howdy everyone and welcome for continuing on with module 7 in this video we're going to finally explore in greater detail what actually causes floods and some of the factors that influence flooding by making them more frequent or larger in magnitude so before we get too deep into our discussion on floods let's first review some more terms that will more than likely come up during our discussion flooding is an overbank flow condition that occurs when the discharge is greater than a channel's holding capacity and the stage refers to the height of the water level in a river at a specific location at a certain time and lag time that I briefly introduced a few videos ago is the amount of time between the occurrence of peak rainfall and the onset of flooding which in the case of flash floods the lag time is very small now flooding is caused by these five main reasons most of our floods are caused by heavier long periods of precipitation but we can sometimes get flooding along coasts forming hurricanes or typhoons especially along deltas because of storm surges and then land subsidance and sea level rise so sinking of the land or rising of the sea can also cause flooding especially in coastal regions and especially along deltas as well and then we have natural dams what do I mean by natural dams i mean these are formed by landslides ice blockages or lava flows so when something happens suddenly to block the flow of a river all that water is going to back up behind the landslide behind the ice blockage behind the lava flow and cause a flood but then we also have our humanmade dam so we flood the landscape on purpose when we make our dams that's what the reservoir is it's a man-made flood it's a man-made reservoir when we made that dam it blocked the flow of the river and it flooded the landscape behind it and we use that we use that for water storage and hydroelectric power and even flood control sometimes we use it to our advantage but sometimes these things can fail and those have caused some of the most catastrophic floods in history so five main um causes heavy long periods of precipitation storm surge land substance or sea level rise natural dams and humanmade dams this might be a question on a quiz FYI this might be a question asking you what causes a flood remember what causes a flood not influences a flood these are the causes of a flood direct causes this is what I'm looking for if it asks you that now those were the causes of floods we also have things that influence floods and what do I mean by influence cause influence what's the difference influence i mean these are the factors that can dictate how often flooding will happen or how large a flood will be they're not the direct cause but they're controlling how big and how often these things are happening so floods are caused and influenced by a variety of factors and they're influenced by the magnitude timing and type of precipitation so we know precipitation can cause flooding but how heavy is the precipitation what's the timing of that precipitation is it long duration short duration and the type of precipitation is it rain or snow those things are going to influence the flooding does that kind of make sense and here I have shown a graph on the left that shows um rainfall over a six-month span on the upper Mississippi River Iowa and the green bars represent the average amount of rain by a month over a 30year year time period between 1961 and 1992 and the blue bar represents the actual amount of rain received in 93 alone and you can clearly see that here in 1993 was an especially wet year especially in the month of July and that produced one of the worst flooding events along the Mississippi in 1993 and on the right is a map showing just one rainfall event that year that some areas received up to 15 cm of rain in one single storm and that area is part of the upper Mississippi and you can see all the white rivers and streams on this map that feed the Mississippi River and all that runoff in that region eventually made its way to the Mississippi River now other factors can influence flooding as well like the size shape and slope of the drainage basin how big is the drainage basin is it steep those things are going to impact how much and how often flooding is going to occur the capability of the soil and the bedrock to temporarily store water is also going to influence it that brings us back to paracity and permeability how easily can the soil infiltrate or are we promoting a lot of runoff and then most importantly the human modifications of the physical landscape we influence flooding a lot we prevent flooding and sometimes we make them worse so again if the quiz asks you what influences flooding not causes influences flooding this is what I'm looking for the nature of the precipitation the geometry of the drainage basin the characteristics of the soil and human modifications of the physical landscape now how we use land is so important when we want to understand these flooding risks and hazards because anytime we change how we're using the land or how the land is being used we ultimately are changing the infiltration rate and if you change the infiltration rate you're going to change the amount of runoff or the amount of water that's flowing into a river and if you change that you're ultimately going to change soil erosion as well you're going to see a change in the amount of sediments in a river or the size of sediments in a river and the amount of water and sediments in a river is going to change the velocity of the river the velocity of the water flow and anytime you have a change in the river's velocity you are ultimately going to change the entire river dynamic system so here are just some common ways that we change the landscape where we have forest and we've turned it to farmland when we do this we're increasing soil erosion which is increasing stream deposition as well so we're increasing the amount of sediment we're increasing the gradient making it steeper which is going to increase the velocity and we're also going to increase the river channel erosion those things are going to impact the entire river dynamic and it's going to impact the entire way that this river system is going to respond in flooding events now probably what we're most familiar with dealing is urban buildup especially even here in Little College Station but especially those of you who live in Houston know this very well when we build up our urbanist areas we are increasing our imperous cover that means we're getting more surface runoff and less infiltration concrete is blocking the ground the water can't soak through that concrete this increases our lower magnitude flood frequency this causes a lot more floods because it reduces the lag time between our peak precipitation and our actual flooding event hence flash floods we see flash floods almost every single time we get an intense rainstorm in Houston do we not and that's because we have such a huge increase in impervious cover that water cannot go anywhere we have blocked the natural way for that water to infiltrate the soil so of course our use of land can severely increase flooding events such as urbanization deforestation but even the destruction of wetlands also severely increases flooding events now urbanization again adds imperous surfaces like that concrete we talked about roads but also roofs and parking lots which leads to the umbrella effect storm sewers are also quickly moving that water directly to the streams that's reducing our lag time the snow melts more rapidly on farmland than it does on forest land because we don't have trees shading it that's going to add water more quickly to our drainage basin tile drains that are used in agriculture and farm fields also reduces infiltration it's blocking the natural way for the water to infiltrate when we destroy our wetlands we're removing places where surface water can be stored and slowly released and the channelization of streams which is one technique we use to um prevent floods actually moves water more rapidly from certain locations from our tributaries to rivers and can cause problems farther downstream so it's important we understand Earth as a system making one change to a system is going to enact changes in almost all the other parts of that system even when the those changes were meant to be good in the first place so here's a powerful figure just showing you um how strong the relationship is between imperous surfaces and runoff so here on the left we have our natural ground cover when we have this natural ground cover and we get a rainfall event about 50% of it actually gets infiltrated so whether that be infiltrated with shallow or deepif about half of it making its way into the soil whereas only 10%'s running off because 40% actually gets evaporated or transpired or used with vegetation so only 10%'s getting run off with our natural ground cover compare that to imperous cover that we see in urban areas we're blocking the soil so we're not getting 50% infiltration we're only getting about 15 and we're not getting 10% runoff anymore we're getting 55% runoff that is a drastic huge difference we've removed vegetation we're seeing less evapo transpiration and a drastic increase in runoff which is why we see a drastic increase in flash floods now why are floods so important well floods can cause immense damage and remember they're the most common and costly natural disaster on our planet they can cause damage to structures farmland and crops on flood planes they can kill livestock contaminate sewage and agricultural um fields with chemicals and petroleum and agricultural chemicals they can cause sediment deposition on the flood plane which is where sediments get dumped and covers and smothers and damages crops they can disrupt river traffic and then the most important reason of all people die and are uprooted and evacuated and left homeless because of these events now again a flood occurs when the stream discharge exceeds the capacity of the stream channel so this is why we monitor that discharge so we know when it's going to exceed its capacity of its stream channel so we can get those warnings out ahead of time now we know rivers are places of constant evolution constant change nothing's going to remain the same along a river and that goes true for its meanders as it sweeps back and forth through time but also the water level this is James River in Virginia at base flow conditions normal day and here is that same river at the same location at flood conditions completely different now here are those two images side by side and you can see the same bridge in both of these images that can help you estimate the height of the river during that flood using those people for scale so how high do you think the water level rose here now flooding causes billions of dollars of damage annually and I cannot stress this enough but they are the most common natural hazards that humans experience as you can see on this map this map is showing you the flood hazards which very nicely correlates with our rivers and streams which is why we spend so much time talking about rivers and streams in this module