CIVE 445 - ENGINEERING HYDROLOGY

CHAPTER 5A: HYDROLOGY OF MIDSIZE CATCHMENTS, RUNOFF CURVE NUMBER

  • Catchments possessing some or all of the following properties are midsize in a hydrologic sense:

    • rainfall intensity varies within the storm duration.

    • rainfall can be assumed to be uniformly distributed in space.

    • storm duration is usually shorter than time of concentration.

    • runoff is by overland flow and stream channel flow.

    • channel storage processes are small and are usually neglected.

  • Catchment response is described by methods that take explicit account of the temporal variation of rainfall intensity.

  • The most widely used method to accomplish this is the unit hydrograph.

  • In the unit hydrograph, a hydrograph for a unit storm is derived, and used as a building block to develop the composite or flood hydrograph.

  • The assumption of uniform spatial rainfall distribution is a characteristic of midsize catchment analysis.

  • This assumption allows the use of a lumped method such as the unit hydrograph.  

     

  • For large catchments, the rainfall varies spatially.

  • Large catchments have a substantial capability for channel storage.

  • This is because large catchments have large rivers, which usually have mild gradients.

  • Mild gradients lead to diffusion and to channel storage.

  • This means that large catchments cannot be analyzed with lumped methods such as the unit hydrograph.

  • Its is necessary to use channel routing and distributed catchment modeling.  

     

  • The upper limit of a midsize catchment is somewhat arbitrary.

  • Values between 1000 km2 (400 sq mi) to 5000 km2 (2000 sq mi) have been used to describe the upper limit of a midsize catchment.

  • No consensus to date, but the current trend is toward the lower limit.

  • There is a range of sizes which constitutes a gray area between midsize and large.

  • The larger the area, the less likely it is that lumping will provide the necessary spatial detail.

  • Unit hydrograph techniques can be used as part as distributed models for large catchments.

  • A large catchment can be viewed as a collection of midzise subcatchments.

  • Distributed models: HEC-1, HEC-HMS, TR-20 (NRCS), HSP-F, SWMM, SSARR (Corps of Engineers), Sacramento (NWS).

  • Other models: SHE (Europe), TANK (Japan).

  • Channel routing is not restricted to large catchments.

  • Channel routing can be used for small and midzise catchments if increased accuracy and data availability warrant more precise calculations.

  • Distributed computations benefit from channel routing.
5.1  RUNOFF CURVE NUMBER METHOD

  • The runoff curve number method is a procedure for hydrologic abstraction developed by the NRCS (ex SCS).

  • Runoff depth is a function of total rainfall depth and an abstraction parameter referred to as "curve number" or CN.

  • The curve number varies in the range 1-100, being a function of the following runoff-producing properties of the catchment:

    • hydrologic soil type

    • land use and treatment

    • ground surface condition

    • antecedent moisture condition.

  • The method is based on 24-hr rainfall data.

  • The method does not account for temporal variations of rainfall.

  • Temporal variations of rainfall are introduced during the conversion of effective rainfall into runoff by means of the unit hydrograph.

  • Runoff curve number equations: See page 156 and page 157

Fig. 5-2

 

Estimation of runoff curve number from tables

  • Tables of CN's for selected hydrologic soil-cover complexes are available.

  • The soil-cover complex describes a specific combination of

    • hydrologic soil type (group),

    • land use and treatment,

    • hydrologic condition, and

    • antecedent moisture condition.

  • All these have a direct bearing on the amount of runoff produced by a catchment.

  • The hydrologic soil group describes the type of soil and its hydraulic conductivity.

  • The land use and treatment describe the type and condition of vegetative cover.

  • The hydrologic condition refers to the ability of the catchment surface to enhance or impede direct runoff.

  • The antecedent moisture condition accounts for the recent history of rainfall and the moisture stored.

  • All soils are classified into four hydrologic soil groups of distinct runoff-producing properties:

    • Group A: soils of low runoff potential, having high infiltration rates even when wetted thoroughly. They are primarily deep, very well drained sands and gravels. High rate of water transmission.

    • Group B: soils with moderate infiltration rates when wetted thoroughly, primarily moderately deep to deep, moderately drained to well drained, with moderately fine to moderately coarse textures. Moderate rate of water transmission.

    • Group C: soils with low infiltration rate when wetted thoroughly, primarily soils having a layer that impedes downward movement of water or soils of moderately fine to fine texture. Slow rate of water transmission.

    • Group D: consists of soils of high runoff potential, having very slow infiltration rates when wetted thoroughly. They are primarily clay soils with a high swelling potential, soils with a high permanent water table, soils with a clay layer near the surface, and shallow soils overlying impervious material. Very slow rate of water transmission.

  • Land use and treatment pertains to the watershed cover, including every kind of vegetation, litter and mulch, fallow (bare soil), nonagricultural uses (lakes, swamps), impervious surfaces (roads, roofs).

  • Land treatment applies mainly to agricultural land uses, and it includes mechanical practices such as contouring or terracing, and management practices such as grazing control and crop rotation.

  • A class of land use/treatment is a combination often found in a catchment.

  • The curve number method distinguishes between cultivated land, grasslands, and woods and forests.

  • Hydrologic condition: Grasslands are evaluated by the hydrologic condition of native pasture.

  • The percent of aerial coverage by native pasture and the intensity of grazing are visually estimated.

    • A poor hydrologic condition describes less than 50% aerial coverage and heavy grazing.

    • A fair hydrologic condition describes 50 to 75% aerial coverage and medium grazing.

    • A good hydrologic condition describes more than 75% aerial coverage and light grazing.

  • Antecedent moisture condition (AMC): The curve number method has three levels of AMC (depending on the total rainfall in the 5-day period preceding a storm).

    • Dry AMC: Lowest runoff potential.

    • Average AMC: Average runoff potential.

    • Wet AMC: Highest runoff potential.

  • Table of corresponding runoff curve numbers for three AMC conditions: See page 165  

  • Estimation of curve numbers from tables:
 

Assessment of runoff curve number method

  • The CN method is simple.

  • The CN values are related to major runoff-producing properties.

  • The method should be used judiciously.

  • The results are sensitive to CN values.

  • Standard tables provide helpful guidelines.

  • Local experience is recommended for increase accuracy.

  • Typical CN's used in design are in the range 55-98.

  • The method is sensitive to AMC.

  • This appears to be a limitation; however, in nature runoff is also very sensitive to AMC.
 

Summary

  • The CN method is a conceptual model to estimate runoff volumes based on established hydrologic abstractions.

  • CN's obtained from tables tend to be conservative for midsize and large in arid and semiarid regions.

  • Channel transmission losses in washes and arroyos are not properly accounted for by the method.

  • Applicability recommended for NRCS-scale catchments: small and midsize.

  • Applicability for large catchments has not been validated fully.

  • Recent review of the CN method: See Ponce and Hawkins paper  link 3137.

  • Interview with Vic Mockus  link 1717.

 

Go to Chapter 5B.

 
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