Site analysis

A subdivision design is only as good as the understanding of the land beneath it. Site analysis is the systematic study of a parcel's physical, environmental, legal and human characteristics before any lot is drawn. Its product is a set of maps and a short report that say where development is easy, where it is costly and where it should not happen at all. For a geomatics engineer this is familiar ground: the data come from surveys, lidar, imagery and public databases, and they are combined in a GIS exactly as in earlier courses. What is new is the purpose. Every layer is read for what it means for lots, roads, pipes and drainage.

The base survey

Site analysis starts with a reliable base. Grant's crew establishes a control network of six monuments around Coulee Ridge, tied to the state plane coordinate system and to the city's control. They survey the boundary, recover existing monuments and locate fences, roads, power poles, the pipeline markers and the irrigation lateral. Ines processes a recent airborne lidar flight into a bare-earth terrain model on a 3 ft grid, checked against 45 ground check points that Grant surveys with RTK GNSS on open, flat ground. The vertical accuracy, reported as an RMSE, is about 0.13 ft, good enough for planning and preliminary grading but not for final pipe design, which will rely on detailed ground surveys along the alignments.

The base plan shows the legal boundary, the registered rights of way and easements, the existing features, contours at 2 ft intervals and spot elevations at key points such as the low corners and the top of the coulee bank. It is the drawing every other consultant will design on, so its datum, units, projection and accuracy are stated in its title block.

Slope and landform

Slope is the single most important physical factor in residential layout. Very flat land, below about 1 to 2 percent, is hard to drain: water sits in yards and gutters, and sewers have to be deep to get enough fall. Gentle slopes of 2 to 5 percent are ideal; roads, lots and pipes all fit without much grading. Slopes of 5 to 10 percent are workable but need more careful grading and may suit walkout basements. Slopes of 10 to 15 percent are expensive to build on and limit road grades. Above about 15 percent, building is usually avoided, both for cost and because steep slopes in soft sediments may be unstable.

Slope is computed from the terrain model as the magnitude of the elevation gradient, using differences between neighboring cells. The result is classified into ranges that match these design thresholds. Theo's script below does this for a small grid near the coulee edge and finds the top of bank, the line where the land steepens beyond 15 percent.

# Slope classes from a small elevation grid near the coulee edge
import math
CELL = 33.0                          # grid spacing (ft)
rows, cols = 10, 12
def z(r, c):                           # a plateau that breaks into the coulee on the east
    x = c * CELL; y = r * CELL
    base = 2992.0 - 0.035 * x - 0.010 * y
    drop = 92.0 / (1 + math.exp(-(x - 313.0 + 0.45 * y) / 20.0))   # the coulee edge bends
    return base - drop
Z = [[z(r, c) for c in range(cols)] for r in range(rows)]
classes = [(2, "under 2 %: too flat, drainage hard"), (5, "2-5 %: ideal for lots and roads"),
           (10, "5-10 %: workable with grading"), (15, "10-15 %: costly, walkout lots"),
           (1e9, "over 15 %: steep, avoid building")]
count = [0] * len(classes)
for r in range(1, rows - 1):
    for c in range(1, cols - 1):
        dzdx = (Z[r][c + 1] - Z[r][c - 1]) / (2 * CELL)
        dzdy = (Z[r + 1][c] - Z[r - 1][c]) / (2 * CELL)
        s = 100 * math.hypot(dzdx, dzdy)
        k = next(i for i, (lim, _) in enumerate(classes) if s < lim)
        count[k] += 1
tot = sum(count)
for (lim, name), n in zip(classes, count):
    print(f"{name:38} {n:3d} cells  {100 * n / tot:5.1f} %")
# top of bank: first cell in each row where the slope steepens past 15 %
tob = []
for r in range(1, rows - 1):
    for c in range(1, cols - 1):
        if 100 * (Z[r][c] - Z[r][c + 1]) / CELL > 15:
            tob.append(c * CELL); break
print(f"top of bank from the west line: {min(tob):.0f} to {max(tob):.0f} ft")
print(f"building line with a {50} ft setback: {min(tob) - 50:.0f} ft from the west line")

under 2 %: too flat, drainage hard       0 cells    0.0 %
2-5 %: ideal for lots and roads         26 cells   32.5 %
5-10 %: workable with grading           11 cells   13.8 %
10-15 %: costly, walkout lots            5 cells    6.2 %
over 15 %: steep, avoid building        38 cells   47.5 %
top of bank from the west line: 132 to 231 ft
building line with a 50 ft setback: 82 ft from the west line

About a third of this sample is in the ideal 2 to 5 percent class, and nearly half is steeper than 15 percent: the cells east of the bank, where the land drops toward the coulee floor. The top of bank wanders across the sample because the coulee edge bends. A straight building line drawn 50 ft west of the farthest-west point of the bank would be simple to administer, but it would waste land where the bank swings east. In practice the setback line follows the bank, offset by the distance the geotechnical engineer recommends.

Soils, geology and groundwater

The coulees of the Missouri valley are cut into glacial and post-glacial deposits: clay till, silts and sands, with lake clays in places. These soils can be stable on gentle slopes but may slump on steep ones, especially when wetted by leaking pipes, irrigation or poor drainage. The geotechnical engineer drills boreholes, installs standpipes to measure groundwater, tests the soils and analyses the stability of the bank. The report recommends a setback from the top of bank within which no buildings, and sometimes no irrigation or stormwater discharge, should occur.

Montana cities typically require a geotechnical evaluation for subdivisions on or near steep slopes before lot lines are fixed. At Coulee Ridge the report recommends a setback of 50 ft from the top of bank for the trail and open space, with a further restriction on deep basements in the first row of lots. The report also flags expansive clay in the north half, which affects foundation design and the subgrade for roads, and a perched water table in spring at about 8 ft depth near the coulee, which affects basement elevations and pipe bedding.

Hydrology and drainage

Water shapes every subdivision. The site analysis maps the existing drainage: which way each part of the land drains, where water leaves the site, where it enters from upstream, and where it ponds. Drainage areas are delineated from the terrain model by flow direction and accumulation, then checked in the field. At Coulee Ridge almost all of the land drains east toward the coulee, through a shallow draw that becomes a small gully at the bank. A culvert under the south road carries some runoff from the neighboring field onto the site.

Two facts follow. First, the new neighborhood must not send more water down the coulee than the land does now, or the gully will erode and the bank may fail; a stormwater pond is needed, and lesson 7 sizes it. Second, the upstream runoff must be carried through or around the site in a safe overland route. These drainage paths constrain the street layout: roads and swales should follow the natural fall of the land wherever possible.

Environment, heritage and existing uses

A biophysical assessment records native prairie, wetlands, wildlife and rare plants. The coulee slopes at Coulee Ridge are native grassland with a few shrubs in the draws, used by deer and nesting birds. The upland is cultivated and has little natural value. State databases are checked for species at risk and for recorded heritage sites; the coulee edges of the region were used for thousands of years, and finds are possible.

Existing and planned uses next to the site matter too. A gravel pit to the south, an arterial road with truck traffic and the farmland that will remain to the north all affect the layout, through setbacks, buffers, noise and access. The city's regulations also require minimum distances from certain facilities, such as sewage lagoons, landfills and some oil and gas facilities, which the site analysis must check.

Views, sun and wind

Some site factors affect value and comfort rather than feasibility. The view east across the coulee is the main selling point of Coulee Ridge, and the layout should give it to as many lots and public spaces as possible, which argues for a trail and a row of lots along the top of bank rather than backyards that hide it. The strong chinook and westerly winds of the region favor streets and fences that give some shelter, and tree planting along the west edge. Sun matters for snow melt on streets and driveways and for rooftop solar. None of these is a hard constraint, but together they help decide among layouts that all meet the rules.

Overlay and suitability

Once the layers are mapped, they are combined. The simplest approach is a constraints map: an overlay of all the areas that cannot be developed, such as the steep slopes, the setback strip, the pipeline right of way and any wetlands. What remains is the developable area. A richer approach is a suitability analysis, in which each layer is scored on a common scale for each cell, from very suitable to unsuitable, and the scores are combined with weights that reflect their importance. The result highlights the best land for different uses: lots, roads, the park or the pond.

The weights are judgments, and they should be stated and tested. Leah runs the suitability analysis twice, once with slope weighted heavily and once with drainage weighted heavily, and shows Dana that the best area for the pond is the same in both: the low ground in the southeast, where the natural draw leaves the site. Robust conclusions, ones that do not change when reasonable weights change, are the ones to build on.

Coulee Geomatics: the site analysis report

The site analysis report has twelve maps and four pages of text. Its conclusions shape everything that follows: the natural area boundary follows the setback line from the geotechnical report; the pond goes at the low southeast corner; the collector road enters from the west and runs along the gentle ridge in the middle of the site; the pipeline right of way becomes a linear park; and the first row of lots at the coulee gets restrictions on basements and irrigation, to be registered as a recorded covenant. Dana adds the costs of the extra studies and the pipeline crossing to her budget.