Hydrologic Modeling of Urban Development Scenarios and Low-Impact Design Systems on an Undisturbed Coastal Forested Watershed under Extreme Rainfall-Runoff Events and Hydro-Meteorological Conditions in a Changing Climate

Abstract

Watershed 80 (WS80), a reference watershed located in the USDA Forest Service Santee Experimental Forest, has been undisturbed since 1937, including from the silviculture that has historically characterized the region. Therefore, the results from this study are assumed to serve as a baseline of the developmental behavior for similar watersheds along the Southeastern Coastal Plain. The purpose of this study was first to analyze and compare the outputs of two rainfall-runoff models, the NRCS program WinTR-55 and the USGS Regional Regression Equations (RREs), with historical data gathered from WS80 to examine which model most accurately fits existing peak flow data. An accurate sense of peak flows is crucial in both the conservation and planning of sites, as proper stormwater management and infrastructure preserve the integrity of both natural resources and humanmade structures. Second, the study sought to analyze the impact of hypothetical development on design peak flow rate with up to 15% watershed imperviousness using each model. Additionally, two hypothetical scenarios of low-impact design (LID) practices such as vegetative rooftops and permeable pavements on development within the watershed were examined using the Purdue University software L-THIA. The USGS RREs overpredicted peak flows by 84% at a 5-yr return period to 12% at a 100-yr return period. WinTR-55 underpredicted peak flows by 31% at a 5-yr return period to 52% at a 100-yr return period. Increases in impervious surfaces led to subsequent increases in modeled design peak flows, with the greatest post-development change in design peak flow rate occurring within the USGS model. Although results showed that neither the USGS nor WinTR-55 models accurately predicted the design peak flow data from the watershed, USGS predictions were closer to the observed values for 50-yr or higher return periods than that from WinTR-55. Though LID practices were only applied up to a hypothetical 15% of the watershed, when fully implemented they were estimated to exert a 98% reduction in runoff which translated to a total reduction in volume by 20% and depth by 16% as compared to traditional design counterparts. This hypothesized evidence indicates the merit for using LID practices for runoff management even in situations of low imperviousness.

Hydrometeorological Trends in a Low-Gradient Forested Watershed on the Southeastern Atlantic Coastal Plain in the USA

“…These cultural and socioeconomic factors associated with increased urbanization are common globally and are likely to continue in the southeastern United States [10]. The Charleston Metropolitan Area in South Carolina is one such urban center, and development within which threatens adjacent, less developed watersheds in coastal South Carolina [11]. For example, long-term watersheds at the US Forest Service Santee Experimental Forest within the Francis Marion National Forest are at the wildland-urban interface of the urbanizing Charleston area [12].…”

“…This implies an effect of a larger storm cell size yielding a longer t c as the watershed size increases [19]. The t c of a near 3 h storm duration was estimated for computing design discharges for a much smaller 160 ha WS80 watershed [11,19] adjacent to this study site. Most large peak discharges likely resulted for events when most watershed had saturated/ponded soil conditions or near surface water table (WT) depths (25-27 October 2008; 4-6 October 2015; 8 October 2016; see the following section) possibly contributing to overland surface or shallow subsurface runoff on these low-gradient poorly drained landscape [52].…”