The next topic in hydraulics and hydrologic systems concerns of hydrology principles. In this segment, we'll look at rainfall runoff relations by the curve number method, the rational formula for predicting runoff, and the concept of hydrographs. There are only a few topics discussed in the reference handbook, as shown here. And the first one is the prediction of runoff from a catchment from rainfall excess. And the method that's given here is the National Resources Conservation Service, which also, which used to be known as the Soil Conservation Service, so-called curve number method, as given here. This is empirical method which is based on observations of runoff from fairly small catchments and hillside plots, slope plots. And in this equation given here, Q is the total runoff in inches. S is the storage capacity, also in inches, which is the change in the stored aquifer water volume. P is the gross rainfall in inches. And CN is the so-called curve number, which classifies the type of land use or the soil type involved. If the watershed varies, for example, if we have some grassland, some parkland some impervious areas such as parking lots, we have to divide it into regions and analyze each one separately, or, alternatively, compute an equivalent curve number by weighting the curve numbers by area. And I'll give an example of that. To illustrate this, here's a question. The gross precipitation on a watershed is 8 inches and the maximum storage is 0.5 inches. The total runoff is mostly nearly which of these? So, here's our basic curve number equation from the previous slide. And in this slide, then, P is equal to, is giving us 8 inches, and the storage is giving us 0.5 inches. So the answer is simply 7.43 inches, and the closest answer is B. The next example, we have a park which consists of 10 acres of lawns which have a curve number of 70, and a one acre parking lot which has a curve number of 98. If the total precipitation is 11 inches, the runoff is most nearly which of these? So in this case, the first step is to compute an equivalent curve number, which is weighted, which is equal to the area 1 times curve number 1 plus area 2 times curve number 2 divided by the total area. So, in this case, we have ten acres of 70, one acres of 98. And the combined area is 10 plus 1, which gives me an equivalent curve number of 73. So, S, then, the storage, is 1000 divided by the curve number minus 10, is equal to 3.7. And now, going to the curve number equation, we have this. So substituting in the numbers, the answer is 7.54 inches, and the closest answer is C. The other major formula is to predict peak runoff from the so-called rational formula. And here the equation Q is equal to CIA, where C is the so-called runoff coefficient, which again varies with the type of land. For example, 0.05 for farmland and parks, up to 0.95 for impervious urban areas. And this equation predicts the peak or the the maximum, the highest runoff you get as a function of rainfall. So in this example, we have, water runs to a collector from two fields. One is two acres with a runoff coefficient of 0.45. The other four acres with a runoff coefficient of 0.75. If a storm with a rainfall intensity of 4.5 inches per hour occurs, the peak runoff is most nearly which of these? So again, because we have different soil types or land types here, we have to compute an equivalent runoff A1 C1 plus A2 C2 divided by A1 plus A2, which is equal to this. So our equivalent runoff coefficient for this combined composite area is 0.65. And now, substituting into the rational formula, we get 0.65 times 4.5 times 6, which is 17.6 cubic feet per second. And the closest answer is B. The other topic which is mentioned in the reference handbook is, is hydrographs. And the hydrograph just gives the variation of flow in a channel, for example, a river, or a stream, et cetera, after a rainfall event. And in particular we have the concept of the unit hydrograph, which is the hypothetical response, in other words, the volume and timing, to a unit input, for example, one inch of rainfall. So most likely, the only questions related here would be conceptual or descriptive relating to these basic definitions. And this concludes the discussion of hydrology.
