PREPARED BY: CIK SAMERA SAMSUDDIN SAH WATER AND SEDIMENT CONTROL STRCUTURE ERT 349 SOIL AND WATER ENGINEERING
A. WATER CONTROL STRUCTURE • 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.
1.0 Temporary and Permanent Structures • 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.
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 erosion-resistant lining may be required.
Figure 1: Profile of a gully stabilized by three types of permanent strcutures.
1.1 Functional Requirements of Control Structures • 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.
1.2 Design Features of Control Structures • 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.
2.0 DROP SPILLWAYS • 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.
2.1 Function and Limitation of Drop Spillways • 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.
2.2 Design features of drop spillways • 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
3.0 Pipe Spillways • 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.
3.1 Function and Limitations of Pipe Spillways • 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.
3.2 Design features of pipe spillways • 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, sncan 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.
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.
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%.
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.
4.0 Chutes • Chutes are designed to carry flow down steep slope through a concrete-lined channel rather than by dropping the water in a free overfall.
4.1 Function and Limitations of Chutes • Chutes may be used for the control of elevation changes up to 6m. • They usually require less concrete than drop-inlet 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.
4.2 Design features of chutes • 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.
5.0 Earthen Embankment • 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.
5.1 Types of earth embankments • 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.
3 major types of earth fill • 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. Figure 8
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
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). • 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 Figure 8
5.2 Requirements for earthen embankments • 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.
5.3 Seepage through the embankment • 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
6.0 Spillways as flood control • 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.
7.0 SEDIMENT CONTROL STRUCTURE • 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.
7.1 Principles of sediment capture • 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 ….Eq.7 where u = velocity in cm/s d = particle dia. in mm
7.2 Major of performance • 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
7.3 Detention Basin Characteristics • 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.