New GIS Tool Models Stormwater Flows in Portsmouth


Pamela Ordung, a recent Penn State graduate and GIS technician for Tighe & Bond an engineering and environmental consulting firm with offices in Portsmouth, has developed an innovative geographic information system (GIS)-based tool to model stormwater flows into the Hodgson Brook in Portsmouth. The modeling tool promises a huge leap forward for managing stormwater in the watershed.


The Hodgson Brook is impaired, meaning levels of certain pollutants like metals, nutrients and organic chemicals are too high to meet the standards of the federal clean water act. For the past six years the Hodgson Brook Restoration Project has been working to help remove these pollutants and mitigate the impervious surfaces which cover 32% of the watershed. Projects have included working with local boards to strengthen development and buffer regulations and partnering with the city of Portsmouth and local businesses to install a number of stormwater treatment devices including tree box filters, engineered and residential rain gardens and to restore sections of stream buffer with native vegetation.

Hodgson Brook as it flows into North Mill Pond

To develop the new watershed analysis GIS tool Ordung first divided up the Hodgson Brook watershed into smaller sub-watersheds using light detection and ranging elevation data (LIDAR) then modeled stormwater surface flow for each area using ArcGIS ModelBuilder which is an application for working with maps and geographic information. Once that was done Ordung then developed a repeatable tool that can be used to define any specific area or property in the watershed. Once the study location is entered, the model then predicts the stormwater flow paths. Knowing the flow patterns of stormwater for a specific site or sub-watershed will help better understand pollutant loading patterns, changes in stream flow, and  allow for better stormwater management plans and treatment practices.

Example of the output from the tool – Red marks the boundaries of the subwatersheds, Yellow is the stormwater basins or catchment areas, where the stormwater flows.

The development of this tool not only improves the ability to monitor stormwater flow and pollution in the Hodgson Brook watershed but it also can also be used on an ongoing basis by watershed planners, managers, developers, area businesses and more to create improved stormwater management plans for existing and new development in the watershed. And importantly, the tool can also be easily transferred and integrated into other area watersheds.


It’s really exciting when new technologies and talents are harnessed to help communities and neighborhoods address the growing issue of stormwater pollution. This sort of project is essential in better managing and protecting our watersheds.


The Hodgson Brook Restoration Project is funded through in part by a Watershed Assistance Grant from the NH Department of Environmental Services with Clean Water Act Section 319 funds from the U.S. Environmental Protection Agency.


For more info on this project and tool feel free to contact Candace Dolan, Hodgson Brook Watershed Coordinator at (603) 559-1529 or via email at


Or Pamela Ordung, GIS Technician at Tighe & Bond, at

(508) 471-9626 or via email at

PREP is part of the U.S. Environmental Protection Agency’s National Estuary Program, a joint program between local, state and federal agencies established under the Clean Water Act with the goal of protecting and enhancing nationally significant estuarine resources. PREP is supported in part by an EPA matching grant and is housed within the School of Marine Science and Ocean Engineering at the University of New Hampshire.