Description

appropriate in cases where equilibrium sediment transport can be assumed, i.e.,

when the project will not significantly change the existing hydraulic or sediment

conditions. When the purpose of the sediment study is to evaluate the effect of a

project on sediment transport characteristics, i.e., the project or a flood will

introduce non-equilibrium conditions, then a multiple-grain-size sediment

transport function should be used. Multiple-grain-size functions are very

sensitive to the grain-size distribution of the bed material. Extreme care must be

exercised in order to ensure that the fine component of the bed-material gradation

is representative of the bed surface for the specified discharge. This is very

difficult without measured data. For this reason Einstein (1950) recommended

ignoring the finest 10 percent of the bed material sample for computation of bed-

material load with a multiple-grain-size function. In HEC-6 and SAM, single-

grain-size functions are converted to multiple-grain-size functions simply by

calculating sediment transport using geometric mean diameters for each size class

in the bed (sediment transport potential) and then assuming that transport of that

size class (sediment transport capacity) can be obtained by multiplying the

sediment transport potential by the bed fraction. This can produce unreliable

results since the assumption is that each size class fraction in the bed acts

independent of other size classes on the bed, thus ignoring the effects of hiding.

Description

SAM.aid is based on the premise that a sediment transport function that

accurately predicts measured sediment in a gaged stream would be an appropriate

predictor in an ungaged stream with similar characteristics. SAM.aid compares

calculated "screening parameters" for a given river to the same screening

parameters from a database of rivers (Brownlie, 1981) that have sufficient

sediment data to determine an appropriate sediment transport function. The

"screening parameters" are velocity, depth, slope, width, and d50. It should be

noted that Brownlie reduced measured bed material gradations to median grain

sizes and geometric standard deviations. This means that this guidance is not

applicable to rivers that have bed gradations that are not log-normally distributed.

When the user inputs velocity, depth, slope, width and d50 for an ungaged

river, SAM.aid compares each of these screening parameters with those of each

river in the Brownlie database. A "match" is identified when a parameter falls

within the range of data for a database river. The d50 must fall within the range

for a river in the database before SAM.aid will examine the other parameters.

The three best sediment transport functions for each database river is then listed,

along with the type of parameter(s) that matched and the name of the data set

matched.

After the matches are displayed, the user can check the description of the

rivers on which SAM.aid based its choices to see how close those descriptions

match the user's river or stream. This is an essential step in ensuring that the

sediment transport functions will actually provide the best predictive capability

for the river in question. This will also narrow the choices when SAM.aid

Chapter 5

Theoretical Basis for SAM.aid