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Chapter 12

- Hydrologic Statistics and Hydraulics

Storm Hydrograph

- Stream Hydrographs
- A plot of discharge ( flow rate) or stage (

water level) versus time. - Stormflow Hydrograph
- A plot of discharge or stage before, during, and

after a specific storm. - Rising Limb
- The steep advance portion of the hydrograph that

reflects the onset of runoff - Falling Limb
- Flow that tapers off gradually following the

peak.

- Peak Stormflow
- Generally produced by surface runoff, either by

partial area contribution or Hortonian overland

flow as well as direct precipitation on the

channels. - Interflow
- Flow that takes longer to reach the channel
- Dominates the falling limb of the hydrograph.
- Baseflow
- The flow before and after the storm
- Generated principally by ground water discharge

and unsaturated interflow. - Stormflow Volume
- Total volume of streamflow associated with that

storm - It can be determined from the area under the

hydrograph when the hydrograph plots flow (not

stage) vs time.

Hydrograph for 1997 Homecoming Weekend Storm

Stream Hydrograph

Flow behavior for different streams

Hydrograph Behavior

Hydrograph Behavior Also related to channel

patterns

Streamflow Variability

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

Useful Conversions

- 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 Construction

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Weir Types

Weir Equations

- Submerged Pipe
- Q c ? r2 h1/2
- Rectangular Weir
- Q c W h3/2
- V-notch Weir
- 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

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

Manning's Equation

- 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

Mannings Equation

- River Stage
- The elevation of the water surface
- Flood Stage
- The elevation when the river overtops the natural

channel banks. - Rating Curve
- The relationship between river stage and discharge

Rating Curve

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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.

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)

Flood Prediction

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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

Bankfull Discharge Qbkf 150 A0.63

Curve Number Method

- Most common method used in the U.S. for

predicting stormflow peaks, volumes, and

hydrographs for precipitation events. - It is useful for designing ditches, culverts,

detention ponds, and water quality treatment

facilities.

Curve Number Method

- 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

- 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 F / S
- r 0 means that the bucket is empty
- r 1 means that the bucket is full
- F P - Q
- the soil fills up as it rains
- Combining equations yields
- Q P (P - Q) / S
- Solving for Q yields
- Q P2 / (P S)

- 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 / (P_a S)
- or by using 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.

Examples

- 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.

Another CN Example

- For a watershed with a curve number of 66, how

much rain must fall before any runoff occurs? - Determine the maximum potential storage, S
- S 1000 / 66 - 10 5.15"
- Determine the initial abstraction, Ia
- 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
- Pa P - Ia 3" - 1.03" 1.97"
- Determine the total runoff volume, Q
- Q 1.972 / (1.97 5.15) 0.545"

Unit Hydrographs

Unit Hydrograph

Flood Routing

Unit Area Hydrographs

Unit Hydrograph Example

- A unit hydrograph has been developed for a 100

hectare watershed - The peak flow rate for a storm that produces 1 mm

of runoff is 67 L/s - 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,
- Qo is peak flow for reference watershed,
- A is the area of watershed,
- 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.

Forest Management

- Forest streams have less stormflow and total flow
- 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

BMPs improve soil and water quality

- 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

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)

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.

- 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.

- 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.

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