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.

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