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Archive for September, 2012

Great minds think alike

Anthony Alvarado was also impressed with the CLF videos and commented on them on his Hydraulically Inclined blog.

http://ayresriverblog.com/2012/09/28/looking-back-at-tropical-storm-irene-what-went-right-video/

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I’ve read quite a few reports and postings about the devastation that Tropical Storm Irene brought to Vermont. It was with great pleasure that I just ran across this page:

http://action.clf.org/site/PageNavigator/hurricane_irene_anniversary.html

Being an alumnus of Middlebury College, I was quite interested to hear about how Otter Creek responded to the storm. When I heard about the wetlands storing so much flood water and that the discharge at Middlebury was actually less than the discharge in Rutland, I was a bit taken aback. I had to check out the gage data based on what Mike Kline pointed out in the video.

Figure 1. Mean daily flow for the Otter Creek at Rutland and at Middlebury

Figure 2. Unit Discharge (cubic feet per second per square mile) for the Otter Creek at Rutland and at Middlebury.

Both Figures 1 and 2 tell an extraordinary story as was pointed out in the CLF video. On August 29, the folks in Rutland were seeing the river at 13,500 cfs whereas in Middlebury, the river was at 3,700 cfs. The next day, the Otter Creek was already receding in Rutland, whereas in Middlebury, the river was still slowly rising, due to all the stored water in the wetlands between the two sites slowly releasing that large volume of water. The Otter Creek at Middlebury doesn’t peak until September 2, three days after the storm! So while Irene has moved up into Canada and the weather in Middlebury was turning sunny, the river is still rising due to all the stored water slowly being released. Figure 2 tells the same story as figure 1, the only difference is that the data have been divided by each gage’s drainage area. The unit runoff in Rutland peaks at 44 csm, which, for this part of Vermont, is a large amount of runoff per unit of area. That the Otter Creek only gets up to 9.7 csm at Middlebury is simply another way of recognizing how much water was indeed stored in those wetlands between the two gages. That those wetlands shaved off roughly 34 csm of runoff should make every resident living within the Otter Creek floodplain extremely grateful for their presence. Intellectually, I’ve always known that wetlands can store floodwaters, but to see an effect this dramatic is something I’m finding rather astonishing.

If all those wetlands didn’t exist, one could make some very basic assumptions and ‘play’ with the data to simply get a very rough idea how bad things could have been in Middlebury and points downstream. The drainage area for the Rutland gage is 307 mi2 and the drainage area for the Middlebury gage is 628 mi2 so the drainage area for the Otter Creek at Middlebury is essentially twice that of Rutland. If we simply take the Rutland mean daily flows and multiply them by two, you can get an idea how bad the devastation could have been further downstream.

Figure 3. Synthetic hydrograph for the Otter Creek at Middlebury vs actual flows at Middlebury.

The synthetic hydrograph in Figure 3 is clearly a worst case, ‘what if’ scenario. It assumes that the Otter Creek watershed at Middlebury generates the exact same amount of unit runoff as the watershed does at Rutland, which we know is clearly not the case. What the synthetic hydrograph points out however is that had those wetlands not been storing so much water (in other words, had those wetlands been drained and developed), it’s conceivable that downtown Middlebury and points downstream could have seen discharges north of 20,000 which as Louis Porter points out in the video could conceivably destroy a bridge and certainly numerous more homes would have been flooded out.

Figure 4. Google Earth screenshot of some of the floodplain wetlands along Otter Creek.

That such large tracts of land have been protected and are allowed to flood so frequently (Figure 4) is a testament to the land protection that has occurred in Vermont. Hopefully this story can be used to promote the state’s Fluvial Erosion Hazard Program and various land protection groups can continue to protect the swamps and wetlands within the Otter Creek floodplain.

h/t: Laura Wildman, ASCE Restoration TC Group on LinkedIn

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