WQ 4.2.2 Rating Curve Washoff

In natural catchments and rivers,
both theory and data support the result

load rate of sediment is proportional to flow rate raised to a power

For instance, sediment data from streams can usually be described by a sediment rating curve of the form

w=K_w × Q × N_W ——Eq4-8

,
but

Vanoni (1975), pp. 220-225,
Graf (1971), pp. 234-241, and
Simons and Senturk (1977), p. 602.

Vanoni (1975), pp. 472-481
of particular relevance to overland flow washoff is the appearance of similar relationships describing sediment yield from a catchment

the functional form is evident in many rivers

due to a hysteresis effect, such relationships may vary during the passing of a flood wave

similarity of Equation 4-8 to the exponential washoff function 4-7 (w=Kw × q^(N_W) × m_B)

  • w：sediment loading rate (mass/sec),
  • Q：flow rate (cfs)
  • KW and NW: coefficients

the total quantity of sediment washed off a largely impervious urban area is likely to be limited to the amount built up during dry weather,
the presence of buildup mB in Equation 4-7 reflects the fact

Natural catchments and rivers from which Equation 4-8 is derived generally have no source limitation

difference between the form of the runoff rate used in the two functions

Exponential washoff
uses a normalized runoff rate, q in (inches/hr), over the total subcatchment surface (both pervious and impervious areas)

Rating curve washoff
uses the volumetric runoff rate Q in cfs, over the fraction fLU of total subcatchment area A (in acres) devoted to the land use being analyzed. That is,Q=q × fLU × A

The rating curve approach, combined with constituent buildup
if desired to limit the total mass that can be washed off

Otherwise, there is no buildup between storms during continuous simulation, nor will measures like street sweeping have any effect. Constituents will be generated solely on the basis of flow rate.

If buildup is simulated when a rating curve is used

the maximum amount can be removed is the amount built up prior to the storm

only if this limit is reached

zero values last

if the buildup limit is reached

Street sweeping will have an effect

until dry-weather time steps occur to allow buildup

time loads and concentrations, drop to zero, suddenly

The rating curve method is generally easiest to use

when only total runoff volumes and pollutantloads are available for calibration.