Chapter Two Hydrologic Analysis

Chapter Two Hydrologic Analysis

Urbanization Minnesota, New Jersey and most other states are facing population and development increases. Flood water is increasingly difficult to handle: little infiltration occurs, floodwaters arrive sooner and are deeper. Natural and agricultural lowlands are being developed: detention basins are being lost. Surface flow is much faster than groundwater. Without permeable

floodplains, infiltration does not occur and floods are much more frequent. Peak flow is higher and arrives much sooner than in wild or rural areas. Lift over mountains

Flash Floods in Rain-Shadow Deserts Normally, windward side gets rain, leeward side gets a desert. Air is forced to rise over a mountain range Air cools with increasing altitude on the windward flank Condensation and precipitation occur at high elevations. As the air descends the lee side, it warms up and can absorb moisture creating a rain shadow Precipitation in the lee is relatively rare.

Flash Floods Desert flash floods can originate when moist air masses overtop a high mountain ridge and rains occur on the leeward side, falling on the normally dry leeward watershed. Rains that fell miles away in the mountains can cause flash floods in the desert. Strong floods are few and far between, so oxidation

Forms impermeable Desert Varnish layer Deserts have few trees(many trees would have slowed the water) So Deserts get floods without a nearby storm Kerio River, Turkana District, Kenya Impermeable surfaces slow infiltration

Cause Flash Floods Desert Varnish caused by microorganism induced oxidation Chapter Two (continued)

Unit Hydrograph Method Here net rain is 1 in/hr x 2 hrs = 2 inches over 2 hr storm We want 1 inch over 2 hr storm for 2 hour UH tp = Lag time or time to peak TR = time of rise

A typical Storm gross hyetograph and gross hydrograph. The gross hydrograph can be converted to a unit hydrograph (1 inch over 1 hour, .5 inches over 2 hours, .33333 inches over 3 hours, etc.) by subtracting the base flow (gives a net hydrograph), calculating the total Direct Runoff flow from the net hyetograph , then scaling the hydrograph ordinates by the same multiplier to give the total flow of 1 inch over the storm duration. D = duration of excess rain Look at hyetograph, here 2 hours

Unit Hydrograph Hydrograph usually consists of a fairly regular lower portion that changes slowly throughout the year and a rapidly fluctuating component that represents the immediate response to rainfall. The lower, slowly changing portion is termed base flow. The rapidly fluctuating component is called direct runoff. This distinction is made because the unit hydrograph is essentially a tool for determining

the direct runoff response to rainfall. net hyetograph net hydrograph A Unit Hydrograph has 1.0 inches of Direct Runoff for the storm duration. To get a unit hydrograph, UH, first we remove the base flow amount from the

hydrograph, as shown, and calculate the net rainfall by removing the infiltration and retention storage from the hyetograph. Then scale the new hydrograph units to yield a unit hydrograph for, say, 1 inch for one hour, or 1/2 in/hr for 2 hours, or 1/3 in/hr for 3 hr, etc. Here we start with 2 inches for 2 hours net rainfall. That is, 1 inch per hour for 2 hours. To get a 2-hour UH, we want 1/2 inches/hour for two hours. So we have to divide the hydrograph ordinates by 2.

Define ordinate Notice hydrograph is not as tall Tb is the time base We divided each hydrograph ordinate by two, resulting in a 2hour Unit Hydrograph, i.e. One inch of direct runoff total from the 2 hour storm make a 1 hour UH.

Example 2-3: Convert the direct runoff hydrograph below to a 2-HR UH. In the hyetograph = 0.5 in/hr In the hydrograph, base flow = 100 cfs Figure E2-3a Draw the Net Hyetograph, and calculate the total direct runoff, in inches, over the watershed.

Net Hyetograph 1 in/hr x 2 hours = 2 inches, so 2 inches for a 2 hour storm. This is twice too big. We want 1 inch total for the storm, so we must divide each NET hydrograph ordinate by 2 We removed the base flow from the gross hydrograph, then divided

each ordinate by two, to get a unit hydrograph for a 2-hour storm. We have characterized our watershed; now we know how it will behave in a storm. Well use the usual handouts to follow along in this example, then you will do a similar homework.

Recently Viewed Presentations

  • Title (Arial bold 30 point)

    Title (Arial bold 30 point)

    VAT is a tax on consumption introduced in Kenya in January 1990 to replace sales tax. VAT Act of 1989 repealed in 2013 giving way for VAT Act, 2013. VAT is levied on consumption of taxable goods and services supplied...
  • Phylum Platyhelminthes - Daley science

    Phylum Platyhelminthes - Daley science

    Radial vs. Bilateral Symmetry. ... (the pseudocoelom) which is not completely lined with mesoderm. The "tube within a tube" body plan. This category is also composed of mostly worms. Coelomates. These animals have a "true coelom" lined with mesodermal peritoneum....
  • Unit 3A: Neural and Hormonal Systems

    Unit 3A: Neural and Hormonal Systems

    Unit 3A: Neural and Hormonal Systems. ... The word "hormone" comes from a Greek word meaning "to stir up or excite." Hormones carry messages to vital body organs. The pituitary regulates metabolism by stimulating other glands.
  • Chapter 16: Visions of National Identity

    Chapter 16: Visions of National Identity

    Under asymmetrical federalism, all provinces and territories would not share power with the federal government in the same way. Political philosopher Will Kymlicka believes that Canadians outside Quebec view Canada as a single national community. This community includes all citizens,...
  • Ionic Equilibria I: Acids & Bases

    Ionic Equilibria I: Acids & Bases

    For the acids in Table 18-4, we see that the order of decreasing acid strength for these five weak acids is: HF > HNO. 2 > CH. 3. COOH > HOCl > HCN. Conversely, in Bronsted-Lowry terminology, the order of...
  • CHAPTER 10 Integrated Marketing Communications  2014 Cengage Learning.

    CHAPTER 10 Integrated Marketing Communications 2014 Cengage Learning.

    Integrated Marketing Communications (IMC) The strategic, coordinated use of promotion to create one consistent message across multiple channels to ensure maximum persuasive impact on the firm's current and potential customers. Takes a 360-degree view of the customer. The Importance of...
  • How to Create a Simple Budget

    How to Create a Simple Budget

    If you are under budget (spending less than you have budgeted in a category) you need to find out why and adjust your budget accordingly . Tracking funds carefully will allow you to be certain to spend your resources in...
  • nomen mihi est _________ Latin 2 Spring Semester

    nomen mihi est _________ Latin 2 Spring Semester

    nomen mihi est _____ Latin 2 Spring Semester Exam Review May 2011 Stages 18-21