# 3.4.5 Runoff Analysis

Last Updated: 10 August 2017          For: OptiSurface Designer V2.4

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 furrows (depth greater than furrow) for the existing topography or a particular landform design.

Also, Follow a video tutorial on this subject here: Runoff Analysis Part 1 & Runoff Analysis Part 2

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Appearance:

3.4.5.1  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.  Be careful not to go too small, calculation will increase dramatically as the grid spacing is reduced.

3.4.5.2  Rainfall

Simulation Event Duration (hrs):  Defines how long the storm event that will be applied to the field.  It
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.

1hr Design Rainfall Depth (mm
):  Defines the rainfall intensity. Best to 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.

24hr Design Rainfall Depth (mm):  Defines the rainfall intensity. Best to 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.

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
Another reference from Slope Stabilization and Erosion Control: A Bioengineering Approach By Roy P.C. Morgan, R.J. Rickson:

Below is how Manning's n value can vary with depth of flow by Claytor and Scheuler (1986).

3.4.5.3 Infiltration

Runoff Coefficient or Runoff Proportion of Rainfall (%):  This is the proportion of rainfall that will become runoff, also known as the Runoff Coefficient.  See graph and table below as reference:

Source: Slope Stabilization and Erosion Control: A Bioengineering Approach By Roy P.C. Morgan, R.J. Rickson

Runoff Curve Number, CN:  The USDA SCS Runoff Equation uses the Runoff Curve Number (CN) to estimate the portion of rainfall that infiltrates and the remainder becomes runoff. For full details see Technical Release 55 Urban Hydrology for Small Watersheds, 210-VI-TR-55, Second Ed., June 1986.

Determination of CN depends on the fields soil type and cover conditions, which the model represents as hydrologic soil group, cover type, treatment, and hydrologic condition.  Use the tables below to guide you on an appropriate CN.

Soils are classified into hydrologic soil groups (HSG’s) to indicate the minimum rate of infiltration obtained for bare soil after prolonged wetting.

3.4.5.4 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 bottom of the furrow as shown in the diagram on the dialog box.

3.4.5.5 Inflow Subzone

Inflow for subzone parameter can be added to define water volume coming in from watershed into the field.

NOTES

Here are the permissible velocities for grassed channels:

Table 7.11: Permissible Velocities for Earth Channels (Source: Handbook for Agrohydrology, 1994)

Table 7.12: Permissible Velocities (m/s) for Channels with Grass Cover (Source: Handbook for Agrohydrology, 1994)

Maximum allowable velocity for cultivated soil (Source: NATURAL RESOURCES CONSERVATION SERVICE CONSERVATION PRACTICE STANDARD TERRACE):

• for erosion-resistant soils (clay textural classification) is 2.5 ft/s (0.75 m/s);
• for average soils (silt textural classification), 2.0 ft/s (0.6 m/s);
• for easily erodible soils (sand textural classification), 1.5 ft/s (0.45m/s).

If Manning’s equation is used to compute velocity, use a maximum n value of 0.035 to determine velocity.