Water Resources Exam 3

Pipe line vs OCF

Similarities: conveyance structure, all fluid mechanics apply

Differences: pipes enclosed, OCF open at the top,

pipes in pressure, OCF no pressure at free surface

Pipes always circular, OCF shape variation

Hydraulic Radius Equation

Rh= A/Pw

A= area

Pw= wetted perimeter

What drives flow in pipes and OCF?

Pipe: change in elevation, pressure, pumps

OCF: change in elevation/ slope

Specific Energy (structure, equation, kind of depths, headloss?)

Sluice Gate or spillway, energy equation, alternate depths, no headloss

Rapid Varied Flow/ Hydraulic Jump (structure, equation, depths, headloss?)

channel or spillway, momentum, conjugate depths, significant head loss

Why is there no head loss with a sluice gate?

there is a very short distance between pt 1 and pt 2

Energy Equation

E= [y+ Q²/2gA²]

Specific Energy Plot

y (depth) vs E

Mirror image across the axis of critical depth

where the two halves meet y axis = critical depths

where the two halves meet x axis = minimum energy state

above yc axis = subcritical flow

below yc axis = supercritical flow

data produced by varying the opening of sluice gate

Physical characteristics of subcritical flow

deeper depth, slower velocity

Physical characteristics of supercritical flow

shallower depths, faster velocity

What does critical depth represent

minimum energy state of a system (Ec)

On a sluice gate schematic where is sub and super critical flow

upstream/ behind sluice gate = subcritical

directly downstream/ after sluice gate = super critical

more downstream = critical

even farther downstream subcritical

Positives and negatives of subcritical flow

positive: slower flow, safer waters for plants, animals, people

negatives: sediment deposition

Positives and negatives of supercritical flow

positive: lots of energy

negatives: dangerous for people and animals (need high retaining walls)

What conditions make RVF/hydraulic jump happen

supercritical, high velocity flow quickly coming to a slow

Ex: down a spillway water moving quickly, hits bottom of channel and water jumps up

T/F: You can use energy equation to model hydraulic jump

False: there is significant head loss with RVF and energy equation does not consider head loss

How does hydraulic jump effect a channel

erodes the channel near the spillway

to correct extend the concrete further out or add obstruction/ energy dissipaters

What- if problems: how to evaluate uniform flow

Mannings equation

What- if problems: how to evaluate specific energy

specific energy equation

What- if problems: how to evaluate rapid varied flow

check by visual hydraulic jump and turbulence

head loss not equal to 0

What- if problems: how to evaluate channel with 2 different depths, short distance apart

use specific energy

why?

no turbulence = not RVF

short distance = not GVF

y1 not equal y2 = not uniform

What- if problems: how to evaluate sloped channel with 2 different depths

specific energy if not rapid varied flow

RVF is very steep slope causes turbulence

How to analyze given just Froude number and direction of flow

Remember Fr number classifications

big difference in Fr and short distance between them = RVF

Characteristics of gradual varied flow (GVF)

y1 not equal y2

gradual change in depths

long, significant difference between depths (delta x)

How to calculate GVF delta x: Direct Step

step down channel in flow direction

vary delta y to calculate delta x

y1 and y2 are known

can calculate area, perimeter, hydraulic radius, velocity

Why use a step method for GVF

Calculated slopes are straight lines

actual slopes are not, so there is a gap in the actual slope vs the calculated

less error

How to calculate GVF delta x: standard step

vary delta x to solve for delta y

known: delta x, slope, n

unknown: depths, area, wetted perimeter, hydraulic radius, etc

Would need computer software to solve (HEC-RAS)

When to use standard step method

width changes of channel or natural channels shapes

What is a check dam?

Temporary structure used during construction

Fr or yc to check flow type

OCF Design: primary jobs

collect volume (v) of water

flow (Q) water in a direction

OCF Design: assumptions

once channel is flowing, assume uniform flow

model with Manning’s equation

Manning’s Equation

Q= (B/n)* A*Rn^(2/3)*So^(1/2)

OCF Design: most hydrological efficient shape and why

trapezoid

easier to dig, easy to make wider as needed

OCF Design: trapezoid design considerations

side slope (m) must follow geotech standards

OCF Design: side slope (m) typical values

solid rock: very very small

sandy: 3

other soils (including clay): 1.5

OCF Design: what makes a shape hydrologically efficient

most Q for the lowest wetted perimeter

want to increase wetted perimeter and head loss, and decrease flow

OCF Design: what kind of flow is designed for and why, how to fix wrong flow type?

subcritical

supercritical can erode/damage channel

supercritical flow can injure people

avoid by making slope more gradual

OCF Design: issues with subcritical flow and how to fix

sediment buildup

minimum velocity = 2fps

OCF Design: what is freeboard

extra space at the top of channel

Freeboard equation

Fb= sqrt(cy)

c coefficient = 1.5-2.5

20 cfs= 1.5

>3000 cfs = 2.5

OCF Design: Lining materials

Rigid: concrete, asphalt, brick

Flexible: bare earth, rock aggregate, grass

Lining materials positives and negatives

Rigid: withstand higher stress, more expensive

Flexible: less expensive, can potentially flex under stress (erosion) and fail

Stress on a channel

shear stress

tau = specific weight * y * So

Where is the maximum shear stress

very bottom of channel

What is critical shear stress (Tau c)

reference number from Table that is relative to the lining material

How to evaluate critical vs max shear stress

max > critical = flexible lining failure

max < critical = lining preserved, no flex, no fail

Types of sewers

sanitary, stormwater, combined

what does sanitary sewer convey

water water, flushables, anything down the drain

what does stormwater sewer convey

stormwater runoff

what does combined sewer convey

wastewater and stormwater

where does sanitary sewer discharge go

wastewater treatment plant

where does stormwater discharge go

natural receiving waters (local watershed), detention/retention basin

where does combined sewer discharge go

wastewater treatment plant and CSO (combined sewer overflow)

Why do we not design combined sewers anymore

high flow event will cause CSO to discharge directly into waterbodies to protecting flooding WWTP = raw sewage into water

Too much stormwater = flooded WWTP and diluted treatment of wastewater

Sanitary sewer jobs

collect flow

flow away from community to discharge

sanitary sewer conveyance options and which is preferred

OCF, pipelines

pipes lines preferred bc of risk of sewage open to public

why are pipelines underground

we can , they are fully enclosed

saves space

won’t freeze

Sanitary pipeline materials

avoid metals: WW very corrosive and will make sulfuric acid

VC (vitrified clay): corrosive resistant, inexpensive, very brittle

Preferred:

PVC

Concrete

T/F: pumps are used in sewer design

F

why are pumps not used in sewer design

WW will clog, damage, and corrode pumps

Sewer design considerations

avoid full sewer pipes (need safety factor for high flow surges)

design partially full pipes (OCF)

driven by elevation/slope

T/F: sewer design is considered OCF

T

Size to fullness ratios for sewer design

½ full = up to 16” diameter

2/3 full = 16”-36” diameter

¾ full = 36” + diameter

T/F: sewer flow is uniform and can be modeled with manning’s equation

T

Qdesign for sewer

Qdesign = Qpeak + QI/I

Peak factor from Figure 16.3

What is QI/I

inflow/infiltration flow

rainwater entering into sanitary sewer (NOT COMBINED)

Qave for sanitary

100 gpcd

Manning coefficient for sanitary sewer

SS coated with slime

Additional design criteria for sanitary sewer

min velocity = 2fps

max velocity = 10fps (erosion)

flow state not relevant

design for slope (Table 14.5)

How to design diameter of sanitary sewer

y/do= Q/Qfull graph

find the Q full value

plug into mannings to find total diameter

High stormwater flow problems

erosion

**loss of life **

damage to property

Goals of stormwater management

understand and model stormwater systems (hydrology)

engineer hydraulic structures to manage stormwater

Hydrology and the water cycle 2 outcomes

1. precipitation

   1. infiltration = no flooding

   2. runoff = flooding

2. storm event

T/F: infiltration is a good thing but the ground has an absorption capacity

T anything not absorbed = runoff

Which aspect of precipitation is used to design hydraulic structures

Runoff

Types of precipitation

rain, snow, hail, sleet, mist

T/F: every community at some point in time WILL experience a bad storm that will cause flooding

T

T/F: weather predictions are accurate

F

Precipitation classification: Volume

amount of precipitation that fell

True volume equation

True Vol = p (in) x surface area

Precipitation classification: duration

length of time of precipitation (hrs or min)

Precipitation classification: intensity

how much precipitation fell/ time

inces/hr

Precipitation classification: frequency

how often/frequent a storm event occurs

return period (2 yr, 10 yr, 25 yr storm)

what does a 10 year storm mean

on average, a community will experience a this kind of storm event every 10 years

T/F: longer return period = bigger storm

T

T/F: a community can experience 2 100 yr storms in the same year

T

Storm design

1. select return period (10 or 25 yr)

2. select duration

what kind of structures are built for a 100 yr storm

interstates, roads to hospitals

what kind of structures are built for 500 yr storms

dams or levees by major rivers close to cities

what is used for precipitation data

measuring it

using classic references (NOAA ATLAS 14)

precipitation trends

increase duration, increase volume

increase return period, increase volume

increase duration and return period, increase volume

increase duration and decrease return period, increases volume

T/F: duration is the more important influencing factor for precipitation trends

T

what is a watershed

area with real boundaries, a real shape, size and defined via an outlet

influential characteristics of watershed

size,area, boundaries

hydrological flow path

hydrologial flow length

flow path characteristics (overland flow, channel flow)

slope

surface cover characteristics

underlying soils

abstractions