Manning’s Roughness Coefficients

Roughness coefficients represent the resistance to flow in channels and floodplains. Roughness is usually presented in the form of a Manning's n value in HEC-RAS. There is extensive research and literature on methods to determine n values; however most of this work is representative of only main channels and not floodplains. Additionally, the literature on Manning's n values is for historically experienced floods, which are much lower than the flood resulting from a dam break. The actual selection of n values to be used for each dam assessment will require judgment by the engineer responsible for hydraulic model development.

A proper perspective is required before establishing a range of n values to be used in USACE risk assessment studies. The following general guidelines of factors that affect n value should be considered in developing representative values.

Base Surface Roughness: Base surface roughness is often represented by the size and shape of surface or channel and floodplain material that produces a friction effect on flow.

Stage and Discharge: The n value in most streams decreases with increase in stage and discharge. However, this is not always the case. If the channel bed is of lesser roughness than the channel banks, then the composite channel n values will increase with channel stage. Once the stage gets higher than the main channel banks, the roughness coefficient could begin to decrease. The main point here is that the variation of Manning's n with stage is site specific.

Obstructions: Objects constructed in the channel or in overbanks such as bridge piers or buildings can potentially cause increases in n value. It is especially difficult to estimate Manning's roughness coefficients to represent buildings in the floodplain, as there are many factors to consider: the area obstructed and the density of the buildings, direction of the flow in relation to the layout of the structures, roughness of all of the other boundaries, slope of the terrain, velocities of the flow, etc…

Irregularities: Irregularities are variations in cross-section size and shape along the floodplain. Irregularities are often caused by natural constrictions and expansions, sand deposition and scour holes, ridges, projecting points and depressions, and holes and humps on the channel bed. Gradual and uniform changes will generally not appreciably affect n value, whereas, areas that have lots of sharp channel irregularities will tend to have higher Manning's roughness coefficients.

Channel Alignment: Smooth curvature with large radius will generally not increase roughness values, whereas sharp curvature with severe meandering will increase the roughness.

Vegetation: Vegetation effects are dependent on height, density, distribution, and type of vegetation. Heavily treed areas can have a significant effect for dam failures. In general a lower average depth results in a higher n value. High velocities can potentially flatten the vegetation and result in lower n values.

Silting, Scouring, and Debris: Silting may change a very irregular channel into a comparatively uniform one and decrease n, and scouring may do the reverse. During a dam break flood wave, there will be a tremendous amount of scouring occurring, as well as lots of debris in the flow. The increased sediment load and debris will cause the flow to bulk up (increase in stage). One way to account for this increased sediment load and debris is to increase the Manning's n values.

The resulting maximum water surface profile associated with the failure of a dam will often be much higher than any historically observed flood profile. In such cases, there is no historical based model data to calibrate to floods of this magnitude. It is therefore incumbent upon the engineer to determine reasonable roughness coefficients for flows and stages that will be higher than ever experienced. To gain a perspective on how each modeling parameter affects results, a bounding type sensitivity analysis can be performed regardless of the methods used to establish n values.

Historical regional knowledge of channels and floodplains should be used along with published guidelines in establishing a base level set of n values. Guidelines for establishing base level Manning's n values can be found in "Basic Data Requirements" of this manual. The base level n values should be adjusted up or down based on factors addressed previously. Calibration to the largest historical events of record should be done whenever possible. Once adjusted roughness coefficients are established, uncertainty analyses should be performed by varying all values (two additional computational runs) by plus or minus 20%. In general, channel n values for risk assessment may be in the range of 0.025 to 0.075. The overbank n values may range between 0.05 and 0.15. Note that higher n values can be used in areas to allow for storage embayments with little to no conveyance.

Manning's n Values Immediately below Dam. Significant turbulence, sediment load and debris should be expected for the immediate reach downstream of a failed dam. This is obvious when viewing the photo of the Teton Dam failure shown in the figure below. Because HEC-RAS does not directly account for high volumes of sediment in the flow, and the extreme turbulence in the water surface caused by the breach, it is often a good idea to increase the Manning's n values just downstream of the dam. The increased sediment and turbulence will cause higher water surfaces to occur. The only way to mimic this is by increasing the roughness coefficients. Proper modification and variation of n values is one of the many uncertainties in dam failure modeling. An accurate assessment can be confidently attained only after previous knowledge of a particular dam failure event. A reasonable modeling approach may be to assume double the normal n value directly downstream of the dam and transition to normal roughness coefficients where failure induced turbulence, sediment load, and debris transport are expected to recede.

Roughness Coefficients for Steep Streams. Many dams are located in mountainous regions, where the slopes of the stream are significantly steep. It is very common to underestimate Manning's n values for steep terrain. Underestimation of the roughness coefficients can cause water surface elevations to be too low, increased velocities, and possibly even supercritical flow. In addition to this, abrupt changes in n values or underestimation of n values can cause the model to go unstable. Dr. Robert Jarrett (Jarrett, 1985) collected some extensive field data on steep streams (slopes greater than 0.002 ft/ft) in the Rocky Mountains. Dr. Jarrett measured cross sectional shape, flow rates, and water surface elevations at 21 locations for a total of 75 events. From this data he performed a regression analysis and developed an equation to estimate the Manning's roughness coefficient of the main channel. The equation from his findings is presented below.

While Dr. Jarrett's equation is not necessarily applicable to all locations, it is often a useful check for reasonableness of the Manning's n values in steep terrain.