drainage design presentation

8/12/2019 drainage design presentation

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The Hydrologic Cycle

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Hydrology

Watersheds

Meteorology Study of the atmosphere including

weather and climate

Surface water hydrology Flow and occurrence of

water on the surfaceof the earth

Hydrogeology Flow and occurrence

of ground water

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Intersection ofHydrology and Hydraulics

Water supplies Drinking water

Industry

Irrigation

Power generation Hydropower

Cooling water Dams

Reservoirs

Levees

Flood protection

Flood plainconstruction

Water intakes

Discharge anddilution Wastewater

Cooling water

Outfalls4

Engineering Uses ofSurface Water Hydrology

Average events (average annual rainfall,evaporation, infiltration...) Expected average performance of a system Potential water supply using reservoirs

Frequent extreme events (10 year flood,10 year low flow)

Levees Wastewater dilution

Rare extreme events (100 to PMF) Dam failure Power plant flooding

Probable maximum flood

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Flood Design Techniques

Use stream flow records Limited data

Can be used for high probability events

Use precipitation records Use rain gauges rather than stream gauges

Determine flood magnitude based onprecipitation, runoff, streamflow

Create a synthetic storm Based on record of storms

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0

10

20

30

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50

60

9/30 12/31 4/1 7/2date

Streamf

low(m3/s)

Forecasting Stream Flows

Natural processes -not easily predicted ina deterministic way We cannot predict the

monthly stream flow

We will use probabilitydistributions insteadof predictions

Seasonal trend with large variation

10 year daily average


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Surface DrainageSurface DrainageSurface Drainage

Design FlowsDesign Flows

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Objectives

Identify drainagerequirements and design

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Surface Drainage surface water is removed from

pavement Redirects water into appropriately

designed drainage systems (channelsor pipes)

Eventually, discharges into naturalwater systems

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Surface Drainage

Two types of water Surface water rain and snow (?)

Ground water can be a problem when awater table is near surface

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Surface DrainageSystem Design

Three phases1. Estimate of the quantity of water toreach the system

2. Hydraulic design of system elements3. Comparison of different materials that

serve same purpose


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Calculating Peak Runoff

Rainfall Runoff Analysis /Rational Method

Qp = CiA

C = constant runoff coefficienti = rainfall intensityA = drainage area

(tc

= time of concentration < rainfall duration)

0

0.2

0.4

0.6

0.8

1

0 1 2 3 4 5 6

t / Tp

Q / Qp

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Rational Formula - Methodto Choose Rainfall Intensity

Intensity = f(storm duration)

Expectation of stream flow vs. Time duringstorm of constant intensity

Watershed

Outflow

Q

t

Qp

tcClassic Watershed

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Rational Formula - Timeof Concentration (Tc)

Time required (after start of rainfallevent) for most distant point in basinto begin contributing runoff to basinoutlet

Tc affects the shape of the outflow

hydrograph (flow record as afunction of time)

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Hydrologic Analysis:Rational Method

Useful for small, usually urban,watersheds (


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Runoff Coefficient rural area

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Runoff Coefficient urban area

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Runoff Coefficient ForHigh Intensity Event

(i.e. 100-year storm)

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Runoff Coefficient ForHigh Intensity Event

(i.e. 100-year storm)

C = 0.16 forlow intensityevent forcultivatedfields

C = 0.42 forhigh intensity

event

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Runoff Coefficient

When a drainage area has distinctparts with different C values

Use the weighted average

C = C1A1 + C2A2 + .. + CnAnAi

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Watershed Area

measured in hectares Combined area of all surfaces thatdrain to a given intake or culvertinlet

Determine boundaries of area thatdrain to same location i.e high points mark boundary Natural or human-made barriers


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Watershed Area

Topographic maps

Aerial photos

Digital elevation models

Drainage maps

Field reviews

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Intensity Average intensity for a selected frequency and

duration over drainage area for duration of storm Based on design event (i.e. 5-year storm)

Overdesign is costly Underdesign may be inadequate

Duration is important Based on values of Tc and T

Tc

= time of concentration T = recurrence interval or design frequency

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Time of Concentration (tc)

Time for water to flow from hydraulically mostdistant point on the watershed to the point ofinterest

Rational method assumes peak run-off rate occurswhen rainfall intensity (I) lasts (duration) >= Tc

Used as storm duration dont use Tc


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Infiltration Measurement:Double Ring Infiltrometer

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Double RingInfiltrometer

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Measuring Infiltrationwith Rainfall Simulator

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Rainfall IntensityIncreased until Surface

Runoff Occurs

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Streamflow Measurements

Flood forecasting

Flood analysis Reservoir operations

Low flows water quality concerns

Design structures culverts, bridges,stormwater systems

Evaluate changes in land use on

watersheds and/or

changes in climatic regimes

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Streamflow & Fluvial Geomorphology

(Adapted from Dunne & Leopold, 1978; Leopold, 1994, 1997)


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Flow velocity varies with depth andchannel width

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Measuring StreamflowDischarge

Current meter method: measureflow & stage (elevation) over timeto establish a discharge ratingcurve: Continuously measurestage (stilling well) and derive Qfrom stage.

Pre-calibrated Structures forsmall streams, ditches &research applications

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Current Meter Method

Q = [Velocity x Area] Area is channel cross-sectional area

Need to know width of channel (w), Depth ofchannel (d), and Velocity of flow (V) (ft/s orm/s)

Procedure Depth varies across a channel

Velocity varies

Therefore need to divide the channel intomanageable segments (slices); Typically use10-20 segments

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Discharge (Q) Measurement

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Discharge (Q) Measurement Compute the Q for each segment (slice)

=

n

iiVAQ1

Sum the Q for each segment to compute the

total Q for the stream

Where on a stream do you collect Q data?

Need a quasi stable section (Control Section)

Look for a relatively straight reach w/uniform

flow such as a riffle section

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Each segment has a fixed width

Identify the midpoint for each segment & Measure:

Channel depth from water surface Velocity

Depth of velocity measurement depends upon channeldepth

IF Depth > 0.5m (1.6 ft) take 2 measurements andcompute the average One @ 20% depth

One @80 % depth

Average the two readings

IF Depth < 0.5m, take one reading @ 60% depth


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Measuring Streamflowwith a Current Meter

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Pygmy meter

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Precalibrated Structures

Weirs

Flumes

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Weirs

Obstruct flow andforce it through a

notch

Stage-Q relationshipestablished

for each type

of notch


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Weirs

Generally used in small streams

Various types V-notch for accurate low flow

Rectangular Handles higher flows

Less accurate at low flows

Trapezoidal -- an intermediate weir

Concerns Sediment & debris are trapped

Leakage

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Cipolletti (Trapezoidal) Weir

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Trapezoidal Weir

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Rectangular Weir

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90 degree V-notch Weir

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90 V-notch Weir

Q = 2.5H2/3


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Flumes

An artificial open channel built to contain flow

within a designed cross-section and length

No impoundment

Water height in flume measured with a stilling

well

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Flumes Used to measure flow in:

water and wastewater treatment plants

irrigation channels

agricultural runoff

runoff plots research applications

small watersheds

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Large Crest Flumes

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Long-throated Flume

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Short-throated Flume

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Parshall Flume


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H Flume

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Floods

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stimating Disc arge Qwhere flow was not directly

measured Large flood events that can not be

measured with conventional methods

Peak discharge in streams wherethere is no gauge

Stage will give us X-sectional area Its the velocity (v) or (u) thats

problematic

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Discharge (Q) MeasurementMannings Equation

2 / 3 1 / 2R S

vn

=

u or v = average velocity (m/s)

R = hydraulic radius

= [Area/wetted perimeter]

S= Energy gradient, Approximated by water surfaceslope

n = Mannings roughness coefficient

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Manning Roughness Coefficients

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Simplified Methods:predicting peak flow

discharge

Simplified Methods:Simplified Methods:predicting peak flowpredicting peak flow

dischargedischarge

Rational MethodRational Method


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Rational MethodEmpirically based method

Qp = CIA (cfs)

Qp = [0.00278]CIA (cms, m3/s)

Most commonly used formula for estimating Qpfrom rainfall in small urban watersheds

Widely accepted method for design of storm

sewer capacity

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Rational Method

Qp = [1/360]CIA (cms)

C = dimensionless runoff coefficient

Vegetation type

Soil type

Amount of impervious area

High C values = high RunOff rates

I = rainfall intensity for the storm of interest

(mm/hr)

A = watershed area (acres)

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Rational Method

Only valid for storms that last as long as the

watersheds Tc

If Tc is < Tc for watershed then Qpeak will be

overestimated

Assumes precipitation is uniformly distributed over

entire watershed

Assumes the RI of the flood peak is the same as the RI

of rainfall

1 in/hr of runoff from 1 acre will yield 1 cfs Designed to be used on watersheds < 200 acres (81

hectares) NOT FOR LARGE WATERSHEDS

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drainage design presentation