Water Resources Collections and Archives

Urbanization increases stormwater run-off volumes through the widespread use of impervious surfaces. This leads to localized flooding, water pollution and morphologically degraded water bodies. Rain gardens – a shallow, vegetated form of bioretention – are one strategy for mitigating these hydrologic consequences of urbanization. Rain gardens in highly urban areas typically require greater infiltration rates due to their smaller volumes. The application of urban rain gardens in a Mediterranean climate is further challenged by contrasting wet and dry seasons. These have significant implications for run-off, infiltration, treatment and plant survival. Soil selection is critical to all of these, because it performs the necessary and competing functions of drainage and plant-available water retention. An appropriate soil mix will sufficiently infiltrate stormwater to reduce run-off, while also holding some plant-available water to reduce irrigation needs during dry periods. Similarly, an appropriate plant palette will utilize species adapted to drought, well-drained soils and periodic flooding. The San Francisco Bay Area has a number of rain garden soil specifications, with little data on relative performance and minimal monitoring of existing sites. In this research, I will examine current soil specifications by regional agencies and municipalities. Furthermore, I will measure infiltration rates at nine locally sampled rain gardens sites, comparing these to cultivation needs of associated plant species. The implications for other Mediterranean climate regions, particularly southern Europe, are significant. While the Water Framework Directive recommends that stormwater be dealt with at the source, the EU and particularly the Mediterranean member states lag in rain garden implementation. A better understanding of the soil requirements for Mediterranean-climate rain gardens will advance their implementation in California and Mediterranean-climate regions generally.