ABE 325 Soil & Water Conservation Engineering. CONSERVATION
STRUCTURES. Purpose of these structures is to transfer runoff from higher to
lower ...
ABE 325 Soil & Water Conservation Engineering CONSERVATION STRUCTURES Purpose of these structures is to transfer runoff from higher to lower elevation over a short distance without allowing excessive soil erosion to occur. Some structures also act to retain earth. Temporary Structures: - only where cheap labor and materials can be used - may be constructed of creosoted planks, rocks, logs, brush, woven wire, sod, earth Permanent Structures: - constructed of more permanent materials, such as concrete - possible uses include: transfer water from vegetated waterway to drainage ditch, control overfall at the head of a large gully, to take up the fall at various points along a channel, or to provide discharge through earthen banks on water storages.
Design of Conservation Structures: two primary features: 1) 2)
Types of Structures: 1) drop spillway
2) chute
feature q space cost life
drop spillway
3) pipe spillway
chute
pipe spillway
Hydraulic components: inlet, conduit, outlet Components to prevent seepage and erosion: side, bottom, toe and head walls, apron Refer to Figures 9.2 & 9.3
DROP SPILLWAYS (Fig 9.3) Flow through a drop spillway passes through a weir opening, drops to an approximately level apron or stilling basin, and then passes into the downstream channel. Components of a drop spillway are: inlet, headwall, and outlet.
Advantages: 1. 2. 3. 4.
Stability Low maintenance costs Ease and economy of construction Standardization
Disadvantages: 1. More costly than some other methods if required capacity is less than 100 cfs and total drop is greater than 8 or 10 feet. (Use flume or drop-inlet pipe spillway for drops of 3+ meters.) 2. Subject to undercutting if a stable slope is not maintained below the structure 3. Skilled labor required to construct unless being built with concrete blocks
CHUTES Designed to carry flow down steep slopes through a sloped, concrete channel, rather than by dropping water in a free overfall. Use weir formula
Advantages: - suitable for large q and moderate elevation change (up to 6m)
Disadvantages: - life expectancy is less than for other structures - need good soil conditions for foundation - usually built as monolithic reinforced concrete structure (skilled labor required)
PIPE SPILLWAY (culverts & conduits) Economically lowers water over large drops in a short distance. Dissipates energy so as to reduce downstream erosion. Constructed with jointed pipe or corrugated metal with a variety of inlet and outlet options. Very versatile - can be used with small reservoirs as a culvert, or as an outlet through the spoil bank of ditches. Need some space above the inlet for temporary storage of runoff. Require good backfill material which must be properly placed and compacted. Inlet is subject to blockage by debris.
Three principle types:
1) Direct Entrance (culvert)
2) Drop-inlet Conduit
3) Inverted Siphon
DESIGN 1. Determine expected peak flow rates and inflow hydrographs. 2. Select frequency (return period) of design. Consider: - intended life of structure - probable extent of damage should spillway fail due to inadequate capacity - relative size and cost of structure 3. When water is open to the atmosphere (drop spillways and chutes) the control section is the inlet so the weir formula holds. Also referred to as free-flow discharge (no submergence). 4. For pipe spillways, need to develop “stage-discharge” relationship for the conduit. Consider
3 types of flow: 1) weir flow 2) orifice flow 3) pipe flow Weir Flow:
Orifice Flow
Pipe Flow
Variations (Page 184) - note where head and length measurements are taken for each situation:
Inlet
Conduit Flow
Tailwater Level
Flow type
submerged
full
below outlet
pipe flow
submerged
partially full
below outlet
orifice flow
submerged
full
above outlet
not submerged
not full / full
below outlet
pipe flow open channel flow OR weir flow (depends on whether conduit or inlet controls flow)
DISCHARGE EQUATIONS
Weir Flow:
q=0.55CLh 3/2 {m3 /s}
(equ 9.6)
where: C = weir coefficient L = weir length (m) h = depth of flow over crest (m L = length over which water flows on the inlet = sum of 3 inflow sides on box inlet, circumference of an arch inlet, crest of a straight inlet Typical design value of C = 3.2 (assumes flow approaching weir is at subcritical velocity). For: sharp edged orifice -> C = 0.6 circular weir -> C = 2.7 broad crested weir -> C = 3.0 Pipe Flow: Occurs when the inlet is submerged, the slope of the culvert is LESS than the neutral slope, and the inlet does not restrict flow capacity. The neutral slope, sn, for small angles is given by:
v2 sn = tanΘ =sinΘ= = Kc 2g L Hf
where:
Hf = friction loss in conduit length L = length of conduit Kc = friction loss coefficient (Appendix C) v = flow velocity g = gravity
Pipe Flow Capacity:
(equ 9.7)
q=
a 2 gH 1 + Ke + Kb + Kc L
(equ 9.8)
where:
a = conduit cross-sectional area H = head causing flow Ke = entrance loss coefficient (Appendix C) Kb = loss coefficient for bends (App. C. For straight conduits, Kb = 0)
Orifice Flow: where:
q=aC(2gh)1/2
(equ 9.9)
a = cross-sectional area C = weir coefficient (same as above) h = head to center of the orifice
Conduit controls flow if the slope of the conduit is too flat to carry the maximum possible inlet flow at the required depth. [Depth = headwater depth above inlet minus static head losses due to entrance losses and acceleration.]
EXAMPLES 1. A straight, 24“ (inside) diameter circular pipe connects two reservoirs such that
the outlet is submerged continuously. The pipe is 100 feet long. The inlet to the pipe is inward projecting. What is the discharge through the pipe if the stage of the upper reservoir is 800 ft and the stage of the lower reservoir is 785 ft?
2. Determine the capacity of a 762-mm diameter, corrugated metal culvert that is 22
m long with a sharp, square edged inlet. Elevation of the bottom of the inlet is 97 m, of the bottom of the outlet is 96 m, of the water surface at the inlet is 100 m and the water surface at the outlet is 95 m.
3. What would be the flow rate for question 2 if the actual slope was greater than the neutral slope (i.e orifice flow conditions)?
