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22.A Hydrologic and Hydraulic Processes

2.B Geomorphic Processes

2.C Physical and Chemical Characteristics

2.D Biological Community Characteristics

2.E Functions and Dynamic Equilibrium

hapter 1 provided an overview of

stream corridors and the many

per-

spectives from which they should be

viewed in terms of scale, equilibrium,

and space. Each of these views can beseen as a “snapshot” of different aspects

of a stream corridor.

Chapter 2 presents the stream corridor in

motion, providin a basic understandin

of the different processes that ma!e the

stream corridor loo! and function the way

it does. "hile Chapter 1 presented still

imaes, this chapter provides “film

footae” to describe the processes, char-

acteristics, and functions of stream corri-

dors throuh time.

Section 2.A: Hydrologic and Hydraulic

Processes

#nderstandin how water flows into and

throuh stream corridors is critical to

restorations. $ow fast, how much, how

deep, how often, and when

water flows are

important basic questions that

must be answered to

Figure 2.1: A stream corridor

in motion. %rocesses, characteris-

tics, and functions shape stream

corridors and ma!e them loo!

the way they do.

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Hydrologic and Hydraulic Pr ocesses 22

ma!e appropriate decisions about

stream corridor restoration.

Section 2.B: Geomorphic Processes

&his section combines basic hydro-

loic processes with physical or

eomorphic functions and charac-teristics. "ater flows throuh

streams but is affected by the

!inds of soils and alluvial features

within the channel, in the

floodplain, and in the uplands. &he

amount and !ind of sediments

carried by a stream larely

determines its equilibrium

characteristics, includin si'e,shape, and profile. (uccessful

stream corridor restoration,

whether active )requirin direct

chanes* or passive )manaement

and removal of disturbance fac-

tors*, depends on an

understandin of how water and

sediment are related to channel

form and function and on what processes are involved with

channel evolution.

Section 2.C: Physical and Chemical

Characteristics

&he quality of water in the stream

corridor is normally a primary ob-

+ective of restoration, either to im-

prove it to a desired condition, or

to sustain it. estoration should

consider the physical and chemical

characteristics that may not be

readily apparent but that are

nonetheless critical to the functions

and processes of stream corridors.

Chanes in soil or water chemistry

to achieve restoration oals usually

involve manain or alterin ele-

ments in the landscape or corridor.

Section 2.D: Biological Community

Characteristics

&he fish, wildlife, plants, and hu-

mans that use, live in, or +ust visit

the stream corridor are !ey ele-

ments to consider in restoration.

&ypical oals are to restore, create,

enhance, or protect habitat to

ben- efit life. t is important tounder- stand how water flows,

how sediment is transported, and

how eomorphic features and

processes evolve however, a

prerequisite to successful

restoration is an understandin

of the livin parts of the system

and how the physical and chemical

processes affect the streamcorridor.

Section 2.: Functions and

Dynamic !uili"rium

&he si/ ma+or functions of

stream corridors are0 habitat,

conduit, barrier, filter, source,

and sin!.

&he interity of a stream

corridor ecosystem depends on

how well these functions

operate. &his section discusses

these functions and how they

relate to dynamic equilibrium.

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cycle.

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!egetati"e #ype $ Precipitation %nter cepted

Forests

Deciduous 13

Coniferous 28

Crops

Alfalfa 36

Corn 16

Oats

Grasses 1!"2!

Precipitation can do one of three things

once it reaches the earth. $t can return

to the atmosphere% move into the soil%

or run off the earth’s surface into a

stream, la&e, wetland, or other water

body. All three pathways play a role in

determining how water moves into,

across, and down the streamcorridor .

This section is divided into two subsec-

tions. The first subsection focuses on

hydrologic and hydraulic processes in

the lateral dimension, namely, the

movement of water from the land into

the channel. The second subsection

concentrates on water as it moves in the

longitudinal dimension, specifically as

streamflow in the channel.

Hydrologic and HydraulicProcesses Across t#e $tr eamCorridor

'ey points in the hydrologic cycle serve

as organiational headings in this sub-

section

■ $nterception, transpiration, and

evapotranspiration.

■ $nfiltration, soil moisture, and

ground water .

■ " unoff.

-nterception, & ranspiration, and

Evapotranspiration

*ore than two-thirds of the precipita-

tion falling over the +nited #tates evap-

orates to the atmosphere rather than

being discharged as streamflow to the

oceans. This short-circuiting of the

hydrologic cycle occurs because of thetwo processes, interception and transpi-

ration.

%nterception

A portion of precipitation never reaches

the ground because it is intercepted by

vegetation and other natural and con-

structed surfaces. The amount of water

intercepted in this manner is determined

by the amount of interception storage

available on the above-ground surfaces.

$n vegetated areas, storage is a function

of plant type and the form and density

of leaves, branches, and stems (Table

2.1). actors that affect storage in

forested areas include

■ /eaf shape. 0onifer needles hold

water more efficiently than leaves.

1n leaf surfaces droplets run togeth-

er and roll off. 2eedles, however ,

&eep droplets separated.

■ /eaf teture. "ough leaves store more

water than smooth leaves.

■ Time of year. /eafless periods provide

less interception potential in the

canopy than growing periods% howev-

er, more storage sites are created by

leaf litter during this time.

■ 3ertical and horiontal density. The

more layers of vegetation that precip-

itation must penetrate, the less li&ely

it is to reach the soil.

■ Age of the plant community. #ome

vegetative stands become more dense

with age% others become less dense.

The intensity, duration, and fre!uency

of precipitation also affect levels of in-

terception.

Figure 2.3 shows some of the pathways

rainfall can ta&e in a forest. "ainfall at

#a"le 2.1: Percentage o$ precipitation inter%

cepted $or &arious &egetation types.

$ource% Dunne and &eopold 1'8.

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the beginning of a storm initially fills

interception storage sites in the canopy.

As the storm continues, water held in

these storage sites is displaced. The dis-

placed water drops to the net lower

layer of branches and limbs and fills

storage sites there. This process is re-

peated until displaced water reaches thelowest layer, the leaf litter. At this point,

water displaced off the leaf litter either

infiltrates the soil or moves downslope

as surface runoff.

Antecedent conditions, such as mois-

precipitation

canopyinterceptionand evaporation

ture still held in place from previous

storms, affect the ability to intercept

and store additional water. 4vaporation

will eventually remove water residing

in interception sites. 5ow fast this t#roug#fall

t#roug#fall

stemflo(

litterinterceptionandevaporation

process occurs depends on climaticconditions that affect the evaporation

rate.

$nterception is usually insignificant in

areas with little or no vegetation. 6are

soil or roc& has some small imperme-

able depressions that function as inter-

ception storage sites, but typically most

mineral soil

t#roug#fall

net rainfall enteringt#e soil

of the precipitation either infiltrates the

soil or moves downslope as surface

runoff. $n areas of froen soil, intercep-

tion storage sites are typically filled

with froen water. 0onse!uently, addi-

tional rainfall is rapidly transformed

into surface runoff.

$nterception can be significant in large

urban areas. Although urban drainage

systems are designed to !uic&ly move

storm water off impervious surfaces, the

urban landscape is rich with storage

sites. These include flat rooftops, par&-

ing lots, potholes, crac&s, and otherrough surfaces that can intercept and

hold water for eventual evaporation.

#ranspiration and E"apotranspiration

Transpiration is the diffusion of water

vapor from plant leaves to the atmos-

phere. +nli&e intercepted water, which

originates from precipitation, transpired

Figure 2.': #ypical path(ays $or $orest rain$all.

portion of precipitation never reaches the

round because it is intercepted by veetation

and other surfaces.

water originates from water ta&en in by

roots.

Transpiration from vegetation and evap-

oration from interception sites and

open water surfaces, such as ponds and

la&es, are not the only sources of water

returned to the atmosphere. #oil mois-

ture also is sub7ect to evaporation.

4vaporation of soil moisture is, how-

ever, a much slower process due to cap-

illary and osmotic forces that &eep the

moisture in the soil and the fact that

vapor must diffuse upward through soil

pores to reach surface air at a lower

vapor pressure.

6ecause it is virtually impossible to sep-

arate water loss due to transpiration

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"ater is sub+ect to evaporation whenever it is

e/posed to the atmosphere. 2asically this process

involves0

■ &he chane of state of water from liquid to

vapor

■ &he net transfer of this vapor to theatmosphere

&he process beins when some molecules in the

liquid state attain sufficient !inetic enery

)primarily from solar enery* to overcome the

forces of surface tension and move into the

atmosphere. &his movement creates a vapor

pressure in the atmosphere.

&he net rate of movement is proportional to the

difference in vapor pressure between the water

surface and the atmosphere above that surface.

3nce the pressure is equali'ed, no more

evaporation can occur until new air, capable ofholdin more water vapor, displaces the old

saturated air. Evaporation rates therefore vary

accordin to lati- tude, season, time of day,

cloudiness, and wind enery. 4ean annual la!e

evaporation in the #nited (tates, for e/ample,

varies from 25 inches in 4aine and "ashinton

to about 67 inches in the desert (outhwest

) Figure 2.) *.

)2! inc#es

2!"3! inc#es

3!"*! inc#es

*!"+! inc#es

+!"6! inc#es

6!"! inc#es

!"8! inc#es,8! inc#es

Figure 2.): *ean annual la+e e&aporation $or the period 1,)-1,//.

$ource% Dunne and &eopold -1'8 modified from /o#ler et al. -1'+'.

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gravitationa

l

force

cap

illary

fo

rce

capilla

ry

force

from water loss due to evaporation, the

two processes are commonly combined

and labeled evapotranspiration. 4vapo-

transpiration can dominate the water

balance and can control soil moisture

content, ground water recharge, and

streamflow.

The following concepts are importantwhen describing evapotranspiration

■ $f soil moisture conditions are limit-

ing, the actual rate of evapotranspira-

tion is below its potential rate.

■ 8hen vegetation loses water to the

atmosphere at a rate unlimited by

the supply of water replenishing the

roots, its actual rate of evapotranspi-

ration is e!ual to its potential rate of

evapotranspiration.

The amount of precipitation in a region

drives both processes, however. #oil

types and rooting characteristics also

play important roles in determining the

actual rate of evapotranspiration.

-nfiltration, (oil 4oisture, and

8round " ater

Precipitation that is not intercepted or

flows as surface runoff moves into thesoil. 1nce there, it can be stored in the

upper layer or move downward through

the soil profile until it reaches an area

completely saturated by water called the

phreatic zone.

%n&iltration

0lose eamination of the soil surface re-

veals millions of particles of sand, silt,

and clay separated by channels of differ -

ent sies (Figure 2.5). These macropores

include crac&s, pipes left by decayed

roots and wormholes, and pore spaces

between lumps and particles of soil.

8ater is drawn into the pores by gravity

and capillary action. 9ravity is the

dominant force for water moving into

rain

rain

(ettedgrains

drygrains

(ettedgrains

drygrains

drygrains

(ettedgrains

the largest openings, such as worm or

root holes. 0apillary action is the domi-

Figure 2./: Soil pro$ile. "ater is drawn into

the pores in soil by ravity and capillary action.

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rainfall.+ inc#es0#r

infiltration.+ inc#es0#r

A. %n&iltration 'ate (rain&all rate) *hich is less thanin&iltration capacity

B. 'uno&& 'ate (rain&all rate minusin&iltration capacity

rainfall1.+ inc#es0#r

infiltration1 inc#0#r

Figure 2.-: 0n$iltration and runo$$. (urface runoff occurs when rainfall intensity e/ceeds infiltration

capacity .

nant force for water moving into soilswith very fine pores.

The sie and density of these pore

openings determine the water ’s rate of

entry into the soil. Porosity is the term

used to describe the percentage of the

total soil volume ta&en up by spaces be-

tween soil particles. 8hen all those

spaces are filled with water, the soil is

said to be saturated.

#oil characteristics such as teture andtilth (looseness) are &ey factors in deter -

mining porosity. 0oarse-tetured, sandy

soils and soils with loose aggregates

held together by organic matter or small

amounts of clay have large pores and,

thus, high porosity. #oils that are tightly

pac&ed or clayey have low porosity.

Infiltration is the term used to describe

the movement of water into soil pores.

The infiltration rate is the amount of

water that soa&s into soil over a givenlength of time. The maimum rate that

water infiltrates a soil is &nown as the

soil’s infiltration capacity.

$f rainfall intensity is less than infiltra-

tion capacity, water infiltrates the soil at

a rate e!ual to the rate of rainfall. $f the

rainfall rate eceeds the infiltration ca-

pacity, the ecess water either is de-tained in small depressions on the soil

surface or travels downslope as surface

runoff (Figure 2.6).

The following factors are important in

determining a soil’s infiltration rate

■ 4ase of entry through the soil surface.

■ #torage capacity within the soil.

■ Transmission rate through the soil.

Areas with natural vegetative cover andleaf litter usually have high infiltration

rates. These features protect the surface

soil pore spaces from being plugged by

fine soil particles created by raindrop

splash. They also provide habitat for

worms and other burrowing organisms

and provide organic matter that helps

bind fine soil particles together. 6oth of

these processes increase porosity and

the infiltration rate.

The rate of infiltration is not constant

throughout the duration of a storm.

The rate is usually high at the begin-

ning of a storm but declines rapidly as

gravity-fed storage capacity is filled.

A slower, but stabilied, rate of infiltra-

tion is reached typically : or ; hours

into a storm. #everal factors are in-

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P r o p o r t i o n b y ! o l u m e

volved in this stabiliation process,

including the following

■ "aindrops brea&ing up soil aggregates

and producing finer material, which

then bloc&s pore openings on the sur -

face and reduces the ease of entry.

■ 8ater filling fine pore spaces and

reducing storage capacity.

■ 8etted clay particles swelling and

effectively reducing the diameter of

pore spaces, which, in turn, reduces

transmission rates.

#oils gradually drain or dry following a

storm. 5owever, if another storm occurs

before the drying process is completed,

there is less storage space for new water .

!.6!

!.+!

!.*!

!.3!

!.2!

!.1!

!

unfilledpore space

&ine

loam

porosity

field

capacity

(iltingpoint

clay

#eavyclay loam

clay loam

Therefore, antecedent moisture condi-

tions are important when analying

available storage.

+oil ,oistur e

sandy loam

sandy loam

fine sand

sand

lig#t clay loam

silt loam

After a storm passes, water drains out of

upper soils due to gravity. The soil re-

mains moist, however, because some

amount of water remains tightly held in

fine pores and around particles by sur-

face tension. This condition, called field

capacity, varies with soil teture. /i&e

porosity, it is epressed as a proportion

by volume.

The difference between porosity and

field capacity is a measure of unfilled

pore space (Figure 2.7). ield capacity

is an approimate number, however, be-

cause gravitation drainage continues in

moist soil at a slow rate.

#oil moisture is most important in the

contet of evapotranspiration. Terrestrial

plants depend on water stored in soil.

As their roots etract water from pro-

gressively finer pores, the moisture con-

tent in the soil may fall below the field

capacity. $f soil moisture is not replen-

ished, the roots eventually reach a point

where they cannot create enough suc-

tion to etract the tightly held interstitial

Figure 2.: ater%holding properties o$ &arious

soils. "ater-holdin properties vary by te/ture.

9or a fine sandy loam the appro/imate dif fer-

ence between porosity, 5.:;, and field

capacity ,

5.25, is 5.2;, meanin that the unfilled pore

space is 5.2; times the soil volume. &he dif fer-

ence between field capacity and wiltin point is

a measure ofunfilled

pore space.$ource% Dunne and &eopold 1'8.

pore water. The moisture content of the

soil at this point, which varies depend-

ing on soil characteristics, is called the

permanent wilting point because plants

can no longer withdraw water from the

soil at a rate high enough to &eep up

with the demands of transpiration, caus-

ing the plants to wilt.

Deep percolation is the amount of water that passes below the root one of

crops, less any upward movement of

water from below the root one ().

Ground -ater

The sie and !uantity of pore openings

also determines the movement of water

within the soil profile. 9ravity causes

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water to move vertically downward.

This movement occurs easily through

larger pores. As pores reduce in sie due

to swelling of clay particles or filling of

pores, there is a greater resistance to

flow. 0apillary forces eventually ta&e

over and cause water to move in any

direction.

8ater will continue to move downward

until it reaches an area completely satu-

rated with water, the phreatic zone or

one of saturation (Figure 2.8). The top

of the phreatic one defines the ground

water table or phreatic surface.

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underground scenarios. or eample,

perched ground water occurs when a shal-

low a!uitard of limited sie prevents

water from moving down to the

phreatic one. 8ater collects above the

a!uitard and forms a mini-phreatic

one. $n many cases, perched ground

water appears only during a storm orduring the wet season. 8ells tapping

perched ground water may eperience a

shortage of water during the dry season.

Perched a!uifers can, however, be im-

portant local sources of ground water .

Artesian wells are developed in con-

fined a!uifers. 6ecause the hydrostatic

pressure in confined a!uifers is greater

than atmospheric pressure, water levels

in artesian wells rise to a level where at-

mospheric pressure e!uals hydrostatic pressure. $f this elevation is above the

ground surface, water can flow freely

out of the well.

8ater also will flow freely where the

ground surface intersects a confined

a!uifer. The piezometric surface is the

level to which water would rise in wells

tapped into confined a!uifers if the

wells etended indefinitely above the

ground surface. Phreatic wells draw

water from below the phreatic one in

unconfined a!uifers. The water level in

a phreatic well is the same as the

ground water table.

Practitioners of stream corridor restora-

tion should be concerned with locations

where ground water and surface water

are echanged. Areas that freely allow

movement of water to the phreatic one

are called recharge areas. Areas where the

water table meets the soil surface orwhere stream and ground water emerge

are called springs or seeps.

The volume of ground water and the

elevation of the water table fluctuate

according to ground water recharge

and discharge. 6ecause of the fluctua-

tion of water table elevation, a stream

channel can function either as a

recharge area (influent or losing

stream) or a discharge area (effluent

or gaining stream).

unof f

8hen the rate of rainfall or snowmelt

eceeds infiltration capacity, ecesswater collects on the soil surface and

travels downslope as runoff. actors

that affect runoff processes include cli-

mate, geology, topography, soil charac-

teristics, and vegetation. Average annual

runoff in the contiguous +nited #tates

ranges from less than : inch to more

than ;> inches (Figure 2.9).

Three basic types of runoff are intro-

duced in this subsection (Figure2.10

)■ 1verland flow

■ #ubsurface flow

■ #aturated overland flow

4ach of these runoff types can occur in-

dividually or in some combination in

the same locale.

"erland Flo*

8hen the rate of precipitation eceeds

the rate of infiltration, water collects onthe soil surface in small depressions

(Figure 2.11). The water stored in these

spaces is called depression storage. $t

eventually is returned to the atmos-

phere through evaporation or infiltrates

the soil surface.

After depression storage spaces are filled,

ecess water begins to move downslope

as overland flow, either as a shallow

sheet of water or as a series of small

rivulets or rills. 5orton (:=??) was the

first to describe this process in the liter-

ature. The term Horton overland flow or

5ortonian flow is commonly used.

The sheet of water increases in depth

and velocity as it moves downhill. As it

travels, some of the overland flow is

trapped on the hillside and is called sur-

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precipitation

precipitation

r oun

(ater tale

saturatedoverlandflo(

Figure 2.15: Flo(

paths o$ (ater o&

a sur$ace. &he potion of precipitati

that runs off or

infiltrates to the

round water tab

depends on the s

permeability rate

surface rouhness

and the amount,

duration, and inte

ty of precipitation

$n some situations, infiltrated storm

water does not reach the phreatic one

because of the presence of an a!uitard.

$n this case, subsurface flow does not

mi with baseflow, but also discharges

water into the channel. The net result,

whether mied or not, is increased

channel flow.

+aturated "erland Flo*

$f the storm described above continues,

the slope of the water table surface can

continue to steepen near the stream.4ventually, it can steepen to the point

that the water table rises above the

channel elevation. Additionally, ground

water can brea& out of the soil and

travel to the stream as overland flow.

This type of runoff is termed quic

return flow.

The soil below the ground water brea&-

out is, of course, saturated. 0onse-

!uently, the maimum infiltration rate

is reached, and all of the rain falling

on it flows downslope as overland

runoff. The combination of this direct

pands further up the hillside. 6ecause

!uic& return flow and subsurface flow

are so closely lin&ed to overland flow,

they are normally considered part of

the overall runoff of surface water .

Hydrologic and HydraulicProcesses Along t#e $tr eamCorridor

8ater flowing in streams is the

collection of direct precipitation and

water that has moved laterally from theland into the channel. The amount and

timing of

this lateral movement directly influencesFigure 2.11: 6&erland $lo( and depression

storage. 3verland moves downslope as an

irreular sheet.


surface

detention dept# andvelocity ofoverland flo(

increasedo(nslope

precipitation and !uic& return flow is

called saturated overland flow. As the

storm progresses, the saturated area e-

depression storage-dept# of depressionsgreatly e9aggerated

streamc#annel

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, e a n

, o n t h l y

D i s c h a r g e / c & s 0

: A$;:Os. +nfortunately, the length of

record regarding wet and dry years is

short (in geologic time), ma&ing it is

difficult to predict broad-scale persis-

tence of wet or dry years.

#easonal variations of streamflow are

more predictable, though somewhat

complicated by persistence factors. 6e-

cause design wor& re!uires using histor-

ical information (period of record) as a

basis for designing for the future, flow

information is usually presented in a

probability format. Two formats are es-

pecially useful for planning and design-

ing stream corridor restoration

■ !low duration" the probability a given

streamflow was e!ualed or eceeded

over a period of time.

■ !low frequency" the probability a

given streamflow will be eceeded

(or not eceeded) in a year .

(#ometimes this concept is modified

and epressed as the average number

of years between eceeding Cor not

eceedingD a given flow.)

Figure 2.12 presents an eample of a

flow fre!uency epressed as a series of

probability curves. The graph displays

months on the -ais and a range ofmean monthly discharges on the y-ais.

The curves indicate the probability that

the mean monthly discharge will be

less than the value indicated by the

curve. or eample, on about percent chance that the

1+!!!

1!!!!

+!!!

!Oct. >ov. Dec. ?an. :e. @ar. April @ay ?une ?uly Aug. $ept.

,onth

Figure 2.12: An e7ample o$ monthly pro"a"ility cur&es. 4onthly probability that the mean

monthly dischare will be less than the values indicated. =a!ima iver near %ar!er, "ashinton.

)>ata from #.(. rmy Corps of Enineers.*


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21 Chapter 23 +tream Corridor Processes) Characteristics) and

discharge will be less than =,>>> cfs

and a B> percent chance it will be less

than ;,>>> cfs.

Ecoloical -mpacts of 9low

The variability of streamflow is a pri-

mary influence on the biotic and abiotic

processes that determine the structureand dynamics of stream ecosystems

(0ovich :==?). 5igh flows are impor-

tant not only in terms of sediment

transport, but also in terms of recon-

necting floodplain wetlands to the

channel.

This relationship is important because

floodplain wetlands provide spawning

and nursery habitat for fish and, later in

the year, foraging habitat for waterfowl.

/ow flows, especially in large rivers,

create conditions that allow tributary

fauna to disperse, thus maintaining

populations of a single species in sev-

eral locations.

$n general, completion of the life cycle

of many riverine species re!uires an

array of different habitat types whose

temporal availability is determined

by the flow regime. Adaptation to this

environmental dynamism allows river-ine species to persist during periods

of droughts and floods that destroy

and recreate habitat elements (Poff

et al. :==E).

2. 4eomorp#ic Pr ocesses

#eomorphology is the study of surface

forms of the earth and the processes

that developed those forms. The hydro-

logic processes discussed in the previ-

ous section drive the geomorphic

processes described in this section. $n

turn, the geomorphic processes are the

primary mechanisms for forming the

drainage patterns, channel, floodplain,

terraces, and other watershed and

stream corridor features discussed in

0hapter :.

Three primary geomorphic processes

are involved with flowing water, as fol-

lows)

■ $rosion, the detachment of soil parti-

cles.

■ %ediment transport , the movement of

eroded soil particles in flowing water .

■

%ediment deposition, settling of erod-ed soil particles to the bottom of a

water body or left behind as water

leaves. #ediment deposition can be

transitory, as in a stream channel

from one storm to another, or more

or less permanent, as in a larger

reservoir .

#ince geomorphic processes are so

closely related to the movement of

water, this section is organied into

subsections that mirror the hydrologic processes of surface storm water runoff

and streamflow

■ 9eomorphic Processes Across the

#tream 0orridor

■ 9eomorphic Processes Along the

#tream 0orridor

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4eomorp#ic Processes Acr osst#e $tream Corridor

The occurrence, magnitude, and distrib-

ution of erosion processes in water-

sheds affect the yield of sediment and

associated water !uality contaminants

to the stream corridor .

#oil erosion can occur gradually over a

long period, or it can be cyclic or

episodic, accelerating during certain

seasons or during certain rainstorm

events (Figure 2.13). #oil erosion can

be caused by human actions or by nat-

ural processes. 4rosion is not a simple

process because soil conditions are con-

tinually changing with temperature,

moisture content, growth stage and

amount of vegetation, and the humanmanipulation of the soil for develop-

ment or crop production. Tables 2.2

and 2.3 show the basic processes that

influence soil erosion and the different

types of erosion found within the water-

shed.

4eomorp#ic Processes Alongt#e $tream Corridor

The channel, floodplain, terraces, and

other features in the stream corridor areformed primarily through the erosion,

transport, and deposition of sediment

by streamflow. This subsection de-

scribes the processes involved with

transporting sediment loads down-

stream and how the channel and

floodplain ad7ust and evolve through

time.

(ediment & ransport

#ediment particles found in the streamchannel and floodplain can be catego-

ried according to sie. A boulder is the

largest particle and clay is the smallest

particle. Particle density depends on the

sie and composition of the particle

(i.e., the specific gravity of the mineral

content of the particle).

2o matter the sie, all particles in the

channel are sub7ect to being trans-

ported downslope or downstream.

The sie of the largest particle a stream

can move under a given set of hy-

draulic conditions is referred to as

stream competence. 1ften, only very

high flows are competent to move thelargest particles.

0losely related to stream competence is

the concept of tractive stress, which cre-

ates lift and drag forces at the stream

boundaries along the bed and ban&s.

Tractive stress, also &nown as shear

stress, varies as a function of flow depth

and slope. Assuming constant density,

shape, and surface roughness, the larger

the particle, the greater the amount of

tractive stress needed to dislodge it andmove it downstream.

The energy that sets sediment particles

into motion is derived from the effect

of faster water flowing past slower

water . This velocity gradient happens

because the water in the main body of

flow moves faster than water flowing at

the boundaries. This is because bound-

Figure 2.1': 8aindrop impact. 3ne of

many types of erosion.

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Erosion4Physical Process

Erosion #ype +heet ConcentratedFlo*

,ass-asting

Combination

$#eet and rill 9 9

Bnterill 9

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3ne way to differentiate the sediment load of a

stream is to characteri'e it based on the immediate

source of the sediment in transport. &he total sediment

load in a stream, at any iven time and location, isdivided into

two parts?wash load and bed-material load. &he prima-

ry source of wash load is the watershed, includin sheet

and rill erosion, ully erosion, and upstream streamban!

erosion. &he source of bed material load is primarily the

streambed itself, but includes other sources in the water-

shed.

"ash load is composed of the finest sediment particles

in transport. &urbulence holds the wash load in

suspension. &he concentration of wash load in

suspension is essentially independent of hydraulic

conditions in the stream and therefore cannot be

calculated usin mea- sured or estimated hydraulic

parameters such as velocity or dischare. "ash load

concentration is normally a function of supply i.e., the

stream can carry as much wash load as the watershed

and ban!s can deliver )for sediment concentrations

below appro/imately @555 parts per million*.

2ed-material load is composed of the sediment of si'e

classes found in the streambed. 2ed-material load

moves alon the streambed by rollin, slidin, or +umpin, and may be periodically entrained into the

flow by turbulence, where it becomes a portion of the

suspended

load. 2ed-material load is hydraulically controlled

and can be computed usin sediment transport

equations discussed in Chapter 6.

iner-grained particles are more easily

carried into suspension by turbulent ed-

dies. These particles are transported

within the water column and are there-

fore called the suspended load . Although

there may be continuous echange of

sediment between the bed load and

suspended load of the river, as long as

sufficient turbulence is present.

Part of the suspended load may be col-

loidal clays, which can remain in sus-

pension for very long time periods,

depending on the type of clay and

water chemistry.

+ediment #ransport #erminology

#ediment transport terminology can

sometimes be confusing. 6ecause of

this confusion, it is important to define

some of the more fre!uently used

terms.

■ %ediment load , the !uantity of sedi-

ment that is carried past any cross

section of a stream in a specified

period of time, usually a day or a

year. %ediment discharge, the mass

or volume of sediment passing a

stream cross section in a unit of time. Typical units for sediment load

are tons, while sediment discharge

units are tons per day.

■ &ed-material load , part of the total

sediment discharge that is composed

of sediment particles that are the

same sie as streambed sediment.

■ 'ash load , part of the total sediment

load that is comprised of particle

sies finer than those found in thestreambed.

■ &ed load , portion of the total sedi-

ment load that moves on or near the

streambed by saltation, rolling, or

sliding in the bed layer .

■ %uspended bed material load , portion

of the bed material load that is trans-

ported in suspension in the water

column. The suspended bed material

load and the bed load comprise the

total bed material load.

■ %uspended sediment discharge (or sus-

pended load ), portion of the total sed-

iment load that is transported in sus-

pension by turbulent fluctuations

within the body of flowing water .

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Classi&ication +ystem

Based on

,echanismo&

Based on

Particle +i5e

# o t a l s e d i m e n t l o a d-ash load +uspended

load-ash load

+uspendedbed6materialload

Bed6materialload

Bed load Bed load

■ (easured load , portion of the total

sediment load that is obtained by the

sampler in the sampling one.

■ )nmeasured load , portion of the total

sediment load that passes beneath

the sampler, both in suspension and

on the bed. 8ith typical suspended

sediment samplers this is the lower >.? to >.F feet of the vertical.

The above terms can be combined in a

number of ways to give the total

sediment load in a stream (Table 2.4).

5owever, it is important not to com-

bine terms that are not compatible.

or eample, the suspended load and

the bed material load are not compli-

mentary terms because the suspended

load may include a portion of the bed

material load, depending on the energy

available for transport. The total sedi-

ment load is correctly defined by the

combination of the following terms

;otal $ediment &oad F

ed @aterial &oad G 5as# &oad

or

ed &oad G $uspended &oad

or

@easured &oad G nmeasured &oad

#ediment transport rates can be com-

puted using various e!uations or mod-

els. These are discussed in the %tream

*hannel +estoration section of 0hapter G.

#a"le 2.): Sediment load terms.

+tream Po*er

1ne of the principal geomorphic tas&s

of a stream is to transport particles out

of the watershed (Figure 2.15). $n this

manner, the stream functions as a trans-

porting machine% and, as a machine,

its rate of doing wor& can be calculated

as the product of available power multi- plied by efficiency.

%tream power can be calculated as

ϕ H γ I #

8here

ϕ H #tream power (foot-lbsJsecond-

foot)

γ H #pecific weight of water (lbsJft?)

I H @ischarge (ft?Jsecond)

# H #lope (feetJfeet)

#ediment transport rates are directly re-

lated to stream power% i.e., slope and

discharge. 6aseflow that follows the

highly sinuous thalweg (the line that

mar&s the deepest points along the

stream channel) in a meandering

stream generates little stream power%

therefore, the stream’s ability to move

sediment, sediment-transport capacity" islimited. At greater depths, the flow fol-

lows a straighter course, which increases

slope, causing increased sediment trans-

port rates. The stream builds its cross

section to obtain depths of flow and

channel slopes that generate the sedi-

ment-transport capacity needed to

maintain the stream channel.

"unoff can vary from a watershed, ei-

ther due to natural causes or land use

practices. These variations may change

the sie distribution of sediments deliv-

ered to the stream from the watershed

by preferentially moving particular par-

ticle sies into the stream. $t is not un-

common to find a layer of sand on top

of a cobble layer. This often happens

when accelerated erosion of sandy soils

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