Hydrologic response in small urban watersheds: Analyses from the Baltimore Ecosystem Study

Flooding in urban environments represents a major societal hazard, especially as the world's population rapidly concentrates in urban regions. In this dissertation hydrologic, hydraulic, and hydrometeorological processes that control flooding in urban environments are examined through empirical analyses and numerical modeling studies centered on the study watersheds of the Baltimore Ecosystem Study (BES), a component of the National Science Foundation's Long Term Ecological Research (LTER) network. The BES watersheds exhibit contrasting patterns and histories of development; the Gwynns Falls watershed, and its Dead Run tributary, are the principal study regions for this work. Central research contributions of this study are closely linked to a series of field campaigns in the Dead Run watershed during the warm seasons (June--August) of 2003, 2004, and 2005. Hydrologic, hydraulic, and urban infrastructure data sets, developed as part of the 2003--2005 field campaigns, play a central role in this dissertation and represent an important contribution to urban hydrology. Urban flood studies have linked the severity of flooding to the percent imperviousness of a watershed. In this dissertation, it is shown that, in many settings, stormwater management infrastructure plays a larger role in determining urban flood response than impervious fraction. A particular focus of this study is the urban drainage network, which can include storm drain pipes, surface channels, and street gutters. It is shown that urban drainage networks, like natural river networks, exhibit characteristic structures and that these features, along with stormwater detention infrastructure, play critical roles in determining urban flood response. It is also shown that rainfall variability at time scales shorter than 15 minutes and spatial scales finer than 1 km are central elements of urban flood response. Rainfall analyses combine storm total observations from a dense rain gage network deployed for the 2003--2005 field campaigns and radar rainfall estimates derived from multiple radars in the Baltimore metropolitan region. Hydrologic modeling studies utilize the Environmental Protection Agency's Stormwater Management Model (SWMM), which provides a flexible platform for examining the impacts of urbanization on flood response. Striking heterogeneity of hydrologic response in the Gwynns Falls watershed, associated with contrasting patterns and history of urban development, is characterized based on the 10-year BES observing record. A new framework for performing flood frequency analyses on a drainage network is introduced and illustrated through empirical and modeling applications in the Gwynns Falls and Dead Run watersheds. Case study analyses of extreme flooding in the Dead Run watershed illustrate the interplay of rainfall variability and urban infrastructure in determining the flood hazards of urban environments. Although analyses are focused on metropolitan Baltimore, they provide a general picture of the hydrologic, hydraulic and hydrometeorological controls of flooding in many urban settings.