Surface Water

Streams & Floods

Civilization has also traditionally relied on surface water for transportation, irrigation, hydropower generation, and recreation. It is no accident that most of the worlds major cities and much of the most fertile farmlands lie along large rivers.

As a society we build floodwalls, dams, reservoirs, and canals in a constant engineering effort to make surface water work for us.

Of course, the benefits we derive from rivers are counterbalanced by their tendency to flood on occasion. We tend to want to think of floods as accidents of nature that can be controlled or prevented, when actually they are normal events in the life of a river, which, as the past summer has shown, we are often powerless to influence.

So, because we rely on surface water for so many things, because we are often trying to engineer surface water to do our bidding, and because we are sometimes at its destructive mercy, we as informed citizens in a democratic society should understand some of the basic principles of stream flow and surface water behavior to help ensure that we as a society act in our own best interests when making policy about water resources in the future.

The basics of surface water

Water always flows downslope. The slope down which the stream flows is called the stream gradient. A stream tends to flow along the shortest path of least resistence in a well defined channel. It continues to flow until it reaches an area of standing water such as a lake or a sea.

The elevation of the standing water is called the base level of the stream. Base level is the level below which the stream does not flow and so cannot erode. When one speaks of base level, one is usually referring to sea level, a sort of universal base level. Other base levels that do not flow into the ocean, such as those associated with lakes or closed basins on land, are called local base levels .

The amount of water flowing past a point along the stream over a given interval of time is called the streamâs discharge. In general, discharge increases downstream as tributaries empty into and add their discharge to the stream.

The relationship of discharge at any point along the stream to its channel and its velocity is given by the equation:

Discharge(cfs) = stream width (ft) X depth (ft) X velocity ( ft )

Also moving with the water are the sediments being transported by the stream, its load.

The streamâs load consists of several components.

Bed load

Suspended load

Dissolved load

The bed load consists of sand through boulder size sediments which move either by traction (rolling sliding, or dragging) or saltation (bouncing)

The amount of load that a stream can carry is related primarily to the amount of water flowing through it (discharge) and the velocity of the flow.

The size of sediment that the stream can carry is related to the velocity of its flow.

The size, shape, velocity, and other characteristics of a stream depend on all of the previously mentioned factors and their interactions. Consequently, there are different kinds of streams, and streams can change radically if their discharge changes, as during a dry spell or a flood.

Streams erode rock and sediment over which it flows in 3 ways

Hydraulic action

Solution

Abrasion

Rocks and sediment can be picked up and moved by hydraulic action. This action can erode deep plunge pools at the base of a waterfall.

Streams flowing over limestone can gradually dissolve the rock carrying it away in solution.

A most effective erosional process on a rocky stream bed is by abrasion, the grinding away of the channel by the sediment load.

Streams change their form and load as they flow from mountains to the sea in response to changes in their gradient. Thus, a stream flowing down through the mountains across a steep gradient will erode and flow along a relatively straight path until it reaches the flatter piedmont where it will slow, widen, and deposit the coarser part of its load. Upon reaching the flat plains below the piedmont, the stream will slow, widen further, and meander to the sea.

Upon reaching the sea, a stream will slow almost to a stop, dropping most of its load. This dumping of sediment at the mouth of the river chokes the mouth with sediment and leads to a branching braided pattern of flow called a distributary system. The sediment from the river and the distributaries build out into the sea to form a river delta.

It is also common for stream discharge to vary over time as well as along the length of the streams course.

For example, many large, braided rivers are found ether at the foot of glaciers where runoff is highly seasonal or in dry areas with highly seasonal rainfall. During times of high water flow, enormous amounts of sediment are added to the river channel and carried along by the flowing water. As discharge decreases, these rivers become sediment choked and take on their braided appearence. As water flows through the small, branching channels sediment is continuously eroded and redeposited, resulting in a constant movement and shifting of the courses of the channels and their branching points over the width of the overall channel.

Meandering rivers also tend to move their channels within the larger floodplain. This process, called meandering occurs because of the tendency for curves in the river to be accentuated as the outside bank of the curve erodes due to the increase in velocity of the water travelling along the outside of the meander. Water moving along the inside slows going around the bend and drops part of its load to form a point bar. The the point bar and the outside of the meander grow, the loop becomes more extreme. Eventually, one loop may catch up with another that is slowed in its growth by more resistant banks, or a flood may cause a the river to cut a more direct channel. In either case, the abandoned loop becomes isolated from the main channel and forms an oxbow lake.

Floods

If a streamâs discharge increases to the point where the channel of the stream cannot contain its flow, the stream will overflow its banks and run out onto the floodplain. Floods are usually associated with above average periods of rainfall.

Several things happen to a river when it floods.

The increase in discharge causes the river to flow much more rapidly than usual, which greatly increases the load capacity of the river and causes it to erode its channel and carry much more and larger sediments than it normally does. Large boulders in river beds are moved during floods, and human structures such as bridge pilings and concrete barriers can be swept away by the rushing floodwaters.

When the river overflows its banks, it moves out onto the floodplain, effectively greatly increasing the area of its channel. This causes an immediate decrease in the velocity of the water as it leaves the channel, resulting in the deposition of sand adjacent to the river channel. This material forms a raised bank along the river called a natural levee. Finer grained silts and clays wash out with the floodwaters and are left to settle out onto the floodplain after the flood receeds. The great natural fertility of floodplain soils is a direct result of their periodic rejuvination by floodwaters bearing silt and clay.

During most floods, the floodplain acts as a temporary reservoir where the excess discharge is dumped and held until it evaporates away. This helps to alleviate flooding farther downstream.

Floods often cause the channel of the stream to straighten and change position within the flood plain. In fact, the flood plain itself can be defined as the expanse of land occupied by and eroded by the river over a long period of time and many shifts of position.

Over long periods of time, changes in base level, annual discharge, or load can cause a stream to erode rapidly downward to a new level that put the old flood plain out of reach of flooding. A new floodplain will form leaving the old floodplain standing above as a terrace.

Flood control and its difficulties

Because rivers are important sources of water, power, and transportation, and because floodplains are so fertile, humans like to live on them. Because we do not want the destruction and loss of life the results from flooding, efforts to control flooding are as old as civilization itself. Two main structures have been engineered for flood control.

The simplest is the artificial levee. This is a wall built along a river to prevent high water from leaving the channel. This works well for small flooding events, but it has the drawback of not letting the floodplain function as a natural resevoir, causing the excess discharge of the flooding river to move downstream until it finds a place not protected by artificial levees.

Another problem is that no levee that can be feasably constructed is high enough to contain rare, very large floods. Witness the events of last summer when many large, expensive levees were unceremoniously washed away.

The Army Corp of Engineers and the USGS keep records of floods and they determine the probability of the water reaching certain heights over certain periods of time. If you buy a house near a river, and a hydrographic map of your area shows that your house is on the hundred year floodplain, it means that, on average, a big enough flood to flood you out occurs once within every hundred years.

The natural reservoir function of floodplains can be taken up by artificial reservoirs created behind dams. In fact, the majority of dams and reservoirs built in the world are built primarily for flood control. The reservoir allows you to store water during times of high discharge and release it during times of low discharge. In effect, you store the floodwaters and let them out slowly in a controlled manner later on. An added feature of dams is that you can use the controlled discharge to generate electricity, and you can use the stored water for irrigation during dry seasons.

Dams have some big problems. First, like levees, they can be overwhelmed by large floods. This can be very dangerous if the dam fails. For example, 2200 people died in 1889 in Johnstown Pennsylvania when a dam on the river failed and a 36 foot high wall of water swept down a populated valley.

Dams also have the problem of their resevoirs being non-permanent. As the stream empties into the resevoir it dumps its load of sediment. The water leaving the reservoir through the sluice gates is mostly sediment free. So, all reservoirs in the world are slowly filling up with sand and mud. Most have projected useful lives of only a few to five hundred years or so. What do we do when they fill up? We donât know.

Dams have other major problems besides these. The mixed blessing of dams is perhaps best illustrated by the High Aswan Dam built along the Nile River in Egypt. The High Aswan Dam was completed in 1970. It greatly increased the supply of irrigation water and electrical power to Egypt, and it stopped the seasonal flooding of the Nile River. But it created unforseen new problems as well, that possibly outweigh its benefits...

The abundance of everwet irrigation canals has caused a sharp increase in the disease called schistosomiasis, caused by a parasite that is spread by snails living in the ditches.

Because the Nile does not flood anymore, it no longer receives the annual supply of fertilizing silt that has made it highly productive farmland for the last 5,000 years. Farmers must now use chemical fertilizers which carry a variety of costs, both environmental and economic.

The Nile is no longer supplying sediment to its delta, so it cannot builtd out into the sea. The sea is eroding the delta back, and normal subsidence of shoreline is causing large areas of former land to sink below sea level.

Finally, irrigation is causing the buildup of dissolved mineral salts in the soil. The annual flooding of the Nile used to flush the fields and cleanse the soils and waterways of salts. Now without this flushing, the irrigation water evaporates, leaving dissolved mineral salts behind. To rid the soils of these salts, the fields must be flushed with excess irrigation water every year. This works, but it causes an additional problem. The salts do not go away, they are simply carried down to the water table below the roots of the crops. However, the water table does not drain, and the excess irrigation is causing it to rise. When it reaches the crop roots, expensive, large scale draining systems will be needed to allow farming to continue.

Most of these problems are not peculiar to the Aswan Dam. Similar problems occur wherever flooding is controlled, or where agricultural water is supplied principally by irrigation and groundwater drainage is poor.

The Mississippi River delta is sinking as upstream dams reduce the supply of sediments and as extractiojn of oil and gas from bneath the delta accelerates subsudence.

So you see, flood control is a mixed blessing. However, the alternative is not clear. Do we move people and cities away from floodplains? Do we accept floods and rely on insurance to repair the damage? Can we sacrifice short term economic gains in agriculture to avoid long term problems, problems that might not develop for hundreds of years?

These are the decisions that our generations will have to make over the next 50 years.