Hey there! 🌧️ Have you ever wondered how just a few inches of rainfall can quickly turn into a few feet of flooding? Let’s dive into this fascinating phenomenon and uncover the science behind it. 🌊💧
## Understanding the Rainfall-Flooding Connection
### Topography Plays a Role
– The terrain of an area can greatly impact how rainwater flows. Steep slopes or areas with poor drainage can lead to rapid runoff and subsequent flooding.
– Low-lying areas are particularly susceptible to flooding as water naturally accumulates in these depressions.
### Soil Saturation
– When rain falls, the soil absorbs a certain amount of water. However, if the soil is already saturated from previous rainfall or due to high water tables, it cannot hold any more water.
– Excess rainfall then runs off the surface, adding to the volume of water flowing into rivers, streams, and other waterways.
### Urbanization and Impervious Surfaces
– Urban areas with extensive pavement and concrete surfaces prevent rainwater from being absorbed into the ground. This runoff leads to increased volumes of water flowing into drainage systems and can overwhelm them, causing flooding.
### River and Stream Capacity
– Small to moderate rainfall may not cause flooding under normal circumstances. However, if the rainfall is intense and sustained, the volume of water can exceed the capacity of rivers and streams, leading to overflow and flooding.
## Real-Life Examples
In 2017, Hurricane Harvey dumped over 60 inches of rain in parts of Texas, leading to catastrophic flooding that inundated homes and displaced thousands of residents. The sheer volume of rainwater overwhelmed the drainage systems and caused rivers to burst their banks, resulting in widespread devastation.
## Conclusion
Next time you see a heavy downpour, remember that even a few inches of rainfall can have significant consequences when it comes to flooding. Understanding the factors that contribute to this process can help us better prepare for and respond to extreme weather events. Stay safe and stay dry! 🌧️💦🚣♂️
The TL;DR version is water flows downhill and accumulates in low areas.
For a little more depth (pun slightly intended), lets imagine a spot experiencing precipitation. We often consider precipitation as a “depth” measurement, i.e., with units of L (like your example of the event with 15.5 inches of rain), or in terms of a rate with units of L/T. What this effectively is telling us is the depth of rain that would fall on some small, representative patch in a given event (or over a given time frame), but of course unless we’re actually considering a perfectly flat area, that depth of precipitation is not going to stay in that spot, it will flow down slope until reaches a river and in turn add to the volume of water in that river (which in turn flows down slope within its channel until it reaches some outlet into either the ocean or an internally drained body of water).
To convert our depth measurement into a volume (i.e., a total amount of discharge), we need to consider the [drainage area](https://en.wikipedia.org/wiki/Drainage_basin) of the system in question, where this demarcates the boundaries of the contiguous area above that spot where if any drop of water falls into that area, it will eventually flow through that spot within the river. I.e., the total discharge magnitude resultant from a given storm event in a given spot along the river will ~ equal the precipitation depth x the drainage area. In turn, we can convert that discharge into an approximate height of the river if we consider a fixed width and unit length of the river.
To start to see how this can add up pretty quick, let’s imagine a scenario like your question where everywhere in a given drainage basin experiences a total of 40 cm of rain (roughly equivalent to 15.5 inches) over a period of time. If we integrate that over some drainage area, e.g., 10 km^(2) (which is a pretty small drainage basin all things considered), that gives us a total volume of water that will flow out through that point on the river of 4 x 10^(12) cm^(3) or 4000 m^(3). If we consider a unit length of the river system (e.g., 1 m) and assume that all of the that water that accumulated from upstream (i.e., the 4000 m^(3)) was flowing through this 1 m length section of the river, to get to 16 m deep (i.e., ~53 feet), you would just need to “spread” that total volume of water over a 250 m wide area (assuming no topography for the area in question).
Now, the above is a gross simplification and makes tons of unreasonable assumptions, for example: (1) All precipitation is converted to runoff, where in reality there will be losses to infiltration and evapotranspiration such that runoff < precipitation, (2) the precipitation is delivered effectively instantaneously to the land surface, (3) the runoff transit time to our representative point on the river is effectively instantaneous, and (4) we completely ignore any geometry associated with the river channel, levees, floodplains, etc. It does however get the general point across that accumulation of rainfall over even a modest area can lead to large volumes of water in river systems, and thus *much larger* depths of flows than the storm depth in a single spot.
Rain falls everywhere, then flows downhill.
The farther downhill you go, the more uphill there is.
The more uphill there is, the more water is flowing towards you.
The farther downhill you go, the less downhill there is.
The less downhill there is, the less space for all of the uphill water to go.
Why can a river have 20 feet of water in it when it’s not raining?
Based on your other posts, I refuse to believe you don’t know how rivers work.
The calculation is 15.5 inches *per surface area* all that runoff would end up in local rivers or lakes. So if its 2 sq mi * 15.5 in. Idk math in cowboy. But its a substantial volume. The larger the watershed the more water there is. All of that will end up downhill/down stream where the flooding impacts are likely to be.
If I drop 15.5 inches of rain over 2 acers of land, it will be covered in 15.5 inches of water(we are assuming nothing absorbs into the soil for the sake of simplification). If I then raise 1 of the 2 acers of land 4 feet above the other, the lower acre now has 31″ of water on it and the upper one has no water. If I keep doing this, eventually you will end up with a small part of land that is covered to a very great depth.
Land is not flat, and water runs downhill. It will collect in the rivers and drive the water level of the river up.
1 mm of water on a square meter is equal to 1 liter.
I don’t americans units, but if you takes the area on which the rain fall, i.e. the watershed, and you c’est
Concentrate everything on a talweg, it makes incredibly high volume of water
>How is it possible that 15.5 inches of rainfall doesn’t translate to 15.5 inches of water level rise?
Because the ground isn’t level. Imagine placing a continuous 15.5 inch thick layer of water *everywhere*, and then releasing it. All the water runs down off of every hill (actually *every area that isn’t level*) and fills up lower areas. It’s the same way normal non-flood rivers work too. All the rain that falls over a big area runs downhill and accumulates in a low spot like a valley, because there’s nothing to stop it.
In your quoted text it’s talking about the Appalachians, which is a mountain range. Imagine covering *a mountain* with a 15.5 inch thick layer of water all over its entire surface. There’s nothing to keep it there so all the water runs down the sides. You end up with 0 inches of water depth on the mountain sides (because it’s steep and rocky) and much *more* than 15.5 inches of water pooled around the base of the mountain.
Think of it this way: Take a football field that receives 3″ of rain…That’s 3″ over 57,600 square feet…so a little over 84,000 gallons.
Now, tilt that football field on one end zone and empty all that water into a 50 yard long, 1 yard wide ditch…that ditch would have to be 300″ deep to hold the 84,000 gallons (or 25 feet, or 8.3 yards).
So, when they talk about 3″ of rain, it’s 3″ EVERYWHERE.