EFFECT OF URBANIZATION ON STORM RUNOFF CHARACTERISTICS OF DHAKA CITY

M. F. Bari

Professor, Dept of Water Resources Engineering, Bangladesh University

of Engineering & Tech Ramna, Dhaka 1000, Bangladesh

Email: bari@wre.buet.edu; Fax: +880 2 8623046, 861 3026; Tel +880 2 966 5631

 M. Hasan

Surface Water Modelling Centre, House 476, Road 32, New DOHS, Dhaka

Fax: +880 2 882 7901; Tel: +880 2 8824590-91

Abstract: During the last 25 years or so rapid urbanization has taken place in Dhaka city. Substantial increase in built-up areas has taken place due to development of residential and commercial areas mostly through private land developers and real estate business. These activities resulted in substantial increase in impervious area, created obstruction to natural drainage pattern, and reduced detention basins, which in turn lead to shortening of the runoff concentration time and an increase of the peak flow. In this paper efforts are made to investigate the increase in built-up area with progression of urbanization and corresponding increase of runoff rates and volume. Peak runoff rates computed by a rainfall-runoff model were compared with the results obtained by the rational obtained. Results showed that simple but widely used rational method produced comparable results.

Keywords: conceptual model, rational method, runoff coefficient, urban runoff

1  INTRODUCTION

The process of urbanization is linked with economic development and makes an increasingly higher contribution to the national economy. However, when the growth of urban population takes place at exceptionally a rapid rate, most cities and towns are unable to cope with changing situations due to their internal resource constraints and management limitations. As population and land values increase, the effect of uncontrolled runoff become an economic burden and poses a serious threat to the health and wellbeing of citizens. Management of runoff from even a minor storm is rapidly becoming an engineering requirement to help reduce water logging, flooding and stream erosion. It is important to realize that very few urban drainage systems are designed and built as a complete system. For the design of an adequate drainage system, it is essential to understand the changes in storm runoff characteristics with land use changes. Urbanization of the land usually results in the highly accelerated removal of storm water with corresponding increases in the volume and peak rate of runoff. The principal effects of land use change have been discussed by Viessman et al (1996) after Leopold (1968). Among these related to hydrology are changes in peak flow characteristics and changes in total runoff. Bras and Perkins (1975) studied the effects of urbanization on catchment response. Methods used in runoff calculation include techniques that allow generation of complete hydrograph from storms occurring over urban areas, and those that provide the magnitude of the peak flow rate. Many applications allow use of the peak in the analysis and the procedures are much less time consuming than the unit hydrograph or conceptual models.

The objective of this study is to investigate the impact of land use changes due to urbanization on storm runoff characteristics in the eastern part of Dhaka City. Firstly increase in built-up area and impervious surface with progression of urbanization is estimated. Runoff is computed using conceptual model to see how runoff volume is increasing with increase of built-up area. The model used in this study is known as NAM, which is a lumped conceptual model developed by Danish Hydraulic Institute. The reason for choosing NAM model is that the model is being used frequently for hydrologic studies in Bangladesh. Also peak runoff rates are computed using the rational method and compared with the values obtained by the lumped conceptual model. An assessment is also made of the possible expansion of built-up area with increased urbanization in future and the expected runoff rate for future urbanization scenario is investigated.

2  DATA COLLECTION AND METHODOLOGY

2.1  Study area

Dhaka is the capital of Bangladesh and the most important center for economic, political and cultural activities. In order to provide better facilities to the city dwellers in future more and more development of infrastructures would be needed. The western part of Dhaka city has developed more or less to the limit. There is opportunity for the city to grow eastwards and such expansion trend is being observed. Eastern part of Dhaka City has been selected for investigation of the effect of urbanization on storm runoff characteristics. The study area, shown in Figure 1, covers 118.62 km². It is bounded on the north by the Turag river, to the east by the Balu river, to the south by the Dhaka-Demra road and to the west by the Dhaka-Mynensingh road, the DIT-Rampura road and Biswa road. The storm drains serve less than

Fig. 1  Study area

25% of the Dhaka City area. The drainage problem is being increasingly aggravated due to unplanned expansion of urban area with concomitant increase in built-up area and metalled roads; filling of low-lying areas and water bodies for construction of buildings; obstruction of natural drainage canals.

2.2  Data collection and compilation

The on going urbanization process in Dhaka City has brought about a major change in the land phase of the hydrologic cycle. This is causing runoff generation in increased rates. Data needed to study the changes in storm runoff characteristics with land use changes include rainfall, evaporation and land use data. Rainfall and evaporation data for the 1986-95 period were used in this study. Groundwater level data for the concurrent period was used for calibration of the selected lumped conceptual rainfall-runoff model. Land use data were digitized from maps and some were available from previous reports (JICA, 1992).

3  ANALYSIS AND RESULTS

The analysis started with compilation of land use data from available maps and other sources. According to land use type, the study area was divided as residential, commercial, agricultural, institutional and water bodies. Area under each category for the years 1987 and 1995 was obtained by digitizing land use maps collected from Survey of Bangladesh. Data for 1990 were obtained from JICA (1992) report. Land use data for the years 1987, 1990, and 1995 are presented in Table 1 and plotted in Figure 2, which shows that the built-up area exhibits a linear variation with time. Extrapolating the line the built-up area in 2010 is found to be 50% and this is 22% higher than the JICA (1972) projected value of 72%. It is observed that amount of agricultural land decreases from 79% in 1987 to 68% in 1995 and is expected to decrease to about 11% in 2010. On the contrary built-up area shows an increase from 15% to 72% over the same period.

Table 1  Area under different land use types and weighted average runoff coefficient for the indicated years  (total area = 11862 ha)

Estimation of runoff coefficient

The runoff coefficient C accounts for all the factors affecting the relation of peak flow to rainfall intensity in addition to catchment area and response time. Estimating the value of the runoff coefficient C is the greatest difficulty and the major source of uncertainty in application of the rational method. Design values of C are normally obtained from tables relating runoff coefficient to the soil type and degree of imperviousness of various types of surfaces. Typical values of runoff coefficients for various types of surfaces are tabulated in handbooks and textbooks. Weighted average runoff coefficient for the study area is computed in Table 1 using the proportion of surface areas given in the same table. The computed runoff coefficients also show a more or less linear variation with time as shown in Figure 2 and with built-up area as shown in Figure 3. Extrapolating the line the C value in 2010 is found to be 0.40, whereas JICA (1992) projected land use data in 2010 yields a C value of 0.524. Weighted average C for other intermediate years was obtained by linear interpolation for latter use in the rational method.

Fig. 2  Variation of built-up area and runoff coefficient with time

Fig. 3  Changes of runoff coefficient with built-up area

3.1  Runoff simulation by rainfall-runoff model

Many continuous and event simulation models have been developed for the purpose of hydrologic analysis and synthesis. The rainfall-runoff model called NAM developed by Danish Hydraulic Institute is frequently being used for hydrologic studies in Bangladesh. It is a lumped parameter conceptual type of continuous simulation model and consists of a set of linked mathematical statements describing, in a simplified quantitative form, the behaviour of the land phase of the hydrological cycle. Water falling on a catchment due to rainfall is split into different hydrological components: evapotranspiration, overland flow, interflow, base flow and infiltration through a set of numerical parameters.

The NAM model parameters include catchment area, upper zone and lower zone storage capacity, time constants for interflow and base flow, base flow parameters, capillary flux, threshold values and time constants for overland flow routing. Rainfall, potential evapotranspiration, and monthly groundwater abstraction are the input for the model. Calibration of NAM is a trial and error procedure and initial estimates of the model parameters is made based on physical conditions in the area such as soil type, topography, vegetation type etc. For a good calibration of a NAM model, calibration periods should cover a minimum of two years to include wide range of conditions, i.e. rainfall and evaporation patterns. Calibration of NAM may be carried out either against ground water hydrograph or against observed discharge runoff from the catchments. For the present application the model was calibrated against ground water hydrograph and calibrations was excellent with respect to both maximum and minimum groundwater levels. Model output includes daily runoff and accumulated runoff. For comparison annual accumulated runoff is divided by annual accumulated rainfall to make it dimensionless for convenience of comparison. The ratio of accumulated runoff to accumulated rainfall against percent impervious area for 1986 to 1995 and 2010 are presented below and plotted in Figure 4.

          Table 2  Ratio of accumulated runoff and rainfall predicted by NAM model

It is seen that the ratio of accumulated runoff and accumulated rainfall increases progressively with an increase in impervious area.

3.2  Runoff estimation by rational method

The rational formula for estimating peak runoff rates was introduced in the United States by Kuichling (1889). The validity of the rational formula is based on the assumption that the rainfall intensity for any given duration is uniform over the entire catchment. Peak flow is computed as Q_p= FCIA, where Q_p is the peak runoff rate (m³/s), C is the runoff coefficient, I is the intensity (mm/h) of rainfall having a duration equal to the time of concentration, A is the drainage area (km²), F is a unit conversion factor (1.008 for English units: ft/s, in/h and acres; and 0.278 for SI units: of m³/s, mm/h, and km²). Peak runoff rates computed by the rational method are summarized in Table 3 together with those computed by the NAM model.

3.3  Comparison of results from rational method and nam

For comparison of peak runoff rates computed by the rational method and rainfall runoff model are tabulated in Table 3 and plotted in Figure 4. The table shows that compared to NAM model results, deviation of runoff estimates obtained by the rational method ranges between +14.23% and – 0.18% except for the year 1989 and 1992. The deviations for these years are seen to be 65.03% and 30.70%. The reason for this large deviation can be explained looking at the yearly accumulated rainfall and peak rainfall intensity for different years as shown in Table 3. NAM considers daily rainfall time series for the entire year whereas rational method considers the peak rainfall intensity for runoff computation. As such years having high intensity but comparatively lower accumulated rainfall, such as 1989 and 1992 tend to exhibit larger deviation in computed peak runoff rates by the two methods. 

Table 3  Comparison of peak runoff rates computed by rational method and rainfall runoff model

Fig. 4  Comparison of peak runoff rates

4  SUMMARY AND CONCLUSIONS

The analyses performed and results obtained herein provide an understanding of the underlying processes associated with urban storm runoff characteristics. The method of analysis that has been developed and presented in this study will be helpful in estimated storm water runoff and peak run off rates for a given land use pattern. As expected, it is seen that both the volume and peak rate of runoff increases with increases in urbanization. Considerable amounts of data are essential for correlation of these two variables. Long-term rainfall data is easily available, but land use data is limited. Only four years of land use data could be gathered for this study. As more information will be available in future, better estimates of runoff can be used using the methodology presented in this study. Computed results show that runoff volume is increasing with increase in built-up area in Dhaka city. Most of the low lying lands, which once acted as retarding basin, have been filled up. Volume of runoff also depends on volume of rainfall. Volume of rainfall varies from year to year. So in this study the changes in the ratio of accumulated runoff and accumulated rainfall with built-up area have been investigated.  Comparison of peak runoff rates computed by the rational method compare with the results obtained by the rainfall runoff model shows that the simple but extensively used rational method produces comparable results.

References

Bras, R.L. and Perkins, F.E. 1975: “Effects of Urbanization on Catchment Response,” Proc. ASCE J. Hyd. Div, 101 (HY3), March.

Japan International Cooperation Agency (JICA), 1992: “Feasibility Study on Greater Dhaka Protection Project”, Bangladesh Flood Action Plan 8A. Supporting Report-1, June.

Kuichling, E, (1889): “The Relation Between Rainfall and Discharge of Sewers in Populous Districts,” Trans. ASCE, vol 20.

Danish Hydraulic Institute (DHI), 1992: MIKE11 Version 3.01, User Manual, Edition 2.

Leopold, L.B. 1968: “Hydrology for Urban Land Planning,” USGS Circular No. 554, Washington, D.C., U.S. Government Printing Office.