HomeMy WebLinkAboutDrainage and Detention Comments"
2551 Texas Ave. South, Ste. A, College Station, TX 77840
Ofc: 979.693 .5359 Fax: 979.693.4243 Email: mdgcstx@yahoo.com
22 May 2006
Mr. Josh Norton, E.I.T.
City of College Station Development Services
College Station, TX 77840
RE: River Ridge Drainage/Detention Comments
Dear Mr. Norton,
Municipal Development Group (MDG) has reviewed your comments and have made the
changes and clarifications on the plan submitted with this letter. An overview is as
follows :
1. Concerning the comment about ultimately passing the 10-year flow of 17.6 cfs under
the sidewalk, the opening under the sidewalk has been expanded to a width of 72 in2 or a
total area of 3 ft2. However, the assumption that the maximum flow under the sidewalk is
17.6 cfs is not entirely accurate. Rather, the outlet structure from this detention basin
does provide some detention itself to slow down the water exiting the detention basin.
For example, when the water first enters the stilling basin, the depth of water can be
assumed to be the normal depth of flow in an open channel, and from Manning's
equation, the depth is calculated to be 0.935 ft. Since the opening under the spillway is
only 0.5 ft, water will back up behind the spillway and discharge only beneath it at first.
This discharge can be modeled as orifice flow which initially has a value of 10.8 cfs (Q =
AC(2gH)"0.5). Now, the water will pass through the baffle blocks, and after the
hydraulic jump, regain a normal depth of 0.85 ft according to Manning's equation. Once
it reaches the splashwall, since the depth beneath the splashwall is only 0.5 feet, the water
will again back up behind the splashwall and discharge only underneath it. This flow can
be modeled as an orifice as well, with an initial value of 7.12 cfs . This flow of 7.12 cfs is
the flow actually passing under the sidewalk, and with an area of 3 ft2, results in a
velocity of 2.38 ft/s. The figure below depicts the peak flow as it passes through the
stilling basin.
17.6 cfs ...
Ogee
--.-""pill way
10.8 cfs
plash wall
Sidewalk
Gutter
-----11JJ>• Q=7. l2 cfs, V = 2.38 ft/s
... . •
Thus, this design slows down the peak flow down because it provides for small amounts
of detention as well as energy dissipations caused by hydraulic jumps. The previous
analysis has considered only a snapshot in time of the outlet structure. It is admitted that
as the water backs up behind the wall, the depth of water will increase and the flow
beneath the wall will increase. However, it is also known that the peak flow of 17.6 cfs is
assumed to only occur for 1 minute and then will decrease, so the water will not back up
behind the wall for very long.
2. The sidewalk supports have been placed at a 45° angle to facilitate moving the water
efficiently down Spring Loop in the gutter and not in the street.
3. Concerning the requested model of water moving under the sidewalk and down the
gutter into the storm sewer inlet, it can easily be calculated that the final spread of water
in the gutter will be 10.5 ft. In other words, as the water drains under the sidewalk and
into the street, it will naturally turn left and move down the hill towards the storm sewer
inlet. As it moves down the hill, it will spread out reaching a maximum width of 10.5
feet. This can be calculated from the equations given in the City of College Station
Drainage Policy and Design Standards on page 30:
Q = co.56{ ~}!scn <&/J)
T = (Z)(Y)
Spring loop has a cross slope of 2%, which gives a Z value of 50. Maiming's n was
estimated at 0.012. Using the City of College Station online GIS maps, the bed slope, S,
was estimated from the elevation at the point of discharge and the elevation of the storm
inlet, which are 302 feet and 276 feet, respectively. This elevation drop occurs over a
distance of roughly 670 feet, resulting in a bed slope of 0.038 ft/ft. For 7.12 cfs, the
depth of flow was found to be 0.21 ft, which corresponds to a spread of 10.5 ft. For a 46
ft wide road, this leaves much more than a 12 ft clearance at the center of the roadway for
passage. Even if the full 17.6 cfs were to move out into the street, the above equation
yields a spread of 16 ft, which also leaves a 12 ft clearance at the center of the road.
Thank you for your input on the design of this detention basin. The revised design
should prove to be better suited to all involved parties. If you have any questions, please
call.
Sincerely,
MUNICIPAL DEVELOPMENT GROUP
Scott ubley, E.I.T.
Graduate Engineer