3.5 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.
Displayed by:
Menu: Tools> Runoff Analysis...
Appearance: 
3.5.1 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.
3.5.2 Hydrology
Hydraulic Roughness (n): This is the same as Manning's ‘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 |
3.5.3 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.