Wednesday, February 6, 2013
On Monday, I attended a seminar given by a faculty member in geology from just down the road at SUNY (State University of New York) Cortland. Cortland is a small city located about 15 minutes up the road from Freeville. The talk was about a relatively rare type of flood that I wasn't familiar with - groundwater floods. They happen in places where the water table can rise to the surface, and floodwaters come not overland from rainfall runoff, but from underground. There are a couple of creeks in the Cortland area - most notably Otter Creek, which runs close to Routes 13 and 281 at one point - that are susceptible to this kind of flooding.
That the Otter Creek watershed contains water that might well to the surface is not new information. Cortland's water supply comes from what used to be known as Otter Creek Springs, not far from the modern-day campus of SUNY Cortland. Water from this area, long fenced off and known as the Waterworks, is pumped into the water tower, and from there into Cortland homes. In the Waterworks, as you can see in the photo above, the stream channel is maybe 100 feet wide, and the surface water normally meanders through the stream bed. But just downstream of the Waterworks, to the northeast, is the Broadway district. Here, the stream has been straightened and channelized, forced into a concrete bed that runs next to a road (Broadway St) and alongside a neighborhood with a population nearly equal to that of the Village of Freeville (about 500 people). The channelization, restricting the stream, makes this area especially vulnerable to flooding. Too much water, and it's pouring over the roads and into people's basements. This area has flooded twice in the last ten years, and the folks at SUNY Cortland are awaiting the next flood so they can better understand how the process works.
How does a groundwater flood work? Well, it goes back to geology. It requires a porous basin. In the case of Cortland, this area was once a large outwash zone for the glaciers that covered much of the surrounding area, including Freeville. As a result, Cortland and the surrounding area is very flat. The course of Otter Creek doesn't have much of a gradient (or slope), once you get out of the hills that feed into it. Also, Otter Creek is not just a surface stream. There are a couple of points at which the stream disappears underground, and the water diffuses out into the groundwater system. When it rains enough, the water table - the level at which the ground is saturated - rises to the surface, at least in the stream channel. This takes some time. In the most recent flood, it took roughly four hours for the heavy rain to be concentrated into a flood wave. Most flash floods that are caused by runoff from the surface occur within an hour or two of the rainfall in a basin the size of Otter Creek. The SUNY Cortland folks wonder if a pressure wave might be the cause of the groundwater floods. Imagine squeezing a half-empty tube of toothpaste from the bottom...it takes some time for the toothpaste to become concentrated enough, as pressure builds up from behind, to surge out of the tube...but when it does, it often squirts out quickly.
Is there anything that can be done? I believe the City of Cortland is constructing a wall in the Waterworks that would divert some of the Otter Creek floodwaters. In the Waterworks, especially during a flood, water is added to the flow from the subsurface. The idea is to store some of this water in the Waterworks area rather than sending it all downstream into the Broadway district. The wall will send some of this water to the east, where it will end up in a duck pond. With the wall diverting some of the flow, this water will be stored for up to several days, and the amount of water flowing on into the Broadway channel will be reduced. The hope is that this alleviates the flooding immediately downstream of the Waterworks, where it tends to be worst.