Executive Summary - Watershed Protection and Flood Mitigation Project Phase II- Technical Analysis
Prepared by: Center for Watershed Protection, Aquafor Beech LTD., Lori Barg, and Robert Kort
This report is the second in a multi-phase project being undertaken by the State of Vermont, Agency of Natural Resources (ANR). Phase I of the project involved conducting a literature search and providing discussion and assessment of the impacts of land use change on stream ecology and how levels of change to a stream's hydrology and morphology affect aquatic ecosystems.
To help quantify the relationships between stream geomorphology and land use activities for Vermont conditions and to provide a technical foundation for possible future guidance governing stormwater management runoff control for growing watersheds, ANR commissioned this study under Phase II, Technical Analysis of the project. It is anticipated that Phase III of the project will involve the development of a stormwater management guidance manual for the State of Vermont and Phase IV will involve training and education on the implementation of the guidance.
ANR's goal for this phase of the project was to determine, in Vermont, the type and size of watershed hydrologic and geomorphic impact that could result from various watershed land use activities including, watershed development in the natural floodplain, various levels of urbanization, and logging activities.
This report documents multiple lines of evidence used to assess the above goal. The study methodology incorporated several complimentary components to derive relationships between and among the watershed land use activities and stream system health. The study methodology incorporated the following analyses in descending order of significance:
A total of 8 subwatersheds were investigated as part of the study. Data were collected in the field at 24 separate stream sampling locations (approximately 3 sampling locations per subwatershed). Land use data were provided by the Vermont Center for Geographic Information (VCGI) at the University of Vermont in combination with aerial photography obtained from the Vermont Mapping Program. Biological monitoring data were provided to the project team by the Vermont Agency of Natural Resources, Biomonitoring and Aquatic Studies Section.
Background on the Scope of the Study:
The first component of the study was to validate the empirical relationship of channel enlargement (as measured by cross-sectional area) as a function of total watershed impervious cover. Past investigations have found that channel enlargement is a function of basin imperviousness as well as the corresponding age of that impervious cover. This relationship can be defined by the function:
where, Re is defined as the channel enlargement ratio, 'A' represents the cross-sectional area of the stream channel and the subscripts BFL, POST, and PRE refer to the bankfull stage, the post-disturbance condition, and pre-disturbance condition, respectively.
The age of the development is also a critical variable in the amount of channel enlargement. In general, the longer a channel is exposed to the forces causing accelerated channel erosion, the larger the channel cross-sectional area. The effect of the age of development is represented by the concept of a "relaxation period." This is defined as the period of time required for a channel to reach an "equilibrium" state in concert with the level of watershed alteration, where the channel erosion processes are in a relative balance with the watershed forces causing erosion.
The results of past investigations for channel enlargement and channel relaxation show strong correlations with basin imperviousness. The equation derived from past investigations for alluvial type (AL-Type) streams for the ultimate channel enlargement ratio is defined as:
(Re)ULT= 0.00135(TIMP)2 + 0.0167(TIMP) + 1.0
R2 = 0.78, (n=38)
where, (Re)ULT is defined as the channel enlargement ratio once a stream is in equilibrium with its watershed hydrologic parameters, and TIMP is the total basin impervious cover, in percent. Note that the square of the correlation coefficient shows a very strong relationship between basin imperviousness and channel enlargement for the 38 sites investigated.
The hypothesis being tested in this part of the study was to evaluate the cross-sectional area to impervious cover relationship for eight Vermont watersheds and statistically compare the findings with those of previous investigations. If it could be shown that channel enlargement ratios for Vermont streams were drawn from the same population as channel enlargement ratios for non-Vermont streams then the existing relationships could be used to help predict and assess stream morphological impacts associated with different land use modifications.
The second component of the investigation utilized a rapid geomorphic assessment (RGA) technique to define the current stability of stream channels. The technique used a number of visually observed factors to provide a semi-quantitative assessment of a stream's current stability, referred to as the stability index (SI). The primary purpose of the RGA was to corroborate the findings of the more quantitative channel enlargement assessment and to help define past or current modes of channel adjustment (i.e., aggradation, degradation, widening and/or plan form adjustment). The RGA notes whether change in channel form has occurred or is still occurring, however, it does not provide a measure of the rate of change.
The third level of investigation involved the comparison of previously collected biological monitoring data with the corresponding level of impervious cover. The Vermont Agency of Natural Resources, Biomonitoring and Aquatic Studies Section and the Vermont Department of Fish and Wildlife provided the project team with macroinvertebrate and fish biological monitoring data covering a twelve year period (1986-1998). This analysis was intended to support the more quantitative geomorphological investigation of channel enlargement and channel stability and was not intended as a statistical evaluation of Vermont biological monitoring data.
The final element of the study involved comparing stream channel riparian cover length for each of the selected streams to assess whether or not riparian cover length was a factor in overall physical or biological condition. The methodology utilized aerial photography to estimate the extent of forest buffers in each subwatershed. The extent of the buffer was defined as the length of the forest buffer divided by the total stream length.
The project team employed a ten step methodology to collect and analyze the data. As stated above, data were collected in eight Vermont subwatersheds. Table E.1 presents the basic project methodology.
The first step was to select an initial candidate list of subwatersheds that met a range of land uses, had past biological monitoring data, and likely had historic stream cross-sectional surveys data (for estimating the pre-disturbance bankfull area, (ABFl)PRE). Next, a data collection effort was conducted to obtain past biomonitoring information, historic cross-sectional information, and current and past land use information. Candidate sites were then field reviewed to eliminate those where possible conflicts existed. The final selection of subwatersheds and streams involved input from the Project Steering Committee and included reference subwatersheds, subwatersheds with a range of urban/suburban development densities, a subwatershed where recent logging activity had occurred, and two subwatersheds where upland development was present. Table E.2 lists the final subwatersheds selected for data collection and assessment.
1 subwatershed impervious cover and drainage area at downstream most sampling location
2 impervious cover estimate includes an "equivalent" impervious value
ABFL= Bankfull channel cross-sectional area; tI = area weighted average age of disturbance;
TIMP = Total Basin Imperviousness; (Re)i = Enlargement Ratio at time tI(i.e., current cross-section);
[(Re)ULT]OBS = Ultimate channel Enlargement Ratio, based on observed survey data;
(ABFL)PRE = Pre-disturbance channel bankfull channel cross-sectional area
The original channel enlargement curve for alluvial type streams was revised by integrating the Vermont data into the original database and undertaking a curve fitting process. The following second order polynomial provided the best fit for the data:
Revised Equation for Channel Enlargement Incorporating Vermont Data
(Re)ULT = 0.0013(TIMP)2 + 0.0168(TIMP) +1.0
(R2 =0.83, n= 52)
Channel Stability Assessment
Results of the channel stability assessment are presented in Table E.4. The RGA process was originally developed for application in older urban watersheds that had been under riparian vegetation management programs and, consequently, largely denuded of wooded species. As such, metrics indicative of early geomorphic alteration were not incorporated into the original RGA Protocol. In consideration of the above, a modified RGA protocol was developed for Vermont to include the additional parameters: the number of Large Organic Debris pieces (NLOD) observed within the channel and riparian zone, the number of debris jams (NJAMS) and the number of complete riffle lines (NRIFF). The results are contained within the modified RGA data presented in Table E.4.
(1) SI = Modified Stability Index for Vermont Conditions
AI = Aggradation Factor; DI = Degradation Factor;
WI = Widening Factor; PI = Planimetric Adjustment Factor;
n/a = not available; AL = Alluvial; Ar = Armored; RB = Rock Bed with alluvial banks;
The RGA protocol was applied to 23 sites surveyed in this study, with the exception of Site DOW3, A simple linear correlation analysis was undertaken relating the Stability Index to Total Basin Imperviousness (TIMP) for 20 of the 23 sites (W. Branch of Little River was excluded from the analysis because of past gravel mining operations) as follows:
SI = 0.158(TIMP)0.413, R= 0.75, n = 20
The above relation was found to be statistically significant at the 95% confidence level for variance and mean.
Biological Monitoring Analysis
Table E.5 lists a generalized assessment of the biological monitoring data for the nine Vermont streams evaluated in this. The results suggest that these Vermont streams can be related to their contributing impervious cover and fall into one of two categories. The generally "good" streams, from a biological community assessment perspective, fall into an impervious cover range of 6% and less. The "poor" streams have impervious cover of 12% or greater.
* represents an average of all biomonitoring presented in Table 4.1
Riparian Cover Analysis
The results of the riparian cover analysis are presented in Table E.6. Forest buffers were identified based on aerial photography for each watershed A simple methodology was used to estimate the extent of forest buffers in each subwatershed. The extent of the buffer was defined as the length of the forest buffer divided by the total stream length. The criteria used to determine the length of stream and buffer were:
Based on methodology performed, the results presented in Table E.6 yield no conclusive results to suggest that the extent of riparian cover has an undue influence on biological or physical stream quality. It should be noted that the assessment was conducted for only those streams with measurable development within a fairly modest range of impervious cover (~6 to 22%).
The methodology and data analyses support a suite
of conclusions on the findings of this study. The project team identified
the following six major conclusions as a result of our work on the geomorphological,
and biological assessments:
An Enlargement Ratio equation and curve developed using stream geomorphological data from outside of Vermont was tested for inclusion with data from the Vermont streams investigated in this study and found to be statistically valid for the total population of data-points. This conclusion supports that there is now a statistically valid tool for Vermont conditions to help predict channel enlargement as a function of watershed imperviousness.
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