Fluvial Processes and Flooding

Summary of Important Concepts• Stream erosion is a very important force shaping the earth’s surface. A stream’s ability to

erode is related to its velocity and its discharge. • Streams transport sediment particles. Streams that end at standing bodies of water (oceans

or lakes) deposit this sediment to form deltas. Intermittent streams in arid mountain areas deposit coarse sediment to form alluvial fans.

• Flooding is the main hazard associated with streams. Flooding is measured using hydrographs -- graphs which plot water level (or discharge) over a period of time associated with the flood.

• Upland floods (upstream) occur in the higher areas of a drainage basin. These are sudden floods (“flash floods”) that move quickly through narrow valleys.

• Lowland floods occur in the lower areas of a drainage basin. These are slower but longer-lasting floods that spread out over broad areas of the stream’s floodplain.

• Urban development increases flooding because buildings, roads and parking lots decrease the infiltration of rainwater into the ground, and increase the speed with which rainwater runs off into channels.

Water on Earth is Recycled: The Hydrologic Cycle• >97% of earth’s water is contained in the oceans and roughly 3% is fresh.• Of that three %, MOST of earth’s fresh water is tied up in ice (~2%), then groundwater (next chapter), lakes, ice and streams.• Weather patterns determine the amount and location of precipitation, and the amount and time over which precipitation occurs is not constant.• Evaluation of Precipitation– Area over which the rain falls– Duration of the rain– Intensity

The Hydrologic Cycle The Fate of Precipitation

Most water evaporates off the ocean (large volume) and enters the atmosphere. Through precipitation of snow, rain and ice, water falls either directly back into the ocean or on the land.

Water that falls onto land enters streams by infiltration through the ground or by runoff.Infiltration

• The movement of water into rocks or soil through cracks and pore spaces.Runoff

• Water that flows over the land.

The Hydrologic CycleHow Water Enters a StreamA cup of land bounded by areas of high relief is called the DRAINAGE BASIN. Any precipitation that

falls within the ‘cup of land’ flows to the stream as either runoff or by infiltration. Infiltration capacity of the soil is controlled by:

– Slope of the land (steeper slopes cause water to flow as runoff rather than infiltrate).– Soil texture (we will see with groundwater, properties of soils may dictate how easily water will infiltrate—think about sand versus clay).– Nature of the vegetative cover (plants can remove water with roots and transpire the water back into the atmosphere).

EX. The United States is divided, so to speak by the Appalachian Mountains on the East and the Rockies in the Midwest. All water falling in between those two mountain ranges is in the Mississippi River Drainage basin.

StreamsA body of running water that is confined in a channel and flows under the influence of gravity.Channel width may vary from a few cm’s to several km’s. The term “stream” in geology refers to any water flowing in a channel, and so includes “rivers”, “streams”, “creeks”, “brooks”, “washes”, “arroyos” and other related features. Throughout this

lesson we will use the term stream to include all these types of features.

In general, streams begin at the headwaters at higher elevations, and discharge into other streams and lakes (relative base level) that will eventually reach the ultimate base level (sea level).

Longitudinal Profile - Elevation changes of a stream from source to mouth.Gradient - the vertical drop of a channel over a horizontal distance.The headwaters are the upper part of the stream near its source in the mountains. Upstream

regions. – Steep Gradient.Lower reaches of a stream are referred to as downstream regions.– Shallow Gradient.

The mouth is the place where a stream channel terminates and enters the sea, a lake, etc.Base level is the theoretical limit to which the stream can erode. It is, in effect, the elevation of the

streams mouth.The ultimate goal of a stream is to erode the land down to sea level!

• Streams BEGIN at headwater regions and either DISCHARGE (flow into) into other streams, into lakes or into the ocean.

• The limit to which a stream can downcut the bottom of its channel (erode) is its BASE LEVEL. The ultimate base level for fluvial processes is the ocean (weather all land to sea level). Many streams have a localized base level.

Stream Gradation• Headwater streams are located at higher elevations with steep slopes or gradients (change in elevation/distance). These streams usually have more ‘work’ (breaking down the mountains) to do in terms of weathering away land and carrying sediments downstream. These streams are termed ‘youthful’ because they are just beginning to carve the land’s surface (DEGRADING STREAMS). It is essentially a collecting system for water and sediment.• As we move further downstream, the land begins to level, slope gradient begins to become more gentle and the water velocity slows down (GRADED STREAMS). It is essentially a transporting system for water and sediment.• Ultimately a stream will discharge into a large lake or ocean. As the stream approaches its ‘base level’, the land is relatively flat, water is flowing extremely slow and sediments are deposited into many landforms (AGGRADING STREAMS). It is essentially a dispersing system for water and sediment.

In order to examine features of these three streams, we need to look at the relationship between stream velocity and how much ‘stuff’ a stream can carry (i.e. sediment). • The VOLUME of sediment a stream can carry is called CAPACITY. Think of the capacity of sediment a stream is capable of carrying. Larger streams (having a larger volume of water has a higher capacity to transport stuff).• A stream’s COMPETENCE is the MAXIMUM grain size a stream can transport. This is directly related to the velocity of the stream. Faster moving water has a higher competence because it can move larger sized materials.

The ability of a stream to erode (competence and capacity) relates to two things:1. Velocity -- the speed of the water, generally measured in feet per second. 2. Discharge -- the total amount (volume) of water carried by the stream. Discharge is

generally measured in cubic feet per second, or cfs.

Stream Velocity Friction along the bottom and sides of a stream (wetted perimeter) decreases the velocity there. The fastest moving water is therefore, in the center of the stream at the surface.

Stream Discharge

• Stream discharge is the VOLUME of water that passes a given point along a stream in a given amount of time.• Discharge (Q) depends on two things—how fast is the water moving (velocity, v) and cross sectional area (A) of the stream (to determine volume of water). • Q = A*v where velocity is measured in feet/sec and area is ft2. ft/sec*ft2= ft3/sec or cubic feet per second or CFS.• For example, if we have a lot of precipitation, stage level in the stream rises, increasing the cross sectional area of the stream (i.e. more water). Increase A and multiply it by v, will increase the discharge Q.• The higher the velocity, the faster the water moves, the more sediments can be transported, and the higher erosive capabilities of the stream.

Stream Gradation• Okay, considering what we have learned, we can examine the features of the different types of streams mentioned earlier. These three types of streams are associated with specific geologic landforms and are located at certain places on land.• DEGRADING STREAMS are streams that are young, located near headwater areas at high elevations.• GRADING STREAMS are located down stream of degrading streams (getting closer to sea level)—are said to be in equilibrium (I’ll address this later).• AGGRADING STREAMS are located closest to base level at the lowest gradients (very gentle slopes so they move slow).

Degrading Streams• Degrading streams are found in headwater regions with steep gradients. On a map view, the streams appear straight as water is flowing STRAIGHT down hill.• Steep slopes contribute to faster water velocity and erosion predominates, especially at the bottom of the stream (downcutting).• The velocity is so rapid that the stream’s sediment load is less than the stream’s capacity to carry load (water can transport more sediments than it does). Fast moving water results in a high competence.• Excessive downcutting results in a V shaped valley with steep valley walls (over time, the valley walls will mass waste into the stream channel, widening the stream—working towards a graded stream system).• These streams are characterized by rapids and may be associated with water falls. These streams DO NOT have floodplains (flat areas of land adjacent to the stream channel). Degrading streams at headwater regions effectively erode mountains at higher elevations. Water velocity is the fastest (steep gradient) and erosion most intense. Load<capacity. Associated with these streams are v shaped valleys, steep valley walls and falls. Streams are straight (not meandering). They are considered ‘youthful’ as they are beginning the erosion process at higher elevations.

Graded Streams• As erosion of degrading streams continue, the valley walls are widened and deepened—and the land surrounding the stream is less ‘steep’. • The gentler gradient of graded streams translates into slower velocity of water flow and a shift in capacity to carry sediment– typically load equals the capacity to carry sediments so the stream is said to be in equilibrium (overall transporting sediment load). Load equals the capacity. Sediments are deposited along point bars and eroded at cut banks (I.e. deposition = erosion). • Stream energy is directed laterally, where the stream slows down, meanders (an S shaped pattern on a map view)—erosion is lateral not at the base of the stream channel.• Graded stream valley shape is wider, as a result (U shaped).• Geologic features associated with graded streams include, point bars, cut banks, FLOODPLAINS, levees, oxbow lakes, etc. (View the next few slides).

Stream channels tend to meander more and more over time. The reason is because streams erode on the outsides of curves (fastest water flow), and deposit sediment on the insides of curves (slowest water flow).

The eroding outside part of the curve is called the cut bank. The inside part of the curve where

sediment is deposited is called the point bar.

Eventually graded streams meander to such an extent that its easier to just cut off the meander. A long-term result of erosion of cut banks (outsides of bends) is that a stream may eventually cut through the neck of a tight meander, abandoning part of its channel, and forming a feature called an oxbow lake. The cutoff horseshoe shaped body of water is no longer part of the stream. Eventually the lake, being stagnant, will fill in with vegetation. Organisms will proliferate and suck the oxygen out of the water (sediments will turn what color?). As it fills in it will become swampy and then finally land when the water dries/evaporates.

Aggrading Streams• Aggrading streams are considered Old Age. • Landscape is very flat as we approach sea level.• The water velocity decreases tremendously at the gentle slopes so that the sediment load exceeds the capacity of the stream to carry it whereby deposition of sediments occurs.• Massive deposits of sediments produce geologic features called channel bars (filling of the stream channel with sediment) producing a BRAIDED stream pattern on a map view.• The channel shape is extremely wide and extremely shallow due to the filling in of sediments.• Generally, the main stream valley branches out as it approaches the ocean or large lake—these are called distributaries.• Downstream near the mouth of the stream, gradients are very low. Load>capacity so much of the sediments are deposited. Water moves very slowly. As sediments deposit, stream channel becomes shallow (sediments fill in). Braided streams heavily loaded with sediment result from channel bars that are deposited. The main channel is lost. Eventually the stream will branch apart into distributaries, right before it discharges into another body of water. These are considered OLD age streams simply because the land is essentially eroded down to base level.

The End of the Line• Ultimately streams will discharge into a larger body of water (lake or ocean).• Sediments deposited into the ocean produce a large fan shaped deposit called a delta.• New Orleans is built on the delta of the Mississippi River—technically, the delta will prograde (grow) out to sea as more and more sediments are deposited in the Gulf by the Mississippi.• Unfortunately, the large volumes of sediment in this local area cause the weight of the lithosphere to be very heavy—this results in isostatic subsidence as the lithosphere sinks into the asthenosphere—New Orleans is not below sea level due to a RISE in sea level, but because it is being PUSHED down under sea level!• If the stream is ephemeral (short lived) as in desert, mountainous areas, sediments are deposited at the base of a mountain producing an Alluvial Fan (similar shape to a delta).

DELTAS: Occur at the ends of a stream. Deltas form when a stream discharges into a lake or ocean basin. Because water velocity decreases rapidly (I.e. into stagnant water such as the Gulf of Mexico or a lake), the sediment is deposited. Deltas move when streams migrate, that is, streams can move side to side.

ALLUVIAL FANS: Similar to deltas, alluvial fans are sediments deposited by ephemeral streams (short lived) in mountainous areas. Material is deposited rapidly at the base of the cliff resulting in coarse, angular grains. Alluvial fans, too appear like a triangle.Alluvial fans can grow very large in size. Material originates from the steep walled valley (top center of picture) and spills out in a fan when deposited. After heavy rains, water charges down the canyons as flash floods, carrying large amounts of sediment as debris flows. As the debris flows slow down, this sediment is deposited to form alluvial fans -- broad sloping sheets of coarse sediment at the mouths of mountain canyons.

HydrographStream flow over time, including during flooding, is measured using hydrographs: graphs which plot water level (or discharge) over a period of time. Hydrographs are derived from instruments that measure discharge and/or stream height at key locations along the stream.

Reminder!

When water falls within a stream’s drainage basin it enters the stream by infiltration/base flow and by runoff contributing to the streams discharge.

The relationship between precipitation and the effect it has on a streams discharge can be shown in graphical form using a hydrograph.

The lag time indicated on the graph is the time between peak precipitation and peak discharge (flooding).

High infiltration = high residence time in the soil before entering the stream as base flow: results in a long lag time and low peak discharge.High runoff = water that falls to the ground enters the stream quickly; results in a short lag time and a high peak discharge.

Urban development increases flooding, because buildings, roads and parking lots decrease the infiltration of rainwater into the ground, and increase the speed with which rainwater runs off into stream channels. After urban development, floods tend to peak earlier after a rainstorm, and have higher peak discharge!

Flooding - an excessive discharge.Flooding is the main hazard associated with streams. The behavior of a flood, and the type of damage it causes, vary depending on where in the drainage basin the flood occurs.

Upstream floods occur in the headland areas, where the valleys are narrow and steep-sided (degrading streams). Upland floods are sudden floods of relatively short duration that move quickly through narrow valleys. They are commonly called flash floods because they occur so quickly. These floods cause damage mainly from the force of the rapidly moving water.

Downstream (Regional) floods occur in the lower areas of a drainage basin, where the valleys are wide and gently sloping, with broad floodplains (graded and aggrading streams). Lowland floods are slower but longer-lasting floods that spread out over broad areas of the stream’s floodplain. These floods typically cause damage from extensive wetting and deposition of sediment, rather than from the shear force of the moving water.

Types of FloodsUpstream Flash Floods• Caused by locally intense rainfall (covering only one or two tributaries) over a short period of time.• Steep Gradient.• High and Rapid runoff• Short lag time and high peak discharge.• Waters rise quickly.• Narrow V-shaped channels. No floodplains or levees.• Q = A X V. Increased discharge increases water velocity resulting in an increase in erosion.• Unpredictable (because it is a function of weather conditions), high loss of life.• Floods rapidly, recedes quickly.• Downstream unaffected.• Covers a smaller area of land.

Downstream Regional Floods• Caused by large amounts of rainfall over an extended period of time over a large portion of the drainage basin.• Shallow gradient.• High infiltration (Natural)• Long lag time and broad discharge curve.• Water rises slowly.• Wide, deep U-shaped channels with floodplains and levees.• Q = A x V. Waters rise as soil becomes saturated. Water spreads out onto the floodplain (large cross-sectional area).

• Predictable, great property damage because it covers a large area of land and flood waters remain high for long periods of time.• Recedes slowly.• Floods affect tributaries.

As a stream rises prior to flooding, its increased velocity and discharge allow it to carry more and more sediment. When the stream crests its banks and spills out onto the floodplain, the water slows down, depositing ridges of sediment along the banks called levees. The levees are often the highest places on the floodplain. When a stream floods and overtops a levee it quickly erodes a gap, leading to massive flooding of the floodplain in the region of the broken levee.

Urban development increases flooding, because buildings, roads and parking lots decrease the infiltration of rainwater into the ground, and increase the speed with which rainwater runs off into stream channels. After urban development, floods tend to peak earlier after a rainstorm, and have higher peak discharge!

Conclusions• Streams are extremely dynamic geologic processes that continually erode and reshape the landscape with the goal of weathering and transporting all continental rocks below sea level (of course tectonics keeps pushing the rocks up).• Streams have various geologic structures associated with them as a result of changes in topography (gradient), velocity, and competence and capacity to carry sediment load.• Flooding differs greatly upstream regions versus downstream regions— upstream floods are unpredictable, triggered by locally intense rain storms, quick to rise and fall and result in high loss of life. On the other hand, downstream floods have larger drainage basins—here precipitation over a large regional area will eventually make its way downstream resulting in long lived, slow to rise and fall floods—predictable so low loss of life, but over a large area and large time, much economic damage.• Urbanization can increase flooding merely by reducing infiltration of water in soils and causing precipitation to reach a stream much quicker.