HYDROLOGY & HYDRAULICSRace Street Dormitory

J. Richard WeggelCAEE201 Lecture23 October 2006

HYDROLOGY(Natural Science)

• Study of the waters of the earth– Oceanography– Meteorology– Limnology– Potomology– Engineering Hydrology

• Surface water hydrology• Ground water hydrology

HYDROLOGIC CYCLE

• Where water exists on the earth and how it moves from place to place – processes

Location:

Ocean

Lakes, rivers, streams

Below ground

Atmosphere

Processes:

Precipitation

Runoff

Evaporation

Transpiration

Infiltration

HYDROLOGIC CYCLE

PRECIPITATION

• Geographic Distribution of Rainfall– National Weather Service – collects rainfall

data all around the United States• Rain gages

– Daily accumulations– Recording gages

• Radar– Current precipitation rates– STP = “storm total precipitation” - for a time period

STANDARD RAINGAGE - ACCUMULATION

RECORDING RAIN GAGE

Weighing type

MEAN ANNUAL RAINFALL IN THE UNITED STATES

100-YEAR, 30 MINUTE RAINFALL

Depth-Duration-Frequency Data - Philadelphia PA

0

1

2

3

4

5

6

7

8

10 100 1000 10000

Duration (minutes)

Dep

th (i

nche

s)

100 yr

50 yr

25 yr

10 yr

5 yr

2 yr

1 yr

RAINFALL STATISTICS FOR THE PHILADELPHIA AREA

TIME OF CONCENTRATION

• The time it takes for water to flow from the farthest reach of a watershed to the point where discharge is measured. For times greater than the “time of concentration” all of the watershed is contributing to the discharge.

• Rainfall durations equal to or greater than the time of concentration give maximum flows.

A WATERSHED, ISOCHRONES & RUNOFF HYDROGRAPH

HYDRAULICS(Engineering Science)

• Moving water around (controlled flows)– Pipe flow– Free surface flow (open channels)

OPEN CHANNEL FLOW• Overland flow –

“sheet flow”• Channel flow

– Manning’s equation for open channel flow

• V = velocity• n = “roughness

coefficient”• R = hydraulic radius• S = slope• A = flow area• WP = wetted perimeter

– Conservation of mass

2/13/249.1 SRn

V =

VAQ =

WPAR =

SOME “n” VALUES

TRAVEL TIME

)(xVxt = Travel time = distance/velocity

Note: Velocity can depend on where you are along the channel, x

ROOFTOP DRAINS

ASSIGNMENT

• Using a 2-year storm having a duration of one hour in the Philadelphia area, determine the volume of water that will fall on the dorm site. Give your answer in ft3. What is the rainfall intensity associated with that storm?

• How much of that water fell on the roof of the building and how much on the surrounding area? (Use the attached approximate building footprint.)

• How much water will flow down each drain in one hour? (Determine tributary drainage area from the attached building footprint and roof drain locations.)

ASSIGNMENT (continued)

• Using the handout site plan and the proposed contours (solid lines), sketch, in some distinctive color, the direction in which water will flow across the site.

• Using the site plan, construct a profile across the site going diagonally from the northwest corner to southeast corner of the lot, i.e., elevation v. distance.

• Determine the average slope of the ground going across the site in this direction.

ASSIGNMENT (continued)

• Determine the average flow velocity across the site. (Use the rainfall depth for the hydraulic radius, the slope you found above and an appropriate “n” of your choosing.)

• Determine the travel time for water to go from the northwest corner of the lot to the southeast corner. How does this value compare with the rainfall duration used? What can you say about the time of concentration?