1. Floodplain Management SESSION 6
Stream Systems on Dynamic Earth
Stream Mechanics
Prepared By
Donald R. Reichmuth, PhD.
Title Page for Session 6

2. Stream Mechanics Framework
Objectives:
1 How floodplains first evolved after glacial retreat.
2 Basic concepts of sediment transport.
3 Stream patterns develop in bedrock areas and in alluvial valleys.
4 Streams are shaped by peak flows, stream gradient and sediment supply.
5 Gravity systems transport water and sediment.
6 Secondary currents and stream features that are formed by their action.
7 In-channel obstructions cause random energy dissipation.
8 Stream mechanics within the context case study floodplains
Objectives

3. Post Glacial Retreat Starting 15,000 Ago
River Systems Reorganize
Glacial Sediment Sources Diminish
Sea Level Raises 400 Ft.
Climate Warms & Becomes Dryer
Isostatic Rebound Begins
Homo Sapiens Colonize The Americas
Megafauna Become Extinct
Post Glacial Retreat –Starting at the end of the Last Glacial Maximum about 15K years ago, river systems began a period of reorganization. Sediment sources diminished; sea level rose; climate changed; human arrived and much of the megafauna became extinct. Once the weight of the ice was removed, isostatic rebound began which altered stream gradients.

4. Ohio River Ice Marginal Formation Utilizes Preglacial Stream Sections
Ohio River -- The Ohio River is an example of a stream that joined together sections of preglacial drainages to form a postglacial system. The new river truncated a major portion of the preglacial Teays river basin.
Diagram used with the permission of Geomax, P.C., Spokane, WA

5. Long Term Regional Trends Post-Glacial
Above 400 Ft. Elev.
Sediment Deficient
Downcutting Prevails
20 To 400 Ft. Elev.
Influenced By Sea Level Change
Valley Filling Dominant
Below 20 Ft. Elev.
Tectonic Behavior Controls
Channels Impacted By Sinking Or Uplift
Long Term Regional Trends – Higher elevation sections of stream systems usually showed a tendency to downcut because higher reaches became sediment deficient. Reaches at lower elevations often were affected by raising sea level which caused deposition to dominate. Those areas only slightly above sea level showed mixed behavior because tectonic conditions were more important.

6. Channel Evolution
Channel Evolution – Many preglacial valleys had fully excavated old fill from previous glacial periods and were graded to sea level base elevations. Additionally, sea level started to drop as glaciation began which caused streams to tend to downcut further. When the glaciers became fully active, large quantities of glacial outwash (boulders to silt sizes) began to enter the stream systems. Deposition then began to fill the valleys with glacially derived sediment. Often side drainages were blocked and lakes formed. After glaciation ended, sediments supplies were greatly reduced and the streams often began downcutting.
Diagram used with the permission of Geomax, P.C., Spokane, WA

7. Blocked Valleys Example
Blocked Valleys Example – The Finger Lakes in northern New York are examples of blocked river valleys that formed lakes.

8. Post Glacial Conditions North Central United States
Post Glacial Conditions – The North Central U.S. area is generally quite flat. Following glaciation, the area was covered with large area of swampy land. Keystone megafauna including mammoths, mastodons and giant beaver had a significant influence on the landscape.

9. Paleo-Floodplain Development Impacts of Extinct Keystone Species
Paleo-Floodplain Development – The large size and strength of the now extinct Keystone species allowed them to modify the vegetative cover. Currently, elephants in Africa have a similar impact of the vegetative cover in their range.

10. Paleo-Indian Hunters Attacking A Huge Bison latifrons With The Help of Domesticated Dogs
Paleo-Indian Hunters upon their arrival in North America hunted the megafauna. Most archeologists now agree the this hunting pressure was the primary cause for many megafauna extinctions.

11. Giant Beaver (Castoroides ohioensis) Became Extinct 10,000 Years Ago Estimated Weight kg. Estimated Length meters Size Relative To Modern Beavers Shown
Giant Beaver -- These large marshland rodents apparently had a major impact of stream development but they too became extinct about 10K years ago.

12. Giant Beaver Fossil Sites (Sites Marked By Red Dots)
Giant Beaver Fossil Sites – Fossil remains of giant beaver have been found throughout eastern U.S. Apparently, they favored less mountainous terrain that could be converted to larger wetland complexes.

13. Pre-Columbus Floodplain Development Floodplain Stabilization Efforts Of Beaver
Pre-Columbus Floodplain Development – Modern beaver replaced the megafauna that became extinct and became the dominate species that operated in floodplains.

14. Historic Beaver Range Prior To Widespread Trapping
Historic Beaver Range – Prior to widespread trapping, beaver ranged across virtually the entire U.S. and Canada except some areas in Nevada and southern California that were too dry.

15. Post Glacial Model Beaver Stabilized Drainage
Post Glacial Model – Beaver rapidly colonized drainages once the sediment supply slowed enough for riparian vegetation to begin to cover the floodplains. Photo used with the permission of Geomax, P.C., Spokane, WA

16. Beaver Stabilized Drainage
Beaver Stabilized Drainage – Beaver stabilized drainages often have the entire floodplain covered with valley wall-to-valley wall with beaver dams and wetlands. Large quantities of water are stored during spring snow melt and released slowly during the summer. The high water peaks are significantly lowered and base flows are increased.
Photo used with the permission of Geomax, P.C., Spokane, WA

17. Beaver Lost Active Vertical & Horizontal Erosion Begins
Beaver Lost – When beaver are removed from a drainage, the stream then becomes unrestrained. Both vertical and horizontal occurs as the stream attempts to develop sufficient frictional resistance to dissipate the available potential energy contained in the system. Photo used with the permission of Geomax, P.C., Spokane, WA

18. Lost Base Flow Storage
Lost Base Flow Storage – Once a stream downcuts into its alluvial valley floor, groundwater storage is decreased. The stream will then become more flashy and have higher peak flow and lower base flow.
Photo used with the permission of Geomax, P.C., Spokane, WA

19. Effects Of Beaver Introduction Tierra del Fuego, Argentina
Effects Of Beaver Introduction – When beaver are introduced into an area that has a sufficient food supply they will begin building dams and creating wetlands. Beaver were introduced into Tierra del Fuego, Argentina in There was ample food and no natural predators. The beaver population expanded rapidly and caused much of the wooded valley floors to be flooded. This flooding has killed large numbers of trees.

20. Basic Concepts
Basic Concepts – Sediment is carried by stream in a number of ways. Material can be carried as dissolved solids; suspended solids or bedload. The larger material (usually bedload and the coarser suspended load are carried in the water closer to the stream bed. It is useful to in River Mechanics discussions to differentate this sediment ladened bottom water (Bottomwater) from the cleaner water in the upper parts of the water column (Topwater). The coarsest available particles often cover the bed and form a Bed Armor that protects the finer underlying material. Diagram used with the permission of Geomax, P.C., Spokane, WA

21. Stream Patterns – Streams can be broken into two categories: bedrock controlled and alluvial. Drainage patterns on bedrock controlled streams tend to follow weaknesses in the bedrock. Streams on alluvial fill tend to develop a pattern that is controlled by the peak flows; sediment loading and stream grade that exists. Diagram used with the permission of Geomax, P.C., Spokane, WA

22. Lower Mississippi River System Aggrading Levees Widespread Local Dams Stop Sediment Movement Channel Change Likely
Lower Mississippi River – The Lower Mississippi River Basin is dominated by the Mississippi Delta and the Atchafalaya River Sub-basin. The river, about 150 miles above its mouth is aggrading but the Delta is being destroyed because it is sediment starved. The Mississippi River is now trying to change course and reach the Gulf using the Atchafalaya River channel. Diagram used with the permission of Geomax, P.C., Spokane, WA

23. Mississippi Valley Cross-Section
Mississippi Valley Cross-Section (16K) – (See PP6.3-2 for Cross-section location). The Mississippi River was entrenched in a bedrock canyon 16K years ago. At this time, the ocean base level was 400 feet lower than today’s sea level.

24. Mississippi Valley Cross-Section
Mississippi Valley Cross-Section (9K) -- (See PP6.3-2 for Cross-section location). By 9K years ago, sea level had risen to only 100 feet below current sea level. The raise caused the Mississippi River to flatten grade and fill its earlier trench with gravel.

25. Mississippi Valley Cross-Section
Mississippi Valley Cross-Section (4K) -- By 4K years ago, sea level had reached its present level. The Mississippi River became more meandering and only slowly aggraded. Most of the newer buildup consisted of silty levee deposits and finer grained backswamp deposits.

26. Mississippi River Delta Evolution Future Shift To The Atchafalaya River Basin Likely
Mississippi River Delta Evolution – During the last 3K years, the Mississippi River has shifted its position on the Delta a number of times. It is now trying to again shift into the Atchafalaya River channel.

27. Stream Characteristics – Stream patterns give diagnostic clues concerning stream stability and behavior over time. The vertical movement of the system is most critical. Stream systems can be classified into four categories depending on their vertical behavior: Erosional (Degrading); Stable Vertically; Transitional (Slight Filling) and Depositional (Extensive Filling). If Peak flows increase; Stream gradient increases and/or Sediment supply decreases the stream will shift to a more Erosional pattern. If the opposite happens, the stream will shift to a more Depositional pattern.
Diagram used with the permission of Geomax, P.C., Spokane, WA

28. Erosional Streams – Erosional streams normally are structurally controlled; occur in “V” shaped valleys and are downcutting. Usually erosional streams are source areas for sediment. Diagram used with the permission of Geomax, P.C., Spokane, WA

29. Erosional Streams (Structure) – Erosional streams tend to follow underlying weaknesses in the bedrock. These weaknesses can be joints, faults, changes in rock type or any other condition that creates strength and/or erodibility differences.
Diagram used with the permission of Geomax, P.C., Spokane, WA

30. Stair-Step Profile – When erosional streams flow through reaches that are so steep that a stable grade cannot form, a stair-step profile develops. Steps are constructed out of any material available in the streams including boulders, trees and rock outcrops. A stable grade normally develops between steps and the system’s excess energy is dissipated at the steps.
Diagram used with the permission of Geomax, P.C., Spokane, WA

31. Meandering Streams – Streams that are vertically stable normally develop meanderings patterns. Valleys usually are wide and flat. The meander loops shift back and forth across the floodplain as the system transports the sediment load downstream that enters the valley. Meandering always flow in alluvial valleys, tend to armor their beds and create slack water deposits on the floodplain. Diagram used with the permission of Geomax, P.C., Spokane, WA

32. Note: Flip-Flopping Cyclic 20-40 Yr. Cycle Typical
Transitional Streams – When conditions in a stream valley change sufficiently to cause a vertically-stable meandering streams to start to shift to becoming a depositional stream, a transition in the stream pattern must take place. This transition zone usually consists of a series of double channels that are separated by stable, vegetated islands. The system will have a dominant channel and the other channel will be mostly used to carry water during peak flow periods. Because the dominant channel carries most of the bottomwater and the other channel carries mostly topwater, the erosion and deposition potential of the two channel will be different. The dominant channel will tend to fill while the other channel will tend to erode. These tendencies continue until a critical point is reached and the channels switch dominance and the cycle begins again.
Diagram used with the permission of Geomax, P.C., Spokane, WA

33. Common Causes For Deposition
Common Causes For Deposition – Normally there are three causes that will cause a stream to become depositional. They are decreased peak flow (especially is topwater is removed from the stream); decreased gradient (less energy) and/or increased sediment supply.
Diagram used with the permission of Geomax, P.C., Spokane, WA

34. Other Depositional Patterns
Other Depositional Patterns (Fans) – Alluvial fans are a depositional landform. Sediment generated in smaller side drainages or from other causes such as landslides often is temporarily deposited on larger valley floors as fan shaped deposits. Flood channels shift across the fans over time and distribute the sediment over the entire fan surface. Generally, there is no flood-free location on fans because the stream channel can easily shift locations to any location on the fan. The main valley stream is often crowded to the far side of the valley by the encroaching fan.
Diagram used with the permission of Geomax, P.C., Spokane, WA

35. Typical Fan Utilization
Typical Fan Utilization -- Alluvial fans are quite common throughout the more arid areas in the Western United States. This diagram depicts typical alluvial fan utilization in this region. Water is usually diverted out on fields from diversions that are located near the Fan Apex. Because alluvial fans are depositional features, they usually are quite permeable and considerable amounts of the irrigation water sinks into the ground. This water then tends to emerge at a spring line at the base of the fan. Flood channels are not well defined and often change paths unexpectedly. Typically in the past, the most dominant flood paths are undeveloped and often considered waste land by the local agricultural community. When land use changes from agricultural to residential and/or recreational development, these dominant flood paths often become highly sought after land and with proper control can become the jewel of the community. Diagram used with the permission of Geomax, P.C., Spokane, WA

36. Alluvial Fan Case Study
Alluvial Fan Case Study -- The Teton Basin, southwest of Yellowstone Park contains a number of coalesced alluvial fans along the eastern side of the basin. The area is semiarid and very active geologically. The Hot Spot activity centered in Yellowstone Park is causing the entire region to tilt to the south. Additionally, this area is part of the Basin & Range and is being pulled apart in an east-west direction. This pull-apart action is causing the mountain range east of the Basin to tilt to the west. Teton Creek is one of the larger creeks that flows westward off the mountain range. The surface drainage area is shown on the diagram. It has formed an alluvial fan that has its apex at the mountain front. Because of the southerly tilting, the stream has a tendency to shift to the southern part of the fan. The southern tilting also has caused Teton River, the primary stream draining the basin, to have a very low gradient. The Teton Dam Failure occurred on this River near where the stream leaves the drawing shown above. This flat gradient has caused extensive wetlands to form at the southern end of the Basin at the base of the Teton Creek Alluvial Fan.
Diagram used with the permission of Geomax, P.C., Spokane, WA

37. Teton Creek Schematic Diagram -- Teton Creek is approximately 22 miles long (13 miles in the mountains; 6 miles across the alluvial fan; and 3 miles through the wetlands below the Spring Line). The mountainous reach flows through glaciated valleys. Once the stream reaches the mountain front, the alluvial fan starts. This point is called the “Topographic Apex”. Since the last glaciation, the creek has cut through the glacial deposits that occur at the mountain front and become incised. Farther downstream, the stream becomes less incised and the currently active alluvial fan begins. This point is called the “Hydrographic Apex”. Once the creek reaches the alluvial fan Spring Line (wetlands), it totally changes character and becomes a single channel meandering stream. Floodplain and Floodway delineation on this short creek must use different models for each section mentioned above to fully account for the different behavior that is expected.
Diagram used with the permission of Geomax, P.C., Spokane, WA

38. Standard FEMA Model -- The Standard FEMA Model is based on having a dominant single channel although it is frequently used where multiple (but stable) channels occur. The dominant channel plus adjoining overbank areas, defined as the Floodway, are supposed to have sufficient conveyance to handle the Basic Design Flood. This Basic Design Flood normally is the “100 Year Flood” (a flood having a 1% change of occurring in any year). Encroachment is then allowed on the floodplain outside of the Floodway boundaries.
On Teton Creek this model will work reasonably well for all reaches except the reach containing the active alluvial fan (i.e. between the Hydrographic Apex and the Spring Line). Unfortunately, when the Standard FEMA Model is applied to active alluvial fans poor results often occur. This is because there is rarely a stable dominant channel crossing the alluvial fan. Even minor channel blockage, such as could be caused by a log jam can cause major shifting of the active channel’s location.

39. Alluvial Fan Floodplain Model
Alluvial Fan Floodplain Model -- FEMA has prepared a booklet titled “Guidelines for Determining Flood Hazards on Alluvial Fans”. This booklet provides a methodology for determining flood risk on alluvial fans. On Teton Creek, this model should be used on the active alluvial fan reach.

40. Fan Apex Area Details
Fan Apex Area Details -- On Teton Creek the Hydrographic Apex occurs near the Idaho-Wyoming border. The Standard Model will provide the best results through the incised section but the model should be switched to the Alluvial Fan Model near the state line. Unfortunately, the Standard Model was used for the entire creek. Currently, the area is experiencing considerable growth due to its proximity to Grand Teton National Park. Land along Teton Creek has become quite expensive and considerable building is taking place near the Creek. Relying on the wrong model on the active alluvial fan has created the potential for serious loss when major floods occur.
Diagram used with the permission of Geomax, P.C., Spokane, WA

41. Typical Flood Channel
Typical Flood Channel -- This photo shows a typical, currently inactive channel on the Teton Creek alluvial fan. During a flooding event, the currently dominant channel could be blocked upstream and the creek could jump to this channel (or others on the floodplain in this area). The currently active floodplain across the alluvial fan is about 1000 feet wide and the expected “100 Year Flood” is less than 2400 cubic feet per second. If 800 feet of the floodplain width was available to convey the flow, the average flow velocities could be as low as 3 feet per second and the average water depth could be about 1 foot. If enlightened building controls were put in place, only minor damage would occur at these expected flow velocities and water depths. It is possible to live on this alluvial fan without serious damage occurring during a major flood but road and house locations must be carefully sited.
Photo used with the permission of Geomax, P.C., Spokane, WA

42. Unique Alluvial Fan Flood Hazards
Unique Alluvial Fan Flood Hazards -- Flood Hazards on alluvial fans have three components that must be addressed if development is to occur in these areas. First allowance must be made to accommodate the uncertainty of not knowing where the water will flow. Second, abrupt depositions and erosions can occur almost any where on the floodplain. Finally, it is not easy to just add fill to raise the structures above some design flood elevation and expect that the structure will not experience the effects of flowing water.

43. Model Comparison
Model Comparison -- This diagram compares Riverine and Alluvial Fan flood hazards. Two sites (A and B) are compared to determine the relative flood hazard of each location. Site A in on the floodplain well away from the active channel (historical flow path). Site B is located within the Floodway (historical flow path). The Riverine Model correctly predicts that Site B is clearly more hazardous than Site A. Because channel blockages easily occur on alluvial fans, water can be diverted to almost anywhere on the active alluvial fan. Therefore, Sites A and B are equally hazardous on alluvial fans.

44. Other Depositional Patterns
Other Depositional Patterns (Fans) – Alluvial fans are a depositional landform. Sediment generated in smaller side drainages or from other causes such as landslides often is temporarily deposited on larger valley floors as fan shaped deposits. Flood channels shift across the fans over time and distribute the sediment over the entire fan surface. Generally, there is no flood-free location on fans because the stream channel can easily shift locations to any location on the fan. In the main valley stream is often crowded to the far side of the valley by the encroaching fan.
Diagram used with the permission of Geomax, P.C., Spokane, WA

45. Other Depositional Patterns
Other Depositional Patterns (Lateral Replacement) -- Occasionally, a stream that is carrying a coarse bedload will pass through an area that contains finer, easily eroded formations. When this occurs a process called lateral replacement can take place. The coarser gravel is deposited on the floodplain and an equivalent space (NOT volume) is created for the channel conveyance. Often, considerably more finer material is mobilized than is deposited. This possible because it is much easier for the stream to transport the finer material that was mobilized than to transport the coarser bedload.
Diagram used with the permission of Geomax, P.C., Spokane, WA

46. Stream Flow Analysis 1st Level – Flow Downhill
Gravity Controls
2nd Level – Secondary Currents
Bed Friction Controls
3rd Level – Random Eddies
Obstructions Control
Stream Flow Analysis – Stream flow patterns are more easily analyzed when the flow components ar broken into three levels of behavior. These levels are flow downhill; secondary currents and random eddies.

47. Gravity Systems Potential Energy (Height) Converts To Kinetic Energy (Velocity) Or Friction Loss
Gravity Systems – Gravity causes all objects (including water) to fall toward the center of earth’s mass. The height of the object or water above some reference point (datum) determines that object’s Potential Energy. When the object is allowed to free fall (or flow in the case of water) without frictional resistance, the Potential Energy is converted to Kinetic Energy. If there is frictional loss in the system, some of the Potential Energy is used up and the Kinetic Energy will be less.
Diagram used with the permission of Geomax, P.C., Spokane, WA

48. Designer’s Toolkit
Designer’s Toolkit – In river systems, friction loss producing components must be added to the channel to keep the kinetic energy buildup from becoming excessive. Typical friction loss producing components include: increasing channel length; varying cross-section size; creating bank friction; installing in-channel obstructions and installing instream drops.
Diagram used with the permission of Geomax, P.C., Spokane, WA

49. Flow Velocity Relationships
Velocity Relationships – Sediment transport is highly sensitive to local flow velocity (turbulence). For example increasing flow velocity from 4.5 feet per sec. to 9 feet per sec. will increase the water’s transport potential from a 1 lb. rock to a 90 lb. rock.
Diagram used with the permission of Geomax, P.C., Spokane, WA

50. Channel Features Determined By Secondary Currents
Vertically Stable (Meandering) Streams:
Meander Shape
Thread (Thalweg) Location
Erosion/Deposition Pattern
Bar Shape
Mid-Channel Undertow
Depositional (Braided) Streams:
Mid-Channel Island Formation
Channel Features – Many channel features are created by secondary currents. In meandering streams these features include meander shape; thread (thalweg) location; erosion/deposition patterns; bar shape and mid-channel undertow. In braided streams the location and formation of mid-channel islands is controlled by secondary currents.

51. Bed Friction Controls
Bed Friction Controls -- Secondary current in streams is driven primarily by bed (and bank) friction. Water in contact with the bed and banks is slowed more the water on the top free surface. This velocity causes a helical flow pattern to develop within the water mass. The bed flow pattern that develops creates a continuous zone within the channel where bed flow sweeps away in both direction. Because this zone is swept by cleaner down-dwelling topwater deposition is restricted and this zone becomes the deepest part of the channel. This zone is called the thread or thalweg.
Diagram used with the permission of Geomax, P.C., Spokane, WA

52. Channel Features Active
Channel Features Active – During bank full flow, there is one continuous pair of secondary flow cells (Continuous Thread Cells) and another set of discontinuous flow cells (Bar Cells). These bar cells start and stop as flow overtops the bars and moves across their surface.
Diagram used with the permission of Geomax, P.C., Spokane, WA

53. Channel Features Stable
Channel Features Stable – During normal flow conditions, nothing much but water moves in the channel except fine sand and silt in heavily overloaded streams. This mobile sand and silt creates a mobile bed of dunes that slowly migrate downstream. In meandering streams, gravel bars are exposed where the bar cells operated on the inside of bends. When the system is overloaded with sediment, midchannel islands appear where the secondary currents were updwelling. This action creates the braided stream pattern.
Diagram used with the permission of Geomax, P.C., Spokane, WA

54. Random Eddy Production
Causes:
In-Stream Obstructions
Downed Trees
Rock Outcrops/Boulders
Manmade Structures
In-Channel Flow
Turbulence
Velocity Variations
Random Eddy Production – The third level of flow within a stream is random eddies. These eddies are caused by in-stream obstructions and in-channel flow variations that cause local turbulence to develop.

55. Random Energy Dissipation Controlled By Site Specific Conditions
Random Energy Dissipation – Local site conditions within streams can be quite variable. In-stream obstructions and channel cross-section changes all cause significant friction losses to develop. These random elements help dissipate potential energy so kinetic energy (flow velocity) does not exceed stable values.
Diagram used with the permission of Geomax, P.C., Spokane, WA

56. Slide Presentation Prepared By Geomax, P. C. Dr. Donald R
Slide Presentation Prepared By Geomax, P.C. Dr. Donald R. Reichmuth, President 1023 W. 30th Ave. Spokane, WA Phone & FAX – –
This slide set for Session 6 was last edited on 12/18/04.