ARGUMENTS AGAINST CONCRETE ARMORED CHANNELS IN
DOG RIVER WATERSHED
Philip R. Jones, Department of Earth
Impermeable surfaces and or saturated soil prevent rainwater from infiltrating into the ground and cause overland flow into creeks and rivers. Under low infiltration circumstances, stream levels rise and larger flows carry more sediment and cause erosion and flooding. Concrete channels are designed to prevent flooding and stabilize a stream bank, but do not offer a perfect solution to storm water runoff.
Many concrete channels have been installed in the
Water quality in a creek is also affected by
concrete channelization. The natural stream flow of
Large sediment deposits in
Channel dimensions in
Engineers use a value called a runoff coefficient to
determine how much runoff occurs in a watershed for a given rainfall
event. It varies by land use and depends
on the permeability of the surface. This
value ranges from 0.05 for flat grassy lawns to 0.95 for downtown business
areas (Fetter, 2001). It is similar to
saying that 5% of water runs off a grass lawn and 95% of water runs off a paved
business area. The headwaters that feed
The best way to understand the characteristics of a watershed is to focus on small basins within that watershed. In order to understand runoff in concrete channels and possibly prevent future dredging projects, a study must be conducted in upstream areas concerning the magnitude of peak flows and a comparison done on their measured runoff coefficients to typical values.
It is evident that concrete lined channels increase
the flow speed for sections of
In the first
part of this study, the peak discharge is calculated from seven rainfall events
for six streams that feed into
Measurements were taken from six locations, on West
Eslava Creek, Bolton Branch West and Spencer Branch for seven rainfall
events. The location on West Eslava
Creek was at
The values needed for calculations were: stream height at maximum discharge, local rainfall in inches per hour, slope of channel, dimensions of the channel where the data was collected, watershed areas, and land use data. The required supplies included chalk, a measuring tape, ruler, level, heavy fishing line, 7.5 minute topographic map, pencil, notepad, calculator, graphing software, transportation, and a few raindrops.
Velocity and area were needed to find a stream’s discharge. Stream velocity was calculated by using the Manning equation for open-channel hydraulics (Fetter 2001):
(V) Velocity = 1.49 ´ ((R^2/3) ´ (S^1/2)) ¸ (n), Where,
V = velocity in feet per second
R = the ratio of the cross-sectional area of flow (in square feet) to the
wetted perimeter (in feet) through the channel
S = the slope of the surface
n = the roughness coefficient
Deriving each of these values consisted of their own complications. The roughness coefficient depends upon the amount of friction between the water and the channel. Normally (n) is obtained from a table where it is 0.012 for smoothed concrete or 0.035 for a winding natural stream with weeds (Fetter 2001). The surface of these concrete channels is rougher from weathering. Therefore, a finite difference calculation of velocity was done using a floating object and a stopwatch. This was only conducted on W Eslava Cr due to lack of water flow in the other streams. With known depth and dimensions, the Manning equation was used to solve for the roughness coefficient (n). The test runs indicate a roughness coefficient greater than smoothed concrete around 0.015.
A lengthy run was needed to accurately calculate the low slopes. First, seven meters of line was secured uphill and connected to a level downhill. The line was made tight and level, then the distance from downhill side to the channel surface was measured for the rise.
Velocity was needed at peak discharge, so the crest of the water in the channel was obtained to calculate (R). Channel walls were chalked and then washed away by the stream flow up to the maximum height. These marks were made beneath bridges, protected from falling rainwater. Best judgment was used in discerning these erased marks. Some washed away with a defining line, others eroded more gradually and plain chalk worked better than colored chalk. Uneven left and right wall measurements were averaged because of stream currents, capillary fringe of water at the surface and or uneven channel floors (Appendix A).
With the channel dimensions and the height of peak stream flow known, R was determined for the Manning equation. Most streams were divided through two or three concrete frames under bridges, so calculations for each channel section were added to obtain the total ratio variable (R) for that stream (Appendix A). A large sand slug had collected in the inside of a bend at Little Spencer Branch, and needed to be integrated out of the channel dimensions (Fig. 4) . The sediment load occupied three quarters of the channel, so one quarter of the measurements were applied.
Measurements were taken from bridges with square concrete frames which conveniently allowed for calculation of exact stream dimensions through the channels (Area = Height ´ Width). Bolton Drainpipe is an exception, where the arc measurement was the wetted perimeter and the cross sectional area was obtained by using the law of cosines and subtracting the triangle formed by the chord from the circle sector (Appendix A).
The discharge, Q, was obtained from multiplying the maximum cross sectional area through the channel by the velocity (Q =V ´ A). The peak discharge values for each stream and corresponding rainfall intensities were listed in a data set (Appendix B). These maximum flow rates were plotted against the rainfall intensity for that peak discharge using the computer software program Mathematica. A function for a curve was applied to each data set depending on its shape. Curves were extrapolated 75% beyond the sets of data to show expected trends at these locations for a given rainfall event.
In the second part of the study, a runoff coefficient was calculated for the streams using the Rational Method equation.
Q = K ´ C ´ I ´ A, where;
(Q) = The peak discharge in cubic ft/ s
(K) = A frequency correction factor
(C) = The Runoff Coefficient (unit less)
(I) = The average rainfall intensity in inches per hour
(A) = The drainage area above the test location in acres
This 100-year-old equation is a simplified, practical
approach to hydrological estimations. It
calculates discharge for a constant intensity rainfall but has many limitations
due to its assumptions (Whipple 1983).
The equation was rearranged to find the runoff coefficient (C). The (K)
variable will equal one for rain events in this study. Variable (I) was taken
from National Weather Service measurements made at
peak discharge and rain intensity for each creek from March 27, plus the land
area, the runoff coefficient was solved for each basin. Each calculated coefficient value was compared
to an estimated one from land use and soil type. Based on data from the City of
Each creek had a degree of exponential growth in it's peak discharge with an increase in rainfall intensity (Fig. 5) , (Fig. 6) , (Fig. 7) , (Fig. 8) , (Fig. 9) , and (Fig. 10) . This is expected as soils become saturated and more rain contributes to a stream's flow. The stream with the largest watershed area (516.6 acres) and largest discharge per rainfall was Spencer Branch, and Bolton Drainpipe had the smallest watershed area (51.66 acres). The watershed with the highest measured runoff coefficient was Spencer Br., followed by Little Spencer Br., then W Eslava Cr. The lowest coefficient value was measured at Bolton Br. West. See Table 1 . The largest margin between measured and estimated runoff coefficients was for Little Spencer Br., then Spencer Br. See Table 2 .
Throughout this study, many
educated assumptions were made pertaining to the measurements and
equations. The rational method technique
is only valid for rainfall duration equal to the watershed’s time of
concentration. This is travel time from
inlet to basin outlet or the time taken for the entire watershed to contribute
to the flow of the stream (Whipple 1983).
For duration times less than this, the entire basin doesn’t contribute.
For most of these observations, the rainfall duration exceeded the time of
concentration. Rainfall values for the
rational method are usually obtained from an intensity-duration-curve based on
recurrence interval for a particular region (
The curves were only extended 75 percent beyond the data set to maintain minimal error. Larger rainfalls are needed in order to show a more accurate trend and the equations used, including the rational method, are better suited to bigger rains. The shape of the curves, in Figure 5 to 10, was expected but the high numbers were not. These large discharge values made an impact on the corresponding measured runoff coefficients.
Spencer Branch, Little Spencer Br and their
tributaries have the longest reach of concrete channels than any other stream
in this study. Spencer Br is channeled northward beyond
The reason for vegetation in Eslava Cr. is due to a weak, steady flow of water that encourages growth. The high runoff coefficient measured for W Eslava Cr. did not reflect the affects of the rough surface because of the many parking lots in the headwater region.
Parking lots were in the lower end of the Bolton Br
West watershed, so runoff was more evenly drained. Lack of parking lot area in the Little Bolton
Br basin resulted in a low runoff coefficient.
Bolton Drainpipe drains an office complex nearby and may drain storm
Results show basins with impervious surfaces concentrated upstream contribute to high runoff rates. Runoff in a watershed is focused along the drainage channel. If this channel is concrete, stream flow increases and more discharge from upstream accumulates with inflow downstream resulting in high water levels. The variance of the roughness coefficient (n) in the Manning equation had a significant impact on stream velocity. Vegetation and rougher concrete were witnessed to have an influence on a stream’s discharge amount. These observations could be implemented to retard water flow in upstream areas and lessen discharge totals downstream in concrete channels. Along with more infiltration in the watershed and riparian boundaries, the risk of flooding would be reduced and the quality of water improved
Future studies could observe runoff to rainfall relationships along the course of one channel and determine the impact due to the extent of the concrete channel. More and larger rain events would aid in the accuracy of the findings.
Mike. 2001. DRCW JAN 2002
Fetter, C. W. 2001. Applied Hydrology, 4th ed. Upper Saddle River, N.J. Prentice-Hall, Inc.
Neilly, Peter P. and UCAR 2001. http://www.met.fsu.edu/weather/
Purdue University 2000. Runoff Coefficients for Rational Equation http://abe.www.ecn.purdue.edu/~engelb/abe526/Runoff/C_table.html
Whipple, William and Randall, C.W. 1983. Stormwater Management in Urbanizing Areas Englewood Cliffs, N.J. Prentice-Hall, Inc.
Much thanks the following people for their unselfish time and assistance.
John Crawford PE, City of Mobile Engineering Department, Scott Kearney and Sam Stutsman, City of Mobile GIS, Dr. Miriam Fearn, Dept of Earth Sciences USA.