750.3 Bridges - Revision history

Smithk: updated several links

2022-06-22T15:38:46Z

updated several links

Smithk: Updated website links for increased security

2021-10-25T14:25:48Z

Updated website links for increased security

Smithk: updated links

2021-06-08T12:10:16Z

updated links

← Older revision		Revision as of 07:10, 8 June 2021
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	\|'''Asset Management'''		\|'''Asset Management'''
	\|-		\|-
	\|[~~http~~://~~library~~.modot.mo.gov/~~RDT~~/~~reports~~/~~ri07002~~/~~or09019~~.pdf Report 2009]		\|[https://spexternal.modot.mo.gov/sites/cm/CORDT/or10010.pdf Report 2009]
	\|-		\|-
	\|'''See also:''' [https://www.modot.org/research-publications Research Publications]		\|'''See also:''' [https://www.modot.org/research-publications Research Publications]
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	\|'''Asset Management'''		\|'''Asset Management'''
	\|-		\|-
	\|[~~http~~://~~library~~.modot.mo.gov/~~RDT~~/~~reports~~/~~ri07002~~/~~or09019~~.pdf Report 2009]		\|[https://spexternal.modot.mo.gov/sites/cm/CORDT/or10010.pdf Report 2009]
	\|-		\|-
	\|'''See also:''' [https://www.modot.org/research-publications Research Publications]		\|'''See also:''' [https://www.modot.org/research-publications Research Publications]

Smithk at 18:54, 23 April 2021

2021-04-23T18:54:37Z

← Older revision		Revision as of 13:54, 23 April 2021
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	===750.3.2.4.1 Normal Water Surface Elevation===		===750.3.2.4.1 Normal Water Surface Elevation===
	The normal water surface elevation is the elevation of the water surface across the ~~flood plain~~ without MoDOT bridges, culverts, or roadway fills in place. This elevation may be obtained using the slope-area method or from a "natural conditions" water surface profile.		The normal water surface elevation is the elevation of the water surface across the floodplain without MoDOT bridges, culverts, or roadway fills in place. This elevation may be obtained using the slope-area method or from a "natural conditions" water surface profile.

	===750.3.2.4.2 Slope-Area Method===		===750.3.2.4.2 Slope-Area Method===

Smithk at 20:13, 18 August 2020

2020-08-18T20:13:46Z

← Older revision		Revision as of 15:13, 18 August 2020
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	\|[http://library.modot.mo.gov/RDT/reports/ri07002/or09019.pdf Report 2009]		\|[http://library.modot.mo.gov/RDT/reports/ri07002/or09019.pdf Report 2009]
	\|-		\|-
	\|'''See also:''' [~~http~~://www.modot.~~gov~~/~~services/OR/byDate.htm Innovation Library~~]		\|'''See also:''' [https://www.modot.org/research-publications Research Publications]
	\|}		\|}
	=750.3.1 Hydraulic Considerations for Bridge Layout=		=750.3.1 Hydraulic Considerations for Bridge Layout=
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	\|[http://library.modot.mo.gov/RDT/reports/ri07002/or09019.pdf Report 2009]		\|[http://library.modot.mo.gov/RDT/reports/ri07002/or09019.pdf Report 2009]
	\|-		\|-
	\|'''See also:''' [~~http~~://www.modot.~~gov~~/~~services/OR/byDate.htm Innovation Library~~]		\|'''See also:''' [https://www.modot.org/research-publications Research Publications]
	\|}		\|}
	Hydraulic analysis of a bridge design requires evaluation of the proposed bridge's vulnerability to potential scour. Unanticipated scour at bridge piers or abutments can result in rapid bridge collapse and extreme hazard and economic hardship.		Hydraulic analysis of a bridge design requires evaluation of the proposed bridge's vulnerability to potential scour. Unanticipated scour at bridge piers or abutments can result in rapid bridge collapse and extreme hazard and economic hardship.

Smithk: /* 750.3.1.1.2.6 Existing Bridges */ Per BR, clarified several items on Bridge Survey Checklist.

2018-09-24T15:56:46Z

750.3.1.1.2.6 Existing Bridges: Per BR, clarified several items on Bridge Survey Checklist.

← Older revision		Revision as of 10:56, 24 September 2018
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	See [[238.3 Route Surveying#238.3.36.3.9 Typical Channel Sections\|EPG 238.3.36.3.9 Typical Channel Sections]] for details		See [[238.3 Route Surveying#238.3.36.3.9 Typical Channel Sections\|EPG 238.3.36.3.9 Typical Channel Sections]] for details

	====750.3.1.1.2.6 ~~Existing~~ Bridges====		====750.3.1.1.2.6 Other Bridges====

	See [[238.3 Route Surveying#238.3.36.3.10 ~~Existing~~ Bridges\|EPG 238.3.36.3.10 ~~Existing~~ Bridges]] for details.		Hydraulically significant data for other bridges on the same stream or in the vicinity of the proposed bridge(s) is required to develop an accurate hydraulic model. When an existing bridge is determined to affect the hydraulics of a proposed structure the following information should be added to the Bridge Survey Location Request:

			:* Survey Data – Additional valley sections and a centerline profile. Location and extents of this survey data is determined the same way as the data for the proposed structure.
			:* Bridge Data – If bridge data is not available from another source (plans for some offsystem bridges are available in TMS) the following data should be requested:

			:::• Number of spans and span length
			:::• Low Chord of Superstructure or superstructure depth (may be omitted if above extreme high water elevation.)
			:::• Substructure type & size of intermediate bent columns or piling.

			See [[238.3 Route Surveying#238.3.36.3.10 Other Bridges\|EPG 238.3.36.3.10 Other Bridges]] for additional details.

	==750.3.1.2 Abutment Layout==		==750.3.1.2 Abutment Layout==

EPGsysop: /* 750.3.2.2.2 Bridge Survey */Per BR, omitted revision related to Nov 3, 2017 revision guidance for district submittal of request for valley section locations

2017-11-14T15:42:13Z

750.3.2.2.2 Bridge Survey: Per BR, omitted revision related to Nov 3, 2017 revision guidance for district submittal of request for valley section locations

← Older revision		Revision as of 10:42, 14 November 2017
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	===750.3.2.2.2 Bridge Survey===		===750.3.2.2.2 Bridge Survey===
	The bridge survey is prepared by district personnel and provides information regarding existing structures, nearby structures on the same stream, and streambed and valley characteristics including valley cross-sections along the centerline of the proposed structure, valley cross-sections upstream and downstream of the proposed structure, and a streambed profile through the proposed structure.		The bridge survey is prepared by district personnel and provides information regarding existing structures, nearby structures on the same stream, and streambed and valley characteristics including valley cross-sections along the centerline of the proposed structure, valley cross-sections upstream and downstream of the proposed structure, and a streambed profile through the proposed structure.

	Location of the surveyed [[238.3 Route Surveying#238.3.36.3.7 Valley Sections\|valley sections]] is an important factor in developing the best possible water surface profile model for the proposed structure. For this reason, inclusion of the bridge survey as an agenda item on an initial core team meeting to discuss appropriate location of the valley sections is recommended.

	Bridge surveys are conducted in accordance with [[:Category:747 Bridge Reports and Layouts\|EPG 747 Bridge Reports and Layouts]].		Bridge surveys are conducted in accordance with [[:Category:747 Bridge Reports and Layouts\|EPG 747 Bridge Reports and Layouts]].

EPGsysop: Per BR, guidance for district submittal of request for valley section locations was expanded

2017-11-03T13:15:34Z

Per BR, guidance for district submittal of request for valley section locations was expanded

Show changes

EPGsysop: /* 750.3.1 Hydraulic Considerations for Bridge Layout */

2017-04-07T19:19:47Z

750.3.1 Hydraulic Considerations for Bridge Layout

← Older revision		Revision as of 14:19, 7 April 2017
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	===750.3.1.1 Abutment Layout===		===750.3.1.1 Abutment Layout===
	Abutments shall be placed ~~such~~ that spill fill slopes do not infringe upon the channel; the toes of the spill fill slopes may be no closer to the center of the channel than the toe of the channel banks. The Soil Survey provided by the the [http://sharepoint/systemdelivery/CM/geotechnical/default.aspx Geotechnical Section] gives minimum spill fill slopes based on slope stability criteria. The minimum bridge length for stability criteria is thus determined by projecting the stability slopes outward from the toes of the channel slopes as shown below. For structures crossing an NFIP regulatory floodway, abutments shall be placed ~~such~~ that the toes of the spill fill slopes are outside the floodway limits.		Abutments shall be placed so that spill fill slopes do not infringe upon the channel; the toes of the spill fill slopes may be no closer to the center of the channel than the toe of the channel banks. The Soil Survey provided by the the [http://sharepoint/systemdelivery/CM/geotechnical/default.aspx Geotechnical Section] gives minimum spill fill slopes based on slope stability criteria. The minimum bridge length for stability criteria is thus determined by projecting the stability slopes outward from the toes of the channel slopes as shown below. For structures crossing an NFIP regulatory floodway, abutments shall be placed so that the toes of the spill fill slopes are outside the floodway limits.

	[[Image:750.3 Abutment and Pier Location Limits.gif\|center]]		[[Image:750.3 Abutment and Pier Location Limits.gif\|center]]

Smithk: /* 750.3.2.5.2 Contraction Scour */ Per BR, irrelevant paragraph removed

2016-09-07T20:07:11Z

750.3.2.5.2 Contraction Scour: Per BR, irrelevant paragraph removed

← Older revision		Revision as of 15:07, 7 September 2016
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	'''Live-Bed Contraction Scour''' - Live-bed contraction scour depths can be determined using the equations in [[#750.3.1.9 List of References\|FHWA HEC-18]].		'''Live-Bed Contraction Scour''' - Live-bed contraction scour depths can be determined using the equations in [[#750.3.1.9 List of References\|FHWA HEC-18]].

	The upstream cross-section is typically located either one bridge opening length or the average length of constriction upstream of the bridge. This is consistent with the required location of the approach cross-section in both HEC-RAS and WSPRO.

	'''Clear Water Contraction Scour''' - Clear-water contraction scour depths can be determined using the equations in [[#750.3.1.9 List of References\|FHWA HEC-18]].		'''Clear Water Contraction Scour''' - Clear-water contraction scour depths can be determined using the equations in [[#750.3.1.9 List of References\|FHWA HEC-18]].

← Older revision		Revision as of 10:38, 22 June 2022
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	==750.3.2.4 Hydraulic Analysis of Bridges==		==750.3.2.4 Hydraulic Analysis of Bridges==
	The Corps of Engineers Hydrologic Engineering Center's River Analysis System (HEC-RAS) shall be used to develop water surface profile models for the hydraulic analysis of bridges. Documentation on the use of HEC-RAS is available in [[#750.3.1.9 List of References\|references (6), (7), and (8)]].		The Corps of Engineers Hydrologic Engineering Center's River Analysis System (HEC-RAS) shall be used to develop water surface profile models for the hydraulic analysis of bridges. Documentation on the use of HEC-RAS is available in [[#750.3.1.10 List of References\|references (6), (7), and (8)]].

	Hydraulic design of bridges requires analysis of both the "natural conditions" and the "proposed conditions" at the site to show that the structure meets [[748.4 Headwater and Backwater\|Backwater requirements]] and [[127.9 Floodplain Management and the Regulatory Floodway#127.9.1.1 National Flood Insurance Program Requirements\|National Flood Insurance Program Requirements]]. It is also necessary to analyze the "existing conditions", when replacing an existing structure, see [[748.5 Matching Existing Structures\|EPG 748.5 Matching Existing Structures]].		Hydraulic design of bridges requires analysis of both the "natural conditions" and the "proposed conditions" at the site to show that the structure meets [[748.4 Headwater and Backwater\|Backwater requirements]] and [[127.9 Floodplain Management and the Regulatory Floodway#127.9.1.1 National Flood Insurance Program Requirements\|National Flood Insurance Program Requirements]]. It is also necessary to analyze the "existing conditions", when replacing an existing structure, see [[748.5 Matching Existing Structures\|EPG 748.5 Matching Existing Structures]].
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	It is extremely important that roughness coefficients in overbank areas be carefully selected to represent the effective flow in those areas. There is a general tendency to overestimate the amount of flow occurring in overbank areas, particularly in broad, flat floodplains. Increasing the roughness coefficients on overbanks will increase the proportion of flow in the channel, with a corresponding decrease in the proportion of flow on the overbanks.		It is extremely important that roughness coefficients in overbank areas be carefully selected to represent the effective flow in those areas. There is a general tendency to overestimate the amount of flow occurring in overbank areas, particularly in broad, flat floodplains. Increasing the roughness coefficients on overbanks will increase the proportion of flow in the channel, with a corresponding decrease in the proportion of flow on the overbanks.

	[[#750.3.1.9 List of References\|References (9), (10), and (11)]] provide guidance on the selection of roughness coefficients.		[[#750.3.1.10 List of References\|References (9), (10), and (11)]] provide guidance on the selection of roughness coefficients.

	===750.3.2.4.4 Hydraulic Gradient (Streambed Slope)===		===750.3.2.4.4 Hydraulic Gradient (Streambed Slope)===
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	Current methods of analyzing scour depths are based mainly on laboratory experiment rather than on practical field data. The results should be carefully reviewed and engineering judgment used to determine their applicability to actual field conditions.		Current methods of analyzing scour depths are based mainly on laboratory experiment rather than on practical field data. The results should be carefully reviewed and engineering judgment used to determine their applicability to actual field conditions.

	HEC-RAS includes the ability to calculate scour depths. The methods used to calculate those depths are based on the [[#750.3.1.9 List of References\|FHWA HEC-18 publication]]. Additional methods for abutment scour, scour in cohesive soils, scour in rock, and for scour in coarse bed streams not found in HEC-RAS are available in HEC-18. Rock scour information in HEC-18 is based on [https://epg.modot.org/index.php?title=750.3_Bridges#750.3.1.~~9_List_of_References~~ NCHRP Report 717].		HEC-RAS includes the ability to calculate scour depths. The methods used to calculate those depths are based on the [[#750.3.1.10 List of References\|FHWA HEC-18 publication]]. Additional methods for abutment scour, scour in cohesive soils, scour in rock, and for scour in coarse bed streams not found in HEC-RAS are available in HEC-18. Rock scour information in HEC-18 is based on [https://epg.modot.org/index.php?title=750.3_Bridges#750.3.1.10_List_of_References NCHRP Report 717].

	===750.3.2.5.1 Long Term Profile Changes - Aggradation and Degradation===		===750.3.2.5.1 Long Term Profile Changes - Aggradation and Degradation===
	Long term profile changes result from aggradation or degradation in the stream reach over time. Aggradation involves the deposition of sediment eroded from the channel and banks upstream of the site. Degradation involves the lowering or scouring of a streambed as material is removed from the streambed and is due to a deficit in sediment supply upstream. Aggradation and degradation are generally the result of changes in the energy gradient of the stream.		Long term profile changes result from aggradation or degradation in the stream reach over time. Aggradation involves the deposition of sediment eroded from the channel and banks upstream of the site. Degradation involves the lowering or scouring of a streambed as material is removed from the streambed and is due to a deficit in sediment supply upstream. Aggradation and degradation are generally the result of changes in the energy gradient of the stream.

	Aggradation and degradation over the life of a structure are difficult to predict. These long term profile changes are typically the result of human activities within the watershed including dams and reservoirs, changes in land use, gravel mining and other operations. [[#750.3.1.9 List of References\|HEC-18 and HEC-20]] provide more information on predicting long term profile changes. Comparison of channel bottom elevations shown on plans for existing bridges to current survey data may be informative.		Aggradation and degradation over the life of a structure are difficult to predict. These long term profile changes are typically the result of human activities within the watershed including dams and reservoirs, changes in land use, gravel mining and other operations. [[#750.3.1.10 List of References\|HEC-18 and HEC-20]] provide more information on predicting long term profile changes. Comparison of channel bottom elevations shown on plans for existing bridges to current survey data may be informative.

	===750.3.2.5.2 Contraction Scour===		===750.3.2.5.2 Contraction Scour===
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	Contraction scour may be one of two types: live-bed contraction scour or clear water contraction scour. Live-bed scour occurs when the stream is transporting bed material into the contracted section from the reach just upstream of the contraction. Clear-water scour occurs when the stream is not transporting bed material into the contracted section. The type of contraction scour is determined by comparing the average velocity of flow in the channel or overbank area upstream of the bridge opening to the critical velocity for beginning of motion of bed material.		Contraction scour may be one of two types: live-bed contraction scour or clear water contraction scour. Live-bed scour occurs when the stream is transporting bed material into the contracted section from the reach just upstream of the contraction. Clear-water scour occurs when the stream is not transporting bed material into the contracted section. The type of contraction scour is determined by comparing the average velocity of flow in the channel or overbank area upstream of the bridge opening to the critical velocity for beginning of motion of bed material.

	'''Live-Bed Contraction Scour''' - Live-bed contraction scour depths can be determined using the equations in [[#750.3.1.9 List of References\|FHWA HEC-18]].		'''Live-Bed Contraction Scour''' - Live-bed contraction scour depths can be determined using the equations in [[#750.3.1.10 List of References\|FHWA HEC-18]].

	'''Clear Water Contraction Scour''' - Clear-water contraction scour depths can be determined using the equations in [[#750.3.1.9 List of References\|FHWA HEC-18]].		'''Clear Water Contraction Scour''' - Clear-water contraction scour depths can be determined using the equations in [[#750.3.1.10 List of References\|FHWA HEC-18]].

	===750.3.2.5.3 Local Scour===		===750.3.2.5.3 Local Scour===
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	Pier scour and abutment scour are considered two distinct types of local scour.		Pier scour and abutment scour are considered two distinct types of local scour.

	'''Pier Scour''' - Pier scour depths can be determined using the equation found in [https://epg.modot.org/index.php?title=750.3_Bridges#750.3.1.~~9_List_of_References~~ FHWA HEC-18].		'''Pier Scour''' - Pier scour depths can be determined using the equation found in [https://epg.modot.org/index.php?title=750.3_Bridges#750.3.1.10_List_of_References FHWA HEC-18].

	The HEC-18 Pier Scour Equation is based on the Colorado State University (CSU) equation. The equation is best suited for non-cohesive soils, but has been used for cohesive soils. Additional equations are provided in HEC-18 for cohesive soils, coarse bed materials and erodible rock.		The HEC-18 Pier Scour Equation is based on the Colorado State University (CSU) equation. The equation is best suited for non-cohesive soils, but has been used for cohesive soils. Additional equations are provided in HEC-18 for cohesive soils, coarse bed materials and erodible rock.
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	Top width of pier scour holes, measured from the pier to the outer edge of the scour hole, can be estimated as 2.0 x the scour hole depth, y<sub>s</sub>.		Top width of pier scour holes, measured from the pier to the outer edge of the scour hole, can be estimated as 2.0 x the scour hole depth, y<sub>s</sub>.

	'''Abutment Scour''' - Abutment scour depths can be determined using the equations found in [https://epg.modot.org/index.php?title=750.3_Bridges#750.3.1.~~9_List_of_References~~ FHWA HEC-18].		'''Abutment Scour''' - Abutment scour depths can be determined using the equations found in [https://epg.modot.org/index.php?title=750.3_Bridges#750.3.1.10_List_of_References FHWA HEC-18].

	The first equation, is the Froelich Abutment Scour Equation.		The first equation, is the Froelich Abutment Scour Equation.
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	The HIRE Abutment Scour Equation, is recommended when the ratio of projected abutment length, L', to flow depth, y<sub>a</sub>, is greater than 25.		The HIRE Abutment Scour Equation, is recommended when the ratio of projected abutment length, L', to flow depth, y<sub>a</sub>, is greater than 25.

	HEC-18 also provides another approach to calculating abutment scour which is based on [https://epg.modot.org/index.php?title=750.3_Bridges#750.3.1.~~9_List_of_References~~ NCHRP Report 24-20].		HEC-18 also provides another approach to calculating abutment scour which is based on [https://epg.modot.org/index.php?title=750.3_Bridges#750.3.1.10_List_of_References NCHRP Report 24-20].

	===750.3.2.5.4 Rock Scour===		===750.3.2.5.4 Rock Scour===

← Older revision		Revision as of 09:25, 25 October 2021
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	==750.3.1.6 Hydraulic Performance Curve==		==750.3.1.6 Hydraulic Performance Curve==
	The hydraulic performance of the proposed structure shall be evaluated at various discharges, including the 10-, 50-, 100-, and 500-year discharges, which are the discharges typically found in a [~~http~~://epg.modot.org/index.php?title=748.9_National_Flood_Insurance_Program_%28NFIP%29#748.9.4.2_Flood_Insurance_Study Flood Insurance Study]. The risk of significant damage to adjacent properties by the resulting velocity and backwater for each of these discharges shall be evaluated.		The hydraulic performance of the proposed structure shall be evaluated at various discharges, including the 10-, 50-, 100-, and 500-year discharges, which are the discharges typically found in a [https://epg.modot.org/index.php?title=748.9_National_Flood_Insurance_Program_%28NFIP%29#748.9.4.2_Flood_Insurance_Study Flood Insurance Study]. The risk of significant damage to adjacent properties by the resulting velocity and backwater for each of these discharges shall be evaluated.

	==750.3.1.7 Flow Distribution==		==750.3.1.7 Flow Distribution==
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	===750.3.2.2.5 Flood Insurance Study Data===		===750.3.2.2.5 Flood Insurance Study Data===
	If a [~~http~~://epg.modot.org/index.php?title=748.9_National_Flood_Insurance_Program_%28NFIP%29#748.9.4.2_Flood_Insurance_Study Flood Insurance Study (FIS)] has been performed for the community in which the structure is proposed, the FIS may provide an additional data source. The FIS may contain information on peak flood discharges, base flood elevations (BFE) water surface profile elevations, and information on regulatory floodways.		If a [https://epg.modot.org/index.php?title=748.9_National_Flood_Insurance_Program_%28NFIP%29#748.9.4.2_Flood_Insurance_Study Flood Insurance Study (FIS)] has been performed for the community in which the structure is proposed, the FIS may provide an additional data source. The FIS may contain information on peak flood discharges, base flood elevations (BFE) water surface profile elevations, and information on regulatory floodways.

	===750.3.2.2.6 Data Review===		===750.3.2.2.6 Data Review===
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	==750.3.2.3 Bridge Hydrologic Analysis==		==750.3.2.3 Bridge Hydrologic Analysis==
	Peak flood discharges are determined by one of the following methods. If the necessary data is available, discharges should be determined by all methods and engineering judgment used to determine the most appropriate. For accuracy of discharges, see [~~http~~://epg.modot.org/index.php?title=751.5_Structural_Detailing_Guidelines#Hydraulic_Data EPG 751.5.2.1, Hydraulic Data].		Peak flood discharges are determined by one of the following methods. If the necessary data is available, discharges should be determined by all methods and engineering judgment used to determine the most appropriate. For accuracy of discharges, see [https://epg.modot.org/index.php?title=751.5_Structural_Detailing_Guidelines#Hydraulic_Data EPG 751.5.2.1, Hydraulic Data].

	===750.3.2.3.1 Historical USGS Stream Gage Data===		===750.3.2.3.1 Historical USGS Stream Gage Data===
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	Current methods of analyzing scour depths are based mainly on laboratory experiment rather than on practical field data. The results should be carefully reviewed and engineering judgment used to determine their applicability to actual field conditions.		Current methods of analyzing scour depths are based mainly on laboratory experiment rather than on practical field data. The results should be carefully reviewed and engineering judgment used to determine their applicability to actual field conditions.

	HEC-RAS includes the ability to calculate scour depths. The methods used to calculate those depths are based on the [[#750.3.1.9 List of References\|FHWA HEC-18 publication]]. Additional methods for abutment scour, scour in cohesive soils, scour in rock, and for scour in coarse bed streams not found in HEC-RAS are available in HEC-18. Rock scour information in HEC-18 is based on [~~http~~://epg.modot.org/index.php?title=750.3_Bridges#750.3.1.9_List_of_References NCHRP Report 717].		HEC-RAS includes the ability to calculate scour depths. The methods used to calculate those depths are based on the [[#750.3.1.9 List of References\|FHWA HEC-18 publication]]. Additional methods for abutment scour, scour in cohesive soils, scour in rock, and for scour in coarse bed streams not found in HEC-RAS are available in HEC-18. Rock scour information in HEC-18 is based on [https://epg.modot.org/index.php?title=750.3_Bridges#750.3.1.9_List_of_References NCHRP Report 717].

	===750.3.2.5.1 Long Term Profile Changes - Aggradation and Degradation===		===750.3.2.5.1 Long Term Profile Changes - Aggradation and Degradation===
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	Pier scour and abutment scour are considered two distinct types of local scour.		Pier scour and abutment scour are considered two distinct types of local scour.

	'''Pier Scour''' - Pier scour depths can be determined using the equation found in [~~http~~://epg.modot.org/index.php?title=750.3_Bridges#750.3.1.9_List_of_References FHWA HEC-18].		'''Pier Scour''' - Pier scour depths can be determined using the equation found in [https://epg.modot.org/index.php?title=750.3_Bridges#750.3.1.9_List_of_References FHWA HEC-18].

	The HEC-18 Pier Scour Equation is based on the Colorado State University (CSU) equation. The equation is best suited for non-cohesive soils, but has been used for cohesive soils. Additional equations are provided in HEC-18 for cohesive soils, coarse bed materials and erodible rock.		The HEC-18 Pier Scour Equation is based on the Colorado State University (CSU) equation. The equation is best suited for non-cohesive soils, but has been used for cohesive soils. Additional equations are provided in HEC-18 for cohesive soils, coarse bed materials and erodible rock.
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	Top width of pier scour holes, measured from the pier to the outer edge of the scour hole, can be estimated as 2.0 x the scour hole depth, y<sub>s</sub>.		Top width of pier scour holes, measured from the pier to the outer edge of the scour hole, can be estimated as 2.0 x the scour hole depth, y<sub>s</sub>.

	'''Abutment Scour''' - Abutment scour depths can be determined using the equations found in [~~http~~://epg.modot.org/index.php?title=750.3_Bridges#750.3.1.9_List_of_References FHWA HEC-18].		'''Abutment Scour''' - Abutment scour depths can be determined using the equations found in [https://epg.modot.org/index.php?title=750.3_Bridges#750.3.1.9_List_of_References FHWA HEC-18].

	The first equation, is the Froelich Abutment Scour Equation.		The first equation, is the Froelich Abutment Scour Equation.
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	The HIRE Abutment Scour Equation, is recommended when the ratio of projected abutment length, L', to flow depth, y<sub>a</sub>, is greater than 25.		The HIRE Abutment Scour Equation, is recommended when the ratio of projected abutment length, L', to flow depth, y<sub>a</sub>, is greater than 25.

	HEC-18 also provides another approach to calculating abutment scour which is based on [~~http~~://epg.modot.org/index.php?title=750.3_Bridges#750.3.1.9_List_of_References NCHRP Report 24-20].		HEC-18 also provides another approach to calculating abutment scour which is based on [https://epg.modot.org/index.php?title=750.3_Bridges#750.3.1.9_List_of_References NCHRP Report 24-20].

	===750.3.2.5.4 Rock Scour===		===750.3.2.5.4 Rock Scour===
	Since scour in rock is mainly an issue for spread footings, and the number of spread footings used in or near streambeds is limited, use the following procedure to request rock scour parameters to prevent unnecessary geotechnical sampling and testing.		Since scour in rock is mainly an issue for spread footings, and the number of spread footings used in or near streambeds is limited, use the following procedure to request rock scour parameters to prevent unnecessary geotechnical sampling and testing.

	:a) Spread footings should be specified on the request for soil properties form when [~~http~~://epg.modot.org/index.php?title=751.1_Preliminary_Design#751.1.2.18_Soundings_.28Borings.29 soundings (borings)] are requested. By default for spread footings and retaining walls, rock erodibility is automatically checked for investigation of rock parameters for scour by the Geotechnical Section.		:a) Spread footings should be specified on the request for soil properties form when [https://epg.modot.org/index.php?title=751.1_Preliminary_Design#751.1.2.18_Soundings_.28Borings.29 soundings (borings)] are requested. By default for spread footings and retaining walls, rock erodibility is automatically checked for investigation of rock parameters for scour by the Geotechnical Section.

	:b) The foundation investigation geotechnical report should provide erodibility index numbers and indicate if scour numbers will be provided for further recommended evaluation. Alternatively, if rock scour is determined to not be a concern it should be noted on the report.		:b) The foundation investigation geotechnical report should provide erodibility index numbers and indicate if scour numbers will be provided for further recommended evaluation. Alternatively, if rock scour is determined to not be a concern it should be noted on the report.