'tormwater Management
'aster Plan For
]avis County, Utah
050
A Case History in 208 Water Quality Management Planning
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EPA-440/3-77-023
May 1978
STORMWATER MANAGEMENT
MASTER PLAN FOR DAVIS COUNTY, UTAH
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U.S. ENVIRONMENTAL PROTECTION AGENCY
* Environmental Research Information Center* Technology Transfer
* Office of Water Planning and Standards Water Planning Division (WH 554)
401 "M" St. S.W., Washington, D.C. 20460
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ACKNOWLEDGEMENTS
This publication contains information prepared for the U.S. Envi-
ronmental Protection Agency, Environmental Research Information
Center, Office of Research and Development; and Water Planning
Division, Office of Water Planning and Standards.
The information in this publication was prepared by Wiggins-Rimer
& Associates, Durham, North Carolina. The case history was au-
thored by Mr. Roger N. Schecter, AIP, under the general direction of
Mr. Alan E. Rimer, P.E. Also providing assistance were Dr. Michael
Miner, 208 Project Director of the Weber River Council and Mr. H.
Eugene Nielsen, P.E., of Nielsen, Maxwell, and Wangsgard.
NOTICE
This publication has been reviewed by the Environmental Research Information Center and the
Water Planning Division, Office of Water Planning and Standards, U.S. Environmental Protection
Agency, and is approved for publication. Approval does not signify that the contents necessarily
reflect the views and policies of the U.S. Environmental Protection Agency, nor does any mention
of trade names or commercial products constitute endorsement or recommendation for use.
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STORMWATER MANAGEMENT MASTER PLAN
FOR DAVIS COUNTY, UTAH
The areawide water quality planning pro-
gram for the tri-county planning area of Weber
River Water Quality Planning Council
(WRWQPC) was initiated in June, 1975. A
major effort of the WRWQPC was directed
toward documenting stormwater runoff prob-
lems, assessing the capacity of stormwater
facilities, identifying water quality related
problems and developing a master plan to
correct these problems. This urban storm-
water runoff study focused on the highly de-
veloped Davis and Weber Counties. Mor-
gan County is and will continue to be a
rural area, relatively unaffected by urban
runoff. Growth in Davis and Weber Coun-
ties has increased significantly over the
last decade. Davis County, located imme-
diately north of Salt Lake City, is one of
the fastest growing areas of the State and
population is expected to increase from
a 1975 level of 115,050 to 227,800 by
1995.
The already rapid growth of the planning
area, coupled witii increased development,
has resulted in significant problems with
stormwater runoff. WRWQPC's storm-
water master plans address the alleviation of
flooding and the design modification of con-
ventional detention basins to control water
quality impacts of urban stormwater runoff.
These plans have now been adopted by Davis
and Weber Counties.
Davis County is currently implementing
its program through a recently adopted
County-wide ordinance. All sixteen munici-
palities in Davis County have, by inter-
local agreement, set maximum runoff limits
and other requirements for new development
and established Davis County as the storm-
water management agency. Weber County is
expected to implement a similar if not an
identical ordinance in the near future. This
study will, however, only focus on Davis
County's experience.
BACKGROUND
The tri-county area of the Weber River
Water Quality Planning Council (WRWQPC)
is shown in Figure 1. Bordered on the east
by the Wasatch Range which slopes west-
ward to the Great Salt Lake, the area has ex-
perienced flooding problems as the result of
the terrain and dense urban development.
The objectives in initiating the study of ur-
ban stormwater problems were to determine
the most cost-effective urban runoff flood
control methods and to provide alternatives
which would also address water quality.
The flooding problem in Davis and Weber
Counties has become significant over the
last decade as development, with the resultant
increase in impervious surfaces, has occurred.
Prior to development, the natural terrain and
vegetation impeded runoff. Natural channels
were able to accommodate the stormwater
flow but now with increased development
in addition to alteration or elimination of
some of the natural channels, the increased
quantity and velocity of storm runoff has
caused major flooding problems.
Urban development is concentrated along
the foothills and the lower, gently sloping
areas between the Wasatch Mountains and the
Great Salt Lake. Development on the foot-
hills is on steeply sloped areas and, conse-
quently, streets running perpendicular to
the foothills carry a large volume of storm-
water toward the lower slopes. The drainage
channels in the intermediate slope area
(which contains the majority of development
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and streets) have a much smaller carrying
capacity than the steep slopes, thereby mak-
ing it difficult to provide sufficient drainage
onto the extremely flat area along the shore
of the Great Salt Lake.
PROJECTING STORMWATER FLOWS
In developing the stormwater master plan
it was first necessary to characterize the vol-
ume of runoff which needed to be controlled
to prevent flooding. Significant runoff in
the Weber River comes from two sources,
snowmelt and cloudbursts. The runoff
produced from melting snows in the moun-
tains lasts for several weeks in the late spring.
Because of the large volume of snowmelt, it
was not considered feasible to control this
flow by structural methods. Cloudbursts
generally occur during the summer and early
fall and produce frequent, high intensity,
short duration runoff events. Peak flow is
considerably higher than snowmelt flow and
cannot be carried in the existing channels.
A study of typical drainage areas in Davis
and Weber Counties indicated that the
average instantaneous runoff from natural un-
developed surfaces is approximately 0.20
cubic feet per second (cfs) per acre (0.014
cubic meters per second per hectare -
m3/sec/ha}. Runoff from highly developed
areas was found to be several times larger.
For each of 180 drainage areas in Davis
County, the peak runoff flow was calcu-
lated based upon drainage area character-
istics. The maximum rainfall intensity fig-
ure was derived from a storm event which
would occur once every 10 years. Once
peak flow was calculated, it was compared to
the average runoff value of 0.20 cfs/ac (0.014
m3/sec/ha) for the drainage area without
development. This comparison indicated
the severity of the excess stormwater
runoff. An example of the calculations
of estimated runoff flows for selected
drainages is shown in Table 1. The last
column indicates the excess runoff in
these areas.
ALTERNATIVE CONTROL METHODS
In assessing alternative methods to control
excessive flows and flooding caused by urban
stormwater runoff, six basic control alterna-
tives were considered and evaluated. These
controls, as depicted in Figure 2, were develop-
ed for their relevance to Davis County and
included many major considerations as de-
tailed in WRWQPC's basic planning document,
"An Inventory of Nonpoint Pollution Control
Strategies."
The alternative of providing no storm drain-
age facilities for urban areas was determined
to be unacceptable because of the potential
for property damage and loss of life. The
construction of storm drains for peak flow
was also considered unacceptable because of
the high cost of installing the large diameter
pipe needed to accommodate the high peak
flow from cloudbursts.
The use of centralized, on-site detention
basins was assessed as a favorable, cost-
effective method for controlling urban storm-
water runoff. With the use of detention, run-
off flows would be significantly reduced.
Even short term detention allows greatly re-
duced conveyance facilities which in turn
greatly reduce costs, often to a point where
local governments are able to finance these
projects.
Detention basins for storing the entire vol-
ume of water from a particular storm were
assessed as impractical in urbanized areas be-
cause of the large storage area required. Ad-
ditionally, the retention basin would general-
ly be limited to single purpose use.
A canal-like drain was considered as an-
other practical means of accommodating ex-
cessive surface runoff as well as providing de-
tention and subsurface drainage for areas
with a high water table. The existing Davis-
Weber Canal could be modified to serve this
purpose.
UTILITY OF DETENTION BASINS
The use of both centralized and on-site
detention basins was determined to be advanta-
geous for several reasons. First, storm runoff
flows could be reduced to about 10 percent
of their peak values which means a significant
reduction in the size of conveyance facilities.
Second, because the flow would be detained
(rather than retained for long periods), de-
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No storm drainage
facilities
Storm drains for
peak flow
Problem not solved
Cost intensive
Centralized
detention basins
Applicable, and permits
multiple uses
On-site
detention basins
Applicable for small
developing tracts
Retention basins
Canal
detention drains
Land intensive for
single purpose
Applicable in select
areas of county
Figure 2. Alternative stormwater control methods.
tention basins could serve as multiple use fac-
ilities such as parks and open space when they
were not being utilized for detention. The
cost of cleaning a detention basin to allow
for continued park use was determined to be
far less than the cost of providing a storm-
water collection system for the peak runoff.
In addition to concluding that large, cen-
tralized detention basins and smaller, on-site
detention basins for individual developments
were the most effective means of controlling
existing and future storm runoff problems,
the WRWQPC and its consulting engineers
recommended a maximum runoff limit.
This criterion was established as 0.20 cfs
per acre (0.014 m3/s/ha) and represents runoff
from a naturally vegetated, gently sloping
area that has no development. Because de-
,velopment increases runoff, it was determined
that the developer should provide on-site
runoff detention, limiting runoff to the
pre-development flow. The advantages of
using a standard maximum runoff rate of
0.20 cfs per acre (0.014 m3/s/ha) were:
Runoff from areas to be developed can
be calculated prior to actual development,
thereby facilitating minimum design and
installation of culverts, bridges, and other
drainage facilities in and around the pro-
posed development, i.e., assuming snow-
melt is projected as less than this peak
flow.
Design capacity for any drainage control
can be easily approximated by multi-
plying the drainage area by 0.20 cfs (0.0056
nvVs).
Storm drainage construction costs will be
reduced in many cases with less peak flow
to consider in design analysis.
Controversy between municipalities would
be avoided because both would be opera-
ting under the same uniform design cri-
terion.
INCORPORATING WATER QUALITY
CONSIDERATIONS IN STORMWATER
MANAGEMENT
The development of potential control meth-
ods was predicated on an assessment of vari-
ables which affect the general quantity and
quality of urban stormwater runoff in the
Davis County area. Although no Utah stan-
dards or criteria for water quality problems
caused by stormwater exist, three considera-
tions for analyzing urban wasteloads were
studied.
Average yearly storm pollutant load
Average pollutant load,within an event
Variation of pollutant load within an event
A determination of the average yearly
storm pollutant load is based on the assump-
tion of consistent storm flows and pollutant
loads. Such predictions are useful in an area
where storm events occur frequently and are
of long duration, but this is not the case in
Davis County. The second consideration indi-
cates the variability of average pollutant loads
from event to event, while the third is indica-
tive of the variability of pollutant loads in a
single event.
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Because rainfall in the area is characterized
by high-in tensity, short-duration storm events
during the summer season, pollutant loads
were believed most significant within a single
event. This set of conditions caused WRWQPC
to focus on the loading encountered during
the initial or "first-flush" washout of pollu-
tion during each storm.
The typical storm event selected for
pollutant load analysis had an intensity of
20 percent of the once-in-10-year storm event.
This storm normally occurs a maximum of
five times during the summer season. Since
these storms also occur during the summer
at the time of lowest stream flow, it was
concluded that summertime first-flush con-
trols would be the most effective in reducing
pollutant loads.
In developing alternatives to control or re-
duce the pollutant loadings of summertime
first-flush, three primary control techniques
were investigated: source controls, collec-
tion systems controls, and storage and treat-
ment. These control methods were eval-
uated in terms of the efficiency of removal,
relative costs, and applicability to the study
area. Table 2 summarizes the conclusions
regarding efficiency of each control measure
in reducing those pollutants studied. It is
noted that physical treatment processes are
the most effective, but as one might expect,
they are also the most expensive. Physical
treatment requires collection; however, no
stormwater collection system or combined
sewers are in use in Davis or Weber County.
Therefore, the WRWQPC concluded that the
prohibitive cost of collection alone made
source controls the more cost-effective
solution. As discussed earlier, detention ba-
sins were assessed as the most desirable meth-
od of source control.
In addition to looking at detention basins
as source controls to reduce the quantity and
improve the quality of urban runoff, other
structural and nonstructural source control
methods were reviewed. Structural alterna-
tives included the following:
POROUS PAVEMENT. The use of porous pave-
ment enhances groundwater recharge, elimin-
ates the need and associated costs for curb
and gutter, and reduces runoff velocity from
pavement surfaces.
PARKING LOT PONDING. Temporary ponding of
water on parking lots provides a slow runoff
rate and because of the low use rate of park-
ing lots in the area for most of the year, the
public would not be greatly inconvenienced.
INFILTRATION SYSTEMS. Such systems con-
sist of excavated holes in the ground filled
with coarse stone or concrete block with
perforations. Infiltration.systems can be used
in areas where limited detention capacity is
required.
DIVERSION STRUCTURES. These structures
have been used to a large extent along major
highways as a means of diverting runoff. They
are useful in some areas but should not be
used on sites where the potential exists for
slumping or landslides.
ROOFTOP PONDING. Stormwater may be
stored on rooftops equipped with detention
drains. In many cases, buildings in the area
are already designed to support this addi-
tional load.
Non-structural control methods which
could reduce pollutant loads from storm-
water discharges were also assessed and in-
cluded the following:
OPEN STORAGE REGULATIONS. Restrictions
on open storage of pesticides, oil and lubri-
cants, paper, solid wastes, etc. for industrial
and commercial enterprises not affected
by existing Federal regulation of oil and
hazardous materials.
ANTI-LITTER LAWS. Provisions for sidewalk
containers and clean-up requirements for con-
centrations of litter at refuse dumping sites,
vacant lots, parking areas, drive-in restau-
rants, etc.
AIR POLLUTION ABATEMENT. Active
enforcement of air pollution regulations
to reduce paniculate fallout.
CATCH BASIN CLEANING. Regular mainten-
ance and cleaning of catch basins.
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IMPROVED DE-ICING METHODS. Operator's
training and efficient equipment to provide
optimum spreading of appropriate sandsalt
mixtures. (De-icing salts which contain
cyanide or chromium compounds as anti-
caking or corrosion inhibitors should be pro-
hibited.)
PUBLIC EDUCATION. Educational programs
to inform citizens of good housekeeping mea-
sures and encourage sponsorship of clean-up
campaigns.
STREET CLEANING. Street cleaning pro-
grams with vacuum sweepers to reduce the
pollutant load in urban runoff (provided
streets are cleaned often enough to limit
first-flush effects).
LAND MANAGEMENT. New development con-
trols, such as erosion and sediment ordinances,
zoning, building codes, and related regula-
tory measures.
DETENTION BASIN DESIGN AND WATER
QUALITY CONSIDERATIONS
The positive aspects of detention basins
for flood control have been outlined pre-
viously. Emphasis in the past has been
oriented toward improving water quality
strictly by flow attenuation and no con-
sideration has been given to the design of
basins as "treatment" facilities.
Detention basins by nature of their de-
sign will afford some pollutant removal due
to settling. The WRWQPC and their con-
sultants felt that if this settling were to
occur in a controlled portion of the basin,
the maintenance problem could be greatly
reduced and a more predictable improve-
ment in water quality could be realized.
Difficulty has been experienced in accu-
rately predicting the pollutant removal
capability of detention basins as they are
presently designed. Many variables are in-
volved including basin volume, depth, shape,
flow patterns in the basin, and inlet-outlet
configuration. In addition, the pollutant
load introduced into the basin is highly
variable. Of prime concern is the first-
flush phenomenon. It was known that in
many urban drainage situations, a large
fraction of the total pollutant loading is
encountered during the early stages of run-
off. This occurs because a disproportionately
high fraction of the runoff load, which con-
sists of materials accumulated on land sur-
faces and in storm drains between rainfall
events, is carried in the first portion of the
runoff., As the storm continues, the rela-
tively high pollutant concentration which
existed during the early runoff stages dimin-
ishes throughout the duration of the storm.
Factors which affect the magnitude of the
first flush are land use and size of the
drainage basin, length and intensity of the
storm, and overall, the time intervals be-
tween the flushing action of each sub-basin.
The settleable solids are the only portion
of the total solids in the runoff which can
be removed to any extent by detention
basins. These solids are normally 10 or
more microns in diameter. Non-settling
solids are normally 1 to 10 microns in
diameter. The settling velocity for settle-
able particles, at 10°C, ranges from 1000
to 63 millimeters per second for gravel
and sands to less than 0.025 millimeters
per second for fine silt. Since the majority
of the pollutant load is associated with
sands and silt, detention basin design must
remove as many of these particles as
possible.
A standard detention basin design is
shown in Figure 3. A typical settling pat-
tern for this type of basin is shown in
Figure 4. As can be seen, none of the flow
of solids goes into the basin during the
first hour. This first flush, which contains
the peak pollutant concentration, goes past
the basin without entering the detention
basin.
A modified detention basin design is
shown in Figure 5. This design provides a
chamber through which all runoff would
flow regardless of the size of the storm.
The chamber incorporates a smaller exit
pipe or weir to permit buildup of settle-
able solids in the chamber and includes a
skimmer to collect "floatables" that would
be retained on the water surface. The cham-
ber would be cleaned periodically in much
the same way as a catch basin. This modi-
fication could, with a combined cleanout
" grate and hydraulic overload safety valve
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Max. water surface
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Section
Settling structure
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Figure 5, Modified detention basin design.
(not shown), simplify the maintenance in-
herent with detention basins and would
also enhance the water quality. Flow atten-
uation would be realized while at the same
time a portion of the pollutant load would
be removed. A detention basin may reduce
suspended solids and biochemical oxygen
demand by 50 to 75 percent and nutrients
and coliform bacteria up to 25 percent.
URBAN STORMWATER MASTER PLAN
The groundwork for acceptance of the
Stormwater Master Plan by the tri-county
area was established early in the WRWQPC
water quality planning efforts. In Davis
County, public and governmental meeting^
with the County and with the sixteen munic-
ipal units within the County greatly facili-
tated subsequent implementation of the plan.
As data was assembled for projected storm-
water flows, pollutant loads, control meth-
ods, and design of necessary drainage pro-
jects, this information was shared with the
planning and regulatory agencies in county
and municipal governments. By maintaining
ongoing communication among the affected
parties, the WRWQPC's recommended plan
for stormwater management was well under-
stood and the need for control was well
established.
To provide a uniform and coordinated
county-wide approach to implement the
Stormwater Management Plan, two basic
management agency approaches for Davis
County were studied.
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County government as the management
agency. Create a flood control fund with
the use of a county-wide property tax,
having the County manage major flood
control projects with City cooperation.
Special improvement district for flood
control. Create a special improvement
district to levy a property tax and pro-
vide funding to handle major flood
control projects.
The special improvement district has the
organizational advantage of providing a
separate governmental entity devoted ex-
clusively to flood control. The creation of
a new and separate agency would, however,
be a difficult approach politically. Also,
such an agency would not have authority
for land use planning and development
regulations or the broad powers of county
or municipal government to further alleviate
runoff problems through planning mechanisms.
An advantage of a county management
agency is that Davis County has under Utah
law the authority over stream channels for
both incorporated and unincorporated
municipalities. It may levy all necessary
flood control taxes on property in the
county. It also has authority, with voter
approval, to issue bonds for capital improve-
ments. In addition, the county government
may use planning and zoning powers to
pass ordinances regulating development,
thereby decreasing the urban runoff prob-
lem, especially in unincorporated areas, and
can develop common ordinances which the
municipalities may adopt on a cooperative
basis. Because of the many advantages and
its political acceptability, Davis County
was selected as the management agency
for the implementation of the Stormwater
Management Master Plan.
The Davis County Commissioners have
willingly accepted the responsibility as the
management agency. A committee has been
established to set drainage project priorities
for the County and the stormwater runoff
control ordinance for new development has
been adopted. A $500,000 fund has been
established to carry out the first year's opera-
tion and to implement high priority drainage
projects.
The stormwater runoff ordinance adopted
by the County sets the maximum runoff
standard at 0.20 cfs per acre (0.014 m3/s/ha)
and requires developers to construct the
necessary detention facilities to store excess
runoff. In addition, the ordinance enables
the County, as the management agency, to
assess a fee on the developer for construc-
tion of any additional County stormwater
facilities which would be needed due to the
development. The fee would be seven per-
cent of the fair market value of the unim-
proved real estate. This was a major step
in improving water quality in Davis County.
The WRWQPC plan for stormwater run-
off management in Davis County has been
adopted. The Davis County program, com-
plete with a stormwater ordinance and man-
agement structure, is now operational. With
the acceptance of a similar stormwater ordi-
nance and management arrangement by
Weber County, both of these rapidly grow-
ing areas will improve drainage and water
quality through the comprehensive areawide
planning process.
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* U.S. GOVERNMENT PRINTING OFFICE: 1978 757-140/6835
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U.S. ENVIRONMENTAL PROTECTION AGENCY
* ENVIRONMENTAL RESEARCH INFORMATION CENTER TECHNOLOGY TRANSFER
* OFFICE OF WATER PLANNING AND STANDARDS WATER PLANNING DIVISION
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