\documentclass[tikz]{standalone}

\usepackage{pgfplots}
\usepackage{amssymb} 

\pgfplotsset{compat=1.14}
\usepgfplotslibrary{fillbetween}

\begin{document}

\begin{tikzpicture}
    \begin{axis}[xmin=0, xmax=1050, ymin=0, ymax=550, width=10.5cm, height=5.5cm, ylabel={$z$}, xlabel={$y$}]
        \addplot [name path=f, domain=0:1000, color=black, forget plot] {100^2/20-10*x^2/(2*100^2)};
        \path[name path=axis] (axis cs:0,0) -- (axis cs:100,0);

        \addplot [thick, color=blue, fill=black, fill opacity=0.05, forget plot]
            fill between[of=f and axis];

        \addplot [color=red, domain=0:1000] {-0.5 * 10 * x^2 / (100^2) * (1+0.577^2)+x*0.577};
        \addplot [color=blue, domain=0:1000] {-0.5 * 10 * x^2 / (100^2) * (1+1.732^2)+x*1.732};
        \legend{$\alpha=30^\circ$,$\alpha=60^\circ$};
    \end{axis}
\end{tikzpicture}

\end{document}
