An analytical approach to ascertain saturation‐excess versus infiltration‐excess overland flow in urban and reference landscapes

Abstract

Uncontrolled overland flow drives flooding, erosion, and contaminant transport, with the severity of these outcomes often amplified in urban areas. In pervious media such as urban soils, overland flow is initiated via either infiltration-excess (where precipitation rate exceeds infiltration capacity) or saturation-excess (when precipitation volume exceeds soil profile storage) mechanisms. These processes call for different management strategies, making it important for municipalities to discern between them. In this study, we derived a generalized one-dimensional model that distinguishes between infiltration-excess overland flow (IEOF) and saturation-excess overland flow (SEOF) using Green-Ampt infiltration concepts. Next, we applied this model to estimate overland flow generation from pervious areas in 11 U.S. cities. We used rainfall forcing that represented low-and high-intensity events and compared responses among measured urban versus predevelopment reference soil hydraulic properties. The derivation showed that the propensity for IEOF versus SEOF is related to the equivalence between two nondimensional ratios: (a) precipitation rate to depth-weighted hydraulic conductivity and (b) depth of soil profile restrictive layer to soil capillary potential. Across all cities, reference soil profiles were associated with greater IEOF for the high-intensity set of storms, and urbanized soil profiles tended towards production of SEOF during the lower intensity set of storms. Urban soils produced more cumulative overland flow as a fraction of cumulative precipitation than did reference soils, particularly under conditions associated with SEOF. These results will assist cities in identifying the type and extent of interventions needed to manage storm water produced from pervious areas. K E Y W O R D Sinfiltrations, storm water run-off, urban hydrology, urban soil

K in an Urban World: New Contexts for Hydraulic Conductivity

“…For example, most models assume that runoff production in urban areas is solely due to infiltration‐excess overland flow. However, Stewart et al (2019) developed rainfall‐runoff models using the same data described here, finding that runoff in urban landscapes is generated via saturation‐excess to a greater extent than previously thought, as urban variable source areas may be common (Miles and Band 2015). These findings suggest that depth to confining layer, soil structure in successively deeper horizons, and slope may be important field measurements to supplement and give context to an accurate estimate of K .…”

“…However, our field data showed that this is a poor assumption in urbanized soils, as our borehole measurements did not reveal any consistent decline in K borehole with depth up to 3 m (Figure 5). Moreover, while recent work has attempted to better account for, and improve model assumptions regarding, spatial variability in K , e.g., by using depth‐weighted profile values (Stewart et al 2019) or log‐normal spatial distributions (Morbidelli et al 2017; Goyal et al 2019), these approaches have not yet been incorporated into urban hydrologic models such as the Stormwater Management Model (Rossman 2010).…”

Event controls on intermittent streamflow in a temperate climate

“…of the system to buffer the incoming precipitation is limited by the infiltration capacity, the ability to channel subsurface storm flow, and the antecedent soil moisture (e.g. Tromp-van Meerveld and McDonnell, 2006;Bachmair and Weiler, 2014;Stewart et al, 2019).…”

Preliminary Characterization of Underground Hydrological Processes under Multiple Rainfall Conditions and Rocky Desertification Degrees in Karst Regions of Southwest China

“…Meanwhile, the timings for the bottom soil (70 cm) to reach saturation were about 100-150 min, 80-100 min and 45-55 min, respectively. Under light rain conditions, the time to generate underground runoff was 60 min after the soil reached the saturation, typical of Dunne overland flows [41,42]. However, under moderate and heavy rain conditions, the initiation of underground runoff happened before the soil reached the saturation, which is typical of Horton overland flows.…”