Why is it that the neutrino experiments are designed by keeping constraints on the energy E of the neutrinos and the length L which they travel from the source to the detector. What decides actually the ratio $L/E$ for a neutrino experiment? To be more specific, for example, why is it that NO$\nu$A experiment operates with $E \approx 2$ GeV (corresponding to the maximum neutrino flux and length $L=810$ Km. The T2K experiment uses $E\approx1$ GeV and $L\approx2$ Km.
Are these experiments designed with some specific aims? If so, how is this ration ($L/E$) going to be so important a factor?