Soils & Hydrology II

swedishstreakMécanique

22 févr. 2014 (il y a 3 années et 10 mois)

81 vue(s)

1

Soils & Hydrology II

9.
Soil Water

10.
Precipitation and Evaporation

11.
Infiltration, Streamflow, and Groundwater

12.
Hydrologic Statistics and Hydraulics

13.
Erosion and Sedimentation

14.
Soils for Environmental Quality and Waste Disposal

15.
Issues in Water Quality

2


What is the significance of understanding streamflow?


Why are we concerned with how it relates to
Landscapes?


Streamflow is important because it is related to:



Construction
of houses, bridges, spillways, and culverts


Surface Runoff

over landscapes, including flooding


The associated processes of
Erosion, Transport
, and
Deposition
.



Drinking

and
Irrigation

water supplies, especially during
droughts


Recreational

activities, such as boating and fishing


Navigation

of commercial shipping and transport


3

Hydrograph:


Plots precipitation and runoff over time.


Runoff can be discharge, flow, or stage


4

Storm Hydrograph

5

Storm Hydrograph

6

Storm Hydrograph

7

Storm Hydrograph

8

Storm Hydrograph

9

Lag

Time

10

Flow behavior for different streams

11

Hydrograph Behavior

12

Hydrograph Behavior:

Related to channel size

13

Hydrograph

for 1997
Homecoming
Weekend

Storm

14

15

16

Hydrograph Behavior:

Also related to channel patterns

17

Measurement Units

cfs: cubic feet per second

gpm: gallons per minute

mgd: million gallons per day

AF/day: Acre
-
Feet per day

cumec: cubic meters per second

Lps: liters per second

Lpm: liters per minute



1 cfs




2 AF/day


450 gpm


28.3 Lps


1 m
3
/s = 35.28 cfs


1 mgd


1.5 cfs


1 gpm = 3.785 Lpm

18

WEIR: Used to provide accurate flow measurements

19

20

Weir Types

Circular opening:

Q = c


r
2

h
1/2

Rectangular:

Q = c W h
3/2

Triangular:

Q = c h
5/2


where

Q is flow, cfs

c are weir coefficients

h is stage, ft

r is the pipe diameter, ft

W is the weir width, ft

21

Rectangular Weir

V
-
Notch (Triangular) Weir

Coweeta
Hydrologic
Station

22

Field Velocity Measurements

Flow Equation:

Q = v A

where

Q is the discharge, cfs

v is the water velocity, ft/s

A is the flow cross
-
sectional
area, ft
2

23

24

Discharge Measurements

25

Manning's Equation


When flow velocity measurements are not available


v = (1.49/n) R
2/3
S
1/2


where


v is the water velocity, ft/s


n is the Manning's hydraulic roughness factor


R = A / P is the hydraulic radius, ft


A is the channel cross
-
sectional area, ft
2


P is the channel wetted perimeter, ft


S is the water energy slope, ft/ft

26


Hydrologic Statistics:


Trying to understand and predict streamflow


Peak Streamflow Prediction:


Our effort to predict catastrophic floods


Recurrence Intervals:


Used to assign probability to floods


100
-
yr flood:


A flood with a 1 chance in 100 years, or a flood
with a probability of 1% in a year.

27

Return Period


T
r

= 1 / P


T
r

is the average recurrence interval, years


P is exceedence probability, 1/years


Recurrence Interval Formulas:


T
r

= (N+1) / m


Gringarten Formula: T
r

= (N+1
-
2a) / (m
-
a)


where


N is number of years of record,


a = 0.44 is a statistical coefficient


m is rank of flow (m=1 is biggest)

28

River Stage:

The elevation of the
water surface

Flood Stage

The elevation when the
river overtops the
natural channel
banks.

29

Bankfull Discharge

Q
bkf

= 150 A
0.63


30

31


Rating Curve


The relationship between river stage and discharge

32

33

34

Peak Flows in Ungaged Streams


Q
n

= a A
x

P
n


where


A is the drainage area, and


P
n

is the n
-
year precipitation depth


Q
n

is the n
-
year flood flow


Q
2

= 182 A
0.622


Q
10

= 411 A
0.613


Q
25

= 552 A
0.610


Q
100

= 794 A
0.605

35

Channel flooding vs upland flooding

36

Curve Number Method


Most common method used in the U.S. for predicting
stormflow peaks, volumes, and hydrographs.


Useful for designing ditches, culverts, detention ponds, and
water quality treatment facilities.

37


P = Precipitation, usually rainfall


Heavy precipitation causes more runoff than light precipitation


S = Storage Capacity


Soils with high storage produce less runoff than soils with little storage.


F = Current Storage


Dry soils produce less runoff than wet soils


38


r = Runoff Ratio => how much of the rain runs
off?


r = Q / P


r = 0 means that little runs off


r = 1 means that everything runs off


r = Q / P = F / S


r = 0 means that the bucket is empty


r = 1 means that the bucket is full


F = P
-

Q

or

r = Q / P = (P
-

Q) / S


the soil fills up as it rains


Solving for Q yields:


Q = P
2

/ (P + S)


39


S is maximum available soil moisture


S = (1000 / CN)
-

10


CN = 100 means S = 0 inches


CN = 50 means S = 10 inches


F is actual soil moisture content


F / S = 1 means that F = S, the soil is full


F / S = 0 means that F = 0, the soil is empty

Land Use



CN


S, inches

Wooded areas


25
-

83


2
-

30

Cropland



62
-

71


4
-

14

Landscaped areas


72
-

92

0.8
-

4

Roads



92
-

98

0.2
-

0.8

40

Curve Number Procedure


First we subtract the initial abstraction, I
a
, from the observed
precipitation, P


Adjusted Rainfall: P
a

= P
-

I
a


No runoff is produced until rainfall exceeds the initial abstraction.


I
a

accounts for interception and the water needed to wet the organic layer
and the soil surface.


The initial abstraction is usually taken to be equal to 20% of the maximum
soil moisture storage, S, => I
a

= S / 5


The runoff depth, Q, is calculated from the adjusted rainfall, P
a
, and
the maximum soil moisture storage, S, using:


Q = P
a
2

/ (P
a

+ S)


or use the graph and the curve number


We get the maximum soil moisture storage, S, from the Curve
Number, CN:


S = 1000 / CN
-

10


CN = 1000 / (S + 10)


We get the Curve Number from a Table.

41

Example


A typical curve number for forest lands is CN = 70, so the maximum soil
storage is: S = 1000 / 70
-

10 = 4.29"


A typical curve number for a landscaped lawn is 86, and so


S = 1000 / 86
-

10 = 1.63”


A curve number for a paved road is 98, so S = 0.20”


Why isn’t the storage equal to zero for a paved surface?


The roughness, cracks, and puddles on a paved surface allow for a small amount
of storage.


The Curve Number method predicts that I
a

= S / 5 = 0.04 inches of rain must fall
before a paved surface produces runoff.


For a CN = 66, how much rain must fall before any runoff occurs?


Determine the maximum potential storage, S = 1000 / 66
-

10 = 5.15"


Determine the initial abstraction, I
a

= S

/ 5

= 5.15” / 5 = 1.03"


It must rain 1.03 inches before runoff begins.


If it rains 3 inches, what is the total runoff volume?


Determine the effective rainfall, P
a

= P
-

I
a

= 3"
-

1.03" = 1.97"


Determine the total runoff volume, Q = 1.97
2

/ (1.97 + 5.15) = 0.545"

42

Unit Hydrographs

43

Unit Hydrograph

44

Unit Area Hydrographs

45

Unit Hydrograph Example


A unit hydrograph has been developed for a watershed


The peak flow rate is 67 L/s for 1 mm of runoff and an area of 100 ha


What is the peak flow rate for this same watershed if a storm produces 3 mm
of runoff?


The unit hydrograph method assumes that the hydrograph can be scaled linearly
by the amount of runoff and by the basin area.


In this case, the watershed area does not change, but the amount of runoff is three
times greater than the unit runoff.


Therefore, the peak flow rate for this storm is three times greater than it is for the
unit runoff hydrograph, or 3 x 67 L/s = 201 L/s.


What would be the peak flow rate for a nearby 50
-
ha watershed for
a 5
-
mm storm?


Peak Flow: Q
p

= Q
o

(A / A
o

) (R / R
o

)

where


Q
p

is the peak flow rate and Q
o

is for a reference watershed,


A is the area of watershed and A
p

is the area of reference watershed.


Q = (67 L/s) (50 ha / 100 ha) (5 mm / 1 mm) = 168 L/s


In this case, the peak runoff rate was scaled by both the watershed area and
the runoff amount.

46

Flood Routing

47


Streamflow and Land Management


BMPs improve soil and water quality


Most of our attention is placed on preventing pollution,
decreasing stormwater, and improving low flows.


Forestry


Forest streams have less stormflow and total flow, but
more baseflow


Forest litter (O
-
Horizon) increases infiltration


Forest canopies intercept more precipitation (higher
Leaf
-
Area Indices, LAI)


Forest have higher evapotranspiration rates


Forest soils dry faster, have higher total storage

48


Forest Management


Harvesting


High
-
lead yarding on steep slopes reduces soil compaction


Soft tires reduces soil compaction


Water is filtered using vegetated stream buffers (SMZs)


Water temperatures also affected by buffers


Roads


Road runoff can be dispersed onto planar and convex slopes


Broad
-
based dips can prevent road erosion


Site Preparation


Burning a site increases soil erosion and reduces infiltration


Leaving mulch on soils increases infiltration


Piling mulch concentrates nutrients into local "hot spots"


Distributing mulch returns nutrients to soils


Some herbicides cause nitrate increase in streams

49

Forestry Compliance in Georgia with Water
Quality Protection Standards (1991
-
2004)

50


Agricultural Land Management


Overland flow is a main concern in agriculture


increases soil erosion, nutrients, and fecal coliform


increases herbicides, pesticides, rodenticides, fungicides


Plowing


exposes the soil surface to rainfall (and wind) forces


mulching + no
-
till reduces runoff and increases infiltration


terracing and contour plowing also helps


Pastures (livestock grazing)


increases soil compaction


reduces vegetative plant cover


increases bank erosion


rotate cattle between pastures and fence streams


Urban Land Management


Urban lands have more impervious surfaces


More runoff, less infiltration, recharge, and baseflow


Very high peak discharges, pollutant loads


Less soil storage, channels are straightened and piped, no floodplains


Baseflows are generally lower, except for irrigation water (lawns & septic)

51

Benefits of Riparian Buffers


Bank Stability:


The roots of streambank trees help hold the banks together.


When streambank trees are removed, streambanks often collapse,
initiating a cycle of sedimentation and erosion in the channel.


A buffer needs to be at least 15 feet wide to maintain bank
stability.


Pollutant Filtration:


As dispersed overland sheet flow enters a forested streamside
buffer, it encounters organic matter and hydraulic roughness
created by the leaf litter, twigs, sticks, and plant roots.


The organic matter adsorbs some chemicals, and the hydraulic
roughness slows down the flow.


The drop in flow velocity allows clay and silt particles to settle out,
along with other chemicals adsorbed to the particles.


Depending on the gradient and length of adjacent slopes, a buffer
needs to be 30
-
60 feet wide to provide adequate filtration.

52


Denitrification:


Shallow groundwater moving through the root zones of floodplains is
subject to significant denitrificiation.


Removal of floodplain vegetation reduces floodplain denitrification


Shade:


Along small and mid
-
size streams, riparian trees provide significant
shade over the channel, thus reducing the amount of solar radiation
reaching the channel so summer stream temperatures are lower and
potential dissolved oxygen levels are higher.


Buffers need to be at least 30 feet wide to provide good shade and
microclimate control, but benefits increase up to 100 feet.


Organic Debris Recruitment:


River ecosystems are founded upon the leaves, conifer needles, and
twigs that fall into the channel.


An important function of riparian trees is providing coarse organic
matter to the stream system.


Buffers only need to encompass half the crown diameter of full
-
grown
trees to provide this function.

53


Large Woody Debris Recruitment:


Large woody debris plays many important ecological functions in
stream channels.


It helps scour pools, a favored habitat for many fish.


It creates substrate for macroinvertebrate and algae growth, and it
forms cover for fish.


It also traps and sorts sediment, creating more habitat complexity.


Woody debris comes from broken limbs and fallen trees.


The width of a riparian buffer should be equal to half a mature tree
height to provide good woody debris recruitment.


Wildlife Habitat:


Many organisms, most prominently certain species of amphibians
and birds use both aquatic and terrestrial habitat in close proximity.


Maintaining a healthy forested riparian corridor creates important
wildlife habitat.


The habitat benefits of riparian buffers increase out to 300 feet.