If you've driven by the Civic Campus recently you have no doubt seen the giant mound of dirt that sits where the old City Hall was located. While some of us would very much like a new City Hall to emerge from the pile, that's not what's going on at the site.
That large pile of dirt is from the large hole that used to be the library parking lot. What's being constructed is a stormwater infiltration facility.
Why do we need stormwater facilities?
When a forested area is developed what was previously trees, brush and undergrowth is turned in to roofs, asphalt, and concrete. Before it was developed, rain would fall and be soaked up by all the vegetation or infiltrate in to the soil. After it is developed, rain falls and is concentrated by downspouts and curbs and eventually in to pipes that take the water away. Well it turns out that there are some pretty significant negative impacts that happen when there is all this extra water with nowhere to go. Urban flooding and erosion are the most visible. Additionally, the water is now full of pollutants from cars and fertilizers. Up until the 1980's developers didn't have to do anything to mitigate these negatives impacts of development. Since then, regulations have become more and more stringent to try to more closely mimic the historical quantity and quality of the stormwater.
Storm water ponds, vaults, filters, rain gardens, and swales are all tools that developers and municipalities now use to prevent flooding and water quality problems. Our Town Center neighborhood is made up of mostly properties that were developed when there were no stormwater requirements. What the new infiltration facility will do is reverse some of the negative effects of the older development in the neighborhood by storing stormwater in the infiltration facility during storms and infiltrating it in to the ground.
What does this thing look like?
The infiltration facility is made up of seven large 10' diameter corrugated metal pipes. The pipes are perforated; they have 3-inch by 1-inch slots all throughout. In between, above, and below the pipes is gravel. On top this pipe and gravel chamber is about 10-feet of soil and on top of that soil will be the new library parking lot. See the images below for a diagram of what it will look like.
When it rains, stormwater from an area of about 13 acres in the Town Center neighborhood will be piped to the facility. It will fill with water while also infiltrating water in to the ground over an extended period of time. This facility will reduce downstream flooding and erosion. Additionally, the act of infiltrating the water in to the ground also cleans the water.
The facility will be somewhat of an incentive to future developers because it will partially fulfil some of the stormwater requirements for future redevelopment.
If you get a chance, try to walk or at least drive by the 25' hole in front of the library soon, it's quite impressive.
Here's more about the project from MLTnews. Stormwater facility construction to impact MLT library parking