The way you have shown it, the boundaries are rather complicated, and not specifically determined, functions. Do you mean to approximate them by circles? If the upper boundary, for example, is a circle with radius r, deflection wpk, and chord 2a, then we can look at the right triangle, with one vertex at the center of the circle, having one leg of length r- wpk, one leg of length a, and hypotenuse of length r. By the Pythagorean theorem, we have a^2= r^2- (r- w_{pk})^2= 2rw_{pk}- w_{pk}^2. More importantly, the angle at the center of the circle is arccos((r-w_{pk})/r) so we have a sector of a circle of radius r covering an angle of 2arccos((r- w_{pk})/r). The has area 2arccos((r- w_{pk}/r) r^2.
For the outer arc, you have the same thing except that instead of "r", the radius is "r+t". Use the same formula, replacing r by r+ t, and subtract the two areas to find the area you want. There is no Calculus required here.
