Hydrologic Processing

Figure 7.10 Full processing setup.

The following data sets are extracted for the specified study area in the ProjView:

“FilledDEM” representing the depressionless DEM.

“FdirGrid” representing the flow direction grid.

“LinkGrid” representing the stream segments grid.

“SmallStrGrid” containing denser stream representation for visualization purposes.

“WaterShd.shp” representing the subbasins.

“River.shp” representing the stream segments.

“Bostanli.shp” containing the project outlet point that defines the study area.

Basin Processing

The Basin Processing tool includes Basin Merge, Basin Subdivision, River Merge, River Profile, Basin Split at Confluences and Batch Subbasin Delineation operations.

Multiple basins can be merged by using the Basin Merge menu, or a single basin can be subdivided into several subbasins, following the rules and methods necessary to obtain the desired modeling area.

Stream & Watershed Characteristics

Topographic characteristics of streams and watersheds are also computed by using the HEC-GeoHMS extension. The computational results should first be compared with the published information before the hydrologic parameters are estimated. The characteristics given in Table 7.1 may be used to compare basins and to estimate hydrological parameters.

Table 7.1 Physical characteristics of streams and subbasins.

Physical Characteristics

Stream Length

Upstream elevation Downstream elevation Slope

Stream Profile

Watershed Area

Centroid Location Centroid elevation Longest Flow Path Longest Flow Length Upstream elevation Downstream elevation Slope between endpoints Slope between 10%-85%

Centroidal Path Centroidal Length

• River Length

This step computes the river length for all subbasins and routing reaches in the

“River.shp”, river shape file. In the attribute table, the river length is computed, using the grid representation of the stream. This step computes the river length more accurately, based on the vector representation of the stream.

• River Slope

The upstream and downstream elevations of a river reach are derived, and the river slope is then computed by using these values.

• Basin Centroid

Four methods are available in GeoHMS for specifying the place of the basin centroid: bounding box, ellipse, flow path, and user-specified methods. Since there is a possibility that the centroid may lie outside a U-shaped or other similar odd-shaped subbasins, the momentum calculation around an X and Y axis, that is commonly used in engineering applications, is not performed here.

• Longest Flow Path

It is possible to have information on the longest flow length, upstream elevation, downstream elevation, slope between the endpoints, and slope between 10% and 85% of the length, that are computed with the longest flow path operation. These characteristics are visualized on the “Watershd.shp” theme.

• Centroidal Flow Path

The length of the centroidal flow path, which is the length between the centroid and the outlet of the subbasin, is computed by using this Menu tool, such that the centroid lies on the longest flow path.

Hydrologic Modeling System

The input files used in the HEC-HMS model, such as background-map file, lumped-basin schematic model file, grid-cell parameter file, and distributed-basin

schematic model file, are created by HEC-GeoHMS. In this part, reaches and subbasins are designated; possible errors in the basin and/or stream connections are determined; a schematic diagram is developed for HMS, and finally HMS related inputs are generated.

• Reach AutoName

In this process, stream reaches are named, starting upstream by using the index

“R” together with a corresponding number. The user can further change these reach names by more descriptive ones.

• Basin AutoName

Similar to the reach autoname process, the basin autoname process provides a naming convertion for subbasins in the upstream - downstream order simply by adding “W+10, 20, etc.” to the receiving reach name. It is possible to alter the names given by the program with real or more descriptive ones.

HMS Model Files

• Map to HMS Units

Although the map unit is the unit of the ArcView data, the physical characteristics of the reaches and subbasins in map units are to be transformed into the HMS units since the terrain is described typically in meters.

• HMS Data Check

This option is used on the data sets in order to make a consistency check for the hydrologic structure of the model. A track of the relationship between the stream segments, subbasins, outlet points, and other entities is kept by the program. Since these relationships may have been disrupted because of the improper use of the tools, it is also suggested here to check the data before continuing the process for the next step.

• HEC-HMS Basin Schematic

In the HEC-HMS basin schematic, the hydrologic basin model with basin elements and their connectivity are shown in GIS visual format. Hence, two files are to be created in this step: a point shape file, so-called HMSPoint.shp, in which a number of point features, such as icon location, outlets, and junctions are available;

and a line shape file, called HMSConnect.shp, that includes some interconnected line features, such as subbasin connectors and reaches.

• HMS Legend

HMS Legend process applies the symbols of the HMS model to represent point and line features for hydrologic elements.

• Background Map File

The background map is simply used to show the boundaries of the basin, subbasins, and stream reaches analyzed. It is in an ASCI text file readable by HEC-HMS.