PRELIMINARY STREAMFLOW REQUIREMENTS FOR HABITAT
PROTECTION
The R2Cross and Wetted-Perimeter standard-setting methods were used to obtain preliminary streamflow requirements for habitat protection for sites on the Assabet River and Charles River for the summer period. Six riffle reaches were identified as being critical areas for determination of streamflow requirements. Three of these sites are on tributaries to the Assabet River, two are on the mainstem of the Charles River, and one is on a tributary to the Charles River. The results of the analyses are considered preliminary, because the flow conditions that were observed for model calibration were generally low due to the dry weather prior to December 2001.
Danforth Brook at Hudson
The riffle section studied on Danforth Brook is about 300 ft upstream from the Route 85 culvert in a conservation area owned by the town of Hudson (fig. 3). Seven cross sections were surveyed in this study reach, six of which were included in the water surface profile model. The cross sections were along an 80-ft length of pool and riffle habitats and were predominantly trapezoidal in shape. The channel takes a shallow bend to the left along the study reach. There are scattered trees and shrubs along both banks, the bed material is primarily cobbles, and the bank material is a mixture of organic silt, sand, and cobbles. The stream has a moderately steep slope. The water surface drops about 1.10 ft along the study reach at measured flows of both 0.005 ft3/s and 0.145 ft3/s.
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| Figure 3. Danforth Brook at Hudson, Massachusetts, looking upstream. |
The HEC-RAS model was run as a mixed-flow regime on the basis of the standard-step energy method (table 8, at end of this report). Two discharge measurements and measurements of water surfaces at the surveyed cross sections were taken at the study reach for the purpose of model calibration. The calibration discharges were modeled at normal (sub-critical) depth for the most downstream section and a slope of 0.016494 ft/ft was input as a boundary condition; this slope was calculated from water-surface altitudes at surveyed reference points at the most downstream end of the reach. Bankfull discharge was estimated from the 1.5-year flood for the streamgaging station at Nashoba Brook near Acton (01097300) adjusted for the differences in drainage areas, and by comparison with the altitudes of observed bankfull indicators surveyed at the study site.
Initial roughness coefficients for each cross section were estimated by solution of Manning's equation on the basis of the measured discharges and surveyed cross-sectional areas and slopes. The roughness coefficients for each cross section were varied as required until calculated water-surface altitudes matched measured water-surface altitudes with reasonable accuracy. The variability in the roughness coefficients between river stations (table 8) is due to the variability of the actual flow length as opposed to the straight-line lengths between river stations. The longer path the water takes in flowing around the bed material at different discharges changes the energy slope in the analysis and is compensated for by increasing the roughness coefficients to obtain acceptable model calibration. The calibration accuracy was calculated as the root mean square of the differences between the observed and modeled water-surface altitudes for all cross-sections and measured discharges used in the modeled reach. The calibration accuracy was 0.041 ft over the entire reach for both measured discharges. The HEC-RAS software occasionally indicated the need for more cross sections to reduce velocity head drops between sections. Addition of interpolated cross sections would reduce the number of these messages, but would not significantly affect the water-surface profile.
The R2Cross analysis to determine the required streamflow for habitat protection was based upon the HEC-RAS model results for cross sections at river stations 60.8 and 68.7 (table 3). The cross sections at river stations 60.8 and 68.7 were identified as being the critical sections in the riffle reach as they are upstream of the point where the riffle is above any downstream backwater influence. The R2Cross criteria at station 60.8 were an average depth of 0.21 ft, a wetted perimeter of 10.57 ft, and an average velocity of 1.0 ft/s. These criteria were met or exceeded at a discharge of 10.5 ft3/s (2.05 ft3/s/mi2). The R2Cross criteria at station 68.7 were an average depth of 0.23 ft, a wetted perimeter of 11.51 ft, and an average velocity of 1.0 ft/s. These criteria were met or exceeded at a discharge of 14.3 ft3/s (2.79 ft3/s/mi2). The limiting R2Cross criteria or last of the three criteria to be met was the mean velocity at both station 60.8 and station 68.7.
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Table 3. Required streamflows for habitat protection determined by means of the R2Cross method [ft, foot; mi, mile; s, second; --, no data] |
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| River and reach | River station |
Drainage area (mi2) |
Discharge | Limiting R2Cross criteria | |||||
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| ft3/s | ft3/s/mi2 | ||||||||
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| Danforth Brook at Hudson | 60 | .8 | 5 | .12 | 10 | .5 | 2 | .05 | Mean velocity |
| 68 | .7 | 5 | .12 | 14 | .3 | 2 | .79 | Mean velocity | |
| Great Brook near Bolton | 109 | .75 | 4 | .47 | -- | -- | -- | ||
| 114 | .57 | 4 | .47 | -- | -- | -- | |||
| Elizabeth Brook near Stow. | 202 | .05 | 18 | .7 | 9 | .2 | .49 | Mean velocity | |
| 213 | .57 | 18 | .7 | 12 | .0 | .64 | Mean depth | ||
| 229 | .45 | 18 | .7 | 7 | .4 | .40 | Mean velocity | ||
| 241 | .46 | 18 | .7 | 15 | .0 | .80 | Mean depth | ||
| Mine Brook near Franklin | 123 | .60 | 10 | .01 | 6 | .75 | .67 | Mean velocity | |
| 164 | .85 | 10 | .01 | 8 | .75 | .87 | Mean velocity | ||
| 173 | .65 | 10 | .01 | 2 | .4 | .24 | Mean depth | ||
| 187 | .25 | 10 | .01 | 7 | .25 | .72 | Mean velocity | ||
| Charles River at Medway downstream | 641 | .1 | 65 | .7 | 57 | .7 | .88 | Mean depth | |
| Charles River at Medway upstream | 1,481 | .8 | 65 | .7 | 45 | .4 | .69 | Mean depth | |
| 1,498 | .9 | 65 | .7 | 52 | .8 | .80 | Mean depth | ||
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A wetted perimeter-discharge relation was determined for cross sections at river stations 60.8 and 68.7 on the basis of the HEC-RAS model simulations for a range of discharges up to bankfull flow. Breaks in the wetted perimeter-discharge relation were used to identify a discharge of about 0.35 ft3/s (0.07 ft3/s/mi2), for station 60.8 and a discharge of about 0.08 ft3/s (0.02 ft3/s/mi2), for station 68.7 (table 4). At these discharges, the average water depth in both cross sections was estimated to be 0.11 ft.
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Table 4. Required streamflows for habitat protection determined by means of the Wetted-Perimeter method [ft, foot; mi, mile; s, second; --, no data] |
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| River and reach | River
station |
Drainage
area (mi2) |
Discharge | |||||
|---|---|---|---|---|---|---|---|---|
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| ft3/s | ft3/s/mi2 | |||||||
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| Danforth Brook at | 60 | .8 | 5 | .12 | 0 | .35 | 0 | .07 |
| Hudson | 68 | .7 | 5 | .12 | .08 | .02 | ||
| Great Brook near | 109 | .75 | 4 | .47 | -- | -- | ||
| Bolton | 114 | .57 | 4 | .47 | -- | -- | ||
| Elizabeth Brook | 202 | .05 | 18 | .7 | 1 | .7 | .09 | |
| near Stow | 213 | .57 | 18 | .7 | 6 | .0 | .32 | |
| 229 | .45 | 18 | .7 | 1 | .5 | .08 | ||
| 241 | .46 | 18 | .7 | 3 | .4 | .18 | ||
| Mine Brook near | 123 | .60 | 10 | .01 | 2 | .4 | .24 | |
| Franklin | 164 | .85 | 10 | .01 | 4 | .0 | .40 | |
| 173 | .65 | 10 | .01 | 5 | .2 | .52 | ||
| 187 | .25 | 10 | .01 | 1 | .0 | .10 | ||
| Charles River at | ||||||||
| Medway downstream | 641 | .1 | 65 | .7 | 8 | .7 | .13 | |
| Charles River at | 1,481 | .8 | 65 | .7 | 1 | .8 | .03 | |
| Medway upstream | 1,498 | .9 | 65 | .7 | 3 | .4 | .05 | |
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Great Brook near Bolton
This study site is the first riffle reach downstream of the Route 117 culvert (fig. 4). Five cross sections were surveyed in this reach. The most upstream section and the most downstream section were about 64 ft apart near the ends of the riffle reach. The streambed is primarily sandy silt overlying gravel, and cobbles. The three most downstream cross sections have a roughly trapezoidal shape. The two upstream sections have been modified with a vertical concrete wall for the right bank, but the left bank is of natural material. The stream bank on the left side of the channel is soft sediment and shrub vegetation. This riffle was selected for study during a period of very low streamflow. However, the riffle becomes a run at moderate flows and its gradient may be below the recommended limits for the application of Manning's equation. The results of the HEC-RAS model simulations (table 9, at the end of this report) indicate that the surveyed sections at this site are subject to backwater conditions during low to moderate flows and are not appropriate for application of the R2Cross method.
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| Figure 4. Great Brook near Bolton, Massachusetts, looking upstream. |
The roughness coefficients for each cross section were varied as required until calculated water-surface altitudes matched measured water-surface altitudes with reasonable accuracy. The calibration accuracy was 0.04 ft over the entire modeled reach for all the measured discharges. The computed water surfaces for the three farthest upstream cross-sections have the same water-surface altitude within 0.02 ft for the calibration discharges, indicating that the sections are in backwater. The R2Cross and Wetted Perimeter analyses were not conducted at this site because riffle conditions are present only during extreme low flows. The site's habitat becomes a run during the range of flows near the streamflow requirements.
Elizabeth Brook near Stow
The riffle section studied on Elizabeth Brook is just downstream of a ford about 0.2 mi south of White Pond Road on an unnamed road in Stow (fig. 5). Six cross sections were surveyed in this study reach; all were included in the water-surface-profile model. The cross sections were along a 70-ft length of riffle. The channel bends slightly to the right and is predominantly trapezoidal in shape. There are shrubs along both banks and larger deciduous trees further back from the channel banks. The bed material is primarily gravel and cobbles; bank material is a mixture of organic soil and cobbles. The reach has a moderate slope and the water surface dropped 0.66 ft to 1.01 ft along the study reach at the measured discharges, which ranged from 1.7 ft3/s to 30 ft 3/s.
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| Figure 5. Elizabeth Brook near Stow, Massachusetts, looking downstream. |
The model was run as a subcritical flow regime using the standard, upstream-step energy method (table 10, at the end of this report). Four discharge measurements were made at the study reach for model calibration, ranging from 1.7 ft3/s to 30 ft3/s. The calibration discharges are modeled at normal depth at the most downstream modeled section and a slope of 0.019737 ft/ft was input as a boundary condition. This slope was calculated from water-surface altitudes measured at the surveyed reference points at the downstream end of the reach. Bankfull discharge was estimated by a drainage area adjustment for the 1.5-year flood determined for the gage at Nashoba Brook near Acton (01097300) and by comparison with the altitude of observed bankfull indicators surveyed at the study site.
The roughness coefficients for each cross section were varied as required until calculated water-surface altitudes matched measured water-surface altitudes with reasonable accuracy. The calibration accuracy was 0.022 ft over the entire reach for all four measured discharges.
The R2Cross analysis was based on the HEC-RAS model results for cross sections at river stations 202.05, 213.57, 229.45, and 241.46 (table 3). The R2Cross criteria for determining streamflow requirements at station 202.05 were an average depth of 0.31 ft, a wetted perimeter of 15.9 ft, and an average velocity of 1.0 ft/s. These criteria were met at a discharge of 9.2 ft3/s (0.49 ft3/s/mi2). The R2Cross criteria at station 213.57--an average depth of 0.27 ft, a wetted perimeter of 13.9 ft, and an average velocity of 1.0 ft/s--were met at a discharge of 12.0 ft3/s (0.64 ft3/s/mi2). The R2Cross criteria at station 229.45--an average depth of 0.26 ft, a wetted perimeter of 13.3 ft, and an average velocity of 1.0 ft/s--were met at a discharge of 7.4 ft3/s (0.40 ft3/s/mi2). Finally, the R2Cross criteria at station 241.46--an average depth of 0.28 ft, a wetted perimeter of 14.5 ft, and an average velocity of 1.0 ft/s--were met at a discharge of 15.0 ft3/s (0.80 ft3/s/mi2). The limiting R2Cross criteria were mean velocity at stations 202.05 and 229.45 and mean depth at stations 213.57 and 241.46.
A wetted perimeter-discharge relation was determined for the cross sections at stations 202.05, 213.57, 229.45, and 241.46 on the basis of the HEC-RAS model results (table 4). Breaks in the wetted perimeter-discharge relation were used to identify a discharge of about 1.7 ft3/s (0.09 ft3/s/mi2) for station 202.05, 6.0 ft3/s (0.32 ft3/s/mi2) for station 213.57, 1.5 ft3/s (0.08 ft3/s/mi2) for station 229.45, and about 3.4 ft3/s (0.18 ft3/s/mi2) for station 241.46. For these discharges at these four stations, the average water depth ranged from 0.17 to 0.27 ft with an average depth of 0.22 ft.
Mine Brook near Franklin
The riffle section on Mine Brook is about 680 ft upstream of a culvert under Route 140 in the town of Franklin (fig. 6). Seven cross sections were surveyed along an 87-ft length of riffle habitat and all were included in the water-surface-profile model. The predominantly trapezoidal cross sections are in a fairly straight reach of riffle channel. Only a few scattered shrubs and trees line both banks because the reach flows through a forest of large deciduous trees. The bed material is primarily cobbles, and the bank material is a mixture of organic soil and cobbles. The water surface dropped 1.08 to 1.10 ft along the study reach at measured discharges ranging from 2.03 ft3/s to 2.30 ft3/s.
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| Figure 6. Mine Brook near Franklin, Massachusetts, looking downstream. |
The model was run as a subcritical flow regime on the basis of the standard, upstream-step energy method (table 11, at the end of this report). Two discharge measurements of 2.03 ft3/s and 2.30 ft3/s were made for model calibration. The calibration discharges are modeled at normal depth for the most downstream section, and a slope of 0.015691 ft/ft was input as a boundary condition. The slope was calculated from water-surface altitudes measured at the surveyed reference points below the modeled area. Bankfull discharge was estimated by a drainage-area adjustment for the 1.5-year flood for the gage at Nashoba Brook near Acton (01097300), and by comparison with the altitude of observed bankfull indicators surveyed at the study site.
The roughness coefficients for each cross section were varied as required until calculated water-surface altitudes matched measured water-surface altitudes with reasonable accuracy. The calibration accuracy was 0.035 ft over the entire reach for measured discharges.
The R2Cross analysis was applied on the basis of the HEC-RAS model results for the cross sections at river stations 123.60, 164.85, 173.65, and 187.25 (table 3). The R2Cross criteria for determining streamflow requirements at station 123.60--an average depth of 0.20 ft, a wetted perimeter of 9.57 ft, and an average velocity of 1.0 ft/s--were met at a discharge of 6.75 ft3/s (0.67 ft3/s/mi2). The R2Cross criteria at station 164.85--an average depth of 0.20 ft, a wetted perimeter of 8.60 ft, and an average velocity of 1.0 ft/s--were met at a discharge of 8.75 ft3/s (0.87 ft3/s/mi2). The R2Cross criteria at station 173.65--an average depth of 0.20 ft, a wetted perimeter of 8.68 ft, and an average velocity of 1.0 ft/s--were met at a discharge of 2.4 ft3/s (0.24 ft3/s/mi2). Finally, the R2Cross criteria at station 187.25--an average depth of 0.21 ft, a wetted perimeter of 10.71 ft, and an average velocity of 1.0 ft/s--were met at a discharge of 7.25 ft3/s (0.72 ft3/s/mi2). The limiting R2Cross criterion was mean velocity at stations 123.60, 164.85, and 187.25 and mean depth at station 173.65.
A wetted perimeter-discharge relation was determined for cross sections at stations 123.60, 164.85, 173.65, and 187.25 on the basis of the HEC-RAS model results (table 4). Breaks in the wetted perimeter-discharge relation were used to identify a discharge of about 2.4 ft3/s (0.24 ft3/s/mi2) for station 123.60, 4.0 ft3/s (0.40 ft3/s/mi2) for station 164.85, 5.2 ft3/s (0.52 ft3/s/mi2) for station 173.65 (fig. 2), and a discharge of about 1.0 ft3/s (0.10 ft3/s/mi2) for station 187.25. For these discharges at the four cross sections, the average water depth ranged from 0.18 to 0.4 ft deep with an average depth of 0.28 ft.
Charles River at Medway
Two riffle sections were studied along a 1,500 ft reach upstream of the gage at Charles River at Medway (01103200). The first riffle is at the upstream end of the reach and the downstream riffle is about 600 ft upstream of the Walker Street Bridge (fig. 7). Of the 11 cross-sections that were surveyed in this study reach, 9 were included in a single water surface profile model spanning the river length from Walker Street Bridge to the upstream riffle with the cross sections at river stations 641.1 and 691.6 at the downstream riffle and at river stations 1466.3, 1481.8, and1498.9 at the upstream riffle. The riffle cross sections are predominantly trapezoidal in shape. The channel takes several shallow bends along the course of the study reach. The vegetation along the southern banks of the reach includes large hemlocks, with deciduous trees, small shrubs, and lawn along the northern side. The bed material is primarily gravel, the bank material is a mixture of organic silt, sand, and cobbles. The surveyed water surface dropped 2.49 ft along the study reach at 8.2 ft3/s and dropped 2.14 ft at 76.2 ft3/s.
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| Figure 7. Charles River at Medway, Massachusetts, looking downstream. Photo (A) is of the upstream riffle and (B) is of the downstream riffle study reach. |
The model was run as a mixed-flow regime on the basis of the standard-step energy method. Several measurements of water-surface altitude were taken at each cross section for model calibration to observed water surfaces at the Medway gage (table 12, at the end of this report). The discharges determined from the Medway gage-rating table could be used as calibration discharges at each cross section because there are no tributaries in the study reach. The water-surface altitudes at the most downstream section were used as initial boundary conditions for the HEC-RAS model.
Bankfull discharge was estimated by a drainage area adjustment for the 1.5-year flood, determined from the records for the gage at Charles River at Dover (01103500) and at Nashoba Brook near Acton (01097300), as well as by comparison with observed altitudes of bankfull indicators surveyed along the study reach.
The roughness coefficients for each cross section were varied as required until calculated water-surface altitudes matched measured water-surface altitudes with reasonable accuracy. The calibration accuracy was 0.097 ft over the entire reach for both measured discharges.
The R2Cross analysis to determine streamflow requirements for habitat protection was based upon the HEC-RAS model results for the cross sections at river station 641.1 in the downstream study riffle and the cross sections at river stations 1481.8 and 1498.9 in the upstream riffle (table 3). The R2Cross criteria at station 641.1--an average depth of 0.61 ft, a wetted perimeter of 49.0 ft, and an average velocity of 1.0 ft/s--were met at a discharge of 57.7 ft3/s (0.88 ft3/s/mi2). The R2Cross criteria at station 1481.8--an average depth of 0.53 ft, a wetted perimeter of 30.3 ft, and an average velocity of 1.0 ft/s--were met at a discharge of 45.4 ft3/s (0.69 ft3/s/mi2). The R2Cross criteria at station 1498.9--an average depth of 0.58 ft, a wetted perimeter of 35.4 ft, and an average velocity of 1.0 ft/s--were met at a discharge of 52.8 ft3/s (0.80 ft3/s/mi2). The limiting R2Cross criterion was mean depth for all three cross sections.
A wetted perimeter-discharge relation was determined for cross sections at stations 641.1, 1481.8, and 1498.9 on the basis of the HEC-RAS model results (table 4). The break in the wetted perimeter-discharge relation was used to identify a discharge of about 8.7 ft3/s (0.13 ft3/s/mi2) for station 641.1, a discharge of about 1.8 ft3/s (0.03 ft3/s/mi2) for station 1481.8, and a discharge of about 3.4 ft3/s (0.05 ft3/s/mi2) for station 1498.9. At these discharges, the average water depth in all three cross sections was estimated to be 0.19 ft.
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