Quasi-Unsteady Temperature

Fall velocity and some sediment transport equations are sensitive to water viscosity, which is a function of water temperature. Therefore, sediment transport analyses require temperature data. Finer systems (systems with finer sediment) are more sensitive to temperature. Currently, users can only specify one temperature per time step for the entire model.

To specify a temperature time series, press the Set Temperature button on the bottom of the Quasi-Unsteady Flow editor (see figure in Boundary Conditions). This will open the temperature series editor depicted in the figure below, an irregular time series editor very similar to the other quasi-unsteady time series editors.Specifying a temperature time series.

Modeling Note: Developing Long-Term Temperature Records

Temperature data are seldom sufficient to generate a daily time series for multi-decadal, historical models. And modelers must make some temperature assumption to simulate the future. There are a few options to define multi-decadal historical or future temperature data in the absence of a long-term temperature record (Options 1 and 3 are most common):

  1. Constant Temperature: Any of the irregular time step editors in the quasi-unsteady flow boundary conditions can define a constant condition by specifying large durations (e.g. 240,000 hrs or 10,000 days/>25 years in the figure below). In coarse systems (rivers with significant gravel and/or cobble), that are less sensitive to temperature, detailed temperature data may be unnecessary. Perform sensitivity analysis to evaluate this assumption.
    Example of a constant temperature time series in the Quasi-Unsteady Temperature Editor.

  2. Interpolating Annual Highs and Lows: If results are sensitive to temperature, HEC-RAS can generate a simple time series with a little more user effort. Estimate max and min water temperatures and input them at approximate dates each year, leaving the intermediate dates blank. Then press the Interpolate Missing Values button (below).  For example a user could create a weekly time series (Durations=7*24 hrs = 168 hrs) that interpolates between an early July high and early December low by defining the high temperature in all of the rows aligned with the first week of July, the low temperature with all the rows associated with the first week of December, and pressing the Interpolate Missing Values button.
  3. Monthly Average Temperatures: Often a gage has some opportunistic temperature readings. You cannot use them as a times series or even interpolate a time series from them. The gage has enough water temperature data to compute monthly averages.   Give each monthly average temperature a 730.48 hour (30.4368 day) durations will create an approximate average monthly temperature time series that you can repeat for multiple years (demonstrated in figure below) and it will remain relatively consistent over long simulations (including leap years).

Repeated sequence of monthly average temperature data developed from opportunistic temperature measurements that only account for a small portion of the historical record. The 730.5 hour duration averages to one month over long time periods.

Unsteady Temperature

Temperature is the only data the unsteady flow editor requires for sediment transport analyses. Specify temperature for an unsteady sediment transport model in the Unsteady Flow Editor. Select the Water Temperature (for Unsteady Sediment)… option from the Options menu (top) to get an unsteady temperature time series editor, similar to the unsteady flow and stage editors (bottom). In the absence of temperature data in the unsteady flow file, HEC-RAS will assume 55o F. The unsteady temperature editor will allow users to specify actual monthly temperatures by setting the Data time interval to 1 Month:

Specifying water temperature for unsteady sediment models.
Unsteady Temperature Editor.