Whether it is agricultural or civil drainage, in order to correctly approach the design of a drainage system, it is necessary to have a set of basic data and knowledge, which can be obtained through case-by-case studies and surveys. To create a draining soil, it is necessary to identify and establish the source of water infiltration, ascertain the percolating characteristics of the soil, determine the permeability coefficient K and the depth of the first impermeable layer.
For the drainage of agricultural or civil land, it is always necessary to have a complete plano-altimetric survey of the area, on which the heights of the land, existing buildings and infrastructures (road network, irrigation network, other watercourses, inhabited areas, etc.) are highlighted. In the case of home perimeter drainage or foundation drainage, for example, information on the extent of the green area around the house or the depth of the foundation can be obtained from the survey.
The first step in designing a drainage system is to determine the specific flow rate to be drained.
Excess water in the soil may originate from:
Here we focus only on water generated by meteoric events.
Water generated by rainfall is classified on the basis of its different final destination into:
The calculation of the amount of rainwater that must be evacuated by the drainage pipes is performed on the interflow: in fact, both surface runoff and deep recharge water, not being stagnant, generally do not decrease the mechanical stability of the soil. The amount of water for each destination varies depending on the type of soil structure and texture; as a guide, it can be stated that 5% of rainfall constitutes surface runoff and 95% filters into the ground, of which 25% is deep recharge and 70% from interflow that is captured by drainage systems. In this case, therefore, the pipes or drainage cells should be sized for a specific flow rate of 70% of the rainwater.
Once the flow rate to be drained has been established, the diameter of the drainage pipes can be defined using Visser’s empirical formula:
d = 0.0209q0.375 A0.375 J-0.375
where:
d: internal diameter of the drainage pipe (cm)
q: specific flow rate of rain to be drained (mm/day)
A: the area of land affected by drainage (m2)
J: slope of the drainage pipe (%)
In addition to the flow rates, the degree of filling (maximum height of the water with which the drainage pipe will operate) must also be established, and the slopes to be adopted, in relation to the altimetric course and the level of the collectors where the drainage pipes enter; all this is necessary to ensure a regular flow of water.
The speed of the water must be such that any deposits inside the drainage pipe are removed (speeds above 0.5 m/sec are generally recommended). This can be achieved by arranging the drainage pipes with slopes that are also dissimilar to the slope of the ground surface but in any case greater than 1%.
The following guidelines must be followed for the correct and effective installation of a cracked pipe: