Archive for the ‘Science’ Category

Scene of the (almost) crime

Remember that discussion from yesterday about stormwater pollution and NPDES permits?  I have to admit, I’m a cynical soul and I’ve had my doubts that the public education requirements would make any real difference.  Tennesseans don’t like the government telling them what is good for them.  So I was actually pretty surprised at a bit of drive-by green heckling I witnessed the other day.

To entirely appreciate this story you have to know, First Avenue in Nashville is essentially the back alley for the tourist traps and bars on Second Avenue.  All the businesses have their dumpsters back there and a lot of random garbage ends up on First and the sidewalks are sometimes unpleasant.  (It’s a really odd choice since First fronts the river with the stadium on the other side and has the potential to be really nice, but that’s a story for another time.)  It’s also a large downhill grade that bottoms out at Broadway, the other tourist trap of downtown Nashville.  So all the garbage from both often ends up at First and Broadway.

I was idling at the traffic light at First and Broadway watching an employee of Hard Rock Café hosing down the sidewalk around their dumpster.  He had quite a pile of random garbage on the sidewalk and I was hoping he wouldn’t wash it into the drain since it would be floating in the Cumberland River a few minutes later (this is literally 100 ft from the river).  Right about that time a guy in a giant pick-up drives past me leaning nearly all the way out of the window and yells at the Hard Rock guy “Why don’t you pick it up instead of washing it out into the street!”.

I was torn between delight that he noticed and anger at the level of asshole required to yell at some poorly paid employee about something that small.  Even better, right after this happened the Hard Rock employee grabbed a dust pail and started scooping up the garbage.  I don’t know if the heckling had an effect or if that was his plan all along, but I was very happy to see both of them paying attention to such a minor detail.  I’m not going to claim my cynical heart grew three sizes that day, but apparently the education efforts are working.


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Photo by Flickr user dogsbylori

Photo by Flickr user dogsbylori

These days just about every city or town in Tennessee has to have a special permit related to stormwater.  Stormwater is runoff from a rainfall, and the part that goes into those ubiquitous road drains is of particular interest because those drains are direct lines to natural waterways. In heavy urban areas stormwater can be just as polluted as the water coming into a sewage treatment plant but it doesn’t get treated before ending up in the river.

Stormwater falls into an EPA permitting program called the National Pollutant Discharge Elimination System, generally known as NPDES.  NPDES covers A LOT of different pollution sources and you pretty much have to be a specialist to understand all the in’s and out’s of the program, but the important thing for the moment…  A few years back nearly every Tennessee city and town of any size was required to get a permit for their stormwater systems (some larger industrial sites and other entities had to get their own permits as well).  The permit requires the municipality to do its best to improve the quality of the storm runoff.  This involves things like litter programs, street sweeping, filters in curb drains, and endless other possibilities.  Cities that have programs for curbside pick-up for fall leaves or grass clippings may even get to count that since it keeps organic pollutants out of natural waterways.

untitledMost municipal NPDES stormwater permits require a certain amount of money be spent on public education.  The idea being that if people know what types of behavior cause pollution they’ll quit doing it.  The earliest efforts involved obvious things like making sure people knew those road drains go to the river because their was a common belief that the road drains went to the sewage treatment plant.  These days towns are getting creative with the public education component.  Farragut (suburb of Knoxville) has one of the more creative outreach and education efforts that I’ve heard about in Tennessee.     One of their programs invites local artists to decorate rain barrels which are then sold to the community.   The watershed signs you’ve probably seen on interstates throughout Tennessee are part of TDOT’s NPDES mandated public education efforts rather than simple government waste as some people think.

It remains to be seen just how successful the public outreach and education programs are.  I’m a bit skeptical, but I did get a free rain gage with Memphis Stormwater Department’s logo on it…

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The free body diagram

The free body diagram is a simple concept that most engineering students learn in their first year of college.  It’s a teaching tool, and the first step toward learning to analyze forces in a high rise building.  I’m going to get a little out of my water/drainage design wheelhouse here and go into an analytical tool used more for structural engineering.

Have I made it sound interesting enough that you’re willing to sit through this next part?  Because it really is an elegant concept.  I’m going to do my best to explain without getting deep enough to put anyone to sleep.  If you get bored, skip to the last two paragraphs. (Feel free to laugh at my MS Paint sourced drawings as well.  It was either that or hand drawn.)

A free body diagram is a conceptual drawing of a physical body with all the forces that are acting on it.  It’s isolated from anything in its environment that doesn’t cause a physical force on it.  The object is at equilibrium (either standing still or moving at a constant velocity).  It’s most widely used as a conceptual tool to teach students how to analyze forces, and generally you use known values to solve for unknown.

The classic teaching example is a block on a surface.

This version is simplistic enough to be of pretty much no use, but it does illustrate an important idea that most everyone instinctively knows but no one actually thinks about.  Gravity pulls your weight down (shown as W in the diagram) while the bulk of the earth holds you up by pushing back (shown as F1 on the diagram).  Since the block isn’t moving, F = W.

fbd2It starts to get more interesting when you incline the surface.  The block is still stationary, but it does introduce another force.  You still have W (weight of the object) and F1 (force holding it up), but now you also have a friction force (F2) keeping the block from sliding down the slope.  Friction is caused by two surfaces pushing against each other and it’s felt at the point where the block touches the surface.

This diagram is overly simplified, but still marginally useful as an example because it illustrates a couple of potential real world questions. (1) How much force do you need to apply to move the block up the incline (A) and (2) how much can you incline the surface before the object slides down it.

Solving either of those requires one last concept.  I’m going to quit for the day (or more likely quit for the week considering my posting frequency) once I get this one done, so stay with me just a little longer.

Notice the weight of the object is applied straight down, but the counteracting force is applied perpendicular to the surface.  Conceptually this means that only a portion of the entire weight of the block is trying to make it slide down the surface.  On a more practical level, we can’t solve the problem while the forces are going in so many directions.  The solution involves breaking the force down into its component vectors. *

* This concept of component vectors is the most basic to any engineering analysis, and it’s why the free body diagram is such a useful teaching tool.

Component vectors are essentially straight lines used to describe a diagonal.  The best example I’ve been able to come up with is directions on a city grid.  Let’s say we want to go from 1st and Broadway to 5th and Church.  As the crow flies that’s fairly straight forward.  But if you’re trying to tell your friend Maggie (who can’t fly) how to get there, you would tell her to go four blocks on Broadway, turn right, and go two blocks on 5th.

mapSo, back to our free body diagram.  W is ‘as the crow flies’ so we have to break it down into ‘four blocks on Broadway (Wx component) and four blocks on Church (Wy component)’.   I’m not going to get into how the sausage is made, suffice to say it involves principles of the geometry of right triangles that’s taught around the high school sophmore level.  I just want everyone to understand it has to be done.  Below is our revised free body diagram with all the forces on our grid system.

fbd3aThe take away here is this:

The weight acts in two directions.  It pushes back against the surface (Wy) and it wants to slide down the surface (Wx).  As you steepen the slope Wx goes up and Wy goes down.  (That’s pretty instinctive.  The steeper the slope, the more likely something is going to slide down it.)  Friction (F2) keeps the block from sliding down the slope meaning Wx is less than F2.  As you steepen the slope Wx gets larger while friction levels off.  When you get to where Wx is larger than friction, the block moves.

Now you have the same training as a freshman engineering student a month into her first Physics class.  From here on they start adding complications like springs, pulleys, diagonal forces, variable friction, and so on with the eventual goal of being able to analyze things like the forces at work in a high rise building frame.

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Today I want to talk about an amazingly useful tool that I’ve used to great effect many times.  Contour maps.  A contour map is a way of visually representing a three dimensional environment on a two dimensional plane (a sheet of paper).  In short, it shows the hills and valleys of the land area on the map.

The basic idea of contour mapping is that the lines are a constant elevation.  So if you were to walk the line depicted on the map you would never go uphill or down.  Each line represents a particular elevation, and the closer the lines are to each other, the steeper the grade.  A circle, or any closed shape, represents a hilltop or sinkhole.  When reading a contour map it’s always important to keep in mind, any sort of V shape means a ditch or channel, and it flows in the direction opposite of the way the V is pointed.

That description probably doesn’t make a whole lot of sense when written long form, but hopefully it will become more clear when you look at the map below.  If you take nothing else from this post always remember, more lines mean more hills and less lines means flatter.

The thin brown lines are contour lines. Thick red and blue lines are added for effect.

Contour maps are used for a lot of things.  In hydrology they’re used for determining watershed boundaries and stream steepness.  They’re indispensable to land developers, and anyone interested in building anything other than a small house, because they help determine the suitability of a plot of land for various uses.  For example, the example map above has a lot of contour lines very close together indicating it’s very hilly.  It’s obviously too steep to farm, and you wouldn’t want to try and put up any large buildings.  A Walmart with a large flat parking area would be out of the question.

The primary use I want to discuss today is watershed delineation for bridge design.  The watershed area is a primary component in determining how much water is flowing to a particular bridge.  The sample map I’ve posted above shows the watershed of Crocker Spring Branch, which is a minor tributary to White’s Creek in northern Davidson County.  I picked this area because it’s local to Nashville and it has lots of hilly topography, making it easier to see for people who aren’t familiar with reading a contour map.

The example is delineating a watershed for the bridge at the bottom right corner of the map.  The overall watershed is in red with smaller interior watersheds drawn in blue.  The general rule is that water flows perpendicular to the contour line.  Determining watershed boundaries is a simple exercise in following the highest ridge.  You start at the bridge location and move out following the highest points.  I started to the left of the structure at the obvious ridge and followed it back from the road.  It tends to get a bit tricky when there are lots of smaller creeks flowing into the mainline, and I drew in several of those for illustrative purposes.  It’s a fairly simple process in a location with easily defined ridges like this, but it can be a bit difficult in flatter areas such as west Tennessee near the Mississippi River.

Here’s an overhead photo of the same area.  It’s very difficult to see much without the contour lines.

I’m going to close with this map clipped from the same general area as Crocker Springs Branch.  (I’ve placed red arrows beside a couple of the hills just to show how the contours close on each other.)  You can see Whites Creek running through the center, with several offshoot tributaries.  The general lack of contour lines around the main creek indicates a fairly wide floodplain and the general steepness of the tributaries indicates runoff is going to get to the main creek pretty quickly.  So based on this map you can conclude that Whites Creek probably floods regularly and it can rise fairly quickly.  You can also see how the roads are generally down in the flatter areas wedged in to one side or another of the floodplain at roughly the bottom of the hills.  This is very common in this type of area because it’s much easier and cheaper to build a road on flattish ground, but you place it at the bottom of the rise so it’s less likely to flood.

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I’ve spent a lot of posts explaining a bit about what engineers do, but let’s dive deep this time.  I’m going to tell you a little about how I spend my days on a project by project basis.  Today I’m going to start with a project that ties into the recent Metrocenter theme I’ve had going on.

After the Corps of Engineers finished raising and rebuilding the Metrocenter levee, they were confident it would stand up just fine to a 100 year storm.  However, someone quickly came to the conclusion that stopping at 100 year protection wasn’t enough given the potential for up to $4 billion.  If Katrina taught us anything, it’s that when a levee fails, it fails hard.  The levee itself is high enough to hold back a 500 year flood on the Cumberland (with the help of the reservoir system), but there are a few holes in the protection at some strategic points.

The primary hole in the flood coverage is actually a bridge on Interstate 65.  If you look at the old maps from my previous posts you can see a drainage stream of some kind and a rail line running under the bridge footprint.  Those may have been in place when the bridge was built in 1969, but these days there’s nothing running under the bridge except a buried gas line.  As best I can tell there may have been two railroad tracks servicing the Marquette Cement Yard, but they were removed when Metro bought the property in the mid 1990’s. Frankly, I’m not sure the tracks were there even then, because the ground under the bridge is at least 25 ft higher than the parking lot just to the north (where a rail line would go). 

The bridge is 20 ft above ground on the Metrocenter side

This is where I come into the picture.  The Corps and Metro asked the state department of transportation if they could build a small ridge under the bridge to keep water out.  It seems the river was backing up the low ground just south of the I-65 embankment and if the river got high enough it could pour through the bridge and into the low ground inside Metrocenter.  During the process of getting this project approved, the May 2010 flood happened and the Corps had to call out volunteers to lay sandbags under the bridge.  They tell me that water got up to the bottom layer of sandbags.

The low spot where water comes up from the Cumberland

My job as a hydraulic/bridge engineer was to look over their plans and make sure that the bridge wouldn’t be compromised and the project wouldn’t create a new flooding issue.  It was a fairly simple assessment.  The area under the bridge was already a high point, it just needed to be raised a little further, and the construction plan just consisted of bringing in soil and compacting it in the right spot.  With the rail line gone the bridge could be completely torn out and filled in if not for the traffic disruption it would cause on I-65.  So state approval was simple enough.

The major problem arose because the Interstate system is actually owned by the federal government.  States only manage them on behalf of the federal government, so this project required approval from the Federal Highway Administration.  They just happen to have a policy against using road embankments as a levee despite the fact that this one already is being used that way.  It took a lot of negotiation (the bureaucratic equivalent of slamming your hand in the car door) but the project was finally approved and construction is essentially complete.  Between this project and Metro’s efforts to replace the pump system Metrocenter is even more protected than it was during 2010.

The final product.

So there you have it.  One project in my life.  It started out quite interesting and ended up with fingers stuck in car doors, but it’s actually one of the simpler projects I’ve been involved with.  Mostly because someone else was doing all the design work.

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Now that I’ve showed you a practical example of a levee system, let’s discuss a technical challenge that a levee system has to address and a short case study using the Metrocenter levee and the biggest  flood to occur in Nashville since it was constructed.

Take a look at the map above.  The red shaded portions represent the heavier urban areas on higher ground south of Metrocenter while the white and green portions are the lowland floodplain areas.  You can see a major stream immediately west of Bush Lake, and another running along the bottom of the hilly area and going into the river next to the Bordeaux Bridge.  You can also see a swampy area near the center of the floodplain area.  In order to provide flood protection you need high ground or the human built equivalent. 

By 1968 the embankment for I-65 (then referred to as I-265) has closed off the southeastern gap between the southern hillside and the river, and the southwestern gap is closed off by the Clarksville Pike embankment.  Roadway embankments aren’t built to be impermeable to water the way a good levee is, but that isn’t necessarily obvious to a non-engineer and they do provide a pretty decent physical impediment to water flow (but more on that later). 

When the levee was built in the early 1970’s the arc was completely closed off.  At this point we have the entire area enclosed by higher ground and we can assume it’s protected from the river.  But, and this an important, we have essentially built a hole that water can’t get out of.  The river is kept out, but anything that does get inside our protection is going to be stuck in there.  That water comes from a combination of rainfall inside the levees, and runoff from higher ground to the south.  It’s not that much water compared to the Cumberland River, but if we don’t get it out it will build over time.  (And interior rainfall can become a problem quickly since so much of the area is paved or covered by building roofs that don’t allow water to percolate into the ground.)

This is what the lake in the center was most likely built to deal with.  The interior drainage is diverted to this lake and a pumping system is provided to pick up the water and dump it over the top of the levee into the Cumberland.  If the pumps do their job properly then all is well.  If the pumps can’t get water out as fast as it comes in, the lake gets bigger and water starts to back up along the canals and ditches inside Metrocenter.  Eventually you reach a break point where things start getting flooded.  The design difficulty is figuring out how much pump capacity to install.  Pumps are expensive, and most of what you’re paying for won’t get used unless there’s a flood.  So you end up having lots of expensive pumping gear sitting idle unless there is a major flood.  That’s not the kind of thing real estate developers want to spend money on.  Not only do you have to buy it, but you have to do keep it in working order, which requires periodic maintenance even if they haven’t been used.

During the May of 2010 flood in Nashville there were reports of some flooding inside Metrocenter.  A significant amount of flow also came in through the road embankments.  When I-65 was built the designers knew there was a flooding problem in the area, so the road was built on a base of rock that allowed water to flow in and out without damaging the road or interrupting traffic.  There was no levee back then and no expensive development to flood so a little water flowing through the embankment wasn’t a big concern.  The rain falling inside Metrocenter, runoff coming down the hill from the south, and water seeping in from the road embankments combined so that a lot more water was coming in than the pumps could handle and caused some flooding inside the levee system.

I’m not slinging any blame here, it’s a problem you have to expect when dealing with large areas behind a levee.  The original pump station was built in 1970 and it just didn’t have the capacity to handle the water coming in.  Metro is currently in the process of expanding the pump station and doubling capacity to handle a 500 year rainfall event.

This was also a significant problem for New Orleans for months post Hurricane Katrina.  Once the water gets in the hole, it’s hard to get out and New Orleans had waaaay more water since their levees didn’t hold up.

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Metrocenter through the years

As a follow up to my last post on some of the history of development and the levees at Metrocenter I wanted to post this sequence of maps from the US Geological Survey.  They’re all on the same scale and cropped to roughly the same area.  You can find them for yourself at the USGS map store.

This is the 1952 map, and it’s the base map all the others get their contours from.  I want to point out Cumberland Airfield in the left-center area, along with the big swamp and the railroad spur.  You’ll also want to note Bush Lake.  A helpful commenter from the Nashville Scene tells me Bush Lake was originally a quarry created by  W.G. Bush & Company.  In a few paragraphs the location of Bush Lake will be important.

Next up we have the 1968 map.  Bush Lake is still there, but Cumberland Airfield is gone and the swampy area in the center seems to have been drained with the exception of a few small ponds.  You can see the proposed location of what is now Rosa Parks Boulevard on the hill south of Buena Visa Park.  This would have been after Cheatham Dam was built, but before the levee.

Now we have the 1983 map with the purple items showing what has changed since the previous map.  This would have been 5-10 years after the levees were built.  Bush Lake is gone and several buildings are where it used to be on the eastern edge of the area.  Most of the streets and drainage canals are in place but only the eastern side seems to have many buildings.  The western portion is now the site of a golf course.  Rosa Parks Boulevard has been built, but it’s north of Buena Vista Park and at the bottom of the hill rather than the proposed location from the previous map.  (I assume this was changed in order to avoid the heavy residential area on top of the hill.)  The large purple blob in the center is much as it is today.

The last map is from 1997 and is largely as it appears today.  Bush Lake is mostly gone with all the buildings along Great Circle Road built over its old location.   If you look closely you’ll see a sliver of blue still on the old Bush Lake site.  Google maps still calls it Bush Lake but these days it’s not much more than a stormwater detention pond.

That’s the geographic history of the Metrocenter as best I’ve been able to trace it.  I’m particularly bummed that the earliest map was 1952, but this type of mapping requires aerial capabilities that weren’t really available for civilian use until post World War II.  If you’re so inclined, you can find the 2010 map at the USGS map store link above.  It’s not significantly different from the 1997 map above other than including

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