1. Drainage and Grading Creating stable ground surfaces
Providing for runoff drainage in desired direction, volume, and speed
Creating buildable and useable ground surfaces (for structures, activities, roads and infrastructure)

2. Creating stable ground surfaces
Maximum angles of repose (modifiable by extra drainage installation):
ivy-planted slopes: 100%
wet sand: 80%
unmown planted banks: 50-60%
wet clay and silt: 30%
grassy slopes: 25%

3. Providing for runoff drainage in desired direction, volume and speed
Minimum slopes for drainage:
at and around buildings: min. grade 2%
for both planted and broad paved areas away from buildings: 1%, though if ponding is permitted, then 0.5%
away from buildings within 3m: 2%
in swales and ditches: 2% (maximum 10%, or 5% if area drained is over 0.2 ha/0.5 acres)
*Maximum length of uniform moderate slope before water will cut rivulets: 150m/500ft.
**Surface drainage systems adequate at densities up to 5 families per ha / 2 families per acre.

4. Creating buildable and useable ground surfaces – roads and walks
Follow principles of street hierarchy and function, esp. horizontal and vertical alignment, as varied for different road surfaces
Cross-section slope determines vehicle tractability
concrete or asphalt: 2%
earth or gravel: 4%
Longitudinal slope determines vehicle tractability, icing, visibility
normal max: 10%
no-icing max: 12%
intersection (to distance of 40 ft. from intersection) max: 4%
need for a smooth transition from one speed to the next: avoidance of car hitting the pavement
as at the bottom of a driveway, if change in slope is more than 9%
parking lot max: 5%
sidewalk max: 10%

13. Slab Grading In-Class Exercise
On the site shown (scale is 1”=20’), modify the existing contours to create a flat, rectangular area 20 feet x 30 feet in Size. Following are further restrictions:
1. Cut and filled slopes may not exceed a 2:1 slope.
2. Balanced cut and fill is desirable, but not required.
3. Retaining walls may not be used.
4. Show revised contours with solid lines.
5. All contour modifications must occur within property lines.
6. All surface water must drain away from slab.
0 ft
20 ft

16. All other things being equal, streets should run roughly parallel with or perpendicular to contours, to improve buildability of lots and manageability of drainage. Building roads diagonal to grade is problematic.
Of course, roads that must ascend a very steep slope can only be built at a diagonal to contours.
Infrastructure is easiest to place under or alongside roads, but must follow principles of sizing to accommodate runoff from impervious surfaces, and slope to allow proper velocity (steep enough to keep water and silt flowing, but not so steep that it scours the pipe. Generally pipes must slope at a minimum of 0.3%, modified to account for capacity.)