Methods of calculating the maximum runoff flow

Methods of calculating the maximum runoff flow

Flood is a natural phenomenon and an inevitable event from the point of view of human societies; But the event, size and frequency of floods are due to several factors that vary depending on the climatic, natural and geographical conditions of each region. What is certain is that floods are caused by rain; But studies show that there is no linear and direct relationship between these two factors. It is very important because of issues such as methods for calculating the maximum runoff flow. In the design of water structures, in addition to the volume or height of runoff due to rainfall, the maximum instantaneous intensity of runoff is also desired. The simplest relation that can be written to estimate the maximum runoff flow is the rational equation. If it rains with intensity i on the basin and the area of ​​the basin is equal to A. If the intensity of rainfall is constant and the duration of rainfall is equal to or more than the concentration time of the basin, assuming that the rainfall covers the entire surface A, the maximum runoff flow according to Runoff coefficient C will be equal to Equation 1, and if the rainfall intensity is in millimeters per hour, the basin area is in square kilometers, and the output flow is in cubic meters per second, it will be Equation 2:

Maximum flow rate

In the reasoning method, the maximum instantaneous flow is calculated on the assumption that the duration of rainfall is equal to the time of concentration of the basin. This means that the maximum rainfall, which is equal to the time of concentration of the basin, is included in the formula. Otherwise, the flow will be less than the above value. Because if the duration of rainfall is more than the concentration time, based on the intensity-duration relationship, such rainfall is less intense, and therefore the flow is reduced for this reason, and vice versa, if the rainfall period is less than the concentration time before the entire basin surface Participate: The rain will end and thus Dubai will be less. Therefore, the maximum flow rate is when the duration of rainfall is equal to the concentration time. Reasoning methods are very simple and are used in small tasks such as calculating the maximum flow of currents that must pass under bridges in road construction.



Another method used to estimate the maximum runoff flow is the Time-Area Method. The duration-area method is actually a more advanced form of the reasoning method. With this method, in cases where the rainfall intensity changes at different hours, the runoff can be estimated more accurately. Suppose it rains in a catchment for several hours and the intensity of the rain in time periods t is as shown above. Draw the lines – the navigation of the basin and the area between the two lines – the navigation A. It should be noted that contour lines are lines that connect points of focus at the same time. The amount of discharge that leaves the basin in different periods according to the reasoning formula is as follows:



It is observed that the discharge will be constantly increasing until we reach the time of basin concentration, at which time the output discharge reaches a maximum, and if the rainfall intensity does not increase, if the rainfall is longer than the basin concentration time, the hydrograph discharge will decrease. کرد.

The advantage of the duration-area method is that there is no assumption that the rainfall is uniform during the rainfall period. On the other hand, the intensity of rainfall depends on the duration of rainfall and we do not know the forecast of rainfall period in advance. Since in most cases the maximum flow forecast is desired to estimate the maximum flow that may leave the basin, it can be assumed that the duration of rainfall is equal to the time of concentration of the basin, but the intensity of rainfall in each period of the pattern Follow the area rainfall distribution. If changes in rainfall intensity over time (Hyetograph) are available, more calculations will be true. Otherwise, conventional rainfall temporal distribution patterns can be used. The simplest method is to assume that the intensity of rainfall per hour is less than the previous hour and its decrease is proportional to the relationship between the intensity and duration of rainfall in the region. For example, if the concentration time is 3 hours, in the first hour, heavy rainfall is assumed to be equal to the maximum one-hour rainfall in the region, in the second hour, heavy rainfall is equivalent to the maximum rainfall of two hours, and in the third hour, heavy rainfall is equivalent to maximum three-hour rainfall. Appearance. Of course, this is not the case, and if it rains continuously for three hours, according to the mentioned cases, its intensity in the first and second hours is more than usual, which can be considered as a factor of confidence. In this way, we can predict the maximum runoff flow that will leave the basin with great confidence. For this purpose, we perform the following operations in order:

Draw contour lines for time periods. When the concentration for small basins is calculated from the formula of the spiral, which is as follows in the metric system, it is better to know that to draw lines – navigation. Water flows in the desired time period.
Estimate the amount and then the intensity of rainfall for each time period from the rainfall distribution pattern of the region. The model proposed by the World Meteorological Organization (WMO) for the distribution of rainfall during rainfall is as follows: Using this model, it is possible to determine the amount and intensity of rainfall in each of these periods.
Calculate the output flow from the basin with the formulas of the duration-area method.