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Horizontal Velocities

Environmental Engineering · FE Reference Handbook section

Environmental Engineering
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10 exam-style examples
~45 min
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Handbook notes for this section

Definitions and conditions exactly as the handbook states them.

  • 1. Water Treatment—horizontal velocities should not exceed 0.5 fpm
  • 2. Wastewater Treatment—no specific requirements (use the same criteria as for water)

Core formulas for this FE topic

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

This section is conceptual; there are no equations to memorise.

Worked exam-style examples

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

Example 1
Horizontal Velocities — solve for horizontal velocity — Horizontal Velocities

horizontal velocities through a rectangular sedimentation basin Given flow rate (Q) = 17,830 m^3/day; basin width (W) = 2.6000 m; basin depth (H) = 5.0000 m, determine the horizontal velocity (v_h) in m/day.

Given

  • flowrate(Q)=17,830m3/dayflow rate (Q) = 17,830 m^3/day
  • basinwidth(W)=2.6000mbasin width (W) = 2.6000 m
  • basindepth(H)=5.0000mbasin depth (H) = 5.0000 m

Find

horizontal velocity (v_h), in m/day

Start with the thinking

  • The governing relation printed in this handbook section is Horizontal Velocities.
  • Everything except v_h is given, so isolate v_h 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.
  • Horizontal velocities through a sedimentation basin must remain low enough to avoid resuspension of settled particles.
sedimentation basin

Figure 1 — schematic for Horizontal Velocities — solve for horizontal velocity — Horizontal Velocities

Step-by-step solution

  1. Step 1 — State the governing relation:

    vh=QWHv_h = \dfrac{Q}{W H}
  2. Step 2 — Rearrange symbolically for v_h:

    vh=QWHv_{h} = \dfrac{Q}{WH}
  3. Step 3 — List the givens: flow rate (Q) = 17,830 m^3/day, basin width (W) = 2.6000 m, basin depth (H) = 5.0000 m.

  4. Step 4 — Substitute the given values:

    vh=17830W5.0000v_{h} = \dfrac{17830}{W5.0000}
  5. Step 5 — Evaluate:

    vh=1372 m/dayv_{h} = 1372\ \text{m/day}
  6. Step 6 — Check: returning v_h = 1,372 m/day to

    vh=QWHv_h = \dfrac{Q}{W H}

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

Answer:
vh=1372 m/dayv_{h} = 1372\ \text{m/day}

Why the other options are there

  • 2,743 — kept a factor of two that cancels in the correct rearrangement.
  • 685.8 — dropped that same factor in the other direction.
  • 1,509 — rounded an intermediate value before the final step.

Reference: FE Handbook — Horizontal Velocities (sedimentation)

Example 2
Horizontal Velocities — solve for basin width — Horizontal Velocities (2)

horizontal velocities check to prevent scour in a settling tank Given flow rate (Q) = 48,430 m^3/day; basin depth (H) = 4.0000 m; horizontal velocity (v_h) = 2,793 m/day, determine the basin width (W) in m.

Given

  • flowrate(Q)=48,430m3/dayflow rate (Q) = 48,430 m^3/day
  • basindepth(H)=4.0000mbasin depth (H) = 4.0000 m
  • horizontalvelocity(vh)=2,793m/dayhorizontal velocity (v_h) = 2,793 m/day

Find

basin width (W), in m

Start with the thinking

  • The governing relation printed in this handbook section is Horizontal Velocities.
  • Everything except W is given, so isolate W 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.
  • Horizontal velocities through a sedimentation basin must remain low enough to avoid resuspension of settled particles.
sedimentation basin

Figure 2 — schematic for Horizontal Velocities — solve for basin width — Horizontal Velocities (2)

Step-by-step solution

  1. Step 1 — State the governing relation:

    vh=QWHv_h = \dfrac{Q}{W H}
  2. Step 2 — Rearrange symbolically for W:

    W=QvhHW = \dfrac{Q}{v_hH}
  3. Step 3 — List the givens: flow rate (Q) = 48,430 m^3/day, basin depth (H) = 4.0000 m, horizontal velocity (v_h) = 2,793 m/day.

  4. Step 4 — Substitute the given values:

    W=48430vh4.0000W = \dfrac{48430}{v_h4.0000}
  5. Step 5 — Evaluate:

    W=4.3349 mW = 4.3349\ \text{m}
  6. Step 6 — Check: returning W = 4.3349 m to

    vh=QWHv_h = \dfrac{Q}{W H}

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

Answer:
W=4.3349 mW = 4.3349\ \text{m}

Why the other options are there

  • 8.6699 — kept a factor of two that cancels in the correct rearrangement.
  • 2.1675 — dropped that same factor in the other direction.
  • 4.7684 — rounded an intermediate value before the final step.

Reference: FE Handbook — Horizontal Velocities (sedimentation)

Example 3
Horizontal Velocities — solve for basin depth — Horizontal Velocities (3)

horizontal velocities computed from basin cross-sectional area and flow Given flow rate (Q) = 39,170 m^3/day; basin width (W) = 3.9000 m; horizontal velocity (v_h) = 4,402 m/day, determine the basin depth (H) in m.

Given

  • flowrate(Q)=39,170m3/dayflow rate (Q) = 39,170 m^3/day
  • basinwidth(W)=3.9000mbasin width (W) = 3.9000 m
  • horizontalvelocity(vh)=4,402m/dayhorizontal velocity (v_h) = 4,402 m/day

Find

basin depth (H), in m

Start with the thinking

  • The governing relation printed in this handbook section is Horizontal Velocities.
  • Everything except H is given, so isolate H 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.
  • Horizontal velocities through a sedimentation basin must remain low enough to avoid resuspension of settled particles.
sedimentation basin

Figure 3 — schematic for Horizontal Velocities — solve for basin depth — Horizontal Velocities (3)

Step-by-step solution

  1. Step 1 — State the governing relation:

    vh=QWHv_h = \dfrac{Q}{W H}
  2. Step 2 — Rearrange symbolically for H:

    H=QvhWH = \dfrac{Q}{v_hW}
  3. Step 3 — List the givens: flow rate (Q) = 39,170 m^3/day, basin width (W) = 3.9000 m, horizontal velocity (v_h) = 4,402 m/day.

  4. Step 4 — Substitute the given values:

    H=39170vh3.9000H = \dfrac{39170}{v_h3.9000}
  5. Step 5 — Evaluate:

    H=2.2816 mH = 2.2816\ \text{m}
  6. Step 6 — Check: returning H = 2.2816 m to

    vh=QWHv_h = \dfrac{Q}{W H}

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

Answer:
H=2.2816 mH = 2.2816\ \text{m}

Why the other options are there

  • 4.5632 — kept a factor of two that cancels in the correct rearrangement.
  • 1.1408 — dropped that same factor in the other direction.
  • 2.5098 — rounded an intermediate value before the final step.

Reference: FE Handbook — Horizontal Velocities (sedimentation)

Example 4
Horizontal Velocities — solve for horizontal velocity (case 2) — Horizontal Velocities (4)

horizontal velocities through a rectangular sedimentation basin Given flow rate (Q) = 8,180 m^3/day; basin width (W) = 12.5000 m; basin depth (H) = 4.5000 m, determine the horizontal velocity (v_h) in m/day.

Given

  • flowrate(Q)=8,180m3/dayflow rate (Q) = 8,180 m^3/day
  • basinwidth(W)=12.5000mbasin width (W) = 12.5000 m
  • basindepth(H)=4.5000mbasin depth (H) = 4.5000 m

Find

horizontal velocity (v_h), in m/day

Start with the thinking

  • The governing relation printed in this handbook section is Horizontal Velocities.
  • Everything except v_h is given, so isolate v_h 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.
  • Horizontal velocities through a sedimentation basin must remain low enough to avoid resuspension of settled particles.
sedimentation basin

Figure 4 — schematic for Horizontal Velocities — solve for horizontal velocity (case 2) — Horizontal Velocities (4)

Step-by-step solution

  1. Step 1 — State the governing relation:

    vh=QWHv_h = \dfrac{Q}{W H}
  2. Step 2 — Rearrange symbolically for v_h:

    vh=QWHv_{h} = \dfrac{Q}{WH}
  3. Step 3 — List the givens: flow rate (Q) = 8,180 m^3/day, basin width (W) = 12.5000 m, basin depth (H) = 4.5000 m.

  4. Step 4 — Substitute the given values:

    vh=8180W4.5000v_{h} = \dfrac{8180}{W4.5000}
  5. Step 5 — Evaluate:

    vh=145.4 m/dayv_{h} = 145.4\ \text{m/day}
  6. Step 6 — Check: returning v_h = 145.4 m/day to

    vh=QWHv_h = \dfrac{Q}{W H}

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

Answer:
vh=145.4 m/dayv_{h} = 145.4\ \text{m/day}

Why the other options are there

  • 290.8 — kept a factor of two that cancels in the correct rearrangement.
  • 72.7111 — dropped that same factor in the other direction.
  • 160.0 — rounded an intermediate value before the final step.

Reference: FE Handbook — Horizontal Velocities (sedimentation)

Example 5
Horizontal Velocities — solve for basin width (case 2) — Horizontal Velocities (5)

horizontal velocities check to prevent scour in a settling tank Given flow rate (Q) = 38,000 m^3/day; basin depth (H) = 2.6000 m; horizontal velocity (v_h) = 3,628 m/day, determine the basin width (W) in m.

Given

  • flowrate(Q)=38,000m3/dayflow rate (Q) = 38,000 m^3/day
  • basindepth(H)=2.6000mbasin depth (H) = 2.6000 m
  • horizontalvelocity(vh)=3,628m/dayhorizontal velocity (v_h) = 3,628 m/day

Find

basin width (W), in m

Start with the thinking

  • The governing relation printed in this handbook section is Horizontal Velocities.
  • Everything except W is given, so isolate W 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.
  • Horizontal velocities through a sedimentation basin must remain low enough to avoid resuspension of settled particles.
sedimentation basin

Figure 5 — schematic for Horizontal Velocities — solve for basin width (case 2) — Horizontal Velocities (5)

Step-by-step solution

  1. Step 1 — State the governing relation:

    vh=QWHv_h = \dfrac{Q}{W H}
  2. Step 2 — Rearrange symbolically for W:

    W=QvhHW = \dfrac{Q}{v_hH}
  3. Step 3 — List the givens: flow rate (Q) = 38,000 m^3/day, basin depth (H) = 2.6000 m, horizontal velocity (v_h) = 3,628 m/day.

  4. Step 4 — Substitute the given values:

    W=38000vh2.6000W = \dfrac{38000}{v_h2.6000}
  5. Step 5 — Evaluate:

    W=4.0285 mW = 4.0285\ \text{m}
  6. Step 6 — Check: returning W = 4.0285 m to

    vh=QWHv_h = \dfrac{Q}{W H}

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

Answer:
W=4.0285 mW = 4.0285\ \text{m}

Why the other options are there

  • 8.0570 — kept a factor of two that cancels in the correct rearrangement.
  • 2.0142 — dropped that same factor in the other direction.
  • 4.4313 — rounded an intermediate value before the final step.

Reference: FE Handbook — Horizontal Velocities (sedimentation)

Example 6
Horizontal Velocities — solve for basin depth (case 2) — Horizontal Velocities (6)

horizontal velocities computed from basin cross-sectional area and flow Given flow rate (Q) = 36,250 m^3/day; basin width (W) = 8.0000 m; horizontal velocity (v_h) = 1,401 m/day, determine the basin depth (H) in m.

Given

  • flowrate(Q)=36,250m3/dayflow rate (Q) = 36,250 m^3/day
  • basinwidth(W)=8.0000mbasin width (W) = 8.0000 m
  • horizontalvelocity(vh)=1,401m/dayhorizontal velocity (v_h) = 1,401 m/day

Find

basin depth (H), in m

Start with the thinking

  • The governing relation printed in this handbook section is Horizontal Velocities.
  • Everything except H is given, so isolate H 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.
  • Horizontal velocities through a sedimentation basin must remain low enough to avoid resuspension of settled particles.
sedimentation basin

Figure 6 — schematic for Horizontal Velocities — solve for basin depth (case 2) — Horizontal Velocities (6)

Step-by-step solution

  1. Step 1 — State the governing relation:

    vh=QWHv_h = \dfrac{Q}{W H}
  2. Step 2 — Rearrange symbolically for H:

    H=QvhWH = \dfrac{Q}{v_hW}
  3. Step 3 — List the givens: flow rate (Q) = 36,250 m^3/day, basin width (W) = 8.0000 m, horizontal velocity (v_h) = 1,401 m/day.

  4. Step 4 — Substitute the given values:

    H=36250vh8.0000H = \dfrac{36250}{v_h8.0000}
  5. Step 5 — Evaluate:

    H=3.2343 mH = 3.2343\ \text{m}
  6. Step 6 — Check: returning H = 3.2343 m to

    vh=QWHv_h = \dfrac{Q}{W H}

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

Answer:
H=3.2343 mH = 3.2343\ \text{m}

Why the other options are there

  • 6.4686 — kept a factor of two that cancels in the correct rearrangement.
  • 1.6171 — dropped that same factor in the other direction.
  • 3.5577 — rounded an intermediate value before the final step.

Reference: FE Handbook — Horizontal Velocities (sedimentation)

Example 7
Horizontal Velocities — solve for horizontal velocity (case 3) — Horizontal Velocities (7)

horizontal velocities through a rectangular sedimentation basin Given flow rate (Q) = 10,660 m^3/day; basin width (W) = 9.2000 m; basin depth (H) = 3.0000 m, determine the horizontal velocity (v_h) in m/day.

Given

  • flowrate(Q)=10,660m3/dayflow rate (Q) = 10,660 m^3/day
  • basinwidth(W)=9.2000mbasin width (W) = 9.2000 m
  • basindepth(H)=3.0000mbasin depth (H) = 3.0000 m

Find

horizontal velocity (v_h), in m/day

Start with the thinking

  • The governing relation printed in this handbook section is Horizontal Velocities.
  • Everything except v_h is given, so isolate v_h 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.
  • Horizontal velocities through a sedimentation basin must remain low enough to avoid resuspension of settled particles.
sedimentation basin

Figure 7 — schematic for Horizontal Velocities — solve for horizontal velocity (case 3) — Horizontal Velocities (7)

Step-by-step solution

  1. Step 1 — State the governing relation:

    vh=QWHv_h = \dfrac{Q}{W H}
  2. Step 2 — Rearrange symbolically for v_h:

    vh=QWHv_{h} = \dfrac{Q}{WH}
  3. Step 3 — List the givens: flow rate (Q) = 10,660 m^3/day, basin width (W) = 9.2000 m, basin depth (H) = 3.0000 m.

  4. Step 4 — Substitute the given values:

    vh=10660W3.0000v_{h} = \dfrac{10660}{W3.0000}
  5. Step 5 — Evaluate:

    vh=386.2 m/dayv_{h} = 386.2\ \text{m/day}
  6. Step 6 — Check: returning v_h = 386.2 m/day to

    vh=QWHv_h = \dfrac{Q}{W H}

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

Answer:
vh=386.2 m/dayv_{h} = 386.2\ \text{m/day}

Why the other options are there

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

Reference: FE Handbook — Horizontal Velocities (sedimentation)

Example 8
Horizontal Velocities — solve for basin width (case 3) — Horizontal Velocities (8)

horizontal velocities check to prevent scour in a settling tank Given flow rate (Q) = 30,670 m^3/day; basin depth (H) = 4.9000 m; horizontal velocity (v_h) = 4,423 m/day, determine the basin width (W) in m.

Given

  • flowrate(Q)=30,670m3/dayflow rate (Q) = 30,670 m^3/day
  • basindepth(H)=4.9000mbasin depth (H) = 4.9000 m
  • horizontalvelocity(vh)=4,423m/dayhorizontal velocity (v_h) = 4,423 m/day

Find

basin width (W), in m

Start with the thinking

  • The governing relation printed in this handbook section is Horizontal Velocities.
  • Everything except W is given, so isolate W 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.
  • Horizontal velocities through a sedimentation basin must remain low enough to avoid resuspension of settled particles.
sedimentation basin

Figure 8 — schematic for Horizontal Velocities — solve for basin width (case 3) — Horizontal Velocities (8)

Step-by-step solution

  1. Step 1 — State the governing relation:

    vh=QWHv_h = \dfrac{Q}{W H}
  2. Step 2 — Rearrange symbolically for W:

    W=QvhHW = \dfrac{Q}{v_hH}
  3. Step 3 — List the givens: flow rate (Q) = 30,670 m^3/day, basin depth (H) = 4.9000 m, horizontal velocity (v_h) = 4,423 m/day.

  4. Step 4 — Substitute the given values:

    W=30670vh4.9000W = \dfrac{30670}{v_h4.9000}
  5. Step 5 — Evaluate:

    W=1.4151 mW = 1.4151\ \text{m}
  6. Step 6 — Check: returning W = 1.4151 m to

    vh=QWHv_h = \dfrac{Q}{W H}

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

Answer:
W=1.4151 mW = 1.4151\ \text{m}

Why the other options are there

  • 2.8303 — kept a factor of two that cancels in the correct rearrangement.
  • 0.7076 — dropped that same factor in the other direction.
  • 1.5567 — rounded an intermediate value before the final step.

Reference: FE Handbook — Horizontal Velocities (sedimentation)

Example 9
Horizontal Velocities — solve for basin depth (case 3) — Horizontal Velocities (9)

horizontal velocities computed from basin cross-sectional area and flow Given flow rate (Q) = 12,450 m^3/day; basin width (W) = 19.5000 m; horizontal velocity (v_h) = 1,310 m/day, determine the basin depth (H) in m.

Given

  • flowrate(Q)=12,450m3/dayflow rate (Q) = 12,450 m^3/day
  • basinwidth(W)=19.5000mbasin width (W) = 19.5000 m
  • horizontalvelocity(vh)=1,310m/dayhorizontal velocity (v_h) = 1,310 m/day

Find

basin depth (H), in m

Start with the thinking

  • The governing relation printed in this handbook section is Horizontal Velocities.
  • Everything except H is given, so isolate H 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.
  • Horizontal velocities through a sedimentation basin must remain low enough to avoid resuspension of settled particles.
sedimentation basin

Figure 9 — schematic for Horizontal Velocities — solve for basin depth (case 3) — Horizontal Velocities (9)

Step-by-step solution

  1. Step 1 — State the governing relation:

    vh=QWHv_h = \dfrac{Q}{W H}
  2. Step 2 — Rearrange symbolically for H:

    H=QvhWH = \dfrac{Q}{v_hW}
  3. Step 3 — List the givens: flow rate (Q) = 12,450 m^3/day, basin width (W) = 19.5000 m, horizontal velocity (v_h) = 1,310 m/day.

  4. Step 4 — Substitute the given values:

    H=12450vh19.5000H = \dfrac{12450}{v_h19.5000}
  5. Step 5 — Evaluate:

    H=0.4874 mH = 0.4874\ \text{m}
  6. Step 6 — Check: returning H = 0.4874 m to

    vh=QWHv_h = \dfrac{Q}{W H}

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

Answer:
H=0.4874 mH = 0.4874\ \text{m}

Why the other options are there

  • 0.9748 — kept a factor of two that cancels in the correct rearrangement.
  • 0.2437 — dropped that same factor in the other direction.
  • 0.5361 — rounded an intermediate value before the final step.

Reference: FE Handbook — Horizontal Velocities (sedimentation)

Example 10
Horizontal Velocities — solve for horizontal velocity (case 4) — Horizontal Velocities (10)

horizontal velocities through a rectangular sedimentation basin Given flow rate (Q) = 32,330 m^3/day; basin width (W) = 19.6000 m; basin depth (H) = 4.4000 m, determine the horizontal velocity (v_h) in m/day.

Given

  • flowrate(Q)=32,330m3/dayflow rate (Q) = 32,330 m^3/day
  • basinwidth(W)=19.6000mbasin width (W) = 19.6000 m
  • basindepth(H)=4.4000mbasin depth (H) = 4.4000 m

Find

horizontal velocity (v_h), in m/day

Start with the thinking

  • The governing relation printed in this handbook section is Horizontal Velocities.
  • Everything except v_h is given, so isolate v_h 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.
  • Horizontal velocities through a sedimentation basin must remain low enough to avoid resuspension of settled particles.
sedimentation basin

Figure 10 — schematic for Horizontal Velocities — solve for horizontal velocity (case 4) — Horizontal Velocities (10)

Step-by-step solution

  1. Step 1 — State the governing relation:

    vh=QWHv_h = \dfrac{Q}{W H}
  2. Step 2 — Rearrange symbolically for v_h:

    vh=QWHv_{h} = \dfrac{Q}{WH}
  3. Step 3 — List the givens: flow rate (Q) = 32,330 m^3/day, basin width (W) = 19.6000 m, basin depth (H) = 4.4000 m.

  4. Step 4 — Substitute the given values:

    vh=32330W4.4000v_{h} = \dfrac{32330}{W4.4000}
  5. Step 5 — Evaluate:

    vh=374.9 m/dayv_{h} = 374.9\ \text{m/day}
  6. Step 6 — Check: returning v_h = 374.9 m/day to

    vh=QWHv_h = \dfrac{Q}{W H}

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

Answer:
vh=374.9 m/dayv_{h} = 374.9\ \text{m/day}

Why the other options are there

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

Reference: FE Handbook — Horizontal Velocities (sedimentation)

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