Date of Award

Winter 2011

Project Type

Thesis

Program or Major

Civil Engineering

Degree Name

Master of Science

First Advisor

Robert Roseen

Abstract

Communities are confronting the effects of rapid development and associated land use transformation, while also dealing with the serious impacts of a changing climate. Both factors influence the frequency and magnitude of flood events. This project presents a method used to assess the flood risk associated with current and projected changes in land use and climate for a 213 square mile coastal New Hampshire watershed. The evaluation includes the use of Low Impact Development (LID) as an adaptation planning tool, and, in particular, as a means for building community resiliency in managing water resources.

The hydrologic and hydraulic modeling methods used include the Army Corp of Engineers Hydraulic Engineering Center software Hydrologic Modeling System and River Analysis System, specialty tool kits, in combination with GIS.

The rainfall-runoff analysis was consistent with guidance for Federal Emergency Management Agency floodplain analysis. The land use conditions were modeled for historic, current and a future climate change scenarios. Revised precipitation data from the Northeast Regional Climate Center was used with 8.5 inches for the 100-year, 24-hr design rainfall depth, a 26% increase along the seacoast area of New Hampshire as of 2011. LID strategies, including infiltration, pervious pavements, bioretention systems, and undisturbed cover, were modeled as a runoff reduction method using revised curve numbers for the distributed storage.

Results of the hydrologic rainfall-runoff analysis, using increased rainfall depth, indicate a 45% increase in the 100-year flood flow at a USGS gaged location on the Lamprey River near Newmarket, NH. The increased flood flows raise the base flood elevations by an average of 2.7 feet along the 36 mile study reach. The conventional build-out scenario indicated an additional 0.3 feet increase in base flood elevation with a 4.3% flood flow increase of 11,109 cfs up from the 2005 flows of 10,649 cfs, and a 2.8% increase with the LID scenario of 10,952 cfs. Differences between conventional and LID build-out scenarios were minimal at the watershed scale because total impervious cover was low (<7.5%); whereas differences were substantial in developed subwatersheds with high impervious cover. Analysis of results from three smaller developed sub basins in urban settings demonstrated that LID had substantial runoff reductions for build-out scenarios and in one instance actually reduced beyond current conditions. Conventional build-out had increases in runoff ranging from 29-36% whereas LID build-out had a range of -2-7%. This last finding is substantial in that it illustrates that LID in a redevelopment scenario can serve to reduce runoff from current conditions.

The long-term watershed management implications of LID zoning as a redevelopment strategy are tremendous. It is important to note that the degree of benefit appears to increase with increasing degree of impervious cover.

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