Effects of Debris on Bridge Pier Scour (2010)

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

D-1 Part 1 Field Data Sheets, D-2 Part 2 South Platte River Site Reconnaissance and Preliminary Data Sheets, D-11 Part 3 South Platte River Case Study: Final Data Sheets and Application of the Guidelines, D-28 Part 4 Debris Scour Calculations, D-47 A P P E N D I X D Field Data Sheets and Case Study

Field Data Sheets 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Upstream Reach Length: feet / miles 17 18 19 20 Yes ____ No ____ Yes ____ No ____ 21 22 Yes ____ No ____ Unknown ____ Evidence of Regular Flooding: Yes ____ No ____ 23 Yes ____ No ____ 24 25 26 27 28 29 30 Historic Ground Photos avaialable for: 31 32 Historic Aerial Photos available for: Stream Flow Data or Records available for: 33 34 Scour Calculations available for: Hydraulic Models available for: 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 Descriptions: Description: Average Upstream Channel Width at Crossings and/or Straight Reaches: feet (top bank to top bank) Width (ft): Number: C - DATA & INFORMATION TO BE COLLECTED DURING A SITE VISIT & FIELD EXAMINATION (1) OBTAIN GROUND PHOTOS AND DETAILED DESCRIPTIONS THAT DOCUMENT THE FOLLOWING Description: LEFT - Distance From Top Bank:Type: Nearby Bridge: Y N Nose: Nearby Bridge: Y N Width (ft): Descriptions: Piers, Pilings, and footings Abutments RIGHT - Distance From Top Bank: Number: Skew to Flow (deg): Bridge Openings Nearby Bridge: Y N Current Bridge Site: Y N Current Bridge Site: Y N Current Bridge Site: Y N Nearby Bridge: Y N Current Bridge Site: Y N (a) Upstream Bridge Face and Structural Elements Nearby Bridge: Y NCurrent Bridge Site: Y N General Vegetation Patterns and Characteristics (describe): Evidence of Active Meander Migration: Evidence of Active Bank Erosion/Retreat: Describe Existing Natural or Man-Made Barriers to Debris Delivery (dams, diversions, bridges, rock outcrops, major constrictions etc.): Upstream Channel Sinuosity: Upstream Channel Slope: Was Reach Channelized: Average Upstream Channel Depth at Crossings and/or Straight Reaches: feet (at bankfull) Presence of : Flow Type: Perennial ______ Sloughs _____ Bridge Inspection Records available for: (2) AVAILABILITY OF BRIDGE INFORMATION FROM STATE, COUNTY, OR LOCAL DOT (circle) (1) SITE INFORMATION FROM TOPO MAPS, AERIAL PHOTOS, AND SURVEYS Ehemeral ______ Flashy ______ (b) Upstream Channel and Flood Plain Characteristics (a) Upstream Watershed Characteristics Land-use: Natural Disturbances (e.g., landslides, forest fires, etc.): Man-made Disturbances (e.g. mining, logging, grazing, etc.): Other _______ County and State: Longitude / Latitude: Urban _______ Agricultural _______ Forested _______ Rural _______ B - DATA & INFORMATION THAT CAN BE COLLECTED PRIOR TO OR FOLLOWING A SITE VISIT A - BRIDGE LOCATION Street / Road Name: Bridge #: River / Stream Name: Date: Project Personnel: FIELD DATA SHEETS FOR ASSESSING WOODY DEBRIS DELIVERY AND ACCUMULATION POTENTIAL AT A BRIDGE SITE (To be used in conjunction with "Guidelines for Assessing Debris Production and Accumulation Potential") Bars and Other Major Sediment Deposits:Meander Cutoffs _____ Current Bridge Site: Y N Nearby Bridge: Y N Visible Evidence of Active Debris Delivery, Transport, and Storage Along Upstream Channel: Current Bridge Site: Y N Nearby Bridge: Y N Bridge Plans available for: Bridge Maintenance Records available for: Nearby Bridge: Y NCurrent Bridge Site: Y N D-2 P A R T 1

D-3 52 53 54 55 56 Sand ___ Gravel ___ Cobble ___ Bldrs ___ 57 58 59 60 61 Bars: 62 63 64 65 General Bank Face Shape: Convex _____ Concave _____ Vertical _____ Undercut/Overhanging _____ 66 LB Toe Sediment Accumulation (Basal Endpoint Control): Significant _____ Moderate _____ Negligible _____ 67 RB Toe Sediment Accumulation (Basal Endpoint Control): Significant _____ Moderate _____ Negligible _____ 68 69 LB: Trees___ Shrubs___ Grass___ Other___ Dense___ Mod.___ Sparse___ None___ Toe ___ Mid ___ Upper ___ 70 RB: Trees___ Shrubs___ Grass___ Other___ Dense___ Mod.___ Sparse___ None___ Toe ___ Mid ___ Upper ___ 71 Berms: LB ____ RB ____ Erosional ___ Depositional ___ Inset Flood Plain: LB ____ RB ____ Stable: Y___ N___ 72 Vegetation on Berms/Inset Flood Plain: Yes ____ No ____ Trees___ Shrubs/Bushes___ Grasses___ Other ____ 73 74 75 76 77 78 Intermittent Continuous In Bends Reach-wide In Toe Whole Bank Fluvial Rotational Cantilever Saturation Piping 79 Left Bank 80 Right Bank 81 82 83 84 85 86 87 88 89 90 91 92 93 94 Evident Debris Delivery Processes in Reach: 95 96 97 98 99 100 Stage: Multi-generational ____ Even-Aged ____ Spacing: Uniform ____ Irregular ____ Density: Dense ____ Mod. ____ Sparse ____ Multistory? Yes____ No____ Typical Age: Young____ Intermediate____ Mature____ Old Growth____ Reach Location: Corridor Width (ft): Avg_______ Max._______ Min._______ Mid-channel Bar____ Point Bar____ Bank Attached Bar____ Aggradation (berms, inset flood plain, overbank sedimentation, etc.): Yes____ No____ Is General Bank Erosion Evident? Left Bank: Yes____ No____ Right Bank: Yes____ No____ Lateral Channel Stability & Active Bank Erosion Characteristics Degradation (headcuts, knickzone, vertical banks, exposed footings, etc.): Yes___ No___ Corridor Length (ft): Continuous ______ Intermittent ______ Debris Available from Flood Plain? Yes___ No___ Healthy? Y___ N___ Bank Erosion/Failure___ Windthrow___ Landslides___ Flood Plain Input___ Disease/Insect Kill___ Logging___ DS Transport___ Typical Species (if known): Estimate Potential for Debris Transport & Delivery:Estimate Potential for Debris Production: Sand____ Gravel____ Cobbles____ Boulders____ (c) General Upstream Riparian Corridor Characteristics Bank Face/Slope Vegetation Type and Location: On Bed ______ On Bars ______ On Bridge Elements ______On Banks ______ Tree Type: Coniferous_____ Deciduous_____ Flow Split _____ Skew to Flow (deg): Scour Holes (Pier #): Bed Material Type: Thalweg Position at Bridge: Scour Hole Sizes (ft): At Abutments (Location and Size): Evidence of Scour (Debris Present ___ / Absent ___ ): (b) General Bridge Reach Characteristics In Meander Bend - On US Limb _____ At Bend Apex: _____ On DS Limb _____Straight _______ Low Flow:________ High Flow:________ Low Chord Height Above Streambed (ft): Min. _____ Max. _____ Bankline Characteristics ( LB = Left Descending Bank, RB = Right Descending Bank ) Evidence: Evidence of Vertical Channel Instability If meander bends are present, are they actively migrating? Yes_____ No _____ Unsure _____ Additional Information: Additional Information: Existing Debris Accumulations: On Flood Plain ______ Additional Information: (See Flowchart A) High _____ Low _____ (See Flowchart B) High _____ Low _____

101 102 103 104 105 106 107 108 109 Pier # Abutment 1 2 3 4 5 6 7 8 9 Abutment 110 P&P Shape / / / / / / / / / / / 111 Width / / / / / / / / / / / 112 Hgt/Depth / / / / / / / / / / / 113 Length / / / / / / / / / / / 114 115 Span/Gap Abut - 1 1-2 2-3 3-4 4-5 5-6 6-7 7-8 8-9 9-10 # - Abut 116 P&P Shape / / / / / / / / / / / 117 Width / / / / / / / / / / / 118 Hgt/Depth / / / / / / / / / / / 119 Length / / / / / / / / / / / 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 Type of Debris Accumulation on Bridge(s): Existing Debris Accumulations ( P lanform & P rofile Descriptions) Span Accumulation____ Pier Accumulation____ Both____ Debris Accumulation Locations: On Bars____ Bank Toe____ Top Bank____ Piers____ Abutments____ (d) Existing Debris Accumulation Characteristics at a Bridge Site (e) Site Plan View Sketch (include important features and dimensions) Additional Information: Estimate Debris Accumulation & Span Blockage Potential (see Flowcharts C and D) Bridge Pier/Abutment (#, see above): Pier-to-Pier Gap/Span (#, see above): Low _____________ Med _____________ High_____________ Chronic ___________ Chronic ___________Low _____________ P&P Shape Abbreviations: Planform : T = Triangular R = Rectangular / Profile : C = Conical Cyl = Cylindrical IC = Inverted Cone Root Wad Sizes (range - ft): At Bridge Piers/Abutments: In Channel or Gap/Span (pier-to-pier): Med _____________ High_____________ Typical Key Log Species: Typical Key Log Length (range - ft): Typical Key Log Diameter (range - ft): D-4

160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 Photo # 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 (f) Cross Section Sketch of Upstream Bridge Face (looking downstream - include important features and dimensions) (g) Field Site Photo Log Description D-5

D-6 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 Item B1b records the general upstream channel and flood plain characteristics. Item B1a records the general characteristics of the watershed upstream of the bridge site. DESCRIPTION OF FIELD DATA SHEET ENTRIES 24 Item C1a records information on the morphology of the channel and structural elements of the upstream bridge face. Existing Debris Accumulations records the location of debris relative to the bridge site. Scour calculations identify estimated scour without debris and can then be used to estimate the potential scour depth associated with debris. Hydraulic models can provide much of the channel and hydraulic information to be recorded on these sheets. Item C1b records information on the general morphology, characteristics, and conditions at the bridge site. Bankline Characteristics provides information on bank stability, vegetative cover, the potential modes of bank failure, how debris may be recruited, and whether of not debris can be stored along the bankline in the upstream channel. The shape of a bank helps define the mode of failure such as undercutting with gravity failure (vertical or overhanging bank) or by rotational failures (concave bank) or by slow fluvial erosion (convex bank). Sediment buildup along the bank toe can temporarily buttress the bank from erosion, but is also indicative of active bank failures. Bank face vegetation can provide some measure of bank stability, but may also be susceptible to recruitment. Depositional berms or narrow floodplains inset into an entrenched floodway are indicative of past incision/degradation followed by recovery. Erosional berms may be indicative of recent incision/degradation. Unstable inset floodpains may be indicative of a new wave of incision. Vegetation on the inset berms and floodplains provide a source for recruitment. Part B2 identifies the availability of existing bridge information that can be obtained from state, county, and/or local DOTs. Data should be obtained for the both the current bridge site, if available, and any nearby bridge, if close enough, to determine if there are any ongoing debris and stability problems in the reach. Reach Location records the general planform of the channel at the bridge site, if it is situated on the upstream or downstream limb of a meander bend, the skew of the bridge to flow, the height of the low chord above the channel bed, the overall channel bed material that is being transported, and the thalweg position in the channel at the bridge site. Evidence of Scour, if present, records the specific dimensions of the scour at the bridge piers and abutments. Scour may be dependent of whether or not debris is present. *** It is highly recommended that the user review and understand HEC-20 prior to conducting this assessment *** Land use plays an important role in defining the types and extent of debris that may potentially be delivered to the site. Urban land use as well as logging, mining, overgrazing, forest fires, and landslides can be significant contributors to channel instability, bank erosion, and subsequently debris delivery. Section A records specific information about the existing, replacement, or new bridge. Record the Date of the project and the main Personnel involved in the project. Provide as much information as possible about the bridge and its location. This information will be useful to others that review these sheets in the future. Part B1 records information that can be acquired or measured from topo maps, aerial photos, or surveys. Section B records information about the bridge site that can be obtained from various sources prior to or following the site evaluation. The bridge plans will provide specific data and measurements for the various structural elements of the bridge. The bridge inspection records can be used to identify any long-term debris or channel stability problems that have occurred since construction. The data collected in this part includes pertinent data and information that defines the type of pier (e.g., column, wall, pile bent, etc.), the nose shape, the pier width, the skew of the piers to flow, and the number of bridge piers, openings, and abutments, their physical condition, their relationship to the channel, banks, and flood plain and any scour and debris problems that may be evident. Bars records whether or not bars are present at the site, the type of bar, and the dominant bar material composition. Bridge maintenance records can provide information on past debris removal. Historic ground photos may show previous debris configurations and locations or past channel conditions that may be useful in identifying long-term changes tha have occurred. Historic aerial photos can be used to identify long-term changes such as active meandering, changes in channel width, land use changes, etc. Stream flow data can be used to determine flood flow depths, extent, duration, velocity, and other factors that influence debris production, transport, and accumulation. Section C records data and information to be collected on the bridge site during a site visit and field examination of the site. Part C1 records the data, information, descriptions, and ground photos acquired during the site reconnaissance. The general characteristics of the upstream channel and flood plain can be acquired or measured from maps, aerial photos, and survey data. Prior to recording this information, the user should determine the appropriate reach length to be evaluated. This will generally be the reach between the current bridge site and the next upstream bridge site, dam, diversion, or other controlling structure OR an estimated distance over which most or all of the upstream area could potentially contribute debris during a major flow event. Sinuosity, which is the ratio of the channel length to the straight line valley length between two points, will control the distance over which debris will move along a channel (i.e., sinuous channels will impede the movement of debris). Slope can be estimated from maps or can be obtained from survey data. Channel width has the greatest control over debris transport, since debris that is longer than the width of a channel will probably not be transported very far from its source area. Depth also controls transport since floating of the debris is required, especially if the debris has root wads/balls attached. Exposed or shallow bars and in-channel sediment deposits can trap debris. Active channel migration and evident bank erosion are the primary contributors to debris delivery. Channels that have been straightened or channelized may be unstable and incising and can be active contributors of debris. Man-made or natural barriers may restrict the downstream movement of debris. Regular flooding can also supply debris from the flood plain.

D-7 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 Item C1f Cross Section Sketch of Upstream Bridge Face is drawn here. The sketch should be viewed in the downstream direction and should include important features and dimensions. Item C1e Site Plan View Sketch is drawn here and should include important features and dimensions. Item C1d records general information on existing debris accumulations at a given bridge site. General information is collected on the location of debris accumulations, the type of accumulation on a bridge, the Typical Key Log type and dimensions, and root wad sizes. Qualitative estimate of debris production and transport and delivery to bridge site is made. Information on the plan and profile (P&P) shape, dimensions, and location of debris along an existing bridge is collected at each bridge element. Provide a qualitative estimate of the potential for debris accumlations on piers/abutments (use Flowchart C) and in gaps/spans (use Flowchart D). Be sure to incorporate information from available bridge data acquired from DOT (lines 27-35) in the debris accumulation estimations (lines 120-122). Lateral Channel Stability & Active Bank Erosion Characteristics identifies the location, degree, and specific mode of active bank retreat and channel migration in the upstream channel. Active bank retreat and lateral migration are the modes of debris recruitment. Vertical Channel Stability identifies whether aggradation or degradation is occurring in the upstream channel since both can contribute to debris recruitment. Degradation results in bank erosion and bank retreat, whereas aggradation can induce overbank flooding and lateral migration. Item C1c records general information on the riparian corridor upstream of the bridge site. The characteristics of the corridor and its composition provide information on the potential extent and size of the debris that could be supplied to the bridge site. Provide a qualitative estimate of the potential for debris production (use Flowchart A) in the upstream reaches and for debris transport and delivery (use Flowchart B) to the bridge site .

D-8 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 Item C1g Field Site Photo Log is used to record photos taken at the site. The descriptions of the photo and any important features in the photo are recorded here as well. Direct evidence of debris production Indirect evidence of debris production Widespread bank erosion Erosion of outer bank in meander bends Localized, minor bank erosion Are trees present along or within 100 feet of bankline? • Channel incision • Lateral migration • Channel widening • Channelization • Dams and diversions • Land use changes • Logging, mining or landslides along channel • Other hydraulic/geomorphic factors • Few trees along or near bankline • Fully stabilized channel HIGH (Significant) Potential for Debris Production LOW (Minimal) Potential for Debris Production Potential for Debris Transport & Delivery POTENTIAL FOR DEBRIS PRODUCTIONFlowchart A Direct evidence of potential for debris transport and delivery Indirect evidence of potential for debris transport and delivery • Documented chronic or frequent accumulations at one or more bridge sites along this channel or upstream tributaries • Abundant debris stored in channel and/or along banks • Ongoing or prior need for debris removal from channel • Upstream channel geometry too small to transport debris • Debris is absent after floods in typical debris accumulation sites other than bridges • Negligible debris delivered to a site following major events • Debris in the channel remains in place following floods because of low flow velocities • Long straight or slightly sinuous reach upstream • Direct or unobstructed transport path to bridge site HIGH Potential for Debris Transport & Delivery LOW Potential for Debris Transport & Delivery Estimate Size of the Largest Debris (Key Log Length) Potentially Delivered to Site POTENTIAL FOR DEBRIS TRANSPORT AND DELIVERY • Highly sinuous reach upstream • Obstructed transport path to bridge site Is it likely that debris will be delivered to the bridge site during subsequent floods? YES NO Flowchart B NO YES

D-9 Sheltered by upstream forest? SHELTERED Location Category DETERMINE LOCATION CATEGORY In channel between tops of banks? YES NO TOP BANK/FLOOD PLAIN Location Category CHANNEL Location Category DEBRIS PATH Location Category Contains or is in close proximity to the thalweg or thread of the stream? NO NO YES LIKELY

D-10 Horizontal / vertical gap / span wider than design log length? LOW Gap/Span Blockage Potential GAP / SPAN IN SHELTERED LOCATION CATEGORY Potential for gap / span blockage due to large adjacent pier accumulations? YESNO MEDIUM Gap/Span Blockage Potential HIGH Gap/Span Blockage Potential HIGH, CHRONIC Gap/Span Blockage Potential YES Flood Plain / Top of Bank In Channel Is pier-to-pier (or pier- to-bank) gap wider than design log length? YES NO Debris Path Flood Plain / Top of Bank In Channel Debris Path Location Category NOYES NO HIGHLOW Location Category Potential for Delivery of Debris Flowchart D LOW Potential LOCATION CATEGORY LOCATION CATEGORY MEDIUM Potential HIGH Potential HIGH, CHRONIC Potential YES NO PIER OR SUBSTRUCTURE DESIGN SOLID WITH APERTURES Potential for Delivery of Debris Low High Low High Low High DEBRIS PATH DEBRIS PATHCHANNEL FLOOD PLAIN /TOP BANK CHANNEL FLOOD PLAIN /TOP BANK Potential for Delivery of Debris Potential for Delivery of Debris PIER OR SUBSTRUCTURE SECTION IN SHELTERED LOCATION CATEGORY Flowchart C

D-11 D.1 Introduction The intent of the field reconnaissance was to investigate potential case study locations on the South Platter River in Colorado. The Field Data Sheets were used to document site characteristics such as channel type and size, channel insta- bility, bank erosion and retreat, and bank vegetation char- acteristics in detail. The purpose of the case study will be to provide an example of how the practitioner should apply the guidelines for assessing debris production and predicting debris accumulation at a bridge site developed in NCHRP Project 24-26. D.2 South Platte River Upstream of the potential study sites, the South Platte River Basin has a drainage area of approximately 14,600 mi2. Figure D.1 shows a location map of the South Platte River Basin and contributing tributaries (see Attachment 1). Head- waters of the South Platte River are located in the central Colorado mountains where the mean annual precipitation is about 30 in., which includes approximately 300 in. of snow- fall (USGS 2006). On the northern Colorado plains between Greeley and Sterling, Colorado, where the study sites are located, the mean annual precipitation is about 12 in., prima- rily in the form of rain that typically falls April through July. The average June flow in the South Platte River in the corri- dor containing the potential study sites is 1,260 ft3/s. During the field reconnaissance, the flow was approximately 90 ft3/s at the USGS gage station in Fort Morgan. Unusually warm tem- peratures in May caused snowpack to melt faster than normal, resulting in an earlier runoff peak. In 2006, the South Platte River at Fort Morgan peaked in early May at 182 ft3/s. Land use in the corridor containing the potential study sites is primarily agriculture and some rangeland. Continuing mod- erate to severe drought conditions in Colorado have resulted in the need to shut off irrigation wells that draw from a shallow aquifer in the area in order to ensure minimum flow condi- tions in the South Platte River are maintained (ABC 2006). During the field visit, flow depths were observed to decrease moving downstream, despite the confluence with several trib- utaries, because of what appeared to be irrigation diversion. D.3 Field Sites Sixteen bridge sites were examined in a corridor of the South Platte River stretching from Greeley, Colorado, to Merino, Colorado; see Figure D.2 for a regional map. Two bridge sites were identified as potential candidates for case study locations (Figure D.3). These sites were the bridges carrying County Road 37 (RD-37) and County Road 50 (RD-50) over the South Platte River and are described in the following paragraphs. The other fourteen sites were eliminated for a number of rea- sons, which included: • Stable banks did not contribute debris to channel. • Several bridges had relief bridges that may reduce exposure to flood flows. • Flow diversion drastically reduced channel flow. • Older riparian vegetation was set back from the banks min- imizing large woody debris in channel. Bridge RD-37 has twelve sharp-nosed piers with narrow spans and a low bridge deck; four piers were located in the channel at the time of the site visit (Figure D.4). The Colorado Division of Water Resources (DWR) maintains a stream gage at this site, historical and recorded discharge data from this gage are shown in Figure D.5. Flow depth under Bridge RD-37 during the field visit was about 3.0 ft, overbank flow in this area occurs at 8.0 ft and bridge overtopping flow occurs at 10.0 ft (DWR 2006). A small amount of debris was observed on Pier 2, abundant amounts of debris were observed in the upstream channel and banks. The upstream banks were erod- ing and large trees were leaning into the channel (Figure D.6). P A R T 2 South Platte River Site Reconnaissance and Preliminary Data Sheets South Platte River between Greeley, CO, and Merino, CO (June 7, 2006)

Bridge RD-50 has three sharp-nosed piers skewed approx- imately 5° to high flows and about 18 in. wide; two piers had debris buildup on the nose and sides (Figure D.7). The left and right abutments had about 1 ft diameter riprap protection, see Figure D.8. Riprap protection on the left bank extended about 300 ft upstream. Pier 1 is located on a mid-channel bar with moderate grass covering. A trian- gular debris pile consisting of several logs was located on the nose of Pier 1, with a small (less than 1 ft) scour hole (Figure D.9). Debris on Pier 2 consisted of a single log approximately 14 in. in diameter with a 4 to 6 ft rootwad aligned upstream (Figures D.10 and D.11). A debris pile and scour hole were observed on the right abutment, the scour hole was approximately 2 to 3 ft deep and 8 to 10 ft wide. Debris accumulation on the right abutment had no discernable key log but comprised several logs approxi- mately 18 in. in diameter; one log had a 6 to 8 ft diameter rootwad aligned upstream (Figure D.12). The bridge is located on the apex of a meander bend; see Figure D.13 for an aerial photograph. Upstream of Bridge RD-50, large woody debris was observed in the channel (Figure D.14). General bank erosion and evidence of bank failure was observed on the upstream right and left banks (Figure D.15). Attachment 2 shows the completed Field Data Sheets for Bridge RD-50. D.4 References ABC News, The Denver Channel website, http://www.thedenverchannel. com/news/9317230/detail.html (June 3, 2006). Colorado Division of Water Resources (DWR) website, http://www.dwr. state.co.us/Hydrology/flow_graph_standard.asp?ID=PLAKERCO& MTYPE=DISCHRG (June 12, 2006) U.S. Geological Survey (USGS) National Water Quality Assessment Program, South Platte River Basin website, http://co.water.usgs.gov/ nawqa/splt/ (June 9, 2006). D-12 Figure D.1. South Platte River basin. Source: USGS 2006 ATTACHMENT 1 Photographs

D-13 Figure D.2. Map showing area of Northern Colorado investigated for potential case study sites. Figure D.3. Map of the South Platte River from Greely to Morin, Colorado. Potential study sites

D-14 Figure D.5. Historic and recorded flow rates for Bridge RD-37. Source: DWR 2006 10000 1000 D is ch ar ge (ft 3 /s ) 100 06 /02 06 /03 06 /04 06 /05 06 /06 06 /07 Date/Time: 06/02/06 TO 06/12/06 23:00 DISCHARGE MEAN DAILY AVERAGE STREAMFLOW BASED ON 101 YEARS OF RECORD PLAKERCO Discharge Graph (Hourly Average) 06 /08 06 /09 06 /10 06 /11 06 /12 Figure D.4. Looking downstream left bank to right bank at Bridge RD-37.

D-15 Figure D.7. Looking upstream left bank to right bank at Bridge RD-50. Figure D.6. Looking upstream from Bridge RD-37.

D-16 Figure D.9. Pier 1 with debris and scour hole. Figure D.8. Left abutment and Pier 1 on Bridge RD-50.

Figure D.11. Looking down on Pier 2 and debris. D-17 Figure D.10. Pier 2 with debris.

D-18 Figure D.13. Aerial view of Bridge RD-50 and the upstream corridor. Figure D.12. Debris on right abutment.

D-19 Figure D.14. Looking upstream from Bridge RD-50. Figure D.15. Looking upstream from Bridge RD-50. Note riprap protection on left bank.

D-20 ATTACHMENT 2 Field Data Sheets Bridge WEL 50/67a for assessing woody debris delivery and accumulation potential at a bridge site

D-21

D-22

D-23

D-24

D-25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 The general characteristics of the upstream channl and flood plain can be acquired or measured from maps, aerial photos, and survey data. Prior to recording this information, the user should determine the appropriate reach length to be evaluated. This w debris. Active channel migration and evident bank erosion are the primary contributors to debris delivery. Channels that have been straightened or channelized may be unstable and incising and can be active contributors of debris. Man-made or natural ba Stream Bed Characteristics records pertinent information on bed material, bedforms, and bar types that can affect how and where debris accumulates in the upstream channel. Large bedforms and bars can migrate into the bridge reach and influence debris acc 26 Part B2 identifies the availability of existing bridge information that can be obtained from state, county, and/or local DOTs. Data should be obtained for the both the current bridge site, if available, and the next upstream bridge, if close enough, to d The bridge plans will provide specific data and measurements for the various structural elements of the bridge. The bridge inspection records can be used to identify any long-term debris or channel stability problems that have occurred since construction Item C1c records general information on the bed and bankline conditions and channel stability upstream of the bridge site. The data collected in this part includes pertinent data and information that defines the type of pier (e.g., column, wall, pile bent, etc.), the nose shape, the pier width, the skew of the piers to flow, and the number of bridge piers, openings, and abutm Bars records whether or not bars are present at the site, the type of bar, and the dominant bar material composition. Bridge maintenance records can provide information on past debris removal. Historic ground photos may show previous debris configurations and locations or past channel conditions that may be useful in identifying long-term changes tha have occurred. Historic aerial photos can be used to identify long-term changes such as active meandering, changes in channel width, land use changes, etc. Stream flow data can be used to determine flood flow depths, extent, duration, velocity, and other factors that i Section C records data and information to be collected on the bridge site during a site visit and field examination of the site. Part C1 records the data, information, descriptions, and ground photos acquired during the site reconnaissance. Land use plays an important role in defining the types and extent of debris that may potentially be delivered to the site. Urban land use as well as logging, mining, overgrazing, forest fires, and landslides can be significant contributors to channel ins Section A records specific information about the existing, replacement, or new bridge Record the Date of the project and the main Personnel involved in the project Provide as much information as possible about the bridge and its location. This information will be useful to others that review these sheets in the future. DESCRIPTION OF FIELD DATA SHEET ENTRIES Item C1b records information on the general morphology, characteristics, and conditions at the bridge site. Reach Location records the general planform of the channel at the bridge site, if it is situated on the upstream or downstream limb of a meander bend, the skew of the bridge to flow, the height of the low chord above the channel bed, the overall channel b Evidence of Scour, if present, records the specific dimensions of the scour at the bridge piers and abutments. Scour may be dependent of whether or not debris is present. Item C1a records information on the morphology of the channel and structural elements of the upstream bridge face. Part B1 records information that can be acquired or measured from topo maps, aerial photos, or surveys. Section B records information about the bridge site that can be obtained from various sources prior to or following the site evaluation *** It is highly recommended that the user review and understand HEC-20 prior to conducting this assessment *** Existing Debris Accumulations records the location of debris relative to the bridge site. Scour calculations identify estimated scour without debris and can then be used to estimate the potential scour depth associated with debris. Hydraulic models can provide much of the channel and hydraulic information to be recorded on these sheets. Item B1b records the general upstream channel and flood plain characteristics. Item B1a records the general characteristics of the watershed upstream of the bridge site.

D-26 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 Vertical Channel Stability identifies whether aggradation or degradation is occurring in the upstream channel since both can contribute to debris recruitment. Degradation results in bank erosion and bank retreat, whereas aggradation can induce overbank f Item C1d records general information on the riparian corridor upstream of the bridge site. The characteristics of the corridor and its composition provide information on the potential extent and size of the debris that could be supplied to the bridge sit Bankline Characteristics provides information on bank stability, vegetative cover, the potential modes of bank failure, how debris may be recruited, and whether of not debris can be stored along the bankline in the upstream channel. The shape of a bank h Item C1f Additional Information that may be deemed important is recorded here. Item C1e records general information on existing debris accumulations at a given bridge site. General information is collected on the location of debris accumulations, the type of accumulation on a bridge, the Typical Key Log type and dimensions, and root wad sizes. Information on the plan and profile (P&P) shape, dimensions, and location of debris along an existing bridge is collected at each bridge element. Lateral Channel Stability & Active Bank Erosion Characteristics identifies the location, degree, and specific mode of active bank retreat and channel migration in the upstream channel. Active bank retreat and lateral migration are the modes of debris rec

D-27 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 Item C1h Cross Section Sketch of Upstream Bridge Face is drawn here. The sketch should be viewed in the downstream direction and should include important features and dimensions. Item C1i Fie ld Site Photo Log is used to record photos taken at the site. The descriptions of the photo and any important features in the photo are recorded here as well. Item C1g Site Plan View Sketch is drawn here and should include important features and dimensions.

D-28 Estimated potential for debris production: HIGH Estimated potential for debris transport and delivery: HIGH Estimated debris accumulation potential: Left Bank Abutment—LOW Pier 1—MEDIUM Pier 2—HIGH Pier 3—HIGH Right Bank Abutment—LOW Estimated span blockage potential: Span 1—LOW Span 2—LOW Span 3—LOW Span 4—LOW P A R T 3 South Platte River Case Study: Final Data Sheets and Application of the Guidelines Summary of Assessment of Debris Production, Transport, Delivery, and Accumulation Potential Bridge WEL 50/67A over South Platte River near Hardin, Weld County, Colorado

D-29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Upstream Reach Length: 17 18 19 20 Yes __X_ No ____ Yes __X_ No ____ 21 22 Yes _____ No __X__ Unknown _____ Evidence of Regular Flooding: Yes ____ No _X__ 23 Yes _X__ No ____ 24 25 26 27 28 29 30 Historic Ground Photos avaialable for: 31 32 Historic Aerial Photos available for: Stream Flow Data or Records available for: 33 34 Scour Calculations available for: Hydraulic Models available for: 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 Descriptions: Vertical abutment cap wall with rock riprap spill-through abutment slopes. Nose: sharpWidth (ft): 1.5 Descriptions: Some riprap protection of piers appears to have been dispersed or displaced. Piers are skewed 70° to bridge. Description: Approx. 1-ft diameter riprap protection extends ~300 ft upstream and downstream. Upstream riprap in 173 ft long guidebank Abutments RIGHT - Distance From Top Bank: 0 Number: 3 Skew to Flow (deg): 5° at flood stage Average Upstream Channel Width at Crossings and/or Straight Reaches: 350 - 400 feet (top bank to top bank) Width (ft): 112.9-112.2-112.9-112.9 Number: 4 C - DATA & INFORMATION TO BE COLLECTED DURING A SITE VISIT & FIELD EXAMINATION (1) OBTAIN GROUND PHOTOS AND DETAILED DESCRIPTIONS THAT DOCUMENT THE FOLLOWING Description: Approx. 1-ft diameter riprap protection extends upstream and downstream. Upstream riprap in 173 ft long guidebank. LEFT - Distance From Top Bank: 0Type: sharp nosed wall piers General Vegetation Patterns and Characteristics (describe): Primarily narrow, intermittent to continuous, riparian corridor along both banks. Relatively wide, grassy floodplain with some grazing. Evidence of Active Meander Migration: Evidence of Active Bank Erosion/Retreat: Describe Existing Natural or Man-Made Barriers to Debris Delivery (dams, diversions, bridges, rock outcrops, major constrictions etc.): Two tight meander bends approx. 2.6 miles upstream; diversion dam approx. 3.8 miles upstream Upstream Channel Sinuosity: 1.2 Upstream Channel Slope: 0.00123 (1994) Was Reach Channelized: Average Upstream Channel Depth at Crossings and/or Straight Reaches: 5 - 6(?) ft (at bankfull), ~12 ft at flood stage Presence of : Sloughs _X__ Bridge Inspection Records available for: (2) AVAILABILITY OF BRIDGE INFORMATION FROM STATE, COUNTY, OR LOCAL DOT (circle) Bars and Other Major Sediment Deposits: BarsMeander Cutoffs _X__ Bridge Plans available for: (attached) Visible Evidence of Active Debris Delivery, Transport, and Storage Along Upstream Channel: Ehemeral ______ Flashy ______ (b) Upstream Channel and Flood Plain Characteristics (a) Upstream Watershed Characteristics Land-use: Natural Disturbances (e.g., landslides, forest fires, etc.): None Man-made Disturbances (e.g. mining, logging, grazing, etc.): some grazing; irrigation diversions Other _______ 1.0 miles Flow Type: Perennial ___X__ Rural _______ B - DATA & INFORMATION THAT CAN BE COLLECTED PRIOR TO OR FOLLOWING A SITE VISIT (1) SITE INFORMATION FROM TOPO MAPS, AERIAL PHOTOS, AND SURVEYS FIELD DATA SHEETS FOR ASSESSING WOODY DEBRIS DELIVERY AND ACCUMULATION POTENTIAL AT A BRIDGE SITE (To be used in conjunction with "Guidelines for Assessing Debris Production and Accumulation Potential") A - BRIDGE LOCATION Bridge ID / #: Bridge WEL 50/67A County and State: Weld County, Colorado Street / Road Name: Weld County Road CR 50 Nearest Town/City: Hardin Date: April 24, 2007 Field Personnel: W. Spitz and L. Girard River / Stream Name: South Platte River Longitude / Latitude: -104.41466 / 40.34886 Urban _______ Agricultural ___X____ Forested _______ Current Bridge Site: Y N Nearby Bridge: Y N Nearby Bridge: Y N Current Bridge Site: Y N Nearby Bridge: Y N Bridge Maintenance Records available for: Nearby Bridge: Y N Current Bridge Site: Y N Current Bridge Site: Y N Nearby Bridge: Y N Nearby Bridge: Y NCurrent Bridge Site: Y N Bridge Openings Current Bridge Site: Y N Piers, Pilings, and footings (a) Upstream Bridge Face and Structural Elements Nearby Bridge: Y NCurrent Bridge Site: Y N Nearby Bridge: Y NCurrent Bridge Site: Y N

D-30 52 53 54 55 56 57 58 59 60 61 62 Bars: 62 63 General Bank Face Shape: 64 LB Toe Sediment Accumulation (Basal Endpoint Control): 65 RB Toe Sediment Accumulation (Basal Endpoint Control): 66 67 LB: Trees___ Shrubs_X_ Grass_X_ Other___ Dense___ Mod.__X_ Sparse___ None___ Toe _X__ Mid ___ Upper ___ 68 RB: Trees_X_ Shrubs_X_ Grass_X_ Other___ Dense___ Mod.__X_ Sparse___ None___ Toe _X__ Mid ___ Upper ___ 69 Berms: LB ____ RB ____ Erosional ___ Depositional ___ Inset Flood Plain: LB __X__ RB __X__ 70 Vegetation on Berms/Inset Flood Plain: Yes _X__ No ____ Trees_X_ Grasses_X__ Other ____ 71 72 73 74 75 76 Intermittent Continuous In Bends Reach-wide In Toe Whole Bank Fluvial Rotational Cantilever Saturation Piping 77 Left Bank X X X X X 78 Right Bank X X X X 79 80 81 82 83 84 85 86 87 88 89 90 91 92 Evident Debris Delivery Processes in Reach: 93 94 95 96 97 98 At Abutments (Location and Size): RB Abutment, 2-3 ft deep, 8-10 ft wide Evidence of Scour (Debris Present X / Absent ___ ): Skew to Flow (deg): Scour Holes (Pier #): Pier #1 - downstream on left at nose Pier #2 - small Scour Hole Sizes (ft): Pier #1 - less than 1 ft Pier #2 - less than 1 ft Estimate Potential for Debris Production: Estimate Potential for Debris Transport & Delivery: Additional Information: Mid-channel Bar_X__ Point Bar_X__ Bank Attached Bar_X__ Tree Type: Conifererous_____ Deciduous___X__Debris Available from Flood Plain? Yes_X_ No___ (c) General Upstream Riparian Corridor Characteristics Evidence: Evidence of Vertical Channel Instability Typical Age: Young____ Intermediate_X__ Mature____ Old Growth____ Corridor Length (ft): Continuous __X___ Intermittent ______ Corridor Width (ft): Avg__500__ Max._1000__ Min.__300__ Bank Erosion/Failure_X_ Windthrow_X_ Landslides___ Flood Plain Input_X_ Disease/Insect Kill___ Logging___ D/S Transport_X_ Typical Species (if known): Cottonwoods, willows Healthy? Y _X_ N___ Stage: Multi-generational ____ Even-Aged _X__ Spacing: Uniform __X__ Irregular ____ Density: Dense ____ Mod. _X__ Sparse ____ Multistory? Yes_X__ No____ Low Chord Height Above Streambed (ft): Min. 7'(pier 3) Max. _____ Flow Split _____On DS Limb _____ Thalweg Position at Bridge: right bank (RB) Straight _______ Reach Location: Bank Face/Slope Vegetation Type and Location: On Bed ___X___ On Bars ___X___ On Bridge Elements ___X___On Banks ___X___ Bankline Characteristics ( LB = Left Descending Bank, RB = Right Descending Bank) Significant _____ Moderate _U/S, D/S_ Degradation (headcuts, knickzone, vertical banks, exposed footings, etc.): Yes___ No__X_ If meander bends are present, are they actively migrating? Yes__X__ No _____ Unsure _____ Additional Information: Stream observed during low flow - braided pattern at low flow. Gage located approx. 4.5 miles upstream. Current meander belt and flood plain inset into older, higher meander belt and flood plain is indicative of previous vertical instability, but is currently vertically stable. Extensive young vegetation on bars and lots of algae growth in channel. Additional Information: Aerial photos show numerous active meander bends upstream and active meander migration and point bar development. Active channel planform is borderline between braided and meandering. Aggradation (berms, inset flood plain, overbank sedimentation, etc.): Yes____ No__X__ Is General Bank Erosion Evident? Left Bank: Yes__X__ No____ Right Bank: Yes____ No____ Lateral Channel Stability & Active Bank Erosion Characteristics Existing Debris Accumulations: On Flood Plain ___X___ Undercut/Overhanging _____ Significant _____ Moderate __U/S_ Negligible _____ Concave _RB, LB_ Vertical _____Convex _____ Low Flow:__15°____ High Flow:___5°____ (b) General Bridge Reach Characteristics In Meander Bend - On US Limb _____ At Bend Apex: __X__ Negligible _____ Stable: Y _X_ N___ Shrubs/Bushes X (See Flowchart A) High __X___ Low _____ (See Flowchart B) High __X___ Low _____

D-31 99 100 101 102 103 104 105 106 107 Pier # Abutment 1 2 3 4 5 6 7 8 9 Abutment 108 P&P Shape / T / NA 1* / / / / / / / / / 109 Width / / / / / / / / / / / 110 Hgt/Depth / 2' / 3' / / / / / / / / / 111 Length / / / / / / / / / / / 112 113 Span/Gap Abut - 1 1-2 2-3 3-4 4-5 5-6 6-7 7-8 8-9 9-10 # - Abut 114 P&P Shape / / / / / / / / / / R / IC 115 Width / / / / / / / / / / 8' / 10' 116 Hgt/Depth / / / / / / / / / / 7' / 8' 117 Length / / / / / / / / / / 25' / 30' 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 Typical Key Log Length (range - ft): 18-20 feetSpan Accumulation_____ Pier Accumulation_____ Both__X__ Debris Accumulation Locations: On Bars__X__ Bank Toe__X__ Top Bank__X__ Piers__X__ Abutments__X__ Typical Key Log Species: Cottonwoods (d) Existing Debris Accumulation Characteristics at a Bridge Site Existing Debris Accumulations ( P lanform & P rofile Descriptions) Type of Debris Accumulation on Bridge(s): (e) Site Plan View Sketch (include important features and dimensions) Low ___All_______ Med ______________ High______________ Chronic ___________ Typical Key Log Diameter (range - ft): 1.4-1.6 feet Med ______1_______ High____2 & 3____ Additional Information: This field visit: Single log accumulation noted on Pier 2, no debris noted on Pier 3. On RB abutment, 3 to 4 18-inch diam. logs piled on riprap in scour hole, one log contains 7-8' diameter root wad. Original bridge inspection records: Indicate consistant debris accumulation problem prior to bridge replacement in 1996. Current bridge inspection records: In 1999, debris noted on upstream face at Pier 2; In 2002, debris noted at Span 4 and RB Abutment and photos show debris on downstream side of Span 3; In 2004, large trees noted at Pier 3 and RB Abutment; In 2006, debris and large trees noted on Piers 3 and 4 and RB Abutment. Estimate Debris Accumulation & Span Blockage Potential (see Flowcharts C and D) Bridge Pier/Abutment (#, see above): Pier-to-Pier Gap/Span (#, see above): In Channel or Gap/Span (pier-to-pier): Low _LB & RB Abut__ P&P Shape Abbreviations: Planform : T = Triangular R = Rectangular / Profile : C = Conical R = Rectangular IC = Inverted Cone Root Wad Sizes (range - ft): >6 ft on Pier #2 At Bridge Piers/Abutments: Chronic ___________ LB Abut RB Abut

D-32 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 Photo # 184 P1/12 185 P1/13 186 P1/14 187 P1/15,16 188 P1/17 189 P1/18 190 P1/19 191 P1/20 192 193 194 195 196 197 198 199 200 (g) Field Site Photo Log View looking down on Pier #1 debris accumulation at nose of pier View TL to TR showing debris on wall piers and upstream face of bridge View looking upstream, note debris in channel and on eroding banks View looking down on log with root wad and other debris on center pier (Pier #2) View of debris on right bank abutment. Note scour hole around debris (2-3' deep, 8-10' wide) (f) Cross Section Sketch of Upstream Bridge Face (looking downstream - include important features and dimensions) View looking toward Pier #1 and log along pier wall, root wad at pier nose Description View downstream showing root wad on Pier #2 = single log accumulation Pan looking upstream

D-33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 55 56 57 58 59 60 61 62 62 63 64 65 66 67 68 69 70 71 72 73 54 Existing Debris Accumulations records the location of debris relative to the bridge site. Records whether or not bars are present at the site and the type of bar since bars can influence debris grounding and accumulation. 24 Item C1a records information on the morphology of the channel and structural elements of the upstream bridge face. Scour calculations identify estimated scour without debris and can then be used to estimate the potential scour depth associated with debris. Hydraulic models can provide much of the channel and hydraulic information to be recorded on these sheets. Item B1b records the general upstream channel and flood plain characteristics. Item B1a records the general characteristics of the watershed upstream of the bridge site. Land use plays an important role in defining the types and extent of debris that may potentially be delivered to the site. Urban land use as well as logging, mining, overgrazing, forest fires, and landslides can be significant contributors to channel instability, bank erosion, and subsequently debris delivery. Section A records specific information about the existing, replacement, or new bridge. Record the Date of the project and the main Personnel involved in the project. Provide as much information as possible about the bridge and its location. This information will be useful to others that review these sheets in the future. Part B1 records information that can be acquired or measured from topo maps, aerial photos, or surveys. Section B records information about the bridge site that can be obtained from various sources prior to or following the site evaluation. DESCRIPTION OF FIELD DATA SHEET ENTRIES *** It is highly recommended that the user review and understand the concepts described in FHWA's HEC 20 prior to conducting this assessment *** Bankline Characteristics provides information on bank stability, vegetative cover, the potential modes of bank failure, how debris may be recruited, and whether of not debris can be stored along the bankline in the upstream channel. The shape of a bank helps define the mode of failure such as undercutting with gravity failure (vertical or overhanging bank) or by rotational failures (concave bank) or by slow fluvial erosion (convex bank). Sediment buildup along the bank toe can temporarily buttress the bank from erosion, but is also indicative of active bank failures. Bank face vegetation can provide some measure of bank stability, but may also be susceptible to recruitment. Depositional berms or narrow floodplains inset into an entrenched floodway are indicative of past incision/degradation followed by recovery. Erosional berms may be indicative of recent incision/degradation. Unstable inset floodpains may be indicative of a new wave of incision. Vegetation on the inset berms and floodplains provide a source for recruitment. Part B2 identifies the availability of existing bridge information that can be obtained from state, county, and/or local DOTs. Data should be obtained for the both the current bridge site, if available, and any nearby bridge, if close enough, to determine if there are any ongoing debris and stability problems in the reach. The bridge plans will provide specific data and measurements for the various structural elements of the bridge. The bridge inspection records can be used to identify any long-term debris or channel stability problems that have occurred since construction. The data collected in this part includes pertinent data and information that defines the type of pier (e.g., column, wall, pile bent, etc.), the nose shape, the pier width, the skew of the piers to flow, and the number of bridge piers, openings, and abutments, their physical condition, their relationship to the channel, banks, and flood plain and any scour and debris problems that may be evident. Bridge maintenance records can provide information on past debris removal. Historic ground photos may show previous debris configurations and locations or past channel conditions that may be useful in identifying long-term changes tha have occurred. Historic aerial photos can be used to identify long-term changes such as active meandering, changes in channel width, land use changes, etc. Stream flow data can be used to determine flood flow depths, extent, duration, velocity, and other factors that influence debris production, transport, and accumulation. Section C records data and information to be collected on the bridge site during a site visit and field examination of the site. The general characteristics of the upstream channel and flood plain can be acquired or measured from maps, aerial photos, and survey data. Prior to recording this information, the user should determine the appropriate reach length to be evaluated. This will generally be the reach between the current bridge site and the next upstream bridge site, dam, diversion, or other controlling structure OR an estimated distance over which most or all of the upstream area could potentially contribute debris during a major flow event. Sinuosity, which is the ratio of the channel length to the straight line valley length between two points, will control the distance over which debris will move along a channel (i.e., sinuous channels will impede the movement of debris). Slope can be estimated from maps or can be obtained from survey data. Channel width has the greatest control over debris transport, since debris that is longer than the width of a channel will probably not be transported very far from its source area. Depth also controls transport since floating of the debris is required, especially if the debris has root wads/balls attached. Exposed or shallow bars and in-channel sediment deposits can trap debris. Active channel migration and evident bank erosion are the primary contributors to debris delivery. Channels that have been straightened or channelized may be unstable and incising and can be active contributors of debris. Man-made or natural barriers may restrict the downstream movement of debris. Regular flooding can also supply debris from the flood plain. Reach Location records the general planform of the channel at the bridge site, if it is situated on the upstream or downstream limb of a meander bend, the skew of the bridge to flow, the height of the low chord above the channel bed, the overall channel bed material that is being transported, and the thalweg position in the channel at the bridge site. Evidence of Scour, if present, records the specific dimensions of the scour at the bridge piers and abutments. Scour may be dependent of whether or not debris is present. Part C1 records the data, information, descriptions, and ground photos acquired during the site reconnaissance. Item C1b records information on the general morphology, characteristics, and conditions at the bridge site.

D-34 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 Item C1f Cross Section Sketch of Upstream Bridge Face is drawn here. The sketch should be viewed in the downstream direction and should include important features and dimensions. Item C1e Site Plan View Sketch is drawn here and should include important features and dimensions. Item C1d records general information on existing debris accumulations at a given bridge site. General information is collected on the location of debris accumulations, the type of accumulation on a bridge, the Typical Key Log type and dimensions, and root wad sizes. Qualitative estimate of debris production and transport and delivery to bridge site is made. Information on the plan and profile (P&P) shape, dimensions, and location of debris along an existing bridge is collected at each bridge element. Provide a qualitative estimate of the potential for debris accumlations on piers/abutments (use Flowchart C) and in gaps/spans (use Flowchart D). Be sure to incorporate information from available bridge data acquired from DOT (lines 27-35) in the debris accumulation estimations (lines 120-122). Lateral Channel Stability & Active Bank Erosion Characteristics identifies the location, degree, and specific mode of active bank retreat and channel migration in the upstream channel. Active bank retreat and lateral migration are the modes of debris recruitment. Vertical Channel Stability identifies whether aggradation or degradation is occurring in the upstream channel since both can contribute to debris recruitment. Degradation results in bank erosion and bank retreat, whereas aggradation can induce overbank flooding and lateral migration. Item C1c records general information on the riparian corridor upstream of the bridge site. The characteristics of the corridor and its composition provide information on the potential extent and size of the debris that could be supplied to the bridge site. Provide a qualitative estimate of the potential for debris production (use Flowchart A) in the upstream reaches and for debris transport and delivery (use Flowchart B) to the bridge site .

D-35 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 Item C1g Field Site Photo Log is used to record photos taken at the site. The descriptions of the photo and any important features in the photo are recorded here as well. Direct evidence of debris production Indirect evidence of debris production Widespread bank erosion Erosion of outer bank in meander b e nds Localized, minor ban k erosion Are trees present along or wi thin 100 feet of bankline? NO YES • Chann e l incisio n • Lateral migration • Chann e l wid e n i ng • Chann e lization • Dams and d i versio n s • Land use chang e s • Logging , mining o r landslides alo n g channel • Other h y draulic/geomo r phic facto r s • Few trees along or near b a nkline • Fully stabi l ized channel HIGH (Significant) Potential for Debris Production LOW (Minimal) Potential for Debris Product ion Potential for Debris Transport & Delivery POTENTIAL FOR DEBRIS PRODUCTIONFlowchart A Direct evidence of potential for debris transport and delivery Indirect evidence of potential for debris transport and delivery • Documented chronic or frequent accumulations at one or more bridg e sites along this channel or upstream tributaries • Abundant debris stored in chann e l and /or along b a n k s • Ongoing o r p r ior n e ed for debris removal from channel and/or b r idges • Upstream chann e l g e o m etry too small to transpo r t d e b r is • Debris is absent after floods in typical debris accumulation sites other than bridges • Negligible debris d e livered to a site following major events • Debris in the channel remains in place following floods because of low flow velocities • Long straight or slightly sinuous reach upstream • Direct or unobstructed transport path to bridge site HIGH Potential for Debris Transport & Delivery LOW Potential for Debris Transport & Delivery Estimate Size of the Largest Debris (Key Log Length) Potentially Delivered to Site POTENTIAL FOR DEBRIS TRANSPORT AND DELIVERY • High ly sinuous reach upstream • Obstructed transport path to bridg e site Is i t likely that debris will be delivered to the bridg e site d u ring subsequent floods? YES NO Flowchart B

D-36 Seasonal High Flow Level Design Flood Flow Level Sheltered Top Bank/ Flood Plain Top Bank/ Flood Plain Channel Debris Path (approx.) Meander Bend Seasonal High Flow Level Design Flood Flow Level Sheltered Top Bank/Flood Plain Top Bank/ Flood PlainChannel DebrisPath (approx.) Straight Reach Channel Sh el te re d Locational Categories

D-37 Sheltered by upstream forest? SHELTERED Location Category DETERMINE LOCATION CATEGORY In channel between tops of banks? YES NO TOP BANK/FLOOD PLAIN Location Category CHANNEL Location Category DEBRIS PATH Location Category Contains or is in close proximity to the thalweg or thread of the stream? NO NO YES LIKELY SPAN 1 LOCATION Sheltered by upstream forest? SHELTERED Location Category DETERMINE LOCATION CATEGORY In channel between tops of banks? YES NO TOP BANK/FLOOD PLAIN Location Category CHANNEL Location Category DEBRIS PATH Location Category Contains or is in close proximity to the thalweg or thread of the stream? NO YES LIKELY SPAN 2 LOCATION NO

D-38 Sheltered by upstream forest? SHELTERED Location Category DETERMINE LOCATION CATEGORY In channel between tops of banks? YES NO TOP BANK/FLOOD PLAIN Location Category CHANNEL Location Category DEBRIS PATH Location Category Contains or is in close proximity to the thalweg or thread of the stream? NO NO YES LIKELY SPAN 3 LOCATION Sheltered by upstream forest? SHELTERED Location Category DETERMINE LOCATION CATEGORY In channel between tops of banks? YES NO TOP BANK/FLOOD PLAIN Location Category CHANNEL Location Category DEBRIS PATH Location Category Contains or is in close proximity to the thalweg or thread of the stream? NO NO YES LIKELY SPAN 4 LOCATION

D-39 Horizontal / vertical gap / span wider than design log length? LOW Gap/Span Blockage Potential GAP / SPAN IN SHELTERED LOCATION CATEGORY Potential for gap / span blockage due to large adjacent pier accumulations? YESNO MEDIUM Gap/Span Blockage Potential HIGH Gap/Span Blockage Potential HIGH, CHRONIC Gap/Span Blockage Potential YES Flood Plain / Top of Bank In Channel Is pier-to-pier (or pier- to-bank) gap wider than design log length? YES NO Debris Path Flood Plain / Top of Bank In Channel Debris Path Location Category NOYES NO HIGHLOW Location Category Potential for Delivery of Debris SPAN 1 BLOCKAGE POTENTIAL Horizontal / vertical gap / span wider than design log length? LOW Gap/Span Blockage Potential GAP / SPAN IN SHELTERED LOCATION CATEGORY Potential for gap / span blockage due to large adjacent pier accumulations? YESNO MEDIUM Gap/Span Blockage Potential HIGH Gap/Span Blockage Potential HIGH, CHRONIC Gap/Span Blockage Potential YES Flood Plain / Top of Bank In Channel Is pier-to-pier (or pier- to-bank) gap wider than design log length? YES NO Debris Path Flood Plain / Top of Bank In Channel Debris Path Location Category NOYES NO HIGHLOW Location Category Potential for Delivery of Debris SPAN 2 BLOCKAGE POTENTIAL Flowchart C

D-40 Horizontal / vertical gap / span wider than design log length? LOW Gap/Span Blockage Potential GAP / SPAN IN SHELTERED LOCATION CATEGORY Potential for gap / span blockage due to large adjacent pier accumulations? YESNO MEDIUM Gap/Span Blockage Potential HIGH Gap/Span Blockage Potential HIGH, CHRONIC Gap/Span Blockage Potential YES Flood Plain / Top of Bank In Channel Is pier-to-pier (or pier- to-bank) gap wider than design log length? YES NO Debris Path Flood Plain / Top of Bank In Channel Debris Path Location Category NOYES NO HIGHLOW Location Category Potential for Delivery of Debris SPAN 3 BLOCKAGE POTENTIAL Horizontal / vertical gap / span wider than design log length? LOW Gap/Span Blockage Potential GAP / SPAN IN SHELTERED LOCATION CATEGORY Potential for gap / span blockage due to large adjacent pier accumulations? YESNO MEDIUM Gap/Span Blockage Potential HIGH Gap/Span Blockage Potential HIGH, CHRONIC Gap/Span Blockage Potential YES Flood Plain / Top of Bank In Channel Is pier-to-pier (or pier- to-bank) gap wider than design log length? YES NO Debris Path Flood Plain / Top of Bank In Channel Debris Path Location Category NOYES NO HIGHLOW Location Category Potential for Delivery of Debris SPAN 4 BLOCKAGE POTENTIAL Flowchart C

D-41 LOW Accumulation Potential LOCATION CATEGORY LOCATION CATEGORY MEDIUM Accumulation Potential HIGH Accumulation Potential HIGH, CHRONIC Accumulation Potential YES NO PIER OR SUBSTRUCTURE DESIGN SOLID WITH APERTURES Potential for Delivery of Debris Low High Low High Low High DEBRIS PATH DEBRIS PATHCHANNEL FLOOD PLAIN /TOP BANK CHANNEL FLOOD PLAIN /TOP BANK Potential for Delivery of Debris Potential for Delivery of Debris PIER OR SUBSTRUCTURE SECTION IN SHELTERED LOCATION CATEGORY LB ABUTMENT ACCUMULATION POTENTIAL LOW Accumulation Potential LOCATION CATEGORY LOCATION CATEGORY MEDIUM Accumulation Potential HIGH Accumulation Potential HIGH, CHRONIC Accumulation Potential YES NO PIER OR SUBSTRUCTURE DESIGN SOLID WITH APERTURES Potential for Delivery of Debris Low High Low High Low High DEBRIS PATH DEBRIS PATHCHANNEL FLOOD PLAIN /TOP BANK CHANNEL FLOOD PLAIN /TOP BANK Potential for Delivery of Debris Potential for Delivery of Debris PIER OR SUBSTRUCTURE SECTION IN SHELTERED LOCATION CATEGORY PIER 1 ACCUMULATION POTENTIAL Flowchart D

D-42 LOW Accumulation Potential LOCATION CATEGORY LOCATION CATEGORY MEDIUM Accumulation Potential HIGH Accumulation Potential HIGH, CHRONIC Accumulation Potential YES NO PIER OR SUBSTRUCTURE DESIGN SOLID WITH APERTURES Potential for Delivery of Debris Low High Low High Low High DEBRIS PATH DEBRIS PATHCHANNEL FLOOD PLAIN /TOP BANK CHANNEL FLOOD PLAIN /TOP BANK Potential for Delivery of Debris Potential for Delivery of Debris PIER OR SUBSTRUCTURE SECTION IN SHELTERED LOCATION CATEGORY PIER 2 ACCUMULATION POTENTIAL LOW Accumulation Potential LOCATION CATEGORY LOCATION CATEGORY MEDIUM Accumulation Potential HIGH Accumulation Potential HIGH, CHRONIC Accumulation Potential YES NO PIER OR SUBSTRUCTURE DESIGN SOLID WITH APERTURES Potential for Delivery of Debris Low High Low High Low High DEBRIS PATH DEBRIS PATHCHANNEL FLOOD PLAIN /TOP BANK CHANNEL FLOOD PLAIN /TOP BANK Potential for Delivery of Debris Potential for Delivery of Debris PIER OR SUBSTRUCTURE SECTION IN SHELTERED LOCATION CATEGORY PIER 3 ACCUMULATION POTENTIAL Flowchart D

D-43 LOW Accumulation Potential LOCATION CATEGORY LOCATION CATEGORY MEDIUM Accumulation Potential HIGH Accumulation Potential HIGH, CHRONIC Accumulation Potential YES NO PIER OR SUBSTRUCTURE DESIGN SOLID WITH APERTURES Potential for Delivery of Debris Low High Low High Low High DEBRIS PATH DEBRIS PATHCHANNEL FLOOD PLAIN /TOP BANK CHANNEL FLOOD PLAIN /TOP BANK Potential for Delivery of Debris Potential for Delivery of Debris PIER OR SUBSTRUCTURE SECTION IN SHELTERED LOCATION CATEGORY RB ABUTMENT ACCUMULATION POTENTIAL Flowchart D 0 50 100 150 200 250 300 350 400 450 DISTANCE FROM LEFT ABUTMENT (ft) 4512 4514 4516 4518 4520 4522 4524 4526 4528 EL EV AT IO N (ft) Bridge WEL 50/67A - Weld County Road No. 50 Soundings Bottom of Girder 1995 (pre-const.) 1996 1997 1999 & 2001 2004 2006 Pi er 1 Pi er 2 Pi er 3 LB A bu tm en t RB A bu tm en t

D-44

D-45 P1/12 P1/13 P1/14 P1/17 P1/15 & 16 P1/18 P1/19

P1/20 D-46

D-47 Steps 1 through 5 The case study of the South Platte River results indicate that there is a high potential for debris production, high potential for debris transport and delivery, and a high potential for accumulation of debris at Pier 2, which is located in the center of the bridge near the middle of the channel. The span length is 112.9 ft (34.4 m), which is much longer than the key log length of 20 ft (6 m). Therefore, the debris is extremely unlikely to bridge between piers to form a raft. The key log diameter is approximately 1.5 ft (0.46 m) and rootwad sizes can exceed 6 ft (1.8 m). Inspection records of the existing bridge, a previous bridge at this location and nearby bridges indicate frequent debris accumulations. The debris accumulation at Pier 2 for an extreme event is assumed to be 30 ft (9.1 m) wide, 6 ft (1.8 m) thick, and 20 ft (6.1 m) long. This size accumulation is based on the high debris accumulation potential, key log length, key log diameter, and rootwad size. The shape is assumed to be rectangular. These assumptions should be confirmed for reasonableness with bridge maintenance and inspection personnel (and/or eval- uating debris size and shape in the photographic archive in Appendix A). The hydraulic conditions are calculated for a 100-year flood with and without debris loading. Without debris loading, the maximum channel velocity is 6.25 ft/s (1.91 m/s) and flow depth is 15.3 ft (4.66 m). When the hydraulic model is run to simulate debris loading, the maximum channel velocity is 6.17 ft/s (1.88 m/s) and the flow depth is 15.5 ft (4.72 m). Because the length of the debris cluster exceeds the flow depth, the debris scour will also need to be computed for a length equal to the flow depth. It is assumed that this shorter debris pile is 26 ft (7.9 m) wide but remains 6 ft (1.8 m) thick and that the hydraulic conditions are the same as for the larger debris cluster. The wall pier at this bridge has a width, a, of 1.5 ft (0.46 m); a length, L, of 43 ft (13.1 m); a 5° angle of attack; and a sharp nose (actually a debris deflector). Because the pier is more than 12 times the pier width, a maximum length of 12 × 1.5 ft = 18 ft (12 × 0.46 m = 5.5 m) is used per HEC-18 guidance. In the pier scour equation, the K1 pier shape factor is 1.0 (because of the skew) rather than 0.9 for a sharp nose. The pier scour equa- tion K2 factor can be calculated based on HEC-18 guidance or the projected width of the pier can be used in lieu of using K2. K3 is 1.1 based on an assumption of plane bed or small dunes expected on the South Platte River during extreme floods, and K4 is 1.0 because armoring is not expected. Guidance on obtaining the above information is contained in Steps 1 through 5 of the methodology outlined in the pre- vious section. Step 6 Compute pier scour without debris: Alternatively, the pier scour can be computed using the pro- jected width and excluding K2 from the pier scour equation. ys = × × × × × ⎛⎝⎜ ⎞⎠⎟2 0 1 5 1 0 1 6 1 1 1 0 15 3 1 5 0 35 . . . . . . . . . 0 28 6 9 0 43 . . .( ) = ( )ft 2.1 m Fr V gy = = × = 6 25 32 2 15 3 0 28 . . . . K L a 2 0 65 5 1 = ( ) + ⎛⎝⎜ ⎞⎠⎟ ( )⎡⎣⎢ ⎤ ⎦⎥ = ( ) + Cos Sin Cos θ θ . 8 1 5 5 1 6 0 65 . . .⎛⎝⎜ ⎞⎠⎟ ( )⎡⎣⎢ ⎤ ⎦⎥ =Sin y aK K K K y a Frs = ⎛⎝⎜ ⎞⎠⎟2 0 1 2 3 4 0 35 0 43. . . P A R T 4 Debris Scour Calculations

The projected width of the pier without debris is: Step 7 Determine the effective pier width with debris for the max- imum debris dimensions. Maximum debris dimensions are W = 30 ft (9.1 m), L = 20 ft (6.1 m), and T = 6 ft (1.8 m) and the projected width of the pier should be used. For a rectan- gular debris cluster Kd1 = 0.79 and Kd2 = −0.79. Step 8 Determine the effective pier width for the debris length equal to the flow depth. For a debris length equal to the flow depth, the debris dimensions are W = 26 ft (7.9 m), L = 15.5 ft (4.7 m), and T = 6 ft (1.8 m), where W and T are assumed based on the guidance in the previous section. Step 9 Calculate scour for a∗d equal to the largest computed value of 10.1 ft (3.1 m) excluding K1 and K2 from the pier scour equation. Fr V gy = = × = 6 17 32 2 15 5 0 28 . . . . y a K K y a Frs d d = ⎛ ⎝⎜ ⎞ ⎠⎟∗ ∗2 0 3 4 0 35 0 43. . . ∗ = × × + − ×( ) =ad 0 79 6 26 15 5 0 79 6 3 1 15 5 10 1 . . . . . . ft 3.1 m( ) ∗ = + −( ) a TW y T a y d 0 79 0 79. . proj ∗ = × × ⎛⎝⎜ ⎞⎠⎟ + − ×( − ad 0 79 6 30 20 15 5 15 5 0 79 6 0 79 . . . . . ) = ( ) 3 1 15 5 9 7 . . . ft 3.0 m a TW L y y T a y d = ⎛ ⎝⎜ ⎞ ⎠⎟ + −( )∗ − 0 79 0 79 0 79 . . . proj ys = × × × × ⎛⎝⎜ ⎞⎠⎟2 0 3 1 1 0 1 1 1 0 15 3 3 1 0 28 0 35 . . . . . . . . . ( ) = ( )0 43 6 9. . ft 2.1 m y a K K K y a Frs = ⎛ ⎝⎜ ⎞ ⎠⎟2 0 1 3 4 0 35 0 43. . . proj proj a a Lproj Cos Sin Cos Sin= ( )+ ( ) = ( )+ ( ) = θ θ 1 5 5 18 5 3 . .1 ft 0.93 m( ) Step 10 For comparison, compute the scour for a triangular debris accumulation with the same dimensions. Maximum debris dimensions are W = 30 ft (9.1 m), L = 20 ft (6.1 m), and T = 6 ft (1.8 m). For a triangular debris cluster, Kd1 = 0.21 and Kd2 = −0.17 For a debris length equal to the flow depth, the debris dimensions are W = 26 ft (7.9 m), L = 15.5 ft (4.7 m), and T = 6 ft (1.8 m). Calculate scour for a∗d equal to the largest computed value of 5.2 ft (1.58 m) excluding K1 and K2 from the pier scour equation. Summary In summary, the pier scour would be 6.9 ft (2.1 m) with- out debris, 14.9 ft (4.5 m) with a rectangular debris cluster, and 9.7 ft (3.0 m) with a triangular debris cluster. The con- trolling condition for the rectangular cluster is when L/y = 1.0 (plunging flow coincident with the pier face) and for the tri- angular cluster the controlling condition is when the debris accumulation is at the maximum size. ys = × × × ⎛⎝⎜ ⎞⎠⎟ ( )2 0 5 2 1 1 1 0 15 5 5 2 0 28 0 35 0 . . . . . . . . .43 9 7= ( ). ft 2.96 m Fr V gy = = × = 6 17 32 2 15 5 0 28 . . . . y a K K y a Frs d d = ⎛ ⎝⎜ ⎞ ⎠⎟∗ ∗2 0 3 4 0 35 0 43. . . ∗ = × × + − ×( ) =ad 0 21 6 26 15 5 0 21 6 3 1 15 5 5 0 . . . . . . ft 1.52 m( ) a TW y T a y d = + −( )∗ 0 21 0 21. . proj ∗ = × × ⎛⎝⎜ ⎞⎠⎟ + − ×( − ad 0 21 6 30 20 15 5 15 5 0 21 6 0 17 . . . . . ) = ( ) 3 1 15 5 5 2 . . . ft 1.58 m a TW L y y T a y d = ⎛ ⎝⎜ ⎞ ⎠⎟ + −( ) ∗ − 0 21 0 21 0 17 . . . proj ys = × × × ⎛⎝⎜ ⎞⎠⎟ ( )2 0 10 1 1 1 1 0 15 5 10 1 0 28 0 35 . . . . . . . . 0 43 14 9 . .= ( )ft 4.54 m D-48