Page 572 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

JWCL344_ch14_500-554.qxd 8/7/10 8:56 PM Page 529

14.12 Layout and Hydraulic Design in Storm Drainage 529
where D is the surface detention, in mm of depth; s is the slope of the ground; L is the dis-
tance of overland flow, in m; Q is the overland supply, in mm/h; i is the intensity of rain-
fall, in mm/h; and c is a coefficient of roughness varying downward from 6.0 10 –2 for
r
pervious areas of turf, through 3.2 10 –2 for bare, packed pervious areas, and 1.2 10 –2
–2
for pavements, to 1.7 10 for flat, gravel roofs.
Variations of c and i with time (time being a function of watershed area a and of other
factors such as surface slope s) can be incorporated into overall runoff formulations devel-
s ),
oped empirically for given localities. An example is McMath’s formula (Q cia 4/5 1/5
for St. Louis, Missouri, which uses the U.S. customary system. If rainfall intensity i and re-
sulting runoff ci are properly determined, overall formulas should yield results comparable
with the rational method.
As storm drains are actually constructed in a given community, it becomes possible to
design stormwater systems for adjacent unsewered areas on the basis of (a) actual runoff
measurements conducted in times of heavy rainfall or (b) surcharge experience with
recorded storm intensities. In necessary calculations, it is important to identify possible
downstream effects on surcharge.

14.12 LAYOUT AND HYDRAULIC DESIGN IN STORM DRAINAGE
The layout of storm drains and sanitary sewers follows much the same procedure. Street inlets
must be served as well as roof and other property drains connected directly to the storm sew-
ers. How inlets are placed at street intersections to keep pedestrian crossings passable is indi-
cated in Fig. 14.8. To prevent the flooding of gutters or to keep flows within inlet capacities,
street inlets may also be constructed between the corners of long blocks. Required inlet capac-
ity is a function of tributary area and its pertinent runoff coefficient and rainfall intensity.
Separate storm drains should proceed by the most direct route to outlets emptying into
natural drainage channels. Easements or rights of way across private property may shorten
their path. Manholes are included in much the same way and for much the same reasons as
for sanitary sewers.
Surface topography determines the area tributary to each inlet. However, it is often assumed
that lots drain to adjacent street gutters and thence to the sewers themselves. Direct drainage of
roofs and areaways reduces the inlet time and places greater load intensity on the drainage sys-
tem. Necessary computations are illustrated in Table 14.7, which accompanies Example 14.3.

EXAMPLE 14.3 DESIGN OF A STORM DRAINS SYSTEM
Determine the required capacity and find the slope, size, and hydraulic characteristics of the system
of storm drains shown in the accompanying tabulation of location, tributary area, and expected
storm runoff.
Capacity requirements are based on the rainfall curves included in Fig. 14.18. The area is
assumed to be an improved pervious one, and the inlet time is assumed to be 20 min. Hydraulic
requirements include a value of N 0.012 in Manning’s formula and drops in manholes equal to
(d h v ) 0.2 h v for the sewers flowing full.

Solution:
Illustrative computations for the storm drainage system are shown in Table 14.7.
Columns 1 through 4 identify the location of the drains. The runs are continuous.
Column 5 records the area tributary to the street inlets discharging into the manhole at the
upper end of the line.