BOARD00165

• Board Meeting Date: 3/24/2003
• Description: Flood Section - Special Contract Provisions for Montezuma County
• Board Meetings - Doc Type: Memo

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Although the old Corps of Engineer maps might be ~onsidered
acceptable in that they are only supposed to be accurate plus or
m:lnus2;S feet, that is only satisfactory for topographical
purposes such as calculation of volumes of cut or fill. However
for hydrological purposes such as predicting floods"such lack of
precision and inaccuracies are clearly unacceptable. Thus the
flood analyses used in issuing permits for mining along the river,
based on those old maps, are highly inaccurate.

In addition the earthworks linked to the gravel pits have,further
changed the topography, including lowered floodplains due to
stripping of topsoil, and the presence of dikes and stockpiles
which interfere with normal flood flows. These all create greater
risks of flooding.

1. LosS of floodplain due to dikes and temporary stockpiles

The large dikes and stockpiles constructed around the Line Camp
pits and at the Koenig site, reduce the flood-absorption capacity
of the floodplain at those sites. Any flood peak coming through
would be restricted to the main channel. This constriction would
cause a back-up in flood level upstream, and an increase in
velocities downstream. The increased velocities could cause the
river to jump its banks downstream, as well.

2. Trapping effect of gravel pits on flood peaks

If the floods inundate the pits, there would be a temporary
diversion of the main flow out of the channel and into the pits.
This would' provide only a temporary storage .off loodwaters,' about
the same as the normal floodplain. But it would run the risk of
diverting the main flow, as described below.

3. Diversion effects of dikes on floods

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In the Line Camp and the Koenig areas, depending on the current
condition of the dikes and stockpiles, the trapping effect could
easily cause a buildup of waters behind the dikes, with the risk
ofa disastrous breakthrough. The dikes ,are not reinforced or
armored, thus they CQuld easily erode and break. This would
release a wall of water doWnstream.

Especiplly at the Line Camp pits, it is likely that the flood flow
would seek an outlet in the low area near the highway, with the
possibility that the flood could jump out of the river channel on
the eastern side near the mountain, and move, to the western side
against the, highway, following the low swale. This would put
several homes and Properties at risk downstream.

Dolores River

9/12/02

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