Irrigation Crossing Structures

Transcription

1 Irrigation Crossing Structures Road Open channel H.R. Open Irrigation Canal Tail escape Water way + Road way Culvert Bridge Water way + Water way Syphon Aqueduct Retaining walls Lining ١

2 1- FUNCTION AND TYPES Irrigation canals are designed and operated to provide water requirements for their command areas at all times, i.e. QS = QD For QS less than QD = water level decrease (shortage of water). For QS more than QD= water level increase If number of water users may shut down after a rain and an overload may result further downstream with surplus of water level. excess canal discharge which have to be diverted ٢

3 To avoid the case of QS more than QD: 1. Improve control gates and introduce automation 2. Use off-line storage ponds if possible (available areas, environmental concerns, ) 3. Spill the excess water to the nearest drain by a structure called escape or spillway. All branch canals must be protected by escapes at the dead end of the canal. According to Egyptian practice for irrigation, the water level in the canals must not exceed MDWL by more than 0.25 or 0.50 m according to the canal size. ٣

4 2-TYPES OF ESCAPES The type of escape used in a given case depends upon: 1. the kind of structure (canal, dam,..), 2. the magnitude of the design discharge, 3. the topography and 4. the layout of the structures. 5. the location of the escape with respect to the canal (tail escape at the end of the canal or intermediate escapeadded to a syphon or aqueduct at the crossing of a canal with a drain). The common types of escapes used in Egypt are: Well escape (circular or rectangular) Straight weir or stepped weir. Tail regulator Siphon spillway. ٤

5 ٠٩/٠٤/١٤٣٣ Well escape (circular or rectangular) For small canals or projects, the well type escape can be suitable for Discharges up to 10 m3/sec and water depths up to 3.0 m. For High different levels between the canal and the drain ٥

6 For Very High different levels between the canal and the drain To excess high amount of discharge ٦

7 Canal Regardless of the type, every spillway (escape) has three basic components: 1. A crest section which is the inlet to the spillway. 2. A conveyance section to convey the flow to level near the downstream water level. 3. A discharge section to enable the flow to enter the required stream (drain). ٧

8 Tail escape (Well Type) Tail escape is a structure constructed at the end of a canal to get rid of the excess water to the nearest drain to avoid overtopping in the canal. Tail escape consists of a well where its crest level at the high water level. It is also equipped with an orifice at its bottom to evacuate all the water in 24 hrs if necessary. A pipe evacuates the canal water to the drain under the road. Hydraulic design of the tail escape consists of three parts: 1. Automatic evacuation (Weir (Crest level, Well Diameter)) 2. Controlled evacuation (Orifice (Level, Diameter)) 3. Discharging excess water (Pipe (Slope, Diameter, material)) ٨

9 1. Automatic Evacuation (Weir = Crest level, Well Diameter) A weir with crest at the top of the well which is provided with curved lip at level exceeds 10 cm above the canal H.W.L. Crest Level = HWL + 10cm (Screen is needed at the top of the well to protect it from floating materials). The maximum acceptable rise in water level above the HWL in the canal is 25 cm. ٩

10 the discharge of water over the weir can be calculated as follows: 1. The excess discharge is usually taken between 10% and 30% of the canal flow. 2. Weir discharge can be calculated as follows: Where: Q w = the weir s discharge Q w = T s. h. V s, V s = 1.17 V w T s T s = the top width of the water surface h = the rise in water level above crest level V s = the surface water velocity Vw= the mean water velocity of the water way. H.W.L. 25 cm The flow towards the well escape is dependent on the head over the crest and may be calculated by the conventional weir Equation: Q = w Cd B 2gh 3 Qw = Excess water to be evacuated through the weir Cd = Coefficient of weir discharge = 0.55 (less than the case of straight weir). h = Head over the weir crest not exceed 15 cm. B = Weir length (for circular type = ¾ the medium perimeter of the well. 3 B = πd w 4 D = D o. m inner w 5 Douter = Dw + o. 5m ١٠

11 Example1: A tail escape is required to be constructed at the end of a waterway according to the following data; Q c = 4.6 m 3 /sec, Canal last reach length = 3000 m, V c =0.45 m/sec, maximum allowable rising in canal water level is 25 cm. 7.6 m (10.00) 5.8 m 1.8 m 4 m Intermediate Escape ١١

12 Crest level = HWL+10cm Q w = 2 Cd B 3 2gh 1.5 ١٢

13 2. Controlled Evacuation An orifice at the bottom of the well which is provided with a steel gate supported on shoulders higher than the crest of the weir is designed to evacuate the waterway in 24 hrs. To determine the size of the orifice: π 2 a = d Q = c a gh 4 o o d o 2 Where: T is the time to empty the waterway = 24 * 60 *60 sec L is the waterway length in m, b e is the waterway average width in m, d is the depth of the waterway, Cd is the discharge coefficient = 0.55 a is the area of the orifice in m 2 and Q o is the discharge through the orifice L ١٣

14 L ١٤