how to construct a spillway for a lake?

Answer 1

Most of the spillways in our area are constructed of concrete.

If you are constructing your own spillway and do not want to use concrete, You could use a layer of Rip-Rap rock layed over a geotextile net, and use a shallow slope.
A Rip Rap bed would probably run about $6/ Sq. Ft., so might cost more than concrete?

See Wikipedia: http://en.wikipedia.org/wiki/Riprap

Riprap (also known as rip rap, revetment, shot rock or rock armour) is rock or other material used to armor shorelines or stream banks against water erosion. Riprap reduces water erosion by resisting the hydraulic attack and dissipating the energy of flowing water or waves. The shape of rock is important. Coarse, angular rock, usually made by crushing or blasting, or from scree, is more effective at ground reinforcement than round river rock. A correct mixture of aggregate size can also aid riprap's ability to create an interlocking structure.

Riprap is graded by size. A specified diameter will have fifty percent of the rock (by weight) larger and fifty percent smaller. The velocity of water flow is generally the determining factor for size of stone. Riprap varies in size from several centimeters to cast concrete shapes several meters across. The size and material will be specified by a civil engineer or local building code.

Riprap is often used in conjunction with a geotextile or in gabion baskets.