Growing Trees in a Gravel Bed Stormwater Retention System as a Novel Approach to Stormwater Management in Urban Sites

Dense urban areas are typically covered by impervious surfaces used to construct roadways, parking lots, and sidewalks. Sealing over soils with impervious surfaces increases stormwater runoff volume and reduces water quality downstream. Green infrastructure technologies are a commonly used stormwater control measure that can capture stormwater runoff generated from impervious surfaces. The inclusion of woody and herbaceous plants in green infrastructure mitigates stormwater runoff through canopy interception, increased soil infiltration, and evapotranspiration. However, planting trees situated amongst impervious surfaces remains difficult because they suffer from slow growth rates and shortened lifespans due to the soil compaction necessary to create a stable pavement, which diminishes the water storage capacity of soils and hampers root system development. These green infrastructure technologies in ultra-urban areas also present extremely harsh growing environments for trees due to soil moisture extremes, inhospitable microclimates, and pollution contamination. The subject of this paper is a gravel bed stormwater retention system that was developed to address the combined needs of a stable hardscape, belowground stormwater storage, and tree root development to lead to large, long-lived trees that both intercept precipitation and transpire captured runoff. The design specifications of this system are intended to be low-cost, technically simple, and highly adaptable based on site configuration and intended stormwater capture. This paper describes a pilot project on the Virginia Tech campus that constructed the first multi-tree gravel bed stormwater retention system, which aims to evaluate the functionality and feasibility of the system. This pilot study will raise awareness of trees in green infrastructure systems and provide information about the cost-effectiveness and practicality of the gravel bed stormwater retention system. Also included in this paper is an investigation of the ecophysiological and morphological adaptations that tree species confer to adapt to the unique hydrologic regime and substrate found in green infrastructure systems. Through a rigorous review of literature, a species suitability model was populated by 75 tree species. The model stratified species into groups based on foliage type and mature size to provide a diverse palette of species to allow practitioners to configure the gravel bed stormwater retention system based on cultural conditions, aboveground space limitations, and site-specific environmental challenges.