TMDL Masthead


Measuring Success

Is the Impervious Cover TMDL a mandate to get out the jackhammers?

We don’t think so. And, perhaps more important is that CT DEP doesn’t think so either, as evidenced by the guidance in the Eagleville TMDL final report (read excerpt) (read report). While biological integrity of the brook is the bottom-line goal, the team interprets the interim goals as related to two key watershed management concepts: impervious cover disconnection and runoff reduction (key references are in the Library).

Without the Impervious Cover TMDL target and a way to measure progress against it, this project would be a simple stormwater retrofit effort. Based on the approach outline above, the team recommends that progress be measured on three levels:

1. Amount of Impervious Cover Disconnected

What’s an IC-TMDL without an accounting of impervious cover?

The tables below summarize impervious cover data about the watershed, and about the effect that the project’s recommended stormwater practices are estimated to have on effective impervious cover. Table 1, included also in the Watershed Characterization page of this site, summarizes our estimates of existing impervious cover and the IC disconnection targets that result.  Table 2, below, shows our estimates of the changes to impervious cover resulting from implementation, both of our “Top 10” retrofits only, and of retrofits at all 51 sites.  On this strict acre-by-acre accounting level, the 11% TMDL goal appears to be achievable.

Table 1. Watershed Impervious Cover Estimates and Targets

Eagleville Brook Watershed TMDL Estimated Adjusted with updated mapping Field Adjusted
Watershed drainage area (acres) 1225 1225 1199
Watershed IC (acres) 145 218 167
Watershed IC (%) 11.8% 17.8% 13.9%
11% IC TMDL target (acres) 111 111 132
IC to disconnect/manage to reach target (acres) 34 107 35

Table 1. Existing conditions in Eagleville Brook. The original estimates were based on modeling using 2002 land cover data with 30m resolution. The second column shows additional impervious cover resulting from updates and improvements using 2008 high resolution satellite imagery. The last column includes field adjustments that decreased the watershed area by 26 acres, and "removed" 51 acres of disconnected IC.

Table 2. Estimated Result of Retrofit Implementation on Impervious Cover

Sites Impervious Area Treated (acres) Watershed IC after implementation (acres) Target IC (11% of watershed) (acres) Watershed IC after implementation (%)
Top 10 Retrofit Sites 31 136 132 11.3%
All 51 Retrofit Sites 61 106 132 8.8%

Table 2. Estimated progress toward the TMDL target of 11% IC, were recommended retrofits to be implemented. The "Top Ten" bring the watershed to 11.3% IC, essentially in compliance with the target. Implementing all 51 retrofits would exceed the target, reducing IC to just under 9%. These estimates do not factor in new IC added with additional building or renovations.

2. Runoff Volume Reduction

Stormwater researchers and managers are increasingly focusing on reduction of runoff volume as a key strategy in nonpoint source pollution control (key reference). This project hopes to look at volume several ways:

1. Monitoring: a research weir on Eagleville Brook, located just south of the main part of campus, has been renovated and daily volume measurements are being made. See information on weir.

2. Estimation using standard formulas: Estimates of runoff volume reduction for a 1” storm were made for each of the 51 retrofits. Together, they total a reduction of about 2.5 million cubic feet of water annually. See table on estimated impacts of retrofit implementation above.

3. Modeling: The UConn Civil Engineering Dept. hopes to do some simple modeling of runoff in the watershed using the SWMM model to determine the volume impact of stormwater practices, both implemented and planned.

Runoff reduction estimates are included in Table 3, below.  Also included are estimates of reduction of key pollutants, and cost estimates.

Table 3. Estimated Benefits of Project Implementation

Sites Drainage Area IC (acres) % of Watershed TP Removed (lb/yr) TN Removed (lb/yr) TSS Removed (lb/yr) Runoff Reduction (ft3/yr) Estimated Cost
Top 10 Retrofit Sites 31 2.5% 32.5 207.5 6433 797,600 $1,350,600
All 51 Retrofit Sites 61 5.1% 72.4 521.5 15030 2,494,150 $5,797,500

Table 3. Estimates of the impact of implementing the “Top 10” retrofits, and of all retrofits.  TP = total phosphorus, TN = total nitrogen, TSS = total suspended solids.  Runoff reduction is presented as cubic feet per year.  For more information on estimation methods, see the Technical Report.

3. Biological Health of the Stream

CT DEP will continue its fish and macroinvertebrate sampling program in Eagleville Brook to assess the “bottom line” of the success of the IC-TMDL effort. More information on CT DEP stream studies.


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