Final Study Guide 8 Flashcards
Hydrology
Sketch the hydrologic cycle. What factors determine whether rainfall will run off or infiltrate?
Whether the water runs off or infiltrates is controlled by a number of mechanisms including precipitation rate (how hard it’s raining), soil water content (is it already saturated?), slope (steeper = more run-off) and vegetation (more veg. holds water).
Draw a graph showing the equilibrium water content of a parcel of air vs. temperature. Use this graph to explain why it rains (2 different causes).
- Two air masses near saturation, but with different temperatures, are mixed
- Air rises, causing it to cool, so cant hold as much water, so precipitates
Draw a cross-section across a river channel in an area of gentle topography. Label the following: bank, levee, flood plain, channel.
- Alter hydrologic budget through changes in vegetation, impermeable area, construction, dams, flood control basins
- Channelization = straighten (and shorten) channels to improve drainage. This increases gradient in channel, leading to higher velocity and more erosion
- Impact on sediment delivery to deltas, beaches. Reduction in flow cuts sediment delivery. Debris basins intercept sand.
Which portion of a stream meander will be eroded? Why? What is the most critical factor for determining whether erosion or deposition will occur?
- the outer side of the curve, because the velocity is faster on the outer side than on the inner curve. The speed/velocity of the river in relation to the curve.
Why are dams constructed?
- Millions of people depend on dams to help provide the right amount of water in the right place at the right time. Lots of dams provide water for growing crops and for farm animals to drink. They also store water for fighting fires and can help control floods. Dams also give us hydroelectric power.
- Renewable energy that does not pollute. A dam can cause floods and thus kill people.
What problems can each solve or cause?
Solve:
- Flood control
- Hydroelectric power
- Water storage for dry times
Cause:
- Loss of water through evaporation and infiltration
- Loss of land and some recreational opportunities
- Seismicity
- Downstream impacts: less water, salinization and erosion
- Siltation in reservoirs
- Barrier to fish migration for spawning
What is the meaning of recurrence interval? How are event frequency and recurrence interval related?
- An estimate of the interval of time between events like an earthquake, flood or a river discharge flow of certain intensity or size
- When geologic events are random or quasi-random, it is helpful to represent their frequency as an average time between past events, a “recurrence interval” also known as a “return time.”
Do you expect floods to occur at regularly spaced intervals? Explain why or why not. How can we predict the size of a “100 year flood” on the basis of 20 years of stream gauging?
- We cannot expected them to be at regularly spaced intervals. Certain environmental, natural phenomena can occur, disrupting interval times
- human activities can also cause floods
- In the 1960’s, the United States government decided to use the 1-percent annual exceedance probability (AEP) flood as the basis for the National Flood Insurance Program. The 1-percent AEP flood was thought to be a fair balance between protecting the public and overly stringent regulation. Because the 1-percent AEP flood has a 1 in 100 chance of being equaled or exceeded in any 1 year, and it has an average recurrence interval of 100 years, it often is referred to as the “100-year flood”.
What is residence time? How can it be calculated, and what assumption is involved? What are the major reservoirs of water in the earth’s crust, and approximately what is the residence time of water in each?
- If a dynamic system is gaining and losing material at equal rates, the mass of the system is in a steady state, meaning that it doesn’t change over time.
- dM/dt=Input - Output=0 (Input is equal to output)
- Residence Time=System Mass/Input or Output
- Reservoirs of Water:
> Atmospheres can accumulate water quickly to produce a new storm with a residence time of 2 weeks.
> Groundwater has long residence times and is not quickly replaced, with a residence time of 10-10,000 y.
> Oceans have the longest residence times at 4,000 y. (10,000 y > 4,000 y)
What factors can lead to flooding?
- Heavy rain
- Snowmelt
- Dam failure
- Modification of flood plains and levees
- Coastal floods from storm surge
How can we try to mitigate the impact of floods? Can upstream flood control increase downstream flooding? Briefly explain. What is channelization?
- Channelization = Straighten (and shorten) channels to improve drainage–modify the channel so that it more efficiently transports water–thus decreasing the chance of flooding
- This increases gradient in channel, leading to higher velocity and more erosion
Write Darcy’s Law and identify terms.
- Q(volume of water)=A(cross sectional area flow) x (K(permeability)x H(vertical drop)/ L(flow distance))
- A law in geology describing the rate at which a fluid flows through a permeable medium. Darcy’s law states that this rate is directly proportional to the drop in vertical elevation between two places in the medium and indirectly proportional to the distance between them. The law is used to describe the flow of water from one part of an aquifer to another and the flow of petroleum through sandstone and gravel.
Which rock types have the greatest hydraulic conductivity? Why?
- Gravel, because they have a high intrinsic permeability
What geologic characteristics make a formation a good aquifer? Why?
An aquifer is a rock material with high permeability that acts as a transport system.
- Sands and gravels
- Jointed Units
- Water occupies pours
- Sandstones and limestones
- Low permeability layers
How is hydraulic head (liquid pressure) measured?
It is usually measured as a liquid surface elevation, expressed in units of length, at the entrance (or bottom) of a piezometer
Flood Plain
- an area of low-lying ground adjacent to a river, formed mainly of river sediments and subject to flooding
Erosion
wearing away, abrasion, attrition; weathering; dissolution, corrosion, decay; deterioration, disintegration, destruction. the process of wearing off or being eroded by wind, water, or other natural agents the gradual destruction or diminution of something
Delta
a landform that is formed at the mouth of a river, where the river flows into an ocean, sea, estuary, lake, or reservoir, Deltas are formed from the deposition of the sediment carried by the river as the flow leaves the mouth of the river.
Ogallala Aquifer
Giant aquifer in Midwest, whose water level changes since 1940 have appeared to be between a decrease in 15 meters and an increase in 3 meters
Drainage basin
catchment area; area of land where surface water from rain converges to a single point at a lower elevation, usually the exit of the basin, where the waters join another waterbody, such as a river, lake, reservoir, estuary, wetland, sea, or ocean
Natural levee
An elongated embankment compounded of sand and silt and deposited along both banks of a river channel during times of flood.
Saltation
Transport of a sediment in which the particles are moved forward in a series of short intermittent bounces from a bottom surface.
Antecedent Stream
A stream that has retained its early course in spite of geologic changes since its course was assumed.
River Terrace
- a step like landform. A terrace consists of a flat or gently sloping geomorphic surface, called a tread that is typically bounded one side by a steeper ascending slope
- throughout time there are different levels of floodplains and forms terraces
Stream Gradient
the grade measured by the ratio of drop in elevation of a stream per unit horizontal distance, usually expressed as feet per mile or meters per kilometer
Rain Shadow
An area having relatively little precipitation due to the effect of a barrier, such as a mountain range, that causes the prevailing winds to lose their moisture before reaching it.
Laminar Flow
also known as streamline flow, occurs when a fluid flows in parallel layers, with no disruption between the layers
Permeability
How easy to move water through rock
Meanders
a winding curve or bend of a river or road
Hydraulic Conductivity
a property of vascular plants, soils and rocks, that describes the ease with which a fluid (usually water) can move through pore spaces or fractures
Competence
the ability to carry a load of a given size; faster rivers have a high competence.
Spring
the source of the river usually at high elevations, where the water starts to collect, arises from a perched water table/aquifer.
Recurrence interval
- RI = (N + 1)/ M
> N=number of years on record
> M=number of recorded occurrences of the event being considered - the time between events such as floods or earthquakes or other natural processes. Often we are interested in the average recurrence interval, which is determined by finding the mean of a series of recurrence intervals between events
Divide
waters on each side of the divide never meet, but flow into the same ocean
Gradient
- the grade measured by the ratio of drop in elevation of a stream per unit horizontal distance
- high gradient means steep slope and rapid flow of water
Surface runoff
the water flow that occurs when the soil is infiltrated to full capacity and excess water from rain, melt water, or other sources flows over the land
Aquitard
- a zone within the earth that restricts the flow of groundwater from one aquifer to another. An aquitard can sometimes, if completely impermeable, be called an aquiclude or aquifuge. Aquitards comprise layers of either clay or non-porous rock with low hydraulic conductivity
- same as a confining layer
Floodplain
- an area of land adjacent to a stream or river that stretches from the banks of its channel to the base of the enclosing valley walls and experiences flooding during periods of high discharge
Alluvial fan
- sediments deposited as the river widens dramatically (and the velocity of the river decreases) upon going from mountainous valley to plain.
Turbulent Flow
- the motion of a fluid having local velocities and pressures that fluctuate randomly
Aquifer
- a body of water that is held under ground and is created by the fact that the rocks that surround it are impermeable by water
Longitudinal Profile
Streamflow everywhere balances input and output (besides during floods); longer term dynamic equilibrium between erosion and sedimentation.
Braided stream
multiple “braids” (paths) to a stream, formed before meandering streams and occur when the river has a high velocity and sediment content.
Base level
the theoretical lowest elevation, to which a river may erode, generally is at sea level
Bed Load
particles in a flowing fluid (usually water) that are transported along the bed. Bed load moves by rolling, sliding, and/or saltating (hopping)
Oxbow lake
formed when a river creates a meander, due to the river’s eroding the bank through hydraulic action, abrasion, and corrosion. over time, the meander becomes very curved, and eventually the neck of the meander will become narrower and the river will cut through the neck at a time of flood, cutting off the meander and forming the oxbow lake
Water table
the ground water table is the boundary between the saturated and non-saturated zones of the Earth.
Infiltration
?
Porosity
the water storage capability in rocks. For example, non-cemented sandstone has more porosity than cemented sandstone.
point bar
a depositional feature made of sand and gravel that accumulates on the inside bend of streams and rivers below the slip-off slope
recharge
- groundwater recharge, a hydrologic process where water moves to groundwater
- surface water recharge, a hydrologic process where water runs off to surface watercourses
Hydraulic Head
a specific measurement of liquid pressure above a geodetic datum
Darcy’s Law
Q(volume of water)=A(cross sectional area flow) x (K(permeability)x H(vertical drop)/ L(flow distance))
Terrace
a step-like landform that borders a shoreline or river floodplain
Vadose Zone
- the part of Earth between the land surface and the top of the phreatric zone, the position at which groundwater is atmospheric pressure
- aka: the ground between the surface and the groundwater
Saturated Zone
the zone in rock layers where all the pores are filled with water
Perched Aquifer
an aquifer that rests on a layer of impermeable rock at a higher elevation.
Settling Velocity
- The fastest settling particles are huge, heavy, spherical molecules. The slowest settling particles, which sometimes cannot be settled accurately or properly, are tiny, light, irregularly shaped molecules. And for anything in between, here is a general guide as to what characteristics increase the rate of thickening
Artesian Aquifer
an artesian aquifer contains very high pressure, so when a well taps into it, the water squirts out without being pumped
Confined Aquifer
those in which an impermeable dirt/rock layer exists that prevents water from seeping into the aquifer from the ground surface located directly above; instead, water seeps into confined aquifers from farther away where the impermeable layer doesn’t exist
Unconfined Aquifer
those into which water seeps from the ground surface directly above the aquifer
Capacity
the total sediment carried by the river.
Groundwater Overdraft
the process of extracting groundwater beyond the safe yield or equilibrium yield of the aquifer
Cone of depression
- occurs in an aquifer when groundwater is pumped from a well
- In an unconfined aquifer (water table), this is an actual depression of the water levels
- xIn confined aquifers (artesian), the cone of depression is a reduction in the pressure head surrounding the pumped well
Tributary
stream/river that flows into a main stream, river or lake but not into the ocean. Deals with mountain divides and drainage networks that eventually lead to oceans.
Dendritic
tree-like, most common form of drainage system. Twigs of the tree are the contributing streams that join together into a river.
Drainage
system where smaller and larger rivers merge into streams, leads to the formation of a divide.
Trellis drainage
characteristic of folded mountains, where smaller tributaries feed into a river valley from steep slopes of the mountains, usually at 90 degrees
Grade
measures steepness of river.
