HomeMy WebLinkAbout009 Stormwater Report BG Mill ;tis>��"`''' `��-�i 8/4/2022
Stormwater Design Report , •;;_, `;
A. Site&Drainage Plan with Toroaranhic Contours
A proposed site plan (Sheet C1.1)and a proposed grading and dr`8i2 ¢_pian (5 �6`�i.2)
are included with this submittal. These plans show the proposed sit ncluding
the proposed drainage structures. Finished grade spot elevations, and existing"one-half-foot
and proposed one-foot interval contours are also provided.
B. Description
Existing Stormwater Conditions:
Presently the 0.5 acre site consists of an existing 6,700 sf structure with gravel parking
surfacing with minimal landscaping. All stormwater runoff generated from this site flows
overland toward the north and east or along the street edge to the existing stormwater catch
basin located in the street at the northeast corner of the property. The existing storm drain
consists of an old 13"x13"square concrete manhole with 12"x12"grate. The project frontage
on Mendenhall contains portions of old broken up concrete curb which does not effectively
drain to the catch basin. The existing curb and gutter along Broadway is built to City
standards and is in good condition. The existing alley right-of-way is gravel surfaced and
slopes towards Mendenhall.
Proposed Stormwater Conditions:
The building being proposed is approximately 12,500 sf. All the runoff generated from this
structure will be captured via a combination of roof drains and yard drains and will be retained
in underground retention chambers located below the structure. The proposed storefront
sidewalk shall consist of a combination of concrete sidewalk with pervious landscaping within
the boulevard, allowing for infiltration of on-site runoff from the hardscaped areas. New curb
and gutter built to City standards will be installed along the entire Mendenhall frontage and a
new city standard 36"diameter storm drain inlet is proposed.The existing storm water pipes
entering and exiting the manhole will be maintained. A new stormwater overflow pipe is
proposed from the retention system sediment catchment manhole to the new storm drain
manhole in the street. The alley right-of-way will be paved with a new concrete approach and
asphalt paving from Mendenhall to the west building entrance. This alley will retain its
original drainage pattern of overland flow to Mendenhall Street.
Underground Retention Chambers:
Due to the limited space available for surface retention, underground retention is proposed.
Ten (10) SC-740 Stormtech Chambers will be installed to provide the necessary storage.
The chambers are traffic rated and will be installed directly beneath the parking garage slab
running down the middle of the building as shown on the grading and drainage plan. The
chamber system will allow infiltration of stormwater runoff into the native gravels below.An
overflow pipe shall be provided as described above. Details of the retention chamber system
are provided on Sheet C2.2.
Trench Drain:
A trench drain will also be located in the center of the garage to capture snow melt water from
vehicular tires as well as wash-down water from periodic cleaning of the parking area.The
small amount of surface runoff that flows into this entrance will be captured via a trench drain
running along the length of the opening. Water will then be piped to the sediment catch basin
and underground retention chambers.
Stormwater Design Report 1 of 2