TM 5820-4/AFM 885, Chap 4
CHAPTER 4
HYDRAULIC STRUCTURES
4-3. Outlet energy dissipators.
41. Manholes and junction boxes. Drainage
systems require a variety of appurtenances to as-
Most drainage systems are designed to op-
a.
sure proper operations. Most numerous appurte-
erate under normal free outfall conditions. Tail-
nances are manholes and junction boxes. Man-
water conditions are generally absent. However,
holes and junction boxes are generally constructed
it is possible for a discharge resulting from a
of any suitable materials such as brick, concrete
drainage system to possess kinetic energy in ex-
block, reinforced concrete, precast reinforced-con-
cess of that which normally occurs in waterways.
crete sections, or preformed corrugated metal sec-
To reduce the kinetic energy, and thereby reduce
tions. Manholes are located at intersections,
downstream scour, outfalls may sometimes be re-
changes in alignment or grade, and at interme-
quired to reduce streambed scour. Scour may oc-
diate joints in the system up to every 500 feet.
cur in the streambed if discharge velocities exceed
Junction boxes for large pipes are located as nec-
the values listed in table 4-1. These values are to
essary to assure proper operation of the drainage
be used only as guides; studies of local materials
system. Inside dimensions of manholes will not be
must be made prior to a decision to install energy
less than 2.5 feet. Inside dimensions of junction
dissipation devices. Protection against scour may
boxes will provide for not less than 3 inches of
be provided by plain outlets, transitions and still-
wall on either side of the outside diameter of the
ing basins. Plain outlets provide no protective works
largest pipes involved. Manhole frames and cover
and depend on natural material to resist erosion.
will be provided as required; rounded manhole and
Transitions provide little or no dissipation of en-
box covers are preferred to square covers. Slab
ergy themselves, but by spreading the effluent jet
top covers will be provided for large manholes and
to approximately the flow cross-section of the nat-
junction boxes too shallow to permit corbeling of
ural channel, the energy is greatly reduced prior
the upper part of the structure. A typical large
to releasing the effluent into the outlet channel.
box drain cover is shown in figure 3-5, TM 5-820-
Stilling basins dissipate the high kinetic energy
3/AFM 88-5, Chapter 3. Fixed ladders will be pro-
of flow by a hydraulic jump or other means. Rip-
vided depending on the depth of the structures.
rap may be required at any of the three types of
Access to manhole and junction boxes without fixed
outfalls.
ladders will be by portable ladders. Manhole and
(1) Plain type.
junction box design will insure minimum hy-
(a) If the discharge channel is in rock or a
draulic losses through them. Typical manhole and
material highly resistant to erosion, no special
junction box construction is shown in figures 4-1
erosion protection is required. However, since flow
through 4-3.
from the culvert will spread with a resultant drop
Table 4-1. Maximum Permissible Mean Velocities to
Prevent Scour
42. Detention pond storage. Hydrologic stud-
ies of the drainage area will reveal if detention
Maximum
Permissible
ponds are required. Temporary storage or pend-
Mean Velocity
Material
ing may be required if the outflow from a drainage
area is limited by the capacity of the drainage
Uniform graded sand
system serving a given area. A full discussion of
1.5 fps
and cohesionless silts
2.5 fps
Well-graded sand
temporary storage or ponding design will be found
3.0 fps
Silty sand
in appendix B, TM 5-820-l/AFM 88-5, Chapter 1.
4.0 fps
Clay
Pending areas should be designed to avoid crea-
6.0 fps
Gravel
tion of a facility that would be unsightly, difficult
U.S. Army Corps of Engineers
to maintain, or a menace to health or safety.
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