100-year Event Analyses of Storm Sewersand Overland Flow Considerations as

Applied in Chapter 9 of theCity of Houston Design Manual

Prepared for the HCEC Drainage Committee

January 6, 2005

2

1. Chapter 9 – Storm Water Design Requirements

2. Comments Received

3. Overland Flow Analysis

INTRODUCTIONINTRODUCTION

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Chapter 9 – Stormwater Design RequirementsRevised Criteria

1. Detention• All calculations based on increased impervious area (IIA)• <1 ac; 0.2 ac-ft/ac IIA; exempt single family residential <15K sf• 1ac to 50 ac; 0.5 ac-ft/ac IIA; release at 0.5 cfs/ac, 2 cfs/ac, 2 cfs/ac (100 yr)• >50 ac; comply w/HCFCD

2. Overland (Sheet) Flow for Extreme Event (100-yr)• HGL at or below natural ground at ROW• Max ponding of 6-inch above curb at high point (current criteria)• Max ponding of 18-inch above curb at low point (current criteria)• Claification/emphasis – no change to existing sheet flow patterns; no

discharge to abutting private property

3. HouStorm• Required analysis method for CIP projects and City funded within City limits • Privately funded in City limits and EJT; HouStorm preferred but not required

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Comment: There is no problem that needs addressing.

Response: The City has determined, as a policy, that more stringent storm water management practices are needed to provide an increased level-of-service to our citizenry.

5

Comment: This criterion and procedure provides no benefit.

Response: Benefit is deemed by the City to include reductions in water surface elevations and the proper management of overland flow during an extreme event, which this criterion addresses. Structural flooding, or the lack thereof, is not the only issue at hand.

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Comment: All you are doing is sizing the storm sewers for the 100-year flow.

Response: This is an incorrect statement. The storm sewer remains sized for a 2-year event. The City criteria simply requires that the storm sewer system be analyzed for behavior under the 100-year event in conjunction with due consideration of overland flow.

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Comment: Increases in head on a storm sewer does nothing to increase the flow. Once the storm sewer reaches its 2-year capacity it will not receive any more flow.

Response: If all conduit conditions remain unchanged, an increase in driving head will result in an increased flow through the conduit. Conversely, an increase in conduit size or reduction in friction loss will result in a decrease in driving head (i.e. upstream water surface elevations).

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Comment: Overland flow is completely ignored and by imposing the concepts applied within TP-101 we are eliminating the consideration of overland flow.

Response: The consideration of overland flow is particularly referenced as a critical element in the design process. TP-101 simply provides a simplified means of adjusting anticipated maximum water surface elevations via adjustments to the storm sewer HGL. Further requirements (i.e. a plotted hydraulic profile) exemplify the study and consideration of overland flow pathways for project areas.

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Comment: No provision was made in TP-101 to address detailed overland flow determination procedures.

Response: The intent of TP-101 is to provide a simplified manner of controlling anticipated maximum water surface elevations. The paper specifically addresses that more elaborate means of analyzing overland flow may be invoked if deemed warranted; however, it is beyond the subject matter of this paper to detail such an analysis process.

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Comment: Areas that are inundated by floodwaters or have high existing 25-year tailwater elevations can never be developed or redeveloped.

Response: In project situations where the 100-year water surface elevation, or less (i.e. 50 or 10-year), already inundates the project area(s), for whatever reason, it is not the intent of these guidelines or this criterion to require the design engineer to resolve problem areas beyond their reasonable control. Chapter 9 stipulates this criterion and allows the submittal of documentation demonstrating this project situation. This applies to cases where lower starting water surface elevations for the 100-year analysis are justified.

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Comment: This criterion and procedure adds too much cost to the project.

Response: Case studies have indicated that the construction cost increases for private projects have ranged from 0 to 11% in storm sewer cost only. The resulting total project cost would be less.

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Comment: This criterion and procedure adds too much additional design time.

Response: The additional design time has been determined to be very minor based upon all test cases.

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ABC Project ProfileA

1

A2

A3

A7

A8

A9

A10

A11

A12

OU

T

PO

ND

A4

78

82

86

90

94

98

-50 150 350 550 750 950 1150 1350

Primary Flowpath (ft)

Hei

gh

t (f

t)

Natural Ground

Top of Curb

Gutterline

2-yr. HGL

100-yr. HGL

100-yr. WSEL

25-yr. WSEL

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Project ABC Storm Sewer Cost

Project ABC

Original Design

Pipe Diameter (ft) Length of Pipe Cost per foot Total Cost

2 925 $46.00 $42,550.00

2.5 493 $56.00 $27,608.00

      $70,158.00

   

Altered Design

Pipe Diameter (ft) Length of Pipe Cost per foot Total Cost

2 925 $46.00 $42,550.00

2.5 493 $56.00 $27,608.00

      $70,158.00

Change in Storm Sewer Cost 0.00%

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LMN Project ProfileA

1

A3

A4

A5

A6

A7

A8

A9

A1

0A

11

A1

2

A1

3

A1

5

I15

1O

UT

PO

ND

A2

A3

6

A3

4

A3

5A

14

30

35

40

45

50

-50 450 950 1450 1950 2450

Primary Flowpath (ft)

Hei

gh

t (f

t)

Natural Ground

Top of Curb

Gutterline

2-yr. HGL

100-yr. HGL

100-yr. Altered

100-yr. WSEL

25-yr. WSEL

16

Project LMN Storm Sewer CostProject LMN

Original Design

Pipe Diameter (ft) Length of Pipe Cost per foot Total Cost

2 3898 $46.00 $179,308.00

4 2050 $110.00 $225,500.00

5 254 $144.00 $36,576.00

3 220 $73.00 $16,060.00

5.5 176 $176.00 $30,976.00

      $488,420.00

   

Altered Design

Pipe Diameter (ft) Length of Pipe Cost per foot Total Cost

2 2916 $46.00 $134,136.00

2.5 982 $56.00 $54,992.00

4 582 $110.00 $64,020.00

4.5 1468 $115.00 $168,820.00

5 195 $144.00 $28,080.00

5.5 235 $176.00 $41,360.00

3 220 $73.00 $16,060.00

      $507,468.00

Change in Storm Sewer Cost 3.90%

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XYZ Project Profile A

1

A2

A3

A4

A5

A11

A18

OU

T

PO

ND

A12

A17

60

65

70

75

80

85

90

-50

50

15

0

25

0

35

0

45

0

55

0

65

0

75

0

85

0

95

0

Primary Flowpath (ft)

Hei

gh

t (f

t)

Natural Ground

Top of Curb

Gutterline

2-yr. HGL

100-yr. HGL

100-yr. WSEL 25-yr.

WSEL

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Project XYZ Storm Sewer Cost

Project XYZ

Original Design

Pipe Diameter (ft) Length of Pipe Cost per foot Total Cost

2 1660 $46.00 $76,360.00

2.5 31 $56.00 $1,736.00

3 217 $73.00 $15,841.00

      $93,937.00

   

Altered Design

Pipe Diameter (ft) Length of Pipe Cost per foot Total Cost

2 1660 $46.00 $76,360.00

2.5 31 $56.00 $1,736.00

3 217 $73.00 $15,841.00

      $93,937.00

Change in Storm Sewer Cost 0.00%

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QRS Project ProfileA

1

A3

A5

A10

A11

A12

A13

A14

OU

TP

ON

D

A2

A6

45

50

55

60

65

70

-50 150 350 550 750 950 1150

Primary Flowpath (ft)

He

igh

t (f

t)

Natural Ground

Top of Curb

Gutterline

2-yr. HGL

100-yr. HGL

100-yr. Altered

100-yr WSEL 25-yr

WSEL

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Project QRS Storm Sewer CostProject QRS

Original Design

Pipe Diameter (ft) Length of Pipe Cost per foot Total Cost

2 1171 $46.00 $53,866.00

2.5 254 $56.00 $14,224.00

3 50 $73.00 $3,650.00

Inlets - total      

15   $1,500.00 $22,500.00

Manholes total      

14   $1,700.00 $23,800.00

  $118,040.00

Altered Design

Pipe Diameter (ft) Length of Pipe Cost per foot Total Cost

2 611 $46.00 $28,106.00

2.5 560 $56.00 $31,360.00

3 304 $73.00 $22,192.00

Inlets - total      

15   $1,500.00 $22,500.00

Manholes total      

14   $1,700.00 $23,800.00

  $127,958.00

Change in Storm Sewer Cost 8.40%

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PQR Project ProfileA

1

A3

A6

A8

A9

OU

T

PO

ND

A2

A7

45

50

55

60

65

70

-50 50 150 250 350 450 550 650 750

Primary Flowpath (ft)

He

igh

t (f

t)

Natural Ground

Top of Curb

Gutterline

2-yr. HGL

100-yr. HGL

100-yr WSEL 25-yr

WSEL

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Project PQR Storm Sewer CostProject PQR

Original Design

Pipe Size (ft) Length of Pipe Cost per foot Total Cost

2 RCP 795 $46.00 $36,570.00

3 x 5 RCB 101 $206.00 $20,806.00

Inlets - total      

8   $1,500.00 $12,000.00

Manholes total      

9   $1,700.00 $15,300.00

  $84,676.00

   

Altered Design

Pipe Size (ft) Length of Pipe Cost per foot Total Cost

2 RCP 795 $46.00 $36,570.00

3 x 5 RCB 101 $206.00 $20,806.00

Inlets - total      

8   $1,500.00 $12,000.00

Manholes total      

9   $1,700.00 $15,300.00

  $84,676.00

Change in Storm Sewer Cost 0.00%

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JKL Project Profile A

1

A2

A3

A4

A5

A7

A8

OU

T

A6

PO

ND

30

35

40

45

-50 50 150

250

350

450

550

650

750

850

950

1050

Primary Flowpath (ft)

He

igh

t (f

t)

Natural Ground

Top of Curb

Gutterline

2-yr. HGL

100-yr. HGL

100-yr. Altered

100-yr. WSEL 25-yr.

WSEL

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Project JKL Storm Sewer Cost

Project JKL

Original Design

Pipe Diameter (ft) Length of Pipe Cost per foot Total Cost

2 922 $46.00 $42,412.00

2.5 233 $56.00 $13,048.00

3 354 $73.00 $25,842.00

      $81,302.00

   

Altered Design

Pipe Diameter (ft) Length of Pipe Cost per foot Total Cost

2 686 $46.00 $31,556.00

2.5 236 $56.00 $13,216.00

3 411 $73.00 $30,003.00

3.5 176 $88.00 $15,488.00

      $90,263.00

Change in Storm Sewer Cost 11.02%