This is a first installment of a two part simple impassive risk analysis of the Renaissance Dam for on Egypt and Ethiopia.
A simplified Risk Analysis Tool (Courtesy of mindtools.com)
Impact of the construction of the Renaissance Dam on Egypt can be anything from Medium to a devastating High. Egypt is a barren desert without Nile and the Blue Nile alone contributes up to 63 % of the water reaching Egypt. The dam Ethiopia is constructing will have a reservoir of 63,000,000,000 meters cube. That is 8 billion meters cube more than the annual share of the water Egypt claims to be its historical and natural due. Assuming Ethiopia will fill the dam over a period of 10 years, each year filling it with 6.3 billion of meters cube (Forget the loss to evaporation due to large surface area of the artificial lake that will be created) the dam will adversely affect Egypt for at least the 10 years time it will take for Ethiopia to fill the dam.
Hence, High Impact
And the probability of occurrence (of the construction of the dam) is certain and already underway. According to Ethiopian sources 23% of the work is already done and the diversion of the river, a landmark point, has taken place in mid May.
Obviously, High (Near Absolute) Probability
Conclusion: In the eyes of Egyptians, the construction of the dam is Critically Risky; and thus, national security issue.
tazabi 
Egypt does not consume all the Nile Water. Some of it is just wasted into the Mediterranean sea! Since the dam will fill up only gradually, the risk to Egypt is only minimal. Ethiopia has every right to construct the Renaissance Dam! Suffice it to say that all the rift valley countries support Ethiopia!