, the project is seeking the services of a consulting firm to undertake Geotechnical and Geophysical investigations of proposed surface, sub-surface and sand dams, for both new and rehabilitation in Somaliland and Puntland, mainly targeting about 273,347 beneficiaries in selected project sites.

The main objective of this assignment is to carry out a geotechnical and geophysical survey at proposed dam sites, for identification of suitable locations of the new dams, with potential rain water harvesting, and to develop feasibility, preliminary and detailed designs for the proposed dams based on the consultant’s experience and on the macro and micro studies.

SCOPE OF WORK

Prior to the construction and rehabilitation of the proposed rain water harvesting systems including Balleys, Earth and Sand Dams; the Consultant will conduct the following technical surveys:

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Drought Resilience and Sustainable Livelihoods Program (DRSLP II)

Geotechnical surveys  this survey is intended to ensure that the water to be stored within the dam will not be lost through leakage, hence, the dam should be built on solid bedrock or /and impermeable layer so as not to allow water to seep beneath the sand dam. The survey will be in two steps:

Step1 – To study the geology of the area and to map the bedrock outcrops and

Step2 – To undertake detailed geotechnical investigation by using probing rod tests and excavation pits or hole tests in the riverbed in order to find the depth of a consolidated layer or bedrock layer or the basement rocks.

Topographic Survey – In each location of the four sites selected, by means of total station or precision GPS. A precision up to 10 cm is required. The survey shall include i) a transversal profile to the stream on each dam axis proposed which shall extend for 50 m beyond each river bank (a point each 5 m), ii) a longitudinal profile from 500 m upstream of the selected location up to 200 m downstream (a point each 10 m), iii) the contour of the dam upstream zone of influence, mapping a point each 10 m along both banks up to 300 m of distance upstream and 100 m downstream of the dam site, iv) in the same area and on the banks all the rocky outcrops shall be mapped. The data shall be presented in: v) a cross-section along the dam layout as described at the point “i)” above, vi) longitudinal cross section as described to the point “ii)” above, vii), perpendicular to the stream axis cross-sections roughly spaced each 50 m from bank to bank including the banks top edge, viii) a map in AutoCAD format with the location and the elevations of all the points, outcrops banks and elevations contour lines 0.5 m spaced, ix) excel worksheet with all points coordinates and elevations.

In addition, and in line with the above surveys, the storage and the extraction capacity of the sand in the area will be tested for pits excavation to investigate the porosity and extractability of the sand. These tests are intended to provide rough estimation about the effective porosity of the sediments as well as amount of water that can be extracted from the sand aquifer. This will also give more understanding of soil erosion and thereby helps the decision of appropriate technical design of silt traps.

MAIN TASKS

The Consultant is expected to carry out the following tasks, diligently and within the prescribed time. They include but not limited to:

Review and evaluate preliminary studies on sand, weir and sub-surface dams locations under the project (both rehabilitation and new) whose locations are specified in the attached Annex 1, taking into account the lessons described in the Micro study.

Carry out field investigations and a topographic survey for each proposed sites in order to acquire geometric, geological and hydrological information necessary for the dam design. The field investigations should include general surveys (geo-morphological and hydrogeological) extended in a radius of 300-500 m from the center of the dam and site studies localized in the stream and nearby zones (mostly the river banks), the latter is subject to a high variability due to a wide range of situations and dam solutions (sand or subsurface).

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Drought Resilience and Sustainable Livelihoods Program (DRSLP II)

For each dam site the consultant will:

Carry out soil investigation within the vicinity of each proposed dam site, in order to locate suitable material for dam construction.

Conduct and develop profile drawings (longitudinal and cross-sectional) of the riverbed. This should be based on longitudinal profiling or ‘probing’ to assessthe extent of the sand reservoir, the existence and continuity of the impermeable bedrock layer, the height and consistency of the banks, and, slope of the wadi.

Measuring the storage capacity of the sand reservoir to locate the most cost effective location for constructing a wells and the subsurface, weir or sand dams identify the largestsand reservoir of the wadi and hence where the maximum amount of water can be stored for the lowest costs given different dam placements

Measuring the volume of extractable water from the sand reservoir – estimate the maximum extractable volume of water (yield) from the planned sand reservoir by calculating the rate at which water can be extracted from the sand. Two to three sand samples were collected at different depths from each probed wadi. The samples should be fully saturated with water in buckets with a fixed volume and the total water that is abstracted reported as a percentage of volume of sand.

Carryout detailed geotechnical and geophysical investigations to confirm the suitability of the:

Dam axis for the dam foundation and dam abutmentin the wadi/river embankment;

The depth, continuity and consistency of the impermeable bedrock layer – testing it for permeability, estimating lossesand proposing mitigation measures if any;

Stability of the upstream wadi embankment

More information about each component of this part of the investigation are detailed as follows:

f.1 Geo-Morphological survey: The geological survey shall be carried out in an area of 500 m of radius from the center of the dam layout. The geological formations with particular attention to the rocky basement and fracture’s degree, to the presence of possible faults and to the extension and grain size of the alluvial deposits on the wadi bottom and banks are described in a geological report. The main morphological features (escarpments, stream diversion or convergence, erosions of any type, farms) should be outlined on a separate map or on the same geological map). The results will be shown on two maps, one based on the topographic map described at the point “b.viii)” above and one reporting geo-morphological features in the enlarged area (500 m of radius) based available high resolution satellite images. During the geological survey the consultant shall extend the survey to the nearest site where construction materials (stones) are available. If there are not suitable locations in a radius of 500 m the survey is extended to the nearest accessible site. In the geological report, as a part of the final technical report, the Consultant shall evaluate the suitability of the bedrock with regard to the dam foundations, to the possible leakage caused by fractures or by an unconsolidated semipermeable layer and a description of possible mitigation measures.

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Drought Resilience and Sustainable Livelihoods Program (DRSLP II)

f.2 Geophysical survey: the proper methodology to verify the presence and the pattern of an impervious rocky basement (where appropriate) is the refraction seismic. The lines (or bases) are carried out perpendicularly to the stream axis, spaced of 50 m each other, starting from the dam layout. A continuous longitudinal line is carried out from 50 m downstream to 300 m upstream. Each line is made with layouts of 12-24 geophones with a spacing of 2 to 5 m according to the width of the stream section covered by alluvial deposits. In the case that the contractor is not able to supply the seismic equipment the survey can be done with VES (vertical electrical soundings). The VES shall be carried out with parallel to the stream axis layout only where a thickness of alluvial deposits beyond 3 m is expected or where the distance between near outcrops exceeds 30 m. The maximum AB/2 spacing (semi-distance between the current electrodes) is 50 m. The maximum AB/2 is reduced where an outcrop is present along the layout: no VES shall be done on outcropping rocks. The number of VES to be carried out on each side shall be defined by the contractor as part of his technical offer. An indicative number of VES is given in Annex 1 to this ToR. The same criteria should be applied to test pits, probing roads and porosity test described below. VES coordinates, data, curves and interpretations are delivered to the end of the interpretation and enclosed to the report. The VES centers or the seismic lines shall be mapped on Google Earth images and on the topographic map. All the indications given above can be adapted in the technical specifications supplied for each site.

f.3 Test Pits:the pits are located along the dam layout, where alluvial deposits cover the basement, with a 5 m spacing; where the layout length exceeds 50 m the spacing will be 10 m. Three additional pits will be excavated along the stream axis upstream of the layout spaced 100 m each other, the first 100 m upstream of the dam layout. It is expected a maximum depth of 5 m but mostly they shall reach the impervious basement before the 3 m of depth. If the excavation is made by hand a dewatering pump shall be used in case water is met before the basement is reached. It is suggested the use of a small excavator for the task. For each pit a stratigraphic column will be done illustrating the variations of grain size and color and the presence and the type of the bedrock at the bottom. A lithological cross-section will be presented as elaboration. During the excavation samples of the alluvial sediments are taken for the or porosity tests.

f.4 Probing rods survey:Where appropriatethis type of investigation is made by iron rods of small diameter to be infixed in the ground and hammered down up to the rocky basement or to the point where it is impossible to proceed for the presence of too compact materials or stones (pebbles, blocks). The profiles are done with a 5 or 10 m spacing (with the same spacing suggested for the pits along the dam layout) for each test, along lines perpendicular to the stream axis. Each line or profile is spaced 100 m from the previous, from 100 m downstream of the dam axis, moving upstream. It is suggested to execute the probing rods survey before the pits excavation to verify the reliability of the methodology and to find out possible basement depressions. In the case of sandy banks tests are done horizontally in the banks, to find the basement distance from the bank flank and on the bank top as specified for the single areas.

Drought Resilience and Sustainable Livelihoods Program (DRSLP II)

f.5 Porosity tests:on samples taken during the test pits excavation are done rough porosity tests finalized to evaluate the effective porosity of the sediments. These type of test can be applied only to get rough information since the sample are not in their natural state but deeply disturbed when taken away from the pit walls. Normally the test results give higher values of porosity since the sample material, once removed from the ground shall have a larger volume of voids. The test is made filling a bucket with sample then saturating it with measured volume and water. Once the saturation is reached (new water is not being absorbed) a seal near the bucket bottom is removed and water can freely drop in a cup. After one hour the quantity of water in the cup is measured and the ratio between the volume of the output and the input water is measured. If the bucket is graduated, also the volume of sand can be measured and a rough effective porosity can be calculated. In the case of fine deposits (fine sand or silty sand) the test is extended until when the output water flow is almost negligible. In the description of the test results is given a description of the sample grain size and the volumes of the sediments and of the input/output water. In the technical report the Consultant shall estimate the maximum volume of extractable water from the sand reservoir and the daily rate (yield) with which it can be extracted. The calculation should be done on the base of the data obtained from the porosity tests and considering the possible recharge of the artificial aquifer. It is suggested that the test id done for each site on five samples taken in the pits excavated at the center of the dam layout and along the stream axis, following the instruction given at the point ‘f.3’ listed above. The samples shall be representative of the material found in the excavation.

Building a hydrological catchment model. Assess historical records (past 15-20 years) of rainfall and runoff flow regime of the river including records of the installed gauging and weather stations to check the present and future water availability and prepare a rainfall-runoff model. Conduct flood analysis including frequency analysis and calculation of Probable Maximum Flood (PMF) to determine the dam design flood and spillway capacity/

Construction Investment Report (CIR) – Based on (a) to (g) make a short 10-page feasibility report on design options with recommendations on which to proceed with. The report shall provide summary information justifying the need for the construction of the dam. Specifically, the report should define the targets for the construction of the dams. Targets are listed as follows:

definition of water use

for agriculture

for watering the cattle

(iii)for domestic uses

definition of beneficiaries

definition of water demand

definition of period supply

continuous water supply

seasonal water supply

Drought Resilience and Sustainable Livelihoods Program (DRSLP II)

Further to that, the report shall provide information summarizing the results of the field investigation. Specifically, it should report the following results of the survey:

geological appraisal of the dam site area and related surroundings

estimation on the potential accumulation of alluvium

appraisal on run-off, frequency and quantities

estimation on the potential water reserves accumulation

Finally, the report should provide results of the costs/benefits preliminary analysis or, more in details:

estimate the investment, running and maintenance costs at preliminary level

draft the preliminary ripening curve and the curves of water availability compared to the water demand.

Detailed dam design –develop detailed drawingsincluding dam depth, spillway height and width. The detailed design should include all necessary civil, structural, architectural, mechanical, geo-technical and foundation designs including specification of quality and amounts of required materials needed for construction. Preparing the final technical document (Design and Bill of Quantities) of the dam including Engineering specification, Work Plan.

Detailed multiple-use design of water reservoirs in the sand dam, abstraction, storage and distribution mechanisms– Depending on the intended multiple uses (for people, livestock and agriculture) of the water storage capacity at the dam site design appropriate mechanisms for abstracting the water the wadi to reservoirs and distribution systems out of the wadi for each purpose. The solutions should put an emphasis on the life-cycle costs of the infrastructure, its operation and maintenance. The designs should also be realistic about the local technical capacity to operate and maintain the system including the availability of spare parts. The detailed design should include all necessary civil, structural, architectural, mechanical, electrical, geo-technical and foundation designs.

EQUIPMENT REQUIREMENT

Surveying equipement (Total Station), measuring tape, GPS, resistivity meter, seismograph and Laboratory and field materials testing of soils machine

PROPOSED TIME/ACTIVITY SCHEDULE

Due to the nature of the proposed work, the survey field mission is expected to take a maximum of two months including the reporting.

Drought Resilience and Sustainable Livelihoods Program (DRSLP II)

RESPONSIBILITIES

Save the Children International

For the execution of the survey, SCI shall ensure that the local authorities are informed of this survey. SCI shall inform and update the consultant on the security situation, or changes therein. SCI shall also make available the following information and facilities to the consultant:

Relevant reports, documents, maps, data at contracting authorities disposal;

Counterpart staff for necessary support in the field.

The consultant

Prepare and deliver consultancy services as per the TOR

Deliver the key deliverables in time;

submit a final report duly approved by SCI after full incorporation of views and editions from the client.

Facilitate 2 debriefing sessions before and after the field mission.

The consultant and his team will make their own arrangements on transport, security and accommodation and meals during the duration of the assignment.

KEY DELIVERABLES

The key deliverable is hydrogeological & geophysical survey report, which should be submitted to STC.

A soft copy on e-mail and 2 CDs with the final report will also be submitted to STC.

The consultant will also be required to deliver to STC all study materials:

Soft copies of all data sets both quantitative and qualitative.

The geophysical interpretation model and the graphical plot of the curve and model.

Any other non-consumable documents/items that will be used in the course of the planned consultancy.

The proposed subsurface dam identified should be clearly marked with concrete markers, shown in a picture plate and indicated on a sketch map to be included in the final report.

REPORTING REQUIREMENTS

S/N

REPORTING

CONTENT/FORMS

NO.OF

DUEDATE

REQUIREMENT

COPY

FOLLOWING

SIGNING

CONTRACT

(Weeks)

1

Inception report

Detailed  work  plan

5

3

and desk review

2

Draft report

All geophysical and

5

6

geotechnical

findings

3

Final report

All geophysical and

5

8

geotechnical

findings

Drought Resilience and Sustainable Livelihoods Program (DRSLP II)

CONSULTANTS QUALIFICATIONS AND EXPERIENCE

The service will be delivered by a consulting firm with at least 3 key experts with Master degree qualifications in

o  Decipline 1: Hydrogeology or Geophysics

Decipline 2 : Water Resources engineering Decipline 3: Environmental Engineering

All experts must have at least a Master degree in Hydrogeology/Geology or Environmental Sciences or engineering or water engineering

At least Seven (7) years proven experience in conducting and interpreting hydrogeological/geophysical survey using diverse techniques

Proven experience in drilling supervision and borehole documentation

A reliable and effective analyst with extensive experience in conducting analyses and a proven record of delivering professional results

Excellent computer skills and desirable skills in Arc GIS and groundwater modelling softwares

Excellent presentation and report writing skills.

CONTACTS, COMMUNICATION AND SUPERVISION

The consultant will be under the direct supervision of the Save the Children technical team for the duration of the contract. The consultant will maintain official communication with the Save the Children Technical Team and Supply Chain unit.

MANDATORY SECURITY TRAINING

The consultant shall be expected to go through mandatory on-line security training and submit the certificate of completion to SC before the commencement of the task. Failure to deliver this will lead to an automatic disqualification. SC will advise on the security plan on appointment. SC will cater the security during the survey days in Puntland and Somaliland.

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