# Runoff Analysis

Overview: The Runoff Analysis functionality allows you to calculate runoff depth and velocity maps during a storm event (e.g. 1 in 5yr storm). The outputs allow the designer to assess the risk of erosion (high velocity) and overtopping of furrow (depth greater than furrow) for a particular landform design or the existing topography.

Tip - Follow a video tutorial on this subject here: Runoff Analysis Part 1 & Runoff Analysis Part 2

Displayed by:

Appearance:

Topography

Surface: The surface that you want to carry the analysis out on.

Calculation Grid: The cell spacing of the grid that the drainage analysis is calculated on. 10m is a typical useful value.

Hydrology

Watch this video for a description: Runoff Analysis Part 2

Hydraulic Roughness (n): This is the same as Mannings ‘n’ used commonly in engineering calculations. Here are some values as measured by Chow (1959):

 Description Minimum Normal Maximum a. Pasture, no brush 1.short grass 0.025 0.030 0.035 2. high grass 0.030 0.035 0.050 b. Cultivated areas 1. no crop 0.020 0.030 0.040 2. mature row crops 0.025 0.035 0.045 3. mature field crops 0.030 0.040 0.050

Furrows or Beds

Furrow or Beds Restrict Water Flow Direction: If furrows or beds exist tick this on.

Furrow/Beds Direction (deg): The direction of the furrows relative to north. Use the ‘Pick’ button to click two points to define the direction or type a number in.

Furrow/Beds Spacing, w (m): The spacing between the furrows/bed. i.e from one crest to the other as shown in the diagram on the dialog box.

Furrow/Beds Height, h (m): The height of the intended furrows in the field. The water will build up in the furrow and only spill across the furrow when it exceeds this depth.

Furrow/Beds Side Slope, s (?h:1v): The side slope of the furrow as shown in the diagram on the dialog box. This is the horizontal distance per 1 vertical. eg 1 mean 45% gradient and 3 would mean 33% gradient.

Furrow/Beds Bottom Width, b (m): The width of the bottow of the furrow as shown in the diagram on the dialog box.