Reservoir spillway surveys

Location:                             Galloway, Scotland                                                                        Client:                                   DRAX

Challenge:

We carried out spillway channel capacity assessments of four dams, and reviewed the likelihood and consequence of out-of-channel flow. Engineering drawings of the structures already existed, however these were pdf versions of the original paper structural drawings. The geometric information they contained was basic and could not be used to create the 3D environment required for modelling.

The reservoir dam structures were large in size, approximately 200m in length with steep drops into the spillway channels and the only unrestricted access was along designated walkways. The complexity of the modelling was going to require the shape, gradients and curves of the channel as well as any subtle changes in the channel bed to be recorded. The survey therefore had to combine the vital importance of working safely with a rapid and comprehensive data collection method.

Solution:

A 3D scanning solution was proposed, using a Trimble SX10 scanning total station. The advantages of using this equipment included:the range in scan mode of up to 300m; the 3D accuracy of 2.5mm at 100m; and the ability to change between total station and 3D laser scan modes within the one machine.

We established survey control stations at each site on stable surfaces and their positions were observed using Network RTK GNSS survey equipment. We followed The Survey Association (TSA) published best practice on this technique i.e. observing two independent periods, each of three minutes and separated by 40 minutes.

Overview Point Cloud, reservoir spillway survey
Overview point cloud data

Each scanner set-up was geo-referenced in real time using the pre-established survey control stations, and then re-observed in total station mode, to check the accuracy of the set-up and orientation. This was vital as each scan would fit together to create the model in real time, with no post registration required afterwards.

This set-up process was repeated around the slipway channel, always keeping to the designated walkways, until a full point cloud data set with a 5cm point resolution was collected for each structure.

The point cloud datasets were transferred into Trimble Business Centre and went through a number of processes to confirm quality and continuity within the scan data.

We created a more user-friendly and efficient data set by:

  • rendering the point cloud points with the photographic images collected simultaneously with the scan data.
  • selectively clipping the point cloud to exclude areas out with the specific channel.
  • creating break lines to better define the channel and then reducing the resolution of the point grid, depending on surface material type and general smoothness.

Finally a set of geospatial files were created for direct input into the Computational Fluid Dynamics modelling of the water flow through the spillway channels.

Benefit:

The major advantage of laser scanning each site is that the data collection can be undertaken from safe, stable positions, remote from the principal hazards of water and height.

The finished survey data set was a high resolution point cloud. This could then be integrated and manipulated to provide various output options for the modelling process.

  • The point cloud resolution was altered, depending on the surface material type. The smooth concrete spillway overflow required a lower point density to model the surface actually, whilst the irregular bedrock features within the channel bed required a higher density for definition.
  • Cross sections and through sections were created to better define the channel geometry and provide the modeller with specific information.
  • The high definition photography that the SX10 combines with the point cloud scan data, provided added value and clarity to features within the channel.

Want to know more?

All our surveyors are trained in Swiftwater and Rescue 3 as well as being First Aid trained, and all our site surveyors have completed the JBA Guide to Working On, In or Near Water.

Email Robbie Cowan, Technical Director Land Survey, for more information about reservoir spillway surveys. You can also find out more about our topographical surveying services here.



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