Date of Award

1-1-2015

Document Type

Thesis

Degree Name

Master of Science in Coastal Marine and Wetland Studies

Department

Coastal and Marine Systems Science

First Advisor

Richard N. Peterson

Second Advisor

Erik Smith

Third Advisor

Richard F. Viso

Abstract

Due to their ability to reduce local flooding and protect receiving waters from intense stormwater pulses, stormwater detention ponds are commonly used stormwater management practices. Stormwater engineers construct ponds to moderate peak flow intensities and to allow residence time of the water within the pond to enhance nutrient removal prior to discharging into downstream ecosystems. Yet rarely, if ever, is the functionality of these ponds verified post-construction. This study aimed to compare hydrologic performance of two stormwater detention ponds located in coastal South Carolina to theoretical design plans by assessing a high resolution water budget. Inflow components of the water budget include surface inflow (sheetflow runoff and engineered drainage networks), groundwater inflow, and direct precipitation. Outflow components include evaporation, surface outflow, and irrigation withdrawal (for the pond located at Cold Stream Cove). Interactions between groundwater and pond water are an important, yet often oversimplified component of water budgets due to their temporal and spatial complexities. We use naturally occurring 222Rn as a tracer to constrain groundwater inputs to the ponds due to its high concentration in groundwater compared to receiving surface waters (often 2-4 orders of magnitude). During rain events, groundwater contributions are minimal in comparison to surface water contributions. However, over the course of the entire study, groundwater represented 4% of all water inputs to the pond at Cold Stream Cove and 30% of all water inputs to the pond at Summerall Oaks. This indicates volumetric contributions are certainly significant. Additionally, runoff generated from rain events showed a correlation to water table height, further emphasizing the importance in understanding groundwater contributions to stormwater ponds. The two studied ponds were designed under the same management regulations but each contains unique characteristics (e.g., weir designs, impervious coverage percentages, topography, pumping for irrigation) by which they respond differently to rain events. We found that the design manuals for both ponds underestimated the inflow values for our monitored rain events, implying the design plans may be significantly underestimating inflow values associated with the rain events after which they were modeled. This may result in the ponds containing post-development discharge values higher than pre-development discharge values for large-scale events.

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