A Study Of Basin Response Using Hec-hms

A STUDY OF BASIN RESPONSE USING HEC-HMS AND SUBZONE REPORTS OF CWC Dweependra Nath Kalita e-mail:[email protected]

Abstract In the northeastern part of India, where data availability is scarce in many areas, the Flood Estimation Reports of North Brahmaputra (Subzone-2a) and South Brahmaputra (Subzone-2b) published by CWC have been serving as handy tools for arriving at viable estimates of design flood. The development of the HEC-1 model to a GUI-based user friendly HEC-HMS model available in the public domain have come as another useful tool for the hydrologists. This study is made to compare the basin response to the same storm for the catchments of Subzone-2a and 2b using the HEC-HMS model and the CWC reports. Watershed characteristics along with unit hydrograph parameters of 21 catchments of Subzone-2a and 14 catchments of Subzone-2b are available in the CWC reports. Unit hydrographs are developed for all these catchments using the unit hydrograph parameters. Considering a 50-year return period storm, flood hydrographs are developed for each of the catchments. For the same storm, flood hydrographs are derived using HEC-HMS model also. While using the HEC-HMS, the Snyder and SCS unit hydrograph transform methods are used. All the resulting flood hydrographs are compared with a view to see whether the HEC-HMS model can be used with the same degree of reliability and viability as the CWC reports. An attempt is made to establish regional parameters for the Snyder and SCS unit hydrograph transform methods so that the HEC-HMS model can be used for the areas under Subzone-2c also where a similar report of CWC is yet to be published. Introduction Estimation of design flood is one of the usual tasks assigned to a hydrologist. Design flood information is required for flood protection measures in a river basin or for the design of water related structures against failure by overtopping. Depending on the project, design flood values are provided by hydrologists as a peak discharge value or as a flood hydrograph corresponding to a fixed return period of a few years in case of an urban storm water drainage scheme, or up to 10,000 years or more in case of a spillway of a large dam. In many dam projects the probable maximum flood (PMF) is used as a design flood against dam overtopping. In all the cases, design floods provide a decisive input into the comprehensive flood risk management policy. Estimation of design flood becomes an arduous task when the hydrologist is faced with limited data from poorly gauged or un-gauged basins. With limited or no data, the quantitative understanding and prediction of the processes of runoff generation and its transmission to the outlet represent one of the most challenging areas of hydrology. Traditional techniques for design flood estimation include the rational method, empirical methods, flood frequency method, unit hydrograph techniques, and watershed models. The rational method, empirical methods and flood frequency method are generally used for estimating the magnitude of the flood peak. The unit hydrograph techniques and watershed models can be used to estimate the design flood hydrograph in addition to the magnitude of the design flood peak. However, in a

poorly gauged or un-gauged basin, construction of a unit hydrograph is the toughest part of the job. This is where the regional studies for the 26 hydro-meteorologically homogeneous subbasins of India done by CWC have been immensely helpful for the field hydrologists. In the northeastern part of India, where hydro-meteorological data availability is scarce in many areas, the Flood Estimation Reports of North Brahmaputra (Subzone-2a) and South Brahmaputra (Subzone-2b) published by CWC have been serving as handy tools for arriving at viable estimates of design flood. A similar report for Subzone-2c covering the states of Tripura, Mizoram, Manipur, and parts of southern Assam and Meghalaya is yet to become available. The proliferation of personal computers and the development of the HEC-1 model of the U.S. Army Corps of Engineers (1998) to a GUI-based user-friendly HEC-HMS model available in the public domain have come as another useful tool for the field hydrologists. Unfortunately, the HEC-HMS model, or any of the so many watershed models for that matter, has not found many takers due to the uncertainty involved in the estimation of parameters of the models. But parameter estimation on a regional scale at least may be possible to enable the hydrology community to switch over to watershed models like HEC-HMS and take advantage of such high speed computer programs instead of sticking to the now traditional spreadsheet exercises. With this in view, this study has been undertaken to make a comparative study of basin response using HEC-HMS and Flood Estimation Reports of CWC in North-East India. Objectives of the study The main objectives of the study are: (I) To see whether the HEC-HMS program can be reliably used in estimation of design flood for the catchments under Subzone-2a and Subzone-2b. (2) Parameter estimation in a watershed model like HEC-HMS is one of the main deterrents in its acceptability in poorly gauged or un-gauged basins. With the available watershed physiographic data available in the Subzone-2a and Subzone-2b reports, can regional model parameters be estimated for Subzone-2a and Subzone-2b? (3) A flood estimation report for Subzone-2c covering the states of Tripura, Mizoram, Manipur, and parts of southern Assam and Meghalaya is yet to become available. Can the regional model parameters estimated for Subzone-2a and Subzone-2b be extended to Subzone-2c also? Design flood hydrographs using Subzone-2a and Subzone-2b reports Design flood hydrographs for 50-year return period storms of the 21 catchments under Subzone2a and 14 catchments under Subzone-2b are obtained following the procedure outlined in the reports. The most important component of the procedure is finding out the unit hydrograph ordinates from a set of parameters. The parameters are to be found out through a set of equations derived from physical characteristics of the basin viz. area ( A ), length of the main stream ( L ), length of the main stream from the point opposite to the center of gravity of the basin ( L c ), and equivalent slope of the basin ( S ). The physiographic parameters of the catchments under Subzone-2a and Subzone-2b are given in Table 1 and Table 2 respectively. Unit hydrograph The set of equations to find out the unit hydrograph parameters for the catchments under Subzone-2a and Subzone-2b are given in Table 3. After finding out the unit hydrograph parameters for each catchment, the unit hydrographs of all the 21 catchments under Subzone-2a and 14 catchments under Subzone-2b are obtained.

Table 1. Physiographic parameters of the catchments under Subzone-2a Sl.

Bridge No.

A (sq km)

L (km)

L c (km)

S (m/km)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

240 521 376 373 8 (B) 486 363 450 6/12 242 210 22 95 285 196 429 114 8(S) 385 24 201

1350.00 1016.00 758.00 595.70 345.30 326.00 323.23 233.10 230.45 229.99 220.87 213.05 119.14 92.46 85.47 69.60 66.00 61.38 46.62 42.10 38.49

108.00 120.75 60.62 75.62 42.34 62.75 69.23 38.62 54.71 34.59 36.06 42.00 27.29 36.06 21.56 32.52 23.00 22.54 24.94 12.88 21.33

61.00 89.36 22.00 47.14 27.37 35.40 42.99 29.77 29.94 18.82 17.07 25.76 16.74 26.89 11.26 15.30 11.00 8.36 16.25 8.21 11.75

10.88 7.41 4.46 1.70 21.28 1.95 2.68 1.84 23.47 22.53 9.10 1.52 34.07 0.88 19.81 1.00 12.67 0.63 0.26 0.77 9.97

Table 2. Physiographic parameters of the catchments under Subzone-2b Sl.

Bridge No.

A (sq km)

L (km)

L c (km)

S (m/km)

1 1 2 3 4 5 6 7 8 9 10 11 12 13 14

2 4 (MOT) 463 414 6(MOT) 160 8 146 184 215 446 160 440 70 3(MOT)

3 1270.00 875.46 553.60 476.00 469.80 284.46 215.90 171.00 135.66 54.00 46.44 45.71 30.80 29.00

4 84.47 36.45 51.49 60.18 56.35 49.90 49.75 25.68 20.00 19.70 17.23 27.40 13.68 9.65

5 41.19 49.63 22.53 27.35 31.39 19.30 25.68 12.20 9.50 9.63 10.06 22.70 6.90 5.23

6 5.326 2.91 1.148 7.39 2.01 4.17 1.47 2.54 4.00 2.26 11.09 1.72 0.38 13.36

Design storm It is proposed to test the basin response to a 50-year rainfall. In the Subzone reports 50-year point rainfall values for 24 hour duration is given. The design storm duration is found out; point rainfall converted to areal rainfall and is distributed hourly as per the time distribution coefficients provided in the reports. After that critical sequencing is done and the rainfall input is made ready for convolution.

Table 3. The set of equations to find out the unit hydrograph parameters Sl. Parameter

Description

1

Unit hydrograph peak

2

3

4

5

6

qp

tp

W50

W75

WR 50

WR 75

Time from the center of unit rainfall duration to the peak of unit hydrograph Width of unit hydrograph measured at 50% of peak discharge ( Q p ) in hours

Equation for Subzone-2a

q p = 2.272(

Equation for Subzone 2b

L ) Lc

t p = 2.164(q p )

0.409

W50 = 2.084(q p )

tp =

3.39 qp

1.065

W50 =

Width of unit hydrograph measured at 75% of peak discharge ( Q p ) in hours

W75 = 1.028(q p )

Width of the rising side of unit hydrograph measured at 50% of peak discharge ( Q p ) in hours

WR 50 = 0.856(q p )

Width of the rising side of unit hydrograph measured at 75% of peak discharge ( Q p ) in hours

WR 75 = 0.440(q p )

1.071

TB

Base width of unit hydrograph in hours

TB = 5.428(t p ) 0.852

8

Tm

Time from the start of rise to the peak of unit hydrograph in hours

Tm = t p +

Qp

A

0.940

7

9

Qp

qp =

W75 =

2.206 qp

qp

WR 50 =

0.918

where t r is the unit rainfall duration in hours Peak discharge of unit Q p = q p A hydrograph in cubic meters per second

1.06

1.270

0.865

tr 2

0.71

WR 75 =

1.008

0.625 qp

1.17

0.380 qp

1.13

TB = 2.245 t p Tm = t p +

1.19

tr 2

Q p = 1.171( A) 0.7

Design Loss rate A design loss rate of 2.4 mm/hour as recommended in the Subzone-2a report and 3.5 mm/hour in the Subzone-2b report is adopted. Base Flow A base flow of 0.05 cubic meters per sq km has been adopted as recommended in both the subzone reports.

Design Flood Hydrograph After convolution, the design flood hydrographs are obtained. Design Flood Hydrograph using HEC-HMS The U.S. Army Corps of Engineers Hydrologic Modeling System (HEC-HMS) is a hydrologic model that supersedes HEC-1 and contains many improvements over its predecessor. The latest version of the program, HEC-HMS 3.1.0 has been used. Unit hydrograph A total of six unit hydrograph models are provided in the HEC-HMS program. The are (i) Clark unit hydrograph, (ii) ModClark unit hydrograph, (iii) SCS unit hydrograph, (iv) Snyder unit hydrograph, (v) User-specified S-graph and (vi) User-specified unit hydrograph. Based on parameter requirement, the Snyder unit hydrograph and the SCS unit hydrograph methods have been selected in this study.

The Snyder unit hydrograph method In the HEC-HMS program, two parameters are required for running the Snyder unit hydrograph method. They are the standard lag t p and a peaking coefficient. The standard lag is defined as the length of time between the centroid of precipitation mass and the peak flow of the resulting hydrograph. The standard lag t p is found out using the equations given in the subzone reports. The peaking coefficient measures the steepness of the hydrograph that results from a unit of precipitation. The implementation used in the HEC-HMS program utilizes a unit hydrograph generated with the Clark methodology such that the empirical Snyder relationships are maintained. Thus, the peaking coefficient to be taken is suggested automatically. It has been found that the mean value of 0.77 as the peaking coefficient can satisfactorily run the Snyder unit hydrograph method for the catchments of Subzone-2a and Subzone-2b. The SCS unit hydrograph method The SCS unit hydrograph method requires only one parameter i.e. the standard lag. The same standard lag values derived from the equations given in the subzone reports as used while using the Snyder unit hydrograph method are used for the SCS unit hydrograph method. Design loss In the HEC-HMS program, there are four alternative models included to account for the rainfall losses. They are (i) the initial and constant-rate loss model, (ii) the deficit and constant-rate model, (iii) the SCS curve number (CN) loss model and (iv) the Green and Ampt loss model. The initial and constant-rate loss model has been selected. The underlying concept of the initial and constant-rate loss model is that the maximum potential rate of precipitation loss is constant throughout an event. An initial loss is added to the model to represent interception and depression storage. Interception storage is a consequence of absorption of precipitation by surface cover, including plants in the watershed. Depression storage is a consequence of depressions in the watershed topography; water is stored in these and eventually infiltrates or evaporates. This loss occurs prior to the onset of runoff. The initial and constant-rate loss model has been selected because during an extreme event such as a 50-year storm, a common assumption is that the antecedent moisture saturates the soil before the event occurs. When this happens, the rate of infiltration approaches a constant value from an initial zero value. This physical condition during an extreme event can be well represented with the initial and constantrate loss model. In the Sub-zone 2(a) report of CWC, a loss rate of 2.4 mm/hour is

recommended while in the Sub-zone 2(b) report of CWC, a loss rate of 3.5 mm/hour is recommended. These rates are adopted in the study. Base flow The HEC HMS program includes three alternative models of base flow. They are (i) the constant, monthly-varying value model, (ii) an exponential recession model and (iii) a linearreservoir volume accounting model. The constant, monthly-varying model is the simplest base flow model included in the program. It represents base flow as a constant flow; this may vary monthly. This user-specified flow is added to the direct runoff computed from rainfall for each time step of the simulation. This model is used with a constant base flow value of 0.05 cubic meters per sq km as recommended in both the subzone reports. Design Flood Hydrograph With these inputs, the program is run to obtain the design flood hydrographs. Table- 4 and Table-5 show the comparative pictures of the design flood peaks obtained for the catchments under Subzone-2a and Subzone-2b. Discussion of results From Table 4 it can be seen that the SCS unit hydrograph method of HEC-HMS can be very well used for the catchments of Subzone-2a. On an average, we can expect the design flood estimate to be about 5% more than what would have been arrived at using the CWC procedure. In Subzone-2b, however, the Snyder unit hydrograph method of HEC-HMS appears to be slightly better suited. Parameter estimation The Snyder unit hydrograph is a synthetic unit hydrograph method. In 1938, Snyder published a description of a parametric unit hydrograph that he had developed for analysis of ungauged watersheds in the Appalachian Highlands in the US. More importantly, he provided relationships for estimating the unit hydrograph parameters from watershed characteristics. Snyder collected rainfall and runoff data from gauged watersheds, derived the unit hydrographs, parameterized these unit hydrographs, and related the parameters to measurable watershed characteristics. He proposed the equation: Standard lag t p = CCt ( LLc ) 0.3

(1)

where t p = standard lag, Ct = basin coefficient; L = length of the main stream from the outlet to the divide; Lc = length along the main stream from the outlet to a point nearest the watershed centroid; and C = a conversion constant (0.75 for SI and 1.00 for foot-pound system). The standard lag is defined as the length of time between the centroid of precipitation mass and the peak flow of the resulting hydrograph. Many relationships for estimating lag from sub-basin characteristics have been developed for different regions. As for example, lag equations are provided in the subzone reports as shown in Table 3. The parameter Ct is best found via calibration, as they are not physically-based parameters. It has been reported that Ct typically ranges from 1.8 to 2.2, although it has been found to vary from 0.4 in mountainous areas to 8.0 along the Gulf of Mexico. Alternative forms of the parameter predictive equations are also available in the literature. Others have proposed estimating tp as a function of t c , the watershed time of concentration.

Table 4. Comparison of design flood peaks for catchments of Subzone-2a Sl.

Bridge No.

Peak using CWC procedure

Increase/ decrease in percentage

2060.0 1274.9 2961.5 1238.8 2213.9 626.1 953.4

Peak using Snyder unit hydrograph method of HEC-HMS 2429.7 1613.8 3140.4 1294.3 2172.3 726.4 1103.3

1 2 3 4 5 6 7

240 521 376 373 8 (B) 486 363

8 9 10 11 12 13 14 15 16 17 18 19 20 21

Increase/ decrease in percentage

17.9 26.6 0.6 4.5 -1.9 16.0 15.7

Peak using SCS unit hydrograph method of HECHMS 2251.6 1514.4 2940.3 1214.9 2025.3 685.3 1041.4

450 6/12

818.1 1719.6

945.7 1904.5

15.6 10.8

893.9 1788.6

9.3 4.0

242 210 22 95 285 196 429 114 8(S) 385 24 201

1916.8 958.7 408.6 1557.9 322.4 1104.1 296.5 451.1 248.2 130.0 219.6 363.4

2040.1 1064.5 473.7 1697.0 374.2 1231.8 338.3 570.4 278.2 152.9 242.6 400.1

6.4 11.0 15.9 8.9 16.1 11.6 14.1 5.4 12.1 17.6 10.5 10.1

1923.2 997.1 444.6 1563.5 352.7 1131.0 318.4 546.8 263.3 143.8 228.3 375.4

0.3 4.0 8.8 0.4 9.4 2.4 7.4 1.0 6.1 10.6 4.0 3.3

9.3 18.8 -0.7 -1.9 -8.5 9.5 9.2

Average increase/decrease in percentage

11.7

5.1

Standard deviation

6.4

5.8

In the present study, using Snyder‟s equation along with the t p values derived from the equations for subzones 2a and 2b, it has been found that for these two subzones, Ct ranges from 0.58 to 3.72. Further, it has been observed that the Ct values bear a strong correlation with the slope of the catchments. Table 6 shows the slope and Ct values of the 21 catchments under Subzone 2a. A plot is made between the S pc (% slope) and Ct values shown in Fig. 1 which gives the following relationship-

C t = 0.9511( S pc )

0.39

(2)

Table 5. Comparison of design flood peaks for catchments of Subzone-2b Sl.

Bridge No.

Peak using Peak using CWC Snyder unit procedure hydrograph method of HEC-HMS 1 4 (MOT) 3989.9 4509.0 2 463 3233.8 3571.5 3 414 2096.4 2192.7 4 6(MOT) 1737.7 1829.0 5 160 1697.0 1782.4 6 8 1104.6 1132.1 7 146 889.9 889.0 8 184 717.6 723.7 9 215 593.8 587.2 10 446 328.6 331.9 11 130 234.9 231.4 12 440 335.1 328.9 13 70 232.8 226.0 14 3(MOT) 225.8 203.9 Average increase/decrease in percentage

Increase/ decrease in percentage

Peak using SCS unit Increase/ hydrograph method decrease in of HEC-HMS percentage

13.0 10.4 4.6 5.3 5.0 2.5 -0.1 0.8 -1.1 1.0 -1.5 -1.8 -2.9 9.7 6.4

4367.1 3468.3 2121.7 1734.2 1688.6 1065.4 840.7 678.3 554.6 311.9 213.3 316.4 216.6 195.2

Standard deviation

4.8

9.5 7.3 1.2 -0.2 -0.5 -3.5 -5.5 -5.5 -6.6 -5.1 -9.2 -5.6 -6.9 -13.6 -3.2 5.9

Table-6. Table showing relation between slope and Ct values in Subzone-2a Sl.

Bridge No.

S (m/km)

S (m/m)

S pc

Ct

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

240 521 376 373 8 (B) 486 363 450 6/12 242 210 22 95 285 196 429 114 8(S) 385 24 201

10.88 7.41 4.46 1.70 21.28 1.95 2.68 1.84 23.47 22.53 9.10 1.52 34.07 0.88 19.81 1.00 12.67 0.63 0.26 0.77 9.97

0.01088 0.00741 0.00446 0.00170 0.02128 0.00195 0.00268 0.00184 0.02347 0.02253 0.00910 0.00152 0.03407 0.00088 0.01981 0.00100 0.01267 0.00063 0.00026 0.00077 0.00997

(%slope) 1.088 0.741 0.446 0.170 2.128 0.195 0.268 0.184 2.347 2.253 0.910 0.152 3.407 0.088 1.981 0.100 1.267 0.063 0.026 0.077 0.997

1.12 0.86 1.16 2.58 0.72 1.98 1.79 1.92 0.74 0.70 0.98 2.05 0.58 2.50 0.67 2.25 0.80 2.48 3.72 2.19 0.88

5 4

Ct

3 2 1 0 0

1

2

3

4

Spc

Figure 1. Plot between S pc (% slope) and Ct values in Subzone-2a For running the Snyder unit hydrograph method in HEC-HMS, the standard lag t p is found out using Eq. 1 in conjunction with Eq. 2, which makes a modified Snyder equation as follows:

t p = 0.9511C ( S pc )

0.39

( LLc ) 0.3

(3)

or, for SI units

t p = 0.7133( S pc )

0.39

( LLc ) 0.3

(4)

The peaking coefficient is taken as 0.77. With these input data for all the 21 catchments, the HEC-HMS program is run and the design flood hydrographs obtained. These hydrographs are superimposed upon the hydrographs obtained using the procedure outlined in the Subzone 2a report. As an example, the superimposed situation in case of catchment no. 114 is shown in Fig. 2. 600 500 400 CWC

300

HEC-HMS

200 100 0 0

20

40

60

Figure 2. Superimposition of HEC-HMS hydrograph upon CWC hydrograph

It can be readily observed from Fig. 2 that both the hydrographs are quite matching. It is interesting to note that that for all the other catchments of Subzone-2a, the matching of hydrographs is very encouraging. For a comparison, Table 7 shows the flood peaks obtained using the CWC report, Snyder unit hydrograph method of HEC-HMS with tp as per Eq. 3, and SCS unit hydrograph method of HEC-HMS with tp as per Eq. 3. Table 7. Comparison of design flood peaks Sl.

Bridge No.

Peak using Peak using CWC Snyder unit procedure hydrograph method of HEC-HMS with t p as

Increase/ decrease in percentage

Peak using Increase/ SCS unit decrease in hydrograph method percentage of HEC-HMS with t p as per Eqn. (3)

1

240

per Eqn. (3) 2060.0 2717.0

2 3

521 376

1274.9 2961.5

1372.4 2940.5

7.65 -0.71

1292.0 2743.7

1.34 -7.35

4 5 6 7 8 9 10 11 12 13 14 15 16 17

373 8 (B) 486 363 450 6/12 242 210 22 95 285 196 429 114

1238.8 2213.9 626.1 953.4 818.1 1719.6 1916.8 958.7 408.6 1557.9 322.4 1104.1 296.5 451.1

1660.2 2197.2 783.9 1208.5 980.5 1967.8 2051.2 1061.2 485.7 1694.6 382.6 1212.4 333.2 475.2

34.02 -0.75 25.20 26.76 19.85 14.43 7.01 10.69 18.87 8.77 18.67 9.81 12.38 5.34

1560.1 2042.1 738.6 1142.2 920.9 1848.0 1927.7 992.7 456.3 1550.2 360.8 1081.8 312.9 529.9

25.94 -7.76 17.97 19.8 12.57 7.47 0.57 3.55 11.67 -0.49 11.91 -2.02 5.53 17.47

18 19

8(S) 385

248.2 130.0

260.1 147.5

4.79 13.46

246.3 138.7

-0.77 6.69

20 21

24 201

219.6 363.4

223.7 388.9

1.87 7.02

210.3 362.1

-4.23 -0.36

Average increase/decrease in percentage

13.19

6.77

Standard deviation

10.03

9.91

31.89

2529.0

22.77

From Table 7 it can be observed that the SCS unit hydrograph method of HEC-HMS gives better results. To sum up, it can be said that if the standard lag is computed using Eq. 3 i.e. t p =

0.9511C ( S pc )

0.39

( LLc ) 0.3 and the SCS unit hydrograph method run in HEC-HMS, we can get

a fairly good estimate of design flood.

Possibility of use of HEC-HMS in Subzone-2c As stated earlier, a subzone report of CWC for Subzone-2c covering the states of Tripura, Mizoram, Manipur, and parts of southern Assam and Meghalaya is yet to published. It is thus worthwhile to see the possibility of application of HEC-HMS for the areas falling under Subzone-2c. Having established the parameters required for running the Snyder or SCS unit hydrograph methods in HEC-HMS for Subzone-2a, these parameters are tested for Subzone-2b. The results are tabulated in Table 8. Table 8. Subzone-2b peaks using Subzone-2a parameters Sl.

Increase/ decrease in percentage

Peak using SCS unit Increase/ hydrograph method decrease in of HEC-HMS percentage

4 (MOT) 2 463 3233.8 3863.7 3 414 2096.4 2455.2 4 6(MOT) 1737.7 2344.9 5 160 1697.0 1661.4 6 8 1104.6 1329.7 7 146 889.9 736.1 8 184 717.6 867.3 9 215 593.8 841.7 10 446 328.6 352.1 11 130 234.9 365.1 12 440 335.1 281.5 13 70 232.8 184.2 14 3(MOT) 225.8 204.2 Average increase/decrease in percentage

33.8

5126.9

28.5

19.5 17.1 34.9 -2.1 20.4 -17.3 20.9 41.7 7.2 55.4 16.0 -20.9 -9.6 15.5

3781.3 2182.4 2213.2 1590.1 1246.3 701.7 804.4 767.0 330.4 331.6 271.1 176.5 197.2

16.9 4.1 27.4 -6.3 12.8 -21.2 12.1 29.1 0.5 41.2 -19.1 -24.2 -12.7 6.4

Standard deviation

21.5

1

Bridge No.

Peak using Peak using CWC Snyder unit procedure hydrograph method of HEC-HMS 3989.9 5339.6

20.3

It can be seen from Table 8 that the SCS method of HEC-HMS could be used for Subzone-2b with Subzone-2a parameters with some degree of reliability, had there been no information in Subzone-2b. By this reckoning, if parameter estimation is done on the basis of information available for both the two subzones, the SCS method of HEC-HMS can be used for Subzone-2c. Parameter estimation for Subzone-2c Fig. 1 showed a plot between the S pc (% slope) and Ct values of the catchments under Subzone2a. Fig. 3 shows such a plot for all the catchments under Subzone-2a and Subzone-2b, so that an equation for standard lag can be developed for Subzone-2c. After considering three points in the plot of Fig. 3 as outliers, a revised plot is drawn which is shown in Fig. 4. The equation obtained is

C t = 0.9817( S pc )

0.35

(5)

4

Ct

3 2 1 0 0

1

2

3

4

Spc Figure 3. Plot between S pc (% slope) and Ct values of Subzone-2a and Subzone-2b

4

Ct

3

2

1

0 0

1

2

3

4

Spc

Figure 4. Revised plot between S pc (% slope) and Ct values of Subzone-2a and Subzone-2b

Eq. 5 in conjunction with Eq. 2, makes the modified Snyder equation for Subzone-2c, in SI units, as follows :

t p = 0.7363( S pc )

0.35

( LLc ) 0.3

(6)

Eq. 6 is proposed as a means to estimate the standard lag parameter for running the HEC-HMS program for catchments under Subzone-2c to obtain design flood.

Conclusions The HEC-HMS program can be reliably used for design flood estimate in Subzone-2a and Subzone-2b. The SCS unit hydrograph transform method is found to give better results in Subzone-2a, while the Snyder unit hydrograph method performs better in Subzone-2b.For the required standard lag parameter in the both the unit hydrograph methods, the equations provided in the subzone reports are to be used. For the parameter „peaking coefficient‟ required in the Snyder unit hydrograph method, a value of 0.77 can be used. The HEC-HMS program can also be used in Subzone-2c with a certain degree of reliability. For the standard lag parameter to be used for Subzone-2c, an equation based on the physical characteristics of the basin viz. L , L c and S pc in the form of Eq. 6 is proposed. References Hydrologic Modeling System (HEC-HMS) user’s manual: version 3.1.0. (2006). U.S. Army Corps of Engineers, Institute for Water Resources, Hydrologic Engineering Center, Davis, CA. Hydrologic Modeling System (HEC-HMS) technical reference manual: (2000). U.S. Army Corps of Engineers, Institute for Water Resources, Hydrologic Engineering Center, Davis, CA. Hydrologic Modeling System (HEC-HMS) applications guide: version 3.1.0. (2008). U.S. Army Corps of Engineers, Institute for Water Resources, Hydrologic Engineering Center, Davis, CA. Flood Estimation Report for North Brahmaputra Basin (Subzone-2a). (1991). Hydrology (Small Catchments) Directorate, Central Water Commission, New Delhi. Flood Estimation Report for North Brahmaputra Basin (Subzone-2b). (1984). Hydrology (Small Catchments) Directorate, Central Water Commission, New Delhi. Kumar,R. (2008). “Design Flood Estimation”. Lecture Notes, Training Workshop on Flood Disaster Management, Center for Flood Management Studies, National Institute of Hydrology, Guwahati.