Drone imagery helps map flood risk

The use of aerial surveys from unmanned aerial vehicles (UAVs) has proved effective in mapping flood risk for the Environment Agency at the River Mole in Surrey.

UAVs – commonly known as drones – are growing in popularity as a method of conducting surveys and inspections in hard-to-reach areas where high quality data and modelling is required.

In response to the storms which hit the River Mole – a tributary of the Thames – in winter 2013/14, Remote Aerial Surveys (RAS) were employed by Royal Haskoning DHV, on behalf of the Environment Agency, to conduct a range of low level aerial surveys of the area. The purpose of this was to locate and identify any issues in the channel, particularly those which could negate flood risk management, such as significant build-up of debris or damage to the river bank.

The River Mole rises in Horsham, West Sussex, and meets the Thames near Hampton Court Palace in South West London. At Island Barn Reservoir in South West London the river splits into two channels; the River Mole and the River Ember. The project involved conducting aerial surveys of the stretch of the river from the Esher-Hersham railway bridge in the south to where the rivers Mole and Ember reconnect above Island Barn Reservoir in the North.

Flooding along the River Mole has the potential to cause significant damage on local, national and even international scales due to the amount of key infrastructure located in its catchment. This includes Gatwick Airport, East Surrey Hospital, two major motorways (the M25 and M23) and the London-Gatwick-Brighton railway line. The winter of 2013/14 was one of the wettest on record, with storms causing major flooding events throughout the entire country, including the River Mole catchment. Specific impacts of these floods included the closure of Gatwick Airport causing delays and cancellations of over 100 flights, widespread power failures and flooding of homes and commercial properties and extensive damage to transport infrastructure.

UAVs were particularly suitable for the River Mole surveys for three main reasons. Firstly, the low levels which UAVs can fly at means they were ideal for capturing very high resolution aerial imagery and translating it into detailed photogrammetric elevation models. Secondly, UAVs can conduct surveys far more quickly than those being undertaken purely on the ground. This was a crucial factor in this project as the survey had to be undertaken whilst the level of the river was dropped, which could only be done for a limited amount of time due to the river’s fish population.

Finally, the vast majority the river channel was not visible from the ground due to steep banks which were largely inaccessible. The use of a UAV allowed for an unobstructed view of the entire river channel without requiring personnel to actually climb into the channel, potentially putting them at risk.

Method

The survey involved two main aerial survey types – videography and elevation modelling – and was enhanced by elements of ground based survey work including photography and GPS measuring.
The videography element of the aerial surveys was collected in two main formats: standard video and 360° videos. Both types of video were very useful in identifying potentially problematic areas of the river channel, such as areas where the bank reinforcements were damaged or where there was significant debris build up.

The collection of the standard videos involved the UAV being flown directly in the river channel, with the camera operator directing the camera to focus on one bank of the river at a time. The video was streamed live back to the camera operator, allowing them to direct the UAV pilot, such as instructing them to fly closer to areas of interest.

These videos were collected using a broadcast quality Panasonic Lumix GH3 mirrorless camera. This camera recorded full HD with up to 60 frames per second and was attached to the UAV by a camera mount which can be controlled with a precision of +/-0.05° around all three axes.

The collection of the 360° videos involved mounting six wide-angle cameras beneath the UAV which have enough overlap between them to create a full 360° image. The cameras used for this were GoPro Hero 3s, recording in 1080p with 25 frames per second. This allows viewers to simultaneously see the condition of both banks as well as the river bed. The 360° camera mount on the six rotor UAV is a lightweight solution: with a maximum weight of just under 7kg, it allowed the project team to fly in controlled airspace without requiring permission from Air Traffic Control.

Elevation

Elevation models were produced using photogrammetry. This process involves taking a series of vertical photographs at pre-defined intervals to produce a dataset with exact levels of overlap. The geometry between matching points on different photographs was used to produce highly accurate elevation models.
The accuracy of these models was achieved through capturing very high resolution imagery using the Canon 5D MK III full-frame DSLR, which has a resolution of 22.3 MP. Flying at 70m, this translated to a ground sample distance of 21.09mm. The accuracy was further enhanced by using a high specification ground GPS station (accurate to +/- 30mm) to georeference the elevation model.

Conclusions

The project was extremely successful in proving the viability of using UAVs to complete diverse aerial surveys of river banks in a time and cost effective manner whilst producing incredibly high quality data. The flexibility of these platforms allowed RAS to provide a broad overview of the site whilst enabling them to focus in on the finer details which contribute to this.

Tom Gravett, Operations Manager at Remote Aerial Surveys, said: “UAVs are an ideal solution when data is required from hard to reach areas. In this case it was a river channel, but UAVs can access a whole host of other sites to carry out surveys & inspections, such as pipe line surveys, infrastructure inspections or even georeferenced aerial photography for asset management.

“Essentially, UAVs are a means of delivering sophisticated image capturing equipment to locations that are hard to reach, in a quicker and more cost effective way. By using thermal or multispectral sensors for example, as well as traditional cameras, they are now solving a wide range of real business problems, delivering better data & imagery at a fraction of the present cost.

“UAVs bring a whole new level to what is possible when capturing data & imagery, with elevation modelling for example, being just one really powerful application. The data captured by using a UAV mounted camera can produce 3D models that make a really engaging tool for communicating information about the landscape.”

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