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Hydrology

Velocity and Discharge at Drew Gardens

2002

The velocity distribution at Drew Gardens was measured to calculate discharge for the Bronx River. Discharge, or streamflow, is the volume of water that flows past a fixed point per unit of time. It is usually expressed in cubic feet per second. Discharge varies seasonally and with weather. It impacts floodplain dynamics, instream habitat and water quality.

Discharge cannot be directly monitored by a fixed instrument because stream velocity varies considerably over channel width and depth. Instead, discharge is calculated by dividing the cross-sectional area of the stream into segments and measuring the velocity for each segment. Velocity is recorded with a flow meter, then multiplied by the segment area. The resulting volumes are summarized across the section.

Methods

Discharge data was obtained by simultaneously measuring depth and point velocity across the entire width of the stream. Velocity measurements were taken at 0.6 the water depth from the surface or the average of 0.2 and 0.8 water depth. Segment discharge is equal to the product of depth and velocity at a point, while whole cross-sectional discharge is the sum of segment discharges. Average cross-sectional velocity is obtained by dividing total discharge by stream cross-sectional area.

Velocity readings were taken with a Marsh McBirney FlowMate Meter.

Measurements were taken on November 14, 2002, between 1:40 pm and 3:15 pm. Weather was clear and sunny, with steady rain during the previous 24 hours.

Results

Discharge Estimate 1

Tape Distance (ft)
Depth (ft)
Velocity (ft/s)
Area (sq. ft)
Discharge (cfs)
0.2
0.6
0.8
Avg
0 0.131 0 0 0 0.000 0.000 0.000
1 0.197 0.03 0 0 0.008 0.197 0.001
2 0.328 0.04 0.01 0 0.015 0.328 0.005
3 0.230 0 0 0 0.000 0.230 0.000
4 0.656 0.1 0.05 0.06 0.065 0.656 0.043
5 0.820 0.23 0.14 0.09 0.150 0.820 0.123
6 0.984 0.58 0.46 0.32 0.455 0.984 0.448
7 1.148 0.58 0.25 0.05 0.283 1.148 0.324
8 1.345 0.69 0.62 0.48 0.603 1.345 0.810
9 1.378 1.04 0.83 0.68 0.845 1.378 1.164
10 1.673 1.17 0.7 0.5 0.768 1.673 1.284
11 1.772 1.36 1.1 0.55 1.028 1.772 1.820
12 1.903 1.4 1.43 1.34 1.400 1.903 2.664
13 2.165 1.42 1.12 0.1 0.940 2.165 2.035
14 2.428 1.45 1.17 1.05 1.210 2.428 2.938
15 2.493 2.15 1.72 1.32 1.728 2.493 4.307
16 2.625 1.84 1.51 1.2 1.515 2.625 3.976
17 2.690 1.97 1.53 0.95 1.495 2.690 4.022
18 2.756 1.86 1.76 1.54 1.730 2.756 4.768
19 2.559 1.75 1.49 1.4 1.533 2.559 3.922
20 2.362 1.7 1.65 1.33 1.583 2.362 3.738
21 1.837 1.67 1.71 1.47 1.640 1.837 3.013
22 2.428 1.67 1.51 0.53 1.305 2.428 3.168
23 2.428 1.63 1.51 1.16 1.453 2.428 3.526
24 2.297 1.45 1.53 1.42 1.483 2.297 3.405
25 2.165 1.52 1.15 0.78 1.150 2.165 2.490
26 1.837 1.4 1.35 0.85 1.238 1.837 2.274
27 1.706 1.41 1.13 0.81 1.120 1.706 1.911
28 1.378 1.44 1.43 1.25 1.388 1.378 1.912
29 1.378 1.35 1.4 1.18 1.333 1.378 1.836
30 1.050 1.26 1.24 1.03 1.193 1.050 1.252
31 1.050 0.92 0.81 0.6 0.785 1.050 0.824
32 0.886 0.28 0.23 0.22 0.240 0.886 0.213
33 0.656 0.35 0.32 0.25 0.310 0.656 0.203
34 0.525 0.59 0.35 0.29 0.395 0.525 0.207
35 0.394 0.13 0.12 0.1 0.118 0.394 0.046
36 0.197 0 0.01 0 0.005 0.197 0.001
37 0.000 0 0 0 0.000 0.000 0.000
Sum of Flows (cfs) =
64.628

Discharge Estimate 2

Tape Distance (ft)
Depth (ft)
Velocity (ft/s)
Area (sq. ft)
Discharge (cfs)
0.2
0.6
0.8
Avg
0 0.131 0 0 0 0.000 0.053 0.000
1 0.197 0.03 0 0 0.000 1.008 0.010
2 0.328 0.04 0.01 0 0.005 0.299 0.005
3 0.230 0 0 0 0.000 0.295 0.000
4 0.656 0.1 0.05 0.06 0.025 0.623 0.044
5 0.820 0.23 0.14 0.09 0.070 0.820 0.126
6 0.984 0.58 0.46 0.32 0.230 0.984 0.446
7 1.148 0.58 0.25 0.05 0.125 1.152 0.338
8 1.345 0.69 0.62 0.48 0.310 1.325 0.790
9 1.378 1.04 0.83 0.68 0.415 1.411 1.199
10 1.673 1.17 0.7 0.5 0.835 1.649 1.302
11 1.772 1.36 1.1 0.55 0.955 1.776 1.782
12 1.903 1.4 1.43 1.34 1.370 1.919 2.668
13 2.165 1.42 1.12 0.1 0.760 2.165 1.905
14 2.428 1.45 1.17 1.05 1.250 2.403 2.940
15 2.493 2.15 1.72 1.32 1.735 2.502 4.328
16 2.625 1.84 1.51 1.2 1.520 2.616 3.968
17 2.690 1.97 1.53 0.95 1.460 2.690 3.991
18 2.756 1.86 1.76 1.54 1.700 2.723 4.684
19 2.559 1.75 1.49 1.4 1.575 2.559 3.958
20 2.362 1.7 1.65 1.33 1.515 2.403 3.749
21 1.837 1.67 1.71 1.47 1.570 1.698 2.745
22 2.428 1.67 1.51 0.53 1.100 2.354 2.911
23 2.428 1.63 1.51 1.16 1.395 2.444 3.503
24 2.297 1.45 1.53 1.42 1.435 2.297 3.368
25 2.165 1.52 1.15 0.78 1.150 2.190 2.518
26 1.837 1.4 1.35 0.85 1.125 1.813 2.175
27 1.706 1.41 1.13 0.81 1.110 1.731 1.933
28 1.378 1.44 1.43 1.25 0.715 1.337 1.836
29 1.378 1.35 1.4 1.18 0.700 1.419 1.859
30 1.050 1.26 1.24 1.03 0.620 1.009 1.187
31 1.050 0.92 0.81 0.6 0.405 1.070 0.831
32 0.886 0.28 0.23 0.22 0.115 0.894 0.218
33 0.656 0.35 0.32 0.25 0.160 0.644 0.197
34 0.525 0.59 0.35 0.29 0.175 0.525 0.215
35 0.394 0.13 0.12 0.1 0.060 0.402 0.047
36 0.197 0 0.01 0 0.005 0.172 0.001
37 0.000 0 0 0 0.000 0.000 0.000
Sum of Flows (cfs) =
63.730

Download Drew Gardens Velocity data (Excel format, 39 KB)

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