In order to protect the public health, safety, and welfare, the regulations contained in this article apply to mountain and hillside development which is any lot, tract or parcel of land for which the average slope, as defined herein, equals or exceeds 30 percent. Applicability shall initially be determined by means of the town's GIS maps. Property owners may provide topographic maps of the property if they disagree with the determination made by reference to the GIS maps.
(1) Determining slope. Average slope shall be determined for each separate land tract in accordance with the methods and procedures contained herein. All slope determinations for the purposes of administering this article shall be the natural slope of the lot to be developed or subdivided, which is to say the slope of the lot prior to any modification due to development activities.
a. Prior to commencing any development or land disturbing activity and prior to making application for any permits and/or other approvals, the calculated average slope for a particular land tract shall be approved by the director. Average slope calculations and supporting documentation shall be submitted to the director for review. Within 20 days of receipt, the director shall:
1. Request additional information;
2. Request revisions to the average slope calculation submittal; or
3. Issue written concurrence with the determination of average slope, as submitted.
b. Each slope calculation submitted to the director for review shall include a scaled map, accurately showing:
1. Topography for the entire land tract;
2. A closed perimeter line delineating a single area proposed for any type of land of land disturbing activity; and
3. The deeded land tract boundary.
The accuracy and detail of the map shall be acceptable to the director for site-specific conditions and the particular land disturbing activities proposed. In certain cases, the director may, at his discretion, require that the slope calculation and associated mapping be prepared by a state professional land surveyor, a state professional engineer, or a state professional landscape architect.
c. For an individual lot, the basis of the average slope calculation shall include the entire lot. For subdivisions, the average slope calculation shall be based on the entire tract to be subdivided unless the subdivider has elected to exclude areas subject to an absolute conservation easement pursuant to subsection (2) of this section or the director has approved a division of area pursuant to subsection (3) of this section.
d. Land slopes shall be calculated based on both:
1. The downstream drainage slope from the highest, most remote point within the delineated area of proposed land disturbance; and
2. The upstream drainage slope from the lowest, most remote point within the delineated area of proposed land disturbance in accordance with the following requirements.
e. Land slope based on the highest, most remote point shall be calculated by determining the maximum horizontal length of drainage travel (D) from the highest, most remote point (Elevation H1) within the delineated area of land disturbance in a downslope, drainage direction and perpendicular to topographic contours for the greatest distance to the lowest point (Elevation H2) at which drainage would exit the delineated area of proposed land disturbance. Slope based on the highest, most remote point shall be calculated using the following formula:
S a | = | H1-H2 100 | x | 100 |
S a = ((H1-H2)/D)(100)
Where:
S a = Slope expressed as a percentage.
H1 = Elevation of highest, most remote point.
H2 = Elevation of the lowest point drainage point below H1.
D = The maximum length of drainage travel between points H1 and H2 expressed as a horizontal measurement (D is not necessarily a straight line distance).
f. Land slope based on the lowest, most remote point shall be calculated by determining the maximum horizontal length of drainage travel (D) from the lowest, most remote point (Elevation L2) within the delineated area of land disturbance in an upslope direction and perpendicular to topographic contours for the greatest distance to the highest point (Elevation L1) at which location drainage to point L2 would begin within the delineated area of proposed land-disturbance. Slope based on the lowest, most remote point shall be calculated using the following formula:
S b | = | L1-L2 100 | x | 100 |
S b = ((L1-L2)/D)(100)
Where:
S b = Slope expressed as a percentage.
L1 = Elevation of highest point above drainage point.
L2 = Elevation of the lowest, most remote point.
D = The maximum length of drainage travel between points L1 and L2 expressed as a horizontal measurement (D is not necessarily a straight line distance).
Average slope shall be the greater of S a or S b rounded off to the nearest one percent.
(2) Conservation easements. Lands subject to an "absolute" conservation easement, that is, an easement in which the landowner retains no development rights, may, at the landowner's option, be excluded when determining average natural slope under this chapter.
(3) Division of area. Where there is a substantial variation in the landform character within one site, the site may, with the director's approval, be divided into two or more distinct areas for the purposes of slope determination. Generally, this provision shall only be used in cases where large tracts of property encompass flat land as well as significant mountain and hillside terrain. Details for each division must be provided.
(Code 1989, § 92.200; Ord. of 11-18-2008; Ord. of 3-10-2009; Ord. of 8-9-2011)