HomeMy WebLinkAboutSP201200029 Review Comments Drainage or Detention Plan and Comps. 2013-04-116,01A
COUNTY OF ALBEMARLE
Department of Community Development
401 McIntire Road
Charlottesville, Virginia 22902
11 Apr 2013
Daniel Hines, P.E.
Bohler Engineering
28 Blackwell Park Lane, Suite 201
Warrenton, Virginia 20186
Re: 5th Street Station CLOMR
Thank you for your submission of the proposed conditional letter of map revision for FEMA. After
consultation with FEMA, I have carefully reviewed your modeling data and summary documents. I
have the following comments regarding the information provided;
1. There seem to be irregularities in the water surface profiles. The hydraulic jumps downstream of
the bridge and near section 14.10 seem to be due to section geometry (channel bottom and widths)
more than expected results. The FIS shows an average slope of 0.002. The new models have
large sections of zero slope that are questionable. In the duplicate effective model, there is a
higher water surface elevation on the downstream side of the bridge. This appears to be an error.
The FIS profiles show water higher on the upstream side.
2. Please change the channel and overbank values to more closely correspond to the original FEMA
models. The original models used 0.7 for overbanks and 0.045 for the channel throughout the
model. You have changed your values to 0.12 and 0.035, with some exceptions. I have found no
basis for such significant changes in value. If a more detailed approach is considered for each
overbank reach, open areas (around 0.05), would need to be considered also, with brush and treed
areas as rough as 0.1. But a wooded density of 0. 12, and a smooth channel of 0.035 do not seem
justified.
3. Please change the contraction and expansion coefficients to more closely correspond to the
original FEMA models. FEMA uses 0.1 and 0.3 for all channel transitions, except on either side
of bridge openings where 0.6 and 0.8 are used. Wingwalls and/or armored transitions will need to
be provided for the bridge abutments to prevent the current scouring problems, and this will need
to be reflected in the model.
4. Please label sections on the plan with the modeled parameters (n values, channel limits, reach
lengths, etc).
5. Clarify how the cross - sections fit on the plans by showing the plotted points. Some of the
sections bend arbitrarily, in combination with the differences in reach length. For example,
section 16.21 ends abruptly on the right side in the model, and not on the plan. It is not clear why
this is a starting point, as the confluence with Biscuit Run will make this section's results
incomplete. Similarly, sections 16.17 and 14.20 seem arbitrarily bent, when adding more cross -
sections or ineffective flow areas would be appropriate to model the transition of overbank
widths.
Albemarle County Community Development
County Engineer letter
Page 2
6. Please remove the channel bank grading shown on the plan at stations 16.17, 16.15, 16.00, 15.00,
15.80, 14.4. It is not clear why this is on the CLOMR submission, or why it needs to occur.
7. Please model ineffective flow areas. For example, the protrusion of fill into the floodplain for the
entrance road and parking area has eliminated the incoming stream valley and some of the
overbank itself. The areas to either side of this protrusion appear to be ineffective flow areas.
The steady state flow analysis assumes these areas are active in conveyance unless they are
designated ineffective. This same approach should apply to other areas of significant width
transitions in the overbanks; for example, on the left side from 16.21 to 16.17, and 15.90 to 15.70,
right side 14.40 to 14.10.
8. Model bridge transitions. Typical bridge transitions define ineffective flow areas upstream and
downstream wide of abutments, typically transitioning 1:1 into and 2:1 out of the bridge opening.
9. Please revise the main channel widths to be narrower. The stream and immediate banks, which
utilize the smoother manning's coefficients, range from 20 to about 45 feet in width. The model
seems to have an overemphasized, overly wide channel, keeping the 100yr flow inside the
smoother roughness area. Most of the immediate banks are already rougher with vegetation.
10. The points of flow contribution don't seem to be updated with the new models. The summary
says the flows "appear to be reasonable ", but it is not clear how this judgment is made. Please
provide computations to verify. A suggestion would be to modify the drainage area map to
separate Biscuit Run and Moores Creek. The FIS provides a flow at the mouth of Biscuit Run
(5650cfs) which could be compared to results of the regression equations for the sub - watershed.
This could lend credence to the combined watershed result. Values could also be added for the
tributary under Rt. 64, and flow combined in the model at the newly created sections. This FIS
seems to indicate these flows were computed by the ACE in the 1970's. These values may be too
low today, as parts of the watershed have developed.
11. Please provide two copies of all application information. I will keep one copy for county records
and return a signed copy to you for the FEMA submission.
Sincerely,
Glenn Brooks, P.E.
County Engineer