# Presentation: 318487

## Design and Construction of Bendway Weirs at US-51 over the Hatchie River

-  Wesley Peck

- Tennessee Department of Transportation
- Hydraulic Design Section

**Notes:**
Introduce Self
Name
Position
Introduce Project
Location- Also called State Route 3
Problem-Moving river 
Solution- Bendway Weirs
Design- 2D Model

## General Information

**Notes:**
This is a portion of the Hatchie River.
Point out US-51
Active, Dynamic River with lots of meanders (point them out)
Land use is mostly agriculture and natural forest
Band of trees along the river

## General Information

- General Characteristics of the Hatchie River
- Mouth at Mississippi River
- Drains 2,600 square miles of west Tennessee and north Mississippi
- Majority of drainage area is agricultural or undisturbed forested area
- Only major river in west Tennessee undisturbed by man
- Low channel gradient of approximately 0.001ft/ft

- Class I Wild and Scenic River for state
- Basin is home to two National Wildlife Refuges

**Notes:**
Some of the general characteristics of the Hatchie River are:
It flows to the Mississippi River.  US-51 is located at river mile 34±
Drains 2,600 square miles in Tennessee and Mississippi.
Very pristine river, no major, and very few small urban areas in the drainage basin.
Only large river in west TN which is undisturbed by man
Tennessee declared it a Class I Wild and Scenic River
Two National Wildlife Refuges along its banks.
Low channel gradient and mostly sand substrate

## Historical Bridge Background

- The original bridge opened in 1936 was 4,201 feet long.

**Notes:**
The first state built bridge at this location was opened in 1936.  It was 4,201 feet long.

## Historical Bridge Background

- A bend of the river was straightened in order to build the new bridge.

**Notes:**
The river was straightened when the original bridge was built so that it could flow through the bridge on  a 90° skew.

The some of the old, partially-silted channel scars are still visible (point them out)

## Historical Bridge Background

- The route was upgraded to four lane divided in 1974.  A new 999 ft long bridge was added for southbound traffic.

**Notes:**
In 1974 the route was upgraded to be a four lane highway with a median.  A new bridge was built alongside the old bridge.  The new bridge took southbound traffic and was 999 feet long.

## Historical Bridge Background

- The northbound bridge (built in 1936) collapsed into the river in 1989.

**Notes:**
In April of 1989 two spans of the old northbound bridge collapsed into the river during a flood.

Happened soon after the failure at Schoharie Creek in New York.
Caused a boom in research into scour and stream instability
Well known bridge failure.  Case study for NHI course on bridge scour and stream instability.

## Historical Bridge Background

- The bridge was replaced with a new structure which matched the one built in 1974.

**Notes:**
A new replacement bridge was built to the same dimensions as the one bridge still in use at the site.

Channel was heavily armored with riprap (point it out) on both banks and on the bottom.

## Problem

- A review by the National Transportation Safety Board (NTSB) determined the cause of the failure to be a combination of migration of the river bank and local scour.

**Notes:**
The National Transportation Safety Board (NTSB) reviewed the failure.

NTSB found that the cause of failure was a combination of pier scour and migration of the river.

## The river bank moved to a point where bents that were previously on the floodplain were in the middle of the channel.  Local scour due to a flood then further exposed the bent piles.

- The piles failed due to buckling and embedment caused by excessive exposure.

- Problem

**Notes:**
Movement of the river caused a bent which was previously on the floodplain to be in the middle of the channel.  The streambed lowered exposing friction piles under the bent.  Pier scour due to highwater further exposed the bents.  The piles failed due to excessive exposure and caused the bent to fail. 

## Problem

- Analysis of Corps of Engineers cross-sections shows significant movement of channel bank since the first bridge was constructed.

- 0.8 foot per year from 1934 to 1975
- 4.5 feet per year from 1975 to 1981

- 1.8 feet per year from 1981 to 1988
- No significant change since replacement of failed bridge

**Notes:**
After the collapse the USGS conducted a study on the channel migration.  They utilized a Corps of Engineers gage station which has been located at this site since the 1930’s.  Cross sections were available as far back as 1941.  Based on these cross-sections the USGS quantified the channel migration.
0.8 foot per year from 1934 to 1975
4.5 feet per year from 1975 to 1981
1.8 feet per year from 1981 to 1988
USGS conclusions

## Current Situation

- The problem has shifted upstream due to riprap at the bridge.
- Active erosion of outside bank of the 90° bend upstream of the bridge.
- Active point bar formation on inside bank of upstream bend.
- Large scour hole in channel at the upstream bend.

**Notes:**
That was then..... And this is now.
When the failed bridge was replaced in 1990, the channel and both banks were heavily armored with riprap under the bridge.  This succeeded in halting the migration, but only at the bridge.
The problem has shifted/continued to the 90° bend upstream of the bridge.
Active erosion of the outside bank of the bend
Active point bar formation on inside bank of the bend.  There are sand deposits as much as 18 inches thick on the vegetation in that area.
A large scour hole approximately 15 feet deeper than the channel bottom.  Located in the middle of the bend in the river.

## Current Situation: Topographical Map

**Notes:**
These next few slides are meant to show the problem up close and personal.
This is a topographical map of the site as it is now.
North
flow direction
road and bridge face
riprap limits
bank erosion and point bar
picked up the channel and scooted it 
scour hole

## Slide 14

**Notes:**
Looking upstream at the outside bank of the bend.  

Note debris
Note leaning trees
Note vertical banks and undercutting
Note tree in the middle of the channel

## Slide 15

**Notes:**
Looking upstream at the inside bend.

Note end of riprap.
Note sand deposition and point bar buildup
Comment on sand buildup in trees at the top of point bar.

## Slide 16

**Notes:**
Looking upstream at outside bank closer to the bridge.

This is the most informative picture of the group.
Note edge of riprap and scalloped area beyond.  
Note leaning trees and debris.
If riprap is undermined the entire channel could move 50-60’ north towards the bridge abutment.

## Five bendway weirs were used due to their low profile and the ability to redirect flow in the channel.  

- Five bendway weirs were used due to their low profile and the ability to redirect flow in the channel.

- The study area was modeled using two dimensional modeling software from FHWA and Brigham Young University.
- Topography and bridge characteristics provided by field survey.
- Manning’s values from field inspection and engineers judgement.

**Notes:**
Developed 2D model using FESWMS and SMS.
2D models are based on a series of 3D points that describe topography.  After all data is entered the software calculates water elevations and unit velocities at each point.
Explain graphic.
Points and bridge location from a field survey.  Manning’s data entered based on field inspection.

## Slide 18

**Notes:**
Where you got the slide.
Quick bendway weir synopsis.
Project into the channel angled upstream.
Weirs are at or near the mean low water elevation.
When higher water passes over the weirs they redirect flow at a 90° upstream.  

## Weir Design Characteristics: Hydrology

**Notes:**
Hydrology- based on stage and discharge data from Corps gage station since 1941.
Plot shows stage data over time for nine years as well as design values.
Point out bank elevation and discuss how it makes it hard to build.  Out of bank multiple times yearly.  Bankfull event is design flood.  Requires large key section into floodplain.
Point out weir design elevation.  Underwater most of the year.

## Weir Design Characteristics: Location, Spacing, and Angle

- Design guidance in HEC-23.  Modified based on information from Derrick, LaGrone, and Whittler.  Also modified based on velocity vectors from FESWMS model.

- Angles vary from 60° in the center of the bend, to 80° at the edge of the weir field.

**Notes:**
Weir design based on guidance from HEC-23, publications by Dave Derrick and WES, and interviews with state and federal agencies which have built them.  Modified slightly as required based on velocity vectors.  All design values fell within the recommended ranges.

This is the topography of our solution model.  Angles vary from 60° to 80°.  Weirs inside the bend are angled more upstream than those near the end of the bend.

## Naming Conventions

- Velocity vector
- Angle of attack
- Angle of departure
- Degree of turning

**Notes:**
Velocity Vector
Angle of Attack
Angle of Departure
Degree of Turning
Expansion angle

## Model Results: Angle of Departure

- Angle of departure varies depending on where along the face of the weir it is measured.  At all five weirs velocity is lower at the tip of the weir.
- The higher angles of attack result in higher angles of departure.

**Notes:**
Angle of departure varies based on
Angle of attack
Where along the weir you measure it.
Angles get higher (closer to 90) as you move along the downstream edge of the weir closer to the bank
Due to velocity being higher at tip of weir.

## Model Results: Angle of Departure

- Degree of turning is a measure of the efficiency of the weir.  It is a measure of the difference between angle of attack and angle of departure.

- Lower velocities near the river bank mean that the weir is more efficient near the bank than at the tip.

**Notes:**
Degree of turning is the difference between angle of attack and angle of departure.

Measure of efficiency of the weir.

Weirs are more efficient closer to the bank.  Something to consider when deciding how long to make the weir.

## Model Limitations

- Floodplain is not modeled.

- FESWMS is a two dimensional model.  This is a three dimensional situation.

**Notes:**
Model limitations
I feel like the results are useful, but you should keep in mind that FESWMS is a 2D model.  We are using it in a situation with 3D effects.   This is especially important when considering the way velocity vectors don’t change as depth increases.
Floodplain was not modeled.
Point out size of floodplain on photo.  Too big too adequately survey.

## Project Summary & Current Status

- System of five weirs built of riprap and rubble stone.
- Permanent maintenance road.
- Permanent cross-section markers for later reference.

**Notes:**
Summary
5 weirs built of large riprap.  
Design values met the ranges previously set forth
Permanent maintenance road for ease of construction in wetlands and for later maintenance and monitoring.
Permanent field monuments to be used as reference sections.  So that we can follow the change in the channel.

## TDOT’s model is reasonably consistent with earlier work on bendway weirs.

- Design guidance provided by LaGrone, and in Hydraulic Engineering Circular-23 is accurate within limitations of model.
- Weir is more efficient nearer to the bank than at tip due to lower velocities.