Precipitation

Precipitation that falls on the land either evaporates, flows across the surface, or soaks into the ground. Water moving across the surface is called runoff. Water that moves across the land as a small body of running water is called stream. A stream drains an area of land known as the stream. A stream drains an area of land known as the stream drainage basin or watershed. The watershed of one stream is separated from the watershed of another by a line called a divide. Water that collects as a small body of standing water is called a pond, and a larger body is called a lake. A reservoir is a natural pond, a natural lake, or a lake or pond created by building a dam for water management or control. The water of streams, ponds, lakes, and reservoirs is collectively called surface water.

Precipitation that soaks into the ground percolates downward until it reaches a zone of saturation. Water from the saturated zone is called groundwater. The amount of water that a material will hold depends on its porosity, and how well the water can move through the material depends on its permeability. The surface of the zone of saturation is called the water table.

The ocean is the single, continuous body of salt water on the surface of the ocean with different characteristics. The dissolved materials in seawater are mostly the ions of six substances, but sodium ions and chlorine ions are the most abundant. Salinity is a measure of the mass of salts dissolved in 1,000 grams of seawater.

An ocean wave is a moving disturbance that travels across the surface of the ocean. In its simplest form a wave has a ridge called a crest and a depression called a trough. Waves have a characteristic wave height, wavelength, and wave period. The characteristics a of waves made by the wind depend on the wind speed, the time the wind blows, and the fetch. Regular groups of low-profile, long-wavelength waves are called swell. When swell approaches a shore, the time the wind blows, and the fetch. Regular groups of low-profile, long-wavelength waves are called swell. When swell approaches a shore, the waves so they approach the shore head-on. When the wave height becomes too steep, the top part breaks forward, forming breakers in the surf zone. Water accumulates at the shore from the breakers and returns to the sea as undertow, as longshore currents, or in rip currents.

Earthquakes or undersea landslides produce large, destructive waves called tsunamis. Tsunamis do not have a large wave height at open sea, but they can do tremendous damage when they reach a low coastal area. A tide moving in or out of a narrow bay can produce a wave called a tidal bore.

Ocean currents are streams of water that move through other seawater over large distances. Some ocean currents are density currents, which are caused by differences in water temperature, salinity, or suspended sediments. Each ocean has a great system of moving water called a gyre that is centered in mid-latitudes. Different parts of a gyre are given different names such as the Gulf Stream or the California Current.

The ocean floor is made up of the continental shelf, the continental slope, and the ocean basin. The ocean basin has two main parts, the abyssal plain and mountain chains called ridges.

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Flood Frequency

Everyone living near a stream needs to understand the frequency with which floods occur. Small floods happen quite frequently, every year or so. Large floods return less often, every score of years, centaury, or longer. Statistically speaking, the larger the floods, the longer are the recurrence times between each. A typical analysis of flood frequency involves a plot of historic flood sizes versus time. Flood-discharge volumes are plotted on the vertical axis, and the recurrence intervals are plotted on the horizontal axis in years between floods of each size. The longer the historical record of floods in an area, the more accurately the curve can be drawn. With a flood-frequency curve, the return times of floods can be estimated. For example, in move upward from 100 years, intercept the curve, then read to the left to obtain the expected flood size. The U.S Federal Emergency Management Agency (FEMA) uses the 100-year flood in establishing regulatory requirements.

Individual flood-frequency curves must be constructed for each stream because each stream has its own characteristic floods. A flood-frequency curve should serve as the basis for designing all structures built on a floodplain and determining where buildings should be located for the highest probability of safety. Planners can decide what size flood (how many years of protection) to accommodate when determining how the land is to be used.

When designing roads, bridges, and buildings, it is seductive to consider only the smaller floods and save large amounts of money on initial construction costs. However, these initial savings are eaten away by higher maintenance and repair costs. In the long run, it is commonly cheaper to build with respect for large floods; this not only saves money in the future but also eliminates much of the human suffering that occurs when homes and other buildings are flooded.

A designer needs to know the likelihood of a given size flood occurring during the expected usage time of a structure. Flood frequency also can be expressed as the statistical probability of stream discharges of given size arriving in any year or number of years. The bigger the flood, the longer the return period and the smaller the probability of experiencing it any one year. Statistically, the 100-year flood will occur once in 100years? The obvious answer of once is wrong; from the probability is only 63 percent. No flood has a 100 percent chance of occurring. By similar analysis, even though a “150-year flood” may occur one year, it is still possible for another of the same size to come again in the following year, or even in the same year. For example, in 1971, the patuxent River between Baltimore and Washington, D.C., had a flood that was 1.6 times bigger than its calculated 100-year flood, or any other size flood, is a statistically average event that occurs by chance, not at regular intervals.

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