Stormwater study : Hazel Run watershed Spotsylvania County, Va

[From the U.S. Government Printing Office, www.gpo.gov]

FY 1992 FINAL PRODUCT Task 61
St=wtr/Flood Prog. & Ord.

C@ -

CSTORMWATER STUDY
HAZEL RUN WATERSHED
SPOTSYLVANIA COUNTY, VIRGRZA

VCRMP GRANT
NO. NA27OZO312-01

Prepared by:

J. K. Timmons & Associates, P. C.
711 N. Courthouse Road
Richmond, Virginia 23236-4099

FINAL DRAFT
December 17, 1993

Table of Contents

Executive Summary ..................... 1

Purpose .............................. 3

Watershed Description ...................... 3
Database Analysis ....................... 7

Hydrologic Analysis ...................... 9
General .......................... 9
Methodology ...................... 10

Alternative Analysis ................... 14
Alternative 1 ...................... 14
Alternative 1 Costs ................. 17
Alternative 2 ..................... 20
Alternative 2 Costs ................ - 22
Alternative 3 ...................... 23

Recommendations ..................... 26
Cn Appendix A - Main Channel Flows ......... 27
Appendix B - Subwatershed Descriptions .... 29

Appendix C - Watershed Soils ............ 33

List of Figures
VD Figure 1 Major Tributary Subwatersheds .... 6
TA-
Figure 2 Alternative 1 Improvements Locations 16
NO
Figure 3 Alternative 2 Improvements Locations2l

Figure 4 Alternative 3 Improvements Location 25

Executive Summary

Review of Problem

Spotsylvania County is looking to alleviate flooding problems in the
Hazel Run watershed. Currently, two to three major storms per year contribute
to major flooding in parts of the seven square mile watershed that includes
Spotsylvania Mall and the Route 3 corridor west of Interstate 95. Flooding in
low lying areas within the City of Fredericksburg has been identified through a
1989 study for the cit , where the major problems are thought to stem from
y
upstream development in the mall area with little or no runoff restrictions in the
past.

Spotsylvania County also lacks a defined flood plain along Hazel Run.
A defined flood plain is useful to the county in placing development restrictions
on property that may suffer property damage in a flood event. This study
includes defining main stream flood plains for that purpose.

Analysis

J.K. Timmons & Associates collected field information for the study
including aerial topography, stream surveys, and stream morphology. This
information was used to analyze the run generated by current development
levels. These flows were then.compared to the flows that would have occurred
in the Hazel Run Watershed prior to any development in the area- (called
predevelopment flows). All stormwater management structures considered
would be designed to reduce today's flows to predevelopment levels.

The flow analyses revealed that today's level of stormwate r flows has
increased over predevelopment flows by 10-15%. This relatively small increase
is due in large part to two factors, the shape of the Hazel Run watershed and
existing basins that occur within the watershed. The elongated shape of the
watershed affects the travel time for the stormwater, while existing basins, such
as the one in Carriage Hills, currently retain runoff from storms that were once
allowed to flow unimpeded.

Alternatives

Several alternatives were analyzed to reduce the rate of flow throughout
the watershed and at Hazel Run's entrance into the City of Fredericksburg. The
alternatives included main stream and non-perennial channel retention facilities.
The high cost associated with the alternatives however, together with the
relatively small reduction in runoff (10-15%) caused these alternatives to be

J. K Timmons & Associatas, P.C.

considered not cost effective.

The Altoona Subdivision located near the Spotsylvania Mall outfall
channel has had a history of spot erosion, and has received complaints from
homeowners in the past. The analysis has shown that while there is little
significant increase in 2- and 10-year flood flows, the higher-frequency low-
volume storms have increased due to the Spotsylvania Mall area development.
Coupled with the natural erodibility of "fall line" streams, the channel outfall
from the mall watershed would be best served by strearnbank stabilization to
preserve the bank integrity of this channel into the City of Fredericksburg.

Recommendations

A surnmary oftecommendations follows:

1) With no significant flooding or erosion problems, development in the
Hazel Run Watershed can best be controlled through a Stormwater
Management Ordinance.

2) The County should maintain the existing retention facilities to include
ponds and lakes that currently exist in the watershed, while limiting
future development to existing runoff conditions down-stream.

3) The Spotsylvania Mall outfall should be stabilized to prevent further low
volume storm erosion though a cooperative effort with the City of
Fredericksburg.

J. X Timmons & Associatas, P.C. 2

PURPOSE

The purpose of this Study is to provide the County of Spotsylvania with a
management program to control flooding and property damage, soil loss and point
and nonpoint source pollution due to stormwater runoff in the Hazel Run
watershed in Spotsylvania County, Virginia.

WATERSHED DESCRIPTION

The Hazel Run watershed is located primarily in the Hazel Run and
Courtland districts of Spotsylvania County. The area of this study is bound
on the north by Route 3 (Germanna Highway), on the west by Routes 620
and 639 (Harrison and Leavell's Road, respectively), on the south by Route
208 (Courthouse Road), and on the east by U. S. Route 1 and the
Fredericksburg City Limits.

The Watershed is composed of three major branches or
subwatersheds, the upper tributary that collects runoff from the Route 3
corridor, Hazel Run, and Long Branch. Each of these subwatersheds will be
described in more detail in the following sections.

Of particular concern to both the County and the City of
Fredericksburg is the impact of Spotsylvania Mall development in the upper
tributary that feeds Hazel Run. This report refers to this tributary as the
Spotsylvania Mall Tributary. The development of the mall with no
impoundment structures, along with other commercial development along
the Route 3 corridor has raised the question as to its impacts on the erosion
of downstream channels.

The study area contains in its reaches some 5015 acres, most of
which fall in the limits of Spotsylvania County. This area contains a mix of
development ranging from varying density residential developments to
commercial and industrial corridors. The study area is comprised of the
three major tributary branches. For this study, these branches are called
the Spotsylvania Mall, Hazel Run, and Long Branch tributaries.

These three major branches of the Hazel Run watershed converge in
the City of Fredericksburg, at which point Hazel Run continues for nearly
three miles before feeding the Rappahannock River just south of downtown
Fredericksburg.

J. K Timmons & Assodates, P.C. 3

Spotsylvania Mall Tributary

The Spotsylvania Mall tributary drains 960 acres and is the
northernmost tributary in the study. This east to west tributary is
bound most approximately to the north by Route 3 and it extends
west to Salem Elementary School. This tributary is fed by the
commercial corridor fronting Route 3 and enters a closed system
under the parking lot of Spotsylvania Mall. It returns to open
channel flow prior to crossing Interstate 95 just south of its ramps at
Route 3. The flow, now in the City of Fredericksburg, travels just
south of the Altoona subdivision upstream of its confluence with
Hazel Run.

The development makeup of the Mall tributary is
predominantly commercial along the Route 3 corridor, where
development is currently 80% built out. A portion of the Shannon
Green development, north of Route 3 and in Fredericksburg,
contributes to the runoff in this tributary. South of the corridor, the
development becomes primarily medium density single family
housing. The average imperviousness over the Mall tributary is
estimated to be 31%.

Three BMPs exist in the Mall tributary, both in the vicinity of
the Maple Grove subdivision. One is-2.8 acres and lies directly south
of Waverly Village Drive, just upstream from Village Square
Shopping Center. The second lies approximately 2700 feet upstream
in Maple Grove, and covers 1.4 acres.

The third is a BMP to serve the south portion of the mall.
Under construction at the time of the study, the hydraulic
computations included the BMP as an existing structure for purposes
of determining stormwater flows. -

Hazel Run 7ributary

The central branch is called the Hazel Run tributary. At 1850
acres, this watershed contains 37% of the study area's coverage. It
runs from its headwaters near the Five Mile Fork intersection at
Route 3 flowing east roughly parallel to Harrison Road to its junction
with Long Branch west of Interstate 95.

With the exception of the uppermost reaches of this tributary

J. K 71mmons & Associatm, PC 4

being COMInffcial land use, roughly 90% of Hazel Run falls in
residential planned areas. Of this area, approximately 27% has been
constructed at the time of this study. The relatively low impervious
percentage (10.7%) for Hazel Run reflects the modest development
over the majority of this watershed.

There are no retention facilities in existence in the main
stream of Hazel Run. In the upper reach, new development includes
retention facilities to regulate the runoff. These developments
include the kr-)ummerlake and Queens Mill sections. Retention
facilities are also in place in the new Kingswood development as well
as the established Beauclaire Plantation.

Long Branch Tributary

The Long Branch Tributary is the southernmost tributary of
the study area. It flows west to east between Leavell's Road and
9r"
Interstate 'D then turns north to its junction with Hazel Run. Its
area is the largest of the study at 2200 acres.

To the west of Interstate 95 and south of Harrison Road, Long
Branch contains much of the single family development as does Hazel
Run. This development includes some multiple family as well as
detached home subdivisions.

The Long Branch tributary can be divided into its upper reach
and lower reach, the division line being Interstate 95. The upper
reach, as described above, contains mostly residential and
agricultural land. The lower reach, to the east of Interstate is
industrially developed, contains both the Four Mile and Spotsylvania
Industrial Parks at the intersection of U.S. Route 1 and Courthouse
Road. TheSE@ parks, though not fully developed, are a large
percentage of the impervious area that averages 22%. Figure 1,
shown on the following page depicts the study area divided into the
three major tributary subwatersheds that flow into the City of
Fredericksburg.

J. K Timmons & Associates, PC 5

FIGURE 1

F e sb

LOW

J.K TIMMONS & ASSOCIATES, P.C.
MAJOR TRIBUTARIES ENGINEERS * ARCHITECTS * SURVEYORS
F THE 711 N. COURTHOUSE RD. RICHMOND, VA
0 880.5.5TAPLES MILL RD. HENRICO CO., VA
4411 CROSSINGS BLVD. PRINCE GEORGE. VA.
HAZEL RUN WATERSHED DA TE, 7-14-9J SCALE. 1-@-JOOO'
DRAWN 8 Y- PM
MAUM =\0RAV\l&Ul\mfmlm\-
jFXV.MV-ll /"- I CHECKED 8 Y. CAL C. CHK.:
PREWOUS JOB NO. 1/08 NO.: 15-321

DATABASE ANALYSIS

To evaluate the appropriate comprehensive stormwater management
program for Spotsylvania County, it is first necessary to collect vital information
pertaining to the characteristics of the watershed itself. This report is the result
of a detailed analysis of the Hazel Run Watershed and its stormwater runoff
characteristics and data. The data used in this study is described in detail below.

Aerial Survey- The topography of the Hazel Run Watershed was
mapped digitally by Photogrammetric Data Services, Inc.
of Sterling, Virginia. Mapping compilation was
completed at 1"=400' scale with 5' contours.

Stormwater features such as stream channels, lakes,
ponds, and dams are included in this mapping. Digital
mapping of improvements, including buildings, roads,
parking lots, and the like are included to allow accurate
estimates of both levels of current development and
percentage of impervious ground coverage in the
watershed.

Soil Mapping- Soil mapping is critical to -the analysis of runoff
characteristics. Highly permeable soils, which include
sandy or gravelly silts, tend to allow high levels of
rainfall to be absorbed into the ground, reducing the
imnoff from the storm event. Highly impervious soils,
like many of Virginia's clays and silty clays, absorb very
little rainfall, forcing higher levels of runoff, affecting the
amount of rainfall that enters the main channels.

This study uses as its soil reference the Soil Surve_v of
Spotsylvania Count_v Virginia, published by the Soil
Conservation Service in cooperation with Virginia Tech.

'.From this survey soils are categorized into four soil
@Irroups, A through D. The soils of the A group (there
'were none found in the study area), consist of the most
highly permeable soils, while the D soils comprised of
mostly impervious clays and silts.

Soils that predominate this study area are the B, C and
D type soils. The D soils dominate the flood plain

J. X Timmons & Assodates, P. C. 7

regions and along the creek channels, while the B soils
are generally found in the upper topography of the study.

@@opies of the Soil Survey can be obtained upon request.

Hydrologic Mapping- ro evaluate the runoff generated in Hazel Run, it is
necessary to compile the development plan with the soil
mapping to further develop runoff characteristics within
each watershed. The resulting inforination allows for
detailed analysis of the runoff trend in each watershed.

J. K 71=ons Assodateq,.RC. 8

HMROLOGIC ANALYSIS
GENERAL To assess the conditions that exist in the Hazel Run Watershed and
determine recommendations for remedies that may be needed, parameters
are established and assumptions made in the analysis of the system. These
parameters are used to create the model that will estimate what happens
during rainfall events.

For the purposes of this study, the modeling utilizes the HEC-1
computer program, described later in this section.

One of the main assumptions of this study is to determine today's
rate of flow verses the rate of flow prior to any development occurring in the
watershed, including the development of ponds and lakes that exist today.
The assumption isbased on the premise that streams that serve the
undeveloped land are adequate to handle the flows, and that those flows
could be matched to current flows to determine the adequacy of today's
channels. Based on the increase in flow the watershed can then be
analyzed for flow attenuation alternatives.

The HEC-1 program is run for the 2, 10 and 100 year frequency
storm events. Points are analyzed throughout the watershed to determine
areas of increased runoff and to determine where, if any, control structures
should be incorporated.

Because the primary concern of this study is that of downstream
flooding in the City of Fredericksburg, four primary analysis points are
chosen for study. The points represent the following stream tributaries:

1) Hazel Run Confluence (City of Fredericksburg)
This analysis point is the first junction point that carries the
entire runoff from the study watershed.

2) Hazel Run T@-ibutary
This analysis point is taken in the Hazel Run tributary just
west of its confluence with the Long Branch tributary.

3) Long Branch 7@-ibutary
This analysis point is taken in the Long Branch just west of
its confluence with the Hazel Run tributary.

J. K Timmons & Associates, R C.9

4) Altoona Subdivision (City of Fredericksburg)
An area previously determined to have potential erosion
problems, the Altoona Subdivision is adjacent to the
Spotsylvania Mall tributary. A single cul-de-sac is within 30
feet of the channel.

These points are analyzed for predevelopment discharges versus the
existing runoff conditions. .

AffiTHODOLOGY

The hydrologic model used in this study is the HEC-1 Flood Analysis
Program developed by the U.S. Army Corps of Engineers, for determining
the peak runoff volumes and rates. The HEC-1 model is an accepted
standard of the practice, and is selected because of its ability to simulate
the study area as well as its ability to interact effectively with the HEC-2
Water Surface Profile program, which is used to determine the main
channel flood plain..

The primary objectives in using HEC-1 are:

1) To accurately model conditions of the watershed, both existing -and
predevelopment.

2) To deliver a model to the County that is in the public domain that
will likely rMain a standard of the practice, allowing future
modifications as the watershed develops.

3) To model the watershed so that many points within the watershed
can be checked for flow rates and flow times.

4) To deliver a model that can be used by engineers who develop sites
within the watershed, allowing them to submit development impact
analyses on the watershed.

This study employs the SCS option in HEC-1 for infiltration
determination and hydrograph creation. Additional information on this
program is available in the HEC-1 Flood Hydrograph Package User's
Manual, September 1990.

HEC-1 employs the use of information relevant to the stream morphology
and makeup to calculate flows in the watershed. This data is described more fully
below:

J. K Timmons & Associates, PC 10

Drainage Areas

Based on the aerial topography and field verification of flow
directions, the drainage areas that make up watersheds within the Hazel
Run watershed are divided for input in the HEC-1 model. Drainage breaks
are chosen in critical analysis points, major road crossings, and in locations
where a significant change in land use occurs. Each division, or
subwatershed is numbered and described in Appendix B.

Hydrologic parameters

The HEC-1 Model uses the following hydrologic input information to
simulate flows in each subwatershed:

1) Channel Slope
2) Channel Hydraulic Length
3) Impervious Fraction
4) SCS Curve Number
5) Mannings Roughness Coefficient "n"
6) Basin Lag Time
7) Loss Parameters

Rainfall Information

This study uses the 2, 10, and 100 year frequency storms with 24
hour durations as the standard storms for analysis.

Reservoir Routing

For both existing and proposed reservoirs, ponds, lakes and other
impoundments, the HEC-1 model simulates the outlet performance using
stage- storage-discharge relationships unique to each.

Reach Routing

HEC-1 uses input that indicates.the time and distance that
subwatershed flows require to flow through the downstream section. This
distance is not the hydraulic length of the downstream section but rather
the distance of thE@ main channel from the reach outfall to the downstream
outfall.

Land Use

The land use, which tells us about the ground cover makeup, is taken

J. K Timmons & Assodates,.P.C.

from the aerial photography of the watershed. The land use is used to
determine the SCS curve number as well as the impervious fraction for the
subwatersheds.

The land -use is used in large part to properly subdivide
(hydrologically) the watershed into its critical subwatersheds for study
using HEC-1. Where possible, the watersheds are divided where the type of
land use (commercial, industrial, residential) changes significantly.

The impervious fraction is the percentage of impervious surface
(rooftops, parking lots, roads, etc.) taken over the entire subwatershed
boundary. Within E@ach drainage area, a ten acre square representative of
the entire site is broken down into its impervious and pervious parts to
determine accurate impervious fractions. This method helps to create a
model that is tailored to the watershed.

Soils

Soil propertiES affect the runoff potential of a watershed.
Permeability, or the ability of soil to absorb rainfall, contributes to reducing
runoff as it increases. The soil slopes affect the amount of water that can
permeate the soil. Steep slopes allow little infiltration, while flat slopes
allow for higher rates of absorption. The ground cover can significantly
affect the ability of rainfall to enter the soil. Brush, grass, and heavy weeds
all help to hold the water to allow for infiltration, while barren ground does
little to assist effective ground interception.

The soils in this study are classified by the Soil Survey of
Spotsylvania Count_y, Virginia published by the U.S. Soil Conservation
Service. The soils are categorized into four hydrologic groups for purposes
of engineering analysis. The groups are:

Group A - Soils having high infiltration rates even when thoroughly
wetted and consisting chiefly of deep, well- to excessively-drained
sands and gravels. These soils can transmit water at high rates.
Highly permeable.

Group B - Soils having moderate infiltration rates when thoroughly
wetted and consisting chiefly of moderately deep to deep, moderately
well to well-drained soils with moderately fine to moderately coarse
textures. Moderately permeable.

Group C Soils having slow infiltration rates when thoroughly
wetted and consisting chiefly of soils with a layer that impedes

J K Timmons & Assodates, PC. 12

downward movement of water, or soils with moderately fine to fine
texture. These soils have a slow rate of water transmission. Slightly
permeable.

Group D - Soils having very slow infiltration rates when thoroughly
wetted and consisting chiefly of clay soils with a high swelling
potential, soils with a permanent high water table, soils with a
claypan or clay layer at or near the surface, and shallow soils over
nearly impervious material. These soils have a very slow water
transmission rate. Impervious.

The groups found in the study area are listed in Appendix C.

J. K Timmom & Associates, P.C. 13

ALMINATNE ANALYSIS

After analysis of the data within the watershed is performed to determine
the stormwater flow rates in the main stream channels, it is necessary to look at
improvement alternatives to achieve desired reductions in those rates. We
determined that the rate of reduction desired for this project is to that of
predevelopment levels. This will allow the stream to carry flows with no further
erosion.

The first of the three improvements analyses consists of choosing multiple
upland (out of major streams and floodplain) BMPs to -attenuate the flows and
reduce pollutant transport,. The second alternative calls for major in-stream
devices constructed to ternporarily slow the main channel flow- prior to its reaching
the City of Fredericksburg. The third alternative consists of county controlled
maintenance of existing lakes and ponds, while making improvements to points
along the system where erosion and sedimentation are occurring.

ALTFRNATM 1

BMPs, either as dry or wet ponds, are first considered for use as flow
attenuation devices. The basins will reduce the amount of runoff from each
of the watersheds served by the BMPs thereby reducing the total flow
entering the City of Fredericksburg.

These basins are schematically located to maximize the value as a
flood control device while minimizing the potential construction and
maintenance costs. Issues such as accessibility, topography, and
development percentage, all play a role in the initial locating of the
potential stormwater basins.

The existing lakes and ponds that are performing stormwater
attenuation functions are included as part of the BMP system. Because of
this inclusion, and to ensure the continued performance of those facilities,
the County will have to take over ownership and maintenance responsibility
for these existing facilities. This cost of ownership is included in the cost
analysis for the alternative.

The key issue regarding basin placement is that of wetland impacts.
The basins will have to receive a permit from the U.S. Army Corps of
Engineers if any inundation, removal, or other type of destruction to the
wetland habitat will occur in the implementation of the flood control plan.
Every effort will need to be taken to avoid and minimize the destruction of
wetlands, and so the basins are placed out of the main channels just

J. K Timmozw & Assodates, PC. 14

upstream of the major wetland areas of the watershed.

When analyzing the hydraulic character of the watershed, it is
important to note that a method to reduce the peak flows downstream is
through a combination of releasing water that is currently attenuated by
stormwater structures, while detaining water in other areas. This
combination will reduce the "delayed peak" phenomenon that occurs on
large watersheds using many control facilities. The delayed peak is a shift
of the downstream -peak flow conditions caused by detaining water and
holding it for a period of time. In many cases, the delayed peak can
actually be greater than the existing peak without detention, which in the
case of this study, will not solve the problem.

The best hydraulic results occur by using the configuration of basins
within the watershed shown on Figure 2 as Alternative 1 Improvements.
The alternative consists of the County overtaking the maintenance and
ownership responsibilities of eleven existing facilities throughout the three
tributaries and constructing twelve new facilities to impede the runoff
levels.

The runoff levels are reviewed downstream at the Hazel Run
Confluence. The mSUltS of Alternative 1 on the watershed system is as
follows:

Storm Chamiel Peak Flow Rates (units in c1s)

2 Year 10 Year 100 Year

Undeveloped 1458 3534 6112
Existing Cond. 1887 4144 6763
With Alt. 1 18,90 4202 6831

Note that the peak flows using flood control features upstream
actually increased the peak flow at the confluence by approximately 1%.

In reviewing potential locations to release water currently detained, it
is determined that the best hydraulic choices are those basins located
closest to the tributary confluence, namely in the area of the Industrial
Parks of Long Braxich. While releasing these flows may have helped reduce
the peak flows somewhat, the more important function that the existing
basins provide, nonpoint pollution reduction, is lost. While solving one
problem, a gre ater one is created.

J. K Timmons & Assodates, JRC. 15

FIGURE 2

46
e sb

J.K TIMMONS 11, ASSOCIATES, P.C.
Location of Improvements ENGINEERS * ARCHITECTS * SURVEYORS
for 711 N. COUR7HOUSERD. RICHMOND, VA
880J STAPLES A41LL RD. HENRICO CO., VA
Alternatkve 1 4411 CROSSINGS BLVD. PRINCE GEORGE, VA.
DA 7E. 7-14-9J SCA L E. IA@-JOOO'
DRAWN 8 K PM

CHECKED 8 Y. CALC.' CHK.:
PREwous 73-B N 0. JOB NO.: 15J21

ALTERNATM 1 (30STS

The following list is an estimate of the cost to Spotsylvania County
for implementing Alternative 1. The list includes both new construction
and upgrading existing facilities to properly attenuate the storms.
Maintenance and estimated land acquisition costs are also included.

IMP. DESCRIPTION LAND CONSTR. MAINT.
NO.

1 West of Cherry Rd. $8000 $60,000 $3000
Construct dry pond

2 East Cherry Rd. $9500 $100,000 $3000
Construct dry pond

3 Chancellor Village $5000 $30,000 $3200
Improve ex. wet pond

J. K nmmons & Associatm, P.C. 17

4 Sheraton Hills $8500 $90,000 $4500
Construct pond

5 Sheraton Oaks West $15,000 $140,000 $4500
Construct (Iry pond

6 SE Cherry Rd,/Rte.620 $9500 $90,000 $3000
Construct dry pond

7 Salisbury $12,000 $150,000 $5000
Construct dry pond

8 Sheraton Oaks $950 0 $100,000 $4500
Construct pond

9 Salem Station $8250 $70,000 $5000
Construct dry pond

10 Queen's Mill West $5000 $50,000 $3500
Improve ex. dry pond

11 Carriage Hills $5000 $50,000 $4500
Improve ex. wet pond

12 Leavell's Ci' -ossing $5000 $10,000 $3000
Improve ex. dry pond

13 Four Mile Fork Ind. Pk. $5000 $16,000 $4000
Improve pond (Qin=Qout)

14 Beauclaire Plantation $5000 $10,000 $4000
Improve ex. wet pond

15 Spotswood Heights $11,250 $90,000 $3000
Construct dry pond

16 VDOT Commuter Lot $2000 $50,000 $4000
Improve ex. dry pond

17 Rte 3/West Salem Ch. $8000 $70,000 $5500
Improve b,-Lsin

J. Y- Tinmons & ASSOCiatW., P.C. 18

18 Salem Ch. Elementary $5000 $110,000 $4000
Construct dr7 pond

19 Old Salem Church $10,000 $50,000 $3000
Improve ex. ivet pond

20 Maple GrovqfVil.Square $12,500 $50,000 $4500
Improve ex. wet pond

2-1 Toys-R-Us (Fred'Burg) - $20,000 -
Improve pond structures

22 Lower Spots. Mall $10,000- $70,000 $4500
Improve ex. dry pond

23 Maple Grove $5000 $40,000 $3000
Improve ex. -wet pond

TOTAL $174,000 $1,516,000

TOTAL ANNUAL MAINTENANCE $86,200

Costs associated with the implementation of this alternative are
extremely high. It is estimated that approximately $1,500,000 will be
required for the construction of the basin facilities in this plan. Annual
maintenance costs are nearly $90,000. Neither of these two costs includes
the high price of land acquisition for the nearly eighty acres of land for
construction and access.

nmmons & Associates, P.C. 19

MMRNATnT 2

The second alternative is to reduce flows in the main channels by
constructing flow constriction devices in the channels. The devices will
allow base stream flows to pass through uninhibited. As stormwater flows
entered the stream channel, the restrictive walls, or dam effect, will take
place, reducing the flows at the outflow while storing the attenuated water
temporarily in the flood plain area.

The delayed peak effect has to be checked using this alternative
because it is essentially the same analysis as Alternative 1, only on a larger
scale.

Alternative 2; Improvements include three flow restriction devices, one
on the Long Branch tributary and two on the Spotsylvania Mall tributary.
Figure 3 shows the locations. The Long Branch device is located just south
of Harrison Road (Route 620), and the two mall tributary devices are
located west of Salem Church Road and Interstate 95.

The runoff IE!vels are reviewed downstream at the Hazel Run
Confluence. The results of Alternative 2 on the watershed system is as
follows:

Storm Channel Peak Flow Rates (units in c.Ls.)

2 Year 10 Year 100 Year

Undeveloped 14:58 3534 6112
Existing Cond. 1887 4144 6763
With Alt. 2 1855 4190 6741

Like AlternELtive 1, the results of this alternative show that with the
improvements in place up stream, the flow in Fredericksburg shows little
reduction, and in the case of the 10 year peak flows, shows an actual
increase.

J. K Timmons & Associates,.P.C. 20

FIGURE 3

F de sb

J.K TIMMONS & ASSOCIATES, P.C.
Location of Im- rovements ENGINEERS * ARCHITECTS * SURVEYORS
P
for 711 N. COURTHOUSE RD. RICHMOND. VA
880J STAPLES A41LL RD. HENRICO CO., VA
Alternative 2 4411 CROSSINGS BLVD. PRINCE GEORGE, VA.
A@_ s
DA TE. 7-14-93 SCALE- -3000

DRAWN BY' PU

CHECKED 3 Y' CAL C. CHK:
PREWOUS J'OB NO. =JOB NO.: 15321

ALTERNATIVE 2 COSTS

Imp. DESCRIPTION LAND CONSTR. MAINT.
No.

1 Hazel RurVSalem Elem. Sch. $15.1000 $90YO00 $7000
Construct weir upstream of road

2 Long Branch/Harrison Rd. $30,000 $175,000 $8000
Construct weir upstream of road

3 Spots. MaIVI-95 Crossing $20,000 $190,000 $10,500

TOTAL $75,000 $455,000

TOTAL ANNUAL ACkUqTENANCE $25,500

Costs for Alternative 2, while less than Alternative 1, will approach
$500,000 in construction costs. Annual maintenance will add another
$25,500 required for the program.

Another complicating factor of using Alternative 2 is the
aforementioned wetlands issues. There is impacted wetlands at the site of
the device, and a mitigation program will likely be required. Also, there is
the fact that storrawater is stored, temporarily "flooding" a portion of land
(primarily wetlands). It is uncertain what the permitting position of the
Corps of Engineers might be, but they will likely consider the limits of
inundation to be ivetland impacts. This position will make in-stream
restrictive devices difficult and costly to permit and construct.

J K Timmons & Assodates, P.C. 22

ALTERNATNE 3

In evaluating the options to achieve the goals of this study, we again
turn to the major, component of the problem. Erosion and sedimentation
problems are occurring in and near Altoona, a subdivision located in the
City of Fredericksburg.

Using results obtained by studying the first two alternatives, we next
look at AlternatiW 3 Improvements. This involves no new basin
construction, takirLg over major existing facilities, and stabilizing the
downstream reach of the Spotsylvania Mall Tributary to alleviate the
erosion problems.

The reason for no new basin construction is as noted in the first two
alternatives. Delayed peaks caused by the basins do not suit the stated
goals of the flood control plan. The basins are a costly, and more
importantly, ineffective solution to the Fredericksburg erosion problem.

The county will maintain the existing major facilities under
Alternative 3 Improvements. This will ensure the continued flow character
of the watershed at its current level. These levels will -remain at current
levels by restricting future development from adding to the downstream
peak flow.

Stabilizinz Altoona Subdivision

Corrective measures to low level flood damage and erosion occurring
in the Spotsylvania Mall outfall include the stabilization of the naturally
erosive stream- banks. As a result the commercial development in the
watershed, the minor storms produce more runoff. It is these low volume
storms that, Occurring at higher frequencies, have caused some of the
erosion problems that exist in the stream. The highly erosive nature of this
fall line channel increases its susceptibility of degradation. To effectively
protect the channel, stabilization must be provided to handle these low flow
storm events.

The stabilization of the Spotsylvania Mall tributary is made
beginning upstream east of its crossing Interstate 95 down to its confluence
with Hazel Run. Most of the problem areas occur downstream of the
Altoona Subdivision. The measures consist of large diameter rip rap (18"
min. diameter) lining the main channel for a distance of approximately
4,000 feet. Figure 4 shows the location where the stream bank

J. Y, Timmons & Associates, P.C. 23

improvements need to be made.

The costs, aside from land acquisition whe re needed, is approximately
$120,000 for stream stabilization in the mall tributary, and maintenance
costs for the stream improvements is estimated to be less than $5,000 per
year.

J. Y, n=ons & Assodatw, P.C. 24

FIGURE 4

Improvements Area

S N

F de sb

J.K. TIMMONS 11, ASSOCIATES, P.C.
Location of Improvements ENGINEERS *ARCBITECT$,* SURVEYORS
f or 711 N. COUR7HOUSE RD. RICHMOND, VA
880J STAPLES MILL. RD. HENRICO CO., VA
Alternative 3 4411 CROSSINGS BLVD. PRINCE GEORGE, VA.
WE- 7-14-9J SCALE. 1`-@-3000'

DRAWN 8 Y- PM

CHECKED 8 Y- CAL C. CHK.:
PREWOUS JOB NO. JOB NO.: 15J21

RECOATMFNDATIONS

Alternatives 1 and 2 involve large amounts of land acquisition, high
construction and ongoing maintenance costs. In place, these improvements
only reduce peak flows at the Hazel Run confluence by 1-2%, in some cases
actually increasing the downstream peak rate. In all, these alternatives do
not serve the stated goals of the study.

Alternative 3 satisfies the goals of the study with one exception. It
reduces the erosive effect of the higher frequency lower-flow storms that are
causing some problems in the Altoona Subdivision area, while maintaining
the integrity of the watershed as it is. The type of erosion protection in the
mall watershed will also promote wetland growth. Sediment that is
deposited among the -stone will create a bedding for many plant species that
can filter pollutants as the stormwater flows through, creating a desirous
dual benefit for both the City of Fredericksburg and the County of
Spotsylvania.

What Alternative 3 does not do is decrease the current runoff levels,
one of the initial goals. However, since the overall increase in the high
volume storms from predevelopment conditions is between 1045%, and
because no major ero'sion problems have been experienced in the rest of the
watershed, protection for the future is best handled through stormwater
ordinance measures.

Protecting Future Develo)ment Flood Concerns

Effective flood control in Hazel Run depends on an effective approach
to the future development techniques used and required by Spotsylvania
County. While a developer may install an onsite Best Management Practice
(BMP) to control the immediate downstream channel, he may actually
accentuate the flows at the point where the stream enters the City of
Fredericksburg. An effective development plan must therefore be sensitive
to both site and regional constraints.

This is best achieved through a stormwater management ordinance
that will require new development to investigate its impacts on not only its
next door neighbor, but in the City of Fredericksburg. This will cause the
County of Spotsylvania to keep a master version of this stormwater
management study available for use by engineering consultants in their
impact assessments. By implementing this ordinance, Spotsylvania County
will be able to protect its own streams, but will be acting in good faith to
prevent any problems to her downstream neighbor, the City of
Fredericksburg.

J. K Timmozw & Assodates, F.C. 26

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I APPENDICK A
I MAIN CHANNEL FLOWS
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I J. K Tin=ons & Aasodates, P. C. 27
I

Main Channel Analysis - HEC-1 Model

The table below shows a comparison of the discharges found in critical areas
of the study area. Both predevelopment and current levels are shown.

Analysis Location Predevelope Current % Increase
(all discharges are in c.f.s.)

Hazel Run Confluence
2 Year 1458 1887 29%
10 Year 3534 4144 17%
100 Year 6112 6763 10.-5%

Hazel Run Trib.
2 Year 508 573 13%
10 Year 1199 1341 12%
100 Year 2082 2352 13%

Long Branch
2 Year 694 907 30%
10 Year 1668 1932 16%
100 Year 2819 3049 8%

Altoona Subdivision
2 Year 542 817 50%
10 Year 1277 1503 17%
100 Year 2132 2372 11%

J. K Tiamons &Aswc&teqP.C. 28

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I APPET@MEK B
I SUBWATERSHED DESCRIPMONS
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I J. IC nnmons & Assodates, P C. 29
1

SUBWATERSHED DES11DRIPTIONS

The following section describes the subwatersheds used in the HEC-1 model
that make up the study area. The subwatersheds are grouped into three
categories corresponding to where they flow. They are the Hazel Run, Long
Branch, and Spotsylvania Mall tributaries.

Hazel Run Tributary - 1001) Series

Subwatershed Location
ID Number Descrip lion

1000 West of Cherry Road

1001 East of Cherry Road

1002 Chancellor Green

1003 Chancellor Village/Five Mile Fork

1004 Chancellor Green

1005 Sheraton Hills

1006 Sheraton Hills

1001 Sheraton Oaks West

1008 Southeast Cherry Road/Harrison Road

1009 Salisbury

1010 Chancellor Landfill

1011 Sheraton Oaks

1012 Hazel Run/West of Salem Church Road

1013 Hazel Run/East of Salem Church Road

1014 Hazel Run/East Twin Springs Estates

1015 Maple Grove

1016 Hazel Run/Maple Grove Outfall

J. K Timmons & Assodates, P C. 30

1017 Hazel Run/Maple Woods Outfall

1018 Hazel RurV`West of Interstate

1019 Waverly Village

1020 WaverlY Village/Interstate

1021 Hazel Ruri/East of Interstate

1022 Hazel Run Outfall

Long Branch, Tributary - 2000 Series

Subwatershed Location
ID Number Descrip'Jon

2000 Summerlake

2001 Salem Station

2002 7 Queens Mill West

2003 Queens Mill East/Carriage Hills West

2004 Carriage Hills East

2005 Long Brancb/West of Interstate

2006 Leavell's Crossing

2007 Rollingwood Drive

2008 Four Mile Fork Industrial Park

2009 Long Brancb/East of Interstate

2010 Spotsylvania Industrial Park West

2011 Long Branch/Spotsylvania Industrial Park

2012 Long Branch

2013 Beauclaire Plantation

2014 Four Mile Fork Shopping Center

J. K Timmons & Associates, PC. 31

2015 Grainger Mobile Home Park

2016 Spotswood Heights/South Route 620

2017 Long Brancb/Kingswood

2018 Long Branch

2019 Long Branch Outfall

Spotsylvania. Mall Tributw-y - 3000 Series

Subwatershed Location
ID Number Descrip @ion

3000 Upper Sheraton I-Tills

3001 VDOT Commuter Lot

3002 Sheraton Oaks/Salem Elementary School

3003 Salem Elementary School

3004 Old Salem Church

3005 Maple Grove United Methodist Church

3006 Village Square,/Upper Spotsylvania Mall

3007 Toys-R-Us/Shannon Green

3008 Maple Grove/Lower Spotsylvania Mall

3009 Lower Spotsylvania Mall Outfall

3010 Interstate East (in City of Fredericksburg)

3011 Gateway Shopping Center (in City of Fredericksburg)

3012 Altoona Subdivision (in City of Fredericksburg)

3013 Greenbrier Shopping Center (in City of Fredericksburg)

3014 Altoona West Subdivision (in City of Fredericksburg)

J. Y, Timmons & Associates, ]:@C. 32

APPENDIX C
WATERSHED SOILS

J.K. Timmons & Associates, P.C. 33

The following is a list of the soils found in the watershed study area as compiled by the
SCS Soil Survey of Spotsylvania County, along with their soil numbe r and hydrologic
group--

soil soil Hydrologic
Number Description Group
1B Abell B
2B Altavista C
4C2 Appling B
4D2 Appling B
5 Aquults D
12B Cecil B
14B Colfax C
17C Dystrochrepts D
17D Dystrochrepts D
17E Dystrochrepts D
18B Emporia C
18C Emporia C
21B Faceville B
21C2 Faceville B
23 Fluvaquents D
24 Goldsboro B
25C Kempsville B
27 Louisburg B
29C2 Masada C
30B Mattaponi C
31C2 Mattaponi C
34B Partlow D
36B Savannah C
37B Spotsylvania C
44 Udorthents 45
B Udorthents D
46 Interstate D

For more information regarding the soils in this study, refer to the Soil Survey of
Spotsylvania County, Virwigia.

J. K Timmons & Associates, PC. 34

NOAA COASTAL SERVICES CTR LIBRARY

3 6668 14111749 -1