-- Carlo Peris, Pangea srl and Tom Hiller, GeoAcoustics Ltd --
GeoSwath 250kHz transducer head on the inflatable’s customised mount.
Surveying a small dam near Volci, Italy.
Example of a side scan pass by a dam structure.
Access to the waterside is sometimes difficult!
Bathymetry results from a small dam.
Small hydroelectric generation stations can provide local, environmentally friendly
power wherever a suitable head of water exists. By the end of 2008 'small-hydro'
was generating over 85 gigawatts of power from installations with capacities of
a few hundred KW up to a few tens of MW (compared to the Hoover dam at over 2000MW).
These installations often use existing dams or are developed alongside new dams
whose primary purpose is river water-level control or irrigation. There are many
thousands of these small dams worldwide (over 95,000 in the USA alone) and maintaining
this infrastructure requires accurate mapping and monitoring. This presents some
novel challenges to the hydrographic surveyor and this article describes the experiences
of one company providing hydrographic surveys of dams in the Apennine mountains
In 2007 Pangea Srl was contracted by Enel SPA to perform bathymetric and topographic
surveys around hydroelectric plants located in the Lazio and Abruzzo regions of
central Italy, including installations on rivers such as the Liri (province of Frosinone)
and the Tiber (upstream of Rome). The project covered many sites but each individual
survey project was fairly limited in area, including some dams only a few tens of
The accuracy requirements for monitoring the sedimentation and shape of a dam
floor are comparable to a ship navigation channel or dredge works. Knowledge of
the water depth and total volume is required for dam maintenance and to maximise
the renewable energy output. Detailed bathymetry allows improved irrigation planning
and flood control, and survey images show the status of the dam infrastructure (such
as the threshold and sluice gates). Accurate depth maps can also can be critical
to the safe operation of the hydroelectric facility and inform the risk assessment
of the dam structure.
The difficulties posed by this kind of survey are in two main areas. First, the
difficulty of mobilisation of survey assets caused by the remoteness of the survey
site, limited access to the banks and the lack of piers. The dam may be some distance
from the nearest road, requiring all the equipment to be carried over rough land.
Second, running the survey lines is difficult because of the limited depth of water
(sometimes less than a meter over much of the dam periphery) along with the presence
of hazards to navigation such as trees or submerged structures. Often during the
survey the boat will need to navigate close to a shore covered by dense vegetation
and around the hydroelectric plant structures. These difficulties exclude the possibility
of using a standard survey launch; something much lighter and man–launchable is
required, but it still has to carry a fully capable hydrographic survey rig.
For this contract Pangea chose two vessels: a 3.7m long aluminium boat and a
4.30m inflatable boat. The advantages of the aluminium boat is its lightness and
strength; it is easily transportable, is straightforward to launch and robust around
obstacles. However it was found to be fairly unstable especially when loaded with
crew and numerous instruments that raised the centre of gravity. The advantage of
the inflatable boat was the stability, allowing good mobility when the boat was
fully loaded and operating. However it was heavier and less resilient, leading to
transport difficulties where waterside access was limited and the greater likelihood
of damage during launching or around obstacles. In order to limit the weight and
the quantity of equipment to be carried to the site a small 2HP outboard engine
was often used (or even oars in the smallest sites). In the larger surveys and in
the rivers a 15HP outboard engine was used.
Choice of a suitable integrated hydrographic package was key to successful completion
of the contracts. The requirements were for wide swath bathymetric mapping to better
than IHO special order specifications along with co-registered side-scan images
for structure inspection, with reasonable productivity in water depths often less
than 1m and data collection up to the waterline. The GeoSwath Plus interferometric
sonar from GeoAcoustics Ltd a Kongsberg Maritime Company was chosen. One of the
great advantages of the GeoSwath Plus for this type of small boat work is its compactness.
Most of the ancillary sensors are integrated into the transducer V-Plate which saves
space, saves time during installation and allows pre-calibration of the offsets
between the sensors. Full calibration on-site would often have been difficult because
of the small extent of the survey areas. Another advantage of the GeoSwath is the
robustness of the V-Plate and transducers; more than once an accidental grounding
left the system undamaged.
The equipment sometimes had to be hand carried over rough terrain to the waterside,
which is only feasible for a compact system like the GeoSwath Plus. The boat was
separately craned in or manhandled to the water. At each survey site the transit
to the waterside, installation of equipment on the vessel and system tests took
about 2 hours in total. Data acquisition usually lasted 1-2 hours (sometimes much
less on the smaller dams), and dismantling took about 1 hour. The biggest problem
usually encountered was how to get the equipment from the van to the waterside and
The data deliverables were bathymetry and side scan data processed by GS+ as
well as depth profiles exported directly from the real-time data collection windows.
The real-time side scan data displays also proved to be very useful for data interpretation
on-site. The images accompanying this article show a few of the installations used
and bathymetric survey results. After carrying out successful survey seasons in
2008 and '09 Pangea can report that the compact GeoSwath Plus is ideal for this
type of small boat hydrography in harsh environments.
This project has shown that modern highly accurate wide swath hydrographic survey
equipment can be economically deployed for restricted area surveys, with rapid mobilisation
and limited time on-site. Even the smallest hydropower dam can be fully mapped quickly
and efficiently. This type of mapping capability can be used to increase the safety
and efficient operation of small-hydro projects worldwide.
Contact details for further information and image requests:
Tom Hiller, Advanced Products Manager
GeoAcoustics Ltd., Gapton Hall Industrial Estate, Great Yarmouth, UK.
GeoAcoustics Ltd. (Great Yarmouth, UK) has been manufacturing marine survey equipment
for more than 25 years, and is a world-leading manufacturer of sonar survey equipment
for engineering geophysics and Naval survey applications. Principle product lines
are swath bathymetry systems for shallow waters, side scan sonars and sub-bottom
profilers. Kongsberg Maritime acquired GeoAcoustics Ltd in September 2008.
The GeoSwath Plus Sonar
The GeoSwath Plus sonar allows wide swath bathymetry and side scan data to be
collected from small vessels of opportunity in water depths up to 200m beneath the
transducers. The system was designed to meet the need for a wide swath system capable
of high productivity surveys in very shallow waters, and can achieve swath widths
over 12 times water depth. The GeoSwath provides high accuracy, high resolution
bathymetry (depth measurement accuracy exceeding the latest IHO specifications),
and co-registered side-scan data. GeoSwath versions are now also available for mounting
on small ROVs and AUVs.