Difference between revisions of "Module Runoff"
From MohidWiki
(→Main Processes) |
(→Main Processes) |
||
| Line 96: | Line 96: | ||
:{| | :{| | ||
| − | |Flow to the river when the water level of the river is lower that soil one | + | |-Flow to the river when the water level of the river is lower that soil one |
|- | |- | ||
| − | |Flow from the river when the water level is higher than the soil one | + | |-Flow from the river when the water level is higher than the soil one |
|} | |} | ||
Revision as of 01:57, 2 March 2011
Contents
Overview
Module Runoff allows the calculation of the overland surface runoff over a regular structured grid as function of the water column and terrain slope. The water column, namely the water located above the terrain, is given by the Module Basin where the precipitation is converting into a water column considering the losses due to the evapotranspiration and the infiltration processes. In particular the overland flow is evaluated by the Manning’s equation.
Main Processes
The overland surface runoff is calculated at the interface of the grid cells and it is obtained by applying the Manning's equation as follow:
- Failed to parse (unknown error): \[Q=\frac{1}{n}\cdot A\cdot R_{h}^{2/3}\cdot s^{1/2} [1]
where:
Q is the overland flow (m3/s) A is the area of the cross-section (m2) n is the Manning coefficient (s/m1/3) Rh is the hydraulic radius (m) S is the slope of the water surface (m/m)
In order to apply the Manning's equation each grid cell is considered as an open channel as shown in the scheme below.
where:
h is the water column (m) w is the cell width (m)
The hydraulic radius is the following:
- Failed to parse (unknown error): \[R_{h}=\frac{w\cdot h}{w+2\cdot h} [2]
In according with the figure 1. the width is 'much' grater than the water column (w>>h) and the hydraulic radius can be considered only Rh=H. Therefore the Manning's equation can be simplified as:
- Failed to parse (unknown error): \[Q=\frac{1}{n}\cdot w\cdot h^{5/3}\cdot s^{1/2} [3]
The slope (s) is calculated by the difference of the water levels at the extremities of the considered cell:
- Failed to parse (unknown error): \[H(i,j)=\h(i,j)+T(i,j) [4]
where:
H is the water level (m) h(i,j) is the water column (m) T(i,j) is the Topography (m) j is X direction i is Y direction
- Failed to parse (unknown error): \[s_{x}=\frac{H(i,j-1)-H(i,j)}{DZX} [5]
where:
sx is the slope in the X direction H(i,j-1) is the water column at the left face of the cell (m) H(i,j) is the water column at the right face of the cell (m) DZX is width of the cell in the X direction (m)
- Failed to parse (unknown error): \[s_{y}=\frac{H(i-1,j)-H(i,j)}{DZY} [6]
where:
sy is the slope in the Y direction H(i-1,j) is the water column at the left face of the cell (m) H(i,j) is the water column at the right face of the cell (m) DZY is width of the cell in the X direction (m)
The Manning coefficient is derived from the land use map. Indeed by using a GIS program it is possible to associate at each cell a land use class in order to obtain by the support of an abacus a Manning value.
The flows obtained by the formula [3] are dived ed into flows in X direction and Y direction according with the Figure 2.
In the eventuality of the presence of a river in the basin analyze is possible to obtain two different configurations:
Other Features
How To Generate Manning Coefficients
Manning coefficients must be provided in runoff data file.
Options:
- Use a constant value
- Define a Manning's grid: One possible option is to associate Manning with land use classes (shape file). In this case can use MOHID GIS going to menu [Tools]->[Shape to Grid Data] and provide: i) the grid (model grid), ii) the land use shape file and iii) the corespondence between land use codes and Manning.
Use Manning inicialization with Module FillMatrix standards in the block:
<BeginOverLandCoefficient> FILE_IN_TIME : NONE INITIALIZATION_METHOD : ASCII_FILE REMAIN_CONSTANT : 1 DEFAULTVALUE : 0.08 FILENAME : ..\..\GeneralData\Runoff\Mannings200m_v2.dat <EndOverLandCoefficient>
