Bucknell University
IN-15: Stormwater Modeling

All new construction projects within the Commonwealth of Pennsylvania are required to adhere to all applicable state and local stormwater requirements. The specific requirements vary depending on the size, type, and location of the project. General intent is that water leaving the site post development can't exceed the pre-development value. Onsite storage/filtering is required to accommodate the delta in the pre and post development discharge in a 2 yr storm. Some of the potentially applicable codes for Bucknell projects are: East Buffalo Township Subdivision and Land Development Ordinance and Stormwater Management Ordinance Lewisburg Borough Subdivision and Land Development Ordinance and Stormwater Management Ordinance - https://ecode360.com/11770416 PA Code Chapter 102 - https://www.pacode.com/secure/data/025/chapter102/chap102toc.html West Branch Susquehanna Act 167 Stormwater Management Plan - http://www.unioncountypa.org/data/uploads/contentblock/Planning/Watershed%20Plans/West%20Branch_FINAL.pdf Consultants typically use SCS TR-55 methodology for calculating peak rate control requirements. This model is an approved modeling method per EPA's SWM guidance. Also, for new campus projects, consultants utilize the runoff curve number methodology for volume modeling. The runoff curve number method, developed by the Soil Conservation Service (now the Natural Resources Conservation Service), is a very commonly used tool for estimating runoff volumes. This is the preferred method of volume analysis of the PA DEP and is the basis for all permitting requirements. DEP provided worksheets and calculations were used to model and assess the impacts of construction activities and impact of proposed green infrastructure. The approach of the volume and rate control analysis is to consider the net change in hydrology resulting from land development and to fully mitigate changes for storm events up to an including the 2 year storm. In addition to mitigating any net change in hydrology as a result of development, the analysis considers that all non-impervious areas along with 20% of the existing impervious area were modeled as meadow in good condition in pre-development. A combination of low impact design and green infrastructure such as rain garden, bio-retention swales, green roofs, and infiltration beds are the primary best management practices use to mitigate the calculated net change in hydrology. In the NRCS curve number volume method, runoff is modeled based on precipitation, curve number, watershed storage, and initial abstraction. NRCS has empirically approximated initial losses due to depression storage, interception, evaporation and infiltration. Therefore, runoff can be calculated using only the curve number and rainfall. Curve numbers are assigned based on land cover type, hydrologic condition, antecedent moisture condition (AMC), and hydrologic soil group (HSG). Curve numbers for the site specific land covers were selected from the SCS Urban Hydrology for Small Watersheds. Because creating a weighted or composite CN value by combining impervious areas with pervious area can sometimes imply significant initial losses that may not take place, runoff from different sub areas was calculated separately and then combined to create the final volumetric runoff volume. This design method including all calculations, worksheets, and details of various types of green infrastructure are further outlined in Pennsylvania Stormwater Best Management Practices Manual. A copy of the manual can be found at http://www.depgreenport.state.pa.us/elibrary/GetFolder?FolderID=4673. The rainfall values for the project and all design computations are based on data from the National Oceanic and Atmospheric Administration’s (NOAA) National Weather Service. NOAA’s Hydrometerorological Design Studies Center provides downloadable information via the Precipitation Frequency Data Server for a given location within the United States. Based on Data provided from NOAA and historical local rainfall data it is known that the 2 year 24 hour storm is equivalent to greater than a 95th percentile rainfall event.

Stormater management is a critical issue in this geographic area. The local codes have very stringent regulations related to stormwater management and water quality. Those standards do not translate directly to the percentile questions as each specific project may yield a different answer.