BAHM (Bay Area Hydrology Model)

BAHM is a software tool for design of flow control structures to meet hydromodification management requirements for new development or redevelopment projects.

flow control structureshydromodificationmanagement



Initial contribute: 2019-12-30


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Application-focused categoriesNatural-perspectiveLand regions
Application-focused categoriesNatural-perspectiveOcean regions

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The California Regional Water Quality Control Board, San Francisco Bay Region, is requiring stormwater programs to address the increases in stormwater runoff rate, volume, and duration created by new and redevelopment projects, known as hydromodification, where those increases could cause erosion of receiving streams. Municipal stormwater discharge permits in the San Francisco Bay area contain a requirement for stormwater programs to develop Hydromodification Management Plans (HMPs) that describe how each program’s agencies will meet this requirement. The hydromodification control standard established in municipal permits is that post-project runoff shall not exceed pre-project rates and/or durations, over a defined range of storm event sizes. Research has shown that, to develop effective measures for control of changes in flow duration, the changes in a project site’s hydrology cannot be evaluated for a single storm event with traditional design storm approaches. The change in hydrology must be evaluated over a longer time frame using a continuous simulation hydrologic model, and the results used to size control measures to match pre-project flow duration patterns. These analysis methods require specialized expertise and are difficult for many developers’ engineers to perform and for municipal staff to review. During development of their HMPs, three stormwater programs in the southern San Francisco Bay area, the Santa Clara Valley Urban Runoff Pollution Prevention Program, the Alameda Countywide Clean Water Program, and the San Mateo Countywide Storm Water Pollution Prevention Program, recognized this problem and decided to jointly fund development of a tool to simplify the analysis of hydromodification effects and to help design flow control measures. The tool, known as the Bay Area Hydrology Model (BAHM), is a Bay area version of the Western Washington Hydrology Model developed by Clear Creek Solutions for the Washington State Department of Ecology. It consists of a user-friendly graphical interface through which the user inputs information about the project and desired control measure (e.g., detention basin or underground vault); an engine that automatically loads appropriate parameters and meteorological data and runs the continuous simulation model HSPF to generate flow duration curves; a module that sizes the control measure to achieve the hydromodification control standard; and a reporting module. The tool uses parameters that have been calibrated for two watersheds in Alameda County, and is in the process of being calibrated for two watersheds in Santa Clara County. This paper describes the background and need for the BAHM, development of the BAHM and appropriate parameters for the southern Bay Area, and examples of the application of the tool to size hydromodification control facilities for two development projects.

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Clear Creek Solutions (2019). BAHM (Bay Area Hydrology Model), Model Item, OpenGMS,


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