From time to time, people ask us about systems installed in floodplains and other areas subject to ponding. They ask how to determine if an area is subject to flooding.

As we view it, there are at least two types of situations an installer may face relative to systems that could flood. The first is the scenario most people think about: building a system in the 100-year floodplain. The second is in areas where temporary ponding occurs from snowmelt or runoff. Both of these situations need to be considered when designing and installing a system.

MAPS WILL HELP

Flooding – for the regulatory purpose of establishing a floodplain – is the temporary covering of the soil surface by flowing water from streams overflowing their banks, runoff from surrounding slopes or any combination of sources. This is distinguished from the temporary ponding of water over a system due to snowmelt or rainfall runoff. From our perspective, though, both situations can negatively impact the long-term performance of
onsite systems.

A designated floodplain consists of the floodway and the flood fringe. The floodway is the area directly adjacent to the channel of a stream and conducts water during a flood. The flood fringe is the remainder of the floodplain where the water is shallow and slow moving. Installation of systems in the floodway is usually not allowed, although in our home state of Minnesota there are systems that have been “grandfathered,” so they do exist in these areas.

Maps and boundaries for the established 100-year floodplains for insurance and land-use regulation purposes are available from federal and state agencies, counties and local municipalities. These maps can provide information on the boundaries and elevations of designated floodplains.

County soil survey information easily accessed through the Web soil survey will indicate soils and areas that are prone to flooding or ponding due to landscape position, as well as indicate drainageways or intermittent streams in a given area. Some states we have worked with, including Arizona and New Mexico, have provisions where intermittent streams, drainageways, washes or arroyos are present and typically have 25- to 50-foot minimum setbacks from the edges of these features. In semiarid climates, water may very seldom run in these features, but when it does run the flow is usually significant.

LANDSCAPE FEATURES

Here are a few keys to identifying areas that may be prone to flooding or ponding. First, use of landscape features. During site evaluations it is important to look beyond the site proposed for the system and recognize where it fits in the bigger picture of the area around it. From a flooding standpoint, features associated with past and present flooding include former river channels, natural levees and terraces oxbows, meander channels, and other similar features. These are usually easily identified and are also captured on the soil survey. For the temporary ponding situation, location at the foot of slopes or in depressions on the landscape would be telling features.

Vegetation can also be a key to identifying flood-prone areas. Survival of different tree species in the area can be telling. Each area has species that can withstand partial or total submersion during the growing season, while there are others that are intolerant and will not survive. A local biologist or forester can help identify species that indicate wet or flood-prone areas. In terms of temporary ponding, if there are grasses and small shrubs and the inundation exceeds a couple of weeks, you may see areas of dead vegetation. You may see a change in plant species from the surrounding areas, reflecting the presence of water.

Evaluation of soil pits can also identify areas subject to flooding. A thin strata of material at the surface different in color and texture from a deeper area would be a good indicator. For instance, a horizon of dark, rich organic matter under a layer of brown material indicates the original soil surface was covered by a more recent deposit. Or a series of soil layers with abrupt boundaries of contrasting materials, say sand and gravel over finer-textured materials, indicates materials were laid down suddenly at different times from different sources. This is something we see often in the Southwest.

WHEN TO AVOID AN AREA

Follow a few specific design guidelines if a system will be placed in a floodplain. First, of course, is avoiding the area if possible. This is probably more relevant for the ponding situation. Moving the location within the lot can usually avoid the problem. But if not, the onsite system should be located on the highest portion of the lot and should have preference over all other improvements other than the well.

There should not be inspection ports or other connections installed to the surface and open to the distribution media or other parts of the system. If a pump is employed in the system, there needs to be a way to make sure it does not run during times of flooding. If time permits, the best approach would be to pull the pump out.

As always, an installer should be familiar with the state and local rules and regulations covering flooded systems in their area.