748.4 Headwater and Backwater - Revision history

Smithk: Updated website link (for increased security)

2021-10-22T18:18:22Z

Updated website link (for increased security)

← Older revision		Revision as of 13:18, 22 October 2021
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	==748.4.2 Backwater from Obstruction to Flow==		==748.4.2 Backwater from Obstruction to Flow==

	Backwater is the increase in the upstream water surface level resulting from an obstruction to flow, such as a roadway fill with a bridge or culvert opening placed on the floodplain. Backwater is measured above the normal water surface elevation, and is the maximum difference between the [~~http~~://epg.modot.org/index.php?title=750.3_Bridges#750.3.2.4.1_Normal_Water_Surface_Elevation normal water surface elevation] and the water surface elevation resulting from the obstruction to flow.		Backwater is the increase in the upstream water surface level resulting from an obstruction to flow, such as a roadway fill with a bridge or culvert opening placed on the floodplain. Backwater is measured above the normal water surface elevation, and is the maximum difference between the [https://epg.modot.org/index.php?title=750.3_Bridges#750.3.2.4.1_Normal_Water_Surface_Elevation normal water surface elevation] and the water surface elevation resulting from the obstruction to flow.

	[[image:748.4.2.jpg\|center\|825px]]		[[image:748.4.2.jpg\|center\|825px]]

EPGsysop: /* 748.4.1 Headwater */

2017-04-07T18:59:29Z

748.4.1 Headwater

← Older revision		Revision as of 13:59, 7 April 2017
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	The lowest elevation determined from the following criteria shall establish the allowable headwater depth for the design of culverts:		The lowest elevation determined from the following criteria shall establish the allowable headwater depth for the design of culverts:

	:1) The elevation shall be ~~such~~ that damage to upstream property will be minimized.		:1) The elevation shall be so that damage to upstream property will be minimized.

	:2) Elevation shall not exceed the elevation determined from the headwater to culvert diameter ratio (H<sub>w</sub>/D):		:2) Elevation shall not exceed the elevation determined from the headwater to culvert diameter ratio (H<sub>w</sub>/D):

Smithk: /* 748.4.2 Backwater from Obstruction to Flow */ Per Bridge, clarification of "normal water surface elevation"

2014-12-09T13:36:14Z

748.4.2 Backwater from Obstruction to Flow: Per Bridge, clarification of "normal water surface elevation"

← Older revision		Revision as of 08:36, 9 December 2014
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	==748.4.2 Backwater from Obstruction to Flow==		==748.4.2 Backwater from Obstruction to Flow==

	Backwater is the increase in the upstream water surface level resulting from an obstruction to flow, such as a roadway fill with a bridge or culvert opening placed on the floodplain~~. The normal water surface elevation is the elevation of the water surface across the flood plain without the bridge, culvert, or roadway fill~~. Backwater is measured above the normal water surface elevation, and is the maximum difference between the normal water surface elevation and the water surface elevation resulting from the obstruction to flow.		Backwater is the increase in the upstream water surface level resulting from an obstruction to flow, such as a roadway fill with a bridge or culvert opening placed on the floodplain. Backwater is measured above the normal water surface elevation, and is the maximum difference between the [http://epg.modot.org/index.php?title=750.3_Bridges#750.3.2.4.1_Normal_Water_Surface_Elevation normal water surface elevation] and the water surface elevation resulting from the obstruction to flow.

	[[image:748.4.2.jpg\|center\|825px]]		[[image:748.4.2.jpg\|center\|825px]]

Smithk: /* 748.4.1 Headwater */ Per Bridge, clarification of when to apply one of the criteria

2014-08-18T17:50:58Z

748.4.1 Headwater: Per Bridge, clarification of when to apply one of the criteria

← Older revision		Revision as of 12:50, 18 August 2014
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	The lowest elevation determined from the following criteria shall establish the allowable headwater depth for the design of culverts:		The lowest elevation determined from the following criteria shall establish the allowable headwater depth for the design of culverts:

	:1) ~~Elevation of the roadway shoulder line at the lowest point, less one foot (for the [[748.2 Roadway Design Criteria\|Roadway Design Criteria]] discharge)~~.		:1) The elevation shall be such that damage to upstream property will be minimized.

	:2~~) The elevation shall be such that damage to upstream property will be minimized.~~		:2) Elevation shall not exceed the elevation determined from the headwater to culvert diameter ratio (H<sub>w</sub>/D):

	:3) Elevation shall not exceed the elevation determined from the headwater to culvert diameter ratio (H<sub>w</sub>/D):

	{\| border="1" class="wikitable" style="margin: 1em auto 1em auto"		{\| border="1" class="wikitable" style="margin: 1em auto 1em auto"
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	\|}		\|}

	:4) Elevation at which water will flow into another watercourse.		:3) Elevation at which water will flow into another watercourse.

			:4) Elevation at which Backwater limits will be exceeded.

	:5) ~~Elevation~~ at ~~which Backwater limits will be exceeded~~.		:5) For the Roadway Design Frequency and more frequent events, this criteria also includes the elevation at the lowest shoulder point, less one foot (for the [[748.2 Roadway Design Criteria\|Roadway Design Criteria]] discharge).

	==748.4.2 Backwater from Obstruction to Flow==		==748.4.2 Backwater from Obstruction to Flow==

Smithk: /* 748.4.1 Headwater */ Per Bridge, minor clarifications

2014-06-16T13:09:34Z

748.4.1 Headwater: Per Bridge, minor clarifications

← Older revision		Revision as of 08:09, 16 June 2014
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	:2) The elevation shall be such that damage to upstream property will be minimized.		:2) The elevation shall be such that damage to upstream property will be minimized.

	:3) Elevation shall not exceed the headwater to culvert ~~depth~~ ratio (H<sub>w</sub>/D):		:3) Elevation shall not exceed the elevation determined from the headwater to culvert diameter ratio (H<sub>w</sub>/D):

	{\| border="1" class="wikitable" style="margin: 1em auto 1em auto"		{\| border="1" class="wikitable" style="margin: 1em auto 1em auto"
	\|+		\|+
	!style="background:#BEBEBE"\|Culvert Diameter, D~~, or Rise~~ !!style="background:#BEBEBE"\|Maximum H<sub>w</sub>/D Ratio		!style="background:#BEBEBE"\|Culvert Diameter/Height, D !!style="background:#BEBEBE"\|Maximum H<sub>w</sub>/D Ratio
	\|-		\|-
	\| align="center"\| 4 ft. or less \|\|align="center"\|1.5		\| align="center"\| 4 ft. or less \|\|align="center"\|1.5

Smithk: /* 748.4.2 Backwater from Obstruction to Flow */

2014-06-10T12:53:22Z

748.4.2 Backwater from Obstruction to Flow

← Older revision		Revision as of 07:53, 10 June 2014
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	[[image:748.4.2.jpg\|center\|825px]]		[[image:748.4.2.jpg\|center\|825px]]

	Backwater for bridges, box culverts and pipes on all routes shall meet the [[127.9 Floodplain Management and the Regulatory Floodway#127.9.1.1 National Flood Insurance Program Requirements\|National Flood Insurance Program ~~Requirements~~]]. The designer must also consider the impacts to upstream improvements, crops and property values as well as the depth, frequency, extent and duration of the backwater impacts. The backwater must be reasonable for the full range of flows less than or equal to the 100-year event. Impact is defined in terms of value and quantity of property that may be affected. Prior to selecting backwater criteria, the potential backwater impacts are evaluated:		Backwater for bridges, box culverts and pipes on all routes shall meet the [[127.9 Floodplain Management and the Regulatory Floodway#127.9.1.1 National Flood Insurance Program Requirements\|National Flood Insurance Program requirements]]. The designer must also consider the impacts to upstream improvements, crops and property values as well as the depth, frequency, extent and duration of the backwater impacts. The backwater must be reasonable for the full range of flows less than or equal to the 100-year event. Impact is defined in terms of value and quantity of property that may be affected. Prior to selecting backwater criteria, the potential backwater impacts are evaluated:

	:1) For bridges and boxes in areas with low levels of potential impact, allow from 0 to 2 ft. of backwater over natural in a 100-year event. In areas with moderate to high potential impact allow 0 to 1 ft. of backwater over natural in a 100-year event.		:1) For bridges and boxes in areas with low levels of potential impact, allow from 0 to 2 ft. of backwater over natural in a 100-year event. In areas with moderate to high potential impact allow 0 to 1 ft. of backwater over natural in a 100-year event.
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	If it can be assumed that the road won’t be raised in the foreseeable future, consider the flow over the road when sizing the structure. Otherwise, size the structure to handle the full flow so that the structure won’t need to be replaced to raise the road in the future. In this case, the backwater must be specifically calculated for the event that just overtops the road in addition to the standard frequencies. This event may produce the maximum backwater impact.		If it can be assumed that the road won’t be raised in the foreseeable future, consider the flow over the road when sizing the structure. Otherwise, size the structure to handle the full flow so that the structure won’t need to be replaced to raise the road in the future. In this case, the backwater must be specifically calculated for the event that just overtops the road in addition to the standard frequencies. This event may produce the maximum backwater impact.

	Choose the upper end of the allowable backwater range only when it is appropriate and reasonable. For new structures the backwater impacts are determined cumulative with bridges left in place. When extending a culvert, a complete hydraulic analysis or documentation of why a complete analysis is not necessary is required. When the existing culvert can be shown to be adequate and impacts from the lengthening are minor, design exceptions can be sought to allow the use of reduced criteria to avoid replacing the culvert. However, a hydraulic analysis is still necessary to determine the impacts.		Choose the upper end of the allowable backwater range only when it is appropriate and reasonable. For new structures the backwater impacts are determined cumulative with bridges left in place. When extending a culvert, a complete hydraulic analysis or documentation of why a complete analysis is not necessary is required. When the existing culvert can be shown to be adequate and impacts from the lengthening are minor, design exceptions can be sought to allow the use of reduced criteria to avoid replacing the culvert. However, a hydraulic analysis is still necessary to determine the impacts.

	==748.4.3 Existing Structures==		==748.4.3 Existing Structures==

Smithk: Per Bridge, max. headwater depth is now to be based on a ration of the headwaters to the dia. of the culvert

2014-06-10T12:49:57Z

Per Bridge, max. headwater depth is now to be based on a ration of the headwaters to the dia. of the culvert

@@ Line 1: / Line 1: @@
-==748.4.1 Backwater from Obstruction to Flow==
+==748.4.1 Headwater==
-[[image:748.6 Figure Design High Water.gif]]
+Headwater depth is measured from the flow line (invert) of the culvert inlet, to the water surface elevation.
-[[image:748.4 Photo Backwater.jpg|right|475 px]]
+[[image:748.4.1.jpg|center|825px]]
-Backwater for bridges, box culverts and pipes on all routes shall meet the [[127.9 Floodplain Management and the Regulatory Floodway|National Flood Insurance Program]] requirements. The designer must consider the impacts to upstream improvements, crops and property values as well as the depth, frequency, extent and duration of the backwater impacts. The backwater must be reasonable for the full range of flows less than or equal to the design event. '''Impact''' is defined in terms of value and quantity of property that may be affected. Prior to selecting backwater criteria, the potential backwater impacts are evaluated:
+The lowest elevation determined from the following criteria shall establish the allowable headwater depth for the design of culverts:
-:1) For bridges and boxes in areas with low levels of potential impact, allow from 0 to 2 ft. of backwater over natural in a 100-year event.  In areas with moderate to high potential impact allow 0 to 1 ft. of backwater over natural in a 100-year event.
+:1)	Elevation of the roadway shoulder line at the lowest point, less one foot (for the [[748.2 Roadway Design Criteria|Roadway Design Criteria]] discharge).
-:2) For pipes with upstream impacts that may be moderate to high, analyze backwater and consider impacts from depth, extent and frequency of flooding for the range of flows.
+:2)	The elevation shall be such that damage to upstream property will be minimized.
-For projects with moderate or high upstream property impacts, analyze and quantify backwater impacts so that design decisions can be made.  This applies even when the District does the design.  In some cases, the District may choose to document reasons why backwater impacts do not need to be estimated instead of performing the calculations.  The Bridge Division will continue to assess the backwater impacts as part of most hydraulic designs performed by Bridge.  Impacts to areas with no existing improvements or anticipated potential for development would be considered minimal, even for relatively large areas.  Impacts to agricultural land can be considered minimal if only a small area is affected, or considered moderate to high for a large area.
+:3)	Elevation shall not exceed the headwater to culvert depth ratio (H<sub>w</sub>/D):
 [[Category:748 Hydraulics and Drainage|748.04]]

EPG-Admin: Article renumbering.

2008-05-05T14:07:09Z

Article renumbering.

New page

==748.4.1 Backwater from Obstruction to Flow==
Backwater is the increase in the upstream water surface level resulting from an obstruction in flow, such as a roadway fill with a bridge or culvert opening placed on the floodplain. The normal water surface elevation is the elevation of the water surface across the flood plain without the bridge, culvert, or roadway fill. Backwater is measured above the normal water surface elevation, and is the maximum difference between the normal water surface elevation and the water surface elevation resulting from the obstruction to flow.

[[image:748.6 Figure Design High Water.gif]]

[[image:748.4 Photo Backwater.jpg|right|475 px]]

Backwater for bridges, box culverts and pipes on all routes shall meet the [[127.9 Floodplain Management and the Regulatory Floodway|National Flood Insurance Program]] requirements. The designer must consider the impacts to upstream improvements, crops and property values as well as the depth, frequency, extent and duration of the backwater impacts. The backwater must be reasonable for the full range of flows less than or equal to the design event. '''Impact''' is defined in terms of value and quantity of property that may be affected. Prior to selecting backwater criteria, the potential backwater impacts are evaluated:

:1) For bridges and boxes in areas with low levels of potential impact, allow from 0 to 2 ft. of backwater over natural in a 100-year event. In areas with moderate to high potential impact allow 0 to 1 ft. of backwater over natural in a 100-year event.

:2) For pipes with upstream impacts that may be moderate to high, analyze backwater and consider impacts from depth, extent and frequency of flooding for the range of flows.

For projects with moderate or high upstream property impacts, analyze and quantify backwater impacts so that design decisions can be made. This applies even when the District does the design. In some cases, the District may choose to document reasons why backwater impacts do not need to be estimated instead of performing the calculations. The Bridge Division will continue to assess the backwater impacts as part of most hydraulic designs performed by Bridge. Impacts to areas with no existing improvements or anticipated potential for development would be considered minimal, even for relatively large areas. Impacts to agricultural land can be considered minimal if only a small area is affected, or considered moderate to high for a large area.

Consider the extent, depth, duration and frequency of flooding when evaluating backwater impacts. The depth and frequency of flooding are the most critical, but MoDOT can be held responsible for unreasonable increases in any of these.

If it can be assumed that the road won’t be raised in the foreseeable future, consider the flow over the road when sizing the structure. Otherwise, size the structure to handle the full flow so that the structure won’t need to be replaced to raise the road in the future. In this case, the backwater must be specifically calculated for the event that just overtops the road in addition to the standard frequencies. This event may produce the maximum backwater impact.

Choose the upper end of the allowable backwater range only when it is appropriate and reasonable. For new structures the backwater impacts are determined cumulative with bridges left in place. When extending a culvert, a complete hydraulic analysis or documentation of why a complete analysis is not necessary is required. When the existing culvert can be shown to be adequate and impacts from the lengthening are minor, design exceptions can be sought to allow the use of reduced criteria to avoid replacing the culvert. However, a hydraulic analysis is still necessary to determine the impacts.

==748.4.2 Backwater from Another Stream==
The term "backwater" is also used to describe the increase in water surface elevations near the confluence of one stream with another, caused by flood conditions on the larger stream. In this case, the water surface elevation of the larger stream causes the obstruction to flow for the smaller stream and results in backwater on the smaller stream. When backwater from another stream causes water surface elevations higher than the design high water surface elevation, both elevations shall be shown on the plans.

[[Category:748 Hydraulics and Drainage|748.04]]