stormwater watershed delineation, analyses, and regulatory

Stormwater watershed delineation, analyses, and regulatory requirements; County of

Monterey

Patricia Cubanski

Graduate Student

Coastal and Watershed Science and Policy, MS

California State University Monterey Bay

Academic Advisor

Frederick Watson PhD

County of Monterey Public Works Department

Thomas Harty P.E.

Watershed Management Experiential Learning for USDA Careers

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Table of Contents Acknowledgments ......................................................................................................................................... 3

Executive Summary ....................................................................................................................................... 4

Project Context ............................................................................................................................................. 5

Stormwater Concerns ............................................................................................................................... 5

Stormwater Regulation ............................................................................................................................. 5

Project objectives .......................................................................................................................................... 6

USDA Career Path ......................................................................................................................................... 7

Project Methods ........................................................................................................................................... 7

Study Area ................................................................................................................................................. 7

Available Data ........................................................................................................................................... 9

Field Methods ......................................................................................................................................... 10

GIS Methods ............................................................................................................................................ 10

Watershed Analyses ............................................................................................................................ 11

Project Results and Discussion .................................................................................................................... 11

Conclusion ................................................................................................................................................... 16

References .................................................................................................................................................. 16


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Acknowledgments This project was supported by Agriculture and Food Research Initiative Competitive Grant no.

2011-38422-31204 from the USDA National Institute of Food and Agriculture.

Additionally, I would like to thank:

Tom Harty, County of Monterey

Heidi Niggenmeyer, Monterey Regional Stormwater Management Program

Fred Watson, California State University Monterey Bay

Rikk Kvitek, California State University Monterey Bay

Gabriella Alberola, California State University Monterey Bay

Kirk Post, California State University Monterey Bay


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Executive Summary

This report provides descriptions of my project funded by the USDA Experiential Learning in

Watershed Management and of my project’s relevance for a future career with USDA agencies.

My project assisted the County of Monterey in fulfilling the requirements of its National

Pollutant Discharge Elimination System Phase II small municipal separate storm sewer system

(MS4) Permit. More specifically, my project addressed spatial analysis requirements of

stormwater outfalls thus providing an understanding of factors contributing to stormwater quality

and of quantity entering receiving waterbodies. The primary objectives of this project were to 1)

gather, develop, and organize data necessary for spatial analyses, 2) develop a model to automate

watershed delineation and watershed land cover analyses, and 3) provide recommendations

concerning methods to complete the new Phase II MS4 requirements (adoption pending).

The presence of urbanized areas in a landscape can have multiple effects on the hydrology and

water quality of the area. Impervious surfaces decrease the capacity for stormwater and dry

weather runoff to infiltrate soils. In addition, these areas and the activities associated with

urbanization can generate pollutants that can be transported to downstream waterbodies or

sensitive areas by runoff. While water discharge is associated with other regulations and

permits, the Phase II MS4 Permit directly involves storm drain network structure and requires

spatial analysis of storm drain outfalls within urbanized areas as defined by Census information.

The County of Monterey has multiple urbanized areas within its jurisdiction. The urbanized

areas of Carmel Valley, located approximately 10 miles south of the Monterey Peninsula,

parallel the Carmel River which is identified as critical habitat for steelhead. In this area, the

storm sewer system empties to the river potentially introducing pollutants to the receiving

sensitive waterbody.

To help the County comply with discharge regulation, I conducted watershed delineations for

County property on the Monterey Peninsula as well as in Lower Carmel Valley. I also created

maps of the watersheds with regard to land cover, soil type, slope, and relative topography.

From this initial study, I developed a model to complete watershed analyses on the other

urbanized areas within County jurisdiction that have dynamic landscapes. To account for the

storm drain system, the watershed delineation model incorporated the storm drain network

structure into the surface digital elevation model (DEM) which channeled the flow direction

toward the network. While this model adequately delineated most outfalls for the regions

examined, the infrastructure of the storm drains for two outfalls prevented using this model for

those specific areas and needs further investigation on accurately estimating these outfall

watershed boundaries.

In reference to watershed management, this internship project allowed me the opportunity to

explore skills related to water discharge management. Water discharge quantity and quality are

concerns for hydrologists within the United States Forest Service. Understanding flow patterns

and potential pollutant sources provides a hydrologist with information to lessen the negative

effects of runoff on waterbodies and sensitive ecosystems.


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Project Context

Stormwater Concerns

Traditional developed areas and the activities associated with these urbanized areas create

concern for the water quality of downstream waterbodies and other sensitive areas. The increase

of impervious surfaces such as pavement, rooftops, and asphalt resulting from development

decreases the opportunity for runoff to infiltrate the landscape and thus runoff from urban areas

increases in volume and pollutant loads. The Environmental Protection Agency has identified a

variety of pollutants that can originate from both point and non-point sources within urbanized

areas. Typical non-point source pollutants include, but are not limited to sediments, nutrients,

pathogens, petroleum hydrocarbons, heavy metals, polycyclic aromatic hydrocarbons (PAHs),

pesticides, and herbicides (SWRCB 2012). These pollutants, deposited by vehicular emissions,

household hazardous wastes, landscape irrigation, and a variety of other sources, drain into storm

sewer systems and receiving waterbodies during storm events and dry weather runoff. In

addition, the increased runoff volume can alter erosion and accretion patterns of receiving

waterbodies and thus disrupt aquatic ecosystems. Due to these concerns, municipalities and

other entities with separate storm sewer systems are required to monitor and mitigate potential

negative effects. A variety of factors influence what mitigation techniques might be appropriate

for an area. For example, the capacity of the soil to infiltrate runoff water and the slope and

relative topography of the land all contribute to what mitigation technique or suite of techniques

to choose.

Stormwater Regulation In the Monterey Bay region of California, federal and state regulations require a variety of

analyses, mitigation, and monitoring of the stormwater discharge to the surrounding waterbodies

and sensitive areas. As currently outlined in the final draft of the permit, based on 2010 Census

blocks, urbanized areas with populations under 100,000 residents must comply with the

requirements of the Phase II MS4. The County of Monterey has multiple areas under its

jurisdiction through which this permit will affect management practices. The permit will require

the county to provide educational information to and participation opportunities for the public

and train appropriate county staff on illicit discharge detection. In addition, the county must

comply with water quality monitoring and implement a construction site stormwater runoff

control management program and a pollution prevention/good housekeeping for permittee


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facilities program. The main requirement addressed by this study involves the detection and

elimination of illicit discharges entering the sewer system and subsequently outletting to

receiving waterbodies. As part of this regulatory requirement, the county at a minimum must

have a map of all stormwater outfalls and the receiving waterbody for the outfall discharges as

well as photo documentation of all outfalls. In addition, the county requested information

concerning the dimensions and the construction material of each outfall to assist in field

identification of outfalls during monitoring activities. The pending regulation also requires the

permittee to determine the watershed area for each outfall and show the land use present within

the outfall watershed. Based, in part, on land use, the permittee may need to impart additional

monitoring if the watershed contains a high risk land use type. Additionally, Permittees must

sample discharge from outfalls occurring over 72 hours after a storm event.

Project objectives In the Monterey Bay Region, the County of Monterey has multiple urban storm sewer outfalls

that discharge to rivers and other waterbodies classified as waters of the US. Currently, the

County does not have a complete inventory of all outfalls under its jurisdiction and as such

cannot complete the stormwater watershed analysis as required by the draft Phase II General

Permit. The main objective for my project with the County Stormwater Program was to design

watershed analytical models based on County of Monterey data and spatial data available from

online sources to assist the County in complying with NPDES outfall requirements and in

selecting post-construction BMPs.

As mentioned above, to meet the minimum requirements for the Phase II MS4 Permit, the

County must delineate outfall watershed boundaries with respect to storm drain networks,

determine percent land use cover of each watershed, identify areas for potential illicit discharges,

and illustrate storm drain networks with catch basin inlets. Additionally, as required by

Attachment J of the draft permit, Permittees within the Central Coast Region must implement

post-construction stormwater BMPs to reduce runoff from new development that creates and/or

replaces over 2,500 ft2 of impervious surface. The attachment includes maps of watershed

management zones that indicate what BMPs should be implemented if the development falls

within a certain zone. However, while this gives further specification on what BMPs are


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appropriate, to select and tailor a BMP specific to the site, information concerning soil properties

and relative topography, among other variables, will also be necessary.

USDA Career Path In reference to USDA career paths, this project emphasizes watershed management with

responsibilities similar to that of a US Forest Service (USFS) Hydrologist. USFS Hydrologists

analyze and monitor surface water and the parameters influencing quality, quantity, and flow of

these waters. My project addressed the flow and management of water through municipal

systems with the goal of reducing pollutants entering receiving water bodies.

Project Methods

Study Area The County of Monterey, located on the central coast of California, has multiple urban properties

with outfalls to a variety of waterbodies. Urban areas under the County of Monterey’s

jurisdiction cover approximately 49.4 square miles of a total 3,771 square miles. These urban

areas drain into rivers containing endangered steelhead (Oncorhynchus mykiss) in the Carmel

River and rivers with Total Maximum Daily Loads programs (TMDLs), Salinas and Pajaro

Rivers. To establish a watershed model, I focused the pilot study on the Lower Carmel River

(LCR) and a singular outfall emptying into Del Monte Lake (DML) on the Monterey Pennisula

(Fig. 1, Fig. 2).


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Figure 1. County storm drain emptying to Del Monte Lake (DML) located on Naval Postgraduate School

(NPS) property.


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Figure 2. County outfalls emptying to the Lower Carmel River illustrating Outfalls A and B for the purposes

of this report.

Available Data Spatial data for the County were acquired from a variety of sources. The watershed analyses

required information regarding

Elevation: Digital Elevation Models (DEMs), 1/9 arc-second or ~3 m resolution,

downloaded from United States Geologic Survey (USGS) National Map Viewer

(06/22/2012)

A B


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Storm Drain Infrastructure: Data on catch basins, culverts, and stormwater sewers

were available from the County of Monterey Geodatabase. However, as mentioned

previously, the storm drain data was incomplete and not field checked.

Soils Data: Spatial information concerning soils was acquired from the United States

Department of Agriculture (USDA) Natural Resource Conservation Service- Soils

Survey Geographic Database (SSURGO)

County of Monterey urban area and land use information: County urban area

boundaries and land use zonation data for County properties were available from the

County of Monterey Geodatabase

Field Methods Due to a lack of data regarding existing storm drain infrastructure on LCR, I conducted field

surveys of storm drain catch basins and networks. I began examining storm drain infrastructure

at the lower reaches of storm drain network. Few building plans were available that identified the

storm sewer system. Where building plans were unavailable, I took GPS coordinates of catch

basins, culverts, and outfalls using a Trimble Geo Explorer and recorded the directions of pipe

connections and other storm drain properties in a data dictionary file I created. Each feature

recorded was based on an average of 30 collected readings.

GIS Methods At both sites, based on field visits, the DEM was verified and appropriately modified to match

existing topography. Prior to delineating watersheds for each outfall, the collected data

concerning the lower Carmel River’s catch basins, culverts, and outfalls were processed to

develop a polyline shapefile representing the storm drain network of the area. I developed the

network based on the pipe direction connections for each storm drain structure and on building

plans when available. To delineate watersheds for each outfall I used ArcGIS 10 (Hydrology

toolset within the Spatial Analyst extension) which based watershed boundaries on DEMs.

To incorporate the storm drain network within both the Lower Carmel River and Monterey

Peninsula regions, the DEMs were conditioned such that the presence of a storm sewer main

would redefine the elevation 10 m below the original elevation. This created a channelization

effect intended to mimic the storm drain system. From this new conditioned DEM, I analyzed

flow direction and flow accumulation to generate drainage patterns and watershed boundaries for


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stormwater outfalls. I selected the last inlet prior to a stormwater outfall as the outfall

delineation point. To allow for consistent analytical methods, I developed a GIS model within

Arc Model Builder through which a user could enter the original DEM and storm drain data and

receive watershed boundaries output.

Watershed Analyses

I performed further analyses on each outfall watershed. The percent and total area of soil type,

land use type, and percent impervious cover within each watershed were determined based on

existing available data. Watersheds were also summarized based on relative topography and

slope to provide information for future BMP stormwater mitigation techniques. I determined

relative topography based on a topographic position index generated by the Land Facet Corridor

toolset (Jenness 2010). This toolset examines the elevation differences between a target pixel

and the pixels in a user defined neighborhood. From this information, I was able to determine

which areas were depressions, peaks, flat or mildly sloped based on relative location.

Project Results and Discussion Monterey Peninsula Area: Based on field visits, I determined that prominent topographic

features within the outfall’s potential drainage area were represented in the DEM and that no

modifications to the DEM were necessary. The analyses indicated that the outfall drains an area

of approximately 840 acres (Fig. 3). The majority of that area consists of medium residential use

(MDR) land cover. For the permit in its current draft form the above data in addition to outfall

diameter and age of outfall are all required for compliance. Understanding the spatial

contributions and characteristics of the outfall watershed provides the foundation from which to

develop other analyses and mitigation techniques. The watershed area also consisted largely of

Pfeiffer fine sandy loam and Santa Lucia shaly clay loam soil type and contained depression

areas potentially suitable for runoff infiltration sites. With additional data on water table height

and hydrologic flow, frequency, and area drainage among other characteristics, a map may be

developed that can identify areas for potential stormwater treatment and flow reduction sites.


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Figure 3. Watershed boundary for the storm drain emptying to Del Monte Lake.

Lower Carmel River: A portion of the urbanized areas contributing to the LCR are located

within the Carmel River flood plain. A comparison of existing ground features affecting

hydrology with the area’s DEM indicated that the DEM resolution was not fine enough to depict

Highway 1, a subtle, but important feature within the flood plain. To incorporate this

topographic feature, I selected the associated stretch of the highway and created a raster layer


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with a 6 m Euclidean distance from the highway. Then, I combined this corrected feature with

the original DEM.

As previously mentioned this region required gathering spatial data to define the current location

of municipal storm drains. The field work revealed that the current sewer network is a result of

multiple decades of overlapping networks which (Fig. 4).

Figure 4. Representation of the storm sewer network of areas surrounding the Lower Carmel River showing

overlapping storm sewers for two different outfalls.

Three of the identified outfalls’ last inlets were determined to be located at a minimum of 500 m

from the respective outfall. Since the regulations are concerned with the end of pipe flow, I

generated watersheds from the last inlet as opposed to the location of the outfall instead of using


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the outfall as the final downstream point. This would also incorporate the area directly

surrounding the outfall, but these areas do not enter the storm sewer system. Adding to the

complexity of this analysis, two of the outfalls had underground inlets that, if using this method,

could not be incorporated into the conditioned surface DEM without introducing area that might

not contribute to end of pipe runoff. As such manually defining the outfall watershed boundaries

of these two outfalls in the flood plain section of the drainage area may be necessary. To

demonstrate a process by which to use both the method employed at DML as well as a manual

delineation, I delineated the watershed for Outfall B. The initial delineation analysis used the

developed model to identify watershed boundaries of the upper reaches where the landscape is

high relief. This provided the general shape the watershed and the boundary then was manually

modified in ArcGIS 10 to reflect stormwater movement within the floodplain (Fig. 5). Previous

studies have also noted the limitations of using surface topography in which the MS4 has

hydraulic modifications such as pump systems (Jankowfsky et al 2012). Augusto et al (2009)

indicated that GIS floodplain watershed analysis may be limited by DEM resolution.


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Figure 5. Watershed for Outfall B on the Lower Carmel River illustrating the approach of first using the

developed model to delineate upper reaches of a watershed (red) and then to manually alter the watershed

(alterations shown in orange for illustration only).

To address the two outfalls with underground inlets, other methods such as creating sinks in the

DEM may yield results that are more accurate. Another option may be the use of software other

than ArcGIS. For example, Watershed Modeling System (Aquaveo 2012) and XPSWMM are


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programs designed to help analyze the flow of stormwater through municipal systems and

delineate watersheds.

Applications for other County of Monterey areas:

While this model may not accurately delineate the complete watershed for some outfalls, the

model still has applications for the county. Running delineations of outfall subwatersheds where

the terrain is high relief, provides a partial automation for delineating watersheds

Conclusion As a pilot study for Phase II MS4 Permit watershed analyses, my project provided a method for

outfall watershed delineations as well as feedback for the Public Works Department of the

County of Monterey concerning the challenges encountered while establishing municipal

watershed boundaries and conducting related analyses. In addition, this internship allowed me

the opportunity to study regulations regarding watershed management and non-point sources as

well as the opportunity to develop methods to fulfill these NPDES regulatory requirements.

These skills and experiences have provided me appropriate background to support a career as a

USDA Hydrologist.

References Aquaveo, LLC. 2012. WMS 9.1.3. Available at: http://www.aquaveo.com/wms.

Augusto CVG, Bonnet MP, Filho OCR, Mansur WJ. 2009. Improving hydrological information

acquisition from DEM processing in floodplains. Hydrological Processes 23: 502-524.

ESRI (Environmental Systems Resource Institute) 2011. ArcMap 10, ESRI, Redlands,

California.

Jankowfsky S, Branger F, Braud I, Gironás J, Rodriguez F. 2012. Hydrological Processes. DOI:

10.1002/hyp.9506.

Jenness, J., B. Brost and P. Beier. 2011. Land Facet Corridor Designer: Extension for ArcGIS.

Jenness Enterprises. Available at: http://www.jennessent.com/arcgis/land_facets.htm.

[SWRCB] State Water Resources Control Board. 2012. May 18, 2012, Draft Water Discharge

Requirements from Small Municipal Separate Storm Sewer Systems (MS4s) (General Permit).

[Internet]. [cited 2012 September 5] Available from: http://www.waterboards.ca.gov.


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XP Software, Inc. 2011. XPSWMM v.2011. Portland, Oregon.

stormwater watershed delineation, analyses, and regulatory

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