> What is Directly Connected Impervious Cover?
Directly Connected Impervious Cover, abbreviated DCIC, is synonymous with Directly Connected Impervious Area (DCIA).
Paraphrased from the US Environmental Protection Agency, DCIA is: the portion of impervious area with a direct hydraulic connection to the discharge point in question or a waterbody via: continuous paved surfaces; gutters; drain pipes; or other conventional conveyance and detention structures that do not reduce runoff volume. (Source: http://goo.gl/o8Fvns)
The connectivity of impervious cover plays a major role in impervious flow.
> Is this parameter a % of total land cover, or a % of % impervious cover?
This parameter is a % of a %. The value you input indicates how much of the impervious cover has the special property of being 'directly connected' to stream flow.
For example: your land covers total to 100%. Impervious cover is 40% of the total land cover. If you set DCIA to 80%, it means that 80% of the 40% impervious cover found in the entire project is directly connected to stream flow. In other words, 32% of the total land cover is directly connected impervious area (based on 0.4*0.8=0.32).
> Can DCIA be greater than Impervious Cover (IC)?
DCIA as a percentage can be greater than the percent amount of IC. In reality DCIA is a fraction of IC, meaning the absolute amount of DCIA will always be less than or equal to the absolute amount of IC.
The example given in the question above demonstrates DCIA set as a greater % value than % IC, simply meaning that the fraction of IC that is DCIA is greater than the amount of project area that is IC.
> Can you suggest a good way to determine DCIA?
The USDA Forest Service uses DCIA values based on Impervious Cover % as general estimates when modeling watersheds near major cities. Here is one example of such values derived for Deer Creek watershed (stream gauge #07010086) near St. Louis in May, 2015:
DCIA (low density) 22.2903757
DCIA (med density) 26.4845026
DCIA (high density) 34.70990685
These values are generated using Sutherland equations. Which equation should be used for a certain project is based on density estimates made when deriving land cover parameters with i-Tree Canopy. More information on the basis and use of the Sutherland equations is available in the following documents:
- R.C. Sutherland (n.d.). Methods for Estimating the Effective Impervious Area of Urban Watersheds. Retrieved October 13, 2014 from http://projects.vhb.com/stormwatersemin ... 0cover.pdf.
- US EPA (2014, April). Estimating Change in Impervious Area (IA) and Directly Connected Impervious Areas (DCIA) for New Hampshire Small MS4 Permit. Retrieved September 15, 2015 from http://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=P100ALMQ.txt.
- The default value for DCIA can be used of course if no better information exists. The three values provided above based on density estimates can serve as an alternative to the default value.
- Rough estimates can be made with local knowledge of a site and good conceptual knowledge of impervious cover and directly connected impervious area.
- An empirical method that requires only impervious cover to estimate DCIA is the Sutherland equations. This is what model developers often use for their Hydro projects. More information on this method is provided above.
- Accuracy can be refined on small areas using i-Tree Canopy surveys and breaking the impervious areas out into different types (connected roads/non-connected roads, connected roofs/non-connected roofs, etc.) The DCIA estimation is then based on local knowledge and i-Tree Canopy results.
- For a most accurate estimation of this parameter, GIS analysis would need to be done using a sewer network map, stream map, impervious cover map, and elevation information for the area.