Modeling Agricultural Land Drains Using Vertical Design

In the video below we show how to use the basic Linestring Editor, the Vertical Design Tools and Potentially the Multi Offset Line tools to assist you to model agricultural land drains.

The basic design concept is that you have a drainage pit into which all water from an area of Agricultural land has to drain. The drainage system comprises main drains that drain into the pit and then slope drains that are spaced 5m apart across the field that drain into the main drains and then into the pit from there.

The design constraints are that the Slope Drains need to be 0.4m below the ground surface, but have to be at a constant slope of approximately 1 to 1.2% towards the main drain. There is a tolerance of +/- 10mm on the depth of cover - but the ground is not absolutely level so you have to find a best fit and a best slope for the slope drains.

The main drains come into the pit at an elevation below the rim e.g. 0.5m below the rim (at ground level) and the main drain has to be 0.5m below ground. The main drains flow at 0.4% (1:250) towards the pit.

There is therefore going to be an elevation difference between the main drain and the slope drains of ~0.1m because the main drains are ~0.5m below ground and the slope drains are 0…4m below ground. This difference where it exists will be accounted for using a vertical curve at the end of the pipes that is ~1m long.

The first step of the process is to define in 2D the pipe network and the drainage pit location. The slope drain pipe lines are then copied and draped over the existing ground and lowered by 0.4m to provide the “Target Locations” for the slope drain pipes.

The elevation for the rim of the drainage pit is determined from the ground elevation e.g. at the center of the drainage pit. The pit circle can be elevated using an elevation instruction in the Vertical Design - this allows it to be parametrically varied if needed to make adjustments to the pipe network as the design evolves.

The pit rim elevation can now be used to set the invert elevation of the Main Drain pipes at the location where they enter the pits 0.5m below rim elevation. This can be done using a connector rule and assigning a delta elevation to the connector so that they take the rim elevation -0.5m. If the pit rim changes then the invert will change accordingly. If the elevation delta is changed then the pipe invert can be raised or lowered in the pit to meet the drainage design needs.

The slope of the main drains can then be applied from the invert Z at the pit to the ends of the pipe at 0.4% slope. The Grade Rule can be used in the vertical design to compute the main drain pipe elevations.

This gives us the end elevations for all of the slope drain pipes that will enter the main drains. The can be parametrically associated using a Vertical Design and connector rules that lock the slope drain pipe to the main drain pipe elevation where they meet.

The next step is to compute the point 1m from the main drain where we want the slope drain elevation to be - at that point it should be at the elevation of the ground grade -400mm. To create that elevation, i offset the main drain pipe line by 1.0m and 0.5m (needed for the vertical curves) and then drape the 1m offset line over the ground grade lines (-400mm) to create a 3D line that represents the elevation for the slope drains where they cross.

The elevation for the 0.5m offset line can be created using a straight offset line with 0% cross slope or it could be calculated using the Vertical Design using a 0% cross slope rule.

To elevate the slope drains at the 1m offset point you can use a connector rule - connect from the 1m offset line to the slope drain line to set the elevation of the slope drain to that of the ground profile -400mm elevation.

Now you can set the elevation of the ends of the pipes using a grade rule starting from the 1m offset location to the end of the pipe at e.g. 1.1% and then check the pipe against the ground profile -400mm line to make sure it is a close match. Adjust the slope of the grade rule to get the desired match. If you need to raise or lower the 1m offset point - change the Z of the 1m offset line at any single pipe and that will adjust the pipe z automatically.

Once you have all the pipe slope and 1m offset points locked in, you can add a VPI rule for each of the vertical curves to smooth the transition of the slope drain into the main drain over the 1m length. Use a Parabolic curve and use a curve length of e.g. 0.94m or 0.9m rather than 1m to allow for vertical design adjustments. If you try to use 1m it will not create a vertical curve because the elevations of the slope drain are already locked in at the 0 and 1m points.

Video shows you how