9568

• UCREFRP Catalog Number: 9568
• Author: Resource Consultants Inc.
• Title: Sediment Transport Studies of the Little Snake, Yampa, and Green River Systems.
• USFW Year: 1991.
• USFW - Doc Type: Fort Collins, CO.
• Copyright Material: NO

5.2.3 Comparison of Rating Curves. In Section 5.2, coefficients for sediment rating curves for
Deerlodge Park and Mathers Hole were derived. Sediment continuity between these two sites was assessed
by comparison of these two sets of sediment data. To maintain sediment continuity, the inflow of sediment
(from Deerlodge Park), less the outflow of sediment (at Mathers Hole), must be equal to the time rate of
change in sediment storage between these two locations. Theoretically, the continuity of sediment can be
evaluated by performing a sediment budget analysis where inflows and outflows of sediment in a selected
reach are summed over a period of time and subtracted. The difference between these two values is then
the volume of sediment stored (If sediment inflows are greater than outflows), or removed (if outflows of
sediment are greater than inflows).

Typically, sediment continuity is evaluated by conducting a sediment budget analysis. Such an
analysis can be conducted using sediment rating curves which relate water discharge to sediment discharge.
Given a water discharge history, the sediment discharge can be computed at the inflow section and outflow
section of a selected reach.

O~ectives

While simple in concept, the application of the sediment continuity principle using a sediment
budget analysis must be carefully considered based upon the objectives of the analysis and the applicability
of the data available to perform the analysis. One objective of such an analysis would be to evaluate existing
conditions to determine whether the reach is aggrading (deposition), or degrading (erosion). For this type
of analysis the measured rating curves for the upstream and downstream locations of the study reach can be
used to derme the sediment inflow and sediment outflow. If the change in sediment volume is small (i. e.,
the inflow and outflow are approximately equal), then it can be concluded that neither significant erosion or
deposition is occurring. However, it is important to recognize that this does not necessarily imply that the
reach is in equilibrium, since equilibrium is dermed as a balance between supply and transport capacity (see
Section 5.1.2). Only if the measured sediment outflow is indicative of sediment transport capacity can a
statement on equilibrium be made.

A second objective would be to evaluate whether or not the reach is in equilibrium and if not,
determine the changes in water discharge or sediment supply which would result in equilibrium. For this
analysis, the sediment outflow relationship must be indicative of the transport capacity of the channel In
the particular case of a supply limited reach, such an analysis would quantify the extent of change that could
be tolerated without inducing changes in overall channel morphology. In other words, in a supply limited
reach, two separate sediment budgets could be completed; one to establish whether or not stable conditions
currently exist in the reach (i. e., no significant erosion or deposition) based on the measured sediment
outflow relationship, and a second to evaluate the potential increase in sediment supply, or alternatively, the
reduction in transport capacity, that could be tolerated without inducing changes in channel morphology,
based on the estimated transport capacity relationship.

The use of sediment rating curves was originally introduced as a means of computing sediment
discharge in sand bed streams where transported sediments are derived primarily from the bed or banks of
the stream. Although some scatter in the data is expected for sand bed streams, clear relationships between
water discharge and sediment discharge can generally be observed. For streams which derive sediments
from other sources (e.g. from the watershed or bank failure), the relationship between water and sediment
discharge is less identifiable (see Colby 1956). Still, a sediment budget can be applied for streams with
coarser bed sediments, if a valid water-sediment relationship exists.

The computation of a sediment budget between two locations on a stream also depends on water
discharge. Since the Yampa Canyon study reach is essentially canyon-bound with no major tributary inflow,
the daily discharge history in the Yampa River between Deerlodge Park and Mathers Hole is considered to
be nearly the same. With this assumption, only the sediment rating curves need to be compared to evaluate
the objectives discussed previously.

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