Modify code accordingly.
<script type="text/javascript">
function redirection()
{
setTimeout("top.location.href = 'your url'",5000);
}
</script>
call this function from code behind wherever you want user to be alerted and redirected.
string script = "<SCRIPT LANGUAGE='JavaScript'> ";
script += "redirection()";
script += "</SCRIPT>";
Page.RegisterClientScriptBlock("ClientScript", script);