ENGINEERING HYDROLOGY 010


1. DEFINITION OF ENGINEERING HYDROLOGY

1.1
Hydrology is one of the earth sciences.

1.2
Its studies the waters of the earth, their occurrence, circulation and distribution, their physical and chemical properties, and their relation to living things.


The hydrologic cycle.

1.3
It encompasses surface and groundwater hydrology.

1.4
Engineering hydrology is an applied earth science.

1.5
It uses hydrologic principles in the solution of engineering problems arising from human exploitation of the water resources of the Earth.

1.6
It seeks to establish relations defining the spatial, temporal, seasonal, annual, regional, or geographical variability of water.

1.7
It aims to establish risks in sizing hydraulic structures and systems.


Emergency spillway at Oroville Dam, Feather river, Northern California.


2. THE HYDROLOGIC CYCLE

2.1
The hydrologic cycle describes the continuous recirculatory transport of the waters of the Earth, linking atmosphere, land and oceans.

2.2
Water evaporates from the ocean surface and moves inland.


The hydrologic cycle (U.S. Geological Survey).

2.3
Atmospheric conditions act to condense and precipitate water onto the land surface.

2.4
The return of water to oceans is through surface runoff, driven by gravitational forces.

2.5
The water-holding elements of the hydrologic cycle are:

    2.6
    Atmosphere: Clouds are contained in the lower atmosphere.


    Clouds in the lower atmosphere.

    2.7
    Vegetation: Tropical rainforests have great quantities of biomass per unit of area, and the biomass contains a lot of water.


    A specimen of Ceiba pentandra on the flood plains of the tropical rainforest, Beni, Bolivia.

    2.8
    Snowpack and icecaps: Glaciers can hold great quantities of water.


    Mt. McKinley or Denali, Alaska.

    2.9
    Land surface: Vegetation gradients reveal differences in moisture.


    Typical geomorphological gradient in the savannah woodlands of Mato Grosso, Brazil.

    2.10
    Soil: Soil can hold significant amounts of water.


    Agricultural drain, Imperial valley, California.

    2.11
    Streams, lakes and rivers: Streams flow by gravity from headwaters to ocean.


    Mississippi river near Bemidji, Minnesota.

    2.12
    Aquifers: Aquifers can hold great amounts of water.


    Aquifer (U.S. Geological Survey).

    2.13
    Oceans: All water originates in the ocean and returns to the ocean.


    Mouth of the Russian river, California.

2.14

The liquid-transport phases of the hydrologic cycle are:

  • Precipitation (from atmosphere to land surface)

  • Throughfall (from vegetation to land surface)

  • Melt (from snowpack to land surface)

  • Surface runoff (from land surface to streams to ocean)


    The Sacramento River at Sacramento, California.

  • Infiltration (from land surface to soil)

  • Exfiltration (from soil to land surface)

  • Interflow (from soil to streams)

  • Percolation (from soil to aquifers)

3. THE CATCHMENT AND ITS HYDROLOGIC BUDGET

3.1
A catchment is a portion of the land surface that collects runoff and concentrates it at one point or outlet.
3.2
The outlet is referred to as the mouth.
3.3
The terms watershed and basin refer to catchments.
3.4
Small catchments are watersheds; large catchments are basins.
3.5
The hydrologic budget is an accounting of the various transport phases of the hydrologic cycle.
3.6
The following budget considers both surface and groundwater:

 


ΔS = P - (E + T + G + Q)

where

ΔS = change in storage

P = precipitation

E = evaporation

T = evapotranspiration

G = groundwater flow out of the catchment

Q = surface runoff out of the catchment.


Hydrologic budget that considers both surface and ground water

3.7
The following budget considers only surface water:


ΔS = P - (E + T + I + Q)

where

I = infiltration


Hydrologic budget that considers only surface water

3.8
Note that there is some double counting, because infiltration I can return eventually as evapotranspiration T.

3.9
Likewise, infiltration can convert to baseflow and eventually return to the surface water as part of surface runoff Q.

3.10
For certain applications, when the change in storage ΔS is zero, and the sum of E + T + I can be taken as the losses to runoff L, then runoff Q is:


Q = P - L

3.11
This equation is very useful in flood hydrology.


4. USES OF ENGINEERING HYDROLOGY

4.1
There are many uses of Engineering Hydrology. Examples are:


The calculation of the Probable Maximum Flood (PMF) at a damsite. All dams require some type of spillway.


Emergency spillway, Morena reservoir, San Diego County, California.

4.2
The variation of water yield from season to season; and from year to year. Most of the water in deserts goes to evaporation, while most of the water in rainforests goes to runoff.


Sahara desert.


Amazon rainforest.

4.3
The relation between surface water and groundwater. The quantities of groundwater are about 100 times greater than the quantities of surface water.


Relations between surface water and groundwater.

4.4
The permanence of low flows, to provide for economically beneficial uses such as hydropower, inland navigation, and water quality.


Mississippi river shipping.

4.5
The multiannual variation of flows for the purpose of sizing large storage reservoirs.


Tarbela dam and spillway, Pakistan.

4.6
Hydrologic field equipment, to measure precipitation and streamflows, including hardware and software.


Campo raingage, San Diego County, California, in continuous operation since 1891.


5. APPROACHES TO ENGINEERING HYDROLOGY

5.1
Models are either material or formal.
5.2
Material models are physical representations of the prototype, that is, physical models.
5.3
Formal models are a mathematical abstraction of an idealized situation.
5.4
Material models are either iconic or analog.
5.5
Iconic models are simplified representations of real systems, such as the lysimeters of Coshocton, Ohio, and San Dimas, California, the rainfall simulator at Colorado State University, and the USDA ARS Walnut Gulch Experimental Watershed near Tombstone, Arizona.

Coshocton weighing lysimeter.


Construction of the San Dimas Experimental Forest lysimeters, 1937-1946.


Land use in the USDA ARS Walnut Gulch Experimental Watershed, surrounding Tombstone, Arizona.


Raingages and flumes in the USDA ARS Walnut Gulch Experimental Watershed, near Tombstone, Arizona.

5.6
Analog models use other substances, such as electricity.
5.7
All formal models are mathematical in nature.
5.8
Mathematical models classify into:
5.9
Deterministic, which are formulated using physical or chemical laws.
5.10
Probabilistic, which are governed by laws of chance.


A pair of dice.

5.11
Conceptual, which are simplified representation of physical processes, intended to work in the mean.
5.12
Parametric, which use algebraic equations and are limited to a certain region.

5.13
The kinematic wave routing is deterministic.
5.14
The Gumbel method of flood frequency is probabilistic.
5.15
The cascade of linear reservoirs is conceptual.
5.16
The runoff curve number is conceptual, but based on parametric data.
5.17
The USGS State equations for flood peak estimation are parametric.

South Coast Region, California

Q100 = 1.95 A0.83 P1.87

5.18
The rational method is conceptual because the peak flow Qp follows the principle of runoff concentration, that is, the product of rainfall intensity I times the drainage area A, and parametric because the runoff coefficient C is empirical.


Qp = C I A

5.19
Hydrologic models can be lumped, distributed, or quasi-distributed.
5.20
The lumped models describe temporal variations but cannot describe spatial variations.
5.21
The distributed models can describe both temporal and spatial variations.


Schematic for a distributed catchment model (United Nations University)
5.22
A quasi-distributed model such as HEC-HMS's Modified Clark is a hybrid, that is, a lumped method in a distributed GIS context.
5.23
The solutions can be analytical or numerical.
5.24
The models used in practice are numerical in nature, such as HEC-HMS.


6. SURFACE RUNOFF, FLOOD HYDROLOGY AND CATCHMENT SCALE

6.1
Surface runoff occurs when rainfall intensity exceeds the abstractive capability of the catchment.
6.2
It also occurs when the soil profile is saturated.
6.3
Large amounts of surface runoff are referred to as floods.


Flood stage in the Chane river, Santa Cruz, Bolivia, on January 19, 1989.
6.4
Rainfall varies in space and time.
6.5
For modeling purposes, rainfall can be assumed to vary as follows:
6.6
Constant in time and space, applicable to small catchments, as with the rational method.
6.7
Constant in space but varying in time, applicable to midsize catchments, as with the unit hydrograph.
6.8
Varying in both space and time, applicable to large catchments, as with reservoir and channel routing.

Method / ScaleSmallMidsizeLarge
A Rational method Usually Not applicable Not applicable
B Unit hydrograph Not applicable Usually Sometimes
C Routing methodologies Sometimes Sometimes Usually

6.9
The larger the catchment, the more likely that it is gaged.
6.10
The probabilistic approach is suitable for large catchments, but also for midsize catchments.
6.11
The parametric approach is suitable for midsize catchments, as with the USGS State equations.
6.12
The unit hydrograph, a conceptual method, is particularly applicable to midsize catchments.
6.13
Routing can be used for all size catchments, but it is required for large catchments, due primarily to the spatial variability of rainfall.
6.14
In flood hydrology, the choice of method is a function of catchment scale.

6.15
Hydrologic methods vary from country to country.
6.16
Some conceptual U.S. methods such as the NRCS runoff curve number are used throughout the world.

Lecturer:  Dr. Victor M. Ponce

Music:  Fernando Oñate

Editor:  Flor Pérez

Credits:  Google

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