As the ponding continues and the Combined Sewer Overflow (CSO) issue is not being resolved, the residents on 77th and 29th Avenues have contacted outside experts for more analysis.
We have been working with a hydrologist at Shelterwood Consulting, who graciously offered to present to SPU and the Task Force at our last meeting. The presentation can be found here: http://www.shelterwoodconsulting.com/Ballard_Bioretention_ponds.html
In summary, the consultant confirmed and emphasized these ponds were installed on top of Vashon Glacial Till (clay or hardpan), and therefore the water does not “perc” (or infiltrate) through this layer once it is saturated. The perc and soil tests performed by SPU indeed revealed the native soils would not be appropriate for infiltration-based bio-retention ponds. Additionally, these tests were conducted during the middle of the summer when soil is dry. SPU also neglected to add a “correction factor” to the test results, which would have shown even worse perc rates. Best Management Practices (BMPs) advise against installing bio-retention ponds with the type of soil and perc test results found in most of this pilot project area.
Additionally, although it cannot be confirmed without more testing, we believe the perch water (or groundwater) could possibly be drained as well if underground drainage were installed, exacerbating the CSO issue.
As a result of the consultant’s analysis and our extensive monitoring since implementation, we on 77th and 29th have changed our viewpoint:
Since these bio-retention ponds are not a viable Combined Sewer Overflow (CSO) solution, we would like to see this pilot project stopped, the ponds removed, the parking strips returned to their original state, and other CSO solutions sought.
Please see our updated issues on the Issues tab, and find our suggested alternative solutions on the new Options tab. Additional suggestions are welcome and will be added to the list.