Chapter 6 - Streams and Flooding

Chapter 6 - Streams and Flooding Streams and Their Features A stream is a body of flowing water confined within a channel, regardless of size. It flows downhill through local topographic lows, carrying away water over the earth’s surface. ● Drainage Basin is the region from which a stream draws water, also known as a watershed. ● A divide separates drainage basins. ● The size of a stream at any point is related in part to the size of the drainage basin upstream from that point, which determines how much water from falling snow or rain can be collected into the stream. ● Stream size is also affected by vegetation, or lack of it. ● Long-term, sustained changes in precipitation, land use, or other factors that change the volume of water flowing in the stream will be reflected in corresponding changes in channel geometry. The size of a stream may be described by its discharge, the volume of water flowing past a given point in a specified length of time. ● Discharge is the product of channel cross section (area) times average stream velocity. ● For a given stream, discharge will vary with season/weather. ← Diagram shows the erosion (and deposition) of a small lake Diagram shows HOW erosion rates are affected by changing land uses over time (specifically in Washington DC) → Sediment Transport Water is a powerful agent for transporting material. Streams can move material in several ways. Heavier debris may be rolled, dragged, or pushed along the bottom of the stream bed as its traction load. Material of intermediate size may be carried in short hops along the stream bed by a process called salation. ● All this material is collectively described as the bed load of the stream. ● The suspended load consists of material that is light or fine enough to be moved along suspended in the stream, supported by the flowing water. ● Suspended sediment clouds a stream and gives the water a muddy appearance. ● Some substances may be completely dissolved in the water to make up the streams dissolved load. The total quantity of material that a stream transports by all these methods is called its LOAD. Stream capacity is a measure of the total load of material a stream can move. Capacity is closely related to discharge - the faster the water flows, and the more water that is present, the more material can be moved. Velocity, Gradient, and Base Level Stream velocity is related partly to discharge and partly to the steepness (pitch or angle) of the slope down which the steam flows. ● The steepness of the stream channel is called its gradient. ● Gradient is calculated as the difference in elevation between two points along a stream, divided by the horizontal distance between them along the stream channel. ● The higher the gradient, the steeper the channel, and the faster the stream flows. Gradient and velocity commonly vary along the length of the stream. Near the stream’s source, the gradient is usually steeper, and it tends to decrease downstream. Velocity may or may not decrease correspondingly. ● Velocity is also influenced by friction between water and the stream bed, and changes in the channel’s width and depth. ● Discharge tends to increase downstream, at least in moist climates, because of water added by tributaries along the way. A stream will eventually reach its end, or mouth, where it flows into another body of water. At this point the gradient is typically low. Near the mouth, the stream is approaching its base level, which is the lowest elevation to which the stream can erode downward. ● For most streams, the base level is the water level of the body of water into which they flow. For example, sea level (reaching the ocean) might be the base level for some streams. ● There may be an increase in discharge at this point due to widening of the channel and a corresponding increase in channel cross section (area). ● The downward pull of gravity not only pulls water down the channel, but also causes a stream to cut down vertically toward its base level. ● Counteracting this EROSION is the influx of fresh sediment into the stream from the drainage basin. Over time, streams tend to balance out, or reach equilibrium between erosion and deposition of sediments. The longitudinal profile of a stream assumes a concave-upward shape. Velocity and Sediment Sorting and Deposition Variations in a stream’s velocity along its length are reflected in the sizes of sediments deposited at different points. The more rapidly a stream flows, the larger and denser are the particles that can be moved. The sediments found in motionless in a stream bed at any point are those too heavy for that stream to move at that point. ● Where the stream flows most quickly, it carries gravel and even boulders along with the finer sediments. ● As the stream slows down, it starts leaving behind the heaviest, largest particles, yet continues to move the lighter material. ● If stream velocities continue to decrease, successively smaller particles are dropped. The relationship between the velocity of water flow and the size of particles moved accounts for one characteristic of stream-deposited sediments - they are commonly well sorted by size or density, with materials deposited at a given point tending to be similar in size or weight. ● If velocity decreases along the length of the stream profile, then so will the coarseness of sediments deposited. IMPORTANT - If a stream flows into a body of standing water, like a lake or ocean, the stream’s flow velocity drops to ZERO and all the remaining sediment is dropped. ● If the stream is still carrying a substantial load as it reaches its mouth, and it then flows into still standing water, a large, fan-shaped pile of sediment, called a delta, may be built up. Channel and Floodplain Evolution When a stream is flowing rapidly and its gradient is steep, downcutting is rapid. The result is typically a steep-sided valley, V-shaped in cross section, and relatively straight. But streams do not ordinarily flow in straight lines for very long. Small irregularities in the channel cause local fluctuations in velocity, which results in a little erosion where the water flows strongly against the side of the channel and some deposition of sediment where it slows down. Bends, or meanders, thus begin to form in the stream. Once meanders form, they tend to enlarge and shift downstream. ● Over time, the combined effects of erosions, meanders, and sediment deposition produce a broad, fairly flat expanse of land covered with sediment around the stream channel. This is the stream's floodplain, the area into which the stream spills over during floods, and is a normal product of streams evolution over time. Flooding The vast majority of stream floods are linked to precipitation (rain or snow). When rain falls or snow melts, some of the water sinks into the ground (infiltration). Below ground, percolation moved it through soil and rock. Most of the water becomes surface runoff, flowing downhill over the surface into lakes, streams, oceans, etc. ● Every stream has a floodplain, that is the volume of the channel sufficient to accommodate the average maximum discharge. ● In times of higher discharge, the stream may overflow its banks and flood. ● Floods occur on floodplains regularly. But more severe floods occur less often. RIVER FLOODING - It’s not just ‘getting wet!’ ● Flood waters can be incredibly powerful - people, houses, and bridges can be swept away. ● Flood water is sediment laden (filled with mud and rocks). ● Bank erosion undercuts property. ● Water borne diseases are often carried and spread. ● Recovery takes YEARS. Flood Severity Many factors together determine whether a flood will occur. The quantity of water involved and the rate at which it enters the stream system are the major factors. ● When the water input exceeds the capacity of the stream to carry that water away downstream within its channel, the water overflows the banks. ● The rate of surface runoff is influenced by the extent of infiltration, which, in turn, is controlled by the soil type and how much soil is exposed. Soils and rocks vary in porosity and permeability. ○ A very porous and permeable soil allows a great deal of water to sink in relatively fast. ○ If the soil is less permeable (meaning that water does not readily sink into or flow through it) or is covered by artificial structures, the proportion of water that runs off over the surface increases. ○ Once even permeable soil is saturated with water, any additional moisture is forced to become part of the surface runoff. ● Topography also influences the extent or rate of surface runoff: the steeper the terrain, the more readily water runs off over the surface and the less it tends to sink into the soil. ○ Water that infiltrates the soil, like surface runoff, tends to flow down-gradient, and may in time reach the stream. ○ Subsurface runoff water moves much slower than surface runoff. ○ The more gradually the water reaches the stream, the better the chances that the stream discharge will be adequate to carry the water away without flooding. ● Vegetation may reduce flood hazards in several ways. The plants may simply provide a physical barrier to surface runoff, decreasing its velocity and thus slowing the rate at which water reaches a stream. ○ Plant roots working into the soil loosen it, which tends to maintain or increase the soil permeability, and hence infiltration. This reduces the proportion of surface runoff. ○ Plants also absorb water, using some of it to grow and releasing some by evaporation. Flood Characteristics During a flood, the water level of a stream is higher than usual, and its velocity and discharge also increases as the greater mass of water is pulled downstream by gravity. The higher volume and velocity together produce the increased force that gives floodwaters their destructive power. ● Stage - Elevation of the water surface at any point. ○ Stream is at flood stage when stream stage exceeds bank height. ○ Magnitude of the flood can be described by the maximum discharge or maximum stage reached. ● Crest - When the maximum stage is reached. May occur within minutes of the influx of water. ● Upstream Floods - Floods that affect only small, localized areas. ○ Often caused by sudden, locally intense rainstorms by events like dam failure. ○ Even if the total amount of water involved is moderate, the rapidity with which it enters the stream can cause it temporarily to exceed the stream channel capacity. ○ Causes a brief but severe flood. ● Flash Flood - Variety of an upstream flood, characterized by especially rapid rise of stream stage. They can occur anywhere that surface runoff is rapid, large in volume, and funneled into a relatively restricted area. ● Downstream Flood - Floods that affect large stream systems and large drainage basins. These floods are more often result from prolonged heavy rains over a broad area or from extensive regional snowmelt. ○ Usually last longer than upstream floods because the whole stream system is choked with excess water. Stream Hydrographs Fluctuations in stream stage or discharge over time can be plotted on a hydrograph. Hydrographs spanning long periods of time are very useful in constructing a picture of the then ‘normal’ behavior of a stream and that stream’s response to flood-causing events. ● A flood shows up as a peak on the hydrograph. The height and width of that peak and its position in time relative to the water-input event depends, in part, on where the measurements are taken relative to where the excess water is entering the system. ● Upstream, where the drainage basin is smaller and the water doesn’t need to travel as far to reach the stream, the peak is likely to be sharper - with a higher crest and more rapid rise and fall of the water level - and to occur sooner after the influx of water. ● The hydrograph will show a broader, gentler peak when the arrival of water spans over a period of time. Hydrographs can be plotted with either stage or discharge on the vertical axis. Which parameter is used may depend on the purpose of the hydrograph. Flood Frequency Curve Another way of looking at flooding is in terms of the frequency of flood events of differing severity. Long-term records make it possible to construct a curve showing discharge as a function of recurrence interval for a particular stream or section of one. ● A flood event can be described by its recurrence interval - how frequently a flood of that severity occurs, on average, for that stream. ● One can refer to the probability that a flood of a given size will occur in any one year, this probability is the inverse of the recurrence interval. Flood frequency curves can be extremely useful in assessing regional flood hazards. Scientists are able to project how much of the region would be flooded in such severe and serious events. Consequences of Development in Floodplains Why would anyone live in a floodplain? One reason might be ignorance of the extent of the flood hazard. Someone living in a 100 or 200 year floodplain may be unaware that the stream could rise that high, if historical flooding has been less severe. ‘ ● Farmers have settled in floodplains since ancient times because flooding streams deposit fine sediment over the lands flooded, replenishing nutrients in the soil and thus making the soil especially fertile. ● Streams are also very scenic, popular places to live near. Nearly 65% of the world’s cities are built on rivers. Building in a floodplain may increase flood heights. The structures occupy volume that water formerly could fill when the stream flooded, and a given discharge then corresponds to a higher stage (water level). ● Then, floods that occur, are more serious. ● Filling in a floodplain land for construction similarly decreases the volume available to stream water and further aggravates the situation. Both farming and urbanization also disturb the land by removing natural vegetation, leaving the soil exposed. ● Vegetation can decrease flood hazards by providing a physical barrier to surface runoff, by soaking up some of the water, and through plants’ root action, which keeps the soil looser and more permeable. ● It can also be critical to preventing soil erosion. When vegetation is removed and erosion is increased, much more soil can be washed into streams. Thee, it can fill in the channel, decreasing the channel’s volume and reducing its capacity to carry water away quickly. Effects of Urbanization on the Hydrologic System Causes changes in… 1. Shape of the unit hydrograph 2. Total amount of runoff 3. Water quality 4. Amount of stream erosion and stream load 5. Water temperature 6. Rate of groundwater recharge and therefore ground water level 7. Stream aesthetics Some areas are completely 100% un-urbanized (like the Amazon rainforest, or in Iowa, where most of the land is farmland). Where urbanization does take place (closer to big cities = highly urbanized, very industrialized, little greenspace), rivers that flow through the area are impacted. Since the ground is impermeable due to the concrete, flooding often reaches the streets. Effects of Urbanization are ALL DUE TO: ● Permeability (percent of ground that is pervious/impervious) ● Sewer Systems (rate at which water enters an area and flows out) ○ Two types of sewer systems - sanitary and storm. Changes in Shape in Unit Hydrograph Urbanization will change the shape of the hydrograph in two specific ways: ● Peak discharge will get HIGHER as there are BIGGER floods. ● Lag time will get SHORTER as the floods get FASTER. This diagram is showing that areas that didn't flood, are now flooding! This is because of humans impact on the environment and excessive urbanization. Important to understand that peak heights increase and lag time decreases! Total Amount of Runoff Peak Discharge = Highest discharge as a result of a storm event. Calculate the peak discharge ratio using this formula... ● If 50% is serviced by storm sewers, and 50% of the ground is made impervious, storm is 2.7 times worse! ● The same storm can have different impacts/flooding depending on porosity and sewage systems. Water Quality As urbanization increases, the amount of dissolved solids in water also increases. Pollutants such as fertilizers, salts, DOC and other materials that are washed into streams reach much higher concentrations in urbanized areas. ● 1948 —> 10 ppm ● 1963 —> 34 ppm ● 1985 —> 59 ppm ● 2005 —> 90 ppm As dissolved solids increase in the water, the amount of dissolved oxygen decreases, again lowering the quality of water. Organisms will not want to live in streams with low amounts of dissolved oxygen, and the organisms that decide to stay could die. Water Temperature Urbanization causes changes in water temperature that are dangerous to the organisms living there. Can cause 10-15 degree warmer spikes in the summer and 5-10 degrees cooler drops in the winter. Organisms cannot always survive the harsh temperature changes. Amount of Stream Erosion and Stream Load Strategies for Reducing Flood Hazards Various approaches can reduce the risk of flood damage. Many of these approaches (restrictive zoning, special engineering practice, etc.) are similar to strategies applicable to reducing damage from seismic and other geologic hazards. The first step is to identify as accurately as possible the area at risk. Careful mapping coupled with accurate stream discharge data allows identification of those areas threatened by floods of different recurrence intervals. Lands that could be inundated often - by twenty five year floods, perhaps - might be restricted to land uses not involving much building. Unfortunately there is often economic pressure to develop and build on floodplain land, especially in urban areas. ● Retention Ponds/Diversion Channels ○ Flood hazards along a stream may be greatly reduced by the use of retention ponds. These ponds are large basins that trap some of the surface runoff, keeping it from flowing immediately into the stream. ○ A similar strategy is the use of diversion channels that come into play as stream stage rises. They redirect some of the water flow into areas adjacent to the stream where flooding will cause minimal damage. The diversion of water might be into farmland or recreational land and away from built-up areas, to reduce loss of life and property damage. ● Channelization ○ General term for various modifications of the stream channel itself that are usually intended to increase the velocity of water flow, the volume of the channel, or both. ○ Modifications thus increase the discharge of the stream and hence the rate at which surplus water is carried away. ○ Channel can be widened or deepen, especially where soil erosion and subsequent sediment deposition in the stream have partially filled the channel. ○ Care must be taken so that channelization does not alter the stream dynamics too greatly elsewhere. ■ Sometimes, channelization causes increased gradient, thereby increasing the stream's velocity. Then discharges upstream from the modifications increased, leading to more streak bank erosion and channel enlargement. ○ A stream channel might be rerouted by deliberately cutting off meanders to provide a more direct path for the water flow. A meandering stream often tends to keep meandering or to revert to old meanders. ○ Channelization is NOT a one-time effort! Constant maintenance is required to limit erosion in the straightened channel sections and to keep the river in the cutoffs. ○ Channelization often increases the likelihood of flooding downstream from the alterations. Effects of Channelization 1. Downcutting in channelized reach (sediments) 2. Widening in channelized reach (river gets too deep, bank collapses, adds more sediment to river) 3. Rejuvenation of Thompson Creek = downcutting and widening (downcutting occurs in a new, untouched region) 4. Headward erosion by all tributaries (adds sediment, damages roads) 5. Entrenchment of river lowers local groundwater table (wells go dry) 6. All this newly eroded sediment builds a delta (dredging is now necessary to travel down the river - needs constant maintenance) —> Sped river up through the channelized section, which caused downcutting because faster moving river = more erosion! By channelizing a river, the gradient changes... Original Gradient = 20 foot drop in 2 miles = 10 feet per mile Channelized Gradient = 20 foot drop in 1 mile = 20 feet per mile By channelizing the river and creating a larger gradient, the river is steeper and will move much faster. Downcutting and widening will cause the ground water table to drop… as the water table drops, wells go dry. ● Levees ○ Raised banks along a stream channel. ○ Some streams form low, natural levees along the channel through sediment deposition during flood events. These leeves may purposely be enlarged or created where none exist naturally to control floods. ○ Since leeves raise the height of the stream banks close to the channel, the water can rise higher without flooding the surrounding area. ○ However, confining the water to the channel, rather than allowing it to flow out into the floodplain, effectively shunts the water downstream faster during high discharge events, thereby increasing flood risks downstream. ○ Levees alter sediment patterns as well. If the stream and its load is confined by leeves, increased sedimentation may occur in the channel. This will raise stream stage for a given discharge. ● Flood Control Dams and Reservoirs ○ Construction of flood control dams at one or more points along the stream. ○ Excess water is held behind a dam in the reservoir formed upstream and may then be released at a controlled rate that does not overwhelm the capacity of the channel beyond. ■ Additional benefit includes the generation of hydroelectric power at the dam sites. ○ However, dams will restrict navigation of humans and aquatic animals downstream. Can displace people and wildlife. ○ Some large reservoirs have been found to cause earthquakes. The reservoir water represents an added load on the rocks, increasing stress, while infiltration of water into the ground under the reservoir increases pore pressure sufficiently to reactivate old faults. ● Flood Warnings