W.Q.Chapter4-Surface Washoff
4.1 introduction
definition for washoff
from a subcatchment surface
during runoff
when water depth is > a few mm
theory1/mechanism1 for erosion
described by sediment transport theory
what & how
applying over
larger channels
pervious areas
in street gutters
mass flow rate of sediment is proportional to
flow
bottom shear stress
a critical shear stress used to
determine incipient motion
of a particle resting on the bottom of a stream channel
by (Graf, 1971; Vanoni, 1975)
for thin overland flow
rainfall energy can also cause particle detachment and motion
This effect is often incorporated into predictive methods for erosion from pervious areas
by(Wischmeier and Smith, 1958; Haan et al., 1994; Bicknell et al., 1997)
may apply to washoff from impervious surfaces
must consider the effect of a limited supply (buildup) of the material
process of erosion or dissolving
4.2 Governing Equations
to represent pollutant washoff
SWMM incorporates 3 different choices of empirical models
(4.2.3) EMC Washoff
sediment transport based theory is often insufficient in practice
reason1: ？？lack of data for parameter (e.g., shear stress) evaluation, sensitivity to time step and discretization
reason2: simpler methods usually work as well & are usually able to duplicate observed washoff phenomena
by Ammon (1979)
reviewed several theoretical approaches for urban runoff washoff
(4.2.2) Rating Curve Washoff
(4.2.1) Exponential Washoff