Guidelines for use of SWMM in rural areas

Prepared by:
Michael F. Schmidt, P.E. BCEE
Vice President
CDM Smith
Jacksonville, Florida

CDM Smith has used SWMM extensively for rural areas in Florida (e.g., Tomoka River, Orlando Lake Hart, Big Slough, North Fork St Lucie River, Anclote River, Lower St Johns River tributaries, Munson Slough, and several water control district agricultural canal systems). The following descriptions come from our University of Florida TREEO SWMM Class (CDM Smith, 1991 - 2002). Here is some guidance in the application:

1) Two runoff planes are simulated for each catchment (basin or hydrologic unit):

the impervious runoff plane based on the directly connected impervious area (DCIA) percentage to represent existing wetlands and water, and
the pervious (and NDCIA) runoff plane (which can be modeled using Horton or Green-Ampt methods). Often NDCIA is a small percentage of the total and can be neglected or accounted for in the weighted soils parameters.

Here are some typical ranges of Horton parameters based on multiple gage calibration:

Final constant infiltration rate, f_c (inches/hour)

15 - 0.30

2 - 0.15

0.25 - 0.05

0.10 – 0

Initial infiltration rate, f_o (inches/hour)

8 - 25

6 - 10

3 - 7

1 – 5

Shape factor, k for infiltration curve (1/hour)

2 - 5

2 – 5

A shape factor of 2/hour is often used in Florida sandy soils.

Typical Ranges of Value

Available storage capacity soil mantle S (inches) for four Antecedent Moisture Conditions (AMCs)

Soil Group

Dry, Condition 1

12.0 - 4.3

9.0 - 3.4

7.0 – 2.3

3.0 - 1.3

Normal, Condition 2

5.4 - 3.4

4.0 - 2.8

3.0 - 1.8

1.3 - 1.0

Wet, Condition 3

2.1 - 1.8

1.5 - 1.3

1.0 - 0.8

0.7 - 0.5

Saturated, Condition 4

This soil storage S can be related to Curve Number (CN) if desired using the standard SCS equation S= (1000/CN)-10.

Defined Antecedent Moisture Conditions (AMCs) for Pervious Areas

Condition

Description

Total rainfall during 5 days preceding storm (inches)

Dry

<1.4

Normal

1.4 - 2.1

Wet

>2.1

Saturated

Saturation

2) Overland flow paths should be estimated using two to three alternate paths and weighted for representative area (fraction of the catchment), overland flow slope, and Manning n roughness. The overland flow paths should reflect the average area-weighted path (which is not the hydraulically most distant path for the entire catchment); hence the multiple paths to reflect the average area weighted value.

Manning n roughness values can vary from 0.1 to 0.6 and higher (pine palmetto flatwoods or other rough surface) since the depth of overland flow are often much less than the roughness feature height.

We have used overland flows paths from 100 to 3,000 feet depending on catchment size. Width is calculated from the digitized areas and the estimated weighted overland flow length using the equation: A= L*W. The resultant width should be checked for general physical equivalence after the calculation and the overland flow paths refined if necessary.

Here is an example table for Manning n for overland flow:

Factors Used For Calculating The Standard Infiltration Curves For Pervious Areas

Source

Ground Cover

Range

Crawford and Linsley (1966)^a

Smooth asphalt

0.012

Asphalt of concrete paving

0.014

Packed clay

0.03

Light turf

0.20

Dense turf

0.35

Dense shrubbery and forest litter

0.4

Engman (1986)^a

Concrete or asphalt

0.011

0.01-0.013

Bare sand

0.01

0.01-0.016

Graveled surface

0.02

0.12-0.03

Bare clay-loam (eroded)

0.02

0.12-0.033

Range (natural)

0.13

0.01-0.32

Bluegrass sod

0.45

0.39-0.63

Short grass prairie

0.15

0.10-0.20

Bermuda grass

0.41

0.30-0.48

^aObtained by calibration of Stanford Watershed Model.
^bComputed by Engman (1986) by kinematic wave and storage analysis of measured rainfall-runoff data.

3) Initial abstractions for pervious areas can range from 0.2 inches to approximately 0.2*S (SCS NEH 4 Hydrology, 1985).

4) Sensitivity analyses should be considered for parameters which are less known (e.g., slope based on USGS 5 foot contour maps or soils storage based on soils type and depth to water table). Previous sensitivity analyses have indicated that for a given rainfall event and area, the most dominant parameters are DCIA, and then soils parameters and slope (although slopes in Florida are generally moderate to flat).

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