New stormwater management ordinance in the works, but don't expect a 4-inch topsoil rule

Bettendorf is in the midst of revising its stormwater management ordinances, but if the initial draft is any indication there won't be any requirement for developers to retain a specific amount of topsoil on each new home site.

The draft ordinance – scheduled for discussion at next week's (April 19) committee-of-the-whole meeting – lists many suggested "water quality practices" intended to capture and treat rainfall events of up to 1.25 inches (90 percent of all eastern Iowa rain events) on-site.

However, developers aren't required to employ any lot-specific rainwater retention practices, like amending soil to retain rain, if they choose to install a single large retention pond to handle rain events up to 2.6 inches for the entire subdivision.

Retention ponds have long been required by the city. The proposed rules would continue that practice only with better engineering requirements to make sure stormwater runoff is held in the detention areas and slowly released over a period of no less than 24 hours.

The runoff in some existing detention ponds simply flows through the pond areas and has caused erosion where the water leaves the detention basins.

In a controversial move last year, the state Environmental Protection Commission scrapped a state rule requiring homebuilders to retain four inches of topsoil on new lots. Some cities, like Cedar Rapids, have proposed adding stormwater regulation to mandate developers restore soil on building sites.

Subdivision developers typically scrape off all topsoil, sell it to landscape or soil marketing firms, and then lay new sod on the hardened clay subsoil which provides little filtration or retention of even small rainfall events.

The only reference to lot top soil in the proposed ordinance is under a listing of nine water quality practices deemed useful for capturing and and treating stormwater runoff.

"Soil amendment – Material added to a soil to improve its physical properties, such as water retention, permeability, infiltration, drainage, aeration and structure. Soil amendments increase the spacing between soil particles so that the soil can absorb and hold more moisture. This in turn reduces runoff. The amendment of soils changes physical, chemical and biological characteristics so the soils become more effective in maintaining water quality. Compared to compacted, unamended soils, amended soils provide greater infiltration and subsurface storage and thereby help to reduce a site's overall runoff volume, helping to maintain the predevelopment peak discharge rate and timing. . . "

Other water quality practices listed in the proposed stormwater management guidelines include infiltration trenches, dry wells, porous pavement, vegetated filter strips, vegetated swales, bioretention, wet ponds and level spreaders.

None of the practices are required by the proposed ordinance. Developers aren't likely to address stormwater retention on a lot-by-lot basis when they can opt to address the entire subdivision stormwater requirements with a single detention pond.

"A typical new residential subdivision will provide a common stormwater detention basin that will control the water quality volume (1.25-inch rain) and the channel protection volume (2.60-inch rain)," according to city officials. "The individual lots are not required to address these measures, but the basin as a whole will be designed for the cumulative storage needs of all the lots."

The water quality standard – removal of 80 percent of the total suspended solids in the runoff – also would be met with the slow release of stormwater from the detention basins, according to city officials.

CLICK HERE to download a copy of the proposed stormwater ordinance changes.