12.1 Contour Rules & Topographic Landform Interpretation

Core Focus: Contour lines and spot elevations represent the fundamental language of site grading and landform manipulation. Mastery of contour characteristics, landform geometry, spot grade conventions, and interpolation mathematics is critical for passing the LARE Section 4 exam and ensuring public health, safety, and welfare through sound site drainage.

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1. Fundamental Characteristics & Mathematical Rules of Contours

A contour line is an imaginary, horizontal planar slice through the three-dimensional terrain connecting continuous points of equal vertical elevation above a recognized vertical datum (such as the North American Vertical Datum of 1988, NAVD 88). The constant vertical distance between successive contour lines on a drawing is the contour interval (CI), typically established at 1 foot or 2 feet for detailed site grading plans, and 5 feet, 10 feet, or 20 feet for large-scale regional master plans.

To interpret and manipulate topography accurately on the LARE, candidates must memorize and apply six immutable mathematical and physical rules of contours:

1. Rule of Continuity (Closure): Every contour line is a continuous, unbroken closed loop that must eventually close on itself. A contour line never terminates abruptly in space; if it does not close within the boundary of a specific drawing sheet, it continues across adjacent sheets and closes off-site.
2. Rule of Non-Intersection and Non-Branching: Contours never split, divide, or branch into two lines of the same elevation. Furthermore, contours cannot cross or intersect one another except under two rare geological conditions:
   • Overhanging Cliff or Cave: Where an upper geological strata cantilevers over lower terrain, contour lines will cross. The lower (hidden) contours must be depicted with dashed or hidden linework beneath the overlapping solid lines.
   • Vertical Wall or Cliff: Where a sheer vertical surface occurs (such as a structural retaining wall, vertical curb, or vertical rock face), contours of different elevations appear to merge into a single coincident line. In plan view, multiple elevations occupy the same horizontal coordinate.
3. Rule of Maximum Slope (Steepest Path): The direction of steepest slope—and consequently the path that surface runoff water flows under gravity—is always oriented exactly perpendicular (90 degrees) to the contour lines. Water never flows parallel to a contour line or obliquely across a slope if free to move unconstrained.
4. Rule of Spacing and Slope Uniformity:
   • Uniform slope: Evenly spaced contour lines indicate a uniform, constant gradient.
   • Steep slope: Closely spaced contour lines indicate a steep gradient (large vertical rise over short horizontal run).
   • Gentle slope: Widely spaced contour lines indicate a flat, gentle gradient.
   • Concave slope: Contours spaced closely at higher elevations and widely at lower elevations indicate a concave landform (steep crest flattening to a gentle base).
   • Convex slope: Contours spaced widely at higher elevations and closely at lower elevations indicate a convex landform (gentle crest steepening into a bluff or bank).
5. Index vs. Intermediate Contours:
   • Index Contours: Every fifth contour line is rendered with a heavier, bolder line weight and periodically broken to insert the numeric elevation value. On a 1-foot CI plan, index contours occur at 5-foot intervals (e.g., 100, 105, 110); on a 2-foot CI plan, they occur at 10-foot intervals (e.g., 100, 110, 120).
   • Intermediate Contours: The four intervening lines between index contours are drawn with a finer, lighter line weight and are typically unlabeled unless grading clarity demands local identification.
6. Existing vs. Proposed Graphic Conventions:
   • Existing Contours: Represent baseline, undisturbed topography and are drawn with dashed or screened lines.
   • Proposed Contours: Represent engineered, modified landforms and are drawn with solid, dark lines.
   • Daylight Line (Tie-in): Where proposed grading terminates and transitions back into natural ground, the proposed solid contour line must meet and seamlessly merge into the existing dashed contour line of identical elevation.

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2. Topographic Landform Interpretation

Recognizing 3D landforms from 2D contour signatures is an essential skill tested extensively through plan-reading items on the LARE.

The Rule of the V's and U's: Swales vs. Ridges

The most tested concept on Section 4 is distinguishing between surface drainage channels (valleys, swales, ditches) and shedding landforms (ridges, spurs, promontories):

• Valleys, Swales, and Draws: When contour lines form a distinct "V" or "U" pattern that points uphill toward higher elevations, the landform represents a valley or swale. Runoff flows down the slopes from both sides and collects along the centerline of the "V" before discharging down-gradient.
• Ridges, Spurs, and Promontories: When contour lines form a "V" or "U" pattern that points downhill toward lower elevations, the landform represents a ridge or spur. Water sheds outward away from the crest or centerline of the "V" toward adjacent slopes.

   [SWALE: Contours point UPHILL]         [RIDGE: Contours point DOWNHILL]
              104                                    100
             ^   ^                                  |   |
         103 |   | 103                          101 |   | 101
             |   |                                  v   v
         102 |   | 102                          102 |   | 102

      Contours point toward HIGH             Contours point toward LOW
      Water collects and flows OUT           Water sheds OUTWARD off ridge

Summits, Knolls, and Hills

A summit or hilltop is represented by one or more concentric, closed, continuous contour loops where the interior elevation values increase toward the center. The highest ground elevation on the knoll lies higher than the innermost contour line but lower than the next unplotted contour interval.

Depressions and Retention Pits

A depression, basin, sinkhole, or retention feature is represented by concentric, closed contour loops where the interior elevation values decrease toward the center. To avoid confusing depressions with hilltops, standard drafting conventions mandate that depression contours must feature hachure marks—short, perpendicular tick marks along the inside edge of the line pointing down into the bottom of the basin.

• Elevation Rule for Depressions: The first depression contour maintains the exact same elevation as the lower adjacent regular contour line before decreasing by standard contour intervals inward.

Saddles and Cols

A saddle (or col) is a depression along a ridge line situated between two adjacent summits, or conversely, a high point between two drainage draws. In a saddle, the ground rises in two opposite directions and falls in the other two perpendicular directions, creating an hourglass contour configuration.

Summary Table of Topographic Landforms

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