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)
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%
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.
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%
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
• 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.)