Cabin John Creek Watershed Assessment Summary Document

Cabin John Creek Watershed Assessment Summary Document

October 2018

Prepared for:

Montgomery County Department of Environmental Protection

255 Rockville Pike, Suite 120

Rockville, MD 20850

Prepared by:


Page i

Table of Contents Acronym List ................................................................................................................................................. ii

1 Introduction .......................................................................................................................................... 1

Implementation Plans ........................................................................................................................... 1

Runoff Management and Impervious Cover Treatment ....................................................................... 1

Total Maximum Daily Loads (TMDLs) ................................................................................................... 1

Trash and Litter Management .............................................................................................................. 1

2 Goals and Existing Conditions ............................................................................................................... 3

2.1 Existing Conditions in the Cabin John Creek Watershed .............................................................. 3

Introduction to Cabin John Creek Watershed ...................................................................................... 3

Subwatershed Summaries .................................................................................................................... 5

Watershed Land Use ........................................................................................................................... 13

Existing Stormwater Management Facilities ...................................................................................... 13

2.2 Problems Facing the Cabin John Creek Subwatersheds ............................................................. 15

Biological and Habitat Conditions ....................................................................................................... 15

Water Quality Issues ........................................................................................................................... 15

2.3 Existing Pollutant Loads and Impervious Surfaces ...................................................................... 17

Sediment Loads per the TMDL for Cabin John Creek Watershed ....................................................... 17

Bacteria Loads per the TMDL for Cabin John Creek Watershed ......................................................... 17

Nutrient Loads .................................................................................................................................... 18

Impervious Surfaces ............................................................................................................................ 18

Existing Trash Loads ............................................................................................................................ 18

3 Inventory of Stormwater Management Opportunities ...................................................................... 20

3.1 Types of Stormwater Management ............................................................................................ 20

Stream Restoration ............................................................................................................................. 20

Regenerative Stormwater Conveyance (RSC) and Outfall Stabilization ............................................. 20

Stormwater Management Facility ...................................................................................................... 20

Stormwater Management .................................................................................................................. 20

Potential Green Streets Corridor ........................................................................................................ 21

3.2 Inventory of Stormwater Management Opportunities .............................................................. 21

3.3 Inventory of Stormwater Management Opportunities .............................................................. 25

4 References .......................................................................................................................................... 26

Appendix A Catchment Assessments

Appendix B Methods Memorandum


Page ii

Acronym List

DA – drainage area

DEP – Department of Environmental Protection

DU – dwelling unit

EPA – US Environmental Protection Agency

ESD – environmental site design

HUC – hydrologic unit code

HOA – homeowners association

IA – impervious area

IP – Implementation Plan

MDE – Maryland Department of the Environment

M‐NCPPC – Maryland‐National Capital Parks and Planning Commission

MS4 – Municipal Separate Storm Sewer System

NPDES – National Pollutant Discharge Elimination System

ROW – right‐of‐way

RSC – Regenerative Stormwater Conveyance

TMDLs – Total Maximum Daily Loads

TN – total nitrogen

TP – total phosphorus

TSS – total suspended solids

WIP – Watershed Implementation Plan

WLAs – Waste Load Allocations

WSSC – Washington Suburban Sanitary Commission


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1 Introduction

In 2016 the Montgomery County Department of Environmental Protection (DEP) initiated a watershed

assessment for the Cabin John Creek watershed. The last time a full watershed assessment had been

completed for Cabin John Creek was in 2004. Desktop and field methodologies (see Appendix B) were

employed in 2017 through early 2018 to update DEP’s understanding of the current conditions within

the watershed. These assessments were used to document future restoration opportunities within the

watershed as an update to opportunities identified in the Cabin John Creek Implementation Plan

submitted to the Maryland Department of the Environment (MDE) in 2012. This Summary Document

provides an overview of the restoration opportunities identified and will assist DEP in complying with

future National Pollutant Discharge Elimination System (NPDES) Municipal Separate Storm Sewer

System (MS4) Permit requirements and in developing an updated Total Maximum Daily Load

Implementation Plan for the Cabin John Creek watershed. Relevant MS4 Permit requirements are

described in further detail below.

Implementation Plans As required by the County’s current MS4 Permit, DEP develops watershed assessments by evaluating

current water quality conditions and then identifying and ranking structural, non‐structural and

programmatic watershed restoration opportunities for each County watershed. Full watershed

assessments include field investigations, prioritized restoration project inventories, and cost estimates.

Implementation plans to identify restoration opportunities, estimate treatment to be provided by those

opportunities, determine watershed restoration potential, evaluate the ability of the watersheds to

meet applicable Total Daily Maximum Loads (TMDLs), and provide schedules and cost estimates.

This Summary Document is not intended to serve as a full implementation plan. It identifies restoration

opportunities and provides estimated treatment provided by restoration opportunities but does not

include evaluation of compliance with applicable TMDLs.

Runoff Management and Impervious Cover Treatment The County’s current MS4 permit requires that the County restore an additional 20% of untreated

impervious cover within the County’s MS4 Permit area which translates to stormwater control for an

additional 3,778 impervious acres. The County has made significant progress toward meeting this

additional requirement for watershed restoration through multiple programs and anticipates that it will

meet this goal by December 2020. The County utilizes watershed assessments, such as the one

documented within this Summary Document, to continue to identify future restoration opportunities.

Total Maximum Daily Loads (TMDLs) The permit requires the County to develop implementation plans to achieve progress toward the

County’s Waste Load Allocations (WLAs) associated with the TMDLs that existed when the permit was

issued in 2010. These plans were developed and submitted within 1 year of the start of the permit, as

required. Additional TMDLs were added after the permit was issued and TMDL implementation plans

either have been completed or are included in a Countywide Coordinated Implementation Strategy

(2012). TMDLs specific to the Cabin John Creek watershed are addressed in Section 2.3.

Trash and Litter Management The County actively participates in multiple programs and partnerships designed to meet the goals of

the Potomac River Watershed Trash Treaty. Initiatives directly related to regional campaigns include


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ongoing education and outreach for recycling and litter reduction, mass media outreach campaigns, and

litter removal from streets, stormwater ponds, and transit stops.


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2 Goals and Existing Conditions

2.1 Existing Conditions in the Cabin John Creek Watershed

Introduction to Cabin John Creek Watershed The Cabin John Creek watershed is a Maryland 8 and 12‐digit code watershed located in southeastern

Montgomery County. As shown in Figure 2.1, nine subwatersheds, comprise the Cabin John Creek

watershed. These subwatersheds include Booze Creek, Buck Branch, Ken Branch, Old Farm Creek,

Snakeden Branch, Thomas Branch, and the Upper, Middle and Lower Mainstems of Cabin John Creek.

The total drainage area of these subwatersheds is approximately 16,303 acres (25.5 square miles). An

overview of the Cabin John Creek watershed is included in Table 2.1. A map depicting existing conditions

is presented in Figure 2.3, and a map depicting resource conditions is presented in Figure 2.4.

Seventy four percent (12,133 acres) of the total Cabin John Creek watershed area is subject to the

County MS4 permit. The MS4 area excludes the City of Rockville, Maryland‐National Capital Park and

Planning Commission (M‐NCPPC) lands, Federal and State property, and Federal and State roads. A total

of 25% (3,208 acres) of the MS4 area is comprised of impervious cover with 75% in pervious cover.

Table 2.1: Cabin John Creek Watershed Profile

Metric Acres Percent of Watershed

Watershed Drainage Area 16,303 100%

Impervious Cover 4,220 26%

Watershed Area Subject to MS4 Permit1 12,132 74%

Impervious Cover Subject to MS4 Permit1 3,018 25% (of MS4 permit area)

Pervious Cover (e.g., forest, turf, meadow, farm fields)

Subject to MS4 Permit1

9,114 75% (of MS4 permit area)

1 MS4 permit areas do not include City of Rockville, rural zoning, M‐NCPPC lands, Federal and State property, and

Federal and State roads

Cabin John Creek Watershed Summary Document

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Figure 2.1: The Cabin John Creek Watershed in Montgomery County, MD, Including its Subwatersheds


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Subwatershed Summaries This section describes the subwatersheds that make up the Cabin John watershed, which include

Snakeden Branch, Buck Branch, Ken Branch, Old Farm Branch, Thomas Branch, Booze Creek, and the

Upper, Middle, and Lower Mainstem of Cabin John Creek.

Within the subwatersheds, smaller drainage areas, referred to as catchments, were identified for

restoration priorities. These Priority Catchments are described within this section and are identified

within each subwatershed (see Figure 2.2).

Priority Catchments are those ranked as a higher priority by the County for implementation of

opportunities in the interest of improving local stream conditions and were selected for their

stormwater management opportunity potential. The intent of prioritizing catchments is to focus

restoration efforts on streams that are particularly in need of restoration. Also, by combining a number

of efforts, these particular streams are likely to receive a greater positive impact than would be realized

by widely scattered projects where the positive impact would be diluted. Priority Catchments and

associated stormwater management opportunities are described in further detail in Appendix A.

In addition to Priority Catchments, priority conservation areas were also identified, referred to as

Conservation Priority Catchments. These catchments include the local small order drainage areas (1st

and 2nd order streams) of locations where high value natural resources have been identified by the

Maryland Department of Natural Resources (DNR) which include critically significant, extremely

significant, and highly significant biodiversity conservation areas. The study does not identify any best

management practices for these areas, but only that they should be further investigated and considered

in restoration of the watershed. Based on the natural resources that are present, these areas could

potentially benefit from certain restoration techniques or could potentially be areas to avoid, to

minimize disturbance and protect the resources present. Consultation with DNR and the Maryland

National Capital Park and Planning Commission (M‐NCPPC) is particularly important for any activities

conducted in these areas.

Booze Creek

Booze Creek drains 2,603 acres (4.07 square miles) to Cabin John Creek. Booze Creek is in the

southeastern corner of the Cabin John Creek watershed. The Booze Creek headwaters start northwest of

Bethesda, running approximately two miles through a partially forested residential area before draining

to the Cabin John Creek Lower Mainstem. The Booze Creek subwatershed contains the Bradley Creek

priority catchment. Booze Creek received a poor score for fish and benthic parameters and good/fair

rating for stream habitat conditions.

Bradley Creek: Bradley Creek (provisional name) is one of the smallest of the Cabin John Creek

catchments of focus at 102 acres. Bradley Creek is about 27% impervious and roughly 80% of

the land use is low density residential. The catchment has a large number of open section

roadways that offer opportunities for stormwater management.


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Booze Creek Conservation Priority Catchment: This catchment contains a highly significant

conservation area. Hydrologic characteristics may be of particular importance to this

conservation area.

Buck Branch

The Buck Branch subwatershed drains 1,613 acres (2.52 square miles) along the Cabin John Creek and is

located between the Snakeden and Ken Branch subwatersheds. Buck Branch is located along the

northwestern side of the Cabin John Creek watershed just north of the Seven Locks Road/Democracy

Boulevard intersection. The headwaters of Buck Branch start just southwest of of Rockville running

about 3 miles before draining into the Middle‐Mainstem of the Cabin John Creek. The Buck Branch

subwatershed contains the Paytley Creek and Crossing Creek priority catchments, which are located

along the southwestern edge of the subwatershed. Buck Branch exhibited good/fair stream habitat

conditions and fair fish and benthic ratings.

Paytley Creek: Paytley Creek (provisional name) is a 336‐acre catchment located in the eastern

portion of the Buck Branch subwatershed. The catchment is about 21% impervious and is mostly

low‐density residential land use. Most of the roadways have open swales on one or both sides

of the road.

Crossing Creek: Crossing Creek (provisional name) is a 127‐acre catchment located on the

eastern side of the Buck Branch subwatershed. The catchment is about 16% impervious cover

and 60% of the land use is medium density residential. Some of the area available for

stormwater management opportunities has steep slopes and trees.


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Figure 2.2: The Cabin John Creek Watershed Priority Catchments and Conservation Priority Catchments


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Ken Branch

The Ken Branch subwatershed drains 1,450 acres (2.26 square miles) to the Cabin John Creek. Ken

Branch is located on the central western side of the Cabin John Creek watershed in Potomac. Ken

Branch is about 2.5 miles long, running through residential area that is partially forested. The

headwaters for Ken Branch begin just east of the town of Potomac. Ken Branch includes the Twin Creek

priority catchment. Ken Branch received a fair rating for fish and benthic parameters and stream habitat

conditions are rated good.

Twin Creek (labeled on map as Inglewood Creek): Twin Creek (provisional name) is a 296‐acre

catchment located on the southwestern edge of the Ken Branch subwatershed. The catchment

is 15% impervious cover and 80% low density residential land use. The area has a large number

of open section roadways.

Old Farm Creek

The Old Farm Creek drains 2,419 acres (3.78 square miles) to the Cabin John Creek. Old Farm Creek is in

the northeastern corner of the Cabin John Creek with headwaters beginning southeast ofof Rockville.

Old Farm Creek runs approximately 2.5 miles through a residential area before entering the Cabin John

Creek Upper Mainstem. Old Farm Creek contains the Rock Spring Run, Ibsen Run, and Rosemont Run

priority catchments, all located in the eastern side of the subwatershed. The Old Farm Creek

subwatershed received a fair rating for fish and benthic parameters and has a good rating for stream

habitat conditions.

Rock Spring Run: Rock Spring Run (provisional name) is a 361‐acre catchment located in the

southern end of the Old Farm Creek subwatershed. The catchment is 33% impervious cover and

39% medium density residential land use. The area is a mix of open and closed section roadway.

Ibsen Run: Ibsen Run (provisional name) is a 457‐acre catchment located just north of Rosemont

Run in the southern end of the Old Farm Creek subwatershed. The catchment contains 41%

impervious cover and is comprised of largely medium and low density residential land use. The

catchment has mild slopes and is one of the larger priority catchments.

Rosemont Run: Rosemont Run (provisional name) is a 361‐acre catchment located in the

southern end of the Old Farm Creek subwatershed between the Ibsen and Rock Spring Run

catchments. The catchment is roughly 25% impervious and 43% of the land use is medium

density residential. The Rosemont Run catchment includes the Tilden Center and Feynman

School.

Snakeden Branch

The Snakeden Branch subwatershed drains 872 acres (1.36 square miles). Snakeden Branch lies in the

northwest corner of the Cabin John Creek watershed. The headwaters of Snakeden Branch start just

southwest of Rockville and run about one mile through a predominantly residential area and enter Cabin

John Creek at the southern end of the Upper Mainstem. Snakeden Branch received a poor rating for fish

and benthic parameters, but stream habitat conditions are rated as fair.


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Thomas Branch

Thomas Branch drains 1,616 acres (2.53 square miles) to the Cabin John Creek. Thomas Branch is

centrally located in the Cabin John Creek watershed with the headwaters starting near Walter Johnson

High School, running approximately three miles through a residential area and along I‐495 before

draining into the Cabin John Creek Lower Mainstem. The Thomas Branch subwatershed rated poor for

fish and benthic parameters and fair for stream habitat conditions.

Cabin John Creek Upper Mainstem

The Cabin John Creek Upper Mainstem is the northern‐most subwatershed in the Cabin John Creek

watershed, draining 2,590 acres (3.92 square miles). The headwaters begin Rockville, running about 3.5

miles before combining with the Snakeden Branch and Old Farm Branch to form the Middle Mainstem.

The Upper Mainstem runs through both urban and suburban residential areas. The Cabin John Creek

Upper Mainstem contains the Bogley Branch priority. The Upper Mainstem subwatershed rated as fair

for fish and benthic parameters and good/fair for stream habitat conditions.

Bogley Branch: Bogley Branch is a 741‐acre catchment located on the western side of the Upper

Mainstem. Bogley Branch is 27% impervious and 55% of the land use is medium density

residential.

Cabin John Creek Middle Mainstem

The Cabin John Creek Middle Mainstem is the centrally located subwatershed draining 2,177 acres (3.40

square miles). The subwatershed begins at the confluence of the Upper Mainstem, Snakeden Branch,

and Old Farm Branch, running south approximately 4 miles before ending just south of the MD 190 and

I‐495 Interchange. The Middle Mainstem includes large residential areas with much of the stream valley

on M‐NCPPC property. The Middle Mainstem subwatershed contains the West Bradley and Westlake

priority catchments. The Middle Mainstem rated as fair for fish and benthic parameters and good/fair

for stream habitat conditions.

West Bradley: West Bradley is a 284‐acre catchment located in the southern portion of the

Middle Mainstem subwatershed. The catchment is 20% impervious cover and is primarily

medium and low density residential land use making up 41% and 44% of the catchment,

respectively.

Westlake: The Westlake catchment is 150 acres in the northern portion of the Middle Mainstem

subwatershed. The catchment is nearly 20% impervious cover and is primarily high and low

density residential land use making up 21% and 25% of the catchment, respectively.

Cabin John Creek Lower Mainstem

The Cabin John Creek Lower Mainstem is the southern‐most subwatershed draining 963 acres (1.5

square miles). The subwatershed begins at the confluence of the Middle Mainstem and Thomas Branch

and runs about two miles through mostly residential area and along Cabin John Parkway before it

reaches the Potomac River. The Lower Mainstem rated as fair for fish and benthic parameters and has

consistently maintained a good score for stream habitat conditions.


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Cabin John Creek Lower Mainstem Conservation Priority Catchment: This catchment contains

critically significant, extremely significant, and highly significant conservation areas. Hydrologic

characteristics may be of particular importance to some portions of these conservation areas.


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Figure 2.3: Existing Conditions and Stormwater Management Facility Locations for the Cabin John Creek Watershed


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Figure 2.4: Stream Resource Conditions for the Cabin John Creek Watershed (2011‐2015)


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Watershed Land Use County MS4 Permit area land uses in the Cabin John Creek watershed are shown in Table 2.2. Medium

density residential is the dominant land use in the watershed, covering approximately 43% of the land

area. This is followed by low density residential land use which comprises 27% of the watershed area.

Table 2.2: County MS4 Permit Area Land Use for Cabin John Creek Watershed

Maryland Department of Planning 2010 Land Cover/ Land Use

MS4 Included Watershed Area

(acres)

Percent of Total (%)

Medium Density Residential (1‐4 du/acre) 5,241 43

Low Density Residential (<1 du/acre) 3,272 27

Municipal/Institutional‐ Intensive1 751 6

Municipal/Institutional‐ Extensive2 727 6

Forest5 581 5

High Density Residential (>4 du/acre) 534 4

Commercial 482 4

Industrial 293 2

Roadway3 109 1

Rural4 118 1

Water 15 <1

Extractive 11 <1

Total 12,133 100

du = dwelling unit 1 Institutional land use (churches, schools, municipal buildings) 2 Open Urban Land and Bare Ground land use (parks, cemeteries, and golf courses) 3 Combined County and private roads (excludes Federal and State roads) 4 Orchards, Vineyards, Horticulture, Feeding Operations, Cropland, Pasture, and Agricultural Buildings 5 Deciduous Forests, Evergreen Forests, Mixed Forest, and Brush

Existing Stormwater Management Facilities There are 617 stormwater management facilities within the Cabin John Creek watershed MS4 Permit

area and 49 facilities are in MS4 excluded area, totaling 666 facilities in the watershed. Facilities capture

drainage areas that vary from over 300 acres for regional ponds to 0.1 acres for small, on‐site facilities.

The total drainage area captured by stormwater management facilities is 1,896 acres, with 737 acres

being impervious.


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The current inventory of stormwater management facilities was categorized according to structure type,

as shown in Table 2.3 below.

Table 2.3: Existing Stormwater Management Facilities in the County MS4 Permit area of the Cabin John Creek Watershed

Stormwater Management Facility Type

No. In MS4

Excluded Area

No. In MS4 Permit Area

Water Quality Inlet 0 1

Vegetated/Grass Swale 0 6

Underground Detention 7 83

Tree Box 0 5

Shallow Wetland 0 1

Sand Filter 3 33

Porous Pavement 0 5

Wet Pond 0 13

Dry Pond 5 45

Infiltration Trench/Basin 10 51

Oil/Grit Separator 3 63

Bioretention1 6 73

Green Roof 1 7

Flow Splitter 5 117

Bioswale 0 3

Dry Swale 0 1

Other/Proprietary2 9 78

Total 49 617

1Includes micro‐bioretention 2Includes Aquafilters, Bay Savers, Bay Filters, Stormfilters, Stormceptors, and Vortsentries


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2.2 Problems Facing the Cabin John Creek Subwatersheds

Biological and Habitat Conditions About 14 monitoring stations in the Cabin John Creek watershed are sampled approximately every 5

years for benthic macroinvertebrates, fish species, and habitat to assess stream resource conditions.

The survey data can be used to classify instream conditions at the monitoring location and can represent

the overall subwatershed water quality.

The most recent survey results at the time this watershed assessment was conducted were from the

2011‐2015 monitoring cycle. The condition of most subwatersheds within Cabin John Creek were rated

as being in fair condition with one subwatershed rated as being in poor condition.

Water Quality Issues As part of its environmental enforcement program, the County tracks citizen complaints regarding

water quality and solid waste dumping. Table 2.6 summarizes the number and type of citizen complaints

recorded for the Cabin John Creek watershed since 2008. Seventy‐six (76) stormwater, groundwater,

and surface water cases were reported. Tables 2.7 includes the same complaint locations summarized

by general zoning type. The complaints recorded were in residential, industrial, and commercial land

uses, with most occurring in residential zones. Most of the complaints were in residential areas.

Reported solid waste trash dumping sites are also logged by the County to identify trash hotspots. Table

2.8 includes a summary of the complaint database by complaint type. Table 2.9 identifies the general

zoning type at the site of the complaint. The majority of complaints were recorded as residential and

public land dumping.

Table 2.6: Recorded Water Quality Complaints1 in the Cabin John Creek Watershed

Type of Water Quality Complaint1 Total # of Cases

Surface Water – Chemical Discoloration/ Unknown 4

Surface Water – Petroleum Product in the Water 2

Surface Water – Sewage 5

Sewer Overflows 65

Unknown 0

Total # of cases 76

1 From Case_Base.shp

Table 2.7: Water Quality Complaints in Cabin John Creek by Zoning1

General Zoning Type2 Total # of

Properties1

Agricultural 0


16


Properties1

Commercial 3

Industrial 2

Residential 53

Unzoned 15

Exempt – Single Family Detached 2

Exempt ‐ Commercial 1

Exempt – Institutional/ Community Facility 0

Exempt – Vacant 0

1 From Case_Base.shp 2 From County PROPERTIES.shp

Table 2.8: Solid Waste Trash Dumping Sites1

Solid Waste Complaint Type Total # of

Cases1

Cleanup 1

Commercial 6

Dumpster 8

No Dumping Signage 11

Medical Waste 1

Public Land Dumping 133

Residential Dumping 71

Unpermitted Solid Waste Disposal Facility 1

Total # of cases 232

1 From Case_Base.shp


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Table 2.9: Solid Waste Trash Dumping Sites by Zoning1


Properties1

Agriculture 0

Commercial 5

Industrial 1

Residential 152

Unzoned 52

Exempt – Single Family Detached 10

Exempt – Commercial 7

Exempt – Institutional/ Community Facility 3

Exempt – Vacant 2

1 From MC_SW_CASES.shp 2 From County PROPERTIES.shp

2.3 Existing Pollutant Loads and Impervious Surfaces

Sediment Loads per the TMDL for Cabin John Creek Watershed MDE prepared the “Total Maximum Daily Load of Sediment in the Cabin John Creek Watershed,

Montgomery County, Maryland” Final Report in September 2011 and it was approved by US EPA in

September 2011. This document establishes a TMDL for the Montgomery County MS4 Permit area

within the Cabin John Creek watershed and addresses the 1996 sediments listing. The MS4 allocation

and percent reduction are shown in Table 2.10.

Table 2.10: Cabin John Creek Watershed Sediment TMDL MS4 Allocation and Percent Reduction for the (Source: MDE’s Maryland TMDL Data Center)

Parameter Baseline Year

Montgomery County MS4

Allocation % Reduction

Sediment / TSS 2005 2,430.10 tons/year 22.7

Bacteria Loads per the TMDL for Cabin John Creek Watershed MDE prepared the “Total Maximum Daily Loads of Fecal Bacteria for the Non‐Tidal Segments of Cabin

John Creek Basin in Montgomery County, Maryland” Final Report on October 13, 2006. EPA approved

the TMDL in March 2007. This document establishes a TMDL for the non‐tidal Cabin John Creek basin

which is entirely within Montgomery County. The MS4 allocation and percent reduction are shown in

Table 2.11.


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Table 2.11. Cabin John Creek Watershed Bacteria TMDL MS4 Allocation and Percent Reduction for the

(Source: MDE’s Maryland TMDL Data Center)

Parameter Baseline Year

Montgomery County MS4 Allocation

% Reduction

Bacteria (E. coli) 2003 108.19 billion MPN/day 30.6

Nutrient Loads In October 2012, MDE established goals for nutrient reductions to meet the Chesapeake Bay TMDL in

Maryland’s Phase II Watershed Implementation Plan (2012). This Plan establishes estimates for load

reductions from baseline conditions for nutrients.

Impervious Surfaces Impervious cover in the Cabin John Creek watershed, as derived from County GIS data, is summarized in

Table 2.12. A shown in Table 2.12, parking lots and roads account for the largest percentage of

impervious cover in the watershed at 55% with rooftops contributing just over 40% of the impervious

surface.

Table 2.12: Cabin John Creek Watershed MS4 Permit Area Impervious Cover (as of 2015)1

Impervious Cover Type Impervious Acres Within

Watershed (%)

Parking Lots/Roads 1,662.6 55.1

Roofs 1,215.0 40.3

Sidewalks 77.4 2.6

Recreation 62.7 2.0

Total Impervious Acres from GIS 3,017.7 100%

1: From GIS feature class, MontCo_All_Impervious_Jan_2014

Existing Trash Loads The Potomac River Watershed Trash Treaty outlines the agreement between local and state elected

officials to commit to a Trash Free Potomac. The agreement includes three major commitments:

Support and implement regional strategies aimed at reducing trash and increasing recycling;

Increase education and awareness of the trash issue throughout the Potomac watershed; and

Reconvene annually to discuss and evaluate measures and actions addressing trash reduction.

In general, trash reduction strategies fall into four categories: (1) Structural; (2) Educational; (3)

Municipal; and (4) Enforcement. Structural stormwater management facilities are generally assigned

95% removal credit for trash from the contributing drainage area. Stormwater management facilities,

while not specifically designed to capture trash, are also not very good at passing trash, and debris is

prone to build up in forebays, around plants and interior elements, and around the outlet structures.

This trash and debris is removed during maintenance activities.


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In addition to trash removal by structural stormwater management facilities, programmatic practices

from the other three categories ‐ educational, municipal, and enforcement ‐ provide trash prevention

and control. These programmatic practices are specially aimed at reducing trash inputs to roads and

streams, including educationally focused programs such as reduce, reuse and recycle campaigns;

dumpster management and storm drain marking; and programs tied to operations such as littering and

illegal dumping enforcement; stream cleanups; and street sweeping. These programmatic practices are

further explored in the Countywide Coordinated Implementation Strategy (2012).


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3 Inventory of Stormwater Management Opportunities

3.1 Types of Stormwater Management

This section describes the potential stormwater management opportunities evaluated for the Cabin

John Creek watershed. Opportunities differ in the mode and manner by which they would be delivered

in the watershed (capital budgets, operating budgets, regulation, etc.). Multiple delivery mechanisms

are needed to implement enough stormwater management opportunities to meet stringent watershed

treatment and pollutant reduction targets set forth in the County’s MS4 permit, TMDLs, and the

Potomac River Watershed Trash Treaty. Future analysis of restoration opportunities could more broadly

look at the potential for retrofitting other stormwater facilities, such as underground oil grit separators,

to gain stormwater treatment and credit.

Several categories of restoration project opportunities were evaluated. These categories include:

Stream Restoration Streams were identified as Priority Streams for Restoration when the stream assessment ranked them

as highly eroded or ecologically deteriorated (See Section 3 of the Methods Memo located in Appendix

B). Restoration consists of techniques or methods to protect infrastructure and improve water quality by

reducing stream bank erosion, minimizing down‐cutting of stream beds, and restoring aquatic

ecosystems.

Regenerative Stormwater Conveyance (RSC) and Outfall Stabilization Outfall areas and channel reaches with intermittent or ephemeral flows and significant erosion were

identified as potential opportunities. RSC consists of a series of step pools and sand filters constructed

to allow surface water to replenish the shallow groundwater. Outfall stabilization is typically

implemented for a short distance from an outfall and uses techniques to minimize erosion in the outfall

area.

Stormwater Management Facility Suitable specific locations with drainage areas of about an acre or more were identified for retrofit, or

new individual or clustered facilities.

New Stormwater Management Facility: Locations where a stormwater management facility

does not currently exist, but where there may be sufficient space for a facility and a drainage

area of approximately one acre or greater.

Retrofit Stormwater Management Facility: Select locations within priority catchments where

there is an existing stormwater management facility, but the facility does not sufficiently treat

runoff from its associated drainage area. Retrofit would consist of upgrades to improve water

quality in the receiving stream.

Stormwater Management Neighborhoods and other areas were assessed for general suitability of introducing various types of

stormwater management which mimic nature to capture and treat stormwater as close to the source as

possible.


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Green Streets: Rain gardens and other low impact practices constructed within the public street

right of way that reduce and filter stormwater runoff.

RainScapes: Low impact design techniques such as raingardens, conservation landscaping, and

permeable pavement that are voluntarily implemented by private property owners. Under this

program, typically on single family residential lots, the owner receives a rebate for a portion of

the implementation costs from the County.

Community Environmental Site Design (ESD): Low impact stormwater management practices,

that are constructed on private property with the approval of the property owner. These could

include capital improvement projects, grant projects, or RainScapes projects on larger parcels.

Public Property Environmental Site Design (ESD): Low impact stormwater management

practices, that are constructed on public property. These could include capital improvement

projects, grant projects, or RainScapes projects on larger parcels.

Potential Green Streets Corridor County arterial and collector roads which may have opportunity for low impact stormwater practices

within the right‐of‐way and for which these practices should be prioritized during the design of roadway

improvement projects.

3.2 Inventory of Stormwater Management Opportunities

The Cabin John Creek watershed has been the subject of multiple watershed assessments including a

2004 watershed study, a 2012 implementation plan, and the most recent of which was the catchment

focused assessment in 2018. Potential restoration strategies for the Cabin John Creek watershed set

forth in this Watershed Assessment Document and the Catchment Plans (Appendix A) were drawn from

site assessments performed from 2016 to spring of 2018. Identified stream restoration, RSC and outfall

stabilization, and stormwater management opportunities are presented in Figures 3.1 and 3.2.


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Figure 3.1: Identified Stream Restoration Opportunity Locations for the Cabin John Creek Watershed


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Figure 3.2: Stormwater Management Opportunity Locations for the Cabin John Creek Watershed


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Figure 3.3: Stormwater Management Opportunity Areas in the Cabin John Creek Watershed


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3.3 Inventory of Stormwater Management Opportunities

This assessment focused on identifying new stormwater management opportunities and larger retrofit

opportunities within priority catchments. Prior assessments that can also be referenced for both new

stormwater management and retrofit opportunities include:

“Cabin John Creek Watershed Study” (2004)

“Montgomery County Low Impact Development Phase II – Final Report” (GPI, 2009) – Task

Order 20

“Inventory of LID Retrofit Opportunities & Stormdrain Systems at Montgomery County Public

Schools and Facilities ‐ Phase 1” (RKK, 2011) – Task Order 9

“Cabin John Creek Implementation Plan” (2012)

GPI Assessments

Montgomery County Pond Retrofit Assessments (BC/Bio Joint Venture, 2014)


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4 References

Biohabitats, Versar, Horsley Witten Group, Chesapeake Stormwater Network, Capuco Consulting

Services, RESOLVE. 2012. Montgomery County Countywide Coordinated Implementation Strategy.

Prepared for Montgomery County Department of Environmental Protection, Rockville, MD.

Brown and Caldwell (BC)/ Biohabitats Joint Venture. 2014. Montgomery County Pond Retrofit

Assessments. Prepared for Montgomery County Department of Environmental Protection, Rockville,

MD.

MDE. 2006. Total Maximum Daily Loads of Fecal Bacteria for the Non‐Tidal Segments of Cabin John

Creek Basin in Montgomery County, Maryland. Baltimore, MD.

MDE. 2010. National Pollutant Discharge Elimination System Municipal Separate Storm Sewer System

Discharge Permit for Montgomery County, Maryland. Permit No. 06‐DP‐3320. Baltimore, MD.

MDE. 2012a. Maryland’s Phase II Watershed Implementation Plan for the Chesapeake Bay TMDL.

University of Maryland, MDP, MDA, MDE, and MDNR.

MDE. 2012b. Total Maximum Daily Load of Sediment in the Cabin John Creek Watershed, Montgomery

County, Maryland. Baltimore, MD

MDE. 2018. Maryland TMDL Data Center: Montgomery County Stormwater WLA. Retrieved from

http://wlat.mde.state.md.us/ByMS4.aspx on October 19, 2018.

Versar, Biohabitats, Chesapeake Stormwater Network, Horsley Witten Group, Capuco Consulting

Services, RESOLVE. 2012. Cabin John Creek Implementation Plan. Prepared for Montgomery County

Department of Environmental Protection, Rockville, MD.

Documents

Cabin John Creek Watershed Assessment Summary Document