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The infiltration pond and subsurface infiltration system will be connected via an equalizer pipe
' and will collect flows from the majority of impervious surfaces on the Site through a catch basin
and pipe network. The infiltration measures are designed to fully infiltrate flows from the 100-
year recurrence interval 24-hour rainfall event. In the event of system failure or of runoff
' exceeding the 100-year design storm,the infiltration system is equipped with an overflow to a
drain manhole and a connection to the off-site drainage system in Route 5/John Fitch Boulevard.
No increase in peak runoff rates will occur as a result of the development.
In addition,the proposed design also seeks to utilize the incorporation of low impact
development(LID) design principles as discussed in the Stormwater Management Report.The
' maintenance of the drainage system as outlined in the Operations and Maintenance Plan
(included in the Site Drainage Report)will help maintain the system and provide for cleaner
stormwater discharge to downstream waters.
Details of the overall stormwater management system and compliance with applicable
regulations are included in the Stormwater Management Report for the site prepared under
' separate cover.
' Details of the mounding analysis for the described infiltration system are in the following
sections.
' Mounding Analysis Methodology
A mounding analysis was performed using Visual Modflow version 2009.1 to evaluate the
temporary rise in the groundwater elevation below the infiltration basins following the 100-year
' storm event. Soil properties were selected using the results of on-site evaluations and literature
values as described in the Assumptions section of this memorandum. Hydraulic loading rates
were taken as 100% of the 100-year recurrence interval 24-hour runoff volume as calculated using
' TR-55 distributed evenly over a 24-hour period. Runoff calculations are included in the Site
Drainage Report. The water table elevation at the end of the loading period and the time for the
water table to recede to the bottom of the infiltration were recorded for the 100-year storm event.
' Because more frequent storm events result in less runoff volume to the system and therefore
would result in a lower groundwater elevation at the end of the loading period,an analysis of
more frequent storm events was not conducted.The results are presented in the attached Table 1:
' Groundwater Mounding Model Results.
A second analysis was performed to evaluate the changes to overall groundwater patterns in the
vicinity of the Project for more frequent storm events. This analysis considers impacts of the
' system under typical operating conditions. For this analysis,a 1-inch rainfall event was used. All
other inputs remained consistent with the 100-year event analysis.
' The input parameters for the model were determined as follows:
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