Code-Compliant Site Drainage

Following an outstanding bachelor’s thesis, the Department of Wastewater and Waste Management has developed two applications (Adobe PDF and MS Excel) that simplify the process of performing design calculations and verification in accordance with the aforementioned regulations. Specifically, these tools address the determination of discharge volumes (stormwater/sewage/combined sewer), the sizing of property drainage channels, and the performance of flood analysis; they also address how to better account for site-specific conditions and terrain models (GIS) and geometries, and how synergies can be achieved in conjunction with ecological stormwater management measures. In this context, secondary land uses on the property (e.g., parking lots, operational areas, green spaces, flood basins, etc.) are identified, which can temporarily retain water at levels of a few centimeters without causing damage during very extreme, rare heavy rainfall events. Some municipalities already require such documentation as part of the building permit process for exposed properties.

Certainly, when consistently applied, taking this property-specific evidence into account provides flood protection for many areas (especially for the property owner and neighboring properties). However, it remains to be seen to what extent such isolated documentation—which is relevant, if at all, only for individual, large properties (>800 m² of built-up area)—and, if so, likely only in the context of new development—can make a significant or decisive contribution to the comprehensive flood protection of an entire municipality. If applied consistently, however, these assessments could lead to a significant (local) reduction in runoff into the public sewer system during extreme heavy rainfall events. According to our own research, in an average municipality, the approximately 10% of the largest impervious properties (paved properties > 800 m²) account for 40–55% of the total impervious area. These areas are often concentrated in city centers or industrial zones, as well as around exposed (public) structures such as schools, gymnasiums, or public squares. Purely residential areas are, in terms of their relevance to flood protection as defined by DIN 1986-100, only marginally affected. While hydrodynamic calculations (in combination with geodetic elevation models and urban water bodies) are likely better suited for large-scale studies, measures for protecting individual buildings / properties—in addition to DIN 1986-100 for larger structures—measures for property protection (e.g., through drainage ditches in front of garage driveways, the elevation of light wells, etc.) can be considered.

© Krueger