Channel-cavity drainage instability ($N_R^{\prime }(Q_R)>|N_K^{\prime }(Q_K)|$)

At fixed total discharge $Q_R+Q_K$, remove a small amount $\epsilon$ from a channel and send it through connected cavities. The equilibrium channel effective pressure drops by $N_R^{\prime }\epsilon$, while the cavity pressure difference drops by $|N_K^{\prime }|\epsilon$. Under the displayed inequality the cavity system leaves a common effective pressure higher than the reduced-flux channel can sustain. Excess creep closure then decreases channel discharge further, amplifying the perturbation.