ANS:D - 25% to 30%

A discharge increase of 25% to 30% due to a clinging nappe of a weir would be an exceptionally high range and would generally not be expected in standard hydraulic conditions. Typically, the clinging nappe phenomenon results in a relatively modest increase in discharge, usually in the range of 6% to 7%. This increase occurs because the nappe adheres closely to the downstream face of the weir, allowing more water to flow over the crest compared to a free-flowing condition where the nappe separates from the weir. If there were a scenario where the discharge increase fell within the 25% to 30% range, it would likely be under highly specific conditions involving factors such as:

1. Unusual weir design: The shape and profile of the weir might be specially engineered to encourage a clinging nappe and achieve such a high discharge increase.
2. Extremely turbulent flow conditions: Turbulence in the flow might cause the nappe to adhere more closely to the weir, resulting in a greater discharge increase.
3. Unconventional hydraulic situation: Certain hydraulic phenomena or configurations might lead to unexpected results, though such cases would be rare and highly specialized.

However, it's important to note that a discharge increase of 25% to 30% due to a clinging nappe would be unusual and would require careful analysis and consideration of the specific hydraulic conditions and weir design. In standard hydraulic engineering practice, the typical range for the increase in discharge with a clinging nappe is around 6% to 7%.