HYDROLOGY I
CHAPTER 2 (1) - ROBERSON ET AL, WITH ADDITIONS

HYDROLOGY

HYDROLOGY IS THE SCIENCE THAT DEALS WITH THE OCCURRENCE OF WATER, PRIMARILY SURFACE WATER.

GROUNDWATER HYDROLOGY RELATES TO WATER OCCURRING UNDER GROUND.

HOWEVER, SURFACE WATER AND GROUNDWATER HYDROLOGY ARE RELATED, BECAUSE MOST OF ALL GROUNDWATER EVENTUALLY BECOMES SURFACE WATER.

PROJECTS SUCH AS BRIDGES, AIRPORTS, POWER PLANTS, BUILDINGS, ROADS, HYDROELECTRIC PLANTS, AND IRRIGATION NEED HYDROLOGY.

EXAMPLE: THE FORD BRIDGE.

HYDROLOGY IS USED TO:
-- DETERMINE PEAK FLOWS
-- DETERMINE ANNUAL VOLUMES
-- MODEL FLOWS
-- MEASURE PARTS OF THE HYDROLOGIC CYCLE
-- SIZE SPILLWAYS
-- SIZE RESERVOIRS

THERE ARE THREE TYPES OF HYDROLOGY:
-- FLOOD HYDROLOGY (MAXIMUM FLOWS)
-- YIELD HYDROLOGY (MEAN FLOWS)
-- DROUGHT HYDROLOGY (LOW FLOWS)

HYDROLOGY HAS MANY FACETS.

HYDROLOGY ASSESSES RAINFALL AND STREAMFLOW QUANTITATIVELY.

HYDROLOGY RELIES ON ANALYSIS AND MEASUREMENT.

SOMETIMES, ONE COMPLEMENTS THE OTHER.

MEASUREMENTS ARE BETTER, BUT THEY ARE EXPENSIVE.

BESIDES, THE 100-YR FLOOD CANNOT BE MEASURED ON DEMAND.

ANALYSIS IS NECESSARY.

ANALYSIS COULD BE BASED ON:
-- STATISTICS (STATISTICAL HYDROLOGY: GUMBEL METHOD)
-- CONCEPTS (CONCEPTUAL MODELS: RUNOFF CURVE NUMBER)
-- PHYSICAL EQUATIONS (DETERMINISTIC: KINEMATIC WAVE)
-- EMPIRICAL EQUATIONS (PARAMETRIC: USGS STATE EQUATIONS).

HYDROLOGIC CYCLE

HYDROLOGIC CYCLE OR WATER CYCLE CONSISTS OF:
-- PRECIPITATION
-- ABSTRACTIONS
-- RUNOFF

PRECIPITATION VARIES
-- SPATIALLY
-- TEMPORALLY
-- SEASONALLY
-- GEOGRAPHICALLY (BASED ON CLIMATE)

PRECIPITATION CAN BE DESCRIBED IN TERMS OF INTENSITY (MM/HR) OR DEPTH (MM).

FOR SMALL CATCHMENTS, MM/HR.

FOR MIDSIZE AND LARGE CATCHMENTS, MM.

100-YR FREQUENCY PRECIPITATION RECURS EVERY 100 YEARS ON THE AVERAGE.

PMP: PROBABLE MAXIMUM PRECIPITATION FOR A REGION.

EVENT VS. ANNUAL PRECIPITATION.

IN SAN DIEGO, IT MAY RAIN 4 INCHES (100 MM) IN ONE DAY.

YET, THE MEAN ANNUAL PRECIPITATION (RAMONA) IS ONLY 15 IN (375 MM).

Ms. Arvila Johnson measuring Campo raingage, San Diego County.

EVENT PRECIPITATION DETERMINES THE PEAK FLOW IN RIVERS.

ANNUAL PRECIPITATION DETERMINES THE MEAN FLOWS (VOLUMES).

PRECIPITATION IS PRODUCED BY COOLING OF AIR MASSES.

THREE PROCESSES IN RAIN FORMATION:
-- OROGRAPHIC LIFTING
-- THERMAL LIFTING
-- FRONTAL LIFTING.

HYDROLOGIC ABSTRACTIONS:
-- INTERCEPTION
-- DEPRESSION STORAGE
-- EVAPORATION
-- EVAPOTRANSPIRATION
-- INFILTRATION.

EQUATIONS OF WATER BALANCE
-- P = S + W
-- W = U + V
-- V = E + T
-- E = E_n + E_w
-- R = S + U
-- P = R + V

INTERCEPTION FORMS PART OF W AND T.

DEPRESSION STORAGE FORMS PART OF E.

INFILTRATION FORMS PART OF W, U AND T.

DIRECT RUNOFF IS S.

BASEFLOW IS U.

RAINFALL EXCESS GOES INTO S.

SHORT TERM: P = S + W

SURFACE RUNOFF IS IMPORTANT IN THE SHORT TERM (FLOODS).

R= S + U

P = R + V

TOTAL RUNOFF R (SURFACE RUNOFF PLUS SUBSURFACE RUNOFF) IS IMPORTANT IN THE LONG TERM (YIELD AND WATER SUPPLY).

P = S + W;

P = R + V

WETTING AND VAPORIZATION ARE REFERRED TO AS LOSSES, IN BOTH SHORT AND LONG TERM.

THEREFORE: PRECIPITATION = RUNOFF + LOSSES

RUNOFF COEFFICIENT K_r = R/P

K_r VARIES FROM 0.02 (ARIZONA) TO 0.60 (AMAZON)
GLOBALLY, K_r AVERAGES 0.40.

Tapeats Creek flowing directly into the Colorado river, Arizona.

Seepage from overlying plains into Vitor valley, Peru.

RUNOFF CONCENTRATION PRINCIPLE

IF IT RAINS I MM/HR ON A CATCHMENT/BASIN OF AREA A (SQ KM) FOR A SUFFICIENTLY LONG PERIOD (AT LEAST EQUAL TO THE TIME OF CONCENTRATION), AND THERE ARE NO LOSSES (INFILTRATION), THE DISCHARGE AT THE OUTLET WILL EVENTUALLY ATTAIN A VALUE:

Q = IA

Q IN UNITS OF [(MM/HR) ⋅ KM²]

WHICH CAN BE CONVERTED TO M³/S

IF THE BASIN IS SMALL, THE RAINFALL EVENT IS PROPERLY CHARACTERIZED BY THE RAINFALL INTENSITY ONLY (DISREGARD TEMPORAL AND SPATIAL VARIATIONS).

THE RATIONAL METHOD FOR ESTIMATING PEAK FLOWS IS SMALL CATCHMENTS IS:

Q_p = CIA

WHERE C IS A RUNOFF COEFFICIENT, INDICATING THE PERCENTAGE OF TOTAL RUNOFF THAT SHOWS UP AT THE BASIN OUTLET.

THE LOSSES ARE DUE TO INFILTRATION.
IN U.S. CUSTOMARY UNITS:

Q_p (CFS) = 1.008 C I (IN/HR) A (ACRES)

IN PRACTICE, THE 1.008 VALUE IS DISREGARDED.

ONLINE RATIONAL METHOD

PRECIPITATION

MOST OFTEN THE HYDRAULIC ENGINEER IS INTERESTED IN RUNOFF.

HOWEVER, THERE IS MORE PRECIPITATION DATA THAN RUNOFF DATA.

IT IS NOW OBVIOUS THAT RUNOFF COMES AS A RESULT OF PRECIPITATION.

NOTE THAT ONLY S COMES FROM P DIRECTLY.

BUT RUNOFF R = S + U.

U COMES FROM THE GROUNDWATER EXFILTRATING AS BASEFLOW.

PRECIPITATION IS DESCRIBED IN TERMS OF DURATION AND DEPTH.

RAINFALL INTENSITY AND AMOUNT DEPEND ON THE HADLEY CELLS.

AIR DESCENDS AT 30 AND 90 DEGREES; ASCENDS AT 0 AND 60 DEGREES.

THERE IS LITTLE PRECIPITATION AT 30^o LATITUDE AND 90^o LATITUDE (AT POLES).

FOR A MAJOR STORM TO DEVELOP, THE ATMOSPHERE MUST CONTAIN A LARGE AMOUNT OF MOISTURE, HAVE A SOURCE OF ENERGY, AND HAVE A LIBERAL INFLOW OF MOISTURE FROM OUTSIDE THE STORM AREA.

ENVELOPE OF MAXIMUM RAINFALL EVENTS:

R = 15.3 D ^0.486

R IN INCHES; D IN HOURS.

I = dR/dD = 15.3 × 0.486 × D^{0.486 - 1} =
I = 7.44 / D^0.514
THE LONGER THE RAIN DURATION, THE LESSER THE RAIN INTENSITY.
SPATIAL DISTRIBUTION OF RAINFALL:
(a) STATION AVERAGE METHOD:
CATCH OF ALL GAUGES IS SIMPLY AVERAGED
(b) THIESSEN POLYGONS METHOD:
PERPENDICULAR BISECTORS TO LINES CONNECTING ALL ADJOINING GAUGES, TO DEFINE AREA OF INFLUENCE OF EACH GAUGE.
(c) ISOHYETAL METHOD:
CONTOURS OF EQUAL PRECIPITATION ARE PLOTTED, WHICH DELIMIT AREAS OF INFLUENCE OF EACH MEAN ISOHYET VALUE.

DEPTH-DURATION-FREQUENCY RELATIONS:

THESE ARE THE ISOPLUVIAL MAPS PUBLISHED BY NOAA.

INTENSITY-DURATION-FREQUENCY IS USED FOR SMALL CATCHMENTS (URBAN HYDROLOGY).

SYNTHETIC DESIGN STORM

USED FOR MIDSIZE AND LARGE CATCHMENTS.

TO CONSTRUCT A DESIGN STORM FOR A GIVEN RETURN INTERVAL, SELECT THE RAINFALL DEPTH FOR EACH DURATION FROM ISOPLUVIAL MAPS.

THE RAINFALL DEPTHS CAN BE PLOTTED AGAINST DURATION FOR VARIOUS RETURN INTERVALS, AS SHOWN IN FIG. 2-20 (BELOW).

TABLE 2-6 IS OBTAINED FROM FIG. 2-20.

THE SEQUENCE OF RAINFALL DEPTHS IS MORE OR LESS ARBITRARY.

THE ENGINEER MUST REARRANGE THE INCREMENTAL VALUES TO REPRESENT A REASONABLE STORM PATTERN.

THE PEAK INCREMENT SHOULD BE PLACED AT ONE-THIRD POINT OF THE STORM DURATION.

CONTINUOUS INCREASE BEFORE, AND DECREASE AFTERWARDS.

POINT RAINFALL IS GREATER THAN AVERAGE RAINFALL OVER AN AREA.

FIG. 2-21 NWS DEPTH-AREA CORRECTION.

PMP: PROBABLE MAXIMUM PRECIPITATION.

EXPRESSED FOR 24-HR.

USED FOR DAMS UPSTREAM OF POPULATED AREAS.

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