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Archive for the ‘hydrology’ Category

Back in the 1970’s and 1980’s, the U.S. Army Corps of Engineers had the foresight to know the value inherent in floodplains. Consequently, over the course of several years, this agency went about purchasing floodplain land in the Charles River watershed. The Army Corps coined the term “Natural Valley Storage Areas” (NVSA’s) which is indeed what intact and functional floodplains do. The homepage for this project can be viewed here.

Fast forward to 2017 to when the Army Corps presented their Draft Master Plan for the project to the public. The Corps conducted an economic analysis of the benefits of these storage areas. The estimated prevented losses from flood damages was almost $12 million in unadjusted dollars. The Corps also estimated the economic benefits of visitors and recreation between $3.2 an $4.6 million.  Based on this analysis, one could say the natural capital in these areas protected areas is at least $15 million.

At the public meeting,  the Army Corps presented the hydrograph for the 2010 floods in late March and early April. As soon as I saw the data, I was reminded of the Otter Creek analysis I had written about.  My only problem with the analysis that the Army Corps presented, was that the discharge was not normalized by drainage area.  After obtaining the data from the gauges in Dover, MA and  Medway, MA, the story appeared to be pretty similar to Otter Creek’s.

The data show that for the upstream gauge in Medway, the peak unit discharge was 26.8 cubic feet per second (cfs) per square mile (csm) on March 31, 2010. Note how much more rapid the rising limb of the Medway unit hydrograph is compared to the Dover gauge.  On April 1st and 2nd the Medway runoff is receding, whereas the Dover runoff is still increasing, albeit at a slow rate.  The peak discharge in Dover occurs on April 2nd, but only at a unit runoff of only 15 csm.  Just as with the Otter Creek analysis, we can do a ‘what if’ analysis and simply assume that Dover had very similar unit runoff as Medway. Under this hypothetical scenario, Dover could have experienced a discharge of just over 4,900 cfs (26.8 csm X 183 square miles).  Fortunately, the peak mean daily discharge at the Dover gauge was 2,760 cfs, which is 56% of the theoretical peak, had the same unit runoff occurred throughout the watershed.  The two USGS gauges and the Natural Valley Storage Areas can be seen in this ArcGIS Online presentation slide.

A cursory review of the NVSA’s indicate that while floodwaters can and indeed are stored, it would be perhaps a bit too generous to attribute all of the flood attenuation between Medway and Dover to these areas. The 8,095 acres of storage area converts to 12.6 square miles, or roughly 6.9% of the Dover gauge’s watershed area.  In addition, at only 65 square miles, one would expect a certain amount of flashiness (i.e. rapidly increasing and decreasing flows) at the Medford gauge.  Nevertheless, as a thought experiment,  had the Natural Valley  Storage Areas been developed and the percent impervious area dramatically increased in these areas, one could safely assume that  the Dover gauge would very likely have seen a unit discharge higher than 15 csm.  The NVSA’s do provide important floodwater storage and the Army Corps analysis does indeed indicate a diminution in flood damages.

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I don’t use this blog too much for advocacy, but on March 1, 2016 the EPA and USGS released a draft report whereby public input is being solicited. Have a read and let your scientific expertise on ecology, hydrology and anthropomorphic alterations to flow be heard:

http://www.regulations.gov/#!docketDetail;D=EPA-HQ-OW-2015-0335

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I live in Massachusetts and really am not terribly knowledgeable about Florida surface water hydrology. I do know that it is a flat state, it has karst geology and that groundwater plays an important role.  I lived in Northern California for a number of years and spent a good bit of time reviewing USGS gage data. The coastal terrain tended to be steep and storms off the Pacific were capable of dropping significant amounts of rain.  The first time I saw a gage’s stage reading increase by over 15 feet in less than a day I was rather surprised, but over time I became more comfortable seeing such rapid stage increases.

Florida recently experienced some staggering amounts of rainfall. I heard reports indicating 22 to 26 inches of rain falling. As such, I had to go to the USGS NWIS Florida site to get a handle on how the rivers were responding.  As of this writing (May 1st, 2014) a number of gages are currently coded in black which the USGS labels as ‘High’). The Shoal River response caught my eye as it reminds me of responses that look like a Humboldt County California gage might look after a Pacific storm hits.

The Shoal River near Mossy Head (drainage area 123 mi2) was running at 348 cfs (2.8 csm) the morning of April 29 and peaked during the late afternoon on April 30 at 7580 cfs (61.6 csm) .  You can also see that the USGS sent hydrographers to the gage during just prior to the peak. Well done USGS and I hope that life and limb were not risked to obtain these data.

348 cfs to 7580 cfs in a little over a day and a half

348 cfs to 7580 cfs in a little over a day and a half

The stage change in 39 hours was just shy of 14 feet!

Nearly 14 feet of stage change in a day and  half

Average stage increase on the rising limb on April 30 was 8.4 in/hour

As impressive as this storm was, the historic data at this site indicate five events that were larger than 8,000 cfs. Even though roughly two feet of rain fell, other storms and antecedent conditions in past have led to even greater storm runoff.

April 30th flood will be the new flood of record

The data indicate five previous floods that were larger

As fascinating as I find these data, this is the classic case of what I refer to as the “hydrologist’s dilemma”.  We find these rare events exciting and interesting, yet at the same time, many people are suffering and are experiencing a life changing natural disaster.  It goes without saying that my thoughts and prayers are going out to the people in Florida who are now facing the challenge of a post flood situation.  May your fellow neighbors, place of worship, elected officials, local businesses and insurance companies all be a source of inspiration and may you be a more flood resilient community in the end.

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This looks like a great resource for water resource managers, urban planners and hydrologists:
http://hydrology.rice.edu/index.html
T
hank you to the authors Dr.  Bedient, Dr. Huber, and Dr. Vieux

 

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Two water related organizations are planning conferences in Hartford, CT in June 2013.

The Association of State Floodplain Managers has scheduled its annual conference for June 9-14.  My former employer, Milone & MacBroom, will be participating in the conference has terrific and knowledgeable staff that will be engaged in the following events:
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Monday, June 10

WORKSHOP: Recovering From Large Floods, 8:00 AM to Noon

James MacBroom, P.E., and Roy Schiff, P.E., Ph.D., will lead a dynamic workshop focusing on post-flood assessment, design, and construction using specific examples from our work in Vermont and New York during the 2011 and 2012 hurricane seasons. In addition to short lectures, interactive activities will be conducted to help attendees gain experience in identifying risks and strategizing flood recovery efforts.

FIELD TOUR: Coastal Connecticut, 8:00 AM to 1:00 PM

Join David Murphy, P.E., CFM, for a tour of neighborhoods in Guilford facing problems relating to sea level rise and recent storm surges that are each pursuing different methods of adaptation. The tour will continue to Madison with a stop at the state’s premier Hammonasset Beach.

Tuesday, June 11

CONCURRENT SESSION: Building a Community Coastal Resilience Plan in Guilford, Connecticut, 1:45 PM

David Murphy, P.E., CFM, will assist the Town of Guilford in discussing the town’s community coastal resilience planning efforts and the results, including adaptation strategies that are already being implemented.

Wednesday, June 12

FIELD TOUR: Coastal Connecticut, 8:00 AM to 1:00 PM

The coastal tour from June 10 repeats on June 12.

CONCURRENT SESSION: Soliciting, Screening and Selecting Mitigation Projects Part II: When the Process is Repeated After a Disaster, 1:30 PM

David Murphy, P.E., CFM, will share his experience in evaluating and prioritizing competing hazard mitigation projects that were developed in Western Massachusetts following the floods of Hurricane Irene.

Thursday, June 13

CONCURRENT SESSION: Two Floodplain Restoration Case Studies, 10:30 AM

Roy Schiff, P.E., Ph.D., will discuss two floodplain restoration projects he completed in Vermont.

CONCURRENT SESSION: Flood Mitigation and Economic Development: Meriden’s Journey to the 21st Century, 4:00 PM

Nicolle Burnham, P.E., CFM, and the City of Meriden will present the Harbor Brook Flood Control and Mitigation project, which includes three miles of channel improvements and daylighting 1,700 feet of culverted channel through the city’s downtown.
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The other conference being held in Hartford is sponsored by the American Water Resources Association which is organizing a specialty conference on environmental flows.

I won’t be able to attend either event. I have taken the approach of directly contacting presenters. It’s usually pretty easy to obtain a presenter’s email address, I then ask them for their PowerPoint poster or slides, sometimes there is an associated paper, and almost without exception, I have been able to receive the information I requested. Generally speaking, presenters seem to be happy when someone shows interest in their work and are asking for follow up information.

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I have reviewed and analyzed USGS mean daily flow data for several years and I have settled into a bit of routine once I save the text file from the NWIS website for a given gage.  This file (https://www.box.com/s/9x7y9rpi6lpnhis3kny6) shows the steps I take to generate a flow duration curve for the Connecticut River at Montague City (#01170500).  This gage happens to have a very nice 108+ year record; clearly this is on the high end for most gages.  One of the first steps I take at this stage is to obtain the data based on water year (WY). As such, I will enter 1904-10-01 for the begin date and 2012-09-30 for the end date.  I always save the data as ‘Tab-separated’ and make sure that the file has a .txt extension.

Once I have the text file, I import it into Excel and the tab separated format makes the import process a breeze.  The next step is to simply copy the date and discharge data and paste the data into a separate tab.  You should be able to run the macro called: Year_Month_Date and this will calculate the year, month and date for each flow value.  Once that is done, I can then generate a pivot table using Year for Columns, and Month followed by Date for the rows.   Since I’m using 24hr mean daily data, I can use the sum function for the flow data, but if I had 15 minute or hourly data, I would use the average of discharge.  Once this table is made, I then copy and paste it into yet another tab.  If the data are pasted into cell A1, then the delete month macro should quickly delete the twelve extra rows that were generated in the pivot table.  It is at this stage that I think the fun analysis can begin.   The PercentileCharts tab shows two different flow duration curves, one for the April 1 to May 30 flows and another one for the June 1 to July 31 flows.  I use the ‘Percentile’ function to generate the data.  When using this function, remember that for the percent of time a flow is equalled or exceeded, you have to use 1-the percent of interest for the data to be plotted correctly.  These dates are somewhat arbitrary but if you are engaged in perhaps a fish study, you can tailor your dates to a specific life stage (e.g juveniles, adults, migration seasons).  If you know that certain decades or time periods were wetter or drier than others, you can modify your array selection for certain years.

While I don’t generate a hydrograph based on percentiles very often, I thought I would include how the percentile function can be used to create an annual hydrograph based on percent exceedance of interest.  It’s simply another way to explore and think about the data.  I plotted the data on a log scale simply to spread out the low flow data, it’s a bit difficult to discern on a linear scale.

All of the steps mentioned above can be done in a matter of minutes.  The internet and Excel make these analyses rather quick and straightforward.  Comments on these techniques or your own methodology are welcome!

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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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