Simplifying Watershed Mapping: A Step-by-Step Approach with Arc Hydro Tools

The objective of creating a watershed boundary is to identify the contributing catchment to any hydraulic structure or new development.

• Hydrological studies in Saudi Arabia require consultants to analyze catchments and assess the impact of new projects on upstream and downstream flooding.
• Developers must ensure new developments do not obstruct flood paths and are protected from 100-year flood events. The wadis with width more than 25.0 m are preserved.
• The sea level rise to taken into account for grading level.
• The rough grading is decided upon taking into account factor such as 100- Year Flood depth, Storm surge, sea level rise, wave height etc. Refer IPCC to know sea water rise in your area.
• The finished ground level (FGL) of buildings is set at least 0.6 m above the 100-year flood line for safety.

Stormwater design criteria:

• Internal networks: Designed for a 10-year return period.
• External systems: Designed for a 100-year return period.
• A 20% rainfall factor is added to account for climate change impacts.
• 2D model is ideal to study accurately the impact of rainfall on new development.

The following catchment properties are typically identified through watershed modeling:

• Catchment Area
• Drainage Line
• Slope of Drainage Line
• Expected Culvert Locations
• Expected Channel Locations

The identified properties help in calculating the below hydrological factors:

• Peak Flow by Catchment
• Time of Concentration
• Flood Depth
• Flood Velocity
• Flood hazard map using DEFRA

The identified watershed can be further processed into HEC-HMS and HEC-RAS, which will help to identify the following results:

• 100-Year Flood Depth
• Culvert Sizing
• Bypass Channel Sizing
• Flood Hazard map
• Rough Grading Level (0.6 m above the 100 year flood line)
• Pond Location (predevelopment-post development)
• 95th Flood capture to satisfy Mostadam requirement

Data Required:

DEM Data This can be downloaded from USGS Earth Explorer by creating an account. The data accuracy is 30 m × 30 m.

SRTM Data The website Open Topography can be used for SRTM data download as it does not require login.

DEM Preparation:

DEM can also be prepared using survey data such as contour or spot elevations with tools in ArcGIS Pro.

1.??????? Download DEM data

2.??????? Extract the study area using extract by mask tool

3.??????? Project the DEM into UTM Coordinate using Project Raster tool into Data management tool

4.??????? Open ArcHydro Tool

5.??????? Use Fill Sink tool in ArcHydro

6.??????? Use Flow Direction tool in ArcHydro

7.??????? Use Flow Accumulation tool in ArcHydro

8.??????? Use Stream Definition tool in ArcHydro

9.??????? Use Stream Segmentation tool in ArcHydro

10.??? Use Catchment Grid Delineation tool in ArcHydro

11.??? Use Catchment Polygon Processing tool in ArcHydro

12.??? Use Drainage Line Processing tool in ArcHydro

13.??? Use Adjoint Catchment Processing tool in ArcHydro

14.??? Use Drainage Point Processing tool in ArcHydro

15.??? Use Batch Watershed Delineation tool in ArcHydro

16.??? Point Delineation Tool can be used if you need catchment for specific drain point such as culvert

17.??? Any Catchment property can be extracted or converted to KMZ/KML for further Studies

18.??? The Shapefiles of catchment and drainage line can be added to HEC-HMS for study

19.??? HEC-HMS can also be used to create watershed however it is time consuming

Feel free to reach out if you have any further questions regarding hydrological studies conducted in Saudi Arabia.