Added documentation for alpha calculation

OpenHPL/Waterway/Gate.mo

@@ -83,6 +83,15 @@ With
 <dt> $$ \\psi_{90^\\circ}= \\frac{1}{1+0.64\\cdot \\sqrt{1-(a/h_0)^2}} \\tag{8.25}$$ </dt>
 <dd>Contraction coefficient radial gate (for \\(a/h_0 \\rightarrow 0\\))</dd>
 <dt> $$ \\psi_0(\\alpha)= 1.3 -0.8\\cdot\\sqrt{1-\\left(\\frac{\\alpha -205^\\circ}{220^\\circ}\\right)^2} \\tag{8.25a}$$ </dt>
+<dd> With the edge angle \\(\\alpha\\) of the gate </dd/
+<dt> $$ \\alpha = \\left( \\frac{\\pi}{2} - \\arcsin(\\frac{h_h-a}{r})\\right) \\cdot \\frac{180^\\circ}{\\pi} $$
+<blockquote>
+<ul> 
+<li> \\(a \\ldots\\) Vertical gate opening </li>
+<li> \\(h_h \\ldots\\) Height of the hinge above gate bottom\"</li>
+<li> \\(r \\ldots\\) Radius of the gate arm </li>
+</dt>
+</blockquote>
 </dl>
 
 <h5>Backed-up discharge</h5>