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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

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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

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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 Behavior:Related to channel size

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Hydrographfor 1997

Homecoming Weekend

Storm

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Hydrograph Behavior:Also related to channel patterns

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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 m3/s = 35.28 cfs• 1 mgd 1.5 cfs• 1 gpm = 3.785 Lpm

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WEIR: Used to provide accurate flow measurements

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Weir TypesCircular opening:

Q = c r2 h1/2

Rectangular:Q = c W h3/2

Triangular:Q = c h5/2

whereQ 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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Rectangular Weir

V-Notch (Triangular) Weir

Coweeta Hydrologic

Station

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Field Velocity MeasurementsFlow Equation:

Q = v A

whereQ 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 = (N+1) / m

– Gringarten Formula: Tr = (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)

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River Stage: The elevation of the

water surface

Flood StageThe 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 => 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, inchesWooded areas 25 - 83 2 - 30Cropland 62 - 71 4 - 14Landscaped areas 72 - 92 0.8 - 4Roads 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, => Ia = S / 5• The runoff depth, Q, is calculated from the adjusted rainfall, Pa , and

the maximum soil moisture storage, S, using:– Q = Pa

2 / (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 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 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.