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Where I have worked on this I have run into two problems. First is how to measure a steep slope. This sounds silly until you get into an on-the-ground situation. Issues are:
Slope is measured as the verticle rise over a horizontal distance. But what is the minimum horizontal distance? 100 feet? 50 feet? And, from where do you start measuring?
Also, there may well be soils in your location that are highly erodible even at lesser slopes.
The solution I like is to use the USDA Soil Survey Erodibility Index. For instance, control development over all soils with the Erodibility Index of IV-e and greater. This has a couple of advantages. First, it pretty much eliminates the "how to measure" problem. Second, it will include soils that are highly erodible on lesser slopes. Third, the soils are already mapped in the local Soil Survey. And finally, the soils are described in detail and can be verified in the field. An additional bonus will come when the National Resource Conservation Service (the old SCS) finishes remapping the soils via GIS as is going on in many areas.
To help address steep slopes, Bath Township, Summit County, Ohio has made conservation development permitted as a use of right while making standard, "cookie cutter" development a conditional use. Bath has a web site that contains its zoning resolution. Good luck.
Good advice. Soils maps have a purpose. Also check out "Planning for Hillside Development," APA Planning Advisory Report No. 466. My former town in the KY hills had a comp plan that mapped areas "susceptible to landslides." When the school district built an athletic track with retaining walls in one of these locations, they wondered why the wall collasped--exempt from zoning review per statute.
The Town of Vail, Colorado has imposed special regulations for steep slopes since 1977. The town hired a consulting geologist (Art Mears) to map areas where debris flow (mudslide), snow avalanche and rockfall hazards exist. Development is prohibited in high hazard areas. In the areas where lesser hazards exist, development applications must be accompanied by a site-specific hazard report prepared by a qualified consultant. If the report states development of the lot would not be hazardous, the town permits the project and the developer signs a hazard acknowledgment agreement. We have been sued in the past for issuing building permits for homes where rocks have fallen or retaining walls have failed. This process takes some of the liability away from the town.
Also, for development on slopes >30% (measured at 4' intervals underneath the proposed building footprint and parking area), allowable site coverage (the footprint) is reduced in an attempt to maintain the integrity of the slope.
We also require soils reports prior to building permit issuance for every lot in town.
Vail's Town Code is online at http://ci.vail.co.us. Go to Section 12-21 for the geologic hazard regulations.
I would like to keep the discussion of this topic alive. Prior to the change in URL location, I had posted about the quarry proposal in Brisbane, CA.
Now I want to ask for a pointer to the Art Mears (above re: Vail) and other legal resources. Here is my current (can we be too! cynical) thinking about the political process in Brisbane:
The land crowning and above the steep 600' slopes of the Summerhill property is owned by the County and State Park system. Summerhill greases chutes and palms and implements a minimal response to the EIR report concerns about landslides, waterfalls, etc. During a big storm mud slides, water flows and homes are destroyed or damaged. Summerhill, or by this time, the homeowners association sues the County and State thinking they have deep pockets. The legal principle is that the uphill owner must control the dirt on his side of the property line; never mind that the crown of the hill is in a natural state and Summerhill side is a precipous departing from that!
Is this too paranoid! I would like to find a geological legal person or firm. If not work, pro bono, then tell me what the bill might be; I will round up funds from my environmentalist friends.
P.S. My offer is still open to students who can pay their way to the San Francisco area to see this monstrosity: a few days free lodging in my home.
I am trying to collect information on developments that have been located on steep slopes and either had problems due to engineering or planning issues. Most of the information I have seen on the web deals with landslides and such, but the area where I am trying to protect is in New Jersey and landslides are very rare. I do have a variety of ordinances, but now am looking for specific examples of existing developments, including photos. If you have any suggestions I would appreciate it.
We have taken a simple approach in a community in New England. That is to remove slopes greater than 20% from the calculation for developable land. That is to say, if a 20 acre parcel is under review for development and 25 percent of the land has slopes greater than 20% then 15 acres is suitable for development (of course one would also exclude wetlands, roads, and power easements as well.)
However, those comments from Rich Townsend is well founded, and probably would hold up better under scrutiny.