What can you do with these data in practice? A captain needs to know two things in order to sail his ship safely on the waterways:
- the water depth
- the height of bridges.
This is also important when calculating the tonnage of cargo that can be carried on that water at that moment, because the weight of the cargo has consequences for the draught of the ship. It goes without saying that a captain wants to transport as many tonnes of goods as possible, because that produces the greatest possible freight yield. But the ship can’t have too much draught!
The concrete threshold of a lock will often be the shallowest (hard) point with which the ship does not have to come into contact. This threshold is a fixed point, the location of which in relation to the national allocation plan can be indicated to the nearest centimetre. This is also the case with the bottom of a bridge. These two fixed points do not change their position in relation to NAP.
The water surface is not a fixed point in relation to the NAP. That plane rises (flooding water) and falls (falling water); think of the height of the water level. When the water falls, the height of passage increases. But at the same time the water depth is reduced.
In the remainder of this chapter, a number of calculations will be made with regard to clearance height and clearance depth. If you have to make such calculations yourself, it is wise to make a drawing on a piece of paper. That often prevents mistakes.
The data in the examples are fictive. In the captain’s practice, such data is extracted from various books and reports. You will be introduced to this in the next chapter.
If you know the height of a bridge and the water level, you can calculate whether you can sail under the bridge with your ship. Let us first give a simple example.
The height of the bridge is NAP + 9.85 metres. The water level is NAP – 0.80 metres.
What’s the height of this bridge?
The easiest way to calculate is to make a drawing of it.
Start by drawing a line indicating NAP. Then sign the bridge. It’s 9.85 metres above NAP. This is indicated by arrow 1. Then draw the water level. It’s 0.8 metres below NAP (you see that?) There’s a “minus” sign). Arrow 2 indicates that.
To calculate the passage height, draw arrow 3. So the height is 9.85 metres plus 0.8 metres is 10.65 metres. If you know the height of your ship, you now know whether or not you can sail under the bridge.
Now a slightly more difficult calculation. If the location of a bridge in relation to the canal level (KP) is known, you can also calculate the clearance height.
Suppose the bridge height is somewhere on the Juliana Canal KP + 6.30 m. On the water level scale, the water is at KP + 0.20 m.
What is the clearance height now?
If the water level were exactly at KP, the bridge height would be 6.30 metres. But the water level is KP + 0.20. The height of the underpass has thus been reduced by 20 centimetres. That’s 6.10 metres. Check this out on the drawing below.
Just a theoretical trip. Suppose the bridge height was given in relation to NAP. On the Juliana Canal, for example, that could be NAP + 51 m. At a vertical clearance of 6.10 m as in our example, the water would be at NAP + 44.90 m. Because the numbers then become so large, one prefers to work with Channel level.
In the following example, a detour is made to calculate the vertical clearance.
A bridge is located at KP + 9.45 m. The KP is located at NAP – 0.70 m. On a NAP scale the water is at NAP – 0.95 m. How much clearance is there under that bridge?
The clearance height is also clearly visible here:
9.45 m + 0.25 m = 9.70 m.
In practice, in such a case, you can usually read the water level on a KP level scale. Then the calculation would have been simpler. In order to practice and to give insight into the principle, we have made it a little more complicated.
The clearance height of the Polbrug is SP + 7.05 metres. A level shows that the water level is SP + 0.5 metres. How high is the passage height under the Polbrug now?
The drawing below shows that this is 6.55 metres. How was that calculated? The passage height is 7.05 metres, and 0.5 metres must be deducted. It’s actually an easy sum.