soil water conservation structure

Conservation Structures

Water Control Structures

MM HASAN,LECTURER,AIE,HSTU

Water Control Structures• 2 types

• Temporary and • Permanent Structures

• Temporary Structures • Temporary structures should be recommended only where inexpensive

labor and materials are available. • Increasing mechanization and higher labor costs have resulted in a

decline in the 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 a stable stream.

• Without such modifications, the problems are likely to recur, progressively degrading the land.

Water Control Structures• Permanent Structures

• Permanent structures of hard materials may be required to dissipate the energy of the water,

• for example, where a vegetated waterway discharges into a 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 9.1 shows the profile of a gully that has been reclaimed by methods involving the use of several types of permanent structures.

Figure 9.1–Profile of a gully stabilized by three types of permanent structures.

Functional Requirements of Control Structures

The design of control structures must address two primary requirements:

(1) adequate capacity to pass the design discharge, and (2) dissipation of the energy of the water within the structure in a manner that protects both the structure and the downstream channel from damage or erosion.

The main causes of failure of permanent control structures are insufficient hydraulic capacity and insufficient energy dissipation capacity. All permanent structures require maintenance, though it may be infrequent. Where maintenance is neglected, small problems can grow and eventually lead to total failure.

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Design Features

The basic components of a hydraulic structure are the inlet, the conduit, and the outlet.

Structures are classified and named in accordance with the form of these three components.

Design Features

In addition to these hydraulic features, the structure must include suitable wing walls, side walls, head wall extensions, and toe walls to prevent seepage under or around the structure and to prevent damage from local erosion.

Design Features

It is important that a firm foundation be secured for permanent structures. Wet foundations should be avoided or provided with adequate artificial drainage. Topsoil and organic material should be removed from the site to allow a good bond between the structure and the foundation material.Many energy dissipation structures make use of a hydraulic jump, which is a transition from a relatively shallow and rapid flow to a relatively deep and slow flow. Flow in the transition zone is highly turbulent and dissipates some of the energy of the water.

Drop spillways

A typical drop spillway is shown in Figure 9.3. Drop spillways may have a straight, arched, or box-type inlet. The energy dissipater may be a straight apron or some type of stilling basin.

Function and LimitationsDrop spillways are installed in channels to establish permanent control elevations below which an eroding stream cannot lower the channel floor. The structures control the stream grade from the spillway crest through the entire ponded reach upstream. Drop structures placed at intervals along the channel can stabilize it by changing its profile from a continuous steep gradient to a series of more mildly sloping reaches. Where relatively large volumes of water must flow through a narrow structure at low head, the box-type inlet is preferred. The curved inlet serves a similar purpose and also gives the advantage of arch strength where masonry construction is used. Drop spillways are usually limited to drops of 3 m; flumes or drop-inlet pipe spillways are used for greater drops.


Design FeaturesCapacity. The free flow (i.e., with no submergence) capacity for drop spillways is given by the weir formula

q = CLh3/2

where q = discharge (L3T-1),C = weir coefficient (L1/2T-1),L = weir length (L),h = depth of flow over crest (L)

The length L is the sum of the lengths of the three inflow sides of a box inlet, the circumference of an arch inlet, or the crest length of a straight inlet. Using C = 1.8 will also give satisfactory results for the straight inlet or the control section of a flood spillway. The inlet should have a freeboard of 0.15 m above h, the height of the water surface.


Design FeaturesApron Protection

The kinetic energy gained by the water as it falls from the crest must be dissipated and/or converted to potential energy before the flow exits the structure. For straight-inlet drop structures the dissipation and conversion of energy are accomplished in either a straight apron or a Morris and Johnson (1942) stilling basin. Dimensions for the Morris and Johnson stilling basin are given in Figure 9.5.



ChutesChutes are designed to carry flow down steep slopes through a concrete-lined channel rather than by dropping the water in a free overfall.


Function and LimitationsChutes may be used for the control of elevation changes up to 6 m. They usually require less concrete than drop-inlet structures of the same capacity and elevation change. However, there is considerable danger of undermining of the structure by burrowing animals and, in poorly drained locations, 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.


Design FeaturesCapacity

Chute capacity normally is controlled by the inlet section. Inlets may be similar to those for straight-inlet or box-inlet drop spillways, for which the capacity formulas already discussed will apply.

Outlet ProtectionThe cantilever-type outlet should be used where the channel grade below the structure is unstable. In other situations, either the straight-apron or St. Anthony Falls (SAF) outlet is suitable. The straight apron is applicable to small structures. Figure 9.8 shows dimensions of the SAF type of stilling basin.


Formless flumesFunction and Limitations

The formless flume structure has the advantage of low-cost construction. It may replace drop spillways where the fall does not exceed 2 m and the width of notch required does not exceed 7 m. The flume is constructed by shaping the soil to conform to the shape of the flume and applying a 0.13-m layer of concrete reinforced with wire mesh. Since no forms are needed, the construction is simple and inexpensive. The formless flume should not be used where water is impounded upstream (due to the danger of undermining the structure by seepage) or where freezing occurs at great depth.


Design FeaturesFigure 9.9 shows the design features and dimensions of the formless flume. The capacity is given by Equation 9.1 using C = 2.2 (SI). This weir coefficient accounts for the increased cross-sectional area because the sides of the weir slope outward rather than vertically and the entrance is rounded. The depth of the notch, D, is h plus a freeboard of 0.15 m.


Pipe spillwaysPipe spillways may take the form of a simple conduit under a fill (Figure 9.10a) or they may have a riser on the inlet end and some type of structure for outlet protection (Figure 9.10b). The pipe in Figure 9.10c, 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 and therefore require special attention to structural design.


Function and LimitationsThe pipe spillway used as a culvert has the simple function of providing for passage of water under an embankment. When combined with a riser or drop inlet, the pipe spillway serves to lower water through a considerable change in elevation and to dissipate the energy of the falling water. Drop-inlet pipe spillways are thus frequently used as gully control structures. This application is usually made where water may pond behind the inlet to provide temporary storage.


Design FeaturesCulverts.Culvert capacity may be controlled by either the inlet section or the conduit. The headwater elevation may be above or below the top of the inlet section. Several possible flow conditions are represented in Figure 9.11. Solution of a culvert problem requires determination of the type of flow that will occur under given headwater and tailwater conditions. Consider a culvert as shown in Figures 9.11a and 9.11b.


Design FeaturesPipe flow (i.e., where the conduit controls capacity) will usually occur where the slope of the culvert is less than the neutral slope and entrance capacity is not limiting. The neutral slope sn is

where Hf = friction loss in conduit of length L (L),L = length of conduit (L),Kc = conduit friction loss coefficient (Tables C.2 and C.3) (L-1),v = velocity of flow (LT-1),g = gravitational acceleration (LT-2).


The capacity of the culvert under conditions of full pipe flow is given by

where q = discharge capacity (L3T-1),A = conduit cross-sectional area (L2),H = head causing flow (L),Ke = entrance loss coefficient,Kb = bend loss coefficient,Kc = conduit loss coefficient.

For full pipe flow, H is taken as the difference between the headwater elevation and the point 0.6 times the culvert diameter above the downstream invert (Figure 9.11a). Values of Kb, Kc, and Ke are given in Appendix C.


If the inlet is submerged, the slope of the conduit is greater than neutral slope, and the outlet is not submerged, then the flow will be controlled by the inlet section on short-length culverts, and orifice flow will control. Discharge capacity is then given by

where q = discharge capacity (L3T-1),A = conduit cross-sectional area (L2),H = head causing flow (L),C = orifice discharge coefficient.


Drop InletsThe discharge characteristics of a drop-inlet pipe spillway (Figure 9.13) are determined by the component of the system that controls the flow rate. At low heads, the crest of the riser controls the flow (as a weir) and discharge is proportional to h3/2. Equation 9.1 should be used to calculate the discharge for these conditions. As the head increases, the capacity of the weir will eventually equal the capacity of the conduit (pipe flow) or the conduit inlet section (orifice flow). The flow will then 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 the discharge.


Hood InletsFor mechanical spillways on ponds and similar small structures the hood inlet provides a relatively simple and inexpensive alternative to the drop inlet. 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%. Hood inlets shown in Figures 9.14a and 9.14b were developed by Blaisdell and Donnelley (1958). Beasley et al. (1960) reported that a hood inlet with an endplate as shown in Figure 9.14c gave satisfactory performance although the entrance loss was somewhat higher than with the other two. The discharge characteristics of these three inlets are shown in Figure 9.14d for a pipe of length 110D. For H/D less than 1, weir flow occurs. Up to H/D of about 1.4, the flow is erratic. Above H/D of 1.4 the vortex is eliminated and pipe flow controls.


Outlet ProtectionFor small culverts or drop-inlet pipe 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 the SAF stilling basin discussed in Section 9.7.


The neutral slope is the slope of energy grade line when the pipe just flows full, i.e., when the momentum due to the inertial force and the momentum loss due to friction are equal.

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