0000038193 00000 n The values of curve number for different land use conditions and hydrologic soil groups are given in Table 2.5. Example 10–5 Comparison of runoff estimation methods 10–13 Example 10–6 Using the runoff equation to determine variation 10–17 of runoff during a storm (210-VI-NEH, July 2004) 10–1 Chapter 10 Estimation of Direct Runoff from Storm Rainfall 630.1000 Introduction Substituting Equation 4 into Equation 3 yields. Its value varies from minimum zero for the most permeable surface to the maximum 100 for impervious (concrete) surface. On the other hand a rain that takes place for the duration less than the time of concentration, the runoff rate will be less than peak value, because the entire watershed area would not be able to yield the discharge, simultaneously to the outlet. iii. % of Runoff Length on Tangent = 67 Calculation of Runoff Length (from zero to full superelevation) = 3.6 x |-2.5-(0)/100| x 1/ 0.007 = 12.857 m Start chainage of the transition length. Vector addition of two forces graphical method, Vector addition of multiple forces graphical method.

For these individual characteristics, numerical values are assigned.

Count the number of downspouts coming from your roof. 0000017027 00000 n It consists of Runout Length and Runoff Length. In this way the eq.

Disclaimer 8. In above equation (i.e., eq. Start chainage of transition = Full superelevation chainage – New transition length, Designed by Elegant Themes | Powered by WordPress. Rainfall occurs at a uniform intensity throughout the water­shed area. VVR111 – Hydrology and Aquatic Ecology. Cook’s Method: This method consists of evaluating the four watershed characteristics, i.e., relief, … The Rational formula involves following assumptions: i. Rainfall occurs with a uniform intensity for the duration at least equal to the time of concentration of watershed; and. It is best for areas less than 100 acres, but is sometimes used for up to 2 mi2 areas. Multiply the Q inches with the drainage area and convert the units to commonly used volume units of 'ac-ft'. Also, the steep soil surface involves greater value of C, as there less water is retained or soaked into the soil.

The “C” value for roof runoff is 1.00, and the “C” for grass is 0.35. 1 0 obj n,7��{n��[΂�6_���zt��+���sy.e��!��U�&hJ|k@!�I�l�Ծ���l)^���B�T�/*H#�|S���&oG�uS����&-�u���"�.�K���tx�ޟ��䯴6]i��}B���gx��e��V�J�;��yᾅ���"�8I2��G�X��I}�� Z�v� The chapter was prepared originally by Mockus (1964), and was revised by Hjelmfelt (1998) with assistance from the NRCS Curve Number work group and H.F. Moody. 0 Unit-hydrograph technique 3. P = uncorrelated value of runoff, i.e., obtained from the runoff-curve. 2.5, against the sum of the numerical value. Before uploading and sharing your knowledge on this site, please read the following pages: 1. endobj Therefore, if the percentage is changed, the transition length will be changed as well.

If all these assumptions are satisfied to a given rainfall, then the relationship between rainfall and runoff for the watershed is being most accurate. stream Multiply the two figures together to obtain the square footage of your roof.Step 2, Determine the percentage of water collected from each downspout (P). The paved surfaces or other impermeable surfaces have a high C value. To get the CN values for other AMCs (i.e., I and III), the correction factors are applied. 2.3 may be used. The sandy or tilled areas have low C values.

This includes the lowest runoff potential, because the soils are dry enough for satisfactory cultivation to take place. �V!�#sg (�78W�%\1�'^��#�W���l� �08��w!�}4 &�{Ҩ�$eP�m�< � t����i$�,��;,~�)�>p�����9������>�h�,$��_#Fń!9a�G(E]C"�:��L�a{��#�{"�6��� ��;`~"�#�/���+��;� �7օo���1(9|%� %��:U" Ay�#&hC��'����$�%����I���G\A�.f����K� �L3� KH����Ip��-�z����� f��O��>T@�I�#$xB���($b��E���L������ ��bIc�!t��$B�4� � ����

<>>> 298 0 obj<> endobj 0000003225 00000 n % of Runoff Length on Tangent = 67 Calculation of Runoff Length (from zero to full superelevation) = 3.6 x |-2.5-(0)/100| x 1/ 0.007 = 12.857 m Start chainage of the transition length. 5 0 obj

Using equation 5 above, calculate the maximum potential retention S. S = average slope of the channel reach, m/m.

The effect of all these points on runoff has been incorporated in equation 2.10. AMC II. stream a���pz ����J ����o������S��3���i���5�Ib at�E�aN$qi��k�P�;��?�@�� PĒMPD�(bK�]Џ��!L�L���(4���p�Q���r;,!Ԓ� �$fCZ�(�E8���g-��p� Solution: Follow the steps below: Using equation 5 above, calculate the maximum potential retention S. Using equation 6 above, calculate the runoff depth. In above equation, it was assumed that the time of con­centration is approximately equal to the lag time of the peak flow. ii.

<> This method assumes that the ratio of direct runoff (surface runoff) (Q) to the rainfall depth minus the initial losses (P—Ia) is equal to the ratio of actual retention of rainfall to the S, i.e. It is defined as the rate of fall of rainfall, expressed as depth per unit time, i.e., mm/hour. ���� JFIF � � �� C �.B��#�B�J��v��8�%|!�m@.��&��8f�%g5 J#"�0�2 ���$�m`y`P�[A������#r�s�,��2B�3�"�p�p$��X�'�$�} Q�cC�26=���O䆤�8+���@h"yY ��D%�Ga��yA�(|��-Բ�E�skS+ЇE0¬�����G�8$aQCK�2r ��-9�����.� To estimate the magnitude of a flood peak the following alternative methods are available: 2. A sample example for computing the weighted runoff coefficient is shown as under – Let, a watershed has been divided into five sub parts on the basis of its soil types and land use, having the area a1, a2, a3, a4 and a5, with the values of runoff coefficient c1, c2, c3, c4 and c5, respectively. rate of (in/hr)*acre differs from cfs by less than 1 percent, the more common units of cfs are used. Haan et al. 0000002685 00000 n 0000003189 00000 n

In includes average condition of the soil regarding runoff generating potential. The computation of peak runoff by this method is shown in example 2.9. It is a dimensionless factor. 0000012962 00000 n km area. In which, A is the total area of watershed. 1. This method does not care the initial losses such as depression storage, initial infiltration, channel storage etc. ii. In the Rational method for computing the peak runoff, the intensity of rainfall should be for the time equal to the time of concentration. Therefore, the start chainage of the transition starts at 90.727. Since the runoff length at tangent is changed, therefore the transition length also will be adjusted. Copyright 9. Enjoy your reading and also feel free to post your questions/comments, which will be helpful to other users. 2 0 obj For example, a CN value of 80 has an initial abstraction of 1/2 inch, which isn't satisfied until 11.2 hours for a 2-inch 24-hour Type II storm, and significant runoff does not occur until about 11.7 hours. stream It is also expressed as the ratio of total amount of rainfall to its duration.

The C values vary from close to zero to 1.0. In which, CN is the curve number. Rainfall intensity is classified into following three categories: The time required to move the surface runoff from remotest point of the watershed to its outlet is known as time of concentration. Given the Percentage of the Runoff Length is 67%, therefore 67% of the runoff length is located at the tangent area and the remaining 33% is located in the curve region. The method is described in detail in National Engineering Handbook (2004). trailer The 0000032111 00000 n The intensity of severest rainfall during a given recurrence interval of a particular region, for the time interval of one hour is called one hour rainfall for that return period/frequency. Rational method assumes that the intensity of rainfall is uniformly distributed throughout the watershed, but it never happen. The equation (2.16) estimates the value of runoff (Q) in depth unit.

The CN method was developed by the SCS to estimate quantities of runoff generated from rainfall based Surface Runoff Calculation Methods. He justified that the small watersheds are dominated by overland flow rather than channel flow. %%EOF It has been reported from several field and laboratory studies, that the overland flow has a maximum travel time and can be maintained up to a distance of 100 to 150 meters in the watershed. 2.2. Content Filtration 6. The SCS Runoff Curve Number method is developed by the United States Department of Agriculture (USDA) Soil Conservation Service (SCS) and is a method of estimating rainfall excess from rainfall (Hjelmfelt, 1991). x�=�rI����ظ[�;����؝ 00p��e#� BH��Y�d�?u�xw�ՙ-����#�=��G���U�Y)���gVd!��d1�ZC)�.23�dRe&�L��xh2�b�hࡃ�

These values are applied to the antecedent moisture condition II only, i.e., for average condition. Again this value of runoff is modified for geographic location of the given watershed and for the desired recurrent interval. Uploader Agreement, Computation of Runoff Rate from a Watershed, Runoff Hydrograph: Meaning, Components and Factors | Geography, Runoff: Meaning, Types and Factors | Rainfall | Geography, Estimation of Runoff from a Catchment | Precipitation | Geography, How to Compute Runoff Using Unit Hydrograph? Terms of Service 7.

0000001781 00000 n Let’s teach you how to calculate and store your property’s rainwater runoff. The runoff coefficient also depends on the atmospheric characteristics like, season and rainfall etc. <> 1 0 obj Example #1: Calculate the peak storm water runoff rate from a watershed of 15 acres, with a runoff coefficient of 0.35, from a storm of intensity 2.4 in/hr. 298 23 ‘Previous Slope’ feature is now available in MiTS 2. The Rational Method is widely used to estimate the peak surface runoff rate for design of a variety of drainage structures, such as a length of storm sewer, a storm water inlet, or a storm water detention pond.

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