The easiest way of doing this is create a div right after the navigation and make curved png with Photoshop and apply it as background to that div. So in that way you have rectangle navigation and div with curved background after navigation.
.nav-bot{
background: url(https://s23.postimg.org/jakqjag8r/nav_bot.png);
background-size: 100% 100%;
background-repeat: no-repeat;
width: 100%;
height: 20px;
}
Here is example fiddle.
EDIT
If you don't want to use image there is another way to do this Using css gradient and box-shadow.
.nav-bot{
background-image: -webkit-linear-gradient(top, rgba(0, 0, 0, 0) 50%, rgba(0, 0, 0, 0) 50%);
width: 104%;
margin-left: -2%;
height: 50px;
border-radius: 50%;
margin-top: -19px;
-webkit-box-shadow: 0px 6px 15px 0px rgba(0,0,0,0.75);
-moz-box-shadow: 0px 6px 15px 0px rgba(0,0,0,0.75);
box-shadow: inset 1px 3px 5px 0px rgba(0,0,0,0.75);
}
Here is example for that.
Hope this helps you.