Tuesday, March 30, 2010

Dynamic versus Level Pool Reservoir Drawdown for Dam Breach Modeling

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © RASModel.com. 2010. All rights reserved.

This is a summary from a paper (Goodell,Christopher;Wahlin, Brian. “Dynamic and Level Pool Reservoir Drawdown: A Practical Comparison for Dam Breach Modeling.” 33rd IAHR Congress Proceedings, Vancouver Canada, 2009) on level pool versus dynamic reservoir drawdown for dam breach modeling. In RAS you can define your reservoir with a series of cross sections (which uses dynamic routing) or a storage area (which uses level pool routing). Dynamic routing is generally assumed to be more accurate, but the size and shape of a reservoir can sometimes make level pool reservoir adequate.

A key component to dam breach modeling is the reservoir drawdown. This has a significant impact on the magnitude and shape of the breach outflow hydrograph, and ultimately the extent of flood inundation in the downstream reach. Drawdown of the reservoir can be modeled with the precise and physically correct dynamic routing method, which uses the full St. Venant equations of Conservation of Mass and Conservation of Momentum. However, this requires detailed bathymetric data for the reservoir, which is frequently very difficult and expensive to obtain for existing reservoirs. Furthermore, dynamic routing is complex and prone to numeric instabilities. A level pool drawdown is a more simplistic, numerically stable approach that can be used successfully under certain circumstances and requires only a simple stage-storage curve for the reservoir.

Two primary characteristics emerge as indicators of a given reservoir’s ability to be described by a level pool analysis. The Compactness Factor, Fc, is simply the ratio of the dam height (H) to the reservoir length (L). The longer and shallower the reservoir, the lower the Compactness Factor and the more the reservoir acts like a river during its drawdown. Thus dynamic routing would be more appropriate in this situation. Short, relatively deep reservoirs are more compact, have a larger Fc value, and can be adequately described using a level pool analysis.

The Translation Factor, Ft, describes the relationship between the speed of the breach development and the ability of the reservoir to supply water to replace the water leaving through the breach. The easier the reservoir can deliver water to the breach, the more it can be described by a level pool analysis. Fast breach developments and long reservoirs are more appropriate to be modeled by dynamic routing. The Translation Factor is computed as:

Ft = ct/L

Where: c = shallow water wave celerity =clip_image002.

d = representative reservoir depth.

and t = time.

A third parameter can be used to help graphically display the results of the various simulations. The Drawdown Number, Dn, is defined as the product of the Translation Factor and the Compactness Factor.

clip_image002




It becomes apparent that for high Drawdown Numbers, the level pool analysis produces results very close to dynamic routing. By enveloping the data points, a 5% threshold Drawdown Number is shown to be 0.41. That means that a reservoir with a Drawdown Number of 0.41 or greater will produce peak outflow results within 5% of a dynamic routing simulation. The 10% threshold Drawdown Number of 0.24 is also indicated on the plot.

You can see the full paper in the referenced proceedings. Also, the Hbox software has an automated utility for determining the appropriateness of level pool reservoir drawdown based on thd Drawdown Number analysis.

Wednesday, March 24, 2010

Dam Breach Modeling Q & A

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © RASModel.com. 2010. All rights reserved.

Some questions and answers related to dam breach modeling in HEC-RAS…

Question. The Sunny Day model has a consistent water surface elevation from the very start of the model – it only decreases once the breach occurs. How is HEC-RAS setting this starting WSEL?

Answer. You define the starting water surface elevation either by equalizing the flow from your outlet works with the reservoir inflow, or by setting an initial conditions water surface elevation in your flow editor and a pilot flow through the dam equal to the reservoir inflow at the beginning of the simulation.

Question. My breach models show a dramatic decrease in max Q from the cross-section immediately downstream of the dam to the end of the model. I know that HEC-RAS has an inherent storage routine that attenuates the flow throughout the model but is it reasonable to have a result that shows a beginning max Q of 12,370 cfs and an ending Q of 275 cfs (the reach is approx. 3.8 miles long with a slope of 0.02 ft/ft upstream and 0.001 ft/ft downstream)? This is an arroyo about 800-900 ft. wide, Manning’s at .055.

Answer. I would be skeptical of those results. Perhaps there is an error somewhere in the simulation, or you have a lot of flow leaving the system. Sometimes, if your model is not properly constructed, you can develop a large “wall of water” in profile view. A lot of times this is due to poorly defined HTAB parameters. This will create an artificial pool of water behind the wall, which will drastically attenuate your flood wave. Look in the profile plot and animate through your simulation. If you see an unexplainable wall of water backing up flow, that would be the cause.

Question. My models are stable but still have inherent errors (max iterations) and critical depth defaults to varying degrees. Does this have a significant effect on the model results? Changing parameters at this point to reduce inherent errors most likely will cause instability.

Answer. Max iterations are not necessarily a problem as long as the associated errors are small and it is not causing visible instabilities or obvious errors in your results. I try to get rid of all max iterations where possible. If not possible, I try to get the errors below 0.1 ft as much as I can (my own rule of thumb). RAS does not typically default to critical depth in unsteady flow (like it does in steady flow). But it sounds like you have areas that have flow close to critical depth. This can cause instability problems. If you believe flow should be close to critical depth in these locations, try turning on the Mixed Flow option and adjusting your LPI factor. If you do not believe flow should be near critical in these locations (most of the time in natural streams you should not see critical or supercritical flow), then you may be underestimating your Manning’s n values. Manning’s n values for the front end of dam breach flood waves and steep reaches are frequently underestimated. Check Jarrett’s equation if in a steep reach. Your reach slope of 2% is quite high. An n value of 0.055 is possibly too low during the low flow period preceding the dam breach flood.

Question. Does the number of vertices defining a cross section matter, in another words, does the model run better with cross sections that have fewer vertices but still accurately define the section, vs. similar sections that have many redundant vertices?


Answer. Better definition is usually advantageous. RAS does not like to have long horizontal portions of cross sections which is common for coarsely-defined cross sections. It can cause numerical problems. These days, having the maximum number of points in a cross section (500) typically does not noticeably slow down computation speed. I recommend getting as much detail as you can in your cross sections.

Tuesday, March 23, 2010

Bridge on a Spillway

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © RASModel.com. 2010. All rights reserved.

I just got an interesting question. How do I model a bridge that has a spillway just downstream of it? Here’s an example of what I’m talking about:

water flowing over the spillway at the Bitan Bridge, Xindian, 8 Aug 2007

As I see it, there are a three ways to approach this. You could go conventional, and model both a bridge and an inline weir and try to squeeze in a couple of cross sections in between. This is probably the easiest, but not always possible if the bridge is on top of, or very close to the weir.

Another approach is to model the bridge as a bridge, and the weir with a series of closely spaced cross sections. This can be problematic if you have a high drop over the spillway with low tailwater elevations. And the cross sections will have to be very tightly spaced, to prevent over-estimation of energy loss over the weir. This is not recommended for vertical drop structures.

A third approach would be to model both the bridge and the weir together as an inline structure. The bridge opening can be simulated using a gate (or series of gates). The space between the gates simulates the piers. The gate invert is the top of the weir. The gate height then simulates the distance from the top of the weir to the bottom chord of the bridge. The upper chord of the bridge can be entered in as the top of dam. Make sure to leave the gate wide open for the entire simulation. Also, a nice advantage is that each gate can have it’s own weir discharge coefficient.

image

Any other suggestions out there?

Friday, March 12, 2010

Adding Help Files and References to RAS

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © RASModel.com. 2010. All rights reserved.

Everyone has their favorite references that they go to for help and guidance when developing hydraulic models. It could be the n-value table in Chow, or Tony Wahl’s Breach Parameter paper, or simply specific exerts in the RAS manuals. Whatever your preference, HEC has provided a very convenient way to easily access reference documents while in the HEC-RAS environment. You simply copy the document and paste it into the \Program Files\HEC-RAS\4.1.0 folder. You have to change the name so that the file begins with “RasHelp_”. That is how RAS recognizes it as a help document. Then when you open RAS, under the Help menu, you’ll see your custom documents listed for easy access. Notice that I have Appendix B of the RAS Hydraulic Reference Manual, a summary of Breach Parameter equations, and a Sediment Gradation Table in the Help menu.

image

image

Monday, March 8, 2010

Earthquake in Chile. February 27, 2010

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © RASModel.com. 2010. All rights reserved.

I know this is an HEC-RAS blog, but I decided to bend the rules a little and post a recap of my experience during the earthquake in Chile last weekend. Hope you find it interesting...

Early Saturday morning, February 27th, around 3:30 am, a massive earthquake rocked south central Chile. The epicenter was just off the coast of Chile’s second largest city, Concepcion. I was in Santiago at the time, approximately 300 miles north of Concepcion, with plans to fly to Concepcion on Sunday to teach a class at the University there. I had just arrived in Santiago Friday morning, February 26th, and spent the day site-seeing and getting used to my “meager” accommodations. Actually, the room is quite nice, for the price and it has a kitchenette. However, the wireless internet was not working in my room. Friday night, I had a great time at dinner with old friends of mine, who are residents of Santiago, Patricio and Macarena Banados. Dinner traditionally happens very late in Chile, and I returned to my hotel around 12:30 am. I was very tired from the overnight flight and the time difference, so I fell asleep very quickly while watching BBC (the only English language channel on the TV).

At about 3:30 am, local Chilean time, I woke up to a rumble. I was lying in bed and it began to shake a little. Living on the west coast, I’ve felt a few earthquakes in the past. There was one in Portland, while I was on spring break from college in 1993. Like the Chilean one, it too abruptly woke me up. However, the Portland earthquake was more of a novelty. I believe it was about a 5.6 on the Richter scale. I woke up, wobbled over to my window and stood there watching for anything to happen while my parent’s house shook a little. It lasted about 30 seconds, if I recall. The Chilean earthquake was a lot different.

Though it woke me up with a little mild shaking, this earthquake quickly began to grow in intensity. Within (probably) about 10 seconds, my entire room was violently shaking. Outside my window, which I had left open before going to bed, I could hear loud cracking and popping sounds over the background rumbling and of car and building alarms that began to sound. It was so violent, that I quickly decided that this was not another “Portland” earthquake. My first thoughts were that I was on the 14th floor of an apartment building, and I’m in Chile-both of which scared me to death. At that point all I could think about was to get as far away from the edge of the building as I could. While I could see the dark outline of the hanging lamp in the kitchenette swinging back and forth, and I could hear stuff toppling over in my room, I made my way to the door. I have to admit that at this time I briefly paused and considered getting dressed first (I was only in my boxer shorts), but I don’t think I could have even put clothes on with the shaking that was happening. Furthermore, it was very dark in my room, since the power was already off. So I decided to put aside my modesty and get out.

Quickly panic set in as I realized that my door was locked. And this was not a matter of twisting the latch, like a typical American hotel room door. This door is the kind that is locked with a key from the inside. To make it worse, there were two locks (of course I had set both of them before going to bed), two different keys, it was dark, and one of the locks had to be “giggled just right” to unlatch. The exercise of finding the keys, and fumbling around with the locks (it’s hard to put a key into a lock, when the lock doesn’t stay in the same place!), felt like it took forever, all while I’m expecting the building I’m in to collapse around me. FINALLY, I got the door opened, briefly thought about locking it behind me (but didn’t) and stumbled through the hallway. I knew from structural engineering class in college that a central “core” elevator shaft is frequently the strongest part of the building. Not sure if that applies in Chile, but that’s where I headed. I made it to the elevator, put my hands on either side of the elevator doorway, took a wide stance, and braced myself. I figured I would never make it down 14 flights of stairs with the kind of shaking I was feeling, so I decided to wait it out on my floor at the elevator. This is when I put my head down and started praying. “Please, God…make this stop.” Over and over. It felt like another 30 to 40 seconds that I was at the elevator bracing myself, before the earthquake finally subsided. All told, the earthquake lasted anywhere from 90 seconds to 2 minutes (I’ve heard both reported).

Even though the earth stopped shaking, my body was trembling uncontrollably. My legs felt like noodles, my hands were shaking, and my heart was beating out of my chest. That was when I realized I had to get out of this building. I didn’t know if there had been serious structural damage, or broken gas lines. Either way, I wanted to get out. So I ran back to my room, happy that no one on my floor had come out to see the half-naked gringo running around, opened the door and frantically looked for clothes to throw on. With my shorts and shirt on, I grabbed my shoes, jacket, phones, and computer bag and left.

This time, when I left the room, people started to emerge from their rooms. I ran down the hall towards the stairwell. As I passed the elevator, it opened up and a Chilean man came out. I was thinking “what the &#$*% are you doing in the elevator?” If I knew how to say that in Spanish, I probably would have. But instead I ran by. The elevator must have been running on backup power. He said something to me in Spanish as I ran by that I think was “Where are you going?” I yelled back, “A bajo!” Apparently, even with my short response, he could tell from my accent that I was a foreigner, so he ran after me telling me in English to stop and come back. So I stopped. The guy seemed like he knew what to do. He asked me “are you scared?” “YES, I’m scared. I’m getting out of here.” Then he told me I should come with him. To my astonishment, he said it was better to take the elevator down. I said “Really?!?!” My mixed tone of sarcasm and confusion didn’t faze him and He said “Yes yes yes. Come on.” So I did. Thinking about that now, what an idiotic thing to do! Fortunately we made it to the bottom floor without incident. On the way down, the Chilean man could tell I was very scared and was nice enough to reassure me that everything was okay. He kept saying over and over, “This is not Haiti. This is not Haiti.” You see, the small, impoverished country of Haiti had just experienced a devastating earthquake a few weeks before, where their cities and towns were left in ruin. “We have earthquakes here a lot. Our buildings are strong.” It was reassuring.

Nevertheless, my immediate goal was to get outside and as far away from the building as possible. I was taken aback that just about everyone was standing in the lobby or just outside the lobby door on the sidewalk. I got to the sidewalk outside the lobby and put on my socks, shoes, and jacket, then crossed the busy 4 lane street to the other side. There were only 3 other people (out of hundreds) who decided to cross the street-all were Asian. I found it very odd that all of the locals had no problem standing at the base of this 21-story high rise.
At this point I tried to make some phone calls. Not surprisingly, the phones system was completely jammed and I couldn’t get a call out to my family. After redialing what must have been 100 times, I finally got my wife, Wendy’s cell phone, but it went to voice mail. By now, it was almost 4:00 a.m. in Chile, and 11:00 p.m. in Portland. I figured everyone had gone to sleep already, knowing nothing of what was going on in Chile. So I left a brief message, saying there was an earthquake, and I was okay. I found this out later, but Wendy did get the message that night and stayed up all night watching the news and trying to call me back. I spent the next hour or so trying to make phone calls to no avail. My cell phone battery was getting very low, so without power to recharge it, I decided to wait until the next day to try calling again, so that I could preserve whatever battery power I had left.

In total, I probably spent a couple of hours outside-standing, sitting, pacing, and praying. Occasionally I would check back in the lobby to see if I could find anything out, but could find no one who spoke English. The lobby was packed with people, some of whom were already making beds for themselves on the floor. At about 6:00, I went back in the lobby again, and noticed that some people were heading back up the stairs. So, I weighed my options. I could remain outside, tired, uncomfortable, and helpless, or go back to my room and try to sleep. The building looked okay from the outside. No cracks or visible damage anywhere. My sleepiness took control and I walked the 14 flights of stairs back to my room. With my iPhone’s Flashlight app (that was a lifesaver!), I managed to get my room prepped for a quick escape in case of any large aftershocks, and fell asleep on my bed, with all my clothes on. I must have lain there for a half-hour or so, still completely wired from the adrenalin, but then fell asleep. Speaking of iphones, I’m still angry at myself for not having my iPhone at the ready, for video recording impending after-shocks.

Sure enough, I was woken at 7:30 am to a big aftershock and a chorus of car alarms. Nothing like the original earthquake, but still much bigger than the Portland earthquake and enough for me to jump out of my bed and run to the door. As I made my way to the door, I noticed the hanging lamp in the kitchen swinging back and forth again. By the time I got the door opened, the aftershock had ended. I took a quick peek outside (by now it was light outside), and seeing no damage, decided to stay in my room. While I was looking outside, I saw some kids playing in a room in the other wing of the building. They seemed to be enjoying the excitement. That somehow, made me feel better. By now, despite the lack of sleep and jetlag, I was fully awake. Now that there was daylight outside, I could see the effects the earthquake had on my room. To my astonishment, it was in very good shape. No visible cracks anywhere. My toiletry items had all fallen off the sink, and some kitchen items had fallen onto the floor, but the room was just fine. (Note, as I’m typing, at 1:40 pm on February 28th, another small aftershock occurs. That makes 7 aftershocks now that I’ve felt so far. I’m sure many more that I didn’t feel. There is a tennis club across the street from my hotel that has an alarm that goes off with the larger aftershocks. It has reliably been going off just before I feel the shaking. It’s nice to have warning.). At this time, the engineer in me took over, and I decided to have a walk around the hotel, to survey the damage. I started on my floor, and worked my way upstairs to the roof, where there is a patio and two pools. On my floor there was not too much damage. Some drywall had crumbled and fallen to the floor around the doors, and some tiles had popped off, but otherwise, nothing much. On the roof, there was much more damage, but still surprisingly little, given the violence of the initial earthquake. No one else was up there. (note, another small aftershock as I type and accompanying tennis club alarm. I can also hear some chimes jingling around-1:45 pm, February 28th). There is a water heater room on the roof and the door was open. Inside the water heater room, entire sections of sheetrock from the roof had come down. Also, ashtray cans on the patio were toppled, and tiles had popped off walls. Next to the pools, there was a lot of standing water in low areas on the patio, where apparently water had sloshed out of the pool the night before. All of the filter caps, that are common around the perimeter of pools, had popped off during the quake, and some had slid (or floated) across the patio as much as 10 feet. I’m assuming the wave action in the pool pushed the filter caps out from the inside. As I was surveying the pool and patio, the second aftershock happened. This one was imperceptible to me, but I knew it was happening because out of nowhere, the pool started making waves, and the air was completely still. The water in the pool was previously glassy smooth, and then there were some small waves. I took video of this with my iPhone. As I glanced around the skyline of Santiago (360 degree view from the roof), I was surprised to see that from my vantage point, there was no damage I could see. All of the buildings that were there the previous day, were still there. Even the older ones.

After the rooftop survey, I decided to take a walk around the Bellavista neighborhood near my hotel. During this walk, I was able to see a lot of damage. Most of the damage was on building facades, or roof tiles sliding off. The first significant damage I saw was a Subway sandwich shop, where the top 10 feet of the facade had crumbled to the sidewalk below. There was a massive pile of bricks and stucco. I couldn’t help but wonder if someone had been below during the quake. They would not have survived. As I continued to walk, many buildings had roof and facade damage, and many windows had popped out and shattered on the ground below. But surprisingly, all of the buildings in this very old neighborhood were still standing, many with no perceptible damage to the exterior at all.

Shortly after I got back to my hotel, around 12:00 noon, the power came back on. So I charged my phone while watching the BBC to see what was going on. This was when I learned that the epicenter was just off the coast of Concepcion and there was significant damage there. Many buildings had collapsed, some were still on fire, a bridge had fallen into the Rio Bio Bio, and reportedly, up to 70 people had died. As I type, the updated death toll is over 700 and I’m very concerned for my friends and their family who live in Concepcion. I still cannot get a phone call through to them. All of Concepcion is cut off. I also learned that the airports in Santiago and Concepcion were both closed indefinitely, and the road to Concepcion was shut down due to collapsed bridges. It looks like I won’t be making it to class. It was also fascinating to see on the news how the entire Pacific ring was bracing for tsunamis. Concepcion itself, being on the sea, was rocked by a big tsunami, shortly after the quake. There was a lot of concern that Hawaii was going to get hit with a large tsunami, but by the time it eventually got to Hawaii, it was rather uneventful. Currently, as I watch the news, seismologists say the earthquake was an 8.8 on the Richter scale at its epicenter near Concepcion, and an 8.0 in Santiago. I am stunned to hear this. An 8.0 is about 400 times stronger than the 5.6 I felt in Portland years ago. It’s amazing that more damage did not happen in Santiago. Hats off to Chilean structural engineers! There were a few more aftershocks while back in my room. The smaller ones are hard to tell if it’s really an aftershock, or just my own perception. So with the swimming pool as my inspiration, I set up a make-shift “earthquake-ometer”: a glass of water. Filled to the top so that any spilled water would tell me if there had been any aftershocks while outside.

With phone charged, I later I walked to the Providencia neighborhood, where there was much more damage. Given that, it’s interesting that Providencia is a newer neighborhood than Bellavista. A church dome was completely destroyed, a modern office building had a deck that had partially collapsed around its 30th floor, and panes of glass had popped out of buildings all over the place. On my way to the Providencia neighborhood, I walked through the parks that line the Rio Mapocho. All thoughout the parks are 10-ft high lamps capped with three lights each-there must be thousands of them. The lights have (had) glass domes on them. Almost every single lamp I walked by had 1 to 3 of the domes shattered on the ground below. While eating lunch in Providencia (at one of only a few restaurants that was open), I felt the third aftershock. The waiters appeared to be having a good time watching “the concerned gringo”.
On my way back to my hotel, while still in Providencia, I was finally able to get through to my wife on my rented mobile phone. What a relief that was, especially when I found out that she had indeed received my message the night before telling her I was okay. She gave me some details about what she was hearing on the news at home.

After spending some more time in my hotel room, and no internet to use, I decided to head out to the Plaza de Armas for a wireless hotspot walkaround with my iPhone. While walking through the Ahumada pedestrian mall, I found an unrestricted wireless connection and was finally able to get on the internet. People must have thought me a gringo loco, since I was pacing around, back and forth, trying to maximize the wireless signal strength. I took care of some email, tried to Skype with my wife (to no avail), and spent some time on Facebook. Though Wendy had let everyone know I was okay, it was good to be connected again to the world, so I could check in.
Well, with the Santiago airport closed indefinitely, and the class almost surely cancelled, my plan is to wait around Santiago until I can get a flight out. My friends Patricio and Macarena have no power or water at their home, so I’m going to stay at the hotel here and make the best of it. Fortunately, I was able to extend my stay without any problem. Now, if they could only get around to fixing the wireless internet connection.

RAS 4.1

HEC-RAS version 4.1 is officially on the grid!!!