Ecological Assessment on Ecological Assessment on Thorp BrookThorp Brook

Presented by: Kate Crawford, Elizabeth McDonald, Keith Jennings, and Maria

Burke

Thorp Brook WatershedThorp Brook Watershed

Located in Charlotte, VT at the southern end of Chittenden County. Slightly west of Route 7.

Approximately 1800 ha or 7 sq. miles

Flows into Lake Champlain

Thorp BrookThorp Brook Located within the Lewis Creek watershed Directly flows into Lake Champlain Three main types of natural habitats

– Valley Clayplain Forest– Lakeside Floodplain Forest– Wetland of Deep Bulrush marsh, Buttonbush

swamp and Shallow Emergent marsh

Riverbed contains sand and gravel Provides habitat to for six freshwater

mussel species

Life Cycle of Freshwater Life Cycle of Freshwater MusselsMussels

Adult mussels mate in

summer/autumn

Glochidia will attach to host fish and act as blood parasite

The juvenile mussel will drop and spend years growing as an adult in the riverbed

Endangered SpeciesEndangered Species

Pink Heelsplitter: Host fish could be

the Freshwater drum

Pocketbook: No known host

species have been found

Threatened SpeciesThreatened Species

Giant floater: Has 4 specific host species

– Longnose gar– Blacknose dace– Blackchin shiner– Iowa darter

Benefits of Freshwater Benefits of Freshwater MusselsMussels

Increases biodiversityPurifies water as it takes up food with a

siphonCan store nutrients and toxins in shells Great water quality

indicator

Problem:Thorp Brook, which directly connects to

Lake Champlain, will soon see an increase in residential development near its riverbanks

Question: Are there alternative ways in reducing

stormwater runoff to protect the endangered freshwater mussel species?

Goal Assess potential effects

associated with excess storm water runoff in order to identify alternative ways in reducing storm water runoff to protect freshwater mussels

Objective Assess how much

additional storm water runoff will occur if residential development is constructed using TR55

GIS MethodsGIS Methods

In order to obtain all of the data to be used by TR55 for the various development scenarios, several GIS processing steps had to be performed.

All of the data was collected from the Vermont Center for Geographic Information except for the ortho photos, which are from the UVM GIS drive.

GIS MethodsGIS Methods

After all the data was collected, the main Thorp Brook watershed was delineated using the ArcHydro extension, which was also used to create the subwatersheds.

After the main watershed was created, the stream layer, the land use layer, and the soil layer were clipped to the watershed’s boundary.

GIS MethodsGIS Methods

Using the Clip function in ArcMap, we were able to pare down the soils information for each subarea in order to determine the dominant hydrogroup.

Then the raster calculator function was used to obtain the landuse data for each of the subwatersheds.

All of the pertinent info was then plugged into TR55.

SubwatershedsSubwatersheds

Divided into 6 subwatersheds based on stream reach delineation and soil classes.

Proposed development lies in Areas 4 and 5

Thorp Brook SoilsThorp Brook Soils

Soils in the main watershed are primarily in the D hydrogroup.

All soils in the proposed development are class D.

HydroGroup Area (m^2) Area (ha)

A 104821 10.4821

B 564471 56.4471

C 5165841 516.5841

D 12355183 1235.518

Thorp Brook SoilsThorp Brook Soils

Most areas in the watershed do not experience flooding, but there are some places near the proposed development that experience frequent and occasional flooding.

Flooding Frequency Area (m^2) Area (ha)

None 17970177 1797.018

Occasional 197201 19.7201

Frequent 290811 29.0811

Thorp Brook LanduseThorp Brook Landuse

Thorp Brook LanduseThorp Brook Landuse

Code Count Area (m^2) Area (ha) Category

3 30 27000 2.7 Brush/Transitional

5 901 810900 81.09 Water

11 1037 933300 93.33 Residential

12 91 81900 8.19 Commercial

14 1167 1050300 105.03 Transportation

17 7 6300 0.63 Other Urban

22 58 52200 5.22 Orchards

24 1488 1339200 133.92 Other Ag. Land

41 2754 2478600 247.86 Deciduous Forest

42 729 656100 65.61 Coniferous Forest

43 2032 1828800 182.88 Mixed Forest

211 3462 3115800 311.58 Row Crops

212 5265 4738500 473.85 Hay/Pasture

TR55 AnalysisTR55 Analysis

Comparative hydrology between development scenarios

Four scenarios– Current– 21 ½-acre lots– 21 1-acre lots– 21 2-acre lots

AssumptionsAssumptions

Subarea Characteristics– Soil class of each subarea– Development lies within subarea 5– Development converts pasture land to residential land

Channel Characteristics– Manning’s Roughness Coefficient = 0.02– Bottom width = 4 m (Reach 6); 2 m (Reaches 2 & 5)– Average side slope = 5:1

Current ScenarioCurrent Scenario Six Subareas Current landuse types Normalized peak flow

= 0.003 mm/s

Four scenarios identical

Explanation

–4% subarea 5 in question

–CN’s only vary between 74 and 76

Localized EffectsLocalized Effects 21 1-acre lots 8.5 hectares Normalized peak flow

= 0.013 mm/s 5-year storm

FindingsFindings

Negligible effect on Thorp BrookLocalized Impacts

Potential Negative Cumulative Effects

Potential Risks Associated Potential Risks Associated with Developmentwith Development

Negative Effects on Mussels– Sedimentation – Decreased DO due to increased turbidity– Habitat Loss

Other Effects– Fish habitat– Geomorphology– Terrestrial habitat loss

RecommendationsRecommendations

During Construction– Maintain separation between construction and Thorp

Brook– Install silt fences– Seasonal variations in soil stability could compound to

yield increased sedimentation

General Recommendations– Leave 100-ft natural vegetation buffer – Stormwater management practices

AcknowledgementsAcknowledgements

Marty Illick, Lewis Creek AssociationErica Gaddis, ENSC 202 InstructorBreck Bowden, ENSC 202 InstructorKevin Behm, Addison County Regional

Planning CommissionChet Hawkins, N. Ferrisberg Zoning

Administrator