howdy everyone and welcome to the final video of module 7 where we will finally look at flood mitigation and how we can live among these rivers and streams where floods are a common occurrence now flood control is very expensive and really just the developed communities are able to afford such a mitigation strategy that means unfortunately our poor communities and our third world countries are at the highest risk of destruction and and loss of life because of these things because mitigation is expensive it requires money now here and in other developed countries we do spend money on flood control and the most common mitigation techniques involves leveies and flood walls those are per those are used to prevent overflow to our flood planes and we saw some examples of those working along the Mississippi River now artificial levies transmit flood problems downstream though because if the water can't over top its banks at that location where the levy is the water is still going to continue flowing somewhere so sometimes this makes a larger disaster farther downstream and also leveies can just be over top sometimes we don't make them high enough sometimes we don't maintain them and they fail and we've seen some catastrophic examples of flooding when levies have failed along the Mississippi River in 1927 and 1993 but also in New Orleans with Hurricane Katrina and in Missouri in 2019 some grave examples so ultimately flood risks are born by homeowners insurance companies lenders and government agencies and we use hydraologic data that are that are used to produce flood risk maps so these stream gauge data is very important and this allows us to make these flood risk maps and these hazard maps that then allow regulatory agencies to manage risk so we could have government agencies restricting building certain things in these flood risk risk zones you can have insurance companies um charging extremely high premiums for areas that are going to flood a lot more frequently and and so forth so in developed countries building in floodprone locations is usually pretty tightly regulated and prevents a lot of loss of life in these flood planes however in developing nations they lack a lot of these regulations or or these knowledge to make such decisions so they are not protected as much now how do we evaluate flood hazards well we use recurrence intervals that reflect the annual flood probability and this is where we get our hundred-year flood or a thousand-year flood those terms that you've probably heard before now a hundred-year flood is larger and more rare than a two-year flood because the occurrence of the severe flood in one year does not actually decrease the risk of a similar one occurring the following year so technically speaking statistically speaking I should say a 100red-year flood happens once every hundred years that means you have a 1% probability every year of a flood of that size happening and in this figure you can see the 100red-year flood is a flood that would inundate the entire um flood plane whereas a 2-year flood that means it happens once every two years is a flood that inundates this area along the the bank so much smaller in size but happens more frequently so this area is much more at higher risk to live in than this area right so a 100red-year flood means you have a one out of 100 chance of it happening every year 1% probability two-year flood you have a one out of two chance of it happening every year so 50% probability this is how we get these statistical averages and evaluate risk so here's a map showing peak discharge for the Embaris River in Illinois so all of these blue bars are discharge on the river this red dash line is the threshold for a 10-year flood so statistically speaking you should have what 10 floods that are at 10ear level in the last 100 years so here's about 100 years worth of data let's count how many times do these blue bars intersect this 10-year flood 1 2 3 4 5 6 7 8 9 10 so that's what makes it a 10-year flood happens about once every 10 years or 10 times in a century and that's exactly what happens so a flood at this river that has a discharge of 31,000 cubic feet per second that's a 10-year flood this line is our level of a 30-year flood so statistically speaking we should see three of these in a 100 years so 1 2 3 we do so our threshold of a 30-year flood is 39,000 cubic feet so our 100-year flood would be even higher if that makes sense so this is how we calculate those they are statistical averages now just because you had a 100red-year flooding event let's say last year does not mean you can't have a 100red-year flooding event this year it's just a lower probability but it's still possible so this is a really great slide for you especially as you do the quiz and activity that helps you calculate annual probabilities of flood risks now recurrence interval is the average number of years between floods of a particular size so again a 100red-year flood means one out of 100 risk of such a flood in a given year so 1% chance every year of a flood of that size happening so questions for you on this hypothetical river we have data we have discharge on the y we have recurrence interval on the x on this hypothetical river what is the discharge of a 20-year flood well let's just look at where 20 years falls this is 20-year recurrence interval lands right here which is about 520ish cubic feet per second so a 520ish cubic feet per second discharge is the threshold of our 20-year flood because we have the data showing us that okay different kind of question what is the chance of a 50-y year old a 50-year flood occurring this year remember 100red-year flood means one out of 100 chance this year so that means a 50-year flood would have one out of 50 chance 1 divided by 50 is 2% you have a 2% chance of a flood at that level occurring this year okay so what about a fiveyear flood five year floods smaller so that means it should happen more frequently right one out of five that's 20% does that make sense so rewind this work through these questions a few times making sure you understand how to do it in case you're asked a similar question on a quiz or activity okay so government agencies use this data especially FEMA um to make these flood hazard maps and assessments so they collect this hydraologic data they figure out where the 1% annual probability occurs where the 500year flood mark occurs and they make these maps which are 100red-year flood areas is all of the areas in blue our 500year flood includes the areas in green as well so this helps them make decisions on um flood risk and and also make decisions on flood mitigation to maintain the integrity of levies that are built here in flood control structures now this is a flood map and in your activity you're asked to be look you're asked to look at flood maps and so it looks a little confusing but this is actually what the government gives to the general public to make decisions when you want to buy a house before you buy your own homes you always want to check these flood hazard maps make sure the home you are purchasing is not on a flood plane because that's going to cause lots of problems not just hazards and risks it's also going to be more expensive because you're going to need flood insurance and if you want to use a mortgage or finance your home you're going to be required to have flood insurance so there's a lot of things that go into it so this is a flood hazard map all of the areas in gray are flood floodway areas so when you look at your flood hazard maps in your activity all the areas in gray with these dashed lines through it these are the areas that would be flooded in a 100redyear flood as the 1% annual chance flood that's a hundred-year flood so make sure you understand that when you're looking at this because they are a little bit confusing and just here is another example of a flood map again this area in gray this is the area that would be underwater during a 100red-year flood so let's take a look at some of those methods that we use to control flooding keeping in mind that even they also can fail here are some of the major methods we use for flood control they include dams and reservoirs channelization or floodways levies and flood walls and we're going to talk about each of these here in a moment starting with dams and reservoirs now dams protect against flooding by collecting and holding waters when they reach a certain level and once collected a dam might be designed to release the water back into the river at a controlled speed or sometimes they divert the water elsewhere for other uses now here is a map on the right or on the left that's showing all of the dams in the Kansas River Basin that are intended to prevent flooding along the Mississippi River with Wilson Lake Dam in central Kansas actually photographed here now here's another map showing the major dams on the Colorado River and the Colorado River is actually the most controlled river in the US and possibly even the world here are two famous dams along the Colorado River the Glen Canyon Dam which forms Lake Pal and the Hoover Dam which forms Lake me and these two dams define the boundaries of the Grand Canyon National Park and they've received a lot of media attention in the last several years maybe in the last decades for their extremely low levels due to prolonged drought in this region and just the high variability of the water levels during our droughts and flooding events now one of the problems with dams is that sometimes they fail now I know it's hard to imagine today because of the extremely low water levels but the Glen Canyon Dam actually came extremely close to failure in 1983 on the top is a photo from 2015 showing low water levels which are probably lower today compared to what was seen in 1983 only sheets of plywood were actually preventing this dam from overflowing that's how close it came now in 1983 as Lake Powell behind the dam was rising about 3 in a day the lake was really close to maximum capacity a record 120,000 cubic feet per second of rain and snow melt was flowing into Lake Powell and to provide additional capacity engineers had to install this 4ft high sheet of plywood to top the spillways these eventually had to be replaced with 8 foot high metal flashboards because they came so close to disaster and they only had a year to repair the damage that occurred underground in their spillways because the following year 1984 set even more records for runoff into Lake Powell the 1983 floods almost collapsed Glen Canyon Dam and the dam was very close to being overtopped and major damage occurred to the dam's underground spillways which came close to causing the dam to collapse from the inside this is a photo showing the erosion that the water caused underneath the spillway some of the huge chunks of rock were blasted out the sides of the concrete spillways by the flood waters in 1983 now if the Glen Canyon Dam did fail completely it would have probably caused other dams downstream such as the Hoover Dam to fail as well downstream towns would have been completely destroyed and one of the main sources of water to Arizona Nevada and Southern California would have been cut off it's estimated that the floor of the Grand Canyon would have probably been massively scoured to possibly depths of hundreds of feet a huge disaster that almost happened another method of flood control is channelization which is the straightening deepening widening clearing or lining of existing stream channels here's an example of what channelization looks like this one's at the Los Angeles River you can see that this is clearly not natural but they have literally widened straightened and lined the stream channel here's another slightly less ugly example of River Rouge in Dearbornne Michigan now channelization can work for flood control but as you can see with these two examples it is kind of ugly but most importantly it actually destroys the stream's ecosystem now leveies are another method of flood control and we already saw how nature can make these things naturally and we liked mother nature's idea so much we actually tried to replicate it by making our own artificial levies now humanmade leveies such as we saw on the Mississippi River a few videos ago that exist along much of its length are a lot taller than the natural levies are and they're constructed to prevent flooding from high discharge events on the river now most of our levies are constructed of piles of rock and soil with a concrete cover on the river side of the levey now human-made levies give us a sense of false security for those living on the flood plane um because they they believe that these levies are going to protect them but as we saw many many examples they do fail and can cause catastrophic flooding when that happens so it's still important to be prepared again these artificial levies work the same way as our natural levies there these raised embankments along a stream or river channel that contain flood waters and protect development on the flood plane like shown here now this is an example along the Battoic River in West Virginia that's protecting these homes to the left and this is another image from the flooding of the Mississippi River in 2011 some wealthy homeowners built personal levies like this one and it actually worked now during floods watered saturated levies are in danger of failure even when they're not overtopped so they can fail even when they're not overttopped and that's because the pressure of the flood waters against the levies can be extremely high and this pressure can cause sand blows some distance from the levey and they can also become so water saturated and fail now some famous examples of levy failures happened during Hurricane Katrina yes just like dams levies can fail and cause catastrophic flooding now a flood wall is a primarily vertical artificial barrier designed to temporarily contain the waters of a river or other waterway which may rise to unusual levels during seasonal or extreme weather events and flood walls are another common method of flood control now here are some examples one on the left of the Johnston flood wall in Pennsylvania and on the right in New Orleans which you can see it's very poorly maintained and yeah did indeed fail this is a concrete flood wall Missouri and when floods are threatened a crane drops a gate into the slot to hold the river behind it engineering is really cool now some other tactics of flood control include relocating settlements to higher ground modifying land use patterns and flood planes restoring wetlands along river banks and avoiding development altogether in areas prone to flooding keeping homes and businesses off the area right along rivers can save lives and damages from floods now this can be done using zoning laws this photo shows a floodway around downtown Winnipeg Canada which they kept off limits for development which ended up being a smart move now constructing these homes to building codes is what saved most of these houses homes in some areas are required to be built on elevated land to concentrate the flooding in the streets instead of the homes themselves thankfully there are some flood laws and regulations and in many areas there are restrictions on what we can actually build on a flood plane some areas have laws that do not allow certain types of buildings below the 100red-year flood level in many places homes and businesses on the flood plane or below the 100red-year flood level requires national flood insurance in order to obtain a loan to purchase the property and the only place to get national flood insurance is from the federal government flood insurance is not available from commercial insurance companies now after repeated floods the people of Valmeer Illinois actually moved their entire town off the flood plane after the 1993 Mississippi River flood now this is certainly one solution at flood control but Valmeer has a population of less than,300 people now is that a realistic solution for some place like Cincinnati or St louis definitely not this again is why it is critical that we understand these processes and try our best to mitigate their impact on us as much as possible in less developed countries humans are particularly sensitive to flood casualties because of the high population density absence of zoning regulations lack of flood control and lack of emergency response infrastructure and early warning systems remember that vulnerability it is an unfortunate reality that those in less developed countries are at higher risk for all natural disasters that we focus on in this class take Bangladesh for example bangladesh that has over 165 million people is one of the most susceptible countries to flooding about 1 half of the land area in Bangladesh is at an elevation of only 8 m above sea level and every single year at least 18% of Bangladesh gets flooded at least and beginning in June of 2020 the flooding of the Brahmautra River caused over 30% of the country to be covered with flood waters now monsoon rains resulted in flooding that washed away assets for some of the world's poorest people it washed away their goats their chickens their houses which are made of mud and tin and even their sacks of stored rice nearly a million homes were inundated and 4.7 million people were affected unfortunately some of the most vulnerable were killed including at least 54 children this is a map that shows just how much of Bangladesh was impacted in this event here in red these are the flooded areas of the country all along the Brahmautra River now Bangladesh was experiencing a pattern of more severe and more frequent river flooding than in the past along the Brahmautra River and they're experiencing this more and more even today as we speak and it's projected to worsen in the years to come because of climate change that is intensifying the rains in this region in industrial countries like the United States the loss of life is usually lower because of flood control structures and zoning regulations that prevent habitation of seriously vulnerable lands and emergency preparedness still even in our country property damage and disruption of life takes a great toll and despite our flood control structures and land use planning floods still do occur especially flash floods in our urban areas so it is crucial that not only in developed nations but industrialized nations that we continue to adapt and mitigate and try to lessen the loss of life and destruction of property as much as we can