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The ID of the upstream and downstream node
The depth of the pipe at the upstream and downstream ends of the pipe. Notice that the value of depth at the downstream end is is less than that at the upstream end. Does this mean that the pipe is going uphill? No, read on.
The upstream and downstream end invert levels which is the height above sea level for the ends of the pipe. Notice that the height above sea level is lower at the downstream end than it is at the upstream end, so this is how we know that the pipe is flowing downhill and the difference in height from one end to the other is 121.626 - 121.403 = 0.223m or 223mm
So, how is the depth of the pipe at the downstream end so much less than the depth of the pipe at the upstream end? This is because the ground is not level, and demonstrates the point that you can never use the pipe depths to establish if the pipe is flowing the correct way. The only way to prove this is by considering and calculating the altitudes of the ends of the pipe above sea level.
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This would normally be done by measuring the altitude of the cover level above seas level using GPS equipment, then measuring the depth of the pipe at each end with a tape measure, entering these values into WinCan VX, and then using the ‘Calculate Invert Levels’ tool in the ‘Tools’ ribbon to do the subtraction without user error.
From this data, and even before we have looked at any charts or graphs, based on the GPS and manhole measurements, we can calculate:
The fall of the pipe from end to end = 121.403 − 121.626 = −0.223 m or −223 mm (minus because the pipe is dropping down in the direction of flow - this is normal for ‘happy’ pipes).
The gradient of the pipe = (−0.223 × 100) ÷ 23.0 = −0.97 %
So, we can calculate the overall gradient and fall of the pipe with only very basic arithmetic, but these values do not describe the shape of the pipe along its length. This is where the inclination data proves extremely useful.
It should always be remembered that drains seldom have a very large gradient except in exceptional circumstances and surface water pipes usually have an even smaller gradient than foul water pipes. This is why when you see matching pairs of manholes side-by-side where one is foul and the other surface, the surface water pipes are usually large diameter and shallow, and the foul water pipes are small diameter and deep.
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