The design of infiltration facilities should be in such a way that the facility will contain the design inflow without overflowing. Generally, infiltration and percolation facilities will work best for small catchment areas and they should be designed conservatively, using low hydraulic loading rates. The design loading rates should be low enough to allow the water to drain away after a rainfall event.

When a site has been judged to be an acceptable candidate for an infiltration basin (see above, 'Preconditions for infiltration'), the next step is to find the required surface area and storage volume for the facility. The size of the infiltration surface depends on the infiltration rate, which differs for different soil types and should be obtained from infiltration tests at each site.

For the design of a percolation facility Darcy's Law is used for the estimation of the percolating water:

U = k * I

with:

U:flow velocity in meters per second

k: hydraulic conductivity in meters per second

I:hydraulic gradient in meters per meter

The hydraulic gradient can be assumed to be I=1m/m. The hydraulic conductivity k can be estimated according to table 2 below, but it is recommended to perform hydraulic conductivity tests at each individual site. The hydraulic conductivity is usually specified for saturated soil. For unsaturated soil the hydraulic conductivity k_u can be assumed to equal k_u = 0.5 * k.

Table 2: Hydraulic Conductivity of Several Soil Types (Source: Urbonas & Stahre, 1993)

Using Darcy's Law and the assumptions mentioned above, the outflow of a percolation facility can be estimated at:

Q_out = 0.5 * k * A_S

with:

A_S:percolation surface area

k: hydraulic conductivity in meters per second

The assumption that Q_out equals Q_in results in the estimation of the required percolation surface area A_s, for example, of an infiltration bed or an open ditch.

with:

A_red:reduced catchment area (A_red = A * C) [m²]

A:catchment area [m²]

C:runoff coefficient (possibly as average) [-]

k: hydraulic conductivity [m/s]

I_T:rainfall intensity for a T-year storm at a storm duration t [l/(s*ha)]

The required storage volume V_req for swales, ditches and percolation basins is the difference of inflow and outflow multiplied by the duration of the rainfall event. Using iterative calculations the following equation is solved for several combinations of the duration-frequency graph until the maximum required volume is found. Usually a recurrence interval of 5 years is used for decentralised facilities and 10 years for central facilities. Another option is to simulate the behaviour of the infiltration facility using a long-term simulation based on real rainfall data for that specific region.

with:

V_req:required storage volume [m³]

A_S:percolation surface area, can be assumed to vary between 0.05 * A_red and 0.2 * A_red [m²]

A_red:reduced catchment area (A_red = A * C) [m²]

k: hydraulic conductivity in saturated zone [m/s]

I_T:rainfall intensity for a T-year storm at a storm duration t [l/(s*ha)]

t:duration of the rainfall event [min]

f_Z:safety factor (e.g. 1.2)

Concerning the calculation of percolation basins and pipe trenches it has to be noticed that A_S does not need to be included into the calculation of the catchment area, because the pipe surface is underground and does, therefore, not contribute to the catchment area.