## Rational method

## Rational method

(OP)

Please explain what the parameter "Percent Impervious Area" means and how to use it when calculating the runoff coefficient?

Why do the values of runoff coefficients for different soil groups (A,B,C,D) differ depending on the land use?

Please give an example of calculating the runoff coefficient for areas with mixed land use, in particular when commercial and industrial areas are used.

In all the documents that I studied, the depth of the pipe depends only on the depth of freezing of the soil or equal to it. Is there really no other circumstances that affect the depth of the pipe and how is it calculated? I correctly understand that the depth of the pipe is always taken equal to the depth of freezing of the soil?

In many documents that I studied, the minimum velocity in a sewer pipe is 2 feet per second, do I understand correctly that a minimum speed of 2 feet per second is suitable for all diameters, and not just minimum ones?

This table shows the minimum slopes for pipes of different diameters, please explain how these minimum slopes are regulated, as I understand it, the minimum slopes give a minimum speed of 2 feet for pipes with appropriate diameter. Or are these minimum diameters dictated by the design features of the pipe?

Why do the values of runoff coefficients for different soil groups (A,B,C,D) differ depending on the land use?

Please give an example of calculating the runoff coefficient for areas with mixed land use, in particular when commercial and industrial areas are used.

In all the documents that I studied, the depth of the pipe depends only on the depth of freezing of the soil or equal to it. Is there really no other circumstances that affect the depth of the pipe and how is it calculated? I correctly understand that the depth of the pipe is always taken equal to the depth of freezing of the soil?

In many documents that I studied, the minimum velocity in a sewer pipe is 2 feet per second, do I understand correctly that a minimum speed of 2 feet per second is suitable for all diameters, and not just minimum ones?

This table shows the minimum slopes for pipes of different diameters, please explain how these minimum slopes are regulated, as I understand it, the minimum slopes give a minimum speed of 2 feet for pipes with appropriate diameter. Or are these minimum diameters dictated by the design features of the pipe?

## RE: Rational method

2. The better Hydrologic Soil Group rating, the lower the runoff potential (for example, less runoff from well drained sand than clays). Also, the more impervious, the greater runoff potential. So, the charts you reference are combining these factors.

3. For mixed land use, I would either:

a. pick a use that approximates the actual impervious percentage.

b. calculate the compound coefficient using individual cover areas (grass, paving, roofs, etc.)

4. The structural integrety of the pipe is a significant determining factor for depth of pipe. There is typically minimum depth requirements for particular loads published by manufacturers and/or regulatory agencies.

5. It is assumed that a minimum velocity of 2 fps will help keep sediments from accumulating in the pipe. This is typically calculated using Manning's equation. Velocity will vary with size, slope, and roughness coefficient for a given flow. The chart you provided is the slope that will provide the minimum at each size when flowing full for a specific roughness coefficient.

## RE: Rational method

## RE: Rational method

## RE: Rational method

## RE: Rational method

In order to calculate the runoff coefficient, you need to know the % of the area of the different Land Use Types; what type of storm event (2 & 10 vs 25, 50 or 100), Hydrologic Soil Type (A, B, C or D) and the slope of the land. Using the example of a watershed consisting of 65% High-density Residential, 5% park (Open Space), 25% commercial and 5% interstate; All of the soils are Type A; for the 25-year storm; and the slope is 2 - 6 percent the runoff coefficient would be the following:

65%(0.60) + 5%(0.16) + 25%(0.89)+5%(0.71) = 0.656

Depending your watershed, you could have multiple soil types, slope ranges, etc. that would cause the formula more complicated (e.g., if the slopes in the residential was 20% <2% and 80% 2-6%, the formula for the residential portion of the runoff coefficient calculation would be 65%(20%*0.58+80%*0.60)). If you are wanting to determine the maximum runoff, you could take the conservative approach and just use the higher number (i.e., 65%(0.60)).

## RE: Rational method

1. I would also like to ask a question about the slopes that are used in the hydraulic calculation of rainwater networks. As far as I understand, the estimated bias of the rainwater drainage network can be accepted by anyone, provided that it provides Vfull>Vmin and Qfull>Qdes For example, 0.001; 0.0015; 0.002; 0.0025; 0.003; 0.0035; 0.004; 0.0045; 0.005 and so on. Or even 0.0011; 0.0012; 0.0013; 0.0014; 0.0015; 0.0016; 0.0017; 0.0018; 0.0019 i.e. a finer ranking than in the example above. That is, modern equipment during the construction of the network can provide such a small ranking (0,0014; 0,0015; 0,0016) and can create any slope of the pipe?

2. And as far as I understand, is the maximum maximum rainfall rate in the rainwater drainage network 12 feet per second?

## RE: Rational method

In order to calculate the runoff coefficient, you need to know the % of the area of the different Land Use Types; what type of storm event (2 & 10 vs 25, 50 or 100), Hydrologic Soil Type (A, B, C or D) and the slope of the land. Using the example of a watershed consisting of 65% High-density Residential, 5% park (Open Space), 25% commercial and 5% interstate; All of the soils are Type A; for the 25-year storm; and the slope is 2 - 6 percent the runoff coefficient would be the following:

65%(0.60) + 5%(0.16) + 25%(0.89)+5%(0.71) = 0.656

Tell me, please, in the end, what is the correct use of land uses, residential development of commercial industrial or types of soils?

## RE: Rational method

2. The maximum rainfall rate should be based upon rainfall frequency maps. Depending on the storm's duration and frequency for the area will determine the amount of water from a storm.

3. In order to determine the run-off coefficient for an area, you need to know the percentage of each type of land use; the design storm event; the soil types and the slope of the soils.

## RE: Rational method

1) 0.001; 0.0015; 0.002; 0.0025; 0.003; 0.0035; 0.004; 0.0045; 0.005 ranking through 0.0005 (that is, it is possible to set the slope multiple of 0.0005 during the calculation)??????

2) 0,0011; 0.0012; 0.0013; 0.0014; 0.0015; 0.0016; 0.0017; 0.0018; 0.0019 ranking through 0.0001 (when calculating, it is possible to accept any slope)??????

!!!!!I'm interested in what slope values can be taken when designing sewage networks!!!!

## RE: Rational method

You will be limited for slope based upon the following: site topography, frost depth, pipe deflection and installation equipment limitations. At a minimum, you probably will be required to maintain the slope of the sewers to meet the site topography, otherwise you may have pipes that are too shallow (e.g., above the frost line). Depending on the accuracy of the installation equipment and operator can also be factors when installing sewers. Finally, the soil geology may impact your slopes, especially if you have shallow bedrock. The cost for installing pipes in bedrock is prohibitively expensive.