The Sarda type fall, also known as a Simple Vertical Drop Fall, is a type of canal drop structure. It was first introduced and used on the Sarda Canal System in Uttar Pradesh (U.P.), India.
 |
| Simple Vertical Drop or Sarda Canal Fall |
Definition and Purpose:
- A Sarda type fall is characterized by a raised crest from which water falls with a vertical impact into a cistern below.
- The primary purpose of this vertical drop is to dissipate surplus energy from the falling water. The cistern acts as a water cushion to absorb this energy and prevent erosion.
- It was developed out of the necessity for economical falls due to the requirement for constructing a large number of smaller falls on the Sarda Canal system, especially where sandy stratum overlaid sandy-clay and a minimum depth of cutting was desired.
Key Characteristics and Design Principles:
- Non-flumed Design: A Sarda type fall should not be flumed. This means the width of the crest is typically the same as the bed width of the canal. Fluming in vertical falls can increase discharge intensity and cause scour downstream.
- Crest Dimensions:
- For canal discharges of 15 cumecs or more, the crest length is equal to the bed width of the canal.
- For distributaries and minors, the crest length is equal to the bed width plus the depth of flow.
- The crest level is fixed such that the upstream flow depth is not affected, calculated as R.L. of crest = F.S.L. on the upstream (u/s) - H, where H includes velocity of approach.
- Body Wall:
- For canal discharges less than 14 m³/sec, the body wall section is rectangular. Its top width 'b' is calculated as 0.552√d.
- For discharges greater than 14 m³/sec, the body wall section is trapezoidal with an u/s batter of 1:3 and a downstream (d/s) batter of 1:8. Its top width 'b' is 0.522√(H+d).
- The edges of the body wall are rounded with a 0.3 m radius.
- Discharge Formula:
- Under free-fall conditions, the discharge (Q) is given by Q = CLH {H/b}¹/⁶, where C is 2 for a trapezoidal crest and 1.85 for a rectangular crest.
- For submerged flow (above 33% submergence), a coefficient of discharge (Cd) of 0.65 is used.
- Cistern Dimensions: The cistern's length (LC) can be fixed using the Bahadurabad Research Institute formula: LC = 5√E.HL and X = ¼ (E.HL)²/³.
- Impervious Floor:
- The total length of the impervious floor is designed based on theories like Bligh's or Khosla's to account for seepage.
- The length of the d/s impervious floor (Ld) is given by Ld = 2D + 2.4 + HL.
- The d/s floor must be thick enough to resist uplift pressures, with a minimum thickness of 0.3 to 0.6 m of concrete, potentially with 35 cm of brick masonry on top.
- Cut-offs: Cut-offs are provided at the u/s and d/s ends of the floor for safety against a steep exit gradient, typically ranging from 1 to 1.5 m in depth at the d/s end.
- Protective Works:
- Upstream wing walls: Splayed at 1:1 for small falls (up to 14 cumec) or segmental with a radius of 6H for higher discharges.
- Downstream wing walls: Kept vertical along the cistern and then flared to a 1:1 slope with an average splay of 1 in 3.
- Staggered blocks: Provided on the cistern floor or at the end of the d/s impervious floor to aid energy dissipation.
- Bed and side pitching: D/s bed pitching (20 cm thick bricks over 10 cm ballast) for a length of 6 m, followed by a down slope. Side pitching (bricks on edge with a 1:1 slope) is provided after the return-wing.
Suitability and Limitations:
- Sarda type falls are suitable for small drops, with drops varying from 0.6 to 2.5 m, and discharge intensity from 1.6 to 3.5 cumec/m.
- This type of fall is not suitable as a meter for measuring discharge due to the potential formation of a partial vacuum under the nappe.
- They are generally considered economical and simple in design.
In summary, the Sarda type fall is a robust and economical vertical drop structure primarily used in canal systems like the Sarda Canal in U.P. to manage changes in ground slope and dissipate excess water energy.
Post a Comment