Infiltration Trenches for Storm water and Water Supply? 2 for 1?


The concept of horizontal wells arises from riverbank filtration concepts.  Riverbank filtration has been practiced for nearly 200 year in Europe, where the concept was to remove debris form polluted waters by drawing through the banks of rivers.  Much of the concepts for groundwater flow are related to the filtration ability of water to move through a porous media.  The concept was to dig trenches along the river and draw water from the trenches as opposed to the polluted rivers.  The concept worked relatively well.  The result is an abundant, dependable supply of high-quality water with a constant temperature, low turbidity, and low levels of undesirable constituents such as viruses and bacteria. Riverbank filtration also provides an additional barrier to reduce precursors that might form disinfection byproducts during treatment.

Now let’s look at this from another perspective, and we’ll pick on southeast Florida as is provides a great case study.  Sea level rise will inundate coastal property, both via coastal flooding and from a rise in groundwater. Since most stormwater drainage depends on gravity flow, drainage capacity will suffer as sea level rises reducing the head differential between interior surface waters and tide. Saltwater intrusion will be exacerbated. Furthermore, reduced soil storage capacity, groundwater flow and stormwater drainage capacity will contribute to increased flooding during heavy rain events in low-lying areas.  In low lying areas, current practices like exfiltration trenches will become impractical, as will dry retention will become wet retention.

Stormwater utilities will be faced with dramatic, currently unanticipated increases in capital expenditures and operating costs, and time will be needed for planning, design, securing permits and compliance. Additional local pumping stations on secondary canals will be needed to supplant the storm drainage system in order to prevent unacceptable ponding. Design capacities of these stations will depend on local rain patterns, drainage basin size and secondary canal system design.  Many will operate continuously, which means ongoing operations will increase substantially. Hundreds of pumping stations may be needed in some communities.

Permits will be a major challenge due to contaminants in the runoff as regulated by MS 4 Stormwater permits, and the inability to treat this water under the current structure. The cost and energy required for stormwater treatment would be a major concern going forward. But what if we sent this continuous flow to water plants as raw water?  All of a sudden we have a solution to two problems – stormwater and raw water supplies.  How often do you see a 2 for 1 solution?

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