The correct answer is: A. slope-area method.
The slope-area method is a method used to estimate flood discharge based on high water marks left over in the past. The method involves measuring the slope of the land and the area of the floodplain, and then using these measurements to calculate the discharge of the flood.
The slope-area method is a simple and relatively accurate method for estimating flood discharge. However, it is important to note that the method is only applicable to floods that have occurred in the past. The method cannot be used to estimate the discharge of a flood that has not yet occurred.
The other options are incorrect because they are not methods that are used to estimate flood discharge based on high water marks left over in the past.
- The area-velocity method is a method used to estimate flood discharge based on the area of the floodplain and the velocity of the floodwater.
- The moving boat method is a method used to estimate flood discharge based on the time it takes for a boat to travel a certain distance in the floodwater.
- The ultra-sonic method is a method used to estimate flood discharge based on the time it takes for an ultra-sonic wave to travel a certain distance in the floodwater.