WATER AND SEDIMENT CONTROL
STRCUTURE
PREPARED BY: CIK SAMERA SAMSUDDIN SAH
Water control structures are being used
to manipulate the flow of water within
the system with a view to improving the
surrounding environment.
 These structures are used to manage
the hydrological regime by modifying
the direction or rate of flow of water,
and to maintain a desired water surface
elevation.

1.
Temporary structures.
 Should be recommended only where
inexpensive labor and materials are available.
 increasing mechanization and higher labor costs
and resulted in a decline in practicality of
temporary channel stabilization structures.
 Practices that make use of temporary materials
such as logs and root wads can be effective if
combined with channel modifications that will
result in stable stream.
 Without such modifications, the problems are
likely to recur, progressively degrading the land.
2.
Permanent Structures
 Permanent structure of hard materials may be
required to dissipate the energy of the water.
 For example: where a vegetated waterway
discharges into drainage ditch, at the head of a
large gully, or in a channel reach where the grade
is too steep to be stable.
 Where flow velocities must exceed the maximum
values for nonerosive conditions, an erosionresistant lining may be required.

2 primary requirements for design of control
structure;
 Adequate capacity to pass the design discharge
 Dissipation of the energy of the water within the
structure in a manner that protect both the
structure and downstream channel from damage
or erosion.


The basic components of a hydraulic
structure are the inlet, the conduit, and the
outlet.
In addition, the structure must include
suitable wing walls, side walls, headwall
extensions, and toe walls to prevent seepage
under or around the structure and to prevent
damage from local erosion.

It is important that a firm foundation be
secured for permanent structures.

Wet foundation should be avoided or
provided with adequate artificial drainage.

Top soil and organic material should be
removed from the site to allow a good bond
between the structure and the foundation
material.



A typical drop spillway is shown in figure.
Drop spillways may have a straight, arched, or
box-type inlet.
The energy dissipater maybe a straight apron or
some type of stilling basin.
Figure 3: Straight drop
spillway showing structural
components.

Are installed in channels to establish
permanent control elevations below which an
eroding stream cannot lower the channel
floor.

Usually limited to drops of 3m

Drop-inlet pipe spillways or flumes are used
for greater drops.

Capacity
 The free flow (with no submergence) capacity for
drop spillways is given by the weir formula;
Q = CLh3/2
…….. Eq. 1
 Where: Q = discharge (L3T-1)
C = weir coefficient (L1/2T-1)
L = weir length (L)
h = depth of flow over crest (L)

Apron protection
 Kinetic energy must be converted to potential
energy before the flow exits the structure.
 For straight-inlet drop structure, the conversion
and dissipation energy can be accomplished in
either a straight apron or a Morris & Johnson
stilling basin.
Figure 4: Design dimensions
for a drop spillway with
straight inlet and Morris
& Johnson outlet

Pipe spillways may take the form of a simple
conduit under fill or have a riser on the inlet
end and some type of structure for outlet
protection.
Figure 5:
(a) simple culvert;
(b) drop-inlet pipe
spillway with
cantilever outlet



The pipe in Figure 6 called an inverted siphon,
is often used where water in an irrigation
canal must be conveyed under a natural or
artificial drainage channel.
Inverted siphons must withstand hydraulic
pressures much higher than those
encountered in other pipe spillways.
Require special attention to structural design.
Figure 6: Inverted siphon.
Used as a culvert; has simple function of providing
for passage of water under an embankment.
 Drop-inlet pipe spillways are frequently used as
gully control structures.

 Usually made where water may pond behind the inlet to
provide temporary storage.


The hydraulic capacity of pipe spillways is related to
the square root of the head, hence they are
relatively low-capacity structures.
This characteristic is desirable where discharge from
the structure is to be restricted.
1.
Culverts
 Culvert capacity controlled by either inlet or the
conduit.
 The headwater elevation may be above or below
the top of the inlet section.
 Solution of a culvert problem requires
determination of the type of flow that will occur
under given headwater and tailwater conditions.
Figure 7: (a) Full: free outfall, pipe flow
(b) Part full: free outfall, orifice flow
(c) Full: outfall submerged, pipe flow
(d) Inlet not submerged: conduit control, open channel
(e) Inlet not submerged: inlet controls, weir flow.
Pipe flow [refer figure 7 (a)] usually occur where
the slope of culvert < the neutral slope and
entrance capacity is not limiting.
 The neutral slope, sn can be calculate using
equation below:
…… Eq. 2

 Where Hf = friction loss in conduit of length (L)
L = length of conduit (L)
Kc = conduit friction loss coefficient (L-1)
ν = velocity of flow (LT-1)
g = gravitational acceleration (LT-2)


Inlet losses may be so great in some situations
that pipe flow will not occur even the slope of
the culvert is shallower than neutral slope.
The capacity of the culvert under conditions of
full pipe flow is given by:
…… Eq. 3
 Where
Q = discharge capacity (L3T-1)
A = conduit cross-sectional area (L2)
H = head causing flow (L)
Ke = entrance loss coefficient (Figure C-1)
Kb = bend loss coefficient (Table C-1)
Kc = conduit loss coefficient (L-1) (Table C-2/C-3)

For full pipe, H is difference between the
headwater elevation and the point 0.6 times
the culvert diameter above the downstream
invert as shown in figure below.
H = HW – 0.6D

For part full [refer figure 7 (b)], discharge
capacity is given by:
………. Eq. 4
 Where Q = discharge capacity (L3T-1)
A = conduit cross-sectional area (L2)
H = head causing flow (L)
C = orifice discharge coefficient (0.6)
H = Headwater – 0.5D



For inlet not submerged (Figure 7d & 7e), the
possibility of controls of flow is either by the
conduit or the inlet section.
The conduit controls if the slope of conduit is
too flat to carry the maximum possible inlet
flow at the required depth.
The inlet controls occur when the slope of the
conduit is greater than that required to move
the possible flow through the inlet.
2.
Drop Inlets
 The discharge characteristics of a drop inlet pipe
spillway are determined by the component of
the system that controls the flow rate.
 At low head, the crest of the riser controls the
flow (as weir) and discharge is proportional to
h3/2.
 Under this condition, the discharge can be
calculate using Eq. 1
 When the head increases, the capacity of the weir
will eventually equal the capacity of conduit (pipe
flow) or the conduit inlet section (orifice flow).
 The flow can be proportional to the square root of
either the total head loss through the structure or
the head on the conduit inlet, depending on
whether pipe flow or orifice flow controls
discharge.
3.
Hood Inlets
 Provides a simple and inexpensive alternative to
the drop inlet for mechanical spillways on ponds
and similar small structures.
 The hood inlet, when provided with a suitable
antivortex device, will cause the pipe to prime and
flow full for spillway slopes up to 30%.
4.
Outlet protection
 For small culvert or drop-inlet spillways, the
cantilever type outlet is usually satisfactory.
 The straight apron outlet may be used in some
instances.
 Large drop-inlet pipe spillways may be provided
with St. Anthony Fall (SAF) stilling basin.

Chutes are designed
to carry flow down
steep slope through a
concrete-lined
channel rather than
by dropping the
water in a free
overfall.



Chutes may be used for the control of
elevation changes up to 6m.
They usually require less concrete than dropinlet structures of the same capacity and
elevation change
However, there is considerable danger of
burrowing animals undermining the
structure, and in poorly drained location
seepage may threaten foundations.

Where there is no opportunity to provide
temporary storage above the structure, the
inherent high capacity of the chute makes it
preferable to the drop-inlet pipe spillway.

The capacity of a chute is not decreased by
sedimentation at the outlet.

Capacity
 Normally is controlled by the inlet.
 Inlets may be similar to those for straight-inlet or
box-inlet drop spillways.

Outlet protection
 The cantilever-type outlet should be used where
the channel grade below the structure is unstable.
 In other situation, either the straight-apron or St.
Anthony Falls (SAF) outlet is suitable.
 The straight apron is applicable to small
structures.




Widely used as dikes, levees and dams.
They are important in water supply and flood
control.
Earthen embankments are subject to
degradation by erosion, sloughing and other
natural processes.
Proper design, construction, and
maintenance will yield a stable and reliable
structure.
Focused on the rolled-fill type of earthen
embankments.
 The material is spread in uniform layers and then
compacted at optimum water content to
achieve maximum density.
 The selection and design of earthen
embankments for water control depends on

 The properties of foundation; stability, depth og
impervious strata, relative permeability and drainage
conditions.
 The nature and availability of the construction
materials.
Figure 8

3 major types of earth fill
i. Simple embankment type:
▪ Constructed of relatively homogeneous material and is either
keyed into an impervious foundation stratum as shown in
Fig. 8 (a), or
▪ Constructed with an upstream blanket of impervious
material as shown in Fig. 8 (b).
▪ This type is limited to low fills and to sites having sufficient
volumes of satisfactory fill materials available.
ii.
Core or zoned type:
 Includes a central section of highly impermeable
materials extending from an impermeable stratum in
foundation to above the water line as shown in Fig.
8(c).
 An upstream blanket is sometimes used in
conjunction with this design.
 Core type construction reduces the percentage of
high-grade fill materials needed.
Figure 8
iii. The diaphragm type:
 Uses a thin wall of plastic, butyl,
concrete, steel or wood to form a
barrier against seepage through the fill.
 A full-diaphragm cutoff extends from
above the water line down to and
sealed into an impervious foundation
stratum as shown in Fig. 8(d).
Figure 8
 A partial diaphragm does not extend
through this full range and is referred to
as a cutoff wall as shown in Fig. 8(e).
 The plastic or butyl rubber was used to
overcome the broken or cracked
problem due to settlement in the
foundation or the fill.
Figure 8

6 basic requirements must be met to ensure
an effective reservoir for water storage:
 Topographic conditions at site must allow
economical construction,
 Soil materials must be available to provide a
stable and impervious fill.
 Storage embankments must have adequate
mechanical and flood spillway facilities to
maintain a uniform water depth during normal
and to safely manage flood runoff.
 Large storage embankments should equipped
with a bottom drain to facilitate maintenance and
fish management
 Appropriate safety equipment must be installed
around drop-inlets and other hazardous portions
of the structure.
 All design specifications must be followed during
construction.
Seepage is the process by which a liquid leaks
through a porous substance
 Seepage line is analogous to a water table; no
hydrostatic pressure above it and there is
hydrostatic pressure below it.
 Its location depends on ;

 the permeability of various fill materials and
foundation.
 The ground water potential at site,
 Type and extent of any core within the embankment.
 Type and placement of drains in downstream portion
of the structure.

Seepage rate through an embankment of
homogeneous fill on an impervious foundation
can be calculated using equation below:
….. Eq.5
 Where q = discharge per unit length (L2T-1)
K = saturated hydraulic conductivity of the fill (LT-1)
h = head of water (L)
** d = adjusted flow length through the embankment (L)
** if there is no drain, flow length is below midpoint of seepage face



All impoundments must be equipped with an
emergency spillway that will safely conduct
flood flows in excess of the temporary storage
capacity of the structure.
The flood spillways consists of an approach
channel, a level control section and an exit
(grassed) section.
Can used Eq. 1 to calculate the discharge rate.



In many situation, such as mining,
construction, and agricultural activities, it is
not possible to protect the soil surface at all
times.
Rainfall onto disturbed soils will detach large
amounts of sediment.
Sediment-trapping devices and basins have
been designed for service lives ranging from a
few weeks to many years.



The transport capacity of flowing water depends
on its velocity.
Higher velocity flow with higher energies are
able to transport larger particles and larger
amounts of sediment.
The main objectives in sediment capture are:
 To reduce the energy of flow to the point that
sediment can settle out of the water column
 To detain the water long enough for that settlement
accumulated during its required service life or a
design cleanout interval.

The rate of spherical particles can be estimate with
Stroke’s law:
u = d2g (ρs – ρw)
18μ
… Eq.6
where u = velocity downward (LT-1)
d = particle diameter (L)
g = gravitational acceleration (LT-2)
μ = absolute viscosity of water (LM-1T-1)
ρs= density of particle (ML-3)
ρw = density of water (ML`)


From Eq.6, it is shows that larger particles
(sand) settle more rapidly tan smaller
particles (silt or clay).
By assuming a water temperature of 20˚C (μ
= 1.002 cp and ρw = 0.998g/mL) and ρs =
2.65g/m3 for quartz, Eq.6 was simplified to;
u = 89.9d2
where u = velocity in cm/s
d = particle dia. in mm
….Eq.7


The performance of a sediment detention
basin or similar structure characterized by its
sediment-trapping efficiency and the
concentration of sediment in effluent from
the structure.
Trapping efficiency is defined as the
percentage of the mass of sediment in the
inflow that is retained in the structure.

The major factors controlling sediment
transport are:
 Physical characteristics of sediment
 Hydraulic characteristics of the basin
 Inflow sediment graph
 Inflow hydrograph
 Chemistry of the water and sediment

To maximize the effectiveness of a sediment
detention structure, it should be designed to:
 Minimize turbulence
 Minimize dead areas
 Maximize residence time

Sedimentation basin was designed to
maintain a permanent pool or to empty
completely between runoff events.

This structure must provide volumes for:
 Storage of accumulate sediment
 Detention storage to give the necessary residence
time
 Flood storage



Most detention basins are earthen structures.
The difference is in the type and placement of
the outlet structure.
Several outlet designs are in common used as
shown in Figure 9.
Figure 9:
Detention
basin types
and outlets
(a)
(b)
(c)
(d)
(e)
dry basin with
perforated
riser,
wet basin with
perforated
riser,
basin with
skimmer
outlet,
basin with rock
filter outlet,
rock filter
outlet with
notch for flood
spillway.




The geometry of the basin influences its
effectiveness.
In general, basin should be long and narrow in
the direction of flow.
The length-to-width ratio should be at least
2:1 to minimize dead zone (Griffin et al.
1985).
Baffles, usually made with geotextiles, can be
inserted into a basin to create a longer flow
path and suppress turbulence.




The storage volume specified by local
regulation.
If hydrological design is to be used, the basin
should be sized to detain the runoff from a
design storm, typically having return period of 2
to 10 years.
The flood storage volume and size of flood
spillway will depend on the design storm
selected for that purpose.
For small structures, the flood spillway is
typically designed to pass the peak inflow rate.

Determine the capacity of a 762mm diameter
corrugated metal (ring) culvert that is 20m
long with a square-edged entrance. The
elevation of the inlet invert is 127.92m and
the elevation of the outlet invert is 127.71m.
The headwater elevation is 129.54 and the
tailwater elevation is 126.80m. The roughness
coefficient, n is 0.025.

H value
 H = HW – (OI+0.6D) = 129.54 – (127.71+0.6*0.762
= 1.37m

Ke value
 Fig. C-1  square-edged entrance, Ke = 0.5

Kc value
 Table C-2  Kc = 0.112

A value
A = πd2/4 = π*0.7622 / 4 = 0.456m2

Assume, pipe flow control: Calculate Q
 Q = A√(2gH) / √ ( 1 + Ke + Kb + KcL)
= [0.456 √ (2*9.81*1.37)] / √( 1 + 0.5 + 0 + 0.112*20)
= 1.22 m3/s

H = 129.54 – (127.92 + 0.5*0.762) = 1.239

Assume orifice flow controls, calculate Q
Q = AC √(2gH) = 0.456*0.6 √ (2*9.81*1.239)]
= 1.349 m3/s
the lesser of the two discharges represents the
controlling condition which is in this case is pipe
flow.
 As a further check, calculate the Sn using Eq. 2:

= 0.112* (1.22/0.456)2 / (2*9.81)
= 0.041

Actual slope, Sa
Sa = (Inlet Invert-Outlet Invert)/ L
= (127.92-127.71)/20
= 0.011
Sn < Sa  pipe flow control.

Calculate the time required for particles of
fine sand (0.2mm), silt (0.01mm) and clay
(0.001mm) to settle out of 1m column of
water. Assume that all particles are spherical
and have the density of quartz.

Using Eq. 7, calculate the settling velocities and
times for each particle size.
u = 89.9d2
Particle
Fine sand
Diameter
(mm)
0.2
Velocity, u
(cm/s)
3.596
Time to
settle 1m
27.81 s
Silt
0.01
0.00899
3.090 h
Clay
0.001
0.0000899
12.874 d
Time to settle = 100( cm/) u (cm/s)