Soils

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Soils Hydrology II

• Soil Water
• Precipitation and Evaporation
• Infiltration, Streamflow, and Groundwater
• Hydrologic Statistics and Hydraulics
• Erosion and Sedimentation
• Soils for Environmental Quality and Waste
  Disposal
• Issues in Water Quality

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• 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

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• Hydrograph
• Plots precipitation and runoff over time.
• Runoff can be discharge, flow, or stage

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Storm Hydrograph
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Storm Hydrograph
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Storm Hydrograph
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Storm Hydrograph
Peak flow rate
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Storm Hydrograph
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Lag Time
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Flow behavior for different streams
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Hydrograph Behavior
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Hydrograph BehaviorRelated to channel size
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Hydrographfor 1997 Homecoming Weekend Storm
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Hydrograph BehaviorAlso related to channel
patterns
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Measurement Units

• 1 cfs ?
• 2 AF/day
• 450 gpm
• 28.3 Lps
• 1 m3/s 35.28 cfs
• 1 mgd ? 1.5 cfs
• 1 gpm 3.785 Lpm

• 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

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WEIR Used to provide accurate flow measurements
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Weir Types

• Circular opening
• Q c ? r2 h1/2
• Rectangular
• Q c W h3/2
• Triangular
• Q c h5/2

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
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Coweeta Hydrologic Station
Rectangular Weir
V-Notch (Triangular) Weir
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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, ft2

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Discharge Measurements
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Manning's Equation

• When flow velocity measurements are not available
• v (1.49/n) R2/3 S1/2
• 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, ft2
• P is the channel wetted perimeter, ft
• S is the water energy slope, ft/ft

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• 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.

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Return Period

• Tr 1 / P
• Tr is the average recurrence interval, years
• P is exceedence probability, 1/years
• Recurrence Interval Formulas
• Tr (N1) / m
• Gringarten Formula Tr (N1-2a) / (m-a)
• where
• N is number of years of record,
• a 0.44 is a statistical coefficient
• m is rank of flow (m1 is biggest)

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River Stage The elevation of the water
surface Flood Stage The elevation when the river
overtops the natural channel banks.
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Bankfull DischargeQbkf 150 A0.63
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• Rating Curve
• The relationship between river stage and discharge

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Peak Flows in Ungaged Streams

• Qn a Ax Pn
• where
• A is the drainage area, and
• Pn is the n-year precipitation depth
• Qn is the n-year flood flow
• Q2 182 A0.622
• Q10 411 A0.613
• Q25 552 A0.610
• Q100 794 A0.605

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Channel flooding vs upland flooding
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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.

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• 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

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• r Runoff Ratio gt 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 P2 / (P S)

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• 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
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Curve Number Procedure

• First we subtract the initial abstraction, Ia,
  from the observed precipitation, P
• Adjusted Rainfall Pa P - Ia
• No runoff is produced until rainfall exceeds the
  initial abstraction.
• Ia 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, gt Ia S / 5
• The runoff depth, Q, is calculated from the
  adjusted rainfall, Pa , and the maximum soil
  moisture storage, S, using
• Q Pa2 / (Pa 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.

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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 isnt 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 Ia 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, Ia 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, Pa P - Ia
  3" - 1.03" 1.97"
• Determine the total runoff volume, Q 1.972 /
  (1.97 5.15) 0.545"

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Unit Hydrographs
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Unit Hydrograph
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Unit Area Hydrographs
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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 Qp Qo (A / Ao ) (R / Ro )
• where
• Qp is the peak flow rate and Qo is for a
  reference watershed,
• A is the area of watershed and Ap 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.

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Flood Routing
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• 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

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• 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

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

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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.

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• 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.

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• 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.

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Chapter 12 Quiz

• 1. A Curve Number of 95 is most likely typical
  of
• a. farmland b. forestland c. suburbs d.
  parking lot
• 2. Manning's equation is used to measure (circle
  any)
• a. flow depth b. flow velocity c. stream
  discharge d. flow area
• 3.What is cross-sectional area, A, and discharge
  of a stream, Q, if the average depth is 9, the
  width is 20 feet, and the velocity is 27 ft/min
  (circle any)
• a. A 15 ft2 Q 6.75 cfs
• b. A 150 in3 Q 4,860 AF/yr
• c. A 1.5 ft Q 400 in/day
• d. A 180 in2 Q 3000 gpm