Fathoms facing new challenges; crocodiles and hippos.

Last Updated 14 years ago by Kenya Engineer

When the Government of Kenya received financial assistance from the International Development Association (IDA) to carry out a Natural Resource Management (NRM) Project, UK survey specialist Fathoms Ltd was contracted to undertake the all-important bathymetric surveys. 

The overall project is being handled by the Kenya Forest Service (KFS), the National Irrigation Board (NIB) and the Water Resources Management Authority (WRMA) and has four main components: Water Resource Management and Irrigation, Management of Forest Resources, Livelihood Investment in the Upper Tana Catchment and Management, Monitoring and Evaluation. A national project coordination office (NPCO) located in the Ministry of Water and Irrigation is responsible for the overall coordination of the implementation of project activities.

This contract’s objectives were to monitor discharge and sediment load; to assess soil erosion via modelling and to estimate reservoir capacity by bathymetric surveys. Proposals were also required for a design for the meteorological and flow/sediment monitoring network; and for measures to reduce sediment load entering the reservoirs.

The consortium in overall control of achieving these objectives awarded Fathoms the contract to undertake the bathymetric surveys.

INTRODUCTION

The bathymetric project centred on four reservoirs; the Masinga, Kamburu, Sasumua and Ndakaini dams. All four reservoirs lie in the Upper Tana Valley to the north east of Nairobi, just south of the Equator and in Kenya’s Eastern Province.

Masinga and Kamburu deliver hydro-electric power to the Kenya Power Company; primarily to Nairobi. Sasumua and Ndakaini supply drinking water to the Nairobi Water Company. The elevation of these reservoirs ranges from 1,006m above sea level at Kamburu to 2,485m above sea level at Sasumua.

SURVEY REQUIREMENTS

To successfully execute the contract, Fathoms’ crucial task was to determine the remaining capacity of the four reservoirs. This would provide indication of the siltation rates since the construction of each reservoir.

The objective of the survey was to conduct a bathymetric survey of the reservoirs and prepare digitised contour maps to estimate the current reservoir capacity and deposited sediment volume and distribution. This bathymetric survey should use automated depth measurement and positioning systems conducted at appropriately spaced data collection traverses.

Fathoms also had to select and survey a series of cross section lines across the reservoirs which would be re-surveyed at intervals and used to compute (by geometric formulae) the inter-survey volume change in each reach. The number and location of the traverses/cross section lines should be determined based on standard survey procedures to efficiently track and compute accumulated sediment volume.

Previous survey results had delivered conflicting conclusions and it was therefore imperative that the Fathoms survey should deliver verifiable, comprehensive and robust data to provide a good baseline for the present contract’s calculations and future surveys.

PREVIOUS SURVEYS

Fathoms undertook a desk study of all available data to ascertain how the results of Fathoms’ new survey could best be compared with previous ones.

On Masinga, Huntings had undertaken an aerial photographic survey of the reservoir basin 1965. This incorporated both Masinga and Kamburu reservoirs before impoundment. This was followed by the first bathymetric survey of Masinga in 1983 (by HR Wallingford) using about 90 transects established by means of beacons on the shores. This was repeated again in 1988 (by Tarda) using the same method. Engineering and Hydrosystems conducted the first survey using modern methods (GPS and echo sounder) in 2001 but did not use the existing transects at all.

Kamburu was first surveyed using 29 transects during the HR Wallingford 1981 survey and again in 1983. Sasumua was originally commissioned in 1956 with the dam being raised in 1968; so it has had no previous bathymetric survey. The Ndakaini reservoir only became operational in 1994 and the only previous survey was done by E&H in 2001.

These surveys’ transects were originally established by land survey methods and mainly consisted of conventional pillar type beacons. Unfortunately, these were surveyed using a local engineering grid, the details of which were not available to the present team. Many of these beacons were found but – given the period of time since they had been established – many had disappeared. The E&H survey was not available for study; merely the conclusions from it.

METHODOLOGY

Since all previous surveys were either unavailable or unsuitable for direct comparison, Fathoms had to design this survey from scratch; despite the need to compare previous results with present ones.

Rather than using a previous local engineering grid, all data were based upon the Kenya 1960 New Arc Datum using transformations from GPS’ WGS84 based data. Because Masinga is some 35Km in length, a mere 90 transects were never going to provide a definitive shape of Masinga; although the first aerial survey provided the baseline topography.

In order to establish an accurate volumetric assessment of the reservoirs, detailed and closely spaced bathymetry was required, in a regular and systematic pattern. However, it was still hoped to be able to locate some of the original beacons and thus be able to run the previous transects. Although these would be additional to the main survey, they would provide a direct graphical comparison where available.

One of the early decisions Fathoms had to make was whether to adopt swathe bathymetric methods or stick with a conventional single beam echo sounder. Given the extra cost of the multibeam system (for probably not too great a gain in output) and the difficulty of mounting such a system with its necessary peripherals in such small boats, Fathoms advised the consortium that a single beam echo sounder would be the more cost effective option. This recommendation was accepted.

Several exceptional factors required careful planning to ensure a safe and successful survey. The two hydro reservoirs were remote; so there was little chance of assistance should any problems have arisen. Survey vessels were required and the presence of usually unfriendly wildlife would be a real hazard. In particular, there were known to be hippopotami and crocodiles in Kamburu and Masinga. This indeed, proved to be the case; so it was decided that the vessel should keep a safe distance from all wildlife if encountered. Although hippos and crocodiles were present on many occasions, once they heard the vessel’s engines they would submerge and keep a safe distance; as did the survey vessel! Consequently the survey was conducted with minimal environmental impact. On one occasion when the team was setting up the vessel early in the morning, one surveyor heard heavy breathing and sighs behind him. He thought it was his team-mate having problems with the equipment. But it was a hippo that had come to have a look!

The Fathoms team carried out the surveys in July and August 2010. Although it was in the middle of summer, some days were cold due to the height of the reservoirs. Indeed, the survey of Ndakaini was conducted in a heavy mist (low cloud) that reduced visibility to less than fifty metres.

Vessels

KenGen kindly supplied a useful GRP craft for use on the larger Masinga and Kamburu reservoirs. This was fitted with twin outboards (which provided a degree of break-down back-up), a central steering console and a sun canopy. This latter was very important and provided protection for the personnel during long, hot and sunny days; and shade for the computer screens.

A smaller boat was provided for the survey of the two smaller reservoirs. Both boats are shown below. A certain amount of improvisation was required in both cases but in the end, both did the job required. The echo sounder was mounted on an over-the-side mount with the GPS antenna fitted on the same pole to avoid inaccuracies.

EQUIPMENT

A standard survey specification high frequency single beam echo sounder was used for depth measurement. Daily calibrations were undertaken including the measurement of the velocity or propagation of sound in water.

The choice of RTK GPS was ruled out for several reasons:
– The presence of wildlife around the edges of rivers and reservoirs;
– The difficult terrain with long sections of river bank densely vegetated and only accessible on foot;
– The line of sight requirements would have required innumerable base stations.

A differential GPS unit was therefore used for all positioning and corrections were supplied direct by satellite. This obviated the need to establish any local stations. Accuracies of 30cm were consistently obtained when checks were undertaken against control points. Specialist equipment – C-Nav 2050M  – proved ideal for the surveys as it provides RTK solutions particularly suited for remote areas by use of satellite based correction signals.

SURVEYING PROCEDURES

One of the critical issues was to be able to survey as close inshore as possible to provide maximum coverage. This was not always feasible, because the water was often very shallow. It became obvious during the surveys and after consultation with the locals that when the reservoirs’ water levels drop, the people living around the lakes use the freed up land to plant seasonal crops. This is obviously fertile land because of the silt deposited into the reservoirs by the rivers that feed them. The following two photographs provide an indication of some of the natural hazards.

Similar difficulties were experienced with numerous fishing lines in the shallow waters.

The weather varied from glorious sunshine to thick mist on the higher reservoirs at nearly 2,500m above sea level. At times the survey team felt very at home with the damp cold conditions; similar to an English day at work.

Fathoms obtained a regular grid of soundings with line intervals generally at 50m or 100m. This provided excellent coverage of the area and kept time on site to a reasonable and manageable level. In addition, where it was possible to see the old transect beacons, extra lines were run along the same transects to provide as close a comparison as possible without having to interpolate.

RESULTS

The primarily aim of these surveys was to provide a baseline survey for future studies and to determine whether there had been much siltation, so the volume of each reservoir was calculated from the survey results. The volumes were calculated using two separate methods and the results compared. In each case, there was excellent agreement.

There were insufficient datasets from previous surveys available to provide a detailed and comprehensive analysis of volumetric comparisons. However, less accurate volumetric comparisons could be made using the previous surveys’ results to provide an indication of the changes in volume. When comparing a modern survey (using rigorous methods) with some of the earlier surveys (based upon a much smaller dataset), there will be inevitable inaccuracies.

However, hard copy profiles from a previous survey of Kamburu allowed a direct comparison between the previous and present surveys, albeit graphically. An example is shown below.

A plot was made of all the results of the bathymetry to cover each reservoir. A typical output is shown below for Masinga.

It is clear from this plot that the boundaries of this reservoir – and the others too – are anything but simple. The great care that was taken in the gathering of the data has enabled a full picture of the reservoir to be obtained.

CONCLUSIONS

Four interesting and challenging surveys were successfully completed by Fathoms under sometimes unusual circumstances. Volume calculations and contour charts were delivered for each reservoir. All four surveys provide a definitive baseline footprint against which future surveys can be measured.

What’s more, none of the Fathoms team members fell prey to the crocs and hippos that watched these intricate surveying procedures.