Equal Conveyance and Compensatory Storage
In 2018, a friend who owns a small strip mall asked me for assistance in dealing with a wetlands issue. A wetland/floodplain is located behind, and abuts, the strip mall property. The floodplain drains via an intermittent stream which runs along the side of the property and crosses beneath the road via a concrete pipe.
The strip mall owner wanted to reconfigure the electrical signage along the road and submitted to the town a site plan for the proposed electrical work. The strip mall owner was familiar with the architect who had created a site plan for the property in 1991 for a previous owner. As a cost-saving measure, the 1991 site plan was updated by the architect, and submitted by the strip mall owner in 2018, for the sole purpose of showing the proposed electrical signage work.
Included in the 2018 site plan was the prior 1991 proposal to extend the front parking lot toward the stream by adding soil to the side of the stream and removing the same quantity of soil from the rear of the property at the edge of the floodplain. In 1991, the parking lot extension was approved and the parking lot was extended toward the stream.
The 2018 site plan submitted for the electrical work was reviewed by the town. Because the details of the 1991 parking lot extension remained on the new 2018 electrical site plan, the town examined the property beyond the limits of the proposed electrical work. The town was unable to identify from where the soil had been removed along the rear of the property in 1991. The town retained a $1500 bond from the strip mall owner and demanded that soil be removed from the rear of the property, as submitted in the 2018 electrical site plan.
FEMA produces a document called Guidance for Flood Risk Analysis and Mapping which is, for the most part, adopted into floodplain management requirements of most states and communities. When you stir the verbal soup that is the environmental ordinances and regulations, the engineering terms related to the fill/removal of soil in a floodplain which float to the top are “equal conveyance” and “compensatory storage”.
Conveyance is the movement of water from upstream to downstream in a floodplain. Obstructions (structures, fill, fences, etc.) placed in the floodplain alter the flow of water, and can cause increased flood heights upstream of the obstruction and increased velocities of floodwaters adjacent to and downstream from the obstruction. Equal conveyance is a condition where the flow and surface elevation of the flood water are not altered by the situation under consideration.
Storage is the volume of water which is retained by the floodplain. Compensatory storage is a condition where the volume of water removed by the situation under consideration (such as extending a parking lot) is added elsewhere in the same floodplain and serves the same capacity.
According to the FEMA document, once a community has adopted a floodway, it must prohibit development in the floodway unless equal conveyance has been demonstrated through hydrologic and hydraulic analyses performed using standard engineering practice that the development will not result in any increase in flood levels during the base flood. FEMA defines “any” as meaning a zero increase (greater than 0.00 feet). This analysis is usually called a “no-rise” or “zero-rise” analysis and results in a “no-rise” or “zero-rise” certification by a qualified registered professional engineer. It does not mean that you can allow a 0.1-foot or even a 0.01-foot increase – it means nothing greater than 0.00 feet. I pose the question, if your child, after looking for salamanders in your yard outside of the floodplain with a little shovel, then dumps their little pail of dirt into the floodplain behind your yard, are you in violation of floodplain ordinances and regulations?
Now back to the strip mall. The volume of fill (approximately 3000 cubic feet) that was to be added to extend the parking lot is the same volume of soil that was to be removed from behind the strip mall to provide compensatory storage. The floodplain is far greater than 16 acres. The calculated rise in the surface elevation of 3000 cubic feet of water (approximately 22,000 gallons – an inground swimming pool quantity) on the 16 acres is less than a 0.004-foot increase. In surveying, elevations and distances are measured in 100ths of a foot, which is comparable to 1/8 inch (1-96th of a foot) on a tape measure. It is reasonable and appropriate that changes of less than 0.00 feet are not measurable and that the 0.004-foot increase in surface elevation can be considered negligible and DOES meet FEMA’s definition of “not any (0.00 feet)”. I concluded that the removal of soil from the rear of the property was negligible and not required.
The day of the appeal, the strip mall owner provided pizza for his brother, me, and a lawyer (who is a personal friend of both myself and the strip mall owner), and brought us to the wetland appeals committee meeting… for the incorrect time! By the time we arrived, the committee had already decided to take the $1500 bond. Thereafter, the town hired a contractor to remove several dump trucks of soil from the rear of the strip mall property, which ended up costing the contractor more than the $1500 it received from the bond forfeiture.
While the bond levied by the town was $1500, the personal time of myself, the strip mall owner, and the lawyer far outweighed the $1500 bond. As of today, the hole in the back of the property is slowly filling back in; just as happened after having been excavated in 1991.
About the Author
Mr. Centurelli provides consultations in the areas of structural analysis, including root cause, scope of damage and value of loss of residential and commercial property. You may contact Tony for your forensic engineering needs at email@example.com or (860) 285-8000.
Learn about how EDT Forensic Engineering & Consulting approaches cause of damage, and forensic engineering by assigning a file today.