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REFERENCE DOCUMENTS

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TABLE OF CONTENTS

Low Impact Development Technical Design Manual

REFERENCE DOCUMENTS

REFERENCE DOCUMENT A: MEAN SEASONAL PRECIPITATION MAP R‐1

REFERENCE DOCUMENT B: K FACTOR R‐2

REFERENCE DOCUMENT C: RUNOFF COEFFICIENT – C FACTOR R‐3

REFERENCE DOCUMENT D: HYDROLOGIC SOIL GROUPS R‐-4

REFERENCE DOCUMENT E: SAMPLE STRUCTURAL SOIL SPECIFICATION R‐5

REFERENCE DOCUMENT F: STRUCTURAL SOIL REFERENCE INFORMATION R‐12

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REFERENCE DOCUMENT A

City of Santa Rosa and County of Sonoma

MEAN SEASONAL PRECIPITATION MAP

REFERENCE DOCUMENT B


K FACTOR

2.5

2.0

1.5

1.0

0.5

2.5

2.0

1.5

1.0

0.5

10 20 30 40 50 60 70 80

PROVIDED BY THE SONOMA COUNTY WATER AGENCY

REFERENCE DOCUMENT B

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K

FACT

OR

MEAN SEASONAL PRECIPITATION - INCHES

K = MEAN SEASONAL PRECIPITATION DIVIDED BY 30 ”

REFERENCE DOCUMENT C


RUNOFF COEFFICIENT‐ C FACTOR

6 Runoff Reduction Areas 6.1 Design Philosophy Using alternative surfaces with a lower coefficient of runoff or “C-Factor” helps reduce runoff from developed areas. The C-Factor is a representation of a surface’s ability to produce runoff. Surfaces that produce higher volumes of runoff are represented by higher C-Factors, such as impervious surfaces. Surfaces that produce smaller volumes of runoff are represented by lower C-Factors, such as more pervious surfaces. See Table 6-1 for typical C-Factor values for various surfaces during small storms.

Table 6-2 compares the C-Factors of conventional paving surfaces to alternative, lower C-Factor paving surfaces. By incorporating more pervious, lower C-Factor surfaces into a development, lower volumes of runoff are produced. Lower volumes and rates of runoff translate directly to lower treatment requirements.

Site design techniques may be used to reduce the C-Factor of a developed area, reducing the amount of runoff requiring treatment, including:

Pervious Concrete

Pervious Asphalt

Turf Block

Brick (un-grouted)

Natural Stone

Concrete Unit Pavers

Crushed Aggregate

Cobbles

Wood Mulch

Other site design techniques such as disconnecting impervious areas, preservation of natural areas, and designing concave medians may be used to reduce the overall C-Factor of development areas.

Table 6-1 Estimated C-Factors for Various Surfaces During Small Storms

Note: C-Factors for frequent small storms used to size water quality BMPs are likely to differ (be lower) than C-Factors developed forinfrequent, large storms used to size flood control facilities. The above C-Factors were produced by selecting the lower end ofthe best available C-Factor range for each paving surface. These C-Factors are only appropriate for small storm treatment design, and should not be used for flood control sizing. Where available, locally developed small storm C-Factors for various surfaces should be utilized.

rotcaF-C ecafruS gnivaP

08.0 etercnoC 07.0 tlahpsA 06.0 etercnoC suoivreP 06.0 selbboC 55.0 tlahpsA suoivreP 52.0 tuorG tuohtiw enotS larutaN 51.0 kcolB fruT 31.0 tuorG tuohtiw kcirB 01.0 dnaS no srevaP tinU 01.0 etagerggA dehsurC 01.0 ssarG 50.0 citsalP suoroP revO ssarG 50.0 citsalP suoroP revO levarG

Provided by BASMA

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REFERENCE DOCUMENT C

REFERENCE DOCUMENT D


HYDROLOGIC SOIL GROUPS

Exhibi t A: Hydr ologi c Soi l Gr oups for t he Uni t ed St at esAppendix B Synt het ic Rainfal l Dist r ibut ions andRainfal l Dat a Sources

The highest peak discharges from small watersheds in the United States are usually caused by intense, brief rain-falls that may occur as distinct events or as part of a longer storm. These intense rainstorms do not usually ex-tended over a large area and intensities vary greatly. One common practice in rainfall-runo� analysis is to developa synthetic rainfall distribution to use in lieu of actual storm events. This distribution includes maximum rainfallintensities for the selected design frequency arranged in a sequence that is critical for producing peak runo�.

Syn t h et i c r a i n f a l l d i st r i b u t i o n s

The length of the most intense rainfall period contributing to the peak runo� rate is related to the time of concen-tration (T c) for the watershed. In a hydrograph created with NRCS procedures, th

Hydrologic Soi l Groups

Soils are classi�ed into hydrologic soil groups (HSG ’s)to indicate the minimum rate of in�ltration obtained forbare soil after prolonged wetting. The HSG ’s, which areA, B, C, and D, are one element used in determiningruno� curve numbers (see chapter 2). For the conve-nience of TR-55 users, exhibit A-1 lists the HSG classi�-cation of United States soils.

The in�ltration rate is the rate at which water enters thesoil at the soil surface. It is controlled by surface condi-tions. HSG also indicates the transmission rate —the rateat which the water moves within the soil. This rate iscontrolled by the soil pro�le. Approximate numericalranges for transmission rates shown in the HSG de�ni-tions were �rst published by Musgrave (USDA 1955).The four groups are de�ned by SCS soil scientists asfollows:

Group A soils have low runo� potential and high in�l-tration rates even when thoroughly wetted. They consistchie�y of deep, well to excessively drained sand orgravel and have a high rate of water transmission(greater than 0.30 in/hr).

Group B soils have moderate in�ltration rates whenthoroughly wetted and consist chie�y of moderatelydeep to deep, moderately well to well drained soils withmoderately �ne to moderately coarse textures. Thesesoils have a moderate rate of water transmission (0.15-0.30 in/hr).

Group C soils have low in�ltration rates when thor-oughly wetted and consist chie�y of soils with a layerthat impedes downward movement of water and soilswith moderately �ne to �ne texture. These soils have alow rate of water transmission (0.05-0.15 in/hr).

Group D soils have high runo� potential. They havevery low in�ltration rates when thoroughly wetted andconsist chie�y of clay soils with a high swelling poten-tial, soils with a permanent high water table, soils with aclaypan or clay layer at or near the surface, and shallowsoils over nearly impervious material. These soils have avery low rate of water transmission (0-0.05 in/hr).

In exhibit A-1, some of the listed soils have an addedmodi�er; for example, “Abrazo, gravelly. ” This refers toa gravelly phase of the Abrazo series that is found inSCS soil map legends.

Di st u r b ed so i l p r o f i l es

As a result of urbanization, the soil pro�le may be con-siderably altered and the listed group classi�cation mayno longer apply. In these circumstances, use the follow-ing to determine HSG according to the texture of thenew surface soil, provided that signi�cant compactionhas not occurred (Brakensiek and Rawls 1983).

HSG Soi l textures

A Sand, loamy sand, or sandy loam

B Silt loam or loam

C Sandy clay loam

D Clay loam, silty clay loam, sandy clay, siltyclay, or clay

Dr ai n age an d gr o u p D so i l s

Some soils in the list are in group D because of a highwater table that creates a drainage problem. Once thesesoils are e�ectively drained, they are placed in a di�er-ent group. For example, Ackerman soil is classi�ed asA/D. This indicates that the drained Ackerman soil is ingroup A and the undrained soil is in group D.

REFERENCE DOCUMENT D

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REFERENCE DOCUMENT E


SAMPLE STRUCTURAL SOIL SPECIFICATIONS

CU-Soil Specifi cation and Mixing Procedure

CU-Soil is a patented material and must be purchased from a licensed supplier. Amereq (http://www.amereq.com/) licenses the manufacturing of CU-Soil to ensure quality control of installations.

REFERENCE DOCUMENT E

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