Locate Restrictive Conditions
Various materials here on the OSU Stormwater Solutions website, including fact sheets and the Choose the Right Rain Garden Decision Tree, indicate that infiltration should be done carefully and not in various areas such as floodplains, but how do you know whether you have a floodplain on your property? This page is intended to provide you with additional resources to help you identify conditions important to deciding what kind of LID facility you may use and where. This is NOT intended to be a list of all the conditions that may impact LID facility placement; for these, refer to the fact sheets.
I want to know more about LID facilities and:
Use the 100-year floodplain area as a minimum. The 25-year floodplain may also be appropriate depending on the development standards and regulatory environment in your jurisdiction.
- Don't place LID facilities in a floodplain or they may be washed out in the next big storm.
- Consult the Oregon Explorer (to find floodplain layer: Click the plus sign and the checkbox next to "Hazards", then click both again under "Flood Hazards", then click the "100 Year Flood" checkbox).
- Enter your address into this website to see if the site is located in the 100-year or 500-year floodplains. The 500-year floodplain is OK, but avoid the 100-year floodplain.
- Some agencies may not allow development in the 25-year floodplain. Contact your city or county's community development, public works, or engineering department to find out more.
- Don't place LID facilities in a low or seasonally wet area.
- In Oregon, the Department of State Lands, Wetlands Program (503-986-5200) can help you identify if and where wetlands may be.
- Wetlands may or may not be recognized and mapped as "jurisdictional". If a wetland has not been mapped, it may still fall under multiple jurisdictions. Fact sheets from the Department of State Lands on how to start to identify wetlands is available. A series of questions about your site and some additional resources for how to answer them can be found on page 2; look up your site on Oregon Explorer (To find the Wetland layer: Click the plus sign and the checkbox next to "Habitats and Vegetation", then click the plus sign and checkbox next to "Wetland", then click both checkboxes next to "Wetlands in Oregon" and "Wetland Inventory Boundary". To find the hydric soils layer, scroll down to "Landscape and Geology" and click on the plus sign and checkbox next to it, click on plus sign and checkbox next to "Soils", then click on the checkbox next to "Wetland Soils".)
- Additional fact sheets are available from the Department of State Lands.
A riparian area can be simply defined as land adjacent to a waterway that either influences or is influenced by that waterway. A rule of thumb to define this width is that it is equal to the average height of the trees adjacent the stream, since this is the range of influence that fallen trees and leaf litter are likely to have.
- Don't place LID facilities in a riparian area or buffer.
- Contact your city or county planning department to help you identify if and where these may be on your site.
- Contact the Department of State Lands (503-986-5200) to see if they have jurisdiction if you do have a waterway on your site.
Numerous above and below ground animal species provide important hydrologic benefits to our watersheds. Your site may have sensitive uplands that provide important habitat to some of our more specialized native animal species. Generally, if your site looks like it's been impacted by development, either recently (i.e. presence of a road, parking lot, fill/overburden, etc), then it's likely that your site is not located in a critical habitat area.
If your site is natural looking, with native plants (i.e. Oak Woodland), then it's possible that you may want to avoid disturbances, like installing a rain garden, in this area. If you have a question:
- Contact your local county or city Planning Department to find out if and where these areas might be.
- Professionals from various state agencies such as Oregon State University Extension Service or biologists at Oregon Department of Forestry or Department of State Lands may be available in your region.
- Consult the Oregon Conservation Strategy and use the Mapper to discover if there are areas targeted for conservation.
- Consult the Oregon Explorer. (To find the Habitat layers: Click the plus sign and the checkbox next to "Habitats and Vegetation", then click the plus sign and checkbox next to "Bays and Estuaries", then click both checkboxes next to "Estuaries" and "Estuarine Habitat". Scroll down to "NW Forest Plan" and click both the plus sign and the checkbox next to it, then click the checkboxes labeled "Northern Spotted Owl Habitat" and "Marbled Murrelet Zone".
- Don't place LID facilities in a critical habitat area.
- Avoid installing LID facilities over a septic field or other infiltration septic system; this could ruin your septic system in a variety of ways.
- Your city or county planning department may be able to help you locate this if you don't know where it is on the site.
Soil & groundwater contamination can come from many different sources including lead paint and leaking underground storage tanks from current and past land uses.
- Search the DEQ Facility Profiler by address. If your site is listed here, check the status of cleanup. A status of "CLEANUP_COMPLETED" means that the site has been brought up to current regulatory requirements for land quality and the suitability of this site for a rain garden is not constrained by this criteria.
- Historic land uses can often be found through your city or county planning department.
- Oregon has a number of Groundwater Management Areas, which are designated areas of groundwater contaminated by nitrate. For infiltration LID facilities, consult a professional to assist with rain garden design to ensure that denitrification occurs before infiltrating to prevent further groundwater contamination.
Infiltration LID facilities should not be uphill of a known landslide area. For all steep areas, infiltration facilities should be set back a minimum of 100 feet from down-gradient slopes of 10% or greater. Add 10’ of setback for each additional percent slope up to 30%.
- Consult the Oregon Explorer. (To find the landslide layers: Click the plus sign and the checkbox next to "Hazards", then click the plus sign and checkbox next to "Geological Hazards", then click both checkboxes next to "Coastal Erosion", "Landslide Areas" and "Historically Active Landslide Areas". Scroll down to "Landscape and Geology".
A wellhead protection area is a designated area of land where a jurisdiction might draw groundwater to supply public drinking water.
- If runoff is from vehicular areas such as parking lots, driveways, and roads, then contact the city or county planning department to assess whether your site is in a wellhead protection area (i.e. Portland's Columbia Slough Wellhead Protection Area).
Groundwater can move twice as fast horizontally as vertically, so infiltrating too close to private drinking wells can contaminate them.
- If runoff is from vehicular areas like parking lots, driveways, and roads, search for nearby wells by address using the DEQ Well Log Query. Click on "Find T-R-S by Address", enter the address and click "Lookup". On the next page, navigate to the "Type of Log" pull down menu and choose "Water Well", then click "Search". The resulting table shows all the water wells within that T-R-S, which stands for Township, Range, and Section. In this table, look for wells with a "Completed Depth" greater than zero. Locate these wells in relation to your site and infiltrate no closer than 2 times the completed depth. For instance, for a completed depth of 200', an infiltration rain garden should be at least 400' away.
Vertical separation is the distance between the bottom of the rain garden ponding area and the top of a subgrade layer in question (i.e. bedrock, groundwater, fragipan (an impermeable soil layer), or previous development) The soil is nature’s pipe that conveys water downhill to seep out to streams; adequate vertical separation provides enough voids and depth for that conveyance beneath infiltration facilities.
Do not rely on groundwater modeling or maps; these are usually done at too large a scale and don’t apply to the site scale practices such as a rain garden. To look for bedrock or other impermeable subsurface layer that may impede infiltration, after infiltration testing has confirmed that soils are suitable for infiltration (additional info is included in this wizard and the fact sheet “Infiltration Testing”), dig a hole 2 feet of depth from the bottom of the proposed rain garden. If the soil is pretty consistent all the way down, then you don’t have a fragipan or bedrock problem. Now look for a seasonal high groundwater table by digging a hole to an additional one foot of depth. If the hole doesn’t fill up with water, then groundwater levels are sufficiently deep. Ideally, this will be done during the rainy season.
A horizontal setback or separation refers to the distance that something should be placed away from something else. In the case of LID facilities, without adequate horizontal separation, infiltration facilities have the potential to flood basements, overturn walls, or otherwise damage other infrastructure. See the following page for an example that applies to horizontal setbacks for rain gardens, swales, stormwater planters, vegetated filter strips, drywells, and soakage trenches. Porous pavements that receive runoff from other areas should also adhere to these setbacks; however porous pavements that manage only rainfall and not runoff from other areas have fewer setback requirements; see the fact sheet "Porous Pavements" for more information. For other LID facilities, see those fact sheets.