Hydrological Surveys

Designers or contractors may be well advised to arrange for a Hydrological survey to be undertaken – at their client’s expense – before starting out on planning a garden design. Without knowledge of the likely movement of water across a site, excessive groundwater may become a major problem during construction.

Using LiDAR to Create Hydrological Flow Models in a Landscape

Lidar (short for ‘light detection and ranging’) is a remote sensing technology that uses lasers to measure distance and other properties of objects. It works by emitting a laser beam and measuring the time it takes for the beam to return after reflecting off an object. The distance to the object can be calculated based on the speed of light and the time it took for the beam to return.

Lidar is often used for mapping and surveying, as well as for autonomous vehicle navigation and robotics. It can provide high-resolution 3D images of objects and environments, allowing it to detect features such as buildings, trees, and other obstacles. Lidar can also be used to measure the distance to objects with high accuracy, making it useful for a variety of applications including self-driving cars, robotics, and geological mapping by collecting data to create terrain models.

A DSM (digital surface model) is a 3D representation of the earth’s surface that includes features such as buildings, trees, and other aboveground objects. It is created using data from a variety of sources, including aerial or satellite imagery, laser scanning (i.e., Lidar), and other types of remote sensing data. DSMs are often used for a variety of applications, including mapping, land use planning, natural resource management, and disaster response. They can provide information about the location and height of aboveground features, allowing users to visualize and analyse the terrain of an area in 3D. DSMs can be created at various scales, from small areas to entire countries, depending on the resolution of the input data and the needs of the application.

DSMs are different from digital elevation models (DEMs), which are similar 3D representations of the earth’s surface but only include the ground surface, not aboveground features. DSMs are typically more detailed than DEMs and provide a more complete picture of the surface of an area.

Although this data can be very useful to create water movement models, there are several limitations to using a DSM to calculate a hydrological flow map:

Aboveground features: A DSM includes aboveground features but does not include information about underground features such as subsurface geology and soil type, which can be important for hydrological modelling.

Spatial resolution: The spatial resolution of a DSM may not be fine enough to accurately represent the small-scale features of a hydrological flow model.

Temporal resolution: A DSM is a snapshot of the earth’s surface at a particular point in time and may not accurately represent changes in the surface over time that could affect hydrological flow.

Accuracy: The accuracy of a DSM can be affected by factors such as the quality of the input data and the algorithms used to generate the model, which can introduce errors into the hydrological flow model.

There are many software programs that can be used to create hydrological flow models, the easiest and most popular is ArcGIS: a powerful GIS (Geographic Information System) software suite developed by Esri. It includes a wide range of tools and features for creating and analysing hydrological flow models, including tools for generating digital elevation models (DEMs), calculating flow direction and accumulation, and defining streams. While relatively user-friendly, this software does have a fairly steep learning curve, but once the user has become familiar with the program’s environment, many processes are quite simple.

To use a DSM in ArcGIS to determine water flow through a garden, you will need to follow these steps:

Obtain DSM data for the area of the garden. This data is typically collected by aeroplanes or drones equipped with LiDAR sensors or by using satellite imagery. You may be able to find free DSM data through a government agency such as DEFRA (https://environment.data.gov. uk/DefraDataDownload/?Mode=survey) or by purchasing it from a private company.

Import the DSM data into ArcGIS. To do this, you will need to use the ‘Add Data’ tool and select the DSM data file. The data will be added to your ArcGIS project as a raster layer.

Use the ‘Fill’ and ‘Flow Direction’ tools to determine the direction of water flow across the DSM. This tool calculates the direction of flow based on the slope of the surface.

Use the “Flow Accumulation” tool to determine the amount of water flowing through the garden. This tool calculates the accumulated flow across the DSM, taking into account the flow direction.

The displayed data can be fine-tuned using the ‘Symbology’ tab in the Layer Properties (‘Classified’ often works best, using a high number of classes). This will map colours onto the data, and if using the right colour range, you will be able to determine levels of low to high accumulation. (The darker the blue, the greater the likely volume of water.)