Liberty Dam

This view shows the Thematic Mapper bands 4, 3, and 1 combined as red, green, and blue. Each pixel is magnified by a factor of 2.

The dark area of this image is the southern part of Liberty Reservoir. This view shows MD Route 26 crossing it in two places. The small white feature at the lowest part of the reservoir in this image is Liberty Dam. The site of dam is open to the public (no fishing south of Rt. 26) and may be reached by hiking along a paved access road for about 2 miles up and down some fairly good hills. A public parking area is available just off of Wards Chapel Road. Wards Chapel Road crosses Rt. 26 near the upper right corner of this image (head south from Rt 26 to the parking area). The road to the dam may be seen in the image as a cut through the trees to the upper right of the dam itself. On the hillside overlooking the dam is a large grassy area with shady pine trees.

Liberty Dam impounds the North Branch of the Patapsco River. The dam is an impressive piece of engineering. Bronze Plaques give the following information: 

     Work Started:   Sep. 1951
     Work Completed: Sep. 1953
     Contract Cost:  $4,900,000

     Patapsco River
     Safe Yield: 95 MGD
     Avg. Stream Flow: 167 CFS
     Spillway Design Flood: 46,000 CFS

     Reservoir
     Surface Area: 3,106 acres
     Length: 8.5 miles
     Perimeter: 82 miles
     Watershed Area: 164 square miles.
     Storage: 43 billion gallons
     Land Aquired: 9,200 acres

     Dam
     Height: 160 ft
     Length of Spillway: 480 ft
     Length of East Abutment: 92 ft
     Length of West Abutment: 132 ft
     Max Thickness at Base: 134.5 ft
     Elevation of Crest: 420 ft MLT
     Total Concrete: 162,000 Cu. Yds.
     Total Excavation: 212,000 Cu. Yds.
     Steel: 417,000 lbs

My wife copied the above numbers down while my brother-in-law and I climbed down the side of the dam. The sides are a series of giant steps. I took a 100 ft tape measure along: the steps are 12 ft 7 in wide, and 9 ft 3 in high. There are 11 steps on each side, I assume they are all the some width and height (they look like they are). I took a few pictures which I'll enter here when they come back.

The Liberty Dam Echo

My main goal for climbing down into this area was to try to capture a very interesting echo effect I discovered on my previous visit. I estimated the echo of a loud clap to take roughly 10 seconds to fade out. The echo gets replicated by the large steps on the far side, each of these copies gets split by the steps on the near side, and so on, until the sound becomes almost a buzz. I took a pocket tape recorder along to try to capture the echo and analyze it on a computer but my recorder has too much noise. I'll try again with better equipment. In the meantime you can think about what this structure does to a pulse of sound. It takes just under half a second for the sound to reach the far side. It bounces off the steps there, getting split into multiple returns separated by about a hundredth of a second. There are actually 12 reflecting surfaces on each side counting the wall above the steps, so the original pulse gets spread out by about a tenth of a second on each side. After just about 2 or 3 round trips, that is, about 2 or 3 seconds, the original sound pulse is split up into a train of subpulses that extend completely across between the steps and gives a continuous buzzing sound until it fades out. The number of times the original pulse gets split grows very quickly. Taking each round trip to be about a second, if the echo lasts 10 seconds that means it makes 10 round trips. Taking roughly 10 steps on each side means the sound is replicated about 10 times each crossing, giving roughly 10^20 splittings. It's hard to be much more precise since much of the sound that hits the upper steps must escape. A good sound recording will help pin down the numbers better.