How can I tell whether a home site is prone to erosion or landslide problems?

Erosion and landslides are two different processes.

Erosion is a function of several variables, including slope length and steepness, rainfall volumes and intensities, the inherent erodability of the soil, and the type of vegetative cover. A home site on silty soils at the bottom of a long, steep hill with no vegetative cover is clearly more erodable than a site with other characteristics.

Landslides are a function of the site geology and are oftentimes more difficult to predict. Some features to look for include hummocky or uneven topography, spring lines and seeps, trees that are bent or twisted, or fence lines that are no longer straight or upright.

If you find a site that you have questions about, look at the available literature on the geology of the site to see if it is in an area that is considered slide-prone. If questions persist about site stability, a review by a licensed geologist or geological engineer is a reasonable next step.

What resources are available to me to help determine the risk?

Your city or county planning and building department is a good place to start research on any parcel. Soil maps are available through them or by contacting the National Resource Conservation Service office nearest you: Clackamas County: 656-3499 Multnomah County: 231-2270 Washington County: 648-3014

Some jurisdictions may also have developed hazard maps that will at least give you preliminary information concerning the potential risks associated with the property in question.

Are records kept on properties to show whether there have been landslides or sinkholes before?

Probably not. Depending on the jurisdiction, you may be able to find records concerning slide repairs on roads or utilities, but the time frame for record-keeping is short compared to the time involved for geologic events to occur.

The slides that occurred in the metro area as a consequence of the 1996 floods have been documented in a booklet prepared for Metro entitled "Landslides in the Portland, Oregon Metropolitan Area Resulting from the Storm of February 1996: Inventory Map, Data Base and Evaluation." These reports are available from Metro for $20 by calling 797-1742.

Are properties with brooks or streams on them more prone to erosional problems?

Properties with drainageways through them are subject to erosion just the same as any other property, using the same factors outlined in the answer to the first question. The only special consideration that needs to be given here is the impact of the drainageway itself on the property.

Such properties could suffer stream bank erosion or even a change in the location of the streambed during flood or near-flood conditions. Assessment of such properties should include a review of the flood plain location with local authorities (usually the city or county planning department) and perhaps additional on-site review by a geologist or hydrologist to assess high-water impacts on a given parcel.

How concerned should I be about erosion on neighboring properties, especially those above my home site?

Before you buy any lot, it is wise to look at the drainage pattern around you as well as on the lot itself. Oregon Water Law requires that you pass water through your property that comes from upslope property. If that water passes through your building site, you will have a drainage problem for your site unless you take corrective measures as part of your construction.

Remember, too, that the same water law does not allow you to discharge water onto a downslope neighbor in a way that will cause that neighbor material harm. Thus, it is always wise to look at drainage as a neighborhood issue, rather than just focusing on the lot itself.

If I decide to purchase a home, how can I prevent future erosion problems on the property?

There are several things that can be done. First, maintain a good vegetative cover over exposed areas. Grass is good at protecting the soil and at slowing the rate of water flow, both key steps in reducing or controlling erosion. Do not create overly steep banks that are difficult to keep vegetated. If water does flow through your property during rainstorms, steps can be taken to slow down the rate of flow (be careful how you do this) and reduce erosion.

Protect your house from the consequences of drainage problems by maintaining the rain drain and footing or foundation drainage systems in working order, and make sure that the water collected in these systems has a place to go that won't cause you or your neighbors a problem.

How big of a problem is erosion in Oregon?

Erosion problems are an integral part of life in Oregon, especially west of the Cascades. We are all aware of the impact of sedimentation on salmon habitat, and farmers always are reminded to protect their topsoil to prevent erosion from their fields.

On the urban scene, the Federal Clean Water Act requires that all developments take measures to minimize erosional impacts. The U.S. Department of Environmental Quality has special rules for the Tualatin River Basin that are intended to minimize phosphorus loading in the river by using erosion control and other methods.

In January, at a statewide conference on erosion control, Gov. John Kitzhaber and members of private industry emphasized the need to minimize erosional impacts.

Richard Polson is building services supervisor for the Building Services Division of the Clackamas County Department of Transportation and Development. He has a background in soil science and is registered as a Certified Professional Soil Scientist with the American Society of Agronomy.