To calculate the gradient of a stream, you need to determine the change in elevation over a given horizontal distance. The gradient is essentially the slope of the streambed, which is calculated by dividing the difference in elevation between two points by the horizontal distance between them.
Here’s the step-by-step process:
1. Identify two points on the stream: Choose two locations along the stream where you can measure both the elevation and the horizontal distance between them. Ideally, these points should be far enough apart to represent the stream’s gradient effectively but not so far that the terrain or streambed changes significantly.
2. Measure the elevation difference: Determine the elevation (height above sea level or another reference point) at both points. The elevation difference is the difference in height between the upstream and downstream points.
– Elevation at point 1 (upstream) = E1
– Elevation at point 2 (downstream) = E2
– Elevation difference = E1 – E2
3. Measure the horizontal distance: Measure the horizontal distance (usually along the ground surface) between the two points. This is typically done using a map, GPS, or by physically measuring the distance along the stream.
– Horizontal distance between the two points = D
4. Calculate the gradient: The gradient is calculated by dividing the elevation difference by the horizontal distance. The formula is:
Gradient = (Elevation at point 1 – Elevation at point 2) / Horizontal distance
– Gradient = (E1 – E2) / D
5. Interpret the result: The gradient is usually expressed in terms of a ratio or percentage. If the result is a ratio (e.g., 1:100), this means that for every 100 units of horizontal distance, the stream drops 1 unit in elevation. If expressed as a percentage, it is calculated by multiplying the ratio by 100.
For example:
If the elevation difference between two points is 10 meters, and the horizontal distance is 200 meters, then:
Gradient = 10 meters / 200 meters = 0.05 or 5%
This means the stream has a 5% gradient, meaning it drops 5 meters in elevation for every 100 meters of horizontal distance.
