1. Legal basis — why a verification is required at all
At the federal level, § 55 (2) WHG provides: stormwater is to be infiltrated, allowed to seep away or discharged into a body of water close to where it falls, directly or via a sewer without mixing with wastewater, unless water-law or other public-law provisions or water-management concerns stand in the way.
In NRW, § 44 LWG concretises this requirement: stormwater from properties first built on, paved or connected to the public sewer after 1 January 1996 is to be disposed of in accordance with § 55 (2) WHG — infiltrated, allowed to seep away or discharged into a body of water close to where it falls, unless provisions or water-management concerns stand in the way (§ 44 LWG NRW).
The water-rights permit per § 8 WHG is issued on the basis of this verification. Without it, the water authority can generally not decide — and the entire project is regularly delayed as a result.
2. What the water authority actually checks in 2026
The review by NRW's lower water authorities (Untere Wasserbehörden — the water authorities at district and city level) follows the codes of practice. Six core points are expected in a standard infiltration verification:
Water authority review grid
- 1Location-specific KOSTRA-DWD 2020 rainfall data extract (5 km grid, durations from 5 min to 7 days) at the return period governing the specific verification (design frequency per DWA-A 138-1: n = 0.5/a to 0.02/a — depending on the protection category, Tables 8 and 12)
- 2kf-value from a geotechnical soil report per DIN 18130-1 — with multiple boreholes in heterogeneous subsoils
- 3Mean highest groundwater level (MHGW) — governs the thickness of the seepage zone; below 1 m clearance between system base and MHGW, the design must be agreed with the permitting authority (DWA-A 138-1, section 5.2.1)
- 4Dimensioned site plan showing infiltration system, connection lines, and elevations
- 5Dimensioning calculation including storage-to-discharge ratio and emergency overflow concept
- 6Conformity declaration with DWA-A 138-1, plus the name of the responsible designer
3. Five typical rejection reasons
When the water authority requests revisions, the issue is typically one of these five:
Where verifications fail
- 1Generic kf-value without a soil report — e.g. 'kf = 1·10⁻⁵ assumed' instead of a measured value. Generally not accepted.
- 2Missing KOSTRA data — dimensioning against generic figures ('15 l/(s·ha)') instead of KOSTRA statistics for the specific location.
- 3No groundwater level — without the MHGW, the required clearance to the system base cannot be substantiated. Regularly re-requested.
- 4Missing emission assessment for discharge — for sealed surfaces with elevated pollution loads (traffic areas, industrial) or direct discharge into water bodies, the AFS63 verification per DWA-A 102-2 is often missing.
- 5Sketch instead of calculation — a contractor's site sketch without hydraulic dimensioning does not meet the formal requirements. The authority expects a calculated design.
4. Pre-submission checklist
Before submitting the infiltration verification, verify these eight points. Each one is a typical trigger for revision requests:
Pre-submission check
- 1Geotechnical soil report on hand, explicitly stating the kf-value with measurement method (DIN 18130-1 or field test)?
- 2KOSTRA-DWD 2020 data extract for the exact location (5 km grid) attached?
- 3Highest groundwater level substantiated — either in the soil report or via LANUK NRW data?
- 4Site plan shows system geometry, connection lines, emergency overflow, and elevation marks?
- 5Dimensioning includes storage-to-discharge ratio and infiltration time as a function of layer thickness?
- 6Conformity declaration with DWA-A 138-1 attached and signed?
- 7For discharge into a water body or surfaces of load categories II/III (DWA-A 102-2, Annex A): treatment and emission verification (AFS63) accounted for?
- 8Is the responsible designer stated with name and signature?
5. What good engineering delivers beyond the minimum
A code-compliant verification is the obligation — sound engineering is the value-add. Three points we include in every project, beyond what DWA-A 138-1 strictly demands:
Climate robustness, honestly documented: DWA-A 138-1 does not require a blanket climate surcharge — according to the standard itself, its design approaches already contain safety margins, and no validated surcharge factor can currently be proposed (DWA-M 104 in preparation). Where we apply additional reserves beyond that, we state the value and rationale explicitly, so the authority can trace the robustness of the design.
Hybrid systems for borderline soils: When the kf-value is at the lower limit (around 5·10⁻⁶ m/s), we combine an infiltration swale with retention storage rather than operating the system right at its edge.
Emergency overflow with destination proof: Where the design storm is exceeded, we substantiate the ordered overflow into a defined receiving feature — not just 'overflow provided'.