# Hydrodynamics of flood

## Methodology

### Surface roughness and constructions

The dimension of the mathematical modelling is not deciding for the precision of the calculation in relation to the flow resistance caused by surface roughness and constructions, but the empirical approach for modelling of the roughness the mathematical model is based on. They have to have a physical foundation and be able to quantify as well continuous as locally acting flow resistances.

Continuous losses occur if the roughness conditions are nearly constant over a longer stream section. These are generally:

• Roughness losses at bed and bank caused by the:
• structure of harder materials,
• structure of the bed forms,
• structural and vibration resistance of overflowed vegetation,
• Separation area roughness between stream channel and flood plain or vegetation area, respectively.
• Form drag caused by the:
• form drag by vegetation in the flow,
• form drag of single rocks in the flow,
• stream line path.

Locally acting flow resistances occur at single stream obstacles like:

bridges, apertures, weirs, drops in the riverbed and bed slides, individual woods and groves.

The selection of a mathematical model has thus to show which types of flow resistances can occur and which empirical approaches for their modelling are needed. Only after this has been done, the decision for a dimension of the mathematical model can be made. This means that, similar to a structural analysis where a assessment of the static system comes before the calculation, the hydraulic system has to be designed prior to the actual calculation.