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As sea level rises, so does groundwater. Preferential pathways (utility trenches, leaky sewer pipes and stormdrains, buried stream channels) are conduits for emerging groundwater inland from the shoreline. (Image: UHM Coastal Geology Group)
Groundwater wicking between concrete slabs reflects shallow groundwater coming to the surface
Shallow groundwater wicks up the concrete wall from below
Sump pumps in the area have shallow groundwater elevation (about 2 ft. below ground surface) and are common among neighbors
During a drought, the gutter discharge pipe is seen with standing water, presumably from pumping shallow groundwater or from over-irrigation
Neighborhoods where almost every house has one or more curb drains suggests shallow groundwater which could become emergent groundwater during "King Tides"
Some neighborhoods have numerous stormdrains within a few hundred feet, indicating a high level of stormwater runoff. Sometimes rapid, high-volume runoff suggests minimal subsurface water storage capacity due to an elevated groundwater table
Of the many hydrogeology data gaps that exist in research on sea level rise (SLR), few are as consequential as the role of buried preferential pathways (in the coastal urban environment) in groundwater rise and subsequent surface flooding. The general concept of emergent groundwater as a function of SLR has been identified by the USGS and others. The emergence of groundwater above the land surface is also called emergent groundwater, groundwater rise, groundwater flooding and groundwater inundation.
The photo shows emergent groundwater after months of drought (Photo taken October 24, 2020, Jim and Olivia Jacobs)
The USGS and others have studied groundwater flooding related to sea level rise, but the scale needed to really understand the groundwater movement and the role of preferential pathways (utility trenches, leaky sewer and stormwater pipes, buried stream channels, French drains, road base gravels, etc.) needs to occur with a Pilot Study Area. Tamalpais Valley is representative of communities built on filled wetlands in the San Francisco Bay Area (SFBA) with similar geology, geomorphology, hydrogeology, groundwater elevation, urban infrastructure and development history.
With approval and encouragement from local leaders and agencies, Tamalpais Valley could be the site of dozens of small-diameter groundwater monitoring wells and stilling wells to measure groundwater and creek/wetland elevation over time. Comparing tidal data and rain data with creek and wetlands water elevations, small variations in vertical groundwater levels in wells will provide a detailed understanding of the subsurface movement of water. Groundwater emerging through preferential pathways is not addressed by current engineering measures to address SLR overland flooding nor is it found in SLR vulnerability assessment documents, however, this proposed study would address many of the scientific data gaps in the sea level rise literature.
We are trying to locate evidence and location of emergent groundwater, especially during the highest tides (commonly called "King Tides."). Neighbors can participate by taking photos of water coming to the surface in unpaved areas, in streets, in cracks in pavement, through drains, or into sump pumps. Please email these photos to email@example.com and please provide a date and time of the photo and take enough photos so the location can be identified. An address or map will also help. Documenting the locations of emergent groundwater will help in the mapping process and on where to install groundwater elevation wells.
Tamalpais Valley is selected for this type of study, it is more likely that implementation of mitigation strategies could occur in our community. The goal is to provide this information to other communities as others are experiencing the same groundwater flooding issues associated with extreme storms and sea level rise. Please call Jim Jacobs at 510-590-1098 if you have any questions or email him at firstname.lastname@example.org Thank you!