# Presentation: 457604

## Floodplain ManagementSESSION 7

**Stream Systems on Dynamic Earth**
**Floodplain Management **
**Principles** **& Practice**
**Prepared By**
**Donald R. Reichmuth, PhD.**

**Notes:**
Title Page for Session 7

## Management Framework

**Objectives:**
**1****	****Stream sediment cycle in terms of source, transport and deposition.**
**2****	****Difference between the engineering and geologic approaches in developing analytical tools for stream management.**
**3****	****Channel blockages and overall stream stability.**
**4****	****Behavior and stabilization methods used in drainages showing vertical instability.**
**5****	****Problems associated with horizontal stream channel migration. **
**6****	****Management practices to determine how to insure low impact development. **
**7  ****	****Present floodplain as a product of the geological floodplain -- case study exercises **
**8 ****	****Module 2 examination.**

**Notes:**
Management Framework – This slide lays out the objectives to be presented in this session.  The instructor should tell the students what they are going to hear; then present the information and finally summarize what was said.

## Elements Of Channel Formation

**Notes:**
Elements of Channel Formation – One key function of streams is to move the products of erosion to lower, downstream locations.  Erosion begins in a Source area that can involve eroding bedrock or reactivating earlier transported material.  Sediment from the Source is then Transported through the stream system  The channel that is formed during this Transportation process will reflect the material balance that exists.  When the system has no net loss or gain of sediment (this condition is often referred to as being “In regime”), the channel will usually be relatively stable vertically but can experience considerable lateral shifting.  This lateral shifting is most common in meandering streams.  Finally, the sediment must be deposited somewhere within the system.  This deposition often occurs just upstream of a site where Base Level Control occurs.  Normally, braiding and multiple channels form in the depositional area and the stream has unstable channels.
Diagram used with the permission of Geomax, P.C., Spokane, WA

## Analytical Methods

**Engineering Approach**
**	****	****Initiated By Irrigation Designers**
**	****	****Mathematical Approximations**
**	****	****Equations Have Limited Utility**
**Geologic Approach**
**	****	****Initiated By Field Explorers**
**	****	****Descriptive Physical Principals**
**	****	****Universal Concepts Developed**

**Notes:**
Analytical Methods – There are two basic approaches that have been used to analyze stream systems.  The Engineering Approach was initiated by irrigation system designers who developed mathematical approximations that helped predict the future behavior of irrigation systems.  These predictions mainly involved flow volumes and sediment transport/deposition.
The other approach, Geologic Approach, was initiated by field explorers who observed natural stream systems.  They then prepared descriptions of the physical principals observed and developed universal concepts to explain the phenomenon that occurred.
There is a fundamental difference between a natural stream system and an irrigation system.  An irrigation system generally attempts to move water to areas as high on the landscape as possible.  This implies that there is minimum energy loss during the water transport process, which typically leads to uniform cross sections with minimum wetted perimeters.
Natural systems, on the other hand, attempt to get the water downhill as directly as possible while still maintaining a stable channel.  These systems require sufficient friction losses to be incorporated into the channel configuration to dissipate all of the excess fall energy.  This leads to a channel configuration that contains meanders, pools, riffles and other instream obstructions.
Unfortunately, attempts to stabilize natural stream systems often fail because this fundamental difference is not understood.

## Energy EquationsNote:Extra Friction Loss For Expanding Sections

**Notes:**
Energy Equations – The basic Bernoulli Equation is the correct relationship that relates potential energy, kinetic energy and friction loss for water flowing downhill under the influence of gravity.  It is vital to observe that energy is neither created nor destroyed in the flow process and that energy can only be taken out of the system by doing work within the channel.  This work can take a number of forms.  Turbulence, heat generation and sediment movement are typical examples of friction loss/work done within the channel.
When the channel has a Variable Section a specific and significant energy loss occurs.  This friction loss is primarily caused by turbulent conditions that develop within the channel.  The above equation is simply a form of the Bernoulli Equation that has been modified by splitting the friction loss term into two parts.  The energy loss specifically associated with the change in cross sectional area has been separated from the other frictional losses and expressed as a separate term.  This friction loss is approximated by taking the difference in velocity head between the two cross sections and multiplying this difference by a constant (K).  Note:  This type of loss only occurs when the cross section enlarges.  There is no loss associated with a contracting cross sectional area but the velocity head normally will increase between the two cross sections that are analyzed. 
Diagram used with the permission of Geomax, P.C., Spokane, WA

## Slide 6

**Notes:**
Manning Equation – This experimentally derived equation was first published by Robert Manning in 1889.  It was based on results obtained in the laboratory using rectangular flumes as shown above.  The number 1.486 incorrectly implies 4 place accuracy for the equation but it is the number needed to convert the value (n) from metric units to English units so that (n) remains the same value in both systems of units. An excellent account of the derivation of this equation is given in “Open-Channel Hydraulics”, 1959 by Ven Te Chow starting on page 98.
The numbers given for the three Case Studies shown above are based on a rigorous error analysis of the equation when various errors are assumed for the equation’s input values.  Even in these somewhat ideal conditions the error in the answer varies between 10 and 20 percent.  In a natural stream system it is expected that the error in the calculated flow (Q) would be much larger because it would be much harder to measure accurate input values.  Even more important, is the observation that the error in (Q) is most influenced by errors in the determination of the values used for (n) and the energy line slope (S).  Errors in the physical channel cross section measurements (W and H) only make small contributions to the error in (Q).
FEMA’s floodplain mapping program that uses some version of the computer program (HEC2) relies heavily on determining floodplain and channel cross sections.  Usually (S) is not measured directly but is averaged from the cross section data.  The value for (n) is then “adjusted” to make water surface elevations match field conditions.  It is possible that considerable money has be spent unnecessarily determining detailed cross sections.
Diagram used with the permission of Geomax, P.C., Spokane, WA

## Stream Problem Types

**Channel Blockage**
**	****	****	****In-channel Obstructions**
**	****	****	****Geologic Accidents**
**	****	****	****Impoundments**
**Vertical Change**
**	****	****	****Headcutting**
**	****	****	****Channel Aggradation**
**Horizontal Change**
**	****	****	****Meander Loop Migration**
**	****	****	****Bank Erosion**
**	****	****	****Channel Cutoffs**
**	****	**

**Notes:**
Stream Problem Types --  Normally stream instability problems can be classified into one of three categories.  1. There are channel blockages that result from in-channel obstructions or geologic accidents such as landslides or other types of impoundments.  2. There are vertical changes in the stream profile that are caused by either headcutting or in-channel deposition.  Finally, 3. there is horizontal channel migration caused by meander loop migration.  This migration often leads to bank erosion and channel cutoffs.

## Channel Blockages

**Blockages can vary in size**
**from a single downed tree**
**to total channel filling.**

**Notes:**
Channel Blockages – Channel blockages come in all sizes from single downed trees to complete blockage of the channel by events such as landslides.

## Effects Of In-stream Obstructions

**Notes:**
Effects of In-stream Obstructions --  Obstructions that cause partial channel blockage usually cause predictable changes in the flow pattern and associated channel modification.  When the obstruction is submerged below the water surface, the downstream face of the obstruction controls how the flow pattern will change.  Flow is turned perpendicular to the downstream face of the obstruction.  In the example shown above, the flow is turned into the bank and increased bank erosion would likely occur.
When the obstruction extends above the water surface (especially during peak flow), turbulence is created as flow is accelerated around the obstruction.  Often this turbulence causes local bed scour to occur adjacent to the obstruction.  An example of such an obstruction would be a midchannel bridge pier.
Diagram used with the permission of Geomax, P.C., Spokane, WA

## Slide 10

**Notes:**
Mount Rainier – On rare occasions channels can be completely obliterated during geologic events such as mudflows.  Streams in geologically active areas, like those near Mount Rainier shown above, can be totally filled by mudflow debris. Prepared by the USGS

## Debris Flow ExampleAmero, Colombia -- 1985

**Notes:**
Debris Flow Example – This aerial photo of Amero, Colombia, was taken in 1985 after the volcanic eruption on Nevado del Ruiz caused a debris flow to pass through the town.  Many buildings were destroyed and 23,000 lives were lost in the disaster.  
USGS Photo

## Regional Uplift Erosion PatternsNote: Colorado River Maintained Original Grade

**Notes:**
Regional Uplift Erosion Patterns – Streams have tremendous erosive power that allows the stream grade to be maintained when tectonic uplift occurs.  The Grand Canyon on the Colorado River is an excellent example of a river maintaining its grade during regional tectonic uplift. One might be tempted to believe that the Colorado River “cut down” creating the Grand Canyon; but what actually happened was that the river maintained its grade as the region was uplifted.
Diagram used with the permission of Geomax, P.C., Spokane, WA

## Cross Section ViewNote: Grand Canyon Formed At TopOf Structural Warping

**Notes:**
Cross-Section View – The Grand Canyon formed along the axis of an anticline that formed during the regional uplift.  Streams often favor anticlinal locations because the bending process of forming the anticline tends to weaken the rock along the anticlinal axis due to the stretching and fracturing that occurs.  The weakened rocks are then more easily eroded and the stream can more easily maintain its grade in these locations.
Diagram used with the permission of Geomax, P.C., Spokane, WA

## FromCanyons of the ColoradoByJ. W. Powell1895

**Notes:**
Canyons of the Colorado – ( See Photo Point “A” on PP7.3-5 Regional Uplift Erosion Patterns for location and orientation).  In arid regions, erosion is most active in over-steepened locations.  These erosive forces often strip away sedimentary formations one layer at a time because the edge of the formation is usually the most steepened section of the formation. As the layers are eroded away an “onion peel” topography develops as is seen in the above drawing.
Drawing taken from J.W. Powell, 1895.

## ErosionPatternsNote:Stream GradientMaintainedDuring Tectonic Uplift

**Notes:**
Erosion Patterns --  The topography that develops in sedimentary rocks as formations are stripped away depends on whether the beds are flat lying or dipping.  When the sedimentary rocks have a low to moderate dip, the stream tends to migrate laterally and undercut the over-steepened edges of the formations.  By attacking the cliff face (scarp slope) the stream does over time lower its grade.
When the beds are lying flat, vertical incision occurs on the main stream channel.  Then secondary gullies tend to headcut laterally away from the main stem.  Each formation is then stripped away “onion peel” fashion. 
Diagram used with the permission of Geomax, P.C., Spokane, WA

## Effects Of Dams

**Notes:**
Effects of Dams --  When a dam is placed on a stream, the sediment transport capability of the stream is disrupted.  The dam becomes a local base level control.  As the water slows at the head of the dam, any sediment that was being transported tends to be deposited and a delta is formed.
The water released immediately downstream from the dam contains considerably less sediment than it did before the dam was built.  This released flow is often termed “hungry water”  because it usually triggers local erosion of the bed and banks of the channel below the dam.  Most dams also tend to reduce peak flow by storing water during high runoff periods.
Unregulated Side Drainages that enter the dammed stream also carry sediment loads.  Prior to dam construction, these sediment loads normally were flushed down the main stem during peak flows.  When the peak flows are reduced by the dam, the sediment coming from the side drainages cannot be flushed away as efficiently.  When this happens an alluvial fan tends to form at the mouth of the unregulated side drainage.  These alluvial fans then pinch the main stem flow and cause a slackwater area to develop above the fan and rapids to form in the pinched section.
An additional problem develops if the dam is used as a peaking power facility.  Peaking power facilities require the flow from the dam to be rapidly changed to generate power at critical times (usually in the morning at breakfast and in the evening at supper time).  These rapid changes of flow cause excessive bank seepage forces to develop when rapid drawdown occurs.  These seepage forces coming out of the bank often destabilize the banks and sloughing can occur.
Diagram used with the permission of Geomax, P.C., Spokane, WA

## Colorado River HydrographBelow Lake Powell

**Notes:**
Colorado River Hydrograph –  The typical hydrograph for Lake Powell on the Colorado River shows how a dam can change the flow regime.  Because there are unregulated side drainages entering the Colorado River in Grand Canyon below Lake Powell, problems have developed with bed and bank scour, with alluvial fan enlargement and with increased rapid size.  During 1996 and 2004 Controlled High-flow releases were attempted to correct the problems that had developed.  Some temporary improvements that were recorded by the fundamental problem of sediment starvation could not be addressed by simply increasing flow because the released flow was clean “hungry water”.
USGS Diagram

## Clean Water ReleasesFrom Lake Powell -- Nov. 2004

**Notes:**
Clean Water Releases – This photo shows clean water release during the Nov. 2004 experiment.  These releases can only cause additional scour of the bed and banks immediately below the dam.  The temporary benefits that were observed further downstream were a result of transporting sediment from immediately below the dam further down stream and from the higher flows attacking the enlarged alluvial fans at the mouths of the downstream unregulated side drainages.
USGS Photo

## Typical Fish Passage ProblemNote:The Extreme Turbulence At The Base Of The Structure

**Notes:**
Typical Fish Passage Problem –  In many areas low head dams have been installed on rivers to divert water into irrigation systems or to create hydropower projects (a very common occurrence in the eastern U.S. during the Industrial Revolution).  In those areas where migratory fish used the streams, the low head dams became a block to fish passage.  Many newer dams (some older dams were retrofitted) were equipped with fish ladders to aid fish passing over the structures.  Often these dams and fish ladders were made out of steel and concrete.  A typical USBR diversion is shown above.  This particular structure was built in the early 1990’s and is now experiencing scour problems along its base because sediment transport was interrupted.
USBR Photo

## Typical Concrete/ Steel Design

_**Yakima River  -- Town Diversion**_
**	****	****U.S. Bureau of Reclamation Design**
**	****	****Age ----------------------  About 15 Yrs.**
**	****	****Drop Height ----------  About 6 Feet**
**	****	****Fish Ladder ----------  4 Cells (5 Drops) **
**	****	****Repair Costs ---------  $200,000 Est.**
**	****	****	****	****	****	****      ****($130,000 Spent)**
**	****	****Other Problems ----- Apron Undercut**

**Notes:**
Typical Concrete/Steel Design --  Repair of only the undermined fish ladder shown previously is estimated to cost over $200,000.  The structure has other problems because the apron is also being undermined.

## Irrigation Diversion DamTypical Concrete Structure With Fish Ladder

**Notes:**
Irrigation Diversion Dam --  This is a ground photo of the fish ladder and dam shown in the previous slide.  The turbulent flow through the fish ladder and over the dam coupled with the lack of sediment from upstream has caused the fish ladder to be undermined and instability to develop.  Because concrete and steel structures are rigid, there is no way the structure can adapt to the new bed conditions that developed.  Repair then becomes very costly.
Photo used with the permission of Geomax, P.C., Spokane, WA

## Slide 22

**Notes:**
Cedar River Example (near Maple Valley, Washington) – A less costly and more esthetically pleasing retrofit for fish passage on a low head barrier is shown above.  Here large rock was used to create a series of small drops below the barrier.  Because rock structures a flexible, they can accommodate considerably more movement than can a concrete/steel structure.  Additionally, the large rocks  and roughened channel create considerable energy dissipation so flow velocities are reduced.
Photos used with the permission of Geomax, P.C., Spokane, WA

## Improved Safety

**Notes:**
Improved Safety --  This photo is of the same structure shown on the previous slide when the flow was considerably higher.  The lawn area in the background is a public park.  The overall safety of the site has been significantly improved (there were drownings caused by the old structure). Now, the site is even used for a kayak course.
Photo used with the permission of Geomax, P.C., Spokane, WA

## Fish Passage Proof

**Notes:**
Fish Passage Proof – The structure shown on the previous two slides now readily passes fish.
This photo shows salmon spawning immediately above the structure.  The areas that show cleaner gravel are redds (spawning beds).
Photo used with the permission of Geomax, P.C., Spokane, WA

## Vertical Change

**   ****The sediment availability causes most channel filling and degradation.  Many regions are experiencing downcutting as earlier glacial deposits are mobilized.**

**Notes:**
Vertical Change – Vertical changes in channel elevation are usually caused by changes in sediment supply.  In many northern areas that experienced glaciation, there is a long term tendency for channels to lower their grade.  This degradation is caused by the current (as compared to glacial times) decrease in sediment supply.  Many irrigation diversions in the West are now approaching 100 years in age and many are in trouble because the bottom is slowly dropping out of the stream.  Typically, many of the channels are now about one foot lower than they were when the irrigation diversions were originally built.

## Headcuts – Cause & should we do anything?

**Notes:**
Headcuts – Headcuts are little water falls that migrate upstream.  They can be triggered by downcutting due to the lack of sediment and by any activity that increases the local stream gradient.
Diagram used with the permission of Geomax, P.C., Spokane, WA

## Headcut-Caused FailureOpen Arch Culvert --- Bed Scoured To Bedrock

**Notes:**
Headcut-Caused Failure – Headcuts can cause structures like bridge footing and open arch culverts (see above) to be undermined when the headcut passes by the structure.  The stream bed in the photo shown above was originally located at the top of the concrete footings.  Scour has now completely removed all alluvial gravel from the stream bed and bedrock is now showing.
Photo used with the permission of Geomax, P.C., Spokane, WA

## Slide 28

**Notes:**
Erosion Control Treatments --  There are numerous inexpensive ways to control headcutting in small side channels.  Staked-in hay bales and low earth berms can be effective in the areas near the top of the headcuts.  In principal, the idea is to keep the overland flow from concentrating and developing sufficient velocity that erosion can begin.  When the headcuts have already passed through an area, it normally requires a more significant structure to control the problem.  Silt/trash fences, cabled trees and keyed rock drop can be effective in reversing the adverse effects of a headcut.
Diagram used with the permission of Geomax, P.C., Spokane, WA

## Vertical Grade StabilizationExcess Energy Removed With Drops

**Notes:**
Vertical Grade Stabilization --  To be stable, the excess gradient needs to be taken out of the stream system.  One way of removing excess grade is to install grade control structures that dissipate the excess vertical elevation.  The drops need to be designed so that they are stable; cannot be skirted and bring the stream grade back to a stable gradient.
Diagram used with the permission of Geomax, P.C., Spokane, WA

## Rock Drop Characteristics

**Stable**
**Fish Friendly**
- 		**Overhead Cover**
**	****	****Low Velocity Eddy**
**	****	****Macro-Invertebrates**
**Safe For Boaters**
**---------------------------------------------**
**Unstable**
**Poor Habitat**
**Standing Wave Unsafe**

**Notes:**
Rock Drop Characteristics – Rock Drops are normally the most cost effective drops that can be utilized on moderate to large streams.  They are flexible; easily repaired and/or modified and can be installed without dewatering the stream channel.  When skimming (subcritical) flow occurs, the primary current flow passes smoothly over the drop.  A secondary flow cell develops underneath the main flow.  This secondary flow cell provides a number of benefits including stabilizing the base of the drop.  Additionally, the secondary flow cell provides excellent habitat for fish.  This configuration is called a “Low Drop” and typically the water surface differential across the drop is less than two feet during peak flows.
Plunging (Supercritical) Flow creates “High Drops” that can become both unstable and unsafe.  In this case the secondary flow cell forms on top of the primary flow creating a standing wave and an associated hydraulic jump.  Many people have been drowned when caught in this secondary flow cell because they cannot get out (It is kind of like being placed in a water filled rotating barrel).  The high drop causes extreme turbulence to develop at the base of the drop.  The scour caused by this turbulence often undermines the drop.
Diagram used with the permission of Geomax, P.C., Spokane, WA

## Slide 31

**Notes:**
Effects of Structure Alignment --  The alignment of the structure (drop) in the active channel determines how the channel will react.  Because flow tends to be directed perpendicular to the downstream face of the structure, different effects can be achieved by adjusting the horizontal alignment and height of the drop.  Often an upstream pointing “V” shape with a lower center is the most effective, because the “V” wings tend to direct the water toward the center of the channel and away form the banks.  Other configurations as shown above can also be used in special circumstances.
Diagram used with the permission of Geomax, P.C., Spokane, WA

## Rock Drop ExampleLow Flow Concentrated At Mid-Channel

**Notes:**
Rock Drop Example --  This photo shows a large river rock drop. It is an upstream pointing “V” shape and is about one foot lower in the center.  The water surface differential across the drop is about 1.1 feet at this flow.  This differential drop height will stay approximately constant at different flow values.  Note that there are large keys located on each bank that prevent flow from cutting around the drop.  The structure was constructed mostly with large rock (typical average dimension of 3 to 4 feet weighing 8,000 to 12,000 pounds) that have been individually placed as tight together as is physically possible and the remaining voids have been fill with smaller rocks and gravel.  The top of the structure also has been covered with gravel so the large rocks barely show.  This covering improves safety for boaters and helps lock the large rock together.
Photo used with the permission of Geomax, P.C., Spokane, WA

## Slide 33

**Notes:**
Summary --  Rock drops have many advantages over concrete/steel structures.  Boater safety, fish passage and structural stability dictate that skimming flow is essential for properly designed drops.  This means that the water surface differential across the structure must be less than 2.5 feet and 1.5 feet is recommended as the best overall design water surface differential.
Diagram used with the permission of Geomax, P.C., Spokane, WA

## Horizontal Change

**   ****Horizontal channel migration**
**is most common in regions**
**that have low stream gradients**
**and have reached an equilibrium grade.****     **

**Notes:**
Horizontal Change --  Problems with lateral stream migration are most common on systems that have low gradients and that have a stable grade.  It is very common for meandering streams to have laterally shifting bends as the outside of the bend erodes and point bars form on the inside of the bends.  Over time these shifting meanders tend to form a meander belt on the floodplain that, if unconstrained, will cause the stream channel to occupy all areas of the meander belt.

## Slide 35

**Notes:**
Downstream Meander Loop Migration --  Meander loops normally migrate downstream as the outside of the bends erode and point bars form on the inside of the bend.  When the position of the channel is locked as the example above (a bridge on a road crossing the floodplain), the meander loop migration cannot shift downstream unconstrained.  Over time, the meander tends to “fold in” against the upstream edge of the road grade.  The road grade is then subject to erosion and the stream entry to the bridge is distorted.  Because the meander loop migration process is normally fairly slow, there is often a opportunity to “turn the meander lose” when the road and/or the bridge is rebuilt.  It is important to always examine this possibility when updating existing stream crossings.
Diagram used with the permission of Geomax, P.C., Spokane, WA

## Slide 36

**Notes:**
Poor Bridge Entry Problems --  When meandering streams “fold in” on roads that cross the floodplain as shown in the previous slide, a number of problems develop.  The sharp curve that occurs causes the inside of the bend to become depositional.  When this happens, it is common to find a loss of conveyance dropping significantly through the bridge opening.  This loss of conveyance can then cause water levels above the bridge/road crossing to become higher during periods of peak flow.
Additionally, upstream bank erosion often accelerates and the bridge abutment on the outside of the bend is subjected to increased flow turbulence and can be undercut by excessive bed scour.
Diagram used with the permission of Geomax, P.C., Spokane, WA

## Meander Loop CutoffLoss Of Energy DissipationMust Be ReplacedFor Stable Conditions To Exist

**Notes:**
Meander Loop Cutoff --  At times meander loops become so sharp that a cutoff forms that causes the loop to be abandoned.  The friction loss that occurred going around the loop is then lost and the stream has additional available energy to do work.  Often the steeper gradient in the cutoff section triggers a headcut that begins migrating upstream.  The extra sediment generated by the headcutting can then cause the stream to become depositional below the cutoff.  The system then becomes more unstable and increased bank erosion and deposition on point bars accelerates.
Diagram used with the permission of Geomax, P.C., Spokane, WA

## Bank ErosionCaused By Uncontrolled Upstream Sediment Source

**Notes:**
Bank Erosion – When upstream sediment sources become active, there is a tendency for the sediment (particularly the bedload) to be flushed downstream until a location is reached where the sediment (bedload) can be deposited.  One common method for depositing this material is by “Differential Displacement”.  This behavior usually occurs at a bend in the stream.  The sediment (bedload) that originated upstream deposits on the bar that occurs on the inside of the bend where there is less velocity and turbulence in the flowing water.  These depositions then cause the channel to be constricted and cross-sectional area is lost.  The water in the altered channel then flows faster and becomes more erosive.  If the material forming the outside bank consists of material that is finer than the new deposits on the inside of the bend, bank erosion begins. This erosion tends to enlarge the channel back to its original size which allows the stream to return back to the more stable original condition.  This problem is best solved by identifying and stabilizing the upstream sediment source.  When there is no loss of channel size due to deposition on the bar on the inside of the bend, there is less erosive pressure along the outside of the bend and the channel remains more stable.
Often, however, the problem of “Differential Displacement” is addressed by armoring the outside of the bend so that erosion of the finer material is eliminated.  This approach, if properly done, can protect the eroding bend but it does not correct the fundamental problem concerning excess sediment (bedload) transport coming from upstream.  When the particular eroding bank is armored, the mobilized sediment (bedload) simply continues to move downstream until another vulnerable site is found and the process is repeated.
Diagram used with the permission of Geomax, P.C., Spokane, WA

## Cumulative Impacts

**Low Impact Development can only occur when long range goals are adopted that minimize cumulative impacts.**

**Notes:**
Cumulative Impacts --  A stream system is the product of the sum of its pieces.  Too often attention is confined to specific problems that develop along the stream corridor.  “Solutions” to these site specific problems too often fail to take into account what is happening in adjacent hydrologically connected reaches.  Putting “band-aids” on site specific problems should be avoided and the behavior of the hydrologically connected systems should be determined before efforts are made to correct site specific problems.  When this approach is used, cumulative impacts are minimized and “Low Impact Development” is more obtainable.

## Quick FlushPoor Management Technique Shifts Problems To OthersEncourages Unsound Development

**Notes:**
Quick Flush --  Too often communities adapt a “Quick Flush” methodology for addressing their floodplain flooding problems.  This methodology typically attempts to get the high water past the community as quickly as possible.  Often this leads to channelization and diking projects in the stream corridor.  These efforts universally cause high water to be pushed onto their neighbors downstream.  Additionally, development is often allowed to occur behind the dike systems that can be seriously damaged if/when the dikes fail.
Diagram used with the permission of Geomax, P.C., Spokane, WA

## Floodplain UtilizationGood Management TechniqueHelps Protect Downstream ReachesIncreases Base FlowEncourages Sustainable Development

**Notes:**
Floodplain Utilization --  Floodplains were built by the geological processes that are/were active in the stream basin.  During peak flow periods, sediment ladened water normally overtops the active flow channel banks and inundates the surrounding flat lying ground adjacent to the active channel.  Two important things happen when overbank flow occurs.  First, the velocity of the overbank flow significantly slows and water is put into temporary storage.  Because most major floods are of short duration, any temporary floodwater storage lessens downstream impacts.
Second, when overbank flow slows, the entrained sediment is able to settle out and deposit on the floodplain.  These deposits are normally fine grained, nutrient rich and salt free.  Some of the most productive land on the planet can be found on floodplains that contain these deposits.
Diagram used with the permission of Geomax, P.C., Spokane, WA

## Slide 42

**Notes:**
Long Range Goals --  There are two competing approaches to floodplain management – The Quick Flush Approach and The Floodplain Utilization Approach.  Almost always the Floodplain Utilization Approach will be preferable if one desires Low Impact Sustainable Development to occur along stream corridors.
Diagram used with the permission of Geomax, P.C., Spokane, WA

## Cumulative ImpactsOn Roaring Fork River Floodplain

**Notes:**
Cumulative Impacts --  This diagram illustrates how multiple encroachments into the active floodplain can result in cumulative impacts that cause potentially serious conditions to develop.  Although each encroachment may have been tolerable, the combined impact of all of the encroachments has put some structures at risk.  The following three slides show details of the area.
Diagram used with the permission of Geomax, P.C., Spokane, WA

## Roaring Fork River1985 Aerial View Before Highway Construction

**Notes:**
Roaring Fork River, A 1985 Aerial View --  The Roaring Fork River like its name implies is a very dynamic river.  Although located in sparsely populated mountains of central Colorado, considerable development has occurred along sections of the river.  The scenic attraction of the river during low flow periods has created a market for development along the river.  This aerial photo, taken in 1985, shows that a mobile home park and a commercial complex have been built close to the river adjacent to Basalt, Colorado.  The Colorado Highway Department proceeded to realign the highway and bypass the town.  All work was done following the then current FEMA regulations.  As seen in the previous drawing, a large section of floodplain was isolated by the highway and all flood water is now forced to pass through a “bottleneck” that is occupied by the mobile home park, a county road bridge and a commercial development built on fill.  Even before the floodplain was constricted, flood flow velocities above town were calculated to be over 12 feet per second and the old highway required armoring in some locations.
Photo used with the permission of Geomax, P.C., Spokane, WA

## Roaring Fork RiverAerial View Of Bottleneck Area

**Notes:**
Roaring Fork River, A Detailed Aerial View --  This aerial photograph shows a more detailed view of the “Bottleneck” area.  The commercial buildings shown in the photo are built on fill that was placed in the original floodplain at an elevation well above the 100-year FEMA designated flood height.  The mobile home park located along the left bank is quite low and is mainly protected from the river by the county road.
The next slide shows conditions along the fill that is occupied by the commercial buildings. Note: the location of the ground photo is marked by a star on the above photo. 
Photo used with the permission of Geomax, P.C., Spokane, WA

## Roaring ForkRiverGround PhotoShowingFailing Gabion BasketWall

**Notes:**
Roaring Fork River, A Ground Photo --  This detailed ground photo, taken before the highway was realigned, shows that the buildings are well above the FEMA designated 100-year flood elevation.  Unfortunately, the retaining wall that protects the site and fill was constructed with rock filled Gabion Baskets.  It has been Geomax’s observation that Gabion Baskets perform poorly when they are subjected to abrasion by bedload moving through the system.  In this case, wires on the baskets showed considerable wear and some had began to break and dump their rock fill.  These local failures tend to destabilize the entire retaining wall and it is even possible that upper tiers of the Gabion Baskets could be dumped into the river, leaving the fill and buildings unprotected.  When the new, realigned highway was completed, additional pressure was placed on this already unstable retaining wall.
Photo used with the permission of Geomax, P.C., Spokane, WA

## Slide Presentation Prepared ByGeomax, P.C.Dr. Donald R. Reichmuth, President1023 W. 30th Ave.Spokane, WA 99203-1324Phone & FAX – 509-838-6390E-mail – geomax@comcast.net

**Notes:**
This slide set for Session 7 was last edited on 6/27/05.