GIS TOOL BAR FOR IDENTYING POTENTIAL FLOODING EVENTS IN CANAL SECTIONS CAUSED BY SURFACE RUNOFF

by

J. J Escurra

Abstract

One of the most common problems in the Northern part of the U.S. is the merging of surface runoff and irrigation water in canals due to rainfall and snow; this generates flood problems because this exceeds the maximum flow rate available in the canals. For this reason, flood problems are known to produce thousands of dollars in damage; therefore, areas with potentially high flooding problems due to the joining of surface runoff and irrigation water need to be identified to plan the most effective protection to these areas. To identify these possible problem areas, a toolbar using GIS 9.1 is being developed. GIS is a tool which helps manage large maps efficiently for different reasons such as water management, water quality, hydrology, groundwater, etc. This toolbar will execute two tasks. First, the user will be able to add information about the canal conditions and characteristics in the attribute table of the canal shapefile. Second, based on these properties, the toolbar will then calculate the maximum allowable flow of the canal and the maximum surface runoff that can flow into the canal and then it will assign two values (0 or 1) based on the difference of these calculated flows.

Keywords: canal, surface runoff, maximum flow permissible in the canal, GIS Toolbar.

Introduction

Irrigation canals in Cache Valley (Logan, Utah) have flood problems in some sections due to (1) large amounts of precipitation, and (2) the construction of malls, shops, parking lots, and homes which increases the amount of impermeable area. From the paper “Water Management Improvement in Cache Valley Irrigation Canals”, one of the main concerns of the canal companies’ presidents is the flooding problems due to stormwater. The canals, which were built early in the 20^th century, do not have enough cross sectional area to carry this extra water. This problem also has been observed in Colorado and Idaho , where the growth of population and the excess of water are increasing the flood prevention budget.

TAMS program, which was developed by Doyt Bolling from the Utah Local Technical Assistant Program Center , uses the Diagnostic Walk-Thru to inventory roads, parking, and signs. This idea was used in generating this toolbar, which stores data that describes a canal section condition and characteristics such as depth, free bord, canal material and more (Inventory Process).

In addition, this toolbar uses as an input, the eight flow direction. This input was generated from the digital elevation model by the TAUDEM program, developed by David Tarboton.

Finally, this toolbar was created to simplify the work of the user to achieve the identification of flooding problems in canal sections from a practical perspective.

Methodology

All the code was being written in visual basic for applications in GIS 9.1, and it considers as input information, for example, canals shapefile where the canal is segmented by sections. The segmentation of the canal should be developed using the following criteria (same as used by the TAMS program for road sections):

• Change of cross sectional shape
• Change of canal material (change of roughness)
• Change of slope
• Change of canal company jurisdiction
• Presence of hydraulic structures
• Every segment or section should have a picture which supports the information surveyed in the field.
• The author strongly suggest the use of GPS to generate this shapefile as a “line” and the properly definition of the coordinate system.

Digital elevation model (DEM) of the basin where the canals flow should be considered because this DEM will be transformed to D8 Flow Direction raster using TAUDEM (http://hydrology.neng.usu.edu/taudem/). The TAUDEM will encode the DEM assigning values from 1 to 8 which represents the directions of flow; for example, 1 East, 2 North East, 3 North, 4 North West, 5 West, 6 South West, 7 South, 8 South East. The DEM can download from certain internet sites. To develop this procedure, click the function D8 Flow Direction inside the Basic Grid Analysis Menu. It is important to use the function Fill Pits to reduce the possible elevation errors in the DEM and avoid crashes in the computer.

Fig1. GIS Tool bar

4. Calculation of the runoff flow using the rational equation

6. Difference between maximum flow rate and the runoff flow rate and classification of possible flood problem in each section.

2. Filling in data about condition and characteristics of the canals and calculation of the maximum flow using the Manning Equation

3. Calculation of the total runoff area which will flow into the canal

5. Observation of the picture for the specific section

The GIS toolbar (Fig 1) was created considering 6 steps that together will reach the final objective which is the identification of the potential flooding events in canal sections caused by surface runoff.

1.Selection of Section:

This function is code to choose the section of the canal which will be selected to start all the processes, the use of the mouse is needed to select the section. Before,this function is used; the canal shapefile and the DEM converted in D8 flowdirection using TAUDEM should be added in the Table of Contents

2.Filling in Data:

This function is code to insert all the information of the canal from the Diagnostic Walk-Thru process. The information will be saved in the attribute table of the canal shapefile. In addition, nine fields will be created and these will contain the walk-thruinformation for the specific analyzed section, for testing purpose, canal’s characteristics data from the Northern Logan Canal was used to run the model. (Fig 2)

Fig2. DiagnosticWalk Thru

Manning Equation is used to calculate the maximum flow rate in the canal and taking a picture of every section is strongly recommended.

3.Calculation of the Surface Runoff Area:

This code was given by David Tarboton. The code works with the D8 Flow Direction raster from TAUDEM and the outlet shapefile which represents the points where the water will run into the canal. Consequently, these points should be created using edit function as new shapefile and separated every 10 meters to cover all the possible inflow of water into the canal. The output of this function will be the numbers of grids which represents the total area of runoff.

4.Calculation of Runoff Flow

This function uses the Rational Equation:

Q = Flow rate (m³/s)

I = Rainfall intensity (in/hr)

A = Area (m²)

C = Runoff Coefficient (0.9) for urban areas

The rainfall intensity table of Logan, UT is shown with the form that contains the data to calculate the runoff flow. This function creates two fields; one contains the area and the other the runoff flow.

5.Observation of the Picture:

This function allows the user to see the picture for the specific section. This function also finds the picture number and it will be visualized in the top ofthe browse form which will be connected to the folder that contains all the photos.

6. Difference between Maximum Canal and Runoff Flow:

This function is coded to calculate the difference between maximum canal and runoff flow. If the output is negative, the model will add a value of“1”or if it is positive, a value of“0”. Two fields are created one contains the difference value and the other the values of 1 and 0.

Results

To test the model, data from Northern Logan Canal was used. The canal was GPSed only5 kilometers from the total 24 kilometers of length. The pictures were taken from the previous works developed in that canal byTiclavilca, Tammali and Merkley (2005) and J.J. Escurra (2003). J.J. Escurra developed a Field Calibration of the Float Method in two sites of the Northern Logan Canal.(Fig 3)

Fig3.Northern Logan Canal

To obtain efficient results the author suggests the following order to identify the possible sections with flooding problems:

Calculation of the Surface Runoff Area

Create the outlet shapefile considering that each point should be separated every 10 meters (Fig4).

Fig4.Creation of outlet shapefile

Then click in the function “Total_Runoff”, a browse form will appear, in this browse form the folder that contains the outlets shapefile and the D8 Flow Directionraster will be located (Fig 5).

Fig5.Browse form

The input box that asks for the name of the input file will appear. The user should use the name of the outlet shapefile. For this example, the name of outletsec will be used (Fig 6).

Fig6.Input box for outlet shapefile name

The input box which asks for the name of the raster output will appear. The raster output contains the information of the runoff area. The user can use any name. For this example, the name“results”is used (Fig 7).

Fig7.Input box for output raster name

Finally, the output raster is generated and the numbers of grids are calculated from the raster calculator using the formula gridshape([results]). For this example, the number of grid is 60073. Consequently, the total area will be 60073 x 100 m². 100 is the area of a single grid (Fig 8).

Fig8. Total surface runoff area.

Selection of Section:

The selection of the section (Fig 9) is executed using the mouse; all the information given will be stored in the fields for that section, so it is important to finish the process for that section before going onto another section.

Fig9.Selection of section

Filling in Data:

The filling data function in the GIS toolbar is called“Inventory”. This button will show a form (Fig 12) that stores data and will save it in the attribute table of the canal shapefile. To run this part of the code, it is necessary that the canal shapefile has the first position over the other rasters and shapefiles. For this example, the name of the canal shapefile is “NorthernCanal” (Fig 10).

Fig 10.Position of the NorthernCanal shapefile

First of all the shapefiles and rasters

The form needs to be filled in with the following information: canal material, freebord, total canal depth (from the bottom to the top of the canal), canal width, slope (should be measured using level or total station), and the values of m right and m left which represent the ratio of the total depth minus free bord over the length from the end of the top width to the end of the bottom width (Fig 11).

Fig11.Parameters used for a trapezoidal canal

For rectangular canals m right and left are zero.

Fig12. Inventoryform store data from Diagnostic Walk-Thru

This form will save the data for the selected section

Calculation of Runoff Flow:

This function calculates the runoff flow rate that will get into the canal from the runoff area using the rational equation detailed before. The button for this function is called “RationalEquation” and it will bring a form which will ask for the runoff area and the rainfall intensity (Fig13).

Fig13. Runoff FlowRate Form

Rainfall intensity table for Logan, UT

For this example, the area is 6007300 m²and the rainfall intensity is chosen from the table from the Utah Climate Center webpage for a 10 years storm with 60 minutes of duration; this value is 0.73 in/hr. It is important to indicate that this table only represents the rainfall intensity in Logan, UT.

Observation of the Picture:

The button for this function is called“Picture”. This button shows a form where the picture will be visualized and then the usershould click“insert”button to bring the browse form to look forthe path of the picture. It is important to indicate that the number of the picture can be observed in the top of the form. This information is obtained from the field “n°_photo” for that specific section and it is filled in during the filling data step (Fig14).

Fig14. Browse form to get the picture

Picture number from the field n°_photo

Difference between Maximum Canal and Runoff Flow:

The button for this function is called“Owners”. This button will obtain the difference of runoff and maximum flow rate and it willgrant values of 0 and 1. 0 when the difference is a positive value or 0 and 1when is negative value. This calculation is only developed for the selected section using the selection of section function and it will be performed in the attribute table of the canal shapefile. Finally, the canal attribute table will be populated with all this calculated data (Fig 15).

Fig15.Final output attribute table

Conclusions and Recommendations

ThisGIS toolbar allows the user to find the possible flooding problems for different canal sections. That knowledge will help in the creation of solutions such as in construction ofsewers that carry excess water from surface runoff and construction of weirs that flow to secure places in case of overflow. It is known that the Manning Equation is only used to uniform flow; therefore, the user can use any other flow rate measurement method and type it in the form. The model will use that flow rate input to get the values of 1 or 0. Backwater effects were not considered in this model.

The runoff coefficient considered in the equation to calculate the runoff flow ratewas 0.9 due to the approximation of the canal to urbanization areas where the infiltration is reduced because the concrete and asphalt covers the surface. The user can change the runoff coefficient based on own criteria.

For future research it is recommended to create another function in the toolbar that generates pages for the different sections which contains the map of the section and the characteristics of the canal for that specific section. This should be viewed in layout vies and ready to be converted in PDF files and printed. Therefore, the report generation of the inventory of this canal can be easily achieved. In addition, the coding of a button to calculate the seepage gain or loss can be developed for each section.

Acknowledgments

Assistance with concepts information and code to calculate surface runoff area was provided by Dr David Tarboton. Logan City Engineering Office provided canal shapefile as input to test this model.

References

American Society of Civil Engineers Hydrology Handbook Second Edition ASCE, New York, 1996.

Burke, R.Getting To Know ArcObjects Programming ArcGIS with VBA Relands, California 2003.

Escurra, JJ .Field Calibration of The Float Method in OpenChannels MasterThesis.BiologicalandIrrigation Engineering Department, Utah State University, Logan, 2003

Merkley, G.P.IrrigationEngineering FundamentalsBiological andIrrigation Engineering Department,UtahState University, Logan-UT, 2005

Rainfall Intensity in Logan-UT WWW.hdsc.nws.noaa.gov/ Retrieved November 25, 2005, from http://hdsc.nws.noaa.gov/cgibin/hdsc/.

Tarboton, D. VBA Homework Code GIS in Water Resource. Civil and Environmental Engineering Department, Utah StateUniversity, Logan-UT 2005.

TAUDEM model WWW.engineering.usu.edu/cee/faculty/dtarb/ Retrieved September 27, 2005 from http://hydrology.neng.usu.edu/taudem/.

Ticlavilca, A, Tammali, B and Merkley, G.P Water Management Improvements In Cache Valley Irrigations Canals Biological and Irrigation Engineering Department, Utah State University, Logan-UT, 2005