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Momentum Depth Diagram

Hydraulics · FE Reference Handbook section

Hydraulics
2 formulas
10 exam-style examples
~49 min
All Hydraulics lectures

Core formulas for this FE topic

Definitions, applicability, units, assumptions and worked examples for each relation.

Worked exam-style examples

The four ways this section is written on the real exam — thoughts first, then equations, then substitution.

Example 1
Momentum Depth Diagram (specific force) — solve for specific force (momentum function) — Momentum Depth Diagram

a momentum-depth diagram used to locate conjugate depths of a hydraulic jump Given unit discharge (q) = 4.9000 m^2/s; gravity (g) = 9.8700 m/s^2; flow depth (y) = 0.5000 m, determine the specific force (momentum function) (M) in m^2.

Given

  • unitdischarge(q)=4.9000m2/sunit discharge (q) = 4.9000 m^2/s
  • gravity(g)=9.8700m/s2gravity (g) = 9.8700 m/s^2
  • flowdepth(y)=0.5000mflow depth (y) = 0.5000 m

Find

specific force (momentum function) (M), in m^2

Start with the thinking

  • The governing relation printed in this handbook section is Momentum Depth Diagram (specific force).
  • Everything except M is given, so isolate M symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • The momentum-depth diagram plots specific force against depth to find conjugate depths across a hydraulic jump.
depth yspecific force MMomentum-depth diagramconjugate depths

Figure 1 — schematic for Momentum Depth Diagram (specific force) — solve for specific force (momentum function) — Momentum Depth Diagram

Step-by-step solution

  1. Step 1 — State the governing relation:

    M=q2gy+y22M = \dfrac{q^2}{g y} + \dfrac{y^2}{2}
  2. Step 2 — Rearrange the relation so that M stands alone on the left-hand side.

  3. Step 3 — List the givens: unit discharge (q) = 4.9000 m^2/s, gravity (g) = 9.8700 m/s^2, flow depth (y) = 0.5000 m.

  4. Step 4 — Substitute the given values into the rearranged relation.

  5. Step 5 — Evaluate:

    M = 4.9902\ \text{m^2}
  6. Step 6 — Check: returning M = 4.9902 m^2 to

    M=q2gy+y22M = \dfrac{q^2}{g y} + \dfrac{y^2}{2}

    reproduces the given quantities, and both sides carry the same units.

Answer:
M = 4.9902\ \text{m^2}

Why the other options are there

  • 9.9805 — kept a factor of two that cancels in the correct rearrangement.
  • 2.4951 — dropped that same factor in the other direction.
  • 5.4893 — rounded an intermediate value before the final step.

Reference: FE Handbook — Momentum Depth Diagram

Example 2
Momentum Depth Diagram (specific force) — solve for specific force (momentum function) (case 2) — Momentum Depth Diagram (2)

the momentum-depth diagram for flow in a rectangular open channel Given unit discharge (q) = 1.6000 m^2/s; gravity (g) = 9.8500 m/s^2; flow depth (y) = 1.8000 m, determine the specific force (momentum function) (M) in m^2.

Given

  • unitdischarge(q)=1.6000m2/sunit discharge (q) = 1.6000 m^2/s
  • gravity(g)=9.8500m/s2gravity (g) = 9.8500 m/s^2
  • flowdepth(y)=1.8000mflow depth (y) = 1.8000 m

Find

specific force (momentum function) (M), in m^2

Start with the thinking

  • The governing relation printed in this handbook section is Momentum Depth Diagram (specific force).
  • Everything except M is given, so isolate M symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • The momentum-depth diagram plots specific force against depth to find conjugate depths across a hydraulic jump.
depth yspecific force MMomentum-depth diagramconjugate depths

Figure 2 — schematic for Momentum Depth Diagram (specific force) — solve for specific force (momentum function) (case 2) — Momentum Depth Diagram (2)

Step-by-step solution

  1. Step 1 — State the governing relation:

    M=q2gy+y22M = \dfrac{q^2}{g y} + \dfrac{y^2}{2}
  2. Step 2 — Rearrange the relation so that M stands alone on the left-hand side.

  3. Step 3 — List the givens: unit discharge (q) = 1.6000 m^2/s, gravity (g) = 9.8500 m/s^2, flow depth (y) = 1.8000 m.

  4. Step 4 — Substitute the given values into the rearranged relation.

  5. Step 5 — Evaluate:

    M = 1.7644\ \text{m^2}
  6. Step 6 — Check: returning M = 1.7644 m^2 to

    M=q2gy+y22M = \dfrac{q^2}{g y} + \dfrac{y^2}{2}

    reproduces the given quantities, and both sides carry the same units.

Answer:
M = 1.7644\ \text{m^2}

Why the other options are there

  • 3.5288 — kept a factor of two that cancels in the correct rearrangement.
  • 0.8822 — dropped that same factor in the other direction.
  • 1.9408 — rounded an intermediate value before the final step.

Reference: FE Handbook — Momentum Depth Diagram

Example 3
Momentum Depth Diagram (specific force) — solve for specific force (momentum function) (case 3) — Momentum Depth Diagram (3)

a momentum-depth diagram analysis at a stilling basin Given unit discharge (q) = 3.8500 m^2/s; gravity (g) = 9.8500 m/s^2; flow depth (y) = 1.5000 m, determine the specific force (momentum function) (M) in m^2.

Given

  • unitdischarge(q)=3.8500m2/sunit discharge (q) = 3.8500 m^2/s
  • gravity(g)=9.8500m/s2gravity (g) = 9.8500 m/s^2
  • flowdepth(y)=1.5000mflow depth (y) = 1.5000 m

Find

specific force (momentum function) (M), in m^2

Start with the thinking

  • The governing relation printed in this handbook section is Momentum Depth Diagram (specific force).
  • Everything except M is given, so isolate M symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • The momentum-depth diagram plots specific force against depth to find conjugate depths across a hydraulic jump.
depth yspecific force MMomentum-depth diagramconjugate depths

Figure 3 — schematic for Momentum Depth Diagram (specific force) — solve for specific force (momentum function) (case 3) — Momentum Depth Diagram (3)

Step-by-step solution

  1. Step 1 — State the governing relation:

    M=q2gy+y22M = \dfrac{q^2}{g y} + \dfrac{y^2}{2}
  2. Step 2 — Rearrange the relation so that M stands alone on the left-hand side.

  3. Step 3 — List the givens: unit discharge (q) = 3.8500 m^2/s, gravity (g) = 9.8500 m/s^2, flow depth (y) = 1.5000 m.

  4. Step 4 — Substitute the given values into the rearranged relation.

  5. Step 5 — Evaluate:

    M = 2.1282\ \text{m^2}
  6. Step 6 — Check: returning M = 2.1282 m^2 to

    M=q2gy+y22M = \dfrac{q^2}{g y} + \dfrac{y^2}{2}

    reproduces the given quantities, and both sides carry the same units.

Answer:
M = 2.1282\ \text{m^2}

Why the other options are there

  • 4.2564 — kept a factor of two that cancels in the correct rearrangement.
  • 1.0641 — dropped that same factor in the other direction.
  • 2.3410 — rounded an intermediate value before the final step.

Reference: FE Handbook — Momentum Depth Diagram

Example 4
Momentum Depth Diagram (specific force) — solve for specific force (momentum function) (case 4) — Momentum Depth Diagram (4)

a momentum-depth diagram used to locate conjugate depths of a hydraulic jump Given unit discharge (q) = 2.5000 m^2/s; gravity (g) = 9.7700 m/s^2; flow depth (y) = 0.8000 m, determine the specific force (momentum function) (M) in m^2.

Given

  • unitdischarge(q)=2.5000m2/sunit discharge (q) = 2.5000 m^2/s
  • gravity(g)=9.7700m/s2gravity (g) = 9.7700 m/s^2
  • flowdepth(y)=0.8000mflow depth (y) = 0.8000 m

Find

specific force (momentum function) (M), in m^2

Start with the thinking

  • The governing relation printed in this handbook section is Momentum Depth Diagram (specific force).
  • Everything except M is given, so isolate M symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • The momentum-depth diagram plots specific force against depth to find conjugate depths across a hydraulic jump.
depth yspecific force MMomentum-depth diagramconjugate depths

Figure 4 — schematic for Momentum Depth Diagram (specific force) — solve for specific force (momentum function) (case 4) — Momentum Depth Diagram (4)

Step-by-step solution

  1. Step 1 — State the governing relation:

    M=q2gy+y22M = \dfrac{q^2}{g y} + \dfrac{y^2}{2}
  2. Step 2 — Rearrange the relation so that M stands alone on the left-hand side.

  3. Step 3 — List the givens: unit discharge (q) = 2.5000 m^2/s, gravity (g) = 9.7700 m/s^2, flow depth (y) = 0.8000 m.

  4. Step 4 — Substitute the given values into the rearranged relation.

  5. Step 5 — Evaluate:

    M = 1.1196\ \text{m^2}
  6. Step 6 — Check: returning M = 1.1196 m^2 to

    M=q2gy+y22M = \dfrac{q^2}{g y} + \dfrac{y^2}{2}

    reproduces the given quantities, and both sides carry the same units.

Answer:
M = 1.1196\ \text{m^2}

Why the other options are there

  • 2.2393 — kept a factor of two that cancels in the correct rearrangement.
  • 0.5598 — dropped that same factor in the other direction.
  • 1.2316 — rounded an intermediate value before the final step.

Reference: FE Handbook — Momentum Depth Diagram

Example 5
Momentum Depth Diagram (specific force) — solve for specific force (momentum function) (case 5) — Momentum Depth Diagram (5)

the momentum-depth diagram for flow in a rectangular open channel Given unit discharge (q) = 3.4500 m^2/s; gravity (g) = 9.7500 m/s^2; flow depth (y) = 1.1000 m, determine the specific force (momentum function) (M) in m^2.

Given

  • unitdischarge(q)=3.4500m2/sunit discharge (q) = 3.4500 m^2/s
  • gravity(g)=9.7500m/s2gravity (g) = 9.7500 m/s^2
  • flowdepth(y)=1.1000mflow depth (y) = 1.1000 m

Find

specific force (momentum function) (M), in m^2

Start with the thinking

  • The governing relation printed in this handbook section is Momentum Depth Diagram (specific force).
  • Everything except M is given, so isolate M symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • The momentum-depth diagram plots specific force against depth to find conjugate depths across a hydraulic jump.
depth yspecific force MMomentum-depth diagramconjugate depths

Figure 5 — schematic for Momentum Depth Diagram (specific force) — solve for specific force (momentum function) (case 5) — Momentum Depth Diagram (5)

Step-by-step solution

  1. Step 1 — State the governing relation:

    M=q2gy+y22M = \dfrac{q^2}{g y} + \dfrac{y^2}{2}
  2. Step 2 — Rearrange the relation so that M stands alone on the left-hand side.

  3. Step 3 — List the givens: unit discharge (q) = 3.4500 m^2/s, gravity (g) = 9.7500 m/s^2, flow depth (y) = 1.1000 m.

  4. Step 4 — Substitute the given values into the rearranged relation.

  5. Step 5 — Evaluate:

    M = 1.7148\ \text{m^2}
  6. Step 6 — Check: returning M = 1.7148 m^2 to

    M=q2gy+y22M = \dfrac{q^2}{g y} + \dfrac{y^2}{2}

    reproduces the given quantities, and both sides carry the same units.

Answer:
M = 1.7148\ \text{m^2}

Why the other options are there

  • 3.4296 — kept a factor of two that cancels in the correct rearrangement.
  • 0.8574 — dropped that same factor in the other direction.
  • 1.8863 — rounded an intermediate value before the final step.

Reference: FE Handbook — Momentum Depth Diagram

Example 6
Momentum Depth Diagram (specific force) — solve for specific force (momentum function) (case 6) — Momentum Depth Diagram (6)

a momentum-depth diagram analysis at a stilling basin Given unit discharge (q) = 3.1000 m^2/s; gravity (g) = 9.9000 m/s^2; flow depth (y) = 1.3000 m, determine the specific force (momentum function) (M) in m^2.

Given

  • unitdischarge(q)=3.1000m2/sunit discharge (q) = 3.1000 m^2/s
  • gravity(g)=9.9000m/s2gravity (g) = 9.9000 m/s^2
  • flowdepth(y)=1.3000mflow depth (y) = 1.3000 m

Find

specific force (momentum function) (M), in m^2

Start with the thinking

  • The governing relation printed in this handbook section is Momentum Depth Diagram (specific force).
  • Everything except M is given, so isolate M symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • The momentum-depth diagram plots specific force against depth to find conjugate depths across a hydraulic jump.
depth yspecific force MMomentum-depth diagramconjugate depths

Figure 6 — schematic for Momentum Depth Diagram (specific force) — solve for specific force (momentum function) (case 6) — Momentum Depth Diagram (6)

Step-by-step solution

  1. Step 1 — State the governing relation:

    M=q2gy+y22M = \dfrac{q^2}{g y} + \dfrac{y^2}{2}
  2. Step 2 — Rearrange the relation so that M stands alone on the left-hand side.

  3. Step 3 — List the givens: unit discharge (q) = 3.1000 m^2/s, gravity (g) = 9.9000 m/s^2, flow depth (y) = 1.3000 m.

  4. Step 4 — Substitute the given values into the rearranged relation.

  5. Step 5 — Evaluate:

    M = 1.5917\ \text{m^2}
  6. Step 6 — Check: returning M = 1.5917 m^2 to

    M=q2gy+y22M = \dfrac{q^2}{g y} + \dfrac{y^2}{2}

    reproduces the given quantities, and both sides carry the same units.

Answer:
M = 1.5917\ \text{m^2}

Why the other options are there

  • 3.1834 — kept a factor of two that cancels in the correct rearrangement.
  • 0.7958 — dropped that same factor in the other direction.
  • 1.7509 — rounded an intermediate value before the final step.

Reference: FE Handbook — Momentum Depth Diagram

Example 7
Momentum Depth Diagram (specific force) — solve for specific force (momentum function) (case 7) — Momentum Depth Diagram (7)

a momentum-depth diagram used to locate conjugate depths of a hydraulic jump Given unit discharge (q) = 4.0500 m^2/s; gravity (g) = 9.9000 m/s^2; flow depth (y) = 2.4000 m, determine the specific force (momentum function) (M) in m^2.

Given

  • unitdischarge(q)=4.0500m2/sunit discharge (q) = 4.0500 m^2/s
  • gravity(g)=9.9000m/s2gravity (g) = 9.9000 m/s^2
  • flowdepth(y)=2.4000mflow depth (y) = 2.4000 m

Find

specific force (momentum function) (M), in m^2

Start with the thinking

  • The governing relation printed in this handbook section is Momentum Depth Diagram (specific force).
  • Everything except M is given, so isolate M symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • The momentum-depth diagram plots specific force against depth to find conjugate depths across a hydraulic jump.
depth yspecific force MMomentum-depth diagramconjugate depths

Figure 7 — schematic for Momentum Depth Diagram (specific force) — solve for specific force (momentum function) (case 7) — Momentum Depth Diagram (7)

Step-by-step solution

  1. Step 1 — State the governing relation:

    M=q2gy+y22M = \dfrac{q^2}{g y} + \dfrac{y^2}{2}
  2. Step 2 — Rearrange the relation so that M stands alone on the left-hand side.

  3. Step 3 — List the givens: unit discharge (q) = 4.0500 m^2/s, gravity (g) = 9.9000 m/s^2, flow depth (y) = 2.4000 m.

  4. Step 4 — Substitute the given values into the rearranged relation.

  5. Step 5 — Evaluate:

    M = 3.5703\ \text{m^2}
  6. Step 6 — Check: returning M = 3.5703 m^2 to

    M=q2gy+y22M = \dfrac{q^2}{g y} + \dfrac{y^2}{2}

    reproduces the given quantities, and both sides carry the same units.

Answer:
M = 3.5703\ \text{m^2}

Why the other options are there

  • 7.1407 — kept a factor of two that cancels in the correct rearrangement.
  • 1.7852 — dropped that same factor in the other direction.
  • 3.9274 — rounded an intermediate value before the final step.

Reference: FE Handbook — Momentum Depth Diagram

Example 8
Momentum Depth Diagram (specific force) — solve for specific force (momentum function) (case 8) — Momentum Depth Diagram (8)

the momentum-depth diagram for flow in a rectangular open channel Given unit discharge (q) = 0.6500 m^2/s; gravity (g) = 9.8500 m/s^2; flow depth (y) = 1.0000 m, determine the specific force (momentum function) (M) in m^2.

Given

  • unitdischarge(q)=0.6500m2/sunit discharge (q) = 0.6500 m^2/s
  • gravity(g)=9.8500m/s2gravity (g) = 9.8500 m/s^2
  • flowdepth(y)=1.0000mflow depth (y) = 1.0000 m

Find

specific force (momentum function) (M), in m^2

Start with the thinking

  • The governing relation printed in this handbook section is Momentum Depth Diagram (specific force).
  • Everything except M is given, so isolate M symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • The momentum-depth diagram plots specific force against depth to find conjugate depths across a hydraulic jump.
depth yspecific force MMomentum-depth diagramconjugate depths

Figure 8 — schematic for Momentum Depth Diagram (specific force) — solve for specific force (momentum function) (case 8) — Momentum Depth Diagram (8)

Step-by-step solution

  1. Step 1 — State the governing relation:

    M=q2gy+y22M = \dfrac{q^2}{g y} + \dfrac{y^2}{2}
  2. Step 2 — Rearrange the relation so that M stands alone on the left-hand side.

  3. Step 3 — List the givens: unit discharge (q) = 0.6500 m^2/s, gravity (g) = 9.8500 m/s^2, flow depth (y) = 1.0000 m.

  4. Step 4 — Substitute the given values into the rearranged relation.

  5. Step 5 — Evaluate:

    M = 0.5429\ \text{m^2}
  6. Step 6 — Check: returning M = 0.5429 m^2 to

    M=q2gy+y22M = \dfrac{q^2}{g y} + \dfrac{y^2}{2}

    reproduces the given quantities, and both sides carry the same units.

Answer:
M = 0.5429\ \text{m^2}

Why the other options are there

  • 1.0858 — kept a factor of two that cancels in the correct rearrangement.
  • 0.2714 — dropped that same factor in the other direction.
  • 0.5972 — rounded an intermediate value before the final step.

Reference: FE Handbook — Momentum Depth Diagram

Example 9
Momentum Depth Diagram (specific force) — solve for specific force (momentum function) (case 9) — Momentum Depth Diagram (9)

a momentum-depth diagram analysis at a stilling basin Given unit discharge (q) = 4.1500 m^2/s; gravity (g) = 9.8100 m/s^2; flow depth (y) = 1.1000 m, determine the specific force (momentum function) (M) in m^2.

Given

  • unitdischarge(q)=4.1500m2/sunit discharge (q) = 4.1500 m^2/s
  • gravity(g)=9.8100m/s2gravity (g) = 9.8100 m/s^2
  • flowdepth(y)=1.1000mflow depth (y) = 1.1000 m

Find

specific force (momentum function) (M), in m^2

Start with the thinking

  • The governing relation printed in this handbook section is Momentum Depth Diagram (specific force).
  • Everything except M is given, so isolate M symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • The momentum-depth diagram plots specific force against depth to find conjugate depths across a hydraulic jump.
depth yspecific force MMomentum-depth diagramconjugate depths

Figure 9 — schematic for Momentum Depth Diagram (specific force) — solve for specific force (momentum function) (case 9) — Momentum Depth Diagram (9)

Step-by-step solution

  1. Step 1 — State the governing relation:

    M=q2gy+y22M = \dfrac{q^2}{g y} + \dfrac{y^2}{2}
  2. Step 2 — Rearrange the relation so that M stands alone on the left-hand side.

  3. Step 3 — List the givens: unit discharge (q) = 4.1500 m^2/s, gravity (g) = 9.8100 m/s^2, flow depth (y) = 1.1000 m.

  4. Step 4 — Substitute the given values into the rearranged relation.

  5. Step 5 — Evaluate:

    M = 2.2010\ \text{m^2}
  6. Step 6 — Check: returning M = 2.2010 m^2 to

    M=q2gy+y22M = \dfrac{q^2}{g y} + \dfrac{y^2}{2}

    reproduces the given quantities, and both sides carry the same units.

Answer:
M = 2.2010\ \text{m^2}

Why the other options are there

  • 4.4020 — kept a factor of two that cancels in the correct rearrangement.
  • 1.1005 — dropped that same factor in the other direction.
  • 2.4211 — rounded an intermediate value before the final step.

Reference: FE Handbook — Momentum Depth Diagram

Example 10
Momentum Depth Diagram (specific force) — solve for specific force (momentum function) (case 10) — Momentum Depth Diagram (10)

a momentum-depth diagram used to locate conjugate depths of a hydraulic jump Given unit discharge (q) = 3.4000 m^2/s; gravity (g) = 9.7400 m/s^2; flow depth (y) = 1.3000 m, determine the specific force (momentum function) (M) in m^2.

Given

  • unitdischarge(q)=3.4000m2/sunit discharge (q) = 3.4000 m^2/s
  • gravity(g)=9.7400m/s2gravity (g) = 9.7400 m/s^2
  • flowdepth(y)=1.3000mflow depth (y) = 1.3000 m

Find

specific force (momentum function) (M), in m^2

Start with the thinking

  • The governing relation printed in this handbook section is Momentum Depth Diagram (specific force).
  • Everything except M is given, so isolate M symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • The momentum-depth diagram plots specific force against depth to find conjugate depths across a hydraulic jump.
depth yspecific force MMomentum-depth diagramconjugate depths

Figure 10 — schematic for Momentum Depth Diagram (specific force) — solve for specific force (momentum function) (case 10) — Momentum Depth Diagram (10)

Step-by-step solution

  1. Step 1 — State the governing relation:

    M=q2gy+y22M = \dfrac{q^2}{g y} + \dfrac{y^2}{2}
  2. Step 2 — Rearrange the relation so that M stands alone on the left-hand side.

  3. Step 3 — List the givens: unit discharge (q) = 3.4000 m^2/s, gravity (g) = 9.7400 m/s^2, flow depth (y) = 1.3000 m.

  4. Step 4 — Substitute the given values into the rearranged relation.

  5. Step 5 — Evaluate:

    M = 1.7580\ \text{m^2}
  6. Step 6 — Check: returning M = 1.7580 m^2 to

    M=q2gy+y22M = \dfrac{q^2}{g y} + \dfrac{y^2}{2}

    reproduces the given quantities, and both sides carry the same units.

Answer:
M = 1.7580\ \text{m^2}

Why the other options are there

  • 3.5159 — kept a factor of two that cancels in the correct rearrangement.
  • 0.8790 — dropped that same factor in the other direction.
  • 1.9338 — rounded an intermediate value before the final step.

Reference: FE Handbook — Momentum Depth Diagram

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