Hydraulic Impact Assessment or Groundwater Mounding Analysis is the estimation of the effects of infiltrating stormwater beneath an infiltration feature. The New Jersey Department of Environmental Protection (NJDEP) regulates the control of stormwater through the New Jersey Stormwater Management Rule. (N.J.A.C. 7:8) Revisions to this rule were enacted in 2020 which includes the hydraulic assessment of how infiltration features impact the groundwater table at a site. An updated draft of the NJ Stormwater Best Management Practices (BMPs) Manual has been issued and includes the addition of Chapter 13 – Groundwater Table Hydraulic Impact Assessments for Infiltration BMPs. This practice requires that “Whenever the stormwater management design includes one or more BMPs that will infiltrate stormwater into subsoils, the design engineer shall assess the hydraulic impact in the groundwater table and design the site, so as to avoid adverse hydraulic impacts.” (NJ DEP, 2020)
What Is Groundwater Mounding?
The introduction of infiltrating stormwater into the subsurface beneath an infiltration BMP can cause what is referred to as groundwater mounding. This concentration of water takes the shape of a temporary mound on top of the water table and in the unsaturated soil zone. This impact is illustrated in the following:
Source: NJ Stormwater Best Management Practices Manual
The height and lateral extents of the groundwater mound must be evaluated to see if there are any potential impacts to nearby structures or facilities. These potential impacts include surface ponding of water, flooding of basements or other buried structures, interference with sewage disposal systems, or the interference with the proper function of the BMP itself. This impact is a function of the Recharge Rate (infiltration / permeability) of the subsoils, the specific yield of the soils, horizontal hydraulic conductivity, geometric dimensions of the BMP, duration of infiltration, and the initial thickness of the saturated zone.
Hydraulic Analysis Key Subsurface Parameters
Two of the key parameters of the subsurface stratigraphy that are required for the hydraulic analysis are the recharge rate of the soils and the initial thickness of the saturated zone. Determination of the recharge rate has been in common practice currently used to assess the drainage capabilities of a given BMP. It is commonly accomplished by field infiltration (e.g. single (double) ring infiltrometer tests) and/or laboratory testing (e.g. permeameter). What is relatively new with this procedure is the determination of the initial thickness of the saturated zone beneath a BMP. When no supporting subsurface data is available, this initial thickness is set at 10 feet in the analysis. For Illustration purposes, using the Hantush (1967) method of analysis provided in the NJ Stormwater Best Management Practices Manual and setting the infiltration rate to 2.0in/hr., horizontal conductivity equal to the infiltration rate and the duration to 72 hours, the basin size was varied to show the impact that the initial thickness of the saturated zone has on the groundwater mound. As can be seen on the following figures, the height of the groundwater mound can be greatly influenced by this thickness parameter.
For Illustration Purposes
In Conclusion
As demonstrated by the example above, the graph of the groundwater mound can vary significantly with respect to the Initial thickness of the saturated zone. Therefore, it is important to have a thorough understanding of this initial thickness as it can have a profound impact on development. With the help of civil, geotechnical, and/or environmental engineering companies in NJ, you can get a handle on the impacts of stormwater infiltration.
If you need guidance on hydraulic analyses, you can get in contact with one of our expert engineers and schedule a meeting. Get in touch today for more information.
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