Hydraulic design of the drainage system

The hydraulic design of a linear drainage system is carried out in two stages:

1. Determination of the inflow rate (surface runoff from the area to be drained),
2. Dimensioning the channel and determining the number/position of the outlets so that this inflow volume can be safely discharged. Additional measures may be required.
3. The design of an infiltration or retention facility is also required if the water cannot simply be directed into the sewer system.

Attention: It is important to distinguish between property drainage (roof and courtyard areas, designed according to building regulations) and street drainage (traffic areas, designed according to road regulations). The design values (rainfall intensity, runoff coefficient) differ depending on the application.

For land areas, a simplified calculation of the design runoff applies according to DIN 1986-100 (formerly DIN 1986-2 ). The following formula is used for the design of drainage systems on land areas: Q = c⋅r _(T,n) ⋅A/10,000

• Q = Discharge l/s
• A = connected area m²
• r _(T,n) ⋅= Design rainfall intensity l/(s*ha) for a rainfall duration T and a return period n
• c = runoff coefficient (dimensionless, ≤ 1)

Examples of the runoff coefficient (ψ):

• Sloping tile roof: 1
• Asphalt: 0.9
• Paving stones with joints or grass pavers: 0.5-0.7 (partial infiltration)

For initial estimates, ψ = 1 is often assumed to be on the safe side. Recommendation: The local rainfall intensity should be obtained from the responsible water management authority or from rainfall maps (KOSTRA-DWD). Example: Underground parking garage entrance in Aichach. Connected area (A): 50 m². Design rainfall intensity: 713.3 l/(s*ha), r (5.100) according to DIN 1986-100. Runoff width: 1, according to DIN 1986-100, as it is a ramp. Result: Maximum inflow Q (l/s) entering the channel (5 m length) is 3.57 l/s.

The size of the channel cross-section determines how much water it can safely drain. Manufacturers provide tables or diagrams that show the flow rates depending on the nominal diameter (DN). Generally speaking, the wider and deeper the channel, the higher the possible flow rate and the longer the channel run can extend to the next outlet before overflowing. Example performance:

• A channel NW 100 with a clear height of 100 mm achieves a hydraulic capacity of approximately 3–4 l/s per meter at a 0.5% gradient and a typical fill level of approximately 80%, before the cross-section transitions into the area of full filling.
• A channel NW 200 with a clear height of 350 mm and a gradient of 0.5% typically achieves a hydraulic performance of > 10 l/s per meter under comparable boundary conditions.
• The actual hydraulic capacity depends on the following points:
  • Roughness coefficient (e.g. polymer concrete k_st ≈ 90),
  • Designed fill level (usually around 80%, to allow for safety reserves against, for example, wave formation and contamination),
  • as well as the cross-sectional geometry

Procedure for planners:

• First, roughly estimate the required nominal size; for small tributaries, NW 100 is usually sufficient, while for larger areas, NW 150 or 200 is more suitable.
• Next, check whether the channel length between two drains can be dimensioned so that no overflow occurs. Manufacturer software or empirical data can be used for this purpose.

The number of required drains depends on the maximum permissible length of a drainage channel. If the existing channel exceeds this length, it must be divided into several sections, each equipped with its own drain. To avoid hydraulic overload, it is advisable to provide additional drains, especially in anticipation of heavy rainfall. Drains should generally be located at the lowest points. With a uniform longitudinal slope, this is at the end of the channel; with concave profiles, such as roads with a central gutter, it is in the middle of the channel. Often, a centrally positioned inlet box can collect water from both sides, thus making drainage more efficient. For example, consider a 60-meter-long drainage channel. One drain is positioned centrally and collects the inflow from approximately 20 meters of channel length on each side. Another drain at the end of the channel drains the remaining 20 meters. In this way, only two inlet boxes are required instead of three inlet boxes spaced 20 meters apart.