Water has many physical properties. The main physical properties utilized in hydrodynamics and sediment transport are the density and viscosity. These are a function of the water temperature and salinity. However, salinity is currently ignored in HEC-RAS. These correspond to the "clear water" values without sediment. The default water temperature in HEC-RAS is 55.4 F.

Density

The water density is a function a water temperature and salinity. There are many formulas in literature for computing the water temperature. The water specific weight or unit weight particles, γ_w, is the density times the gravitational constant

The default water unit weight in HEC-RAS is 62.4177 lb/ft³. If the user specifies a time series of water temperature, the water density is calculated using the UNESCO (1981) equation (at standard atmospheric pressure)

where

$//<![CDATA[ \begin{array}{l}\rho\end{array} //]]>$ = water density [kg/m³]

$//<![CDATA[ \begin{array}{l}\rho_0 = a_0 + a_1T + a_2T^2 + a_3T^3 + a_4T^4 + a_5T^5\end{array} //]]>$  = freshwater density

$//<![CDATA[ \begin{aligned} a_{0}=&+999.842594 \\ a_{1}=&+6.793952\times 10^{-2} \\ a_{2}=&-9.09529\times 10^{-3} \\ a_{3}=&+1.001685\times 10^{-4} \\ a_{4}=&-1.120083\times 10^{-6} \\ a_{4}=&+6.536332\times 10^{-9} \\ B_{1}=&~ b_{0}+b_{1}T+b_{2}T^{2}+b_{3}T^{3}+b_{4}T^{4} \\ b_{0}=& ~0.824493 \\ b_{1}=&-4.0899\times 10^{-3} \\ b_{2}=&+7.6438\times 10^{-5} \\ b_{3}=&-8.2467\times 10^{-7} \\ b_{4}=&+5.3875\times 10^{-9} \\ C_{1}=& ~c_{0}+c_{1}T+c_{2}T^{2} \\ c_{0}=&-5.72466\times 10^{-3} \\ c_{1}=&+1.0227\times 10^{-4} \\ c_{2}=&-1.6546\times 10^{-6} \\ d_{0}=&4.8314\times 10^{-4} \end{aligned} //]]>$

$//<![CDATA[ \begin{array}{l}T\end{array} //]]>$ = water temperature [ºC]

$//<![CDATA[ \begin{array}{l}S\end{array} //]]>$ = water salinity [PSU]

The above equation is valid for temperatures between 0 and 40^∘C and salinities between 0.5 and 43 PSU. The salinity above is assumed to be 0.5 PSU.

Viscosity

The dynamic water viscosity of water may be calculated using the curve fit formula presented by Neumeier et al. (2008) based on the data from Riley and Skirrow (1965)

where

$//<![CDATA[ \begin{array}{l}μ_w\end{array} //]]>$ = water dynamic viscosity [kg∙s/m]

$//<![CDATA[ \begin{array}{l}T\end{array} //]]>$ = water temperature [ºC]

$//<![CDATA[ \begin{array}{l}S\end{array} //]]>$ = water salinity [PSU]

The error compared to the data of Riley and Skirrow (1965) is less than 0.5% over the range (0 38 PSU and 8 24^∘C). The error is less than 1.0% over the range 0 28^∘C. Currently, in HEC-RAS the salinity is assumed to be 0.5 PSU.

The water viscosity is a measure of the resistance to deformation at a given rate. When a fluid has zero viscosity it called inviscid. The flows resulting from water with and without viscosity are referred to as viscous and inviscid flows respectively. In hydrodynamics and sediment transport the kinematic viscosity, μ_w, is often utilized. It is related to the dynamic viscosity by