Surface Water Routing:Overland Boundary Conditions

Overland Boundaries

Overland boundaries may be used to model coastal storm surges or areas around a standing water body. In essence, GSSHA will take a map of the boundary conditions (constant slope, constant head, or time-varying head) and apply that condition at the beginning of each time step.

Specified discharge and cell to cell hydraulic rating curves can be added at interior portions of the watershed, in addition to the actual watershed boundaries.

Setting up overland boundaries consists of 1) creating an index map describing the boundary condition of all cells, 2) creating the overland boundary mapping table, 3) if needed, creating the time series index mapping table, 4) if needed, creating the time series file, 5) if needed, create the rating curve index mapping table 6) if needed, create the rating curve input file 7) adding appropriate project file cards.

Index Map

The index map is a standard index map. All cells must have a value. Usually, all cells will begin as a uniform value and then changes along the boundaries will be done to create specific instances of boundary conditions. Each specific instance of a boundary condition will have a unique ID.

Overland Boundary Mapping Table

The overland boundary mapping table is very similar to the standard style mapping tables. The table has two parameters, with the meaning of the second parameter dependent upon the first. The first parameter is a boundary type code. The second parameter is a value corresponding to the meaning of the boundary type code.

Boundary Type Code	Second Parameter Meaning
0 (Normal cell)	Put 0.00
1 (Specified slope)	Slope (e.g. 0.001)
2 (Constant specified head)	Constant water surface elevation of water in the cell
3 (Time variable specified head)	Time series index of water surface elevation
4 (Hydrograph - cfs)	Time series index of hydrograph
5 (Hydrograph - cms)	Time series index of hydrograph
6 (Rating curve - head difference - cms)	Table of difference in stage verses discharge rating curve
7 (Rating curve - upstream depth- cms)	Table of depth verses discharge rating curve
99 (Receiving cell - Downstream cell for rating curve structure )	Put 1

The overland boundary table identifier is OVERLAND_BOUNDARY.

Time Series Index Mapping Table

The overland boundary table refers to a time series index table. This table links a unique ID value to a time series name. This table is not in the standard table format. It does not link to an index map, nor does it have any descriptions. The format is shown in the following example.

TIME_SERIES_INDEX
NUM_IDS  ##
ID    Time Series Name (text line)
[##]     “name here”
[##]     “name here”
…
[##]     “name here”

The names referred to are defined in the individual time series.

Time Series Files

Time series files contain as many time series as desired. Time series files are defined in the GSSHA wiki site. Two examples of time series are shown below.

GSSHA_TS
levee_breach
RELATIVE
0 0 0 0 0 140.0
0 0 1 0 0 139.5
END_TS


GSSHA_TS
storm_surge
ABSOLUTE
2005 8 26 4   0 2.4
2005 8 26 8   0 3.2
2005 8 26 13  0 3.4
2005 8 27 0   0 2.4
END_TS

Rating Curve Files

Rating curve files contain as many rating curves as desired. Rating curve files contain one set of rating curves for each structure. Each rating curve has the following format.

GSSHA_ES "Rating curve name" depth (m) discharge (cms) depth (m) discharge (cms) depth (m) discharge (cms) END_ES

Note: The first input is cell depth (m) for type 7 and is the difference between cell water surface elevations (m) for type 6.

As in the following example:

GSSHA_ES "Rating Curve50" 0.000000 0.000000 0.818143 24.994654 3.272572 49.989309 7.363287 74.983963 13.090288 99.978617 END_ES

Project Cards

To turn on the overland boundary process, include the card

OV_BOUNDARY

in the project file. To include the time series files, include the card

TIME_SERIES_FILE  "filename.ext"

in the project file. This card may be repeated for as many time series files as are needed.

Example

Using the two time series defined above, the mapping tables would look like this:

INDEX_MAP  "bdy.idx" "bdy"


TIME_SERIES_INDEX
NUM_IDS 2
ID    Time series name
1     "levee_breach"
2     "storm_surge"


OVERLAND_BOUNDARY "bdy"
NUM_IDS 3
ID    desc…     				bdy_type   	bdy_param
1      normal cells				0		0.00
2      specified slope				1		0.001
3      storm surge				3		2

Assuming the time series are in a file called 'flood.ts', the project file would have the following cards:

OV_BOUNDARY
TIME_SERIES_FILE  "flood.ts"

GSSHA User's Manual

5 Surface Water Routing

5.1     Channel Routing

5.2     Overland Flow Routing

5.3     Channel Boundary Conditions

5.4     Overland Boundary Conditions

5.5     Embankments

5.6     Overland/Channel Interaction

5.7     Introducing Discharge/Constituent Hydrographs

5.8     Overland Routing with Snow

5.9     Overland Routing with BMPs