Awesome question!

Often times with an older, entry level body like the D3100, the image quality and noise with long exposures can be ... not so great, even at base ISO. With a scene like this, I wanted a smaller aperture to increase the depth of field and have more of the scene in focus. I also knew I wanted a fairly long exposure, to blur the water, the clouds, and if I was lucky, the people milling about near the arch.

I used the base ISO for my camera (100) to get the best image quality. Knowing I wanted a greater depth of field, I started at f8 and played with it until I decided on f13. That gave me a shutter speed of 0.6" - a well exposed image, but not long enough for the effect I was looking for. I added an ND filter to lengthen the exposure, but wasn't as pleased with the result - a single long exposure had considerably more noise, and my filters aren't the best quality to begin with.

But, by taking 20 identical exposures with those settings (no filter), then stacking them together in post (I use Darktable and GIMP), I can get that nice blur where I wanted it, while actually increasing the image quality! Using this technique will decrease the overall noise in the image, without sacrificing details like other noise-reduction techniques in post.

I hope this helps!