In order to understand which direction to adjust model parameters to get the desired results, the following is a discussion of general trends that occur when specific variables are adjusted. These trends assume that all other geometric data and variables will be held constant, except the specific variable being discussed.
Impacts of Increasing Manning's n. When Manning's n is increased the following impacts will occur:

  1. Stage will increase locally in the area where the Manning's n values were increased.
  2. Peak discharge will decrease (attenuate) as the flood wave moves downstream.
  3. The travel time will increase.
  4. The loop effect will be wider (i.e. the difference in stage for the same flow on the rising side of the flood wave as the falling side will be greater). An example of this is shown in Figure 7-39.


Figure 7 39. Example of Wider Loop for Higher Manning's n
Impacts of Increasing Storage. When storage within the floodplain is increased, the following impacts will occur:

  1. Peak discharge will decrease as the flood wave moves downstream.
  2. The travel time will increase.
  3. The tail of the hydrograph will be extended.
  4. The local stage (in the area of the increased storage) may increase or decrease. This depends upon if you are trading conveyance area for storage area, or just simply increasing the amount of storage area.