This article takes you step by step through the Runoff Analysis procedure in OptiSurface Designer V1.4.5 or higher.
1. Start OptiSurface Designer (OSD) Program.
2. Open the file to be analyzed.
File > Open > navigate and select the osd file.
3. Start Runoff Analysis.
Tools > Runoff Analysis
The Runoff Analysis Window will prompt as shown below.
4. Modify and add parameters for the analysis.
Tick 'Existing Surface' to calculate using the Existing Topography.
Tick 'Proposed Surface' to calculate using the Proposed Topography.
a. Calculation Grid Spacing - a good start is 20, lower spacing will take more time to calculate.
For this example, 10 ft was used and calculation time was about 30 mins.
b. Simulation Event Duration (hrs) - needs to be large enough for the field to generate the highest runoff that it can. 1.5 hrs is generally long enough for fields of 500 m or yds long, for larger fields you need to go proportionally longer. It is the time for water from one end of the field to run-off to the other end of the field. This is also know as the Critical Duration.
c. 1hr Design Rainfall (in or mm) - use the rainfall data from your location.
A good number to use is the maximum rainfall you get every 5 years on average in 1 hour. This information has been analyzed by meteorological institutes around the world and a lot of information is
available. For the Australian BOM Rainfall Intensity Frequency Duration Data:
old: http://www.bom.gov.au/cgi-bin/hydro/has/CDIRSWebBasic
new: http://www.bom.gov.au/water/designRainfalls/ifd/
For the US: www.nws.noaa.gov/oh/hdsc/PF_documents/TechnicalPaper_No40.pdf
Or follow this link: https://www.dropbox.com/s/9mgg6moodyop3sk/US_Rainfall_Atlas.pdf
For this example, the US Rainfall Atlas is used and the location of the field is marked with a red circle.
It can be estimated that the 5-year 1-hr rainfall is near 2.1 inches.
d. 24hr Design Rainfall (in or mm) - use the rainfall data from your location.
A good number to use is the maximum rainfall you get every 5 years on average in 24 hour.
For this example, the US Rainfall Atlas is used and the location of the field is marked with a red circle.
It can be estimated that the 5-year 24-hr rainfall is near 4.5 inches.
e. Runoff Proportion of Rainfall (%) - the portion of rainfall that will not infiltrate into the soil and becomes runoff. For this example, 75% of the rainfall was assumed to become runoff, this is a typical and maybe conservative figure for erosion analysis. See the table below as reference:
Source: Slope Stabilization and Erosion Control: A Bioengineering Approach By Roy P.C. Morgan, R.J. Ricksonf. Hydraulic Roughness (n) - also known as Manning's 'n'.
Here are some values as measured by Chow (1959)
Another reference from Slope Stabilization and Erosion Control: A Bioengineering Approach
By Roy P.C. Morgan, R.J. Rickson:
In this example, we selected 0.02 which is applicable to cultivated bare soil and typical for erosion analysis.
5. Add parameters for Furrows or Beds. Tick the box and add values as illustrated in the Runoff Analysis Window. Click the 'Pick' button and draw the direction of the furrows on the field. Otherwise, untick the box if the field has no furrows or beds.
6. Hit 'Ok' and wait for OSD to finish calculating.
Note: The analysis only incorporated water input from rainfall on the field. This assumes there is no inflow from areas outside the field. Using the inputs as mentioned in the Runoff Window Analysis shown previously, here are images of an example of Runoff Analysis.
Proposed Topography
Runoff Velocity Map
The results indicate that the velocities are not excessive, except maybe in the red area in the midpoint edge of the Western side. The velocities are getting upto 2 ft/sec, slopes can be reduced in that area to reduce erosion risk but the rest of the field is looking quite good.
Runoff Depth Map.
