AN INQUIRY SYSTEM FOR DESIGN DISCHARGE IN UPLAND SMALL WATERSHEDS

Kwan Tun Lee, Chung-Chieh Meng, Ming-Sang Yang

Department of River and Harbor Engineering

National Taiwan Ocean University

Keelung 202, Taiwan China

E-mail: ktlee@mail.ntou.edu.tw, Fax: 886-2-24622192 ext 6121

Abstract: The paper presents a geomorphic and hydrologic information system for soil and water conservation work in upland small watersheds.  Since future landslide areas are unpredictable, geomorphic information of any location within the watershed is required.  While using the inquiry system, users can interactively select a point only by clicking on the mouse at the desired location, and then consequent calculation procedures will be conducted.  A compound module, which is based on SPOT image classification, digital elevation model, frequency analysis theory, and kinematic-wave approximation, can provide enough information for design discharge for different return period conditions.  Moreover, this system allows users to modify land cover condition and watershed topography to realize the influences of the environment change on watershed hydrologic conditions.  Future work of the inquiry system is to include detail hydrograph routing modeling for midsize and/or large watersheds design work.

Keywords: design discharge, inquiry system, geographic information system

1    INTRODUCTION

The geographic information system (GIS) has significantly changed the way in acquiring and using spatial data, and hence provides the potential for the use of physical based watershed hydrologic modeling.  The attempts to incorporate GIS into hydrologic analysis can be grouped into four categories (Greene and Cruise, 1995): (1) calculation of input parameters for existing hydrologic models; (2) mapping and display of hydrologic variables; (3) watershed surface representation; and (4) identification of hydrologic response units.  Several attempts have been made the GIS techniques applied for watershed hydrologic analysis such as Djokic and Maidment (1991), Greene and Cruise (1995) for urban watersheds; Garrote and Bras (1995) as well as Levy and Baecher (1999) for midsize or large watersheds. 

Due to the diverse training of the practical engineers, we found that an inquiry system with highly intuitive and easily operated characteristics is required.  This paper presents the development of a system which incorporates the spatial analysis capabilities of the GIS technique into watershed hydrologic modeling.  The topography, land cover, and rainfall characteristics were coded into separate layers, and the attribute information for each layer was used to construct database attribute tables.  In contrast to other GIS applications for hydrologic analysis, the coordinate values that define the location of features in the database were used directly to include the spatial heterogeneity of watershed characteristics in the modeling process.  The design discharge is based on the kinematic-wave approximation.  The spatial data sets were used directly in the hydrologic model to determine the design discharge for different return periods. 

2    WATERSHED GEOMORPHIC FACTORS CALCULATION

Channel network extraction from digital elevation data in this study followed the algorithms developed by Jenson and Domingue (1988).  The algorithms were performed on a spatial grid system (raster data structure).  These techniques are based on neighborhood operations, where calculations and decisions are made for a cell according to the elevations in the eight cells that are spatially adjacent in the raster.  The Horton-Strahler ordering system was adopted in this study.  The channel network can be extracted using an adequate threshold-area value, and then the length and slope for different order of overland areas and streams as well as other geomorphic factors can be directly obtained from the data set generated by a digital elevation model (DEM).  The detail procedure for geomorphic factors calculation can be found in Lee (1998).

3    DESIGN DISCHARGE CALCULATION

For small watersheds, storm duration usually exceeds watershed time of concentration; that is, the flow equilibrium state is usually reached.  The magnitude of watershed peak discharge at equilibrium can be expressed as

                                          (1)

in which Q= peak discharge, i_e = rainfall excess intensity, A= watershed area. Once time of concentration is determined, the rainfall intensity can be determined from an intensity-duration-frequency curve storm using rainfall duration equal to the time of concentration.  In this study, the estimation of the time of concentration is based on kinematic-wave theory.  The application of the kinematic-wave approximation for watershed runoff simulation has been considered acceptable for small watersheds provided the watershed slope is not too flat (Ponce, 1989; Fread, 1993). 

Lee and Yen (1997) considered a watershed consisting of two identical rectangular overland-flow planes as a V-shape model.  For an Ωth-order watershed, the time of concentration is the summation of runoff travel time in overland-flow states and in channel-flow states, which is (Lee and Yen, 1997)

              (2)

in which  is the time of concentration of the watershed; n_o and n_c are the roughness coefficient for overland planes and channels, respectively; and are the mean ith-order overland and channel slopes, respectively; and are the mean ith-order overland-flow length and channel-flow lengths, respectively; B_i is the width of the ith-order channel; m is an exponent (=5/3); i_e is the rainfall excess intensity; and is the inflow depth of the ith-order channel due to water transported from upstream reaches.  The value of  is equal to zero for i=1 because no channel flow is transported from upstream.  For 1 < i £ W,  can be expressed as (Lee and Yen, 1997)

                      (3)

in which = the number of ith-order channels, and = the mean of the drainage area of order i.  By means of Eqs. 2 and 3, the travel time for different order subwatersheds can be estimated from overland and channel hydraulics instead of relying on watershed empirical formulas. 

Inquiry System and User Interface

From the user’s point of view, the inquiry system should provide a convenient figure interface for design work.  In order to make the system useful as a decision-support tool, its implementation must allow users for various design conditions.  Through the interface, the user can easily access watershed database and model output for a specified design condition.  In this study, the interface code was written in Borland C++ language, following an object-oriented design methodology (Cox, 1986), which is predominantly in most of geographic information systems.  The GIS functions were used to assign all attributes to corresponding location of the watershed.  The strategy in this system to deal with abundant watershed information is to let the user select preferred figures in several independent windows, which are interactively managed by the user.  Following the analytical procedure mentioned previously, the core of the inquiry system should be composed of data acquisition, geomorphic factors calculation module, and design discharge calculation module.  Each module consists of several executable Fortran programs that operated in the same database environment.

The inquiry system can be run in a PC 486 or higher environment.  The user interface module can extract high-level descriptions of watershed state from the database.  The inquiry system is flexible to offer detailed information at any point within the study watershed for users request. Once the user specified the location of watershed outlet, the watershed geomorphic module will delineate the watershed boundary out.  Consequently, abundant calculation work is conducted to generate the geomorphic factors for time of concentration calculation.  Since the kinematic-wave time of concentration varies with rainfall intensity, users need to specify a design return period which is correspond with a generated intensity-duration regression equation.  

4    CASE STUDY

The inquiry system is flexibly applied to any watershed if geomorphic and hydrologic databases are prepared for the input requirement of the system.  In this study, the Wu-Se Reservoir Watershed located at central Taiwan was used as an example to illustrate the capability of the inquiry system.  The size of the whole watershed is 204 km², and the stream network of the watershed is fifth-order.  The Wu-Se Reservoir is the major flow supply for the Wan-Da hydroelectric power station in peak-load periods.  To avoid serious sedimentation reducing reservoir storage, soil conservation work is required in this watershed.  Therefore, it is important to establish a convenient inquiry system providing detailed geomorphic and hydrologic information for soil conservation design works.

5    RESULTS AND DISCUSSION

The resolution of the digital elevation data set we obtained is 40m×40m.  By applying DEM technique for depression elimination, the stream network is extracted from the data set, and then the geomorphic factors can be calculated.  The geomorphic factors for the entire Wu-Se Reservoir watershed are listed in Table 1.  As mentioned previously, the inquiry system can provide geomorphic information not only for the entire watershed, but also for any sub-area within the watershed.  As shown in Fig. 1a, consequent calculation module was conducted while an arbitrary point was assigned, and then the interface automatically delineated the subwatershed boundary on screen.

In applying Eqs. 2 and 3, the channel roughness coefficient n_c and overland roughness coefficient n_o need to be determined.  Adequate channel roughness coefficients can be estimated through field investigation using the values suggested by Chow (1959).  In this study, SPOT images were used for land cover classification, and then the overland roughness coefficient was linked to different land cover conditions provided by HEC (1990).  Based on August 1999 SPOT image classification, the land cover distribution for the entire Wu-Se Reservoir watershed is 1% impervious area, 5% grassed and agricultural land, 2% bare ground, and 92% forest.  Once the subwatershed is delineated, the percentage of different land covers within the subwatershed can be calculated based on the land cover database to obtain a corresponding overland roughness coefficient n_o. 

  Table 1    Geomorphic factors of the entire Wu-Se watershed

The Tsui-Feng station is the only rainfall station in the Wu-Se watershed.  A three-parameter rainfall intensity-duration regression equation was used to represent the analytical results as

                              (4)

in which i is the rainfall intensity (mm/hr); t_d is the rainfall duration (min); a, b, and c are constants for different return periods which are listed in Table 2.  For a specified return period, the design rainfall intensity i is determined by using the intensity-duration equation combining with the time of concentration equation.  The design rainfall-excess intensity i_e is then obtained by deducing 1 mm/hr for rainfall abstraction, which is usually adopted in the Taiwan Water Conservancy Institute. Consequently, the design discharge can be estimated using Eq. 1. 

   Table 2    Regression constants for intensity-duration-frequency analysis

As shown in Fig. 1b, the geomorphic information and corresponding design discharge are shown on windows for a specified subwatershed (Fig. 1a) for 10yr return period.  The user-driven interface can easily access the results and store the information in the database.  Moreover, the user can run several locations of subwatershed outlet to consult different aspects and possibility for further soil conservation work.

6    CONCLUSIONS

This paper presents a user-friendly inquiry system for hydrologic design in upland watersheds. The system integrates a geomorphic and hydrologic database within a graphic computer environment.  The computational modules are implemented using object-oriented design techniques; therefore, the modules and database can be used for further various purposes.  In this paper, we present a case study to show the possibility of the inquiry system to implement design work for any location of an upland watershed.  This inquiry system also presents the possibility for future improvement to generate complete hydrographs not only for upland small watersheds but also for midsize watershed design work.

Acknowledgements

This study is a research work supported by the National Science Council, Taiwan,China, under grants NSC 88-TPC-E-019-006 and NSC-89-TPC-7-019-011.  Financial support from the National Science Council is gratefully acknowledged.

References

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Chow, V. T. (1959). Open-Channel Hydraulics, McGraw-Hill, New York, N. Y.

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Hydrologic Engineering Center (1990). HEC-1 flood hydrograph package: user’s manual, U. S. Army Corps of Engineers, Davis, California.

Lee, K. T. and Yen, B. C. (1997). “Geomorphology and kinematic-wave based hydrograph derivation”, J. Hydraulic Engrg., ASCE, 123(1), 73-80.

Lee, K. T. (1998). “Generating design hydrographs by DEM assisted geomorphic runoff simulation: a case study”, J. Am. Water Resour. Assoc., 34(2), 375-384.

Levy, B. S. and Baecher, G. B. (1999). “Nilesim: a windows-based hydrologic simulator of the Nile River basin”, J. Water Resour. Planning and Management, ASCE, 125(2), 100-106.

Ponce, V. M. (1989). “Section 4.2, Overland flow” and “Chapter 9, Stream channel routing”, Engineering Hydrology, Prentice Hall Book Co., Englewood Cliffs, N. J.

(a) Wu-Se watershed map and a specified subwatershed

(b) Geomorphic and hydrologic information output

Fig. 1    Geomorphic and hydrologic information inquiry system