1984 City Storm Water ManagementA Report
wet
STORM WATER MANAGEMENT
Prepared for
The City of Blair, Nebraska
by
Blair Engineering Company
Blair, Nebraska
May 1984
TABLE OF CONTENTS
Section
Number Description Page Number
1. SCOPE OF THE WORK 1
2. STORM WATER DRAINAGE MAPS 2
3. EXISTING PROBLEM AREAS 3
4. EXISTING POLICIES AND PROCEDURES 9
5. STORM WATER MANAGEMENT MASTER PLAN 10
6. SUMMARY AND RECOMMENDATIONS 14
FIGURE.NO. 1
FIGURE NO. 2
TABLE 1
TABLE 2
1. SCOPE OF THE WORK
The purpose of this study is to provide the information
necessary for the officials of the City of Blair, Nebraska to
develop and implement a Storm Water Management Master Plan. The
workAone to develop the information necessary for the creation
of the Haster Plan was divided into two phases.
Phase No. 1
Phase No. 1 consisted of the gathering of all available
existing data related to the existing storm water drainage
system. Once this information was assembled the Storm Water
Drainage Maps were developed.
Phase No. 2
Phase No. 2 consisted of the determination of existing
problem areas, a review of existing storm water management
policy, and recommendations to the City of Blair for the
implementation of a Storm Water Management Master Plan.
1
2. STORM WATER DRAINAGE MAPS
Phase One entailed the gathering of all available existing data
related to the existing Storm Water Collection System and the
development of the Storm Water Drainage Maps. The information
gathered during this phase and shown on these maps includes a
centerline profile of all streets, the location of all existing
storm sewer structures, culverts, the direction of flow of the
runoff on all streets and at all intersections, the existing
contours as derived from U.S.G.S.
7 1/2 minute quadrangle maps, and determination of the drainage and
major sub -drainage areas within the City of Blair.
With the aid of these maps it will now be possible to
determine the drainage and sub -drainage area, flow direction, and
eventual outlet for runoff originating from any point within the
City of Blair. Consequently, these maps will serve the city as a
guide, when determining and/or evaluating future or existing storm
water drainage requirements.
For these maps to remain useful, they must he updated to
reflect new development within the jurisdiction of the city,
including all new storm water drainage structures and all
modifications to the existing system.
2
3. EXISTING PROBLEM AREAS
Within the limits of the City of Blair, Nebraska there exists
many storm water drainage problem areas. The severity of these
problems ranges from minor to relatively severe. There are,
however, two specific problem areas that have been a concern to
city officials and are as yet unresolved.
Problem Area Number 1
By .far the most serious problem exists in the area shown in
Figure No. 1. This drainage system consists of; 1) a 4' x 4'
concrete culvert under Tenth Street, just north of Washington; 2)
a 3' x 4' concrete culvert at the railroad tracks, just downstream
of the 4' x 4' culvert and; 3) a storm sewer, which consists of
36" C14P and 42" CMP, that passes beneath Highway 30 and then under
the shopping center south of Highway 30.
These structures are individually and collectively causing
problems which need to be corrected.
Of the three structures, the 4' x 4' concrete box is
hydraulically the largest. As a result, the runoff problems
associated with the other two downstream drainage structures, the
3' x 4' concrete box and the storm sewer, may not be readily
apparent.
3
4' x 4' Concrete Box Culvert. At the present time the storm
water runoff problems upstream of Tenth Street are caused by the 4'
x 4' concrete box culvert. The storm water drainage is restricted
at the culvert which causes a backwater problem upstream of this
crossing. At times, this backwater has flooded the basement of
the Dairy Queen which is located just southwest of the culvert.
Currently this culvert has a capacity of 128 cfs. As
a result, this culvert does not have the capacity to pass the
runoff from a storm of even a 10 year reoccurrence interval. The
10 year flow at this culvert is approximately 190 cfs. This is in
spite of the fact that a portion of the runoff in this drainage
area will be controlled by the soon to be completed Jackson Street
Storm Sewer and the existing storm sewer along Sixteenth Street
between Lincoln and Colfax.
3' x 4' Box Culvert. The 3' x 4' box culvert under the
railroad tracks currently appears to be adequate. However, if the
culvert at Tenth Street was enlarged, backwater would be caused by
the restriction of flow at this culvert and could result in the
same problems upstream of Tenth Street that are currently
occurring This culvert has capacity of 96 cfs.
4
Storm Sewer. The storm sewer which is located under Highway
No. 30 and the shopping center just south of Highway 30 also
appears to be adequate, however the hydraulic calculations indicate
that this structure has a capacity of only 60 cfs. This sewer is
constructed of both 36" and 42" Cr1P.
It should be noted that this sewer passes under a building in
the shopping center south of Highway 30. This is a very
undesirable situation.
Solution. Correction of the storm sewer should occur before
the other problems are corrected. To do otherwise would only
continue to create backwater problems upstream of this structure
and could possibly create new problems in the area around Highway
30 and the shopping center.
First, if possible, a ditch should be constructed along the north
side of Highway No. 30. The ditch would convey the runoff from a
point near the inlet of the storm sewer to an outlet at South
Creek. The inlet to the storm sewer on the north side of Highway
30 could then be plugged. The storm sewer could then be used to
drain only the runoff collected by the inlets at 9th Street and
Highway 30.
It is possible that construction of the ditch can be done'in
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conjunction with the reconstruction of Highway 30, which may
result from the relocation of the bridge over the Missouri River.
In lieu of the construction of the ditch another storm sewer
would have to be constructed to help eliminate problems with the
existing storm sewer.
After the problems with the storm sewer are corrected, the
box culvert under the railroad and the box culvert under Tenth
Street should be replaced, respectively.
Problem Area No.
A second problem area exists in the area highlighted in
Figure No. 2. In this area, drainage is collected from the Larsen
Heights subdivision and outlets to the ditch along U.S. Highway 73
via a concrete flume. A part of this runoff then takes a right
turn and flows southeasterly to the inlet of the 30" concrete
culvert under Highway 73. After passing through this culvert the
water is forced to take a sharp left turn and flow
northwesterly in the ditch along the north side of Highway 73
until it can find another path to the drainageway shown in Figure
No. 2.
9
The problem with this drainage system does not appear to
be the hydraulic capacity of either the ditch or other drainage
structures. The problem is primarily a potential erosion and/or
flooding problem caused when the flow is forced to take a sharp
left turn at the outlet of the culvert under Highway 73. This
situation appears to have been created by the construction of
a house and placement of fill north of Highway 73.
Solution. There appears to be several solutions to this
problem including; 1) Construction of a new culvert under Highway
73 near the outlet of the concrete flume; 2) Diversion of the water
from the flume to the northwest in the highway ditch; 3)
Construction of a drainage channel at the outlet of the 30" culvert
under Highway 73 and; 4) do nothing.
It is our opinion that the City of Blair has no responsibility
to correct this problem since it was caused by the construction
done by the landowner. Therefore we believe that option Number 4
is appropriate.
7
However, if the city feels some responsibility to correct the
problem, then we recommend option No. 2. This option will require
the approval of the Nebraska Department of Roads. To gain the
Department of Road's approval it will have to be shown that there
is no adverse effect on any drainage structures located under the
highway by diverting the water at the flume outlet. I£ the
Department of Road's approval cannot he obtained then we
recommend that the city pursue option No. 3.
Other Problem Areas
Table Number 2 provides a location and description of other
storm water related problems that we have identified in the City
of Blair, including the two problems discussed above.
The cost to solve these problems can vary greatly depending
upon the method chosen to solve the problem. The city should
review each of these problem areas, and some type of priority
system should be established that would determine in which order
these and future problems would be corrected.
4. EXISTING POLICIES AND PROCEDURES
Currently the City of Blair has no established policies
governing storm water drainage. Also, there are no adopted design
standards or guidelines to be used in conjunction with the design
and construction of storm water drainage structures. As a result,
The City of Blair has no mechanism to determine the impact of a
project or the runoff in a particular drainage area before the
project is constructed.
9
5. STORM WATER MANAGEMENT MASTER PLAN
To help minimize problems related to storm water drainage
caused by future development, the City of Blair needs to establish_.
and carry out a policy, or master plan, for storm water management.
Hopefully in the future, the enactment of this plan would help to
bring to light and correct potential problems before they become
real problems.
Review Requirements
As part of the Storm Water Management Master Plan, the City of
Blair should require that all future projects or improvements,
whether they be public or privately .financed, be reviewed by the
Planning Commission and the City Engineer. The purpose of this
review would be to determine not only compatibility with planning
goals but also how the existing and future storm water drainage
would be affected by the proposed project. It would also provide a
determination of the adequacy of any storm sewers, ditches or other
structures proposed to control the runoff in the drainage and/or
sub -drainage areas.
10
The master plan should also state what types of information should
be submitted along with the project application before a project
can be reviewed and approval can be granted. The material
submitted should include; 1) plans of the development, including
drainage structures; 2) the computations related to the drainage
structures and; 3) any other material that may be related to the
storm water drainage for the project. This data could then•be used
by the Planning Commission and the City Engineer to,,.evaluate the
affect of the project on storm water drainage in that drainage
area.
As stated previously, both the Planning Commission and the
City Engineer should review all projects. The Planning
Commission's review should include a determination of the
compatibility of the project with the zoning and/or development
planned for the area as contained in the City's Development Plan.
The commission should also estimate the anticipated development
that will occur in the affected drainage area during the life of
the proposed project. This information will be needed to
adequately review the project. Failure to consider estimated
future development in an affected drainage area can result in the
construction of inadequate drainage structures which, at some later
date, may prove costly to the City of Blair.
11
The City of Blair's development plan should serve as a guide
to the Planning Commission and developers when trying to gage the
estimated future development in the drainage area affected by this
proposed project.
The City Engineer, using the input from the Planning
Commission, will analyze the technical aspects of a project. In
particular, the engineer will review the proposed drainage
structures, and determine their adequacy in light of existing and
anticipated future conditions in the affected drainage area.
After their respective reviews, the Planning Commission and/or
the City Engineer can recommend approval or disapproval of the
project. Along with these recommendations should be any comments
or suggestions that they have related to the project.
Design Criteria
The city should adopt a set of design criteria to be used for the
design of storm drainage structures. At a minimum, these criteria
should spell out the design frequency for all drainage structures.
The recommended frequencies are shown in Table No. 2.
12
The city should also give some thought to material
requirements for future storm drainage structures. Corrugated
metal pipe (CMP) or reinforced concretepipe(RCP) have been used
`extensively in most of the existing structures within the City of
Blair. The installed cost of CMP is less than RCP,. however the
expected life. of CMP, on the whole, is less than RCP.
Consequently, we feel that the city should give serious
consideration to RCP for all future storm water drainage.structures
which are located under paved streets, sidewalks, etc. The use of
RCP, even though it is more costly than CMP, is highly desirable
due to its long life expectancy.
13
6. SUMMARY AND RECOMMENDATIONS
ExistinR.Conditions
There are existing storm water drainage problems within the
City of Blair. These problems have varying degrees of severity.
However, all the problems may cause inconvenience and/or economic
loss to at least a few of the residents of Blair. Consequently, it
is desirable to solve each of these problems.
The City of Blair needs to establish a priority for the
correction of each of these problems, and any future problems that
may develop. Then, as funds become available, each of the problems
can be corrected.
Costs. There has been no attempt to derive a cost for the
solution to each of the problems listed in Table No. 1. Each
problem is unique. Consequently, each problem should be examined
by city officials to determine the most desirable cost effective
solution available.
14
FUTURE POLICY
Review Requirements. In the future it should be the policy of
the City of Blair that all projects be reviewed by the Planning
Commission and the City Engineer. The review should determine and
evaluate the possible effects of a proposed project on existing and
anticipated future runoff in the effected drainage and sub -drainage
areas.
Technical Requirements. Requirements outlining the design
frequency and materials to be used for storm water drainage
structures should be adopted by the City of Blair.
Funding. There are several methods to provide funds for storm
water drainage projects undertaken by the City of Blair, including;
1) establishment of storm water drainage districts; 2) general
obligation bonds and; 3) increase the tax levy by a small
percentage with these moneys being set aside in a fund which can be
used for storm water drainage projects.
As a result of discussion with various public officials, it
appears that the sale of general obligation bonds is the most
favorable method to provide .funds for storm water drainage
improvements.
15
SUMMARY
Currently there exists many storm water drainage problems in
the City of Blair. These problems have largely varying degrees of
severity. To correct these problems, some type of priority system
must be established which should dictate the order in which these
problems are addressed.
In the future, all Projects, including publicly and privately
financed projects, should be reviewed by the city to determine
their impact upon the storm water drainage system.
These projects should be designed in accordance with criteria
established by the city.
Funding for public projects should be financed, except in
special circumstances, by the sale of General Obligation Bonds.
16
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MINIMUM DESIGN FREQUENCY
REOCCURRENCE INTERVAL TYPE OF STRUCTURE
10 yearl Storm Sewers, Inlets,
25 year2 Structures draining depressed
areas, surface flow, ditches
100 year3 Any structure in a major
drainageway
Note: 1. These criteria may be modified to meet
desired conditions in certain areas.
2. The 25 year frequency should serve as a
minimum frequency for drainage structures in
this category. Frequencies as high as 100
year may be necessary to protect life and
property.
3. Culverts shall be designed to pass the
50 -year runoff with a two foot freeboard and
no flow over the roadway. The drainage
system shall accommodate a 100 -year frequency
flood including provision for limited
overflows at bridges and culverts without
loss of life or major property damage.
STORM WATER DRAINAGE
MINIMUM DESIGN FREQUENCY
TABLE 2