low DCIA

Hi SWMM users,

SWMM (RUNOFF) was originally developed for urban area where infiltration is not significant. It should be cautious when it is applied to a place where the percentage of impervious area is low. If the groundwater module is not turned on, infiltrated water lost in the system maybe significant and thus leads to unrealistic calibration.

Regarding "antecedent conditions" and " leaf on/off", these features have been implemented in ILWAS (Integrated Lake-Watershed Acidification Study, developed by Tetra Tech and Systech) and IWMM (Integrated Watershed Management Model, the simulation engine of a decision support system WARMF developed by Systech). A briefly description of the hydrology module of IWMM/WARMF is attached (iwmmhydr.txt). Some approaches employed in IWMM/WARMF can be adapted to SWMM. However, I have been debating myself for expanding SWMM to a real watershed model or leaving it as an urban drainage model. Perhaps more good ideas can be developed through this discussion.

Ching L. Chen
Senior Engineer/Manager
Systech Engineering, Inc.

SWMM is capable of modeling rural, or undeveloped, water sheds. Although I agree that it is not intended for this purpose and I cannot verify its accuracy for large, undeveloped watersheds. However it is capable of simulating most of the same phenomena as IWMM/WARMF.

It is true that infiltration can be significant which raises an interesting point. I have had difficulty in the past modeling sanitary sewers. This is primarily due to the unpredictable nature of I/I, particularly rain induced infiltration. I have not had much success calibrating unit hydrographs to measured flow data for multiple events either. However, I have had very good success in calibrating sanitary sewers and I/I, including rain induced infiltration, by including the groundwater component in my simulation. This component of the RUNOFF computational engine is intended for simulating the movement of flows through the soil to a surface water body such as a stream or lake. However by manipulating the groundwater infiltration coefficients, I was able to get a good fit between measured and simulated flows. Has anyone else had much success simulating these phenomena?

Steve Sticklen
ssticklen@hntb.com

It's right to turn on the groundwater module when the RUNOFF Block is adapted to a place with low DCIA. However, the existing RUNOFF does not simulate the interception process of canopies. The features that "antecedent conditions" including the "leaf off" (winter) / "leaf on" and seasonally varied interception storage suggested by Mr. Don Waye of NVPDC (12/15/98) cannot be accomplished by just tuning the coefficients in groundwater module.

If the model cannot simulate a series of storm events and several continuous rain data sets, it is difficult to justify that the model is able to simulate the physical system. Particularly one time curve-fitted results can not guarantee the model calibration is realistic.

Ching Lin Chen