• Engineering hydrology is an applied earth science.

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

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

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

• Describes the continuous recirculatory transport of the waters of the Earth, linking atmosphere, land and oceans.

• Water evaporates from the ocean surface and moves inland.

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

• Return of water to oceans is through surface runoff, driven by gravitational forces.

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

  • Atmosphere

  • Vegetation

  • Snowpack and icecaps

  • Land surface

  • Soil

  • Streams, lakes and rivers

  • Aquifers

  • Oceans

• The liquid-transport phases of the hydrologic cycle are:

  • Precipitation (atmosphere to land surface)

  • Throughfall (vegetation to land surface)

  • Melt (snow to land surface)

  • Surface runoff (land surface to streams to ocean)

  • Infiltration (land surface to soil)

  • Exfiltration (soil to land surface)

  • Interflow (soil to streams)

  • Percolation (soil to aquifers)

Fig. 1-1

Fig. 1-2

• A catchment collects runoff and concentrates it at the outlet.

• The outlet is referred to as "the mouth."

• The terms watershed and basin refer to catchments.

• Small catchments are watersheds; large catchments are basins.

• The hydrologic budget is an accounting of the various transport phases of the hydrologic cycle.

• The following budget considers both surface and groundwater:

Δ S = P - (E + T + G + Q)

• Δ S = change in storage

• P = precipitation

• E = evaporation

• T = evapotranspiration

• G = groundwater flow out of the catchment

• Q = surface runoff out of the catchment.

• The following budget considers only surface water:

Δ S = P - (E + T + I + Q)

• I = infiltration

• Notice some double counting, because I can return immediately as T and can convert to groundwater and eventually return as Q.

• For certain applications (flood calculations), when change in storage Δ S 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

1.4  USES OF ENGINEERING HYDROLOGY

• Examples:

  • Probable maximum flood (PMF) at a damsite, to design spillway.

  • Variation of water yield from season to season; and from year to year.

  • Relation and quantity of surface water and groundwater (baseflow vs. flood flows).

  • Quantity and permanence of low flows (hydropower, water quality).

  • Sizing storage reservoirs (multiannual flows).

  • Hydrologic hardware (field equipment) and software (computer programs).

• Hydrologists use flow rates (m³/sec) and volumes (m³).

• Volumes are often expressed as units of depth (m) over the catchment area.

• Models are either material or formal.

• Material models are physical representations of the prototype.

• Formal model is a mathematical abstraction of an idealized situation.

• Material models are either iconic or analog.

• Iconic models are simplified representations of real systems, such as lysimeters, rainfall simulators, and experimental watersheds.

• Analog models use other substances, such as electricity.

• All formal models are mathematical in nature.

• Mathematical models classify into:

  • Deterministic: Formulated using physical or chemical laws.

  • Probabilistic: Governed by laws of chance.

  • Conceptual: Simplified representation of physical process; works in the mean.

  • Parametric: Uses algebraic equations and is limited to a certain region.

• Kinematic wave routing is deterministic (page 277).

• Gumbel method of flood frequency is probabilistic (page 223).

• Cascade of linear reservoirs (CLR) is conceptual (page 313); runoff curve number is conceptual (page 155);

• USGS State equations for flood determination are parametric (page 519).

• Rational method is conceptual and parametric (page 110).

• Rational method is conceptual because it simulates runoff concentration; parametric because the runoff coefficients are empirical.

• Hydrologic models can be lumped of distributed.

• Lumped models cannot describe spatial variations.

• Distributed models can describe spatial and temporal variations.

• Quasi-distributed model such as Modified Clark is a hybrid: a lumped method in distributed GIS context.

• Solutions can be analytical or mumerical.

• Most models used in practice are numerical in nature (example: HEC-HMS).

1.6  SURFACE RUNOFF, FLOOD HYDROLOGY, AND CATCHMENT SCALE

• Surface runoff occurs when rainfall intensity exceeds the abstractive capability of the catchment.

• It also occurs when the soil profile is saturated.

• Large amounts of surface runoff are referred to as "floods."

• Rainfall varies in space and time.

• Rainfall can be assumed to vary as follows:

  • Constant in time and space: Small catchments, the rational method.

  • Constant in space but varying in time: Midsize catchments, the unit hydrograph.

  • Varying in both space and time: Large catchments, reservoir and channel routing.

• The larger the catchment, the more likely to be gaged.

• Probabilistic approach is suitable for large catchments.

• Parametric approach suitable for midsize catchments (USGS State equations).

• Routing can be used for all size catchments, but is needed for large catchments (due primarily to the spatial variability of rainfall).

Fig. 1-3