Mark Andre, Judy Wartella
DEVELOPMENT OF A STORMWATER DRAINAGE MASTER PLAN
FOR A SMALL MUNICIPALITY
Abstract
The City of Arcata is completing a several year effort building spatial
data layers by incorporating GIS as a key tool in developing a Stormwater
Drainage Master Plan. The purpose of the master plan is to consider various
maintenance, repair and design alternatives in order to maximize the capability
of the system at the lowest cost, within the context of broad resource-management
policies. The goal is to provide realistic technical solutions that are
economically, socially, environmentally and institutionally acceptable to
the community.
In order to achieve this goal it was necessary to first identify the existing
stormwater drainage and discharge system to evaluate known problems. The
city's GIS system was used to analyze eight watersheds to predict runoff
due to past and future urbanization. Slope, soils, impervious surface areas
and other parameters are used to model various intensity rainfall events
and determine projected flows.
Development of this plan has required mapping multiple spatial data layers
for the entire planning area including parcels, zoning, soils, contours,
drainage infrastructure, building footprints, impervious area, streets,
creeks, ponds and wetlands. Digital elevation models and slope class maps
were created using GRID. Watersheds were further divided into sub-basins
in order to evaluate the capacity of all drainage infrastructure with a
diameter of 24 inches or larger. GIS-derived parameters for each of the
eight urban watersheds, such as flow length and gradient, were exported
into a spreadsheet for calculation of surface, channel and pipe flows which
are a function of a particular rainfall event and time of concentration.
Flow at the outlet of each defined sub-basin was calculated based upon
the rational method which relates flow as a function of rainfall intensity,
area of the basin, and a C coefficient based upon basin characteristics.
The evaluation of existing capacity compared with the projected increased
drainage capacity requirements based upon future growth, form the basis
of fees applied to new developments in order to financially prepare for
future city-wide infrastructure costs. Funding for implementation of this
plan is derived from a utility fee assessed each city parcel and determined
by the actual square footage of impervious surface. Using GIS in this planning
process has allowed the city to create a dynamic database and modeling approach
which considers changing conditions such as the availability of land, new
urban planning strategies, and opportunities for natural resource enhancement.
Background
The City of Arcata is located in Humboldt County, California approximately
300 miles north of San Francisco. Arcata is situated on Humboldt Bay and
covers an area of 10 square miles with a population of approximately 16,000.
Elevations range from 1,200 feet to sea level. The region is characterized
by heavy winter rainfall.
Urban streams originating within the planning area convey the majority of
the city's run-off through town, via a network of channels, pipes, culverts,
ponds, wetlands and tide gates into Humboldt Bay. Arcata's creeks, which
contain fish and other aquatic organisms, enter the Bay where a large commercial
oyster operation is located. Pollutants include typical non-point urban
runoff compounds as well as increased sediment loads due to channel erosion
caused by increased peak flows due to urbanization. Eight watersheds intersect
the city with a total impervious surface area per watershed ranging from
roughly 6 to 35%. Present urban impervious area has increased nearly 40%
from 1978.
Since the passage of State Proposition 13 in 1978, property tax revenues
(traditionally used to fund public works improvements), have been inadequate
to fund maintenance and replacement. Sixty percent of storm drains are
over 30 years old and forty percent are over 80 years old. Recognizing
that an adequate funding base was necessary in order to maintain the deteriorating
storm drainage system, a utility fee was implemented by the city council
which is calculated based upon actual square footage of impervious surface
for each parcel. A city-wide average residential building footprint plus
paved area per parcel was used to determine residential fees. Commercial
and industrial parcels are billed based upon an estimate of the relative
amount of runoff each contributes to the system as a function of the surface
conditions. Billing based upon impervious surface area provides a means
to more equitably distribute the costs according to how runoff is generated,
and it further encourages minimal paving.
This stormwater utility fee funds
the following programs: easement acquisition, remedial maintenance, public
education, Adopt-A-Creek program and habitat/riparian restoration, monitoring
and sampling, routine maintenance, utility billing and administration,
interim capital improvement projects and master planning. A provision for
fee waiver or reduction is available on a case-by- case basis for landowners
wishing to build and maintain on-site detention/retention facilities. This
incentive can help to incrementally improve water quality and decrease peak
flows in urban streams if properly applied and planned with consideration
of the entire drainage system.
The purpose of the Master Plan is to consider various maintenance, repair
and design alternatives to maximize the capability of the system at the
lowest cost, within the context of broad resource management policies.
Specifically, staff have been directed to recommend appropriate management
strategies which are economically, socially, environmentally and institutionally
acceptable to the community. These measures include both structural and
non-structural approaches.
Inventory and Modeling Approach
Arcata purchased pcArcInfo in 1987, and by 1990 had started using this
tool for a variety of municipal applications including planning, natural
resource and infrastructure management. The City presently also uses ArcCad
and ArcView. Both PC and workstation ArcInfo were utilized as tools in
this master planning process.
The first step was to identify, map and digitize the existing stormwater
system of channels, culverts and pipes. The Stormwater Master Planning
process required and was a catalyst for completion of many planning area-wide
digital spatial data themes including building footprints, impervious surfaces,
roads, contours, streams and soils. Most coverages were digitized from
1:1,200 and 1:600 orthophotos. Digital elevation models and slope classification
coverages were generated using GRID from the contour data and spot elevations.
Watershed basins were delineated based upon topography, stream channels,
surface flow and drainage structures which re-route flow. Typical of urban
watersheds, flows are re-routed from one topographically-defined basin to
another via culverts and pipes which limits the applicability of some GIS-based
watershed delineation algorithms. Each major stream basin was further divided
into sub-basins in order to evaluate the capacity of all drainage structures
with a diameter of 24 inches or larger. Sub-basins were overlaid onto other
themes including soils, slopes, etc. to provide input for various modeling
equations. This process provided abundant descriptive data concerning each
sub-basin, which was used to model flow runoff from varying intensity storms
under current and future build- out conditions. These basin statistics
are available to the general public and staff for future projects.
GIS-defined parameters such as flow length and gradient was exported from
the GIS into an EXCEL spreadsheet for iterative calculation of flows which
are a function of a particular rainfall event and time of concentration.
Flow at the outlet of each sub-basin was calculated by the rational method
(which relates flow as a function of rainfall intensity, area of the basin
and a dimensionless runoff coefficient C based upon basin characteristics.)
The time of concentration is based upon the runoff route as it travels
across the landscape via surface, channel and pipe flow. In estimating
flow through a pipe or channel at a given point, the contributing upstream
sub-basin area was considered, with the longest route in a varied flow network
determining the time of concentration for estimating flow at a given point.
This approach of exporting initial variable information to a spreadsheet
was chosen in order to facilitate the calculations and provide access to
the significant amount of data within each basin in an alternative format.
Sensitivity analysis and "what-if" scenarios were used to check
and calibrate flow model assumptions such asC values for various
soil types and Mannings n. Calculations of flow at various points
can be then linked back to the infrastructure coverage. Future monitoring
efforts outlined in the master plan will be used to upgrade some of the
model assumptions.
Conclusion - Benefits of this Approach
There are multiple benefits to the City of Arcata in developing a GIS-based
Stormwater Drainage Master Plan. These include:
- This cost-effective project provided the stimulus and the means to
complete significant work in spatial database development for all watersheds
within the planning area.
- Detailed input parameters and assumptions to flow calculations can
be easily modified as conditions change to allow for rapid and accurate
re-evaluation.
- This process required that future build-out be evaluated for the planning
area which provides justification and a basis for development fees.
- This approach provides the means to evaluate impacts of future development
projects on a case-by-case basis, as well as long-term cumulative effects.
- This process combines various types of data useful to numerous other
applications.
- The dynamic database and modeling approach on a sub-basin scale allows
the possibility of considering changing conditions such as availability
of land and new urban planning strategies.
Acknowledgements
We wish to thank the Arcata City Council and Alice Harris, City Manager
for their vision and support of the GIS program; Scott Baker and Kevin Masters
for their efforts inventorying the drainage infrastructure; Brent Siemer,
P.E. for his detailed review and engineering input; Kim Steele, Anthony
Ambrose, Gary Emery, Rob Beachler and the many other student interns who
participated over the yeas in diligently digitizing various layers of this
city .
References
Binney, Peter D. (1981), Urban Storm Drainage-A Municipal Experience
presented at the Second International Conference on Urban Storm
Drainage, Urbana, Illinois.
Brater, Ernest F. (1976), Handbook of Hydraulics for the Solution of
Hydraulic Engineering Problems, McGraw-Hill.
Charlotte, NC. Storm water credit manual
Los Angeles, CA. (1994), City of Los Angeles Storm Water Information Management
System, presented at the 1994 Esri User Conference.
USGS. (1971) Suggested Criteria for Storm-Drainage Facilities in S.F.
Bay Area
Baruth and Yoder. (1971) Mid-Humboldt County Urban Planning Program Storm
Drainage A report to the Humboldt County Board of Supervisors, City of
Arcata, Humboldt County Califonia. A Study Regarding Storm Drainage
for Mid-Humboldt County Activity 5 of the Mid-Humboldt County Urban Planning
Program. , Engineers and Planners.
U.C. Davis. (1965) Soils of Western Humboldt County, California. Dept.
of Soils & Plant Nutrition, U.C. Davis, in cooporation with the County
of Humboldt, California.
Soil Conservation Service. (1986) Urban Hydrology for Small Watersheds,
Technical Release No. 55, Water ResourcesPublications, Soil Conservation
Service 210-VI-TR-55.
Authors:
Judy Wartella
GIS Specialist
eri@northcoast.com
Mark Andre
Deputy Director - Environmental Services
msaarc@aol.com
City of Arcata
736 F Street
Arcata, CA 95521
(707)822-8184