HomeMy WebLinkAboutA002 - Council Action Form dated October 26, 1993 4 '
COUNCIL ACTION FORM
33a��
Item # _
Date: October 26. 1993
SUBJECT: Approval of the Final Plat of 10th Addition and drainage plan for
Northridge Parkway Subdivision.
BACKGROUND:
The owners and Developers of the Northridge Parkway Subdivision have completed
the work necessary to meet the development requirements of the City for the 10th
Addition of the subdivision. This final plat was initially denied waiting for the
development of a stormwater management plan for the entire subdivision because it
could have had an effect on the plat documents. Although the stormwater plan has
not been implemented, the Developers have committed to constructing three
stormwater detention basins on property within the 9th, 10th, and the future 11th
Addition. These basins are critical to the stormwater program "downstream" within the
subdivision. Please note the stormwater drainage staff report attached.
The attached drainage report outlines the problem and potential solutions. Staff
presented the information to a neighborhood group on October 20th, 1993. Concerns
were expressed that the potential stormwater damage to homes and the problems with
access because of the stormwater build up in the low point of Northridge Parkway
were critical. Although other alternatives will address the problem, the two most viable
alternatives are to grade the open area (Lot A and Lot B) between Northridge Parkway
and Moore Park to pass the major storm runoff. This project would cost approximately
$20,000 with a cost share of roughly $13,500 for the City and $6,500 for the
Developers. Advantages and disadvantages of the concept are discussed in the
report attached.
The second alternative is to develop a detention basin to control the major storm
runoff in the large open space area of Lot A, as well as shaping of the north half of
the downstream area and the area south of Ridgetop Circle to provide for overland
flow. This alternative is also further discussed in the attached report.
Residents adjacent to the area where the detention basin is proposed were concerned
about the aesthetics of the area and the perceived loss of "usable" open space. Staff
believes that these concerns can be addressed within the needs of both the area
residents and the program of a detention basin. By constructing a vertical wall of
attractive material, the basin would be an average of 1'/2 feet deep with a maximum of
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CAF - Northridge
October 26, 1993
Page 2
two feet deep. The area would be generally flat except that it would be sloped to
maintain drainage. The slope would be one foot of vertical change in 200 feet.
The estimated cost of the detention basin project is $50,000. The cost sharing
proposal calls for the City to pay the Developers $24,990 to accomplish the grading,
retaining wall, and detention basin work with the City accomplishing the reconstruction
of the walkway at an additional estimated cost of $15,010. The funding source for the
City's share would be a portion of the FEMA reimbursement for flood clean up
activities that were already budgeted.
ALTERNATIVES:
1. Approve the plat of the 10th Addition and the concept of controlling the major
storm events with detention basins. The plan is subject to receipt of easements
from the Northridge Homeowners Association.
2. Approve the plat of the 10th Addition and the concept of passing the major
storm events by grading Lot A and Lot B. The plan is subject to receipt of
easements from the Northridge Homeowners Association.
3. Deny the plat of the 10th Addition and direct staff to work with the Developers
and residents on a new stormwater plan.
MANAGER'S RECOMMENDED ACTION:
It is recommended by the City Manager that the City Council adopt alternative number
one and take the following action:
a. Approve the final plat of the 10th Addition of Northridge Parkway
Subdivision.
b. Approve the concept of controlling the major storm event with detention
basins and direct staff to prepare a design that meets the needs for
stormwater control and addresses the neighborhood concern for
aesthetics.
COUNCIL ACTION:
NORTHRIDGE DRAINAGE
Over the past two wet years, Northridge Parkway has experienced flooding events when
the rainfall and subsequent runoff has exceeded the 5-year design flow capacity of the
storm sewer system. The most significant problem is located on Northridge Parkway east
of Valley View Drive. During rainfall/runoff events that exceed the 5-year design capacity
of the underground storm sewers, the stormwater is channeled to the street areas and
then flows to the low point in the street on Northridge Parkway. The stormwater builds
up in the street because the flow arriving at the low point is coming faster than the pipe
can carry the flow away. Complicating the situation is that area grading does not allow
the stormwater to get away until it builds to a depth greater than three feet. At this depth,
existing homes are exposed to damage and have actually experienced damage.
General management techniques available to address the situation relate to handling the
major storm event and it's associated runoff by two methods. The first relates to slowing
the runoff where possible through a series of basins called detention basins. A detention
basin is an area that is designed to collect stormwater runoff and release it at a rate that
any downstream system can handle. As the arrival rate exceeds the discharge rate, the
stormwater is stored for short periods to time. The second method of control is to
construct "conduits" to carry the water away and discharge it to a system large enough
to carry it. Conduits can be either channels or underground pipes. The difference
between channels and pipes is that a pipe system has a finite capacity, whereas a
channel is usually capable of carrying differing storm runoff amounts.
In order to evaluate the problem of water accumulating at the low point in Northridge
Parkway, different management techniques were reviewed. The usual design frequency
that is used for major storm events is a 100-year frequency runoff. In the case of
Northridge Parkway Subdivision, this calculates to a runoff amount of 114 cubic feet per
second (cfs) reaching the low point if no controls were implemented.
The pipe size needed to carry 114 cfs away from the low point would be a 54" pipe. The
most direct route to carry the stormwater would be directly south along the bikepath and
the gas main. The cost of the alternative would be $85,000 to lay the pipe and probably
an additional $15,000 for construction of an entrance structure. Easements would be
required to construct the pipe across Lots 5, 6, 7, 8, 9, and 10 of the 8th Addition.
Advantages of this proposal are that this buildup of stormwater in the low point is handled
for events up to and including the 100-year runoff event and that following restoration of
the area, the system will be out of sight except for the entrance structure. Disadvantages
include the fact that the pipe has a definite capacity and that any runoff event higher than
the capacity will again create ponding with no positive outlet. The aesthetics of the pipe
entrance structure would also be of a concern at the main entrance to the development.
It should also be noted that pipe systems are inherently susceptible to plugging from
debris. This could cause backup and ponding as well.
The other method of discharge is to grade a channel so that the stormwater that is
collected in the low point is not allowed to build up. This is possible by shaping the
existing open space areas (Lot A and Lot B) of the subdivision. The fact that the water
will be discharged overland means that there is potential for private property damage
along the channel due to quantity of water and the speed of the flow. The area would
generally be graded to elevation 109 so that the water would not reach more than 1%
feet deep in the low point of Northridge Parkway. Channel grading south of Ridgetop
Circle (Lot B) must be done so that the stormwater buildup problem is not just moved
from Northridge Parkway to Ridgetop Circle. The estimated cost of this grading is
$20,000. The advantage of this alternative is that protection is provided the properties
on the north side of Northridge Parkway and it will serve any level of runoff event. The
disadvantage is that a large amount of stormwater (114 cfs) will be channeled through
the housing area.
The above methods of addressing the problem only allowed for directing the stormwater
runoff problem, not controlling it. As discussed earlier, the use of detention basins can
slow the rate of runoff thus making it easier to deal with. Detention basins can be
developed within the 9th, 10th, and 11 th Additions in areas designated for open space
uses. These detention basins will be shaped into the surrounding areas with 5:1 slopes
(5 feet horizontal distance to 1 foot vertical change) or 10:1 slopes. The maximum depth
that the water will pond in these basins is three feet. The water is stored until the existing
pipe system will be able to carry it away. The stormwater is generally not expected to
be ponded more than % hour in these basins before the pipe network will carry it away.
If all three of these basins are constructed in the 9th, 10th, and 11 th Additions, the
overland flow during the 100-year runoff event that could likely be collected in the low
point of Northridge Parkway is reduced to 46 cfs. From the low point, an area could be
graded along Lot 1, 1st Addition to another detention basin constructed within Lot A
behind Lots 1, 2, and 3 of the 1st Addition and Lot 5 of the 8th Addition. Detention
basins are effective because of their ability to store water. Thus the volume of the basin
is the critical item. The shape of the basin is not critical. To be effective, the detention
basin in Lot A must contain a volume of 33,100 cubic feet below elevation 109. The
original proposal was to grade the basin using 5:1 slopes. In addition, shaping would be
needed to develop a channel for any overtopping that might occur when rainfall or runoff
was greater than the basin. This would necessitate removal and replacement of the
walkway in Lot A and Lot B.
- 2 -
The overall cost of this alternative is $50,000 which includes the grading of the basin,
landscaping removal and replacement, removal and replacement of the walkway after
grading and associated pipe work. The advantages of the system of detention basins
include the fact that the major storm runoff is controlled and not just directed. Also
storms in excess of the 100-year runoff are handled through the same system and thus
additional "insurance" is obtained. The system is such that damage will not occur if pipes
are blocked. Disadvantages include that the shape of the basins may be of a concern
to area residents.
The recommended solution was to develop the detention basin system as a means of
controlling the major runoff volume, not just to pass it. The major concern expressed by
area residents was the aesthetics and perceived loss of use of the basin in Lot A behind
Lots 1, 2, and 3 of the 1st Addition and Lot 5 of the 8th Addition. In consideration of
those concerns, an alternative shape was developed. The alternative would be to create
a "flat" area for open space use by the use of an attractive wall that would be two feet
high at its highest point. Steps could be provided for the adjacent properties. The
"terrace" area would be available for recreational use at any time other than during major
storm events. The attractive wall would be constructed in a horseshoe type shape from
the bikepath area on the east side of Lot A along the lot lines of Lots 1, 2, and 3 (1 st
Addition) and Lots 4 and 5 (8th Addition) with the walkway area being the west end of the
"terrace" basin. The basin would average a depth of 1'/z feet and be two feet deep at the
deepest point at the outlet near the walkway.
Drawings of the subdivision area, the detention basin layout for the 10th and 11th
Additions, and the general layout of the detention system as proposed are attached.
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