---
title: "Design procedure: cross and distributary head regulator"
subject: "Irrigation and Drainage Engineering"
chapter: 7
key: "semester_6/irrigation_and_drainage_engineering:cross-distributary-regulator-design-procedure"
source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/cross-distributary-regulator-design-procedure
site: BCT Engineering
---

# Design procedure: cross and distributary head regulator

Irrigation and Drainage Engineering · Chapter 7: Regulating Structures

- Asked in: 2074 Bhadra, 8 marks
- Video: [DESIGN OF CROSS REGULATORY STRUCTURE | MOST IMPORTANT | IRRIGATION NUMERICAL SOLUTION](https://www.youtube.com/watch?v=n8Pzjuhh5N8) — Sudip Dhakal
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/cross-distributary-regulator-design-procedure
- Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-7

## Question

Write the stepwise design procedure of cross regulator and distributary head regulator with supporting sketches.

## Short answer

For cross regulator, compute Qcr=Qp-Qd, crest at d/s bed, waterway from orifice flow, scour and floor by Khosla. Distributary head regulator: sill at distributary bed, waterway from head difference, same scour and floor design. Both need cut-offs, inverted filter, and stone apron.

## Answer

A cross regulator is placed across the parent canal d/s of an off-take, and a distributary head regulator is placed at the off-take. Both are gated structures with similar design steps.

#### Cross regulator
1. **Data:** parent canal discharge $Q_p$, off-take discharge $Q_d$, u/s and d/s FSL, bed levels and widths.
2. **Discharge through the regulator:** $Q_{cr} = Q_p - Q_d$.
3. **Afflux:** $\Delta h = $ u/s FSL $-$ d/s FSL (usually 0.15 to 0.3 m).
4. **Crest level:** at the d/s bed (or raised 0.3 to 0.5 m if the canal is silty). Provide gates and piers.
5. **Waterway:** from orifice flow, $B = \dfrac{Q_{cr}}{C_d\,D\sqrt{2g\,\Delta h}}$ with $C_d \approx 0.8$, or from the weir formula $Q = 1.7LH^{3/2}$; provide bays and piers.
6. **Scour and cut-offs:** Lacey's $R = 1.35(q^2/f)^{1/3}$; d/s cut-off at $1.5R$ below the d/s water level and u/s cut-off about $R$.
7. **Floor:** length by Khosla's exit gradient for the maximum static head $H_s = $ u/s FSL $-$ d/s bed; thickness from uplift.
8. **Protection works:** inverted filter, stone apron.

#### Distributary head regulator
1. **Data:** distributary discharge, parent FSL, distributary FSL and bed level.
2. **Sill level:** at the distributary bed level (raised 0.3 m to reduce silt entry if needed).
3. **Waterway:** orifice flow, $B = \dfrac{Q_d}{C_d\,D\sqrt{2g\,\Delta h}}$, $\Delta h = FSL_{parent} - FSL_{dist}$; provide bays and piers.
4. **Cut-offs, floor length, thickness and protection** as for the cross regulator, using the head $H_s = $ parent FSL $-$ distributary bed.
5. **Alignment:** the head regulator is set at an angle of about 60° to 90° to the parent canal, and a cross regulator is placed d/s to hold the water level.

```text
 Plan:   parent canal  ===>===[CR]===>===
                         \ [HR]
                          \==> distributary
 Section: gates | piers | crest | floor | cut-offs
```

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What discharge passes the cross regulator?
   A. Qp + Qd
   B. Qd only
   C. Parent discharge minus off-take: Qcr = Qp - Qd
   D. Qp only
   Answer: C. The off-take takes Qd. (Key point: Cross regulator discharge)

2. What afflux is used?
   A. 5 m
   B. Zero
   C. 1 to 3 m
   D. u/s FSL minus d/s FSL, usually 0.15 to 0.3 m
   Answer: D. A small afflux holds up the level. (Key point: Afflux)

3. Which formula gives waterway from orifice flow?
   A. B = Q/(Cd D sqrt(2g dh)), Cd about 0.8
   B. B = 4.75 sqrt(Q)
   C. B = Q/(Cd D)
   D. B = 0.06Q
   Answer: A. The gates operate as orifices. (Key point: Waterway)

4. At what angle is the distributary head regulator set to the parent canal?
   A. 0 degrees
   B. About 60 to 90 degrees
   C. 10 degrees
   D. Parallel
   Answer: B. A cross regulator is placed downstream to hold the level. (Key point: Alignment)

## Related in Irrigation and Drainage Engineering

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